- Email: [email protected]
Bipolar electrostatic charge and mass distributions of powder aerosols – Effects of inhaler design and inhaler material
Journal of Aerosol Science 95 (2016) 104–117
Contents lists available at ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Bipolar electrostatic charge and mass distributions of powder aerosols – Effects of inhaler design and inhaler material Jennifer Wong a, Philip Chi Lip Kwok b, Ville Niemelä c, Desmond Heng d, John Crapper e, Hak-Kim Chan a,n a
Advanced Drug Delivery Group, Faculty of Pharmacy, The University of Sydney, New South Wales, Australia Department of Pharmacology and Pharmacy, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong, China Dekati Limited, Kangasala, Finland d Institute of Chemical and Engineering Sciences, AnSTAR (Agency for Science and Technology and Research), Jurong Island, Singapore e Pharmaxis Limited, Frenchs Forest, New South Wales, Australia b c
a r t i c l e in f o
abstract
Article history: Received 22 July 2015 Received in revised form 11 December 2015 Accepted 3 February 2016 Available online 10 February 2016
This study investigated the effect of modifying the Aerolizers inhaler design and material on bipolar charging of a powder aerosol. The modified inhalers included various grid configurations, mouthpiece lengths, air inlet sizes, and were also constructed of different materials (acrylonitrile butadiene styrene, acetal, low density polyethylene, nylon, polypropylene, and stainless steel). The novel BOLAR™ (Dekati Ltd., Kangasala, Finland) was used to characterise bipolar charge and mass distributions. All of the tested inhalers showed spray-dried mannitol powder carried significantly different positive and negative charges that had higher magnitudes than the measured net charge. The estimated elementary charges per particle indicated the aerosols carried significant levels of charge that may potentially influence particle deposition in the lungs. Increasing the grid voidage decreased the magnitude of charging, as did reducing the air inlet size, whereas mouthpiece length had no obvious effect. The observed effects on charging due to modifications to the inhaler design could be explained by a difference in the level of contacts due to impaction between the particles and the inhaler internal surface. Bipolar charge of mannitol did not differ when dispersed by inhalers constructed of different materials. This could be attributed to various potential factors such as the very brief contact time between the powder and the inhaler material, as well as coating of the inhaler interior by the powder which effectively renders the surface to be mannitol-like. Overall, the comparable q/m profiles across all the inhaler designs and inhaler materials indicated charging correlated with the mass distributions and modifications to the inhaler did not affect the qualitative bipolar charge characteristics of mannitol powder. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Dry powder inhaler Aerolizers Spray-dried mannitol powder Triboelectrification Inhalation Bipolar charge analyzer (BOLAR™)
1. Introduction Electrostatic charge is one of five mechanisms that governs particle deposition in the lungs (Finlay, 2001; Hinds, 2012; Yang, Chan, & Chan, 2014). The literature reports a number of theoretical and experimental studies that demonstrated
n Correspondence to: S341, Pharmacy and Bank Building A15, University of Sydney, New South Wales 2006, Australia. Tel.: þ61 2 9351 3054; fax: þ61 2 9351 4391. E-mail address: [email protected] (H.-K. Chan).
http://dx.doi.org/10.1016/j.jaerosci.2016.02.003 0021-8502/& 2016 Elsevier Ltd. All rights reserved.
Table 1 Theoretical and experimental studies on charged particles. Year
Study
Charging method Particles
Particle size (lm)
Elementary charges per particle
Findings
Ref.
Induction charging
Di-2-ethylhexyl sebacate (DEHS)–ethanol solution
3–6
0–10,000
Azhdarzadeh, Olfert, Vehring, and Finlay (2014a)
2014 In silico –
–
0.005–30
–
2014 In vitro
Induction charging
DEHS–ethanol solution
3–6
0–10,000
2014 In vitro
Induction charging
DEHS–ethanol solution
3–6
0–25,000
2012 In vitro
Induction charging
DEHS–ethanol solution
1
0–24,000
2012 In silico –
–
0.3–1
40–130
2008 In vitro
0.5–10
–
1998 In silico –
Sodium chloride solution –
1
0–3,000
1997 In silico –
–
1–6
1–5,000
In vitro study on nasal deposition. Electrostatic charge effects were largest for the lowest flow rate, smallest particle size, and highest charge level. Deposition efficiency of charged particles can be up to three times that for neutral particles. This phenomenon was explained by the less important effect of impaction for smaller particle sizes. Modelling study on nasal deposition. While electrostatic charge exerted a discernible but insignificant effect on deposition for ultrafine aerosols, remarkably enhanced depositions were observed for charged microparticles compared to uncharged particles. Deposition efficiency of charged particles increased in comparison to the neutral aerosols, and electrostatic charge enhanced deposition of smaller particles due to the less dominant effect of inertial impaction. Higher deposition values were observed in the child oral-extrathoracic airways compared to the adult model in the previous study. This phenomenon was partly explained by higher velocities for a specific volume flow rate in the child replica, due to its smaller diameter compared to the adult model, which resulted in high impaction. The small geometry of the child model was also a favourable parameter in increasing deposition due to electrostatic charges because of the smaller distance between the particles and the walls of the airways. Particle charge substantially affected deposition, especially for smaller particle sizes where the impaction mechanism of deposition was less dominant. Deposition efficiency increased with increasing aerosol charge and tube length due to stronger image force and longer retention time, respectively. The enhanced deposition of charged particles in the alveolar region was up to five times higher than in the tracheobronchial region. Electrostatic charge enhanced deposition compared to uncharged aerosols. Simulations for particles with 0.5 mm diameter showed that as charge was increased to the relatively low level of 200 electrons the deposition in the alveolar region increased dramatically. Charge can also increase deposition in the upper airways and this effect increased with the level of charge. For particles with diameters between 1.5 and 2.5 mm, the effect of space charge force was significant in the first 10 generations of the respiratory tract, while in the last 13 generations the effect of image charge forces became more important as the airway diameters decreased.
2015 In vitro
Azhdarzadeh, Olfert, Vehring, and Finlay (2014c)
Azhdarzadeh, Olfert, Vehring, and Finlay (2014b) Chang et al. (2012)
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
Corona charging
Xi, Si, and Longest (2014)
Majid, Madl, Hofmann, and Alam (2012)
Ali, Reddy, and Mazumder (2008) Bailey, Hashish, and Williams (1998)
Balachandran, Machowski, Gaura, and Hudson (1997) 105
106
Table 1 (continued ) Year
Study
Charging method Particles
Particle size (lm)
Elementary charges per particle
–
1
0–500
1983 In vivo
Corona charging
Carnauba wax
1978 In vitro
Spinning disc aerosol generator
Iron oxide
0.3 0.6 1 2–7
12–29 230 to 34 99–142 360–1,000
1975 In vivo
Corona charging
Carnauba wax
0.33 0.65 1.1
29 45 95
Ref.
Simulation showed that electrostatic forces became predominant for particles with 200 elementary charges per particle. Particles with higher charge tend to deposit in the upper region of the respiratory tract, while particles with negligible degree of charge deposited preferentially in the lower region or alveolar sacs. Deposition in the lung is enhanced by increasing the level Hashish, Bailey, and Williams (1994a, of charge applied to aerosol particles. However, if narrow 1994b) airways are to be targeted, undesirable levels of deposition may arise in the larger airways. Fractional deposition of aerosol particles is enhanced by increasing particle size, as a result of the overall deposition efficiency being higher, though the relative effect of charge is reduced if charge level is not increased accordingly. Total deposition increased with increasing particle Melandri et al. (1983) charge. Chan, Lippmann, Cohen, and Schlesinger Electrostatic charges on particles significantly increased the deposition of monodispersed aerosols in a hollow cast (1978) of the human larynx-tracheobronchial tree. Melandri et al. (1975) Electrostatic charge increased deposition efficiency by 18.5–26.7% depending on particle size. For particles of the same size (0.75 mm), deposition efficiency increased with the elementary charges per particle.
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
1994 In silico –
Findings
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
107
electrostatic charge enhances deposition in the lungs, throat, and nasal passages for particles with diameters in the range 0.005–30 mm (Table 1). Electrostatic enhancement of particle deposition usually occurs through an ‘image charge’ between a charged particle and the grounded lining of the respiratory tract (Yu & Chandra, 1977). These attractive forces are greater as the particle comes into close proximity to a surface, and predominantly takes place in the alveolar region (Balachandran, Machowski, Gaura, & Hudson, 1997; Yu, 1985). Bailey et al. (1998) predicted that relatively low levels of 200 elementary charges could dramatically enhance deposition in the alveolar region, and even higher levels of 3,000 elementary charges may cause significant deposition in the upper airways. Previously, Kwok and Chan (2008) and Kwok, Glover, and Chan (2005) reported more than 600 and 40,000 elementary charges per particle for commercial dry powder inhalers (DPIs) and metered dose inhalers (MDIs), respectively. Such high levels of charge are likely to influence particle deposition in the lungs and warrants the need for investigating electrostatic charge in inhalable pharmaceutical aerosols. At present, there is no published data on how the fundamental design of an inhaler affects the bipolar charging of powder aerosols. While a series of studies by Coates, Fletcher, Chan, and Raper (2006, 2004) investigated how modifications to the Aerolizers inhaler grid structure, mouthpiece length, and air inlet size changed the air flow field within the device and affected the overall performance of the inhaler in terms of fine particle fraction, the effect on electrostatic charge was not considered. Pharmaceutical aerosols delivered by DPIs are known to carry bipolar charges (Balachandran, Kulon, Koolpiruck, Dawson, & Burnel, 2003; Beleca, Abbod, Balachandran, & Mille, 2010a, 2010b; Kannosto, Isherwood, Niemelä, & Ukkonenr, 2013; Mazumder et al., 2002; Saini, Biris, Srirama, & Mazumder, 2007). The solid particles tend to generate and accumulate bipolar charge when they undergo triboelectrification with each other and the surfaces of the inhaler (Matsusaka, 2011; Wong, Chan, & Kwok, 2013). Charge transfer occurs between particles of various sizes and surfaces made from different materials due to a difference in work function, which is defined as the minimum amount of energy required to remove an electron from the surface of a solid to a distance infinitely far (Gallo & Lama, 1976; Lowell & Rose-Innes, 1980). The theory of work function describes the direction of electron transfer from the material with lower work function to the one with higher until the electron energies reach equilibrium (Gallo & Lama, 1976; Lowell & Rose-Innes, 1980). To date, the lack of instruments with which to quantitatively measure bipolar charge by mass-based aerodynamic diameter has meant this area of research remains relatively unexplored. The recent availability of the Bipolar Charge Analyzer (BOLAR™, Dekati Ltd., Kangasala, Finland) offers a new platform for investigating bipolar charge in pharmaceutical aerosols (Wong et al., 2015b). It is the first commercially available impactor capable of separating and measuring positively and negatively charged particles according to aerodynamic particle size fractions by mass at air flow rates of 30–90 L/min. The concept, instrument design, and calibration of the BOLAR™ have been discussed elsewhere (Yli-Ojanperä et al., 2014). The present study aimed to use the novel BOLAR™ (Dekati Ltd., Kangasala, Finland) to determine the influence of modifying the Aerolizer inhaler (both device design and inhaler material) on bipolar charging of spray-dried mannitol powder, which is commercially available for the diagnosis of asthma and the treatment of cystic fibrosis. This formulation
Fig. 1. Schematic diagrams of the various Aerolizers inhaler designs. Diagrams not drawn to scale.
108
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
was selected because it contains only pure drug and the relatively large dose of 20 mg per capsule allows rapid mass assay from a single dose, thereby eliminating error due to inter-dispersion variations from multiple doses.
