- Email: [email protected]
Electrohydrodynamic atomization technique for applying phospholipid coatings to titanium implant materials
Materials Letters 97 (2013) 81–85
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrohydrodynamic atomization technique for applying phospholipid coatings to titanium implant materials David A. Prawel a,c, Matt J. Kipper b,c, Ketul C. Popat a,c, Susan P. James a,c,n a
Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80523, USA Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, CO 80523, USA c School of Biomedical Engineering, Colorado State University, Fort Collins, CO 80523, USA b
a r t i c l e i n f o
abstract
Article history: Received 29 September 2012 Accepted 20 January 2013 Available online 25 January 2013
Phospholipid coatings on titanium implants have been shown to enhance osteoblast activity, promote mineralization, and facilitate implant osseointegration in vivo. To date, dip and drip coating techniques have been used to apply these coatings. These coating techniques are easy to perform, but present difficulties in controlling the characteristics of the coating on the final surface, resulting in inconsistent, non-uniform, non-conformal coatings that are too thick for in vivo use. Electrohydrodynamic atomization (electro-spraying or e-spraying) is a versatile and easy method of creating thin, uniform and consistent coatings by atomizing a liquid with electrical forces. e-Spraying provides the advantage of being able to create coatings with relatively high efficiencies, while providing good control of coating morphology, especially on rough and intricately shaped surfaces, which is an important consideration for cell attachment and growth. Other advantages of this technique are low cost and easy setup. The purpose of this study was to develop an e-spraying technique for applying phospholipid coatings on commercially pure titanium implant materials. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electrospray Coating Biomaterial Orthopedic implant Titanium Phosphatidylserine
1. Introduction More than 4.4 million people have at least one metallic orthopedic implant, and the number of revisions is growing substantially [1]. Roughly a third of total joint replacements fail due to mechanical loosening from various causes [2]. Bone cements are often used to address these issues; however, cemented implants are prone to cement failure, leading many surgeons to prefer cementless implants, particularly for younger, more active patients. Poor mineralization at an implant/bone interface leads to the development of a thin fibrous layer between the implant and bone, which frequently correlates with implant failure. With our increased understanding of the complex biological processes that underlie implant failure in vivo, there has been a shift in focus from bone cements and mechanical fixation techniques to the design of novel mechanical/biomimetic strategies that promote natural physiological integration. Calcium phosphate-based bio-ceramics, primarily in the form of hydroxyapatite, have become popular cementless implant coatings [3]. However, long-term success of these coatings on loadbearing implants has been hindered by sub-optimal mechanical
n Correspondence to: Engineering A101, Campus Delivery 1374, Fort Collins, CO 80523-1374, USA. Tel.: þ 1 970 491 6559; fax: þ 1 970 491 3827. E-mail address: [email protected] (S.P. James).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.01.091
properties [4], delamination of the coating from the substrate [4] and poor mineralization and integration of newly forming bone with the implant surface [5]. As a result, considerable research effort has focused on development of new coating materials and techniques for popular implant materials. Phospholipids are one such candidate material. Phospholipids are broadly implicated in the development of new bone. The phospholipid bilayer of osteoblast vesicles is believed to be the primary nucleation and mineralization site of new bone [5,6]. Recent studies indicate that titanium surfaces coated with phospholipid alone (i.e. not in the presence of the additional constituents found in the matrix vesicle) enhance osteoblast activity, promote mineralization, and facilitate implant osseointegration in vivo [7–9]. These studies found that synthetic phosphatidylserine (PS) achieved the best in vitro biomineralization and in vitro osseointegration. PS coatings also present challenges: newly grown bone did not make intimate contact with the implant surface. Numerous techniques have been proposed for applying bioactive coatings to implant materials, including but not limited to plasma spraying [4] and sputtering [3]. High temperature processes cannot be used with phospholipids, and prior dip [10] and drip [7,11] coating processes resulted in relatively thick (approximately 100 mm) layers of soft PS that tended to form 3-dimensional gels in simulated body fluid. Such gels were not well adhered to the titanium substrate, resulting in mechanical instability [9]. These issues led to recommendations for
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thinner coatings to ensure mechanical stability of the implant [7], thus motivating improved coating processes. Electro-hydrodynamic atomization (electrostatic, electro-spraying or e-spraying) is a versatile method of creating thin, adherent coatings by atomizing a liquid by means of electrical forces [12]. Electrospraying provides the advantage of being able to create coatings with relatively high efficiencies [13] (i.e. most or all of the material sprayed becomes part of the coating on the target) because the charged liquid source material is carried by the electrical field rather than being mechanically atomized or carried on another liquid, as in typical pressure-based (e.g. aerosol) spraying techniques [14]. This is especially advantageous for more costly coating materials, and it enables good control of coating uniformity and morphology, especially on rough and intricately shaped surfaces. Other advantages of electrospraying are low cost and easy setup. Detailed technical explanations and reviews are found elsewhere [12]. The goal of this study was to develop a technique to electro-spray thin, conformal, and consistent PS coatings on flat titanium implant materials and titanium surgical screws.
