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ZnO transducers on printed graphene-cellulose electrodes
Sensors and Actuators A 302 (2020) 111800
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Disposable piezoelectric vibration sensors with PDMS/ZnO transducers on printed graphene-cellulose electrodes Dogan Sinar, George K. Knopf ∗ Department of Mechanical and Materials Engineering, The University of Western Ontario, London, ON, N6A 3K7, Canada
a r t i c l e
i n f o
Article history: Received 26 August 2019 Received in revised form 3 December 2019 Accepted 16 December 2019 Available online 18 December 2019 Keywords: Disposable electronics Environmentally benign manufacturing Graphene-derivative inks Zinc oxide Polydimethylsiloxane Piezoelectric sensors
a b s t r a c t The individual components and circuitry of single-use flexible electronic sensors must be designed and fabricated to maintain stable functionality during normal operation, but once the usefulness of the device has concluded it is often incinerated or disposed of in a landfill. It is critical, therefore, that environmentally benign materials and fabrication processes be used to create these sensors such that manufacturing and disposal processes do not result in toxic or hazardous by-products. This paper introduces a novel flexible piezoelectric vibration sensor based on interdigitated electrodes (IDEs) printed on polymercoated paper substrates using nontoxic graphene nanoparticle (G) and carboxymethyl cellulose (G-CMC) aqueous suspensions as the electrically conductive ink. The piezoelectric transducer consists of environmentally benign zinc oxide (ZnO) nanoparticles dispersed in a polydimethylsiloxane (PDMS) matrix. During fabrication, the PDMS/ZnO composite is spin coated on the inkjet printed G-CMC interdigitated electrodes forming a thin piezoelectric layer. The fabricated sensors are tested, without additional signal amplification, for direct force response and low amplitude vibrations. A repetitive 6.3 N impact force, at very low frequencies (1–2.37 Hz), generated up 541 mVp-p . A further study of low-amplitude vibrations, over the frequency range of 50 Hz to 2.5 kHz, produced voltage outputs from 25 mVp-p to 452 mVp-p . Single-use force and vibration sensors over these low frequency ranges can be used for intelligent packaging, temporary monitoring of the environment and disposable wearable technologies. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Disposable single-use mechanically flexible electronic sensors will significantly impact the future of healthcare, food safety inspection, intelligent packaging, environmental monitoring, public security, and consumer wearable technologies. Often these applications do not require sophisticated multi-functional devices or high precision measurements rather simple robust sensors that perform one or two well-defined functions within acceptable limits. The market opportunity for these inexpensive and robust with short product life cycles has grown significantly in recent years. For example, the shipment of commercial wearable devices from US manufacturers, excluding smart clothing, has increased more than 500 % between 2013 and 2018 to staggering 66 million units a year [1]. In general, the physical structure of the constituent electronic components and circuitry, transducers, substrates, and protective enclosures must be designed and fabricated to maintain stable functionality during normal operation but once the useful-
∗ Corresponding author. E-mail address: [email protected] (G.K. Knopf). https://doi.org/10.1016/j.sna.2019.111800 0924-4247/© 2019 Elsevier B.V. All rights reserved.
ness of the device has concluded the product is physically destroyed in an incinerator or disposed of in a landfill. It is critical, therefore, that environmentally benign materials and fabrication processes be used to create these sensors such that manufacturing and disposal does not result in toxic or hazardous by-products. Advances in electrically conductive inks and drop-on-demand (DoD) printing have enabled a wide variety of flexible electronic sensors and electrochemical transducers to be fabricated on non-rigid polymers, natural fiber composites, textiles, and paper. Although conductive polymer (e.g., PEDOT:PSS) and metal-based (e.g., Au, Ag) inks have high electrical conductivity, these liquid dispersions are synthesized using a variety of toxic solvents and additives. For example, pure PEDOT:PSS has low conductivity properties and must be doped with an organic solvent such as dimethylformamide, ethylene glycol, or dimethylsulfoxide (DMSO). The dopant solvent remains in the fabricated thin film and becomes an integral part of the device. The presence of these toxic chemicals become an environmental hazard [2] when the disposed product is incinerated under high temperature or dumped in landfill. Similarly highly conductive metal-based inks such as silver (Ag) induce oxidative damage to biological tissues and cells, and disrupt the natural lifecycle of bacteria in landfill soil.
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As an alternative, a nontoxic aqueous carbon-based conductive ink based on a hydrophilic carboxymethyl cellulose (CMC) suspension with graphene (G) nanoparticles was recently developed [3]. The novel water-based G-CMC ink can be directly deposited on different functionalized rigid (e.g., glass) and flexible (e.g., PDMS, paper) substrates using DoD inkjet printing technology. Prior studies reported in the literature have shown that this deposition method could create conductive thin G-CMC films at varying thicknesses (100–2000 nm) with sheet resistances as low as ∼660 /sq. [3]. In the work described in this paper, the conductive graphene-based ink and DoD printing technology is used to create the electrodes on a paper substrate for a simple multi-layered piezoelectric sensor that responds to mechanical strain and low amplitude vibrations. The proposed sensor is created by printing G-CMC interdigitated electrodes (IDEs) on conventional paper that has been coated with an ultrathin non-absorbent polydimethylsiloxane (PDMS) layer. A thin piezoelectric polymer composite layer comprised of PDMS and zinc oxide (ZnO) nanoparticles is then deposited on top of the electrodes. Various nontoxic materials and environmentally benign chemical processes are used to synthesize the conductive ink, prepare the PDMS/ZnO piezoelectric composite, and deposit the thin films. Only a small quantity of chloroform (CHCl3 ) is used to disperse the ZnO nanoparticles in the PDMS matrix and majority of CHCl3 is removed by evaporation during the curing process. Futhermore, the simple multi-layered design ensures that all components of the single-use flexible electronic sensor maintain stable functionality during normal operation, but once the usefulness of the device has concluded it can safely incinerated or disposed of in a landfill. The decomposition of the paper substrate will release the nontoxic carbon particles (i.e., G-CMC), PDMS material and ZnO nanoparticles into the immediate environment. Although not biodegradable (i.e., decomposed directly by micro-organisms), the chemical and mechanical properties of PDMS are considered to be nontoxic and environmentally benign [4,5]. If hydrophobic PDMS enters the municipal waste water system it becomes part of the sludge that can be safely incinerated or buried in a landfill [6]. Once in the soil, PDMS can hydrolyze to small water soluble monomeric dimethylsilanediol (DMSD) which can microbially degrade to CO2 and inorganic silicate which is naturally present in soil [7,8]. Studies have also shown that ZnO is harmless to animals with only a negative effect on certain bacterial cultures [9]. Section 2 introduces the selected materials used to create the sensor, describes the interdigitated electrode (IDE) design and fabrication process, and provides details about the preparation of the thin PDMS/ZnO piezoelectric composite layer. The functionality and performance of the proposed sensor for measuring impact forces and low-amplitude vibrations is explored in Section 3. Finally, Section 4 summarizes the key performance capabilities of the fabricated sensor and briefly discusses the notion of disposable electronics.