2. Materials and methods 2.1. Materials Spray-dried mannitol powder was supplied by Pharmaxis Ltd., Frenchs Forest, Australia; de-ionised water (electrical resistivity42 MΩ cm at 25 °C) was obtained from a Modulab Type II De-ionisation System (Continental Water Systems, Sydney, Australia); Slipicones silicone release spray was from DC products, Mount Waverly, Australia; 45 mm glass fibre filters were from MicroAnalytix, Pty. Ltd., Sydney, Australia; the commercial Aerolizer inhaler made of acrylonitrile butadiene styrene (ABS) was supplied by Novartis Australia Pty. Ltd., North Ryde, Australia; the modified Aerolizer inhalers with various grid configurations, mouthpiece lengths, and air inlet sizes (detailed in Fig. 1 as described by Coates et al. (2006, 2004)), and constructed of different polymer materials (acetal, low density polyethylene (LDPE), nylon, and polypropylene (PP)) were supplied by Plastiape S.p.A., Osnago, Italy; and the stainless steel Aerolizer was produced by Explorer Solutions Pte. Ltd., Singapore. 2.2. Particle size A Mastersizer 2000 coupled to a Scirocco 2000 dry dispersion unit (both Malvern Instruments, Worcestershire, UK) was used to measure the particle size distribution by laser diffraction. Approximately 20 mg of spray-dried mannitol powder was dispersed by compressed air at 4 bar of pressure (n ¼3). Measurement parameters were: obscuration of 0.5–6%; particle refractive index of 1.52; particle absorption of 0.10; dispersant (air) refractive index of 1.00. The diameters corresponding to the cumulative volume under 10%, 50%, and 90% were expressed as the d10, d50, and d90, respectively. The span, which describes the breadth of the particle size distribution, was calculated using the formula (d90 d10)/d50. 2.3. Scanning electron microscopy A Zeiss Ultra Plus Field Emission Microscope (Carl Zeiss SMT Ag, Oberkochen, Germany) was used to observe the particle morphology. Samples were dispersed onto sticky carbon tape, which were mounted on SEM stubs, and sputter coated with an approximately 15 nm thick layer of gold (K550X, Quorom Emitech, Kent, UK). Images were obtained at 2 kV. 2.4. Charge measurements The BOLAR™ (Dekati Ltd., Kangasala, Finland) was used to characterise inherent bipolar charge and mass distributions of mannitol powder. Briefly, the instrument consists of two main components, namely, a flow divider and six collection tubes. The flow divider evenly separates air flow into six branches, five of which lead to bipolar charge detection tubes and one to a simple Faraday pail (also called the reference chamber). Each bipolar charge detection tube is composed of an impactor on the top that has a specific aerodynamic cut-off diameter but does not measure charge, an inner cylinder at positive potential ( 1000 V), a grounded outer cylinder, and a filter stage at the bottom. Charge is measured on the particles that travel past the impactor stage and enter the cylinders. When a particle travels through the gap between the two cylinders, positively charged particles deposit on the outer cylinder, negatively charged particles deposit on the inner cylinder, and neutral particles pass through the gap and deposit on the filter stage. Individual electrometers connected to both cylinders measure the positive and negative charges separately. The reference chamber has no impactor stage on the top and measures the net charge of all incoming particles. In order to minimise particle bounce and re-entrainment, impactor stages were sprayed with silicone and the propellant was allowed to evaporate. The BOLAR detection tubes were not sprayed with silicone and glass fibre filters were placed in the bottommost filter stage before the unit was re-assembled. The air flow rate was calibrated using a flow metre (TSI 3063, TSI Instruments Ltd., Shoreview, USA), which was fitted with rubber adapters to form an airtight seal and attached to an inhaler loaded with a pierced empty capsule. Spray-dried mannitol powder (20 mg) was filled into a size 3 hydroxypropyl methylcellulose capsule (Vcapss, Capsugel Australia Pty. Ltd., West Ryde, Australia), loaded into the inhaler, and pierced prior to experiments. A single actuation was discharged for each measurement and experiments were performed in triplicate without gloves to simulate patient use. Ambient environmental conditions were also monitored (35 719% RH, 21.5 72.5 °C). The BOLAR was operated according to the manufacturer’s instructions. Firstly, a self-check test was performed before each measurement to ensure the BOLAR’s electrical control system was operating properly. Secondly, the loaded inhaler was attached to the USP throat, and the vacuum pump was turned on. Thirdly, the automated measurement sequence was initiated to open the internal valves that allow air to flow through the inhaler to disperse the powder into the BOLAR where the charge was measured. One measurement sequence took approximately 60 s and the inhaler was held tightly at the USP throat for the entire duration. Blank measurements were conducted in a similar way, except an inhaler loaded with a
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
109
pierced empty capsule was held tightly at the USP throat. These blank readings were subtracted from the powder charge measurements to give the actual charge and to account for disturbances in the electrical signals due to the presence of the inhaler at the USP throat. 2.5. Mass assay After the charge measurement, the detection tubes were carefully disassembled. The deposited powder on each component was exhaustively washed with 2.5–25 mL of de-ionised water. The specific volumes of the solvent were: 2.5 mL for the capsule; 5 mL each for the inhaler, adaptor, USP throat, flow divider inlet, impactor stages, and glass fibre filter; 10 mL for the filter stage; and 25 mL each for the flow divider, inner detection tube, and outer detection tube. The glass fibre filters were washed with de-ionised water, centrifuged at 13,400 rpm for 10 min (Minispins, Eppendorf, Westbury, USA), and the supernatant was collected for assay. Aliquots of the samples were chemically assayed using high performance liquid chromatography (HPLC). The Shimadzu HPLC system used included an RID-10A detector, LC-20AT pump, and SIL-20A HT autosampler controlled by LCSolution software (all Shimadzu Scientific Instruments, Kyoto, Japan). The mobile phase consisted of de-ionised water filtered through a 0.45 mm polyamide filter membrane (Sartorius Stedim Biotech GmbH, Göttingen, Germany). Samples were injected (20 mL) into a Resolve™ C18 5 mm 3.9 150 mm column (Waters, Milford, USA) at a flow rate of 1 mL/min, resulting in a retention time of 2 min. A calibration curve was constructed by serial dilution using freshly prepared standard solutions in de-ionised water at concentrations of 0.1 to 1,000 mg/mL (R2 ¼0.999). 2.6. Data analysis The charge detection cylinders in the BOLAR have no specific aerodynamic cut-off diameters. They simply collect particles that are not captured by the impactor stages upstream (Table 2). The charge and mass for a particular aerodynamic size range were calculated by subtracting the data between two consecutive detection tubes (Table 3). To facilitate graphical presentation and data discussion, a mid-point diameter representing each size range was calculated as the arithmetic mean of the upper and lower limits of a given size range (Table 3). Since only one sixth of the total dose after the flow divider was sampled by each detection tube, the magnitude of charge and mass equalled to the corresponding measured value multiplied by six. The BOLAR detects charge as current where the total charge of the positive or negative particles was obtained by integrating the electrical current signals on the outer and inner detection tubes, respectively. The net charge in a particular detection tube was the sum of the positive and negative total charges. The charge-to-mass ratio (q/m) was the quotient of the total charge and total drug mass on a particular detection tube, while the net q/m was the quotient of the net charge and total drug mass in a particular detection tube. The number of elementary charges per particles (n) in a particular size fraction was estimated using the following equation from Kwok and Chan (2008): n¼
q ρV : m e
ð1Þ
where ρ is the true density of the particles, V the volume of a particle, and e the elementary charge (1.602 10 19 C). The particles were assumed to be non-agglomerated, spherical, non-porous, and have an average true density of 1.5 g/cm3. For a given detection tube, V was calculated from the physical particle diameter, which was derived using the following equation: dp ¼da(ρ) 0.5 where dp is the physical particle diameter, and da the mid-point aerodynamic diameter for the detection tube, and ρ the true density. The emitted dose (ED) was defined as the total mass of drug recovered from all the components assayed excluding the capsule and inhaler. The fine particle dose (FPD) was defined as the mass of particles with an aerodynamic diameter o4.2 mm, which was given by the total amount of powder assayed in detection tube 3 (cut-off diameter 4.2 mm) multiplied by six. The fine particle fraction percentage loaded (FPF%Loaded) was defined as the percentage of FPD in the loaded dose, and fine particle fraction percentage emitted (FPF%Emitted) was the percentage of FPD in the emitted dose. Table 2 Aerodynamic diameter of the particles collected in the BOLAR™ bipolar charge detection tubes at 60 L/min (Dekati Ltd., 2014). Tube
Aerodynamic diameter (mm)
1 2 3 4 5
0.1o dp o 0.95 0.1o dp o 2.60 0.1o dp o 4.17 0.1o dp o 7.29 0.1o dp o 11.57
110
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
Table 3 Charge and mass calculations for BOLAR at 60 L/min (Dekati Ltd., 2014). Aerodynamic diameter range (mm)
Mid-point diameter (mm)
Charge, Q (pC)
0.1–0.95 0.95–2.60 2.60–4.17 4.17–7.29 7.29–11.57
0.525 1.78 3.39 5.73 9.43
6QTube 1 6(QTube 2 QTube 6(QTube 3 QTube 6(QTube 4 QTube 6(QTube 5 QTube
Mass, m (mg)
1) 2) 3) 4)
6mTube 1 6(mTube 2 mTube 6(mTube 3 mTube 6(mTube 4 mTube 6(mTube 5 mTube
1) 2) 3) 4)
Fig. 2. Scanning electron micrograph of spray-dried mannitol powder at 5000 magnification.
2.7. Statistical analysis GraphPad Prism 6 (GraphPad Software Inc., La Jolla, USA) was used to determine differences between the means within each size fraction. One-way analysis of variance (ANOVA), followed by the Tukey multiple comparisons post hoc test, were conducted with p o0.05 considered statistically significant.
3. Results 3.1. Physical characteristics Fig. 2 shows spray-dried mannitol powder consisted of microparticles, with a size distribution (volume diameter7one standard deviation) of d10 ¼ 1.14 70.00 mm, d50 ¼2.36 70.01 mm, d90 ¼4.52 70.03 mm, and span¼1.44. 3.2. Charge and mass profiles The recovered dose from all measurements ranged from 85% to 106%, which fell within the acceptable mass balance specified in the US Pharmacopoeia as 75–125% (US Pharmacopoeial Convention, 2010). Some components of the BOLAR had low amounts of deposited powder, with some below the detection limit of the HPLC. This could be expected for detection tubes with smaller cut-off diameters since the mannitol powder had relatively few particles in the submicron range, as depicted in the SEM and laser diffraction data (d10 ¼1.14 mm). Figs. 3–5 show the in vitro bipolar charge, mass distribution, and q/m profiles for mannitol powder aerosolised using Aerolizer inhalers with various grid structures, mouthpiece lengths, and air inlet sizes, respectively. The powder carried positively and negatively charged particles where the magnitude of charge depended on the particle size and inhaler design. For various grid structures, the particles charged positively in the range þ1,215 to þ 10,444 pC, negatively in the range 809 to 13,405 pC, and the charge magnitude decreased as the grid voidage increased. For different mouthpiece lengths, the particles carried similar positive (þ1,413 to þ 11,859 pC) and negative ( 809 to 13,517 pC) charges, though no obvious trend was observed. Changes to the air inlet size also led to similar levels of positive ( þ1,400 to þ 10,444 pC) and negative ( 239 to 13,022 pC) charges, where the magnitude of charge decreased with decreasing air inlet size. Hence, the net charge from all measurements was demonstrated to be due to the cancellation of significantly higher positive and negative charges. The resulting net charge profiles exhibited a bipolar trend where small particles charged positively and large particles charged negatively, with a change in polarity observed around the 2.60–4.17 mm cut-off diameter.
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
111
Fig. 3. Mean electrostatic charge, mass distribution, and charge-to-mass profiles of mannitol powder aerosolised using different grid structures. Data presented as mean 7 one standard deviation (n¼ 3). Components with o 50 mg assayed mannitol are not shown.
Fig. 4. Mean electrostatic charge, mass distribution, and charge-to-mass profiles of mannitol powder aerosolised using different mouthpiece lengths. Data presented as mean 7 one standard deviation (n¼ 3). Components with o 50 mg assayed mannitol are not shown.
112
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
Fig. 5. Mean electrostatic charge, mass distribution, and charge-to-mass profiles of mannitol powder aerosolised using different air inlet sizes. Data presented as mean7 one standard deviation (n¼ 3). Components with o 50 mg assayed mannitol are not shown.