2. Materials and methods 1,2-Dioleoyl-sn-glycero-3-phospho-L-serine (‘‘DOPS’’) (Avanti Polar Lipids, Alabaster, AL) was dissolved in chloroform to a 20 mM concentration and drawn into a glass syringe (10 cc, Hamilton) which was then mounted on a syringe pump (Kent Scientific) (see Fig. 1). A double hub syringe tube (12 in., 20 gauge, Hamilton) connected the syringe to a blunt needle (22 gauge, Hamilton). Target titanium samples (flat, commercially pure Ti, 0.016 in. thickness, 0.75 cm 0.75 cm), were cleaned by successive 30 min ultrasonic baths in 2% liquinox, chloroform, acetone
and distilled water (DIH2O). Samples were mounted with a backing of silicone putty (UHU Tac, Basel, Germany) on a ‘‘target board’’ consisting of an insulated circuit board with copper wires directly contacting the Ti samples, and protruding through the target board rearward (away from the spray direction). This target board was positioned at a measured distance from the needle. The needle and the copper wires backing the target board (contacting the Ti samples) were connected to the positive and negative (ground) poles, respectively, of a controlled DC voltage source (Gamma High Voltage Research, Ormond Beach, Florida). The pump, syringe tube, syringe body and all mounting hardware were also grounded. When the voltage is applied, the source material is moved in the electric field from the needle to the target as a very fine, nearly indiscernible, mist. The electro-spraying process depends on many parameters such as the concentration and viscosity of the material sprayed, the electric field strength (induced by voltage over a distance) and the rate at which material is introduced into the electric field (by the syringe pump). Table 1 shows representative values explored experimentally in this study, as well as the values for other parameters held constant throughout the study, and indicates which parameters correspond to figures shown below. DOPS coatings were characterized with SEM; samples were coated with 8–15 nm of gold and imaged at 5–8 kV in a JOEL JSM6500F field emission SEM (Tokyo, Japan). Coating thickness was assessed using SEM by scratching the coating through to the titanium substrate and viewing the scratch at a steep angle. A titanium surgical screw (1.9 mm diameter, 6 mm length, ELI titanium alloy ASTM F136—Bioplate, Los Angeles, California) was e-sprayed in a similar manner, using parameters from Fig. 2D, shown in Table 1. SEM images were taken from ‘‘front’’ (facing directly at the oncoming e-spray), ‘‘back’’ and sides of the screw (i.e. midway between front and back) to determine whether unidirectional spray toward the ‘‘front’’ of a 3D part would result in even coating over the entire part.
3. Results Consistent with prior studies [14], small variations in e-spray parameters led to dramatically different coating morphologies. Fig. 2 presents SEM images of representative DOPS coatings e-sprayed at parameters shown in Table 1. Higher DOPS concentration and pump rate (Fig. 2A) produced a relatively smooth, uniform coating surface devoid of pores, particles or distinct surface features. Reducing the amount of material in the electric field (e.g. by decreasing DOPS concentration and pump rate), while increasing time and voltage, produces a rougher yet still uniform coating surface, also devoid of pores but with numerous particles and distinct surface features (Fig. 2B). Other parameters being constant, increasing voltage led to an increase in particles and features on the surface (compare Fig. 2B and D). Similarly, decreasing distance generally led to a smoother coating (compare Fig. 2C and D). Coatings were 6–8 mm thick.