2. Materials and fabrication 2.1. Material selection The materials selected to create the novel piezoelectric sensor is based on the design principle of mitigating the impact of toxic chemicals during manufacturing, product use, and final disposal. In particular, it is critical that when the short life-cycle device is eventually disposed of in a landfill, or incinerated, it does not release toxic residue or harmful by-products. The constituent materials for synthesizing the electrically conductive ink, substrate, piezo-
electric polymer composite, and coatings must be biologically and environmentally benign. The electrodes for the piezoelectric sensor are fabricated using a nontoxic aqueous carbon-based ink where naturally hydrophobic graphene (G) nanosheets are suspended in de-ionized water at high concentrations using a nontoxic hydrophilic cellulose derivative, carboxymethyl cellulose (CMC), to facilitate liquid phase exfoliation and stabilization [3]. Electrical conductivity of the deposited films are adjusted by varying the film thickness. The DoD deposited conductive G-CMC ink produces a thin film with numerous few-layered carbon sheets that naturally separate when the underlying substrate decomposes because graphene nanosheets are held together by weak van der Waal forces. The remaining residue after decomposition is carbon in the form of graphene nanosheets, chemicals used to create the degradable substrate, and any benign coatings used during manufacturing. The substrate material for all prototypes is standard printer paper (80 gsm) spin coated with an ultrathin layer of polydimethylsiloxane (Sylgard 184). The thin polydimethylsiloxane (PDMS) layer provides a non-absorbent barrier on the naturally porous substrate surface in order to prevent the conductive G-CMC ink droplets from soaking into the substrate during the inkjet printing process. The PDMS is also used as the polymer matrix for creating the piezoelectric ZnO composite. In general, PDMS has well-defined chemical and mechanical properties and is considered to be nontoxic [4,5]. For example, if a PDMS sample is disposed of in soil it will often hydrolyze into water soluble siloxanes and dimethylsilanediol (DMSD). Furthermore, DMSD can biodegrade to CO2 microbially or through sunlight-induced oxidation. Researchers have shown that the biodegradation of PDMS did not harm the microorganisms in the soil or negatively affect the crop growth [8]. The strain responsive polymer composite contains ZnO nanoparticles because it is considered nontoxic [8], exhibits very good piezoelectric properties [10], and easy to synthesis with chemical processes that are not harmful to the environment. Studies have shown that ZnO is harmless to animals with only a negative effect on certain bacterial cultures [9]. In addition, the process of synthesizing ZnO nanoparticles is largely benign and does not involve the use of harmful chemicals and solvents. In terms of fabrication, the ZnO nanoparticles are initially synthesized through a chemical process where the zinc acetate dihydrate seeds are dissolved in 20 % methanol solution and allowed to react with NaOH at 90◦ C. To synthesize the ZnO nanoparticles in this research sodium hydroxide (NaOH) pellets were acquired from ACP (ACS, 97.0 %) and Zinc acetate dihydrate (Zn(OOCCH3 )•2H2 O) was acquired from Alfa Aesar (ACS, 98/0–101.0 %). The resulting ZnO nanoparticles are mixed with PDMS base after purification to create the piezoelectric polymer composite. The PDMS/ZnO composite is then spin coated on the G-CMC electrodes and allowed to cure at 95◦ C for one hour. The ZnO nanoparticles dispersed in the polymer PDMS matrix used to create the thin multi-layered piezoelectric sensor can work both in d33 and d31 modes at acceptable conversion efficiencies due to their geometrical shape. This property makes a planar interdigitated electrode (IDE) design a viable alternative to parallel plate electrodes positioned above and below the polymer composite. This is an advantage from a fabrication perspective because the planar IDE design allows for a single step electrode deposition and avoids the additional assembly steps often required for creating discrete multi-layered structures. 2.2. Electrode fabrication The electrodes for the prototypes are inkjet printed using a water based graphene-derivative conductive ink and a custom built
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Fig. 1. The IDE dimensions as designed (a). Inkjet printed G-CMC electrodes on paper substrates (b). The dimensions of printed samples were confirmed under optical microscope to be in good agreement with the design (error margin of ∼5 %).
DoD printer with an HP C6602a cartridge. The paper substrate is initially spin coated with Sylgard 184 polymer (1:10 curing agent ratio). Once cured, the paper substrate with an ultra-thin PDMS coating is corona treated for about a minute and then placed with the printable face down onto a polyethyleneimine (PEI) solution (4.5 mg/mL, 0.5 M NaCl, DI water). After a thorough wash in deionized (DI) water and pressurized air dried, the substrate is placed on to poly(sodium 4-styrenesulfonate) (PSS) solution (7.5 mg/mL, 0.5 M NaCl, DI water). The substrate is further washed with DI water, air dried, and placed onto the PEI solution for the final time. After a final wash and dry period, the paper substrate is ready for the inkjet deposition process. Note that the polyelectrolyte coating on the substrate forms a positively charged and hydrophilic surface that allows for homogeneous deposition of electrically conductive graphene-derivative ink. The printable graphene-cellulose (G-CMC) ink is synthesized and prepared based on the methodology described in a prior publication [3]. In a typical synthesis procedure, 2 mg/mL of graphene powder was mixed with a 50 % 1-propanol/MilliQ water solution. This mixture was sonicated for 4 h. In a separate container, 2 mg/mL CMC (250 kDa, 0.7 DS) was mixed with 100 mL MilliQ water. The CMC solution was slowly mixed with the graphene mixture and bath sonicated for another 4 h. The resulting dispersion was centrifuged at 20,000 G for 15 min, sediment was collected, and reconstituted in MilliQ water. This process was repeated 2 more times in order to remove free CMC from the functional ink. Final product was reconstituted in 50 mL of MilliQ water. Once synthesized, the G-CMC ink is mixed with 1-propanol (∼ 28 %v) in order to reduce surface tension of the ink. Using the modified G-CMC ink, the IDEs are printed directly on the paper substrate with the ultrathin polymer layer. The interdigitated electrode (IDE) patterns are inkjet printed at 4800 mm/min feedrate and 144 m droplet spacing. The IDE design parameters and photograph of the fabricated electrodes are presented in Fig. 1. The inkjet printed IDEs have 38 digits with d =600 m line width, h = 7.6 mm line length, and g =400 m gap between digits. Overall width and length of the IDEs are w = 10 mm and l = 40 mm, respectively. Two 3 mm × 3 mm contact pads are created at the end of the micro-trace for connecting the samples to the measurement equipment. Based on previous literature [3], the thickness of the deposited thin film is expected to be ∼1.5 m.
2.3. Preparation of PDMS/ZnO piezoelectric composite Under ideal conditions, zinc oxide nanostructures favor the wurtzite crystal structure due to higher stability [11]. In a wurtzite structure, Zn2+ and O2− atoms’ are arranged in tetrahedral coordination. The lack of inversion symmetry under mechanical strain due to the tetrahedral structure results in the desired piezoelectric properties [12]. Hence, a larger crystal domain can result in better piezoelectric performance. The crystal domain size can be manipulated simply through increasing particle size, assuming impurities are non-significant. This phenomenon has been observed in the case of zinc oxide nanowires and nanorods where nanostructures can have lengths in micrometer range and exhibit high piezoelectric conversion efficiency [13–15]. On the other hand, increased nanoparticle size is associated with poor dispersion in polymer/nanoparticle composites. Poor dispersion in a polymer may result in reduced performance and heterogeneous behaviour along the composite. In this regard, good control over nanoparticle dimensions is an important factor for predictable piezoelectric composite performance. In this study, the chemical synthesis route is chosen for growth of zinc oxide nanoparticles due to its simplicity and potential for scaling. The chemical synthesis route, and its many variations, are frequently utilized for quick and easy growth of nanoparticles. In the case of zinc oxide nanoparticle growth from zinc acetate dihydrate, NaOH salt is used to form zinc oxide nanoparticles in the following simplified Eq. (1): (Zn(OOCCH3 ) • 2H2 O) + 2NaOH → ZnO + 2NaCH3 COO + H2 O
(1)
In the above equation, zinc oxide nanoparticles are formed through removal of acetate groups from zinc and formation of sodium acetate. During the reaction, nucleation of newly formed zinc oxide molecules have a strong influence on the nanoparticle size. Conditions that increase the overall reaction speed, such as higher synthesis temperature [16,17], more polar solvents [18], and increased precursor concentrations [19] have been associated with smaller crystallites. Based on these studies, a modified chemical synthesis route is formulized. In a typical synthesis process, zinc acetate dihydrate (0.1 M) is dissolved in ultrapure water with 20 % methanol content. In a separate container, NaOH (0.1 M) is dissolved in ultrapure water with 20 % methanol content. Zinc
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Table 1 Sample list and parameters of initial ZnO/CHCl3 dispersions.