Fig. 6. Mean fine particle fraction of mannitol powder aerosolised using different Aerolizers inhaler designs. Data presented as mean7 one standard deviation (n¼ 3). Asterisks indicate statistical significance between the ‘Original’ and ‘Grid 2’ structures.
When the charge was corrected for mass, q/m of positively charged particles ranged from þ1.95 to þ469 pC/mg (grid), þ1.72 to þ45.1 pC/mg (mouthpiece length), þ1.53 to þ145 pC/mg (air inlet size); and q/m of negatively charged particles ranged from 3.32 to 29.74 pC/mg (grid), 1.63 to 73.75 pC/mg (mouthpiece length), 1.63 to 48.8 pC/mg (air inlet size); leading to a net q/m of 4.86 to þ20.1 pC/mg (grid), 7.48 to þ20.1 pC/mg (mouthpiece length), 14.43 to þ20.1 pC/ mg (air inlet size). Overall, the electrostatic charge correlated with the mass and modifying the inhaler design did not affect the qualitative bipolar q/m profiles of mannitol powder. In this study, the fine particle dose was defined as the mass fraction of particles o4.2 mm, which is slightly different from that defined in Coates et al. (2006, 2004) as o6.8 mm. Nonetheless, the FPF%Loaded and FPF%Emitted exhibited trends consistent with the previous findings (Fig. 6). The inhaler grid played a significant role, whereas both the mouthpiece length and air inlet size had a less significant effect on the aerosol performance. These results suggest the BOLAR is suitable for investigating DPI device performance.
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
113
Fig. 7. Mean electrostatic charge, mass distribution, and charge-to-mass profiles of mannitol powder aerosolised using Aerolizers made of different inhaler materials. Data presented as mean7 one standard deviation (n¼ 3). Components with o 50 mg assayed mannitol are not shown. Abbreviations are defined as acrylonitrile butadiene styrene (ABS), low density polyethylene (LDPE), and polypropylene (PP).
Fig. 8. Mean fine particle fraction of mannitol powder aerosolised using Aerolizers made of different inhaler materials. Data presented as mean 7 one standard deviation (n¼3). Abbreviations are defined as acrylonitrile butadiene styrene (ABS), low density polyethylene (LDPE), and polypropylene (PP).
The in vitro bipolar charge, mass distribution, and q/m profiles of mannitol powder aerosolised using Aerolizer inhalers of the original design but constructed of different materials is shown in Fig. 7. Interestingly, the inhaler material had no statistically significant effect on either the charge and mass distributions (Fig. 7), or aerosol performance (Fig. 8). The particles charged positively in the range þ1,701 to þ 14,737 pC, negatively in the range 1,043 to 15,969 pC, and the resulting net charge profile ranged from 11,948 to þ 11,053 pC. The q/m of positively charged particles was þ1.53 to þ442.32 pC/mg and negatively charged particles was 1.31 to 72.8 pC/mg, leading to a net q/m of 6.43 to þ32.96 pC/mg. Again, the net charge profiles were found to change polarity around the 2.60–4.17 mm cut-off diameter. It is important to note that while charge profiles were remarkably reproducible throughout the study, mass distributions varied substantially within certain size fractions and these contributed to variations in q/m. The number of elementary charges per particle in size fractions with detectable drug mass are given in Table 4. Since the calculations using Eq. (1) made some assumptions about the particles, these values may be overestimates (Kwok & Chan, 2008).
114
Table 4 Estimated number of elementary charges per particle from Eq. (1). Positive charge
Negative charge
1 0.525 0.429
2 1.78 1.45
3 3.39 2.76
4 5.73 4.68
1 0.525 0.429
2 1.78 1.45
3 3.39 2.76
4 5.73 4.68
Inhaler configuration Full grid
55 (33)
3,280 (491)
15,419 (4,068)
25 (10)
16,657 (3,264)
N/A
2,850 (302)
26,072 ( 20,887)
115 (147)
4,267 (3,625) 838 (107)
4,310 (260)
Grid 1
2,952 (1,402) 2,338 (181)
4,689 (255)
Grid 2
652 (1003)
3,743 (698)
15,227 (4,135)
N/A
927 (364)
3,965 (310)
Full mouthpiece
55 (33)
3,280 (491)
15,419 (4,068)
25 (10)
4,310 (260)
¾ mouthpiece
N/A
3,229 (976)
14,361 (2,004)
N/A
4,267 (3,625) 1,251 (1,174)
18,799 (3,429) 18,300 (3,530) 16,657 (3,264)
3,947 (736)
16,537 (2,253)
½ mouthpiece
N/A
3,146 (179)
13,188 (4,034)
N/A
N/A
15,525 (1,161)
Full air inlet
55 (33)
3,280 (491)
15,419 (4,068)
25 (10)
2/3 air inlet
73 (63)
2,212 (208)
13,715 (7,838)
65 (54)
4,267 (3,625) 597 (326)
4,053 (1,414) 4,310 (260)
1/3 air inlet
N/A
3,551 (4,801)
2,000 (205)
11,603 (2,375)
N/A
N/A
3,322 (1,314)
Inhaler material ABS
N/A
1,175 (627)
3,497 (367)
14,990 (2,921)
N/A
792 (282)
Acetal LDPE
N/A N/A
1,061 (120) 1,759 (658)
3,428 (561) 3,538 (565)
11,007 (2,408) 15,511 (4,288)
N/A N/A
16,568 (3,853) 15,480 (1,675) 18,101 (2,197)
Nylon
N/A
1,448 (726)
4,439 (848)
N/A
N/A
N/A 5,150 (8,290) N/A
3,904 (1,375) 3,451 (844) 4,158 (250)
16,589 (1,991)
PP Stainless steel
86 (77) N/A
2,503 (320) 1,862 (1,131)
4,422 (423) 7,757 (5,851)
20,857 (2,696) 26,290 (23,667)
38 (24) N/A
3,445 (1,063) 5,039 (549) 4,750 (446)
2,540 (1,106) 2,952 (1,402) 2,069 (1,219) 1,953 (1,520) 2,952 (1,402) 1,579 (473)
Data presented as mean ( 7 one standard deviation) (n¼3). N/A: not calculated for components with o 50 mg assayed mannitol.
1,148 (148) 1,482 (1,687)
2,903 (219)
16,657 (3,264) 12,784 (2,532) 10,347 (646)
21,493 (2,135) 22,501 (1,671)
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
Detection tube Mid-point aerodynamic diameter (lm) Physical diameter (lm)
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
115
4. Discussion All of the inhalers used in this study showed spray-dried mannitol powder carried significantly different positive and negative charges that had higher magnitudes of charge than the measured net charge. The estimated elementary charges per particle (Table 4) indicated high levels of charge that may potentially influence particle deposition in the lungs. Additionally, small particles charged positively and large particles charged negatively. While the exact reasons for this behaviour are unknown (Wong, Kwok, & Chan, 2015a), Lacks and Sankaran (Lacks & Sankaran, 2011) have discussed several mechanisms including particle-size dependent differences in work function, capacitance, presence and direction of external electric fields, non-equilibrium states, and adsorption of contaminants that contribute to the interparticle contact. Overall, these results agree with previous studies that reported charge bipolarity in pharmaceutical aerosols (Balachandran et al., 2003; Beleca et al., 2010a, 2010b; Kannosto et al., 2013; Mazumder et al., 2002; Saini et al., 2007), which could have important implications during the development of products for inhalation. The observed effect of inhaler design on the bipolar charging of mannitol could be explained by a difference in the contacts due to impaction between the particles and the inhaler internal surface. According to computational fluid dynamics analysis performed by Coates et al. (2006, 2004), the number of impactions the particles have with the inhaler grid will decrease from 62% for the ‘Full grid’ to 20% for ‘Grid 2’ as the grid voidage is increased. The decreased number of contacts with the grid would result in the decreased charge magnitude observed. Even though increasing the grid voidage also increases particle impaction with the mouthpiece walls (22% for ‘Full grid’ compared to 88% for ‘Grid 2’), the particle– mouthpiece impactions occur potentially at a reduced impact intensity due to the lower air flow velocity and a much shallower impact angle than the particle–grid interactions (Coates et al., 2006, 2004). The length of the mouthpiece does not substantially change the particle contacts or impactions because the particle-grid and particle–wall interactions resulting from the air flow field generated in the swirling chamber of the devices remain the same and does not depend on the mouthpiece length, leading to similar charge profiles. As for the air inlet size, the charge magnitude decreased with decreasing air inlet size because reducing the air inlet at high flow rates (such as 60 L/min) causes a large amount of powder to be released from the device before the turbulence levels and particle impaction velocities could be fully developed (Coates et al., 2006, 2004). Overall, the q/m profiles across all the inhaler designs were comparable, indicating that the charge distribution correlated with the mass distributions and modifications to the inhaler design did not affect the qualitative bipolar charge of mannitol powder. Further to studying the inhaler design, we investigated the potential role of the inhaler material on charging. According to electrostatic theory, the triboelectric series enables preliminary prediction of net charge based on the work function of materials on the list. Indeed, Adi et al. (2010) correlated predictions from the triboelectric series with net charges of mannitol that had been tumbled inside containers made of different materials (including aluminium, copper, glass, nylon, polyethylene, polyvinyl chloride, stainless steel, and Teflon). However, contrary to this theory and the tumbled mannitol results, the bipolar charge of mannitol did not differ when dispersed by inhalers constructed of different materials. The results are consistent with the previous study by Kwok, Tran, and Chan (2010), who utilised the same set of polymeric Aerolizers at 30 L/min and examined the net charge using a modified Electrical Low Pressure Impactor (ELPI™). The authors observed no difference in the net charge profiles between the five inhaler materials (ABS, acetal, LDPE, nylon, and PP). The present study using the BOLAR has further demonstrated that inhaler material had an insignificant effect on the bipolar charging of spray-dried mannitol powder. The BOLAR results presented in this study indicate the electrification of mannitol during aerosolisation was not dependent on the inhaler material. It is possible that the crystalline nature of mannitol powder could contribute to a more consistent charging profile (Wong et al., 2014). Another possible reason for the lack of material dependence may be the short contact time between the powder and the internal inhaler surface during dispersion (o1 s) did not allow significant charge transfer. Due to the confined interior of the inhaler, it was also possible that a layer of powder became coated on the internal surface of the inhaler at the initial stage of dispersion. As a result, despite the different inhaler materials used, this coating would render the surface mannitol-like, leading effectively to the same type of mannitol–mannitol interactions instead of mannitol interactions with different inhaler materials. A time-dependent study may be able to support the above hypotheses. In practice, it is difficult to perform on the inhaler because the dispersion process takes place in only a few seconds. Future work using other pharmaceutical powders with different physicochemical properties (such as amorphous and crystalline polymorphic forms of other active ingredients and excipients) is needed to determine if the results are applicable to different formulations and to further establish the effect of inhaler material on charging during aerosolisation. A major limitation of the BOLAR impactor was the substantial variation in mass within certain size fractions. This can be attributed to the unconventional parallel detection tube assembly of the BOLAR where the charge and mass of a particle size fraction is calculated as a subtraction between two consecutive detection tubes. Furthermore, only one sixth of the total dose is sampled by each detection tube and is further distributed to multiple parts that may have relatively large surface areas. This type of experimental setup also contributed to problems with the calculation of q/m. For example, when the difference in mass between two subsequent detection tubes was negative (i.e. mtube1 4mtube2 in Table 3), these values were treated as zero mass and led to incomplete q/m profiles. More sensitive drug detection techniques, such as ultra performance liquid chromatography (UPLC) or liquid chromatography–mass spectrometry (LC–MS), may enhance the successful quantification of mass data. Apart from these potential drawbacks, the BOLAR has enabled the characterisation of bipolar charge in powder aerosols and the assessment of DPI device performance. In time, the knowledge gained may assist the
116
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
development of optimised inhaled products and lead to possible regulatory requirements on the characterisation of electrostatic properties of pharmaceuticals.