Fig. 1. Electro-spraying apparatus.
Table 1 Representative values explored experimentally in this study. Image #
2.A 2.B 2.C 2.D
Target area (nominal) (cm2)
3 3 3 3
Process parameters Needle size (ga)
[DOPS] (w/v) (%)
Pump rate (ml/h)
Time (min)
Distance (cm)
Voltage (kV)
22 22 22 22
2.5 1.3 1.3 1.3
20 14 14 14
2.25 3 3 3
8 8 6 8
8 10 10 12
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Fig. 2. SEM images (all 1000 ) of DOPS coating on flat titanium samples—(A) e-sprayed with pump rate 20 ml/h, voltage 8 kV, 8 cm distance. (B) e-Sprayed with pump rate 14 ml/h, voltage 10 kV, 8 cm distance. (C) Pump rate 14 ml/h, 10 kV, 6 cm distance. (D) pump rate 14 ml/h, 12 kV, 8 cm distance.
We also demonstrated that e-spraying can coat 3-dimensional (3D) objects, such as surgical screws, more uniformly than lineof-sight spray coating techniques. e-Spraying can coat all faces of a 3D object in a single unidirectional e-spray event. Fig. 3 shows front and back images of the screw, showing that coatings on both front and back are consistent, uniform and visually similar in morphology to e-spray coatings on flat samples created with the same e-spray parameters (shaded row in Table 1).
4. Discussion Kumbar and others [14,15] found that holding certain parameters constant while varying others produced PLG and PEG coatings with widely varying morphologies and thicknesses. Higher electric field strength increases field current [12], which increases mass flow rate from the needle to the target, all other parameters held constant (to the limit of the amount of material available in the electric field at a given pump rate). A higher mass flow rate increases the number of charged particles in the e-spray jet [12], which increases both electrostatic and Coulombic forces, inducing a stretching force on material droplets in the jet, similar to increasing surface tension. This results in greater repulsion between adjacent droplets [16], which causes formation of a relatively rougher surface texture consisting of more particles and features on the surface. This effect is clearly seen as voltage is reduced between Fig. 2A and B, and between Fig. 2C and D. DOPS is an amphiphilic molecule with a polar head and nonpolar acyl chain tails. Varying electric field parameters causes differential rearrangement of alignment and conformation of the
DOPS molecules, along with interactions between DOPS molecules, as discussed in the following. This effect also plays a role in the morphology of e-sprayed coatings, as multiple factors interact to form a particular coating. For example, decreasing DOPS concentration and pump rate (from Fig. 2A and B) reduces the amount of sprayed material in the electric field, while simultaneously increasing voltage and time produces a rougher, uniform coating surface, devoid of pores, with numerous particles and distinct surface features. Less material available in the electric field results in a smoother surface texture (fewer charged particles in the e-spray jet), whose effect is slightly overshadowed by the effect of higher voltage (increasing particles and surface features). Furthering this argument, decreasing distance with all other parameters held constant (from Fig. 2B to C) also increases field current, which would be expected to result in formation of more current-induced particles and features on the surface. However, the shorter distance also results in shorter time-of-flight, which appears to overshadow the current-induced effect by reducing time for formation of particles and features, resulting in a smoother but less uniform surface with relatively large pores and pockets. Increasing voltage, with all other parameters held constant (from Fig. 2B to D), increases current-induced formation of more particles and features on the surface. Similarly, increasing voltage and distance simultaneously (from Fig. 2C to D) increases currentinduced formation of more particles and features on the surface, while allowing more time for formation of such features. Although longer distance would by itself reduce field strength, this effect is overshadowed by the effect of increased voltage at this relative increase in distance.
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Fig. 3. SEM images of DOPS coating on surgical screw—(A) (50 ) shows the ‘‘front’’ side of the screw (facing directly at the oncoming e-spray). (B) (50 ) shows the ‘‘back’’ side of the same surgical screw (facing away from the oncoming e-spray). (C,D) Front and back of the same surgical screw at 1000 , respectively. Note the similarity of coatings on both sides.