3. Results and discussion
ZnO Weight Percentage in 2 mL of PDMS Base
CHCl3 [mL]
ZnO [mg]
Sample PZ-2 - 2 %w Sample PZ-4 - 4 %w Sample PZ-6 - 6 %w Sample PZ-8 - 8 %w
2 4 6 8
38.6 77.2 115.8 154.4
acetate solution is heated under magnetic stirring. Once the solution reached 90 ◦ C, NaOH solution is added slowly to the reaction vessel. In about 5 min, formation of white colloids is observed. The dispersion with zinc oxide nanoparticles is kept at 90 ◦ C and under magnetic stirring for an additional hour. Then, the dispersion is left to cool down at room temperature for 12 h. At the end of the cooling period, the zinc oxide sediment is collected, washed with ultrapure water and centrifuged. Next, the sediment is washed with methanol and centrifuged for a second time. Finally, sediment is reconstituted in methanol and left on a hot plate to remove all solvent. The white dry powder is collected and heated to 630 ◦ C in an oven (in ambient air atmosphere) for one hour to remove any leftover impurities and optimize crystal structure. A small part of the nanoparticles were collected for TEM and XRD analysis. The TEM and XRD analysis were conducted to confirm nanoparticle dimensions and crystal structure, respectively. Performance of polymer/nanoparticle composites are impacted by dispersion homogeneity of nanoparticles in the polymer matrix. Non-cured PDMS is a highly viscous liquid and is generally regarded to be chemically inert. Although mechanical mixing of ZnO nanoparticles in PDMS have been reported [20–22], it was deemed to be an inefficient route for this study. In fact, attempts at mechanical mixing of ZnO in PDMS had resulted only in a partial dispersion with large amount of agglomerated particles visibly present. Such an inefficient dispersion would make it problematic to analyze effects of ZnO weight percentage in the polymer, as well as negatively affect device performance. Hence, a common solvent mixing route was developed. In this approach, chloroform (CHCl3 ) was utilized because it can both solvate PDMS and disperse ZnO. Although tetrahydrofuran ((CH2 )4 O) is a better solvent than CHCL3 for this application, tetrahydrofuran’s persistence on the zinc oxide nanoparticles [23] after synthesis implies that the moderately toxic (CH2 )4 O may become a source of long-term contamination during disposal. The synthesis process first required 2 mL PDMS base to be mixed with 4 mL of chloroform. This mixture was thoroughly mixed until all PDMS base was solvated. Next, a certain amount of ZnO was mixed with chloroform in a separate container and bath sonicated for 1 h. Zinc oxide does not have a large number of good solvents and its stable dispersions often require functionalization or extensive sonication. On the other hand, chloroform can temporally disperse ZnO nanoparticles. The bath sonicated ZnO/CHCl3 dispersion was mixed with the PDMS/CHCl3 solution and thoroughly mixed, resulting in a homogeneous PDMS/ZnO/CHCl3 dispersion. Finally, the dispersion was placed on a hot plate at 90 ◦ C for 2 h in order to evaporate CHCl3 . Resulting PDMS/ZnO dispersion was still homogeneous due high viscosity of PDMS base (Fig. 2a). Zinc oxide weight percentages and corresponding polymer composites are presented in Table 1. The piezoelectric PDMS/ZnO composite is finalized by the addition of PDMS curing agent at the 1:10 mixing ratio. After a thorough mixing, a small amount of mixture (1 mL per sample) was spin coated on the G-CMC IDEs at a speed of approximately 820 rpm. Spin coated device was carefully placed in an oven and cured at 95 ◦ C for 1.5 h (Fig. 2b). Fig. 3 summarizes the key steps of the vibration sensor fabrication process.
3.1. Characterization of ZnO nanoparticles Prior to fabricating the PDMS/ZnO piezoelectric composite, a small quantity of ZnO nanoparticles were collected for TEM and XRD analysis. The TEM images reveal that nanoparticles had 50–200 nm lateral dimensions. Most of the nanoparticles exhibit polyhedral form with only a few rectangular shapes (Fig. 4). XRD pattern (Fig. 5) exhibited expected peaks from the wurtzite crystal structure (compared to ICDD database). Lattice parameters were a =3.25 Å and c =5.20 Å, which has the c/a ratio of ∼ 1.6, indicating a hexagonal cell structure. Based on XRD pattern, synthesized ZnO was mostly free of impurities with only small residual peaks observable at 72.54, 81.36, and 92.75 degrees.
3.2. Repetitive impact force tests The fabricated flexible piezoelectric sensors are initially tested for force measurements. The force sensing is possible because of the compressive strain experienced by the thin PDMS/ZnO transducer deposited on the G-CMC electrodes. In these experiments, the voltage potential generated by the compressed devices are monitored and logged with an Agilent DSO5014a oscilloscope. The flexible sensors are secured with epoxy glue on a flat test surface and the force is applied directly on the sensor surface using a vertically aligned positioning system (Fig. 6). The force applicator is a 1 cm diameter flat tip metal rod attached to a multi-axis positioning system that had been vertically aligned with the test surface. In addition, a commercially available force sensing resistor (Interlink Electronics FSR-402) is attached to the test surface to act as the control. The external forces are applied at low frequencies (1.0–2.37 Hz) in order to simulate biomechanical movements. Equipment limitations restricted the choice of applied forces. The magnitude and frequency of the applied force was controlled by adjusting the travel distance of the applicator. The speed (200 mm/min) and acceleration (200 mm/s2 ) of the applicator tip was fixed for all experiments. In addition, surface pressure could be determined because the contact area of the fixed-diameter applicator is known. For the first set of tests, the start position of the applicator was adjusted to produce an applied force of ∼ 3.7 N (∼ 46.8 kPa). The value was confirmed with the Interlink force sensing resistor. For the second set of tests, the initial applied force was increased to 6.3 N (∼79.7 kPa) by moving the start point of the applicator 0.2 mm towards the sensor. The force values of 3.7 N and 6.3 N were selected based on adult foot pressure values as reported in the literature [24]. The peak-to-peak voltage outputs for all measurements were recorded and averaged over 5–12 data points, depending on the frequency. The standard deviations were calculated using individual measured data points. The voltage produced by the piezoelectric composite is based on change in force and, therefore, the peak values will occur at the instant maximum force is applied or removed from the sensor. Fig. 7 shows the voltage output at various frequencies for the maximum applied loads of 3.7 N (Fig. 7a) and 6.3 N (Fig. 7b). The different curves correspond to increasing the ZnO weight percentage as summarized in Table 1. PZ-2, PZ-4, PZ-6, and PZ-8 correspond to 2 %w, 4 %w, 6 %w and 8 %w ZnO, respectively. The graphs show that the PZ-8 sample generated a higher voltage than the other samples for both the 3.7 N and 6.3 N forces. As well, the data confirms that the output voltage increases with the magnitude of the applied force. In contrast, the output voltages for both applied loads did not change significantly to minor variations in the frequency of repeated impact (i.e, 1.0–2.37 Hz). The standard deviation was observed for both the proposed piezoelectric device and the off-
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Fig. 2. Use of chloroform as a common solvent allows for homogenous dispersion of ZnO inside PDMS matrix (a). Inkjet printed piezoelectric sensor after spin coating of PDMS/ZnO composite (b).
Fig. 3. Summary of the fabrication process used to create the piezoelectric vibration sensors based on spin coating PDMS/ZnO material on inkjet printed G-CMC electrodes.
Fig. 4. Synthesized ZnO nanoparticles exhibit polyhedral form. Nanoparticles as small as 50 nm edge length was observed. Larger nanoparticles were approximately 300 nm in edge length.
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Fig. 5. XRD pattern of ZnO nanoparticles. Wide angle analysis confirms wurtzite crystal structure.