5. Conclusions This study investigated the influence of modifying the Aerolizer inhaler design and material on bipolar charging of spraydried mannitol powder. All of the tested inhalers showed that spray-dried mannitol powder carried significantly different positive and negative charges that had higher magnitudes of charge than the measured net charge. The estimated elementary charges per particle indicated that mannitol powder carried significant levels of charge that may potentially influence particle deposition in the lungs. Increasing the grid voidage decreased the magnitude of charging, as did reducing the air inlet size, whereas mouthpiece length had no obvious effect. Overall, the comparable q/m profiles across all the inhaler designs and inhaler materials indicated charge correlated with the mass distributions so modifications to the inhaler did not affect the bipolar charge characteristics of mannitol powder.
Acknowledgements The authors thank Mauro Citterio and Alberto Colombo from Plastiape S.p.A., Italy, for supplying the modified Aerolizers inhalers; Dr. Joseph Khachan from the School of Physics, University of Sydney, for his valuable discussions; Dr. Alun Pope from the Statistical Consulting Service, University of Sydney, for advice on statistical and multivariate analysis; the Australian Microscopy & Microanalysis Research Facility, University of Sydney, for providing facilities for SEM imaging; Kevin Samnick for feedback on the manuscript; as well as John Gar Yan Chan, Yu-Wei Lin, and Wenbo Wang for help with manual handling of the BOLAR. Jennifer Wong was a recipient of the Australian Postgraduate Award, and this work was supported by the Australian Research Council’s Discovery Projects funding scheme (DP120102778 and DP150103953).
References Adi, H., Kwok, P. C. L., Crapper, J., Young, P. M., Traini, D., & Chan, H.-K. (2010). Does electrostatic charge affect powder aerosolisation? Journal of Pharmaceutical Sciences, 99(5), 2455–2461, http://dx.doi.org/10.1002/jps.21996. Ali, M., Reddy, R. N., & Mazumder, M. K. (2008). Electrostatic charge effect on respirable aerosol particle deposition in a cadaver based throat cast replica. Journal of Electrostatics, 66(7–8), 401–406, http://dx.doi.org/10.1016/j.elstat.2008.02.005. Azhdarzadeh, M., Olfert, J. S., Vehring, R., & Finlay, W. H. (2014a). Effect of electrostatic charge on deposition of uniformly charged monodisperse particles in the nasal extrathoracic airways of an infant. Journal of Aerosol Medicine and Pulmonary Drug Delivery, 28(1), 30–34, http://dx.doi.org/10.1089/ jamp.2013.1118. Azhdarzadeh, M., Olfert, J. S., Vehring, R., & Finlay, W. H. (2014b). Effect of electrostatic charge on oral-extrathoracic deposition for uniformly charged monodisperse aerosols. Journal of Aerosol Science, 68, 38–45, http://dx.doi.org/10.1016/j.jaerosci.2013.11.002. Azhdarzadeh, M., Olfert, J. S., Vehring, R., & Finlay, W. H. (2014c). Effect of induced charge on deposition of uniformly charged particles in a pediatric oralextrathoracic airway. Aerosol Science and Technology, 48(5), 508–514, http://dx.doi.org/10.1080/02786826.2014.896989. Bailey, A. G., Hashish, A. H., & Williams, T. J. (1998). Drug delivery by inhalation of charged particles. Journal of Electrostatics, 44(1–2), 3–10, http://dx.doi.org/ 10.1016/s0304-3886(98)00017-5. Balachandran, W., Kulon, J., Koolpiruck, D., Dawson, M., & Burnel, P. (2003). Bipolar charge measurement of pharmaceutical powders. Powder Technology, 135, 156–163, http://dx.doi.org/10.1016/j.powtec.2003.08.011. Balachandran, W., Machowski, W., Gaura, E., & Hudson, C. (1997). Control of drug aerosol in human airways using electrostatic forces. Journal of Electrostatics, 40-1, 579–584, http://dx.doi.org/10.1016/S0304-3886(97)00106-X. Beleca, R., Abbod, M., Balachandran, W., & Miller, P. R. (2010a). Investigation of electrostatic properties of pharmaceutical powders using Phase Doppler anemometry. IEEE Transactions on Industry Applications, 46(3), 1181–1187, http://dx.doi.org/10.1109/Tia.2010.2045332. Beleca, R., Abbod, M., Balachandran, W., & Miller, P. R. (2010b). Measurement of bipolar charge distribution of lactose monohydrate using Phase Doppler Anemometry. Paper presented at the Respiratory Drug Delivery, Orlando, Florida. Chan, T. L., Lippmann, M., Cohen, V. R., & Schlesinger, R. B. (1978). Effect of electrostatic charges on particle deposition in a hollow cast of the human larynxtracheobronchial tree. Journal of Aerosol Science, 9(5), 463–468, http://dx.doi.org/10.1016/0021-8502(78)90009-5. Chang, K.-N., Chen, Y.-K., Huang, S.-H., Chen, C.-W., Lai, C.-Y., & Chen, C.-C. (2012). Penetration of charged particles through metallic tubes. Journal of Aerosol Science, 48(0), 10–17, http://dx.doi.org/10.1016/j.jaerosci.2012.01.006. Coates, M. S., Chan, H.-K., Fletcher, D. F., & Raper, J. A. (2006). Effect of design on the performance of a dry powder inhaler using computational fluid dynamics. Part 2: Air inlet size. Journal of Pharmaceutical Sciences, 95(6), 1382–1392, http://dx.doi.org/10.1002/jps.20603. Coates, M. S., Fletcher, D. F., Chan, H.-K., & Raper, J. A. (2004). Effect of design on the performance of a dry powder inhaler using computational fluid dynamics. Part 1: Grid structure and mouthpiece length. Journal of Pharmaceutical Sciences, 93(11), 2863–2876, http://dx.doi.org/10.1002/jps.20201. Dekati Ltd. (2014). Dekati BOLAR user manual version 1.1. Kangasala, Finland: D. Ltd. Finlay, W. (2001). The mechanics of inhaled pharmaceutical aerosols. San Diego: Academic Press. Gallo, C. F., & Lama, W. L. (1976). Some charge exchange phenomena explained by a classical model of the work function. Journal of Electrostatics, 2(2), 145–150, http://dx.doi.org/10.1016/0304-3886(76)90005-X. Hashish, A. H., Bailey, A. G., & Williams, T. J. (1994a). Modelling the effect of charge on selective deposition of particles in a diseased lung using aerosol boli. Physics in Medicine and Biology, 39(12), 2247. Hashish, A. H., Bailey, A. G., & Williams, T. J. (1994b). Selective deposition of pulsed charged aerosols in the human lung. Journal of Aerosol Medicine, 7(2), 167–171, http://dx.doi.org/10.1089/jam.1994.7.167. Hinds, W. C. (2012). 9.3 Deposition mechanisms aerosol technology: Properties, behavior, and measurement of airborne particles (2nd ed.). New York: John Wiley and Sons, Inc.191–196 Kannosto, J., Isherwood, H., Niemelä, V., & Ukkonen, A. (2013, May 21 24). High bipolar electrical charge levels of lactose particles measured from DPI. Paper presented at the Respiratory Drug Delivery, Berlin, Germany.
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
117
Kwok, P., Tran, P., & Chan, H.-K. (2010). Effect of Aerolizers and capsule materials on the electrification of spray dried mannitol particles. Paper presented at the American Association of Aerosol Research 29th Annual Conference, Portland, Oregon. Kwok, P. C. L., & Chan, H. K. (2008). Effect of relative humidity on the electrostatic charge properties of dry powder inhaler aerosols. Pharmaceutical Research, 25(2), 277–288, http://dx.doi.org/10.1007/s11095-007-9377-2. Kwok, P. C. L., Glover, W., & Chan, H.-K. (2005). Electrostatic charge characteristics of aerosols produced from metered dose inhalers. Journal of Pharmaceutical Sciences, 94(12), 2789–2799, http://dx.doi.org/10.1002/jps.20395. Lacks, D. J., & Sankaran, R. M. (2011). Contact electrification of insulating materials. Journal of Physics D: Applied Physics, 44(45), 453001. Lowell, J., & Rose-Innes, A. C. (1980). Contact electrification. Advances in Physics, 29(6), 947–1023, http://dx.doi.org/10.1080/00018738000101466. Majid, H., Madl, P., Hofmann, W., & Alam, K. (2012). Implementation of charged particles deposition in stochastic lung model and calculation of enhanced deposition. Aerosol Science and Technology, 46(5), 547–554, http://dx.doi.org/10.1080/02786826.2011.645957. Matsusaka, S. (2011). Control of particle tribocharging. Kona Powder and Particle Journal, 29, 27–38. Mazumder, M. K., Yurteri, C. U., Ahuja, G., Grable, N., Chan, H. K., & Chew, N. (2002). Bipolar electrostatic charging of fine pharmaceutical powders by dispersion and transport processes and its implications to lung deposition of inhaled DPI aerosols. Paper presented at the Respiratory Drug Delivery VIII, Tucson, Arizona. Melandri, C., Prodi, V., Tarroni, G., Formignani, M., De Zaiacomo, T., Bompane, G. F., & Maestri, G. (1975). On the deposition of unipolarly charged particles in the human respiratory tract. Inhaled Part, 4(Pt 1), 193–201. Melandri, C., Tarroni, G., Prodi, V., Dezaiacomo, T., Formignani, M., & Lombardi, C. C. (1983). Deposition of charged-particles in the human airways. Journal of Aerosol Science, 14(5), 657–669, http://dx.doi.org/10.1016/0021-8502(83)90070-90078. Saini, D., Biris, A. S., Srirama, P. K., & Mazumder, M. K. (2007). Particle size and charge distribution analysis of pharmaceutical aerosols generated by inhalers. Pharmaceutical Development and Technology, 12(1), 35–41, http://dx.doi.org/10.1080/10837450601166536. US Pharmacopoeial Convention (2010). G-08 Inhalation United States Pharmacopoeia 33-National Formulary 28. Rockville, MD, USA. Wong, J., Chan, H.-K., & Kwok, P. C. L. (2013). Electrostatics in pharmaceutical aerosols for inhalation. Therapeutic Delivery, 4(8), 981–1002, http://dx.doi.org/ 10.4155/tde.13.70. Wong, J., Kwok, P., Noakes, T., Fathi, A., Dehghani, F., & Chan, H.-K. (2014). Effect of crystallinity on electrostatic charging in dry powder inhaler formulations. Pharmaceutical Research, 31(7), 1656–1664, http://dx.doi.org/10.1007/s11095-013-1270-6. Wong, J., Kwok, P. C. L., & Chan, H.-K. (2015a). Electrostatics in pharmaceutical solids. Chemical Engineering Science, 125(0), 225–237, http://dx.doi.org/ 10.1016/j.ces.2014.05.037. Wong, J., Lin, Y.-W., Kwok, P. C. L., Niemelä, V., Crapper, J., & Chan, H.-K. (2015b). Measuring bipolar charge and mass distributions of powder aerosols by a novel tool (BOLAR). Molecular Pharmaceutics, 12(9), 3433–3440, http://dx.doi.org/10.1021/acs.molpharmaceut.5b00443. Xi, J., Si, X., & Longest, W. (2014). Electrostatic charge effects on pharmaceutical aerosol deposition in human nasal–laryngeal airways. Pharmaceutics, 6(1), 26–35, http://dx.doi.org/10.3390/pharmaceutics6010026. Yang, M. Y., Chan, J. G. Y., & Chan, H.-K. (2014). Pulmonary drug delivery by powder aerosols. Journal of Controlled Release, 193, 228–240, http://dx.doi.org/ 10.1016/j.jconrel.2014.04.055. Yli-Ojanperä, J., Ukkonen, A., Järvinen, A., Layzell, S., Niemelä, V., & Keskinen, J. (2014). Bipolar charge analyzer (BOLAR): A new aerosol instrument for bipolar charge measurements. Journal of Aerosol Science, 77(0), 16–30, http://dx.doi.org/10.1016/j.jaerosci.2014.07.004. Yu, C. P. (1985). Theories of electrostatic lung deposition of inhaled aerosols. Annals of Occupational Hygiene, 29(2), 219–227, http://dx.doi.org/10.1093/ annhyg/29.2.219. Yu, C. P., & Chandra, K. (1977). Precipitation of submicron charged-particles in human lung airways. Bulletin of Mathematical Biology, 39(4), 471–478, http: //dx.doi.org/10.1007/Bf02462925.