Electric fields meet surfaces at perpendicular angles. Thus it is not surprising that e-spraying completely coats the surgical screw in a single unidirectional spray event, given appropriate e-spray parameters. The field wraps around the screw, depositing material on all grounded surfaces. It is likely that coating thickness is self-adjusting, due to the insulating nature of the phospholipid. As more material arrives at the screw surface it seeks the area of highest charge density (thinnest coating), resulting in consistent, uniform surfaces completely around the screw. A known limitation of the method can occur when the intricate surface being coated contains large concavities which could result in the formation of a Faraday Cage effect, potentially preventing material from entering the concavity [17]. With appropriate mitigation in these instances, and assuming the target object is, or could be made, conductive, e-spraying can be an excellent method for coating intricate 3D shapes. It is interesting to note the smooth appearance of the thread edges in Fig. 3. It is possible that these edges failed to coat, or that the coating assumes a different (smooth) morphology on these features. Further investigation is required to determine if this is the result of disturbance of the electric field lines, similar to or resulting from a Faraday Cage effect [17].
5. Conclusions The worldwide need for thin, conformal, osseointegrative coatings on orthopedic implants is well documented. e-Spraying is a suitable method for creating DOPS coatings on titanium. Compared to commonly used drip and dip techniques, e-sprayed DOPS coatings are much thinner, which could lead to enhanced
mechanical stability for un-cemented implants [7]. And, e-sprayed coatings are more consistent and uniform, enabling more dependable and predictable device coverage. Manipulation of the key extrinsic e-spray process parameters such as voltage and distance, which result in changes in the electric field strength, provide effective, predictable control of coating characteristics such as morphology, micro-topology and porosity, all of which are important for successful cell growth and adhesion, and therefore to long-term success of the orthopedic implants upon which the coatings reside. e-Spraying is highly repeatable between tests on both flat and 3D samples, as confirmed by SEM, enabling potential commercial success of the technique. Finally, e-spraying is easy to perform and requires only a few thousand dollars in equipment investment. Acknowledgments This paper was supported by the Colorado State University Cancer Supercluster Translational Research Grant Program and Colorado Bioscience Discovery Evaluation Grant Program. Neither funding source had any role in study design, collection, analysis or interpretation of data; in the writing of the report; or in the decision to submit this article for publication. References [1] Kurtz S, Ong K, Lau E, Mowat F, Halpern M. Projections of primary and revision hip and knee arthroplasty in the United States from 2005 to 2030. J Bone Joint Surg Am 2007;89(4):780–5.
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[2] Alegre-Rico F. Infection in primary total hip arthroplasty. Acta Ortop Mex 2004;18:235–9. [3] Yang Y, Kim Kyo-Han, Ong Joo L. A review on calcium phosphate coatings produced using a sputtering process—an alternative to plasma spraying. Biomaterials 2005;26:327–37. [4] Tranquilli PL, Merolli A, Palmacci O, Gabbi C, Cacchioli A, Gonizzi G. Evaluation of different preparations of plasma-spray hydroxyapatite coating on titanium alloy and duplex stainless steel in the rabbit. J Mater Sci: Mater Med 1994;5:345–9. [5] Braun G, Kohavi D, Amir D, Luna M, Caloss R, Sela J, et al. Markers of primary mineralization are correlated with bone-bonding ability of titanium or stainless steel in-vivo. Clin Oral Implants Res 1995;6:1–13. [6] Raggio CL, Boyan BD, Boskey AL. In vivo hydroxyapatite formation induced by lipids. J Bone Miner Res 1986;1:409–15. [7] Bosetti M, Lloyd AW, Santin M, Denyer SP, Cannas M. Effects of phosphatidylserine coatings on titanium on inflammatory cells and cell-induced mineralisation in vitro. Biomaterials 2005;26:7572–8. [8] Satsangi N, Satsangi A, Glover R, Ong JL, Satsangi RK. Osteoblast response and calcium deposition on phospholipid modified surfaces. J Mater Sci: Mater Med 2004;15:693–7. [9] Merolli A, Bosetti M, Giannotta L, Lloyd A, Denyer S, Rhys-Williams W, et al. In vivo assessment of the osteointegrative potential of phosphatidylserinebased coatings. J Mater Sci: Mater Med 2006;17:789–94.