Fig. 6. Experimental apparatus used to measure force impact at low frequencies (1.0–2.37 Hz). The force applicator is attached to computer controlled xyz-positioning system.
the-shelf force sensing resistor indicating reasonable repeatability for the printed sensor. The magnitude and frequency response of the PDMS/ZnO composite on the G-CMC electrodes is comparable to other piezoelectric devices described in the literature. However, a direct comparison is not possible because most authors illustrate device functionality by rapidly tapping the surface with a finger [19,21]. Similar ad hoc tests showed that the proposed sensor comprised of ZnO at 4 w% did generate 13–20 V at around 3−4 Hz with finger tapping. This is reasonable considering that the repetitive contact of the finger is significantly faster than the force applicator used in this study. In addition, the contact surface area of the finger is larger than the mechanical applicator. 3.3. Low-amplitude vibration tests Vibration stimulus is created using a shaker (Agac-Derritron) with an analog amplifier (Crown CE2000). Sinusoidal stimulus signal was provided by an HP 35670A Dynamic Signal Analyzer (Fig. 8). Output voltage from the test sample was collected with Agilent DSO5014a oscilloscope. The fabricated devices are fixed on the stainless steel beam of the shaker with the help of an adhesive for additional robustness. The selected frequency range (50 Hz to
2.5 kHz) demonstrates the effectiveness of the unique sensor (i.e., PDMS/ZnO transducers on G-CMC electrode) to measuring external environmental influences such as workspace and vehicle vibrations on the human body. Frequency response analysis for 10–50 Hz is not included due to limitations of the instruments used in the study. The response of the flexible piezoelectric sensor to low amplitude vibrations over the range from 50 Hz to 2.5 kHz was also investigated. The magnitude of the vibrations were fixed for all tests. Fig. 9 shows the increase in voltage potential for all samples (PZ-2, PZ-4, PZ-6 and PZ-8) over the test range. At lower frequencies (< 200 Hz) the PZ-2 and PZ-4 quickly increase and show a higher voltage output. These two samples with lower ZnO content then produce a near linear response for frequencies greater than 200 Hz. In contrast, the higher ZnO content samples PZ-6 and PZ-8 start with a lower voltage output at frequencies < 300 Hz but then increase significantly doubling the voltage output of PZ-2 and PZ-4. The higher voltage output with increasing ZnO percentage is expected and is a result of efficient dispersion of nanoparticles inside the PDMS matrix. Yet, higher performance of PZ-2 and PZ-4 samples for lower frequencies cannot be explained by nanoparticle concentration directly. Although a more in-depth investigation may be necessary to understand this trend, it is assumed that changing mechanical properties of PDMS (e.g., stiff-
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Fig. 7. Voltage output due to applied force of 3.7 N/46.8 kPa (a) and 6.7 N/79.7 kPa (b). Error bars are shown for 5–12 data points (frequency dependent). Applied force has a direct and positive impact on sensor output while change in frequency did not affect the output.
Fig. 8. Experimental apparatus used to measure low amplitude mid-range vibrations (50 Hz to 2.5 kHz). The sensors are attached with an adhesive to the metal arm of the shaker.
ness) with increasing ZnO amount may have an impact. It has been shown that ZnO weight percentage has a direct effect on composite elastic modulus. Hence, higher ZnO concentration is associated with higher composite stiffness [21]. Furthermore, internal screening effects may become more evident as concentration of ZnO increases which is a result of charge transfer between individual nanoparticles. This would result in reduced electrical output. A layer-by-layer polyelectrolyte coating procedure can be utilized to shield nanoparticles from each other and reduce screening. This procedure was not implemented due to reduce fabrication complexity.
The time varying output signal of the four sensors for three different excitation frequencies (100, 1000 and 2500 Hz) is shown in Fig. 10. Signal output for excitation and relaxation regimes were symmetrical for each sample throughout the tested frequency range. This is the expected output for piezoelectric sensors and also an indication of sensor and test setup robustness. Sensor hysteresis was analyzed from recorded oscilloscope data. The ZnO percentage had no significant impact on sensor hysteresis. On the other hand, time delay between excitation signal and sensor output were reduced across all samples as the frequency was increased. For all samples, a peak-to-peak time delay of 7.1–8.3, 0.84-0.88, and 0.37-
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4. Conclusion
Fig. 9. All samples exhibit repeatable and predictable performance with increasing frequency without any need for signal amplification.
0.375 ms were observed at 100, 1000, and 2500 Hz, respectively. Note that this time delay includes the response time of the amplifier and shaker. Hence, actual time delay of the sensors would be lower.
A disposable and novel mechanically flexible piezoelectric sensor based on inkjet printed G-CMC interdigitated electrodes (IDEs), PDMS coated paper substrates, and a thin PDMS/ZnO piezoelectric polymer composite was introduced. The constituent carbon electronics, natural fiber substrate and biocompatible polymer transducer are composed of nontoxic materials and the method of fabrication did not generate any hazardous by-products. The simple fabricated piezoelectric G-CMC sensors were tested without signal amplification for direct contact force response and low amplitude vibrations. A repetitive 6.3 N impact force, at very low frequencies (1–2.37 Hz), produced up 541 mVp-p . A further study of low-amplitude vibrations over the frequency range of 50 Hz to 2.5 kHz produced voltage outputs from 25 mVp-p to 452 mVp-p . All devices exhibited robust behaviour due to mechanical stress and did not lose functionality upon bending strain or pressure. Considering the small magnitude of the stimulus in all cases, it can be concluded that the proposed device has the capability of providing accurate signal output as a sensor for both low and high frequency applications. Furthermore, proposed sensors demonstrated remarkable mechanical robustness considering delicate substrates they were fabricated on.
Fig. 10. Excitation and relaxation output for all samples were symmetrical for all tested frequencies. Uneven excitation/relaxation output may indicate artifact inherent to the test setup or instability of device structure.
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The proposed sensor was designed and fabricated around the fundamental principle of a disposable electronic device created from environmentally benign materials and fabrication processes that could be incinerated or disposed of in a landfill without introducing additional toxic or hazardous by-products. The paper substrate provides the majority of the support for the device while the PDMS layer provides the non-absorbent surface required for inkjet deposition. PDMS is non-toxic to living beings and known to be degradable overtime [8]. Zinc oxide is also known to be nontoxic and not a major environmental concern in low quantities [4]. Consequently, these types of flexible sensors can be embedded in a variety of wearable short-lifecycle products, intelligent packaging, or distributed in an environment for short term monitoring applications. The same materials and fabrication methods can be scaled for larger or smaller sized sensors, or utilized in different applications by modifying the direct deposited transducer. For example, a similar pressure sensor can be fabricated by printing a simple electrode block of G-CMC thin film instead of an IDE. The PDMS/ZnO composite would be prepared and spin coated as explained in this study. A second layer of solid electrode block would then be inkjet deposited directly on the PDMS/ZnO composite, effectively sandwiching it between two graphene electrodes. Hence, substrate preparation, electrode fabrication, and chloroform based PDMS/ZnO polymer composite methods introduced in this paper are transferable for fabrication of sensors with different geometries and material compositions. CRediT authorship contribution statement Dogan Sinar: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. George K. Knopf: Conceptualization, Resources, Writing - review & editing, Funding acquisition. Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgement This work has been supported, in part, by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (Number: RGPIN/05858-2014) and Western’s Strategic Support Bridge Grant. Portions of this research were carried out with the support of the staff at Western’s Biotron Facility and Surface Science Western (SSW). References ¨ Consumer Technology Sales and [1] Consumer Technology Association, US ¨ 2019. Forecast, 2013 -2018/Mintel, [2] F. Yan, E. Parrott, B. Ung, E. Pickwell-MacPherson, Solvent doping of PEDOT/PSS: effect on terahertz optoelectronic properties and utilization in terahertz devices, J. Phys. Chem. C 119 (2015) 6813–6818, http://dx.doi.org/ 10.1021/acs.jpcc.5b00465. [3] D. Sinar, G.K. Knopf, Cyclic liquid-phase exfoliation of electrically conductive graphene-derivative inks, IEEE Trans. Nanotechnol. 17 (2018) 1020–1028, http://dx.doi.org/10.1109/TNANO.2018.2849264. [4] E. Hobbs, M. Keplinger, J. Calandra, Toxicity of polydimethylsiloxanes in certain environmental systems, Environ. Res. 10 (1975) 397–406, http://dx. doi.org/10.1016/0013-9351(75)90035-3. [5] S. Ertel, B. Ratner, A. Kaul, M. Schway, T. Horbett, In vitro study of the intrinsic toxicity of synthetic surfaces to cells, J. Biomed. Mater. Res. 28 (1994) 667–675, http://dx.doi.org/10.1002/jbm.820280603. [6] R.J. Watts, S. Kong, C.S. Haling, L. Gearhart, C.L. Frye, B.W. Vigon, Fate and effects of polydimethylsiloxanes on pilot and bench-top activated sludge reactors and anaerobic/aerobic digesters, Water Res. 29 (10) (1995) 2405–2411, http://dx.doi.org/10.1016/0043-1354(95)00067-U. [7] R.G. Lehmann, J.R. Miller, G.E. Kozerski, Degradation of silicone polymer in a field soil under natural conditions, Chemosphere 41 (5) (2000) 743–749, http://dx.doi.org/10.1016/S0045-6535(99)00430-0.