Contents lists available at ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Bipolar electrostatic charge and mass distributions of powder aerosols – Effects of inhaler design and inhaler material Jennifer Wong a, Philip Chi Lip Kwok b, Ville Niemelä c, Desmond Heng d, John Crapper e, Hak-Kim Chan a,n a
Advanced Drug Delivery Group, Faculty of Pharmacy, The University of Sydney, New South Wales, Australia Department of Pharmacology and Pharmacy, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong, China Dekati Limited, Kangasala, Finland d Institute of Chemical and Engineering Sciences, AnSTAR (Agency for Science and Technology and Research), Jurong Island, Singapore e Pharmaxis Limited, Frenchs Forest, New South Wales, Australia b c
a r t i c l e in f o
abstract
Article history: Received 22 July 2015 Received in revised form 11 December 2015 Accepted 3 February 2016 Available online 10 February 2016
This study investigated the effect of modifying the Aerolizers inhaler design and material on bipolar charging of a powder aerosol. The modified inhalers included various grid configurations, mouthpiece lengths, air inlet sizes, and were also constructed of different materials (acrylonitrile butadiene styrene, acetal, low density polyethylene, nylon, polypropylene, and stainless steel). The novel BOLAR™ (Dekati Ltd., Kangasala, Finland) was used to characterise bipolar charge and mass distributions. All of the tested inhalers showed spray-dried mannitol powder carried significantly different positive and negative charges that had higher magnitudes than the measured net charge. The estimated elementary charges per particle indicated the aerosols carried significant levels of charge that may potentially influence particle deposition in the lungs. Increasing the grid voidage decreased the magnitude of charging, as did reducing the air inlet size, whereas mouthpiece length had no obvious effect. The observed effects on charging due to modifications to the inhaler design could be explained by a difference in the level of contacts due to impaction between the particles and the inhaler internal surface. Bipolar charge of mannitol did not differ when dispersed by inhalers constructed of different materials. This could be attributed to various potential factors such as the very brief contact time between the powder and the inhaler material, as well as coating of the inhaler interior by the powder which effectively renders the surface to be mannitol-like. Overall, the comparable q/m profiles across all the inhaler designs and inhaler materials indicated charging correlated with the mass distributions and modifications to the inhaler did not affect the qualitative bipolar charge characteristics of mannitol powder. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Dry powder inhaler Aerolizers Spray-dried mannitol powder Triboelectrification Inhalation Bipolar charge analyzer (BOLAR™)
1. Introduction Electrostatic charge is one of five mechanisms that governs particle deposition in the lungs (Finlay, 2001; Hinds, 2012; Yang, Chan, & Chan, 2014). The literature reports a number of theoretical and experimental studies that demonstrated
n Correspondence to: S341, Pharmacy and Bank Building A15, University of Sydney, New South Wales 2006, Australia. Tel.: þ61 2 9351 3054; fax: þ61 2 9351 4391. E-mail address: [email protected] (H.-K. Chan).
http://dx.doi.org/10.1016/j.jaerosci.2016.02.003 0021-8502/& 2016 Elsevier Ltd. All rights reserved.
Table 1 Theoretical and experimental studies on charged particles. Year
Study
Charging method Particles
Particle size (lm)
Elementary charges per particle
Findings
Ref.
Induction charging
Di-2-ethylhexyl sebacate (DEHS)–ethanol solution
3–6
0–10,000
Azhdarzadeh, Olfert, Vehring, and Finlay (2014a)
2014 In silico –
–
0.005–30
–
2014 In vitro
Induction charging
DEHS–ethanol solution
3–6
0–10,000
2014 In vitro
Induction charging
DEHS–ethanol solution
3–6
0–25,000
2012 In vitro
Induction charging
DEHS–ethanol solution
1
0–24,000
2012 In silico –
–
0.3–1
40–130
2008 In vitro
0.5–10
–
1998 In silico –
Sodium chloride solution –
1
0–3,000
1997 In silico –
–
1–6
1–5,000
In vitro study on nasal deposition. Electrostatic charge effects were largest for the lowest flow rate, smallest particle size, and highest charge level. Deposition efficiency of charged particles can be up to three times that for neutral particles. This phenomenon was explained by the less important effect of impaction for smaller particle sizes. Modelling study on nasal deposition. While electrostatic charge exerted a discernible but insignificant effect on deposition for ultrafine aerosols, remarkably enhanced depositions were observed for charged microparticles compared to uncharged particles. Deposition efficiency of charged particles increased in comparison to the neutral aerosols, and electrostatic charge enhanced deposition of smaller particles due to the less dominant effect of inertial impaction. Higher deposition values were observed in the child oral-extrathoracic airways compared to the adult model in the previous study. This phenomenon was partly explained by higher velocities for a specific volume flow rate in the child replica, due to its smaller diameter compared to the adult model, which resulted in high impaction. The small geometry of the child model was also a favourable parameter in increasing deposition due to electrostatic charges because of the smaller distance between the particles and the walls of the airways. Particle charge substantially affected deposition, especially for smaller particle sizes where the impaction mechanism of deposition was less dominant. Deposition efficiency increased with increasing aerosol charge and tube length due to stronger image force and longer retention time, respectively. The enhanced deposition of charged particles in the alveolar region was up to five times higher than in the tracheobronchial region. Electrostatic charge enhanced deposition compared to uncharged aerosols. Simulations for particles with 0.5 mm diameter showed that as charge was increased to the relatively low level of 200 electrons the deposition in the alveolar region increased dramatically. Charge can also increase deposition in the upper airways and this effect increased with the level of charge. For particles with diameters between 1.5 and 2.5 mm, the effect of space charge force was significant in the first 10 generations of the respiratory tract, while in the last 13 generations the effect of image charge forces became more important as the airway diameters decreased.
2015 In vitro
Azhdarzadeh, Olfert, Vehring, and Finlay (2014c)
Azhdarzadeh, Olfert, Vehring, and Finlay (2014b) Chang et al. (2012)
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
Corona charging
Xi, Si, and Longest (2014)
Majid, Madl, Hofmann, and Alam (2012)
Ali, Reddy, and Mazumder (2008) Bailey, Hashish, and Williams (1998)
Balachandran, Machowski, Gaura, and Hudson (1997) 105
106
Table 1 (continued ) Year
Study
Charging method Particles
Particle size (lm)
Elementary charges per particle
–
1
0–500
1983 In vivo
Corona charging
Carnauba wax
1978 In vitro
Spinning disc aerosol generator
Iron oxide
0.3 0.6 1 2–7
12–29 230 to 34 99–142 360–1,000
1975 In vivo
Corona charging
Carnauba wax
0.33 0.65 1.1
29 45 95
Ref.
Simulation showed that electrostatic forces became predominant for particles with 200 elementary charges per particle. Particles with higher charge tend to deposit in the upper region of the respiratory tract, while particles with negligible degree of charge deposited preferentially in the lower region or alveolar sacs. Deposition in the lung is enhanced by increasing the level Hashish, Bailey, and Williams (1994a, of charge applied to aerosol particles. However, if narrow 1994b) airways are to be targeted, undesirable levels of deposition may arise in the larger airways. Fractional deposition of aerosol particles is enhanced by increasing particle size, as a result of the overall deposition efficiency being higher, though the relative effect of charge is reduced if charge level is not increased accordingly. Total deposition increased with increasing particle Melandri et al. (1983) charge. Chan, Lippmann, Cohen, and Schlesinger Electrostatic charges on particles significantly increased the deposition of monodispersed aerosols in a hollow cast (1978) of the human larynx-tracheobronchial tree. Melandri et al. (1975) Electrostatic charge increased deposition efficiency by 18.5–26.7% depending on particle size. For particles of the same size (0.75 mm), deposition efficiency increased with the elementary charges per particle.
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
1994 In silico –
Findings
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
107
electrostatic charge enhances deposition in the lungs, throat, and nasal passages for particles with diameters in the range 0.005–30 mm (Table 1). Electrostatic enhancement of particle deposition usually occurs through an ‘image charge’ between a charged particle and the grounded lining of the respiratory tract (Yu & Chandra, 1977). These attractive forces are greater as the particle comes into close proximity to a surface, and predominantly takes place in the alveolar region (Balachandran, Machowski, Gaura, & Hudson, 1997; Yu, 1985). Bailey et al. (1998) predicted that relatively low levels of 200 elementary charges could dramatically enhance deposition in the alveolar region, and even higher levels of 3,000 elementary charges may cause significant deposition in the upper airways. Previously, Kwok and Chan (2008) and Kwok, Glover, and Chan (2005) reported more than 600 and 40,000 elementary charges per particle for commercial dry powder inhalers (DPIs) and metered dose inhalers (MDIs), respectively. Such high levels of charge are likely to influence particle deposition in the lungs and warrants the need for investigating electrostatic charge in inhalable pharmaceutical aerosols. At present, there is no published data on how the fundamental design of an inhaler affects the bipolar charging of powder aerosols. While a series of studies by Coates, Fletcher, Chan, and Raper (2006, 2004) investigated how modifications to the Aerolizers inhaler grid structure, mouthpiece length, and air inlet size changed the air flow field within the device and affected the overall performance of the inhaler in terms of fine particle fraction, the effect on electrostatic charge was not considered. Pharmaceutical aerosols delivered by DPIs are known to carry bipolar charges (Balachandran, Kulon, Koolpiruck, Dawson, & Burnel, 2003; Beleca, Abbod, Balachandran, & Mille, 2010a, 2010b; Kannosto, Isherwood, Niemelä, & Ukkonenr, 2013; Mazumder et al., 2002; Saini, Biris, Srirama, & Mazumder, 2007). The solid particles tend to generate and accumulate bipolar charge when they undergo triboelectrification with each other and the surfaces of the inhaler (Matsusaka, 2011; Wong, Chan, & Kwok, 2013). Charge transfer occurs between particles of various sizes and surfaces made from different materials due to a difference in work function, which is defined as the minimum amount of energy required to remove an electron from the surface of a solid to a distance infinitely far (Gallo & Lama, 1976; Lowell & Rose-Innes, 1980). The theory of work function describes the direction of electron transfer from the material with lower work function to the one with higher until the electron energies reach equilibrium (Gallo & Lama, 1976; Lowell & Rose-Innes, 1980). To date, the lack of instruments with which to quantitatively measure bipolar charge by mass-based aerodynamic diameter has meant this area of research remains relatively unexplored. The recent availability of the Bipolar Charge Analyzer (BOLAR™, Dekati Ltd., Kangasala, Finland) offers a new platform for investigating bipolar charge in pharmaceutical aerosols (Wong et al., 2015b). It is the first commercially available impactor capable of separating and measuring positively and negatively charged particles according to aerodynamic particle size fractions by mass at air flow rates of 30–90 L/min. The concept, instrument design, and calibration of the BOLAR™ have been discussed elsewhere (Yli-Ojanperä et al., 2014). The present study aimed to use the novel BOLAR™ (Dekati Ltd., Kangasala, Finland) to determine the influence of modifying the Aerolizer inhaler (both device design and inhaler material) on bipolar charging of spray-dried mannitol powder, which is commercially available for the diagnosis of asthma and the treatment of cystic fibrosis. This formulation
Fig. 1. Schematic diagrams of the various Aerolizers inhaler designs. Diagrams not drawn to scale.
108
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
was selected because it contains only pure drug and the relatively large dose of 20 mg per capsule allows rapid mass assay from a single dose, thereby eliminating error due to inter-dispersion variations from multiple doses.