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[10] Merolli A, Bosetti M, Giannotta L, Lloyd AW, Denyer SP, Rhys-Williams W, et al. In vivo assessment of the osteointegrative potential of phosphatidylserine-based coatings. J Mater Sci: Mater Med 2006;17:789–94. [11] Santin M, Rhys-Williams M, O’Reilly J, Davies MC, Shakesheff K, Love WG, Lloyd AW, Denyer SP. Calcium-binding phospholipids as a coating material for implant osteointegration. J R Soc Interface 2006;3:277–81. [12] Grace JM, Marijnissen JCM. A review of liquid atomization by electrical means. J Aerosol Sci 1994;25:1005–19. [13] Siefert W. Corona spray pyrolysis: a new coating technique with an extremely enhanced deposition efficiency. Thin Solid Films 1984;120:267–74. [14] Kumbar SG, Bhattacharyya S, Sethuraman S, Laurencin CT. A preliminary report on a novel electrospray technique for nanoparticle based biomedical implants coating: precision electrospraying. J Biomed Mater Res Part B: Appl Biomater 2007;81B:91–103. [15] Magarvey RH, Outhouse LE. . Note on the breakup of a charged liquid jet. J Fluid Mech 1962;13:151–7. [16] Deitzel JM, Kleinmeyer J, Harris D, Beck Tan NC. The effect of processing variables on the morphology of electrospun nanofibers and textiles. Polymer 2001;42:261–72. [17] Hudson HK. Penetrative osseointegrative phospholipide coatings on 3D titanium lattice structures. Ft. Collins, Colorado: Colorado State University; 2012.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrohydrodynamic atomization technique for applying phospholipid coatings to titanium implant materials David A. Prawel a,c, Matt J. Kipper b,c, Ketul C. Popat a,c, Susan P. James a,c,n a
Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80523, USA Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, CO 80523, USA c School of Biomedical Engineering, Colorado State University, Fort Collins, CO 80523, USA b
a r t i c l e i n f o
abstract
Article history: Received 29 September 2012 Accepted 20 January 2013 Available online 25 January 2013
Phospholipid coatings on titanium implants have been shown to enhance osteoblast activity, promote mineralization, and facilitate implant osseointegration in vivo. To date, dip and drip coating techniques have been used to apply these coatings. These coating techniques are easy to perform, but present difficulties in controlling the characteristics of the coating on the final surface, resulting in inconsistent, non-uniform, non-conformal coatings that are too thick for in vivo use. Electrohydrodynamic atomization (electro-spraying or e-spraying) is a versatile and easy method of creating thin, uniform and consistent coatings by atomizing a liquid with electrical forces. e-Spraying provides the advantage of being able to create coatings with relatively high efficiencies, while providing good control of coating morphology, especially on rough and intricately shaped surfaces, which is an important consideration for cell attachment and growth. Other advantages of this technique are low cost and easy setup. The purpose of this study was to develop an e-spraying technique for applying phospholipid coatings on commercially pure titanium implant materials. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electrospray Coating Biomaterial Orthopedic implant Titanium Phosphatidylserine
1. Introduction More than 4.4 million people have at least one metallic orthopedic implant, and the number of revisions is growing substantially [1]. Roughly a third of total joint replacements fail due to mechanical loosening from various causes [2]. Bone cements are often used to address these issues; however, cemented implants are prone to cement failure, leading many surgeons to prefer cementless implants, particularly for younger, more active patients. Poor mineralization at an implant/bone interface leads to the development of a thin fibrous layer between the implant and bone, which frequently correlates with implant failure. With our increased understanding of the complex biological processes that underlie implant failure in vivo, there has been a shift in focus from bone cements and mechanical fixation techniques to the design of novel mechanical/biomimetic strategies that promote natural physiological integration. Calcium phosphate-based bio-ceramics, primarily in the form of hydroxyapatite, have become popular cementless implant coatings [3]. However, long-term success of these coatings on loadbearing implants has been hindered by sub-optimal mechanical
n Correspondence to: Engineering A101, Campus Delivery 1374, Fort Collins, CO 80523-1374, USA. Tel.: þ 1 970 491 6559; fax: þ 1 970 491 3827. E-mail address: [email protected] (S.P. James).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.01.091