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[8] R.G. Lehmann, S. Varaprath, C.L. Frye, Degradation of silicone polymers in soil, Environ. Toxicol. Chem. 13 (1994) 1061–1064, http://dx.doi.org/10.1002/etc. 5620130707. [9] A. Sirelkhatim, S. Mahmud, A. Seeni, N. Kaus, L. Ann, S. Bakhori, H. Hasan, D. Mohamad, Review on zinc oxide nanoparticles: antibacterial activity and toxicity mechanism, Nano-micro Lett. 7 (2015) 219–242, http://dx.doi.org/10. 1007/s40820-015-0040-x. [10] Z. Wang, Nanostructures of zinc oxide, Mater. Today 7 (2004) 26–33, http:// dx.doi.org/10.1016/s1369-7021(04)00286-x. [11] J.L.G. Fierro, Metal Oxides: Chemistry and Applications, Taylor & Francis, Boca Raton, Flo, 2006. [12] Z.L. Wang, J. Song, Piezoelectric nanogenerators based on zinc oxide nanowire arrays, Science 312 (2006) 242–246, http://dx.doi.org/10.1126/science. 1124005. [13] D.A. Scrymgeour, J.W.P. Hsu, Correlated piezoelectric and electrical properties in individual ZnO nanorods, Nano Lett. 8 (2008) 2204–2209, http://dx.doi.org/ 10.1021/nl080704n. [14] S.-H. Shin, Y.-H. Kim, M.H. Lee, J.-Y. Jung, J.H. Seol, J. Nah, Lithium-doped zinc oxide nanowires–polymer composite for high performance flexible piezoelectric nanogenerator, ACS Nano 8 (2014) 10844–10850, http://dx.doi. org/10.1021/nn5046568. [15] A. Khan, M. Ali Abbasi, M. Hussain, Z. Hussain Ibupoto, J. Wissting, O. Nur, M. Willander, Piezoelectric nanogenerator based on zinc oxide nanorods grown on textile cotton fabric, Appl. Phys. Lett. 101 (2012), 193506, http://dx.doi. org/10.1063/1.4766921. [16] A. Moezzi, M. Cortie, A. McDonagh, Aqueous pathways for the formation of zinc oxide nanoparticles, Dalton Trans. 40 (2011) 4871–4878, http://dx.doi. org/10.1039/C0DT01748E. [17] A.N.P. Madathil, K.A. Vanaja, M.K. Jayaraj, Synthesis of ZnO nanoparticles by hydrothermal method, in: Nanophotonic Materials IV. Presented at the Nanophotonic Materials IV, International Society for Optics and Photonics, 2007, http://dx.doi.org/10.1117/12.730364, p. 66390J. [18] R. Kac¸ar, S.P. Mucur, F. Yıldız, S. Dabak, E. Tekin, Highly efficient inverted organic light emitting diodes by inserting a zinc oxide/polyethyleneimine (ZnO:PEI) nano-composite interfacial layer, Nanotechnology 28 (2017), 245204, http://dx.doi.org/10.1088/1361-6528/aa6f55. [19] V. Koutu, L. Shastri, M.M. Malik, Effect of NaOH concentration on optical properties of zinc oxide nanoparticles, Mater. Sci. 34 (2016) 819–827, http:// dx.doi.org/10.1515/msp-2016-0119. [20] W. Rahman, S. Garain, A. Sultana, T. Ranjan Middya, D. Mandal, Self-powered piezoelectric nanogenerator based on wurtzite ZnO nanoparticles for energy harvesting application, Mater. Today Proc. 5 (2018) 9826–9830, http://dx.doi. org/10.1016/j.matpr.2017.10.173. [21] A. Singh, S. Das, M. Bharathkumar, D. Revanth, A. Karthik, B. Sastry, V. Rao, Low cost fabrication of polymer composite (h-ZnO + PDMS) material for piezoelectric device application, Mater. Res. Express 3 (2016), 075702, http:// dx.doi.org/10.1088/2053-1591/3/7/075702. [22] H. Sun, H. Tian, Y. Yang, D. Xie, Y. Zhang, X. Liu, S. Ma, H. Zhao, T. Ren, A novel flexible nanogenerator made of ZnO nanoparticles and multiwall carbon nanotube, Nanoscale 5 (2013) 6117, http://dx.doi.org/10.1039/c3nr00866e. [23] M. Saba, V. Calzia, C. Melis, L. Colombo, A. Mattoni, Atomistic investigation of the solid–liquid interface between the crystalline zinc oxide surface and the liquid tetrahydrofuran solvent, J. Phys. Chem. C 116 (2012) 12644–12648, http://dx.doi.org/10.1021/jp302700b. [24] M. Birtane, H. Tuna, The evaluation of plantar pressure distribution in obese and non-obese adults, Clin. Biomech. 19 (2004) 1055–1059, http://dx.doi.org/ 10.1016/j.clinbiomech.2004.07.008.
Biographies Dogan Sinar is a Post-doctoral Research Associate in the Department of Mechanical and Materials Engineering at The University of Western Ontario, Canada. His research interests include synthesis of functional materials, nanomaterials, inkjet deposition methods, laser material processing of thin films, and sensor design. He received his B.Sc. in Mechatronics Engineering from Sabanci University, Turkey, and M.Eng. and pH.D. in Mechanical and Materials Engineering from The University of Western Ontario. His past industrial work experience includes technical and managerial positions for several automotive companies including the Ford Motor Company. George K. Knopf is a Professor in the Department of Mechanical and Materials Engineering at the University of Western Ontario, London, Canada. His research interests include bioelectronics, biosensors, laser materials processing, and flexible optical sheets. Dr. Knopf’s current work involves the development of conductive graphene-derivative inks and novel fabrication processes for printing electronic and optoelectronic circuitry on a variety of mechanically flexible surfaces. Applications involve flexible bioelectronic sensors, wearable electronics and soft polymer actuators. Over the past two decades he has acted as a technical reviewer for numerous academic journals, conferences, and granting agencies and has co-chaired several international conferences. In addition, he has recently co-authored a SPIE E-Book on Biofunctionalized Photoelectric Transducers for Sensing and Actuation (2017) and a CRC Press book Light Driven Micromachines (2018).
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Disposable piezoelectric vibration sensors with PDMS/ZnO transducers on printed graphene-cellulose electrodes Dogan Sinar, George K. Knopf ∗ Department of Mechanical and Materials Engineering, The University of Western Ontario, London, ON, N6A 3K7, Canada
a r t i c l e
i n f o
Article history: Received 26 August 2019 Received in revised form 3 December 2019 Accepted 16 December 2019 Available online 18 December 2019 Keywords: Disposable electronics Environmentally benign manufacturing Graphene-derivative inks Zinc oxide Polydimethylsiloxane Piezoelectric sensors
a b s t r a c t The individual components and circuitry of single-use flexible electronic sensors must be designed and fabricated to maintain stable functionality during normal operation, but once the usefulness of the device has concluded it is often incinerated or disposed of in a landfill. It is critical, therefore, that environmentally benign materials and fabrication processes be used to create these sensors such that manufacturing and disposal processes do not result in toxic or hazardous by-products. This paper introduces a novel flexible piezoelectric vibration sensor based on interdigitated electrodes (IDEs) printed on polymercoated paper substrates using nontoxic graphene nanoparticle (G) and carboxymethyl cellulose (G-CMC) aqueous suspensions as the electrically conductive ink. The piezoelectric transducer consists of environmentally benign zinc oxide (ZnO) nanoparticles dispersed in a polydimethylsiloxane (PDMS) matrix. During fabrication, the PDMS/ZnO composite is spin coated on the inkjet printed G-CMC interdigitated electrodes forming a thin piezoelectric layer. The fabricated sensors are tested, without additional signal amplification, for direct force response and low amplitude vibrations. A repetitive 6.3 N impact force, at very low frequencies (1–2.37 Hz), generated up 541 mVp-p . A further study of low-amplitude vibrations, over the frequency range of 50 Hz to 2.5 kHz, produced voltage outputs from 25 mVp-p to 452 mVp-p . Single-use force and vibration sensors over these low frequency ranges can be used for intelligent packaging, temporary monitoring of the environment and disposable wearable technologies. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Disposable single-use mechanically flexible electronic sensors will significantly impact the future of healthcare, food safety inspection, intelligent packaging, environmental monitoring, public security, and consumer wearable technologies. Often these applications do not require sophisticated multi-functional devices or high precision measurements rather simple robust sensors that perform one or two well-defined functions within acceptable limits. The market opportunity for these inexpensive and robust with short product life cycles has grown significantly in recent years. For example, the shipment of commercial wearable devices from US manufacturers, excluding smart clothing, has increased more than 500 % between 2013 and 2018 to staggering 66 million units a year [1]. In general, the physical structure of the constituent electronic components and circuitry, transducers, substrates, and protective enclosures must be designed and fabricated to maintain stable functionality during normal operation but once the useful-
∗ Corresponding author. E-mail address: [email protected] (G.K. Knopf). https://doi.org/10.1016/j.sna.2019.111800 0924-4247/© 2019 Elsevier B.V. All rights reserved.