2. Materials and methods 2.1. Materials Spray-dried mannitol powder was supplied by Pharmaxis Ltd., Frenchs Forest, Australia; de-ionised water (electrical resistivity42 MΩ cm at 25 °C) was obtained from a Modulab Type II De-ionisation System (Continental Water Systems, Sydney, Australia); Slipicones silicone release spray was from DC products, Mount Waverly, Australia; 45 mm glass fibre filters were from MicroAnalytix, Pty. Ltd., Sydney, Australia; the commercial Aerolizer inhaler made of acrylonitrile butadiene styrene (ABS) was supplied by Novartis Australia Pty. Ltd., North Ryde, Australia; the modified Aerolizer inhalers with various grid configurations, mouthpiece lengths, and air inlet sizes (detailed in Fig. 1 as described by Coates et al. (2006, 2004)), and constructed of different polymer materials (acetal, low density polyethylene (LDPE), nylon, and polypropylene (PP)) were supplied by Plastiape S.p.A., Osnago, Italy; and the stainless steel Aerolizer was produced by Explorer Solutions Pte. Ltd., Singapore. 2.2. Particle size A Mastersizer 2000 coupled to a Scirocco 2000 dry dispersion unit (both Malvern Instruments, Worcestershire, UK) was used to measure the particle size distribution by laser diffraction. Approximately 20 mg of spray-dried mannitol powder was dispersed by compressed air at 4 bar of pressure (n ¼3). Measurement parameters were: obscuration of 0.5–6%; particle refractive index of 1.52; particle absorption of 0.10; dispersant (air) refractive index of 1.00. The diameters corresponding to the cumulative volume under 10%, 50%, and 90% were expressed as the d10, d50, and d90, respectively. The span, which describes the breadth of the particle size distribution, was calculated using the formula (d90 d10)/d50. 2.3. Scanning electron microscopy A Zeiss Ultra Plus Field Emission Microscope (Carl Zeiss SMT Ag, Oberkochen, Germany) was used to observe the particle morphology. Samples were dispersed onto sticky carbon tape, which were mounted on SEM stubs, and sputter coated with an approximately 15 nm thick layer of gold (K550X, Quorom Emitech, Kent, UK). Images were obtained at 2 kV. 2.4. Charge measurements The BOLAR™ (Dekati Ltd., Kangasala, Finland) was used to characterise inherent bipolar charge and mass distributions of mannitol powder. Briefly, the instrument consists of two main components, namely, a flow divider and six collection tubes. The flow divider evenly separates air flow into six branches, five of which lead to bipolar charge detection tubes and one to a simple Faraday pail (also called the reference chamber). Each bipolar charge detection tube is composed of an impactor on the top that has a specific aerodynamic cut-off diameter but does not measure charge, an inner cylinder at positive potential ( 1000 V), a grounded outer cylinder, and a filter stage at the bottom. Charge is measured on the particles that travel past the impactor stage and enter the cylinders. When a particle travels through the gap between the two cylinders, positively charged particles deposit on the outer cylinder, negatively charged particles deposit on the inner cylinder, and neutral particles pass through the gap and deposit on the filter stage. Individual electrometers connected to both cylinders measure the positive and negative charges separately. The reference chamber has no impactor stage on the top and measures the net charge of all incoming particles. In order to minimise particle bounce and re-entrainment, impactor stages were sprayed with silicone and the propellant was allowed to evaporate. The BOLAR detection tubes were not sprayed with silicone and glass fibre filters were placed in the bottommost filter stage before the unit was re-assembled. The air flow rate was calibrated using a flow metre (TSI 3063, TSI Instruments Ltd., Shoreview, USA), which was fitted with rubber adapters to form an airtight seal and attached to an inhaler loaded with a pierced empty capsule. Spray-dried mannitol powder (20 mg) was filled into a size 3 hydroxypropyl methylcellulose capsule (Vcapss, Capsugel Australia Pty. Ltd., West Ryde, Australia), loaded into the inhaler, and pierced prior to experiments. A single actuation was discharged for each measurement and experiments were performed in triplicate without gloves to simulate patient use. Ambient environmental conditions were also monitored (35 719% RH, 21.5 72.5 °C). The BOLAR was operated according to the manufacturer’s instructions. Firstly, a self-check test was performed before each measurement to ensure the BOLAR’s electrical control system was operating properly. Secondly, the loaded inhaler was attached to the USP throat, and the vacuum pump was turned on. Thirdly, the automated measurement sequence was initiated to open the internal valves that allow air to flow through the inhaler to disperse the powder into the BOLAR where the charge was measured. One measurement sequence took approximately 60 s and the inhaler was held tightly at the USP throat for the entire duration. Blank measurements were conducted in a similar way, except an inhaler loaded with a
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
109
pierced empty capsule was held tightly at the USP throat. These blank readings were subtracted from the powder charge measurements to give the actual charge and to account for disturbances in the electrical signals due to the presence of the inhaler at the USP throat. 2.5. Mass assay After the charge measurement, the detection tubes were carefully disassembled. The deposited powder on each component was exhaustively washed with 2.5–25 mL of de-ionised water. The specific volumes of the solvent were: 2.5 mL for the capsule; 5 mL each for the inhaler, adaptor, USP throat, flow divider inlet, impactor stages, and glass fibre filter; 10 mL for the filter stage; and 25 mL each for the flow divider, inner detection tube, and outer detection tube. The glass fibre filters were washed with de-ionised water, centrifuged at 13,400 rpm for 10 min (Minispins, Eppendorf, Westbury, USA), and the supernatant was collected for assay. Aliquots of the samples were chemically assayed using high performance liquid chromatography (HPLC). The Shimadzu HPLC system used included an RID-10A detector, LC-20AT pump, and SIL-20A HT autosampler controlled by LCSolution software (all Shimadzu Scientific Instruments, Kyoto, Japan). The mobile phase consisted of de-ionised water filtered through a 0.45 mm polyamide filter membrane (Sartorius Stedim Biotech GmbH, Göttingen, Germany). Samples were injected (20 mL) into a Resolve™ C18 5 mm 3.9 150 mm column (Waters, Milford, USA) at a flow rate of 1 mL/min, resulting in a retention time of 2 min. A calibration curve was constructed by serial dilution using freshly prepared standard solutions in de-ionised water at concentrations of 0.1 to 1,000 mg/mL (R2 ¼0.999). 2.6. Data analysis The charge detection cylinders in the BOLAR have no specific aerodynamic cut-off diameters. They simply collect particles that are not captured by the impactor stages upstream (Table 2). The charge and mass for a particular aerodynamic size range were calculated by subtracting the data between two consecutive detection tubes (Table 3). To facilitate graphical presentation and data discussion, a mid-point diameter representing each size range was calculated as the arithmetic mean of the upper and lower limits of a given size range (Table 3). Since only one sixth of the total dose after the flow divider was sampled by each detection tube, the magnitude of charge and mass equalled to the corresponding measured value multiplied by six. The BOLAR detects charge as current where the total charge of the positive or negative particles was obtained by integrating the electrical current signals on the outer and inner detection tubes, respectively. The net charge in a particular detection tube was the sum of the positive and negative total charges. The charge-to-mass ratio (q/m) was the quotient of the total charge and total drug mass on a particular detection tube, while the net q/m was the quotient of the net charge and total drug mass in a particular detection tube. The number of elementary charges per particles (n) in a particular size fraction was estimated using the following equation from Kwok and Chan (2008): n¼
q ρV : m e
ð1Þ
where ρ is the true density of the particles, V the volume of a particle, and e the elementary charge (1.602 10 19 C). The particles were assumed to be non-agglomerated, spherical, non-porous, and have an average true density of 1.5 g/cm3. For a given detection tube, V was calculated from the physical particle diameter, which was derived using the following equation: dp ¼da(ρ) 0.5 where dp is the physical particle diameter, and da the mid-point aerodynamic diameter for the detection tube, and ρ the true density. The emitted dose (ED) was defined as the total mass of drug recovered from all the components assayed excluding the capsule and inhaler. The fine particle dose (FPD) was defined as the mass of particles with an aerodynamic diameter o4.2 mm, which was given by the total amount of powder assayed in detection tube 3 (cut-off diameter 4.2 mm) multiplied by six. The fine particle fraction percentage loaded (FPF%Loaded) was defined as the percentage of FPD in the loaded dose, and fine particle fraction percentage emitted (FPF%Emitted) was the percentage of FPD in the emitted dose. Table 2 Aerodynamic diameter of the particles collected in the BOLAR™ bipolar charge detection tubes at 60 L/min (Dekati Ltd., 2014). Tube
Aerodynamic diameter (mm)
1 2 3 4 5
0.1o dp o 0.95 0.1o dp o 2.60 0.1o dp o 4.17 0.1o dp o 7.29 0.1o dp o 11.57
110
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
Table 3 Charge and mass calculations for BOLAR at 60 L/min (Dekati Ltd., 2014). Aerodynamic diameter range (mm)
Mid-point diameter (mm)
Charge, Q (pC)
0.1–0.95 0.95–2.60 2.60–4.17 4.17–7.29 7.29–11.57
0.525 1.78 3.39 5.73 9.43
6QTube 1 6(QTube 2 QTube 6(QTube 3 QTube 6(QTube 4 QTube 6(QTube 5 QTube
Mass, m (mg)
1) 2) 3) 4)
6mTube 1 6(mTube 2 mTube 6(mTube 3 mTube 6(mTube 4 mTube 6(mTube 5 mTube
1) 2) 3) 4)
Fig. 2. Scanning electron micrograph of spray-dried mannitol powder at 5000 magnification.
2.7. Statistical analysis GraphPad Prism 6 (GraphPad Software Inc., La Jolla, USA) was used to determine differences between the means within each size fraction. One-way analysis of variance (ANOVA), followed by the Tukey multiple comparisons post hoc test, were conducted with p o0.05 considered statistically significant.
3. Results 3.1. Physical characteristics Fig. 2 shows spray-dried mannitol powder consisted of microparticles, with a size distribution (volume diameter7one standard deviation) of d10 ¼ 1.14 70.00 mm, d50 ¼2.36 70.01 mm, d90 ¼4.52 70.03 mm, and span¼1.44. 3.2. Charge and mass profiles The recovered dose from all measurements ranged from 85% to 106%, which fell within the acceptable mass balance specified in the US Pharmacopoeia as 75–125% (US Pharmacopoeial Convention, 2010). Some components of the BOLAR had low amounts of deposited powder, with some below the detection limit of the HPLC. This could be expected for detection tubes with smaller cut-off diameters since the mannitol powder had relatively few particles in the submicron range, as depicted in the SEM and laser diffraction data (d10 ¼1.14 mm). Figs. 3–5 show the in vitro bipolar charge, mass distribution, and q/m profiles for mannitol powder aerosolised using Aerolizer inhalers with various grid structures, mouthpiece lengths, and air inlet sizes, respectively. The powder carried positively and negatively charged particles where the magnitude of charge depended on the particle size and inhaler design. For various grid structures, the particles charged positively in the range þ1,215 to þ 10,444 pC, negatively in the range 809 to 13,405 pC, and the charge magnitude decreased as the grid voidage increased. For different mouthpiece lengths, the particles carried similar positive (þ1,413 to þ 11,859 pC) and negative ( 809 to 13,517 pC) charges, though no obvious trend was observed. Changes to the air inlet size also led to similar levels of positive ( þ1,400 to þ 10,444 pC) and negative ( 239 to 13,022 pC) charges, where the magnitude of charge decreased with decreasing air inlet size. Hence, the net charge from all measurements was demonstrated to be due to the cancellation of significantly higher positive and negative charges. The resulting net charge profiles exhibited a bipolar trend where small particles charged positively and large particles charged negatively, with a change in polarity observed around the 2.60–4.17 mm cut-off diameter.
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
111
Fig. 3. Mean electrostatic charge, mass distribution, and charge-to-mass profiles of mannitol powder aerosolised using different grid structures. Data presented as mean 7 one standard deviation (n¼ 3). Components with o 50 mg assayed mannitol are not shown.
Fig. 4. Mean electrostatic charge, mass distribution, and charge-to-mass profiles of mannitol powder aerosolised using different mouthpiece lengths. Data presented as mean 7 one standard deviation (n¼ 3). Components with o 50 mg assayed mannitol are not shown.
112
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
Fig. 5. Mean electrostatic charge, mass distribution, and charge-to-mass profiles of mannitol powder aerosolised using different air inlet sizes. Data presented as mean7 one standard deviation (n¼ 3). Components with o 50 mg assayed mannitol are not shown.
Fig. 6. Mean fine particle fraction of mannitol powder aerosolised using different Aerolizers inhaler designs. Data presented as mean7 one standard deviation (n¼ 3). Asterisks indicate statistical significance between the ‘Original’ and ‘Grid 2’ structures.