properties [4], delamination of the coating from the substrate [4] and poor mineralization and integration of newly forming bone with the implant surface [5]. As a result, considerable research effort has focused on development of new coating materials and techniques for popular implant materials. Phospholipids are one such candidate material. Phospholipids are broadly implicated in the development of new bone. The phospholipid bilayer of osteoblast vesicles is believed to be the primary nucleation and mineralization site of new bone [5,6]. Recent studies indicate that titanium surfaces coated with phospholipid alone (i.e. not in the presence of the additional constituents found in the matrix vesicle) enhance osteoblast activity, promote mineralization, and facilitate implant osseointegration in vivo [7–9]. These studies found that synthetic phosphatidylserine (PS) achieved the best in vitro biomineralization and in vitro osseointegration. PS coatings also present challenges: newly grown bone did not make intimate contact with the implant surface. Numerous techniques have been proposed for applying bioactive coatings to implant materials, including but not limited to plasma spraying [4] and sputtering [3]. High temperature processes cannot be used with phospholipids, and prior dip [10] and drip [7,11] coating processes resulted in relatively thick (approximately 100 mm) layers of soft PS that tended to form 3-dimensional gels in simulated body fluid. Such gels were not well adhered to the titanium substrate, resulting in mechanical instability [9]. These issues led to recommendations for
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D.A. Prawel et al. / Materials Letters 97 (2013) 81–85
thinner coatings to ensure mechanical stability of the implant [7], thus motivating improved coating processes. Electro-hydrodynamic atomization (electrostatic, electro-spraying or e-spraying) is a versatile method of creating thin, adherent coatings by atomizing a liquid by means of electrical forces [12]. Electrospraying provides the advantage of being able to create coatings with relatively high efficiencies [13] (i.e. most or all of the material sprayed becomes part of the coating on the target) because the charged liquid source material is carried by the electrical field rather than being mechanically atomized or carried on another liquid, as in typical pressure-based (e.g. aerosol) spraying techniques [14]. This is especially advantageous for more costly coating materials, and it enables good control of coating uniformity and morphology, especially on rough and intricately shaped surfaces. Other advantages of electrospraying are low cost and easy setup. Detailed technical explanations and reviews are found elsewhere [12]. The goal of this study was to develop a technique to electro-spray thin, conformal, and consistent PS coatings on flat titanium implant materials and titanium surgical screws.
2. Materials and methods 1,2-Dioleoyl-sn-glycero-3-phospho-L-serine (‘‘DOPS’’) (Avanti Polar Lipids, Alabaster, AL) was dissolved in chloroform to a 20 mM concentration and drawn into a glass syringe (10 cc, Hamilton) which was then mounted on a syringe pump (Kent Scientific) (see Fig. 1). A double hub syringe tube (12 in., 20 gauge, Hamilton) connected the syringe to a blunt needle (22 gauge, Hamilton). Target titanium samples (flat, commercially pure Ti, 0.016 in. thickness, 0.75 cm 0.75 cm), were cleaned by successive 30 min ultrasonic baths in 2% liquinox, chloroform, acetone
and distilled water (DIH2O). Samples were mounted with a backing of silicone putty (UHU Tac, Basel, Germany) on a ‘‘target board’’ consisting of an insulated circuit board with copper wires directly contacting the Ti samples, and protruding through the target board rearward (away from the spray direction). This target board was positioned at a measured distance from the needle. The needle and the copper wires backing the target board (contacting the Ti samples) were connected to the positive and negative (ground) poles, respectively, of a controlled DC voltage source (Gamma High Voltage Research, Ormond Beach, Florida). The pump, syringe tube, syringe body and all mounting hardware were also grounded. When the voltage is applied, the source material is moved in the electric field from the needle to the target as a very fine, nearly indiscernible, mist. The electro-spraying process depends on many parameters such as the concentration and viscosity of the material sprayed, the electric field strength (induced by voltage over a distance) and the rate at which material is introduced into the electric field (by the syringe pump). Table 1 shows representative values explored experimentally in this study, as well as the values for other parameters held constant throughout the study, and indicates which parameters correspond to figures shown below. DOPS coatings were characterized with SEM; samples were coated with 8–15 nm of gold and imaged at 5–8 kV in a JOEL JSM6500F field emission SEM (Tokyo, Japan). Coating thickness was assessed using SEM by scratching the coating through to the titanium substrate and viewing the scratch at a steep angle. A titanium surgical screw (1.9 mm diameter, 6 mm length, ELI titanium alloy ASTM F136—Bioplate, Los Angeles, California) was e-sprayed in a similar manner, using parameters from Fig. 2D, shown in Table 1. SEM images were taken from ‘‘front’’ (facing directly at the oncoming e-spray), ‘‘back’’ and sides of the screw (i.e. midway between front and back) to determine whether unidirectional spray toward the ‘‘front’’ of a 3D part would result in even coating over the entire part.