ness of the device has concluded the product is physically destroyed in an incinerator or disposed of in a landfill. It is critical, therefore, that environmentally benign materials and fabrication processes be used to create these sensors such that manufacturing and disposal does not result in toxic or hazardous by-products. Advances in electrically conductive inks and drop-on-demand (DoD) printing have enabled a wide variety of flexible electronic sensors and electrochemical transducers to be fabricated on non-rigid polymers, natural fiber composites, textiles, and paper. Although conductive polymer (e.g., PEDOT:PSS) and metal-based (e.g., Au, Ag) inks have high electrical conductivity, these liquid dispersions are synthesized using a variety of toxic solvents and additives. For example, pure PEDOT:PSS has low conductivity properties and must be doped with an organic solvent such as dimethylformamide, ethylene glycol, or dimethylsulfoxide (DMSO). The dopant solvent remains in the fabricated thin film and becomes an integral part of the device. The presence of these toxic chemicals become an environmental hazard [2] when the disposed product is incinerated under high temperature or dumped in landfill. Similarly highly conductive metal-based inks such as silver (Ag) induce oxidative damage to biological tissues and cells, and disrupt the natural lifecycle of bacteria in landfill soil.
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As an alternative, a nontoxic aqueous carbon-based conductive ink based on a hydrophilic carboxymethyl cellulose (CMC) suspension with graphene (G) nanoparticles was recently developed [3]. The novel water-based G-CMC ink can be directly deposited on different functionalized rigid (e.g., glass) and flexible (e.g., PDMS, paper) substrates using DoD inkjet printing technology. Prior studies reported in the literature have shown that this deposition method could create conductive thin G-CMC films at varying thicknesses (100–2000 nm) with sheet resistances as low as ∼660 /sq. [3]. In the work described in this paper, the conductive graphene-based ink and DoD printing technology is used to create the electrodes on a paper substrate for a simple multi-layered piezoelectric sensor that responds to mechanical strain and low amplitude vibrations. The proposed sensor is created by printing G-CMC interdigitated electrodes (IDEs) on conventional paper that has been coated with an ultrathin non-absorbent polydimethylsiloxane (PDMS) layer. A thin piezoelectric polymer composite layer comprised of PDMS and zinc oxide (ZnO) nanoparticles is then deposited on top of the electrodes. Various nontoxic materials and environmentally benign chemical processes are used to synthesize the conductive ink, prepare the PDMS/ZnO piezoelectric composite, and deposit the thin films. Only a small quantity of chloroform (CHCl3 ) is used to disperse the ZnO nanoparticles in the PDMS matrix and majority of CHCl3 is removed by evaporation during the curing process. Futhermore, the simple multi-layered design ensures that all components of the single-use flexible electronic sensor maintain stable functionality during normal operation, but once the usefulness of the device has concluded it can safely incinerated or disposed of in a landfill. The decomposition of the paper substrate will release the nontoxic carbon particles (i.e., G-CMC), PDMS material and ZnO nanoparticles into the immediate environment. Although not biodegradable (i.e., decomposed directly by micro-organisms), the chemical and mechanical properties of PDMS are considered to be nontoxic and environmentally benign [4,5]. If hydrophobic PDMS enters the municipal waste water system it becomes part of the sludge that can be safely incinerated or buried in a landfill [6]. Once in the soil, PDMS can hydrolyze to small water soluble monomeric dimethylsilanediol (DMSD) which can microbially degrade to CO2 and inorganic silicate which is naturally present in soil [7,8]. Studies have also shown that ZnO is harmless to animals with only a negative effect on certain bacterial cultures [9]. Section 2 introduces the selected materials used to create the sensor, describes the interdigitated electrode (IDE) design and fabrication process, and provides details about the preparation of the thin PDMS/ZnO piezoelectric composite layer. The functionality and performance of the proposed sensor for measuring impact forces and low-amplitude vibrations is explored in Section 3. Finally, Section 4 summarizes the key performance capabilities of the fabricated sensor and briefly discusses the notion of disposable electronics.
2. Materials and fabrication 2.1. Material selection The materials selected to create the novel piezoelectric sensor is based on the design principle of mitigating the impact of toxic chemicals during manufacturing, product use, and final disposal. In particular, it is critical that when the short life-cycle device is eventually disposed of in a landfill, or incinerated, it does not release toxic residue or harmful by-products. The constituent materials for synthesizing the electrically conductive ink, substrate, piezo-
electric polymer composite, and coatings must be biologically and environmentally benign. The electrodes for the piezoelectric sensor are fabricated using a nontoxic aqueous carbon-based ink where naturally hydrophobic graphene (G) nanosheets are suspended in de-ionized water at high concentrations using a nontoxic hydrophilic cellulose derivative, carboxymethyl cellulose (CMC), to facilitate liquid phase exfoliation and stabilization [3]. Electrical conductivity of the deposited films are adjusted by varying the film thickness. The DoD deposited conductive G-CMC ink produces a thin film with numerous few-layered carbon sheets that naturally separate when the underlying substrate decomposes because graphene nanosheets are held together by weak van der Waal forces. The remaining residue after decomposition is carbon in the form of graphene nanosheets, chemicals used to create the degradable substrate, and any benign coatings used during manufacturing. The substrate material for all prototypes is standard printer paper (80 gsm) spin coated with an ultrathin layer of polydimethylsiloxane (Sylgard 184). The thin polydimethylsiloxane (PDMS) layer provides a non-absorbent barrier on the naturally porous substrate surface in order to prevent the conductive G-CMC ink droplets from soaking into the substrate during the inkjet printing process. The PDMS is also used as the polymer matrix for creating the piezoelectric ZnO composite. In general, PDMS has well-defined chemical and mechanical properties and is considered to be nontoxic [4,5]. For example, if a PDMS sample is disposed of in soil it will often hydrolyze into water soluble siloxanes and dimethylsilanediol (DMSD). Furthermore, DMSD can biodegrade to CO2 microbially or through sunlight-induced oxidation. Researchers have shown that the biodegradation of PDMS did not harm the microorganisms in the soil or negatively affect the crop growth [8]. The strain responsive polymer composite contains ZnO nanoparticles because it is considered nontoxic [8], exhibits very good piezoelectric properties [10], and easy to synthesis with chemical processes that are not harmful to the environment. Studies have shown that ZnO is harmless to animals with only a negative effect on certain bacterial cultures [9]. In addition, the process of synthesizing ZnO nanoparticles is largely benign and does not involve the use of harmful chemicals and solvents. In terms of fabrication, the ZnO nanoparticles are initially synthesized through a chemical process where the zinc acetate dihydrate seeds are dissolved in 20 % methanol solution and allowed to react with NaOH at 90◦ C. To synthesize the ZnO nanoparticles in this research sodium hydroxide (NaOH) pellets were acquired from ACP (ACS, 97.0 %) and Zinc acetate dihydrate (Zn(OOCCH3 )•2H2 O) was acquired from Alfa Aesar (ACS, 98/0–101.0 %). The resulting ZnO nanoparticles are mixed with PDMS base after purification to create the piezoelectric polymer composite. The PDMS/ZnO composite is then spin coated on the G-CMC electrodes and allowed to cure at 95◦ C for one hour. The ZnO nanoparticles dispersed in the polymer PDMS matrix used to create the thin multi-layered piezoelectric sensor can work both in d33 and d31 modes at acceptable conversion efficiencies due to their geometrical shape. This property makes a planar interdigitated electrode (IDE) design a viable alternative to parallel plate electrodes positioned above and below the polymer composite. This is an advantage from a fabrication perspective because the planar IDE design allows for a single step electrode deposition and avoids the additional assembly steps often required for creating discrete multi-layered structures. 2.2. Electrode fabrication The electrodes for the prototypes are inkjet printed using a water based graphene-derivative conductive ink and a custom built
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Fig. 1. The IDE dimensions as designed (a). Inkjet printed G-CMC electrodes on paper substrates (b). The dimensions of printed samples were confirmed under optical microscope to be in good agreement with the design (error margin of ∼5 %).