When the charge was corrected for mass, q/m of positively charged particles ranged from þ1.95 to þ469 pC/mg (grid), þ1.72 to þ45.1 pC/mg (mouthpiece length), þ1.53 to þ145 pC/mg (air inlet size); and q/m of negatively charged particles ranged from 3.32 to 29.74 pC/mg (grid), 1.63 to 73.75 pC/mg (mouthpiece length), 1.63 to 48.8 pC/mg (air inlet size); leading to a net q/m of 4.86 to þ20.1 pC/mg (grid), 7.48 to þ20.1 pC/mg (mouthpiece length), 14.43 to þ20.1 pC/ mg (air inlet size). Overall, the electrostatic charge correlated with the mass and modifying the inhaler design did not affect the qualitative bipolar q/m profiles of mannitol powder. In this study, the fine particle dose was defined as the mass fraction of particles o4.2 mm, which is slightly different from that defined in Coates et al. (2006, 2004) as o6.8 mm. Nonetheless, the FPF%Loaded and FPF%Emitted exhibited trends consistent with the previous findings (Fig. 6). The inhaler grid played a significant role, whereas both the mouthpiece length and air inlet size had a less significant effect on the aerosol performance. These results suggest the BOLAR is suitable for investigating DPI device performance.
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
113
Fig. 7. Mean electrostatic charge, mass distribution, and charge-to-mass profiles of mannitol powder aerosolised using Aerolizers made of different inhaler materials. Data presented as mean7 one standard deviation (n¼ 3). Components with o 50 mg assayed mannitol are not shown. Abbreviations are defined as acrylonitrile butadiene styrene (ABS), low density polyethylene (LDPE), and polypropylene (PP).
Fig. 8. Mean fine particle fraction of mannitol powder aerosolised using Aerolizers made of different inhaler materials. Data presented as mean 7 one standard deviation (n¼3). Abbreviations are defined as acrylonitrile butadiene styrene (ABS), low density polyethylene (LDPE), and polypropylene (PP).
The in vitro bipolar charge, mass distribution, and q/m profiles of mannitol powder aerosolised using Aerolizer inhalers of the original design but constructed of different materials is shown in Fig. 7. Interestingly, the inhaler material had no statistically significant effect on either the charge and mass distributions (Fig. 7), or aerosol performance (Fig. 8). The particles charged positively in the range þ1,701 to þ 14,737 pC, negatively in the range 1,043 to 15,969 pC, and the resulting net charge profile ranged from 11,948 to þ 11,053 pC. The q/m of positively charged particles was þ1.53 to þ442.32 pC/mg and negatively charged particles was 1.31 to 72.8 pC/mg, leading to a net q/m of 6.43 to þ32.96 pC/mg. Again, the net charge profiles were found to change polarity around the 2.60–4.17 mm cut-off diameter. It is important to note that while charge profiles were remarkably reproducible throughout the study, mass distributions varied substantially within certain size fractions and these contributed to variations in q/m. The number of elementary charges per particle in size fractions with detectable drug mass are given in Table 4. Since the calculations using Eq. (1) made some assumptions about the particles, these values may be overestimates (Kwok & Chan, 2008).
114
Table 4 Estimated number of elementary charges per particle from Eq. (1). Positive charge
Negative charge
1 0.525 0.429
2 1.78 1.45
3 3.39 2.76
4 5.73 4.68
1 0.525 0.429
2 1.78 1.45
3 3.39 2.76
4 5.73 4.68
Inhaler configuration Full grid
55 (33)
3,280 (491)
15,419 (4,068)
25 (10)
16,657 (3,264)
N/A
2,850 (302)
26,072 ( 20,887)
115 (147)
4,267 (3,625) 838 (107)
4,310 (260)
Grid 1
2,952 (1,402) 2,338 (181)
4,689 (255)
Grid 2
652 (1003)
3,743 (698)
15,227 (4,135)
N/A
927 (364)
3,965 (310)
Full mouthpiece
55 (33)
3,280 (491)
15,419 (4,068)
25 (10)
4,310 (260)
¾ mouthpiece
N/A
3,229 (976)
14,361 (2,004)
N/A
4,267 (3,625) 1,251 (1,174)
18,799 (3,429) 18,300 (3,530) 16,657 (3,264)
3,947 (736)
16,537 (2,253)
½ mouthpiece
N/A
3,146 (179)
13,188 (4,034)
N/A
N/A
15,525 (1,161)
Full air inlet
55 (33)
3,280 (491)
15,419 (4,068)
25 (10)
2/3 air inlet
73 (63)
2,212 (208)
13,715 (7,838)
65 (54)
4,267 (3,625) 597 (326)
4,053 (1,414) 4,310 (260)
1/3 air inlet
N/A
3,551 (4,801)
2,000 (205)
11,603 (2,375)
N/A
N/A
3,322 (1,314)
Inhaler material ABS
N/A
1,175 (627)
3,497 (367)
14,990 (2,921)
N/A
792 (282)
Acetal LDPE
N/A N/A
1,061 (120) 1,759 (658)
3,428 (561) 3,538 (565)
11,007 (2,408) 15,511 (4,288)
N/A N/A
16,568 (3,853) 15,480 (1,675) 18,101 (2,197)
Nylon
N/A
1,448 (726)
4,439 (848)
N/A
N/A
N/A 5,150 (8,290) N/A
3,904 (1,375) 3,451 (844) 4,158 (250)
16,589 (1,991)
PP Stainless steel
86 (77) N/A
2,503 (320) 1,862 (1,131)
4,422 (423) 7,757 (5,851)
20,857 (2,696) 26,290 (23,667)
38 (24) N/A
3,445 (1,063) 5,039 (549) 4,750 (446)
2,540 (1,106) 2,952 (1,402) 2,069 (1,219) 1,953 (1,520) 2,952 (1,402) 1,579 (473)
Data presented as mean ( 7 one standard deviation) (n¼3). N/A: not calculated for components with o 50 mg assayed mannitol.
1,148 (148) 1,482 (1,687)
2,903 (219)
16,657 (3,264) 12,784 (2,532) 10,347 (646)
21,493 (2,135) 22,501 (1,671)
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
Detection tube Mid-point aerodynamic diameter (lm) Physical diameter (lm)
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
115
4. Discussion All of the inhalers used in this study showed spray-dried mannitol powder carried significantly different positive and negative charges that had higher magnitudes of charge than the measured net charge. The estimated elementary charges per particle (Table 4) indicated high levels of charge that may potentially influence particle deposition in the lungs. Additionally, small particles charged positively and large particles charged negatively. While the exact reasons for this behaviour are unknown (Wong, Kwok, & Chan, 2015a), Lacks and Sankaran (Lacks & Sankaran, 2011) have discussed several mechanisms including particle-size dependent differences in work function, capacitance, presence and direction of external electric fields, non-equilibrium states, and adsorption of contaminants that contribute to the interparticle contact. Overall, these results agree with previous studies that reported charge bipolarity in pharmaceutical aerosols (Balachandran et al., 2003; Beleca et al., 2010a, 2010b; Kannosto et al., 2013; Mazumder et al., 2002; Saini et al., 2007), which could have important implications during the development of products for inhalation. The observed effect of inhaler design on the bipolar charging of mannitol could be explained by a difference in the contacts due to impaction between the particles and the inhaler internal surface. According to computational fluid dynamics analysis performed by Coates et al. (2006, 2004), the number of impactions the particles have with the inhaler grid will decrease from 62% for the ‘Full grid’ to 20% for ‘Grid 2’ as the grid voidage is increased. The decreased number of contacts with the grid would result in the decreased charge magnitude observed. Even though increasing the grid voidage also increases particle impaction with the mouthpiece walls (22% for ‘Full grid’ compared to 88% for ‘Grid 2’), the particle– mouthpiece impactions occur potentially at a reduced impact intensity due to the lower air flow velocity and a much shallower impact angle than the particle–grid interactions (Coates et al., 2006, 2004). The length of the mouthpiece does not substantially change the particle contacts or impactions because the particle-grid and particle–wall interactions resulting from the air flow field generated in the swirling chamber of the devices remain the same and does not depend on the mouthpiece length, leading to similar charge profiles. As for the air inlet size, the charge magnitude decreased with decreasing air inlet size because reducing the air inlet at high flow rates (such as 60 L/min) causes a large amount of powder to be released from the device before the turbulence levels and particle impaction velocities could be fully developed (Coates et al., 2006, 2004). Overall, the q/m profiles across all the inhaler designs were comparable, indicating that the charge distribution correlated with the mass distributions and modifications to the inhaler design did not affect the qualitative bipolar charge of mannitol powder. Further to studying the inhaler design, we investigated the potential role of the inhaler material on charging. According to electrostatic theory, the triboelectric series enables preliminary prediction of net charge based on the work function of materials on the list. Indeed, Adi et al. (2010) correlated predictions from the triboelectric series with net charges of mannitol that had been tumbled inside containers made of different materials (including aluminium, copper, glass, nylon, polyethylene, polyvinyl chloride, stainless steel, and Teflon). However, contrary to this theory and the tumbled mannitol results, the bipolar charge of mannitol did not differ when dispersed by inhalers constructed of different materials. The results are consistent with the previous study by Kwok, Tran, and Chan (2010), who utilised the same set of polymeric Aerolizers at 30 L/min and examined the net charge using a modified Electrical Low Pressure Impactor (ELPI™). The authors observed no difference in the net charge profiles between the five inhaler materials (ABS, acetal, LDPE, nylon, and PP). The present study using the BOLAR has further demonstrated that inhaler material had an insignificant effect on the bipolar charging of spray-dried mannitol powder. The BOLAR results presented in this study indicate the electrification of mannitol during aerosolisation was not dependent on the inhaler material. It is possible that the crystalline nature of mannitol powder could contribute to a more consistent charging profile (Wong et al., 2014). Another possible reason for the lack of material dependence may be the short contact time between the powder and the internal inhaler surface during dispersion (o1 s) did not allow significant charge transfer. Due to the confined interior of the inhaler, it was also possible that a layer of powder became coated on the internal surface of the inhaler at the initial stage of dispersion. As a result, despite the different inhaler materials used, this coating would render the surface mannitol-like, leading effectively to the same type of mannitol–mannitol interactions instead of mannitol interactions with different inhaler materials. A time-dependent study may be able to support the above hypotheses. In practice, it is difficult to perform on the inhaler because the dispersion process takes place in only a few seconds. Future work using other pharmaceutical powders with different physicochemical properties (such as amorphous and crystalline polymorphic forms of other active ingredients and excipients) is needed to determine if the results are applicable to different formulations and to further establish the effect of inhaler material on charging during aerosolisation. A major limitation of the BOLAR impactor was the substantial variation in mass within certain size fractions. This can be attributed to the unconventional parallel detection tube assembly of the BOLAR where the charge and mass of a particle size fraction is calculated as a subtraction between two consecutive detection tubes. Furthermore, only one sixth of the total dose is sampled by each detection tube and is further distributed to multiple parts that may have relatively large surface areas. This type of experimental setup also contributed to problems with the calculation of q/m. For example, when the difference in mass between two subsequent detection tubes was negative (i.e. mtube1 4mtube2 in Table 3), these values were treated as zero mass and led to incomplete q/m profiles. More sensitive drug detection techniques, such as ultra performance liquid chromatography (UPLC) or liquid chromatography–mass spectrometry (LC–MS), may enhance the successful quantification of mass data. Apart from these potential drawbacks, the BOLAR has enabled the characterisation of bipolar charge in powder aerosols and the assessment of DPI device performance. In time, the knowledge gained may assist the
116
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
development of optimised inhaled products and lead to possible regulatory requirements on the characterisation of electrostatic properties of pharmaceuticals.
5. Conclusions This study investigated the influence of modifying the Aerolizer inhaler design and material on bipolar charging of spraydried mannitol powder. All of the tested inhalers showed that spray-dried mannitol powder carried significantly different positive and negative charges that had higher magnitudes of charge than the measured net charge. The estimated elementary charges per particle indicated that mannitol powder carried significant levels of charge that may potentially influence particle deposition in the lungs. Increasing the grid voidage decreased the magnitude of charging, as did reducing the air inlet size, whereas mouthpiece length had no obvious effect. Overall, the comparable q/m profiles across all the inhaler designs and inhaler materials indicated charge correlated with the mass distributions so modifications to the inhaler did not affect the bipolar charge characteristics of mannitol powder.