3. Results Consistent with prior studies [14], small variations in e-spray parameters led to dramatically different coating morphologies. Fig. 2 presents SEM images of representative DOPS coatings e-sprayed at parameters shown in Table 1. Higher DOPS concentration and pump rate (Fig. 2A) produced a relatively smooth, uniform coating surface devoid of pores, particles or distinct surface features. Reducing the amount of material in the electric field (e.g. by decreasing DOPS concentration and pump rate), while increasing time and voltage, produces a rougher yet still uniform coating surface, also devoid of pores but with numerous particles and distinct surface features (Fig. 2B). Other parameters being constant, increasing voltage led to an increase in particles and features on the surface (compare Fig. 2B and D). Similarly, decreasing distance generally led to a smoother coating (compare Fig. 2C and D). Coatings were 6–8 mm thick.
Fig. 1. Electro-spraying apparatus.
Table 1 Representative values explored experimentally in this study. Image #
2.A 2.B 2.C 2.D
Target area (nominal) (cm2)
3 3 3 3
Process parameters Needle size (ga)
[DOPS] (w/v) (%)
Pump rate (ml/h)
Time (min)
Distance (cm)
Voltage (kV)
22 22 22 22
2.5 1.3 1.3 1.3
20 14 14 14
2.25 3 3 3
8 8 6 8
8 10 10 12
D.A. Prawel et al. / Materials Letters 97 (2013) 81–85
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Fig. 2. SEM images (all 1000 ) of DOPS coating on flat titanium samples—(A) e-sprayed with pump rate 20 ml/h, voltage 8 kV, 8 cm distance. (B) e-Sprayed with pump rate 14 ml/h, voltage 10 kV, 8 cm distance. (C) Pump rate 14 ml/h, 10 kV, 6 cm distance. (D) pump rate 14 ml/h, 12 kV, 8 cm distance.
We also demonstrated that e-spraying can coat 3-dimensional (3D) objects, such as surgical screws, more uniformly than lineof-sight spray coating techniques. e-Spraying can coat all faces of a 3D object in a single unidirectional e-spray event. Fig. 3 shows front and back images of the screw, showing that coatings on both front and back are consistent, uniform and visually similar in morphology to e-spray coatings on flat samples created with the same e-spray parameters (shaded row in Table 1).