DoD printer with an HP C6602a cartridge. The paper substrate is initially spin coated with Sylgard 184 polymer (1:10 curing agent ratio). Once cured, the paper substrate with an ultra-thin PDMS coating is corona treated for about a minute and then placed with the printable face down onto a polyethyleneimine (PEI) solution (4.5 mg/mL, 0.5 M NaCl, DI water). After a thorough wash in deionized (DI) water and pressurized air dried, the substrate is placed on to poly(sodium 4-styrenesulfonate) (PSS) solution (7.5 mg/mL, 0.5 M NaCl, DI water). The substrate is further washed with DI water, air dried, and placed onto the PEI solution for the final time. After a final wash and dry period, the paper substrate is ready for the inkjet deposition process. Note that the polyelectrolyte coating on the substrate forms a positively charged and hydrophilic surface that allows for homogeneous deposition of electrically conductive graphene-derivative ink. The printable graphene-cellulose (G-CMC) ink is synthesized and prepared based on the methodology described in a prior publication [3]. In a typical synthesis procedure, 2 mg/mL of graphene powder was mixed with a 50 % 1-propanol/MilliQ water solution. This mixture was sonicated for 4 h. In a separate container, 2 mg/mL CMC (250 kDa, 0.7 DS) was mixed with 100 mL MilliQ water. The CMC solution was slowly mixed with the graphene mixture and bath sonicated for another 4 h. The resulting dispersion was centrifuged at 20,000 G for 15 min, sediment was collected, and reconstituted in MilliQ water. This process was repeated 2 more times in order to remove free CMC from the functional ink. Final product was reconstituted in 50 mL of MilliQ water. Once synthesized, the G-CMC ink is mixed with 1-propanol (∼ 28 %v) in order to reduce surface tension of the ink. Using the modified G-CMC ink, the IDEs are printed directly on the paper substrate with the ultrathin polymer layer. The interdigitated electrode (IDE) patterns are inkjet printed at 4800 mm/min feedrate and 144 m droplet spacing. The IDE design parameters and photograph of the fabricated electrodes are presented in Fig. 1. The inkjet printed IDEs have 38 digits with d =600 m line width, h = 7.6 mm line length, and g =400 m gap between digits. Overall width and length of the IDEs are w = 10 mm and l = 40 mm, respectively. Two 3 mm × 3 mm contact pads are created at the end of the micro-trace for connecting the samples to the measurement equipment. Based on previous literature [3], the thickness of the deposited thin film is expected to be ∼1.5 m.
2.3. Preparation of PDMS/ZnO piezoelectric composite Under ideal conditions, zinc oxide nanostructures favor the wurtzite crystal structure due to higher stability [11]. In a wurtzite structure, Zn2+ and O2− atoms’ are arranged in tetrahedral coordination. The lack of inversion symmetry under mechanical strain due to the tetrahedral structure results in the desired piezoelectric properties [12]. Hence, a larger crystal domain can result in better piezoelectric performance. The crystal domain size can be manipulated simply through increasing particle size, assuming impurities are non-significant. This phenomenon has been observed in the case of zinc oxide nanowires and nanorods where nanostructures can have lengths in micrometer range and exhibit high piezoelectric conversion efficiency [13–15]. On the other hand, increased nanoparticle size is associated with poor dispersion in polymer/nanoparticle composites. Poor dispersion in a polymer may result in reduced performance and heterogeneous behaviour along the composite. In this regard, good control over nanoparticle dimensions is an important factor for predictable piezoelectric composite performance. In this study, the chemical synthesis route is chosen for growth of zinc oxide nanoparticles due to its simplicity and potential for scaling. The chemical synthesis route, and its many variations, are frequently utilized for quick and easy growth of nanoparticles. In the case of zinc oxide nanoparticle growth from zinc acetate dihydrate, NaOH salt is used to form zinc oxide nanoparticles in the following simplified Eq. (1): (Zn(OOCCH3 ) • 2H2 O) + 2NaOH → ZnO + 2NaCH3 COO + H2 O
(1)
In the above equation, zinc oxide nanoparticles are formed through removal of acetate groups from zinc and formation of sodium acetate. During the reaction, nucleation of newly formed zinc oxide molecules have a strong influence on the nanoparticle size. Conditions that increase the overall reaction speed, such as higher synthesis temperature [16,17], more polar solvents [18], and increased precursor concentrations [19] have been associated with smaller crystallites. Based on these studies, a modified chemical synthesis route is formulized. In a typical synthesis process, zinc acetate dihydrate (0.1 M) is dissolved in ultrapure water with 20 % methanol content. In a separate container, NaOH (0.1 M) is dissolved in ultrapure water with 20 % methanol content. Zinc
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Table 1 Sample list and parameters of initial ZnO/CHCl3 dispersions.
3. Results and discussion
ZnO Weight Percentage in 2 mL of PDMS Base
CHCl3 [mL]
ZnO [mg]
Sample PZ-2 - 2 %w Sample PZ-4 - 4 %w Sample PZ-6 - 6 %w Sample PZ-8 - 8 %w
2 4 6 8
38.6 77.2 115.8 154.4
acetate solution is heated under magnetic stirring. Once the solution reached 90 ◦ C, NaOH solution is added slowly to the reaction vessel. In about 5 min, formation of white colloids is observed. The dispersion with zinc oxide nanoparticles is kept at 90 ◦ C and under magnetic stirring for an additional hour. Then, the dispersion is left to cool down at room temperature for 12 h. At the end of the cooling period, the zinc oxide sediment is collected, washed with ultrapure water and centrifuged. Next, the sediment is washed with methanol and centrifuged for a second time. Finally, sediment is reconstituted in methanol and left on a hot plate to remove all solvent. The white dry powder is collected and heated to 630 ◦ C in an oven (in ambient air atmosphere) for one hour to remove any leftover impurities and optimize crystal structure. A small part of the nanoparticles were collected for TEM and XRD analysis. The TEM and XRD analysis were conducted to confirm nanoparticle dimensions and crystal structure, respectively. Performance of polymer/nanoparticle composites are impacted by dispersion homogeneity of nanoparticles in the polymer matrix. Non-cured PDMS is a highly viscous liquid and is generally regarded to be chemically inert. Although mechanical mixing of ZnO nanoparticles in PDMS have been reported [20–22], it was deemed to be an inefficient route for this study. In fact, attempts at mechanical mixing of ZnO in PDMS had resulted only in a partial dispersion with large amount of agglomerated particles visibly present. Such an inefficient dispersion would make it problematic to analyze effects of ZnO weight percentage in the polymer, as well as negatively affect device performance. Hence, a common solvent mixing route was developed. In this approach, chloroform (CHCl3 ) was utilized because it can both solvate PDMS and disperse ZnO. Although tetrahydrofuran ((CH2 )4 O) is a better solvent than CHCL3 for this application, tetrahydrofuran’s persistence on the zinc oxide nanoparticles [23] after synthesis implies that the moderately toxic (CH2 )4 O may become a source of long-term contamination during disposal. The synthesis process first required 2 mL PDMS base to be mixed with 4 mL of chloroform. This mixture was thoroughly mixed until all PDMS base was solvated. Next, a certain amount of ZnO was mixed with chloroform in a separate container and bath sonicated for 1 h. Zinc oxide does not have a large number of good solvents and its stable dispersions often require functionalization or extensive sonication. On the other hand, chloroform can temporally disperse ZnO nanoparticles. The bath sonicated ZnO/CHCl3 dispersion was mixed with the PDMS/CHCl3 solution and thoroughly mixed, resulting in a homogeneous PDMS/ZnO/CHCl3 dispersion. Finally, the dispersion was placed on a hot plate at 90 ◦ C for 2 h in order to evaporate CHCl3 . Resulting PDMS/ZnO dispersion was still homogeneous due high viscosity of PDMS base (Fig. 2a). Zinc oxide weight percentages and corresponding polymer composites are presented in Table 1. The piezoelectric PDMS/ZnO composite is finalized by the addition of PDMS curing agent at the 1:10 mixing ratio. After a thorough mixing, a small amount of mixture (1 mL per sample) was spin coated on the G-CMC IDEs at a speed of approximately 820 rpm. Spin coated device was carefully placed in an oven and cured at 95 ◦ C for 1.5 h (Fig. 2b). Fig. 3 summarizes the key steps of the vibration sensor fabrication process.
3.1. Characterization of ZnO nanoparticles Prior to fabricating the PDMS/ZnO piezoelectric composite, a small quantity of ZnO nanoparticles were collected for TEM and XRD analysis. The TEM images reveal that nanoparticles had 50–200 nm lateral dimensions. Most of the nanoparticles exhibit polyhedral form with only a few rectangular shapes (Fig. 4). XRD pattern (Fig. 5) exhibited expected peaks from the wurtzite crystal structure (compared to ICDD database). Lattice parameters were a =3.25 Å and c =5.20 Å, which has the c/a ratio of ∼ 1.6, indicating a hexagonal cell structure. Based on XRD pattern, synthesized ZnO was mostly free of impurities with only small residual peaks observable at 72.54, 81.36, and 92.75 degrees.
3.2. Repetitive impact force tests The fabricated flexible piezoelectric sensors are initially tested for force measurements. The force sensing is possible because of the compressive strain experienced by the thin PDMS/ZnO transducer deposited on the G-CMC electrodes. In these experiments, the voltage potential generated by the compressed devices are monitored and logged with an Agilent DSO5014a oscilloscope. The flexible sensors are secured with epoxy glue on a flat test surface and the force is applied directly on the sensor surface using a vertically aligned positioning system (Fig. 6). The force applicator is a 1 cm diameter flat tip metal rod attached to a multi-axis positioning system that had been vertically aligned with the test surface. In addition, a commercially available force sensing resistor (Interlink Electronics FSR-402) is attached to the test surface to act as the control. The external forces are applied at low frequencies (1.0–2.37 Hz) in order to simulate biomechanical movements. Equipment limitations restricted the choice of applied forces. The magnitude and frequency of the applied force was controlled by adjusting the travel distance of the applicator. The speed (200 mm/min) and acceleration (200 mm/s2 ) of the applicator tip was fixed for all experiments. In addition, surface pressure could be determined because the contact area of the fixed-diameter applicator is known. For the first set of tests, the start position of the applicator was adjusted to produce an applied force of ∼ 3.7 N (∼ 46.8 kPa). The value was confirmed with the Interlink force sensing resistor. For the second set of tests, the initial applied force was increased to 6.3 N (∼79.7 kPa) by moving the start point of the applicator 0.2 mm towards the sensor. The force values of 3.7 N and 6.3 N were selected based on adult foot pressure values as reported in the literature [24]. The peak-to-peak voltage outputs for all measurements were recorded and averaged over 5–12 data points, depending on the frequency. The standard deviations were calculated using individual measured data points. The voltage produced by the piezoelectric composite is based on change in force and, therefore, the peak values will occur at the instant maximum force is applied or removed from the sensor. Fig. 7 shows the voltage output at various frequencies for the maximum applied loads of 3.7 N (Fig. 7a) and 6.3 N (Fig. 7b). The different curves correspond to increasing the ZnO weight percentage as summarized in Table 1. PZ-2, PZ-4, PZ-6, and PZ-8 correspond to 2 %w, 4 %w, 6 %w and 8 %w ZnO, respectively. The graphs show that the PZ-8 sample generated a higher voltage than the other samples for both the 3.7 N and 6.3 N forces. As well, the data confirms that the output voltage increases with the magnitude of the applied force. In contrast, the output voltages for both applied loads did not change significantly to minor variations in the frequency of repeated impact (i.e, 1.0–2.37 Hz). The standard deviation was observed for both the proposed piezoelectric device and the off-
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Fig. 2. Use of chloroform as a common solvent allows for homogenous dispersion of ZnO inside PDMS matrix (a). Inkjet printed piezoelectric sensor after spin coating of PDMS/ZnO composite (b).
Fig. 3. Summary of the fabrication process used to create the piezoelectric vibration sensors based on spin coating PDMS/ZnO material on inkjet printed G-CMC electrodes.
Fig. 4. Synthesized ZnO nanoparticles exhibit polyhedral form. Nanoparticles as small as 50 nm edge length was observed. Larger nanoparticles were approximately 300 nm in edge length.
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Fig. 5. XRD pattern of ZnO nanoparticles. Wide angle analysis confirms wurtzite crystal structure.
Fig. 6. Experimental apparatus used to measure force impact at low frequencies (1.0–2.37 Hz). The force applicator is attached to computer controlled xyz-positioning system.
the-shelf force sensing resistor indicating reasonable repeatability for the printed sensor. The magnitude and frequency response of the PDMS/ZnO composite on the G-CMC electrodes is comparable to other piezoelectric devices described in the literature. However, a direct comparison is not possible because most authors illustrate device functionality by rapidly tapping the surface with a finger [19,21]. Similar ad hoc tests showed that the proposed sensor comprised of ZnO at 4 w% did generate 13–20 V at around 3−4 Hz with finger tapping. This is reasonable considering that the repetitive contact of the finger is significantly faster than the force applicator used in this study. In addition, the contact surface area of the finger is larger than the mechanical applicator. 3.3. Low-amplitude vibration tests Vibration stimulus is created using a shaker (Agac-Derritron) with an analog amplifier (Crown CE2000). Sinusoidal stimulus signal was provided by an HP 35670A Dynamic Signal Analyzer (Fig. 8). Output voltage from the test sample was collected with Agilent DSO5014a oscilloscope. The fabricated devices are fixed on the stainless steel beam of the shaker with the help of an adhesive for additional robustness. The selected frequency range (50 Hz to
2.5 kHz) demonstrates the effectiveness of the unique sensor (i.e., PDMS/ZnO transducers on G-CMC electrode) to measuring external environmental influences such as workspace and vehicle vibrations on the human body. Frequency response analysis for 10–50 Hz is not included due to limitations of the instruments used in the study. The response of the flexible piezoelectric sensor to low amplitude vibrations over the range from 50 Hz to 2.5 kHz was also investigated. The magnitude of the vibrations were fixed for all tests. Fig. 9 shows the increase in voltage potential for all samples (PZ-2, PZ-4, PZ-6 and PZ-8) over the test range. At lower frequencies (< 200 Hz) the PZ-2 and PZ-4 quickly increase and show a higher voltage output. These two samples with lower ZnO content then produce a near linear response for frequencies greater than 200 Hz. In contrast, the higher ZnO content samples PZ-6 and PZ-8 start with a lower voltage output at frequencies < 300 Hz but then increase significantly doubling the voltage output of PZ-2 and PZ-4. The higher voltage output with increasing ZnO percentage is expected and is a result of efficient dispersion of nanoparticles inside the PDMS matrix. Yet, higher performance of PZ-2 and PZ-4 samples for lower frequencies cannot be explained by nanoparticle concentration directly. Although a more in-depth investigation may be necessary to understand this trend, it is assumed that changing mechanical properties of PDMS (e.g., stiff-
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Fig. 7. Voltage output due to applied force of 3.7 N/46.8 kPa (a) and 6.7 N/79.7 kPa (b). Error bars are shown for 5–12 data points (frequency dependent). Applied force has a direct and positive impact on sensor output while change in frequency did not affect the output.
Fig. 8. Experimental apparatus used to measure low amplitude mid-range vibrations (50 Hz to 2.5 kHz). The sensors are attached with an adhesive to the metal arm of the shaker.
ness) with increasing ZnO amount may have an impact. It has been shown that ZnO weight percentage has a direct effect on composite elastic modulus. Hence, higher ZnO concentration is associated with higher composite stiffness