Acknowledgements The authors thank Mauro Citterio and Alberto Colombo from Plastiape S.p.A., Italy, for supplying the modified Aerolizers inhalers; Dr. Joseph Khachan from the School of Physics, University of Sydney, for his valuable discussions; Dr. Alun Pope from the Statistical Consulting Service, University of Sydney, for advice on statistical and multivariate analysis; the Australian Microscopy & Microanalysis Research Facility, University of Sydney, for providing facilities for SEM imaging; Kevin Samnick for feedback on the manuscript; as well as John Gar Yan Chan, Yu-Wei Lin, and Wenbo Wang for help with manual handling of the BOLAR. Jennifer Wong was a recipient of the Australian Postgraduate Award, and this work was supported by the Australian Research Council’s Discovery Projects funding scheme (DP120102778 and DP150103953).
References Adi, H., Kwok, P. C. L., Crapper, J., Young, P. M., Traini, D., & Chan, H.-K. (2010). Does electrostatic charge affect powder aerosolisation? Journal of Pharmaceutical Sciences, 99(5), 2455–2461, http://dx.doi.org/10.1002/jps.21996. Ali, M., Reddy, R. N., & Mazumder, M. K. (2008). Electrostatic charge effect on respirable aerosol particle deposition in a cadaver based throat cast replica. Journal of Electrostatics, 66(7–8), 401–406, http://dx.doi.org/10.1016/j.elstat.2008.02.005. Azhdarzadeh, M., Olfert, J. S., Vehring, R., & Finlay, W. H. (2014a). Effect of electrostatic charge on deposition of uniformly charged monodisperse particles in the nasal extrathoracic airways of an infant. Journal of Aerosol Medicine and Pulmonary Drug Delivery, 28(1), 30–34, http://dx.doi.org/10.1089/ jamp.2013.1118. Azhdarzadeh, M., Olfert, J. S., Vehring, R., & Finlay, W. H. (2014b). Effect of electrostatic charge on oral-extrathoracic deposition for uniformly charged monodisperse aerosols. Journal of Aerosol Science, 68, 38–45, http://dx.doi.org/10.1016/j.jaerosci.2013.11.002. Azhdarzadeh, M., Olfert, J. S., Vehring, R., & Finlay, W. H. (2014c). Effect of induced charge on deposition of uniformly charged particles in a pediatric oralextrathoracic airway. Aerosol Science and Technology, 48(5), 508–514, http://dx.doi.org/10.1080/02786826.2014.896989. Bailey, A. G., Hashish, A. H., & Williams, T. J. (1998). Drug delivery by inhalation of charged particles. Journal of Electrostatics, 44(1–2), 3–10, http://dx.doi.org/ 10.1016/s0304-3886(98)00017-5. Balachandran, W., Kulon, J., Koolpiruck, D., Dawson, M., & Burnel, P. (2003). Bipolar charge measurement of pharmaceutical powders. Powder Technology, 135, 156–163, http://dx.doi.org/10.1016/j.powtec.2003.08.011. Balachandran, W., Machowski, W., Gaura, E., & Hudson, C. (1997). Control of drug aerosol in human airways using electrostatic forces. Journal of Electrostatics, 40-1, 579–584, http://dx.doi.org/10.1016/S0304-3886(97)00106-X. Beleca, R., Abbod, M., Balachandran, W., & Miller, P. R. (2010a). Investigation of electrostatic properties of pharmaceutical powders using Phase Doppler anemometry. IEEE Transactions on Industry Applications, 46(3), 1181–1187, http://dx.doi.org/10.1109/Tia.2010.2045332. Beleca, R., Abbod, M., Balachandran, W., & Miller, P. R. (2010b). Measurement of bipolar charge distribution of lactose monohydrate using Phase Doppler Anemometry. Paper presented at the Respiratory Drug Delivery, Orlando, Florida. Chan, T. L., Lippmann, M., Cohen, V. R., & Schlesinger, R. B. (1978). Effect of electrostatic charges on particle deposition in a hollow cast of the human larynxtracheobronchial tree. Journal of Aerosol Science, 9(5), 463–468, http://dx.doi.org/10.1016/0021-8502(78)90009-5. Chang, K.-N., Chen, Y.-K., Huang, S.-H., Chen, C.-W., Lai, C.-Y., & Chen, C.-C. (2012). Penetration of charged particles through metallic tubes. Journal of Aerosol Science, 48(0), 10–17, http://dx.doi.org/10.1016/j.jaerosci.2012.01.006. Coates, M. S., Chan, H.-K., Fletcher, D. F., & Raper, J. A. (2006). Effect of design on the performance of a dry powder inhaler using computational fluid dynamics. Part 2: Air inlet size. Journal of Pharmaceutical Sciences, 95(6), 1382–1392, http://dx.doi.org/10.1002/jps.20603. Coates, M. S., Fletcher, D. F., Chan, H.-K., & Raper, J. A. (2004). Effect of design on the performance of a dry powder inhaler using computational fluid dynamics. Part 1: Grid structure and mouthpiece length. Journal of Pharmaceutical Sciences, 93(11), 2863–2876, http://dx.doi.org/10.1002/jps.20201. Dekati Ltd. (2014). Dekati BOLAR user manual version 1.1. Kangasala, Finland: D. Ltd. Finlay, W. (2001). The mechanics of inhaled pharmaceutical aerosols. San Diego: Academic Press. Gallo, C. F., & Lama, W. L. (1976). Some charge exchange phenomena explained by a classical model of the work function. Journal of Electrostatics, 2(2), 145–150, http://dx.doi.org/10.1016/0304-3886(76)90005-X. Hashish, A. H., Bailey, A. G., & Williams, T. J. (1994a). Modelling the effect of charge on selective deposition of particles in a diseased lung using aerosol boli. Physics in Medicine and Biology, 39(12), 2247. Hashish, A. H., Bailey, A. G., & Williams, T. J. (1994b). Selective deposition of pulsed charged aerosols in the human lung. Journal of Aerosol Medicine, 7(2), 167–171, http://dx.doi.org/10.1089/jam.1994.7.167. Hinds, W. C. (2012). 9.3 Deposition mechanisms aerosol technology: Properties, behavior, and measurement of airborne particles (2nd ed.). New York: John Wiley and Sons, Inc.191–196 Kannosto, J., Isherwood, H., Niemelä, V., & Ukkonen, A. (2013, May 21 24). High bipolar electrical charge levels of lactose particles measured from DPI. Paper presented at the Respiratory Drug Delivery, Berlin, Germany.
J. Wong et al. / Journal of Aerosol Science 95 (2016) 104–117
117
Kwok, P., Tran, P., & Chan, H.-K. (2010). Effect of Aerolizers and capsule materials on the electrification of spray dried mannitol particles. Paper presented at the American Association of Aerosol Research 29th Annual Conference, Portland, Oregon. Kwok, P. C. L., & Chan, H. K. (2008). Effect of relative humidity on the electrostatic charge properties of dry powder inhaler aerosols. Pharmaceutical Research, 25(2), 277–288, http://dx.doi.org/10.1007/s11095-007-9377-2. Kwok, P. C. L., Glover, W., & Chan, H.-K. (2005). Electrostatic charge characteristics of aerosols produced from metered dose inhalers. Journal of Pharmaceutical Sciences, 94(12), 2789–2799, http://dx.doi.org/10.1002/jps.20395. Lacks, D. J., & Sankaran, R. M. (2011). Contact electrification of insulating materials. Journal of Physics D: Applied Physics, 44(45), 453001. Lowell, J., & Rose-Innes, A. C. (1980). Contact electrification. Advances in Physics, 29(6), 947–1023, http://dx.doi.org/10.1080/00018738000101466. Majid, H., Madl, P., Hofmann, W., & Alam, K. (2012). Implementation of charged particles deposition in stochastic lung model and calculation of enhanced deposition. Aerosol Science and Technology, 46(5), 547–554, http://dx.doi.org/10.1080/02786826.2011.645957. Matsusaka, S. (2011). Control of particle tribocharging. Kona Powder and Particle Journal, 29, 27–38. Mazumder, M. K., Yurteri, C. U., Ahuja, G., Grable, N., Chan, H. K., & Chew, N. (2002). Bipolar electrostatic charging of fine pharmaceutical powders by dispersion and transport processes and its implications to lung deposition of inhaled DPI aerosols. Paper presented at the Respiratory Drug Delivery VIII, Tucson, Arizona. Melandri, C., Prodi, V., Tarroni, G., Formignani, M., De Zaiacomo, T., Bompane, G. F., & Maestri, G. (1975). On the deposition of unipolarly charged particles in the human respiratory tract. Inhaled Part, 4(Pt 1), 193–201. Melandri, C., Tarroni, G., Prodi, V., Dezaiacomo, T., Formignani, M., & Lombardi, C. C. (1983). Deposition of charged-particles in the human airways. Journal of Aerosol Science, 14(5), 657–669, http://dx.doi.org/10.1016/0021-8502(83)90070-90078. Saini, D., Biris, A. S., Srirama, P. K., & Mazumder, M. K. (2007). Particle size and charge distribution analysis of pharmaceutical aerosols generated by inhalers. Pharmaceutical Development and Technology, 12(1), 35–41, http://dx.doi.org/10.1080/10837450601166536. US Pharmacopoeial Convention (2010). G-08 Inhalation United States Pharmacopoeia 33-National Formulary 28. Rockville, MD, USA. Wong, J., Chan, H.-K., & Kwok, P. C. L. (2013). Electrostatics in pharmaceutical aerosols for inhalation. Therapeutic Delivery, 4(8), 981–1002, http://dx.doi.org/ 10.4155/tde.13.70. Wong, J., Kwok, P., Noakes, T., Fathi, A., Dehghani, F., & Chan, H.-K. (2014). Effect of crystallinity on electrostatic charging in dry powder inhaler formulations. Pharmaceutical Research, 31(7), 1656–1664, http://dx.doi.org/10.1007/s11095-013-1270-6. Wong, J., Kwok, P. C. L., & Chan, H.-K. (2015a). Electrostatics in pharmaceutical solids. Chemical Engineering Science, 125(0), 225–237, http://dx.doi.org/ 10.1016/j.ces.2014.05.037. Wong, J., Lin, Y.-W., Kwok, P. C. L., Niemelä, V., Crapper, J., & Chan, H.-K. (2015b). Measuring bipolar charge and mass distributions of powder aerosols by a novel tool (BOLAR). Molecular Pharmaceutics, 12(9), 3433–3440, http://dx.doi.org/10.1021/acs.molpharmaceut.5b00443. Xi, J., Si, X., & Longest, W. (2014). Electrostatic charge effects on pharmaceutical aerosol deposition in human nasal–laryngeal airways. Pharmaceutics, 6(1), 26–35, http://dx.doi.org/10.3390/pharmaceutics6010026. Yang, M. Y., Chan, J. G. Y., & Chan, H.-K. (2014). Pulmonary drug delivery by powder aerosols. Journal of Controlled Release, 193, 228–240, http://dx.doi.org/ 10.1016/j.jconrel.2014.04.055. Yli-Ojanperä, J., Ukkonen, A., Järvinen, A., Layzell, S., Niemelä, V., & Keskinen, J. (2014). Bipolar charge analyzer (BOLAR): A new aerosol instrument for bipolar charge measurements. Journal of Aerosol Science, 77(0), 16–30, http://dx.doi.org/10.1016/j.jaerosci.2014.07.004. Yu, C. P. (1985). Theories of electrostatic lung deposition of inhaled aerosols. Annals of Occupational Hygiene, 29(2), 219–227, http://dx.doi.org/10.1093/ annhyg/29.2.219. Yu, C. P., & Chandra, K. (1977). Precipitation of submicron charged-particles in human lung airways. Bulletin of Mathematical Biology, 39(4), 471–478, http: //dx.doi.org/10.1007/Bf02462925.