4. Discussion Kumbar and others [14,15] found that holding certain parameters constant while varying others produced PLG and PEG coatings with widely varying morphologies and thicknesses. Higher electric field strength increases field current [12], which increases mass flow rate from the needle to the target, all other parameters held constant (to the limit of the amount of material available in the electric field at a given pump rate). A higher mass flow rate increases the number of charged particles in the e-spray jet [12], which increases both electrostatic and Coulombic forces, inducing a stretching force on material droplets in the jet, similar to increasing surface tension. This results in greater repulsion between adjacent droplets [16], which causes formation of a relatively rougher surface texture consisting of more particles and features on the surface. This effect is clearly seen as voltage is reduced between Fig. 2A and B, and between Fig. 2C and D. DOPS is an amphiphilic molecule with a polar head and nonpolar acyl chain tails. Varying electric field parameters causes differential rearrangement of alignment and conformation of the
DOPS molecules, along with interactions between DOPS molecules, as discussed in the following. This effect also plays a role in the morphology of e-sprayed coatings, as multiple factors interact to form a particular coating. For example, decreasing DOPS concentration and pump rate (from Fig. 2A and B) reduces the amount of sprayed material in the electric field, while simultaneously increasing voltage and time produces a rougher, uniform coating surface, devoid of pores, with numerous particles and distinct surface features. Less material available in the electric field results in a smoother surface texture (fewer charged particles in the e-spray jet), whose effect is slightly overshadowed by the effect of higher voltage (increasing particles and surface features). Furthering this argument, decreasing distance with all other parameters held constant (from Fig. 2B to C) also increases field current, which would be expected to result in formation of more current-induced particles and features on the surface. However, the shorter distance also results in shorter time-of-flight, which appears to overshadow the current-induced effect by reducing time for formation of particles and features, resulting in a smoother but less uniform surface with relatively large pores and pockets. Increasing voltage, with all other parameters held constant (from Fig. 2B to D), increases current-induced formation of more particles and features on the surface. Similarly, increasing voltage and distance simultaneously (from Fig. 2C to D) increases currentinduced formation of more particles and features on the surface, while allowing more time for formation of such features. Although longer distance would by itself reduce field strength, this effect is overshadowed by the effect of increased voltage at this relative increase in distance.
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Fig. 3. SEM images of DOPS coating on surgical screw—(A) (50 ) shows the ‘‘front’’ side of the screw (facing directly at the oncoming e-spray). (B) (50 ) shows the ‘‘back’’ side of the same surgical screw (facing away from the oncoming e-spray). (C,D) Front and back of the same surgical screw at 1000 , respectively. Note the similarity of coatings on both sides.
Electric fields meet surfaces at perpendicular angles. Thus it is not surprising that e-spraying completely coats the surgical screw in a single unidirectional spray event, given appropriate e-spray parameters. The field wraps around the screw, depositing material on all grounded surfaces. It is likely that coating thickness is self-adjusting, due to the insulating nature of the phospholipid. As more material arrives at the screw surface it seeks the area of highest charge density (thinnest coating), resulting in consistent, uniform surfaces completely around the screw. A known limitation of the method can occur when the intricate surface being coated contains large concavities which could result in the formation of a Faraday Cage effect, potentially preventing material from entering the concavity [17]. With appropriate mitigation in these instances, and assuming the target object is, or could be made, conductive, e-spraying can be an excellent method for coating intricate 3D shapes. It is interesting to note the smooth appearance of the thread edges in Fig. 3. It is possible that these edges failed to coat, or that the coating assumes a different (smooth) morphology on these features. Further investigation is required to determine if this is the result of disturbance of the electric field lines, similar to or resulting from a Faraday Cage effect [17].
5. Conclusions The worldwide need for thin, conformal, osseointegrative coatings on orthopedic implants is well documented. e-Spraying is a suitable method for creating DOPS coatings on titanium. Compared to commonly used drip and dip techniques, e-sprayed DOPS coatings are much thinner, which could lead to enhanced
mechanical stability for un-cemented implants [7]. And, e-sprayed coatings are more consistent and uniform, enabling more dependable and predictable device coverage. Manipulation of the key extrinsic e-spray process parameters such as voltage and distance, which result in changes in the electric field strength, provide effective, predictable control of coating characteristics such as morphology, micro-topology and porosity, all of which are important for successful cell growth and adhesion, and therefore to long-term success of the orthopedic implants upon which the coatings reside. e-Spraying is highly repeatable between tests on both flat and 3D samples, as confirmed by SEM, enabling potential commercial success of the technique. Finally, e-spraying is easy to perform and requires only a few thousand dollars in equipment investment. Acknowledgments This paper was supported by the Colorado State University Cancer Supercluster Translational Research Grant Program and Colorado Bioscience Discovery Evaluation Grant Program. Neither funding source had any role in study design, collection, analysis or interpretation of data; in the writing of the report; or in the decision to submit this article for publication. References [1] Kurtz S, Ong K, Lau E, Mowat F, Halpern M. Projections of primary and revision hip and knee arthroplasty in the United States from 2005 to 2030. J Bone Joint Surg Am 2007;89(4):780–5.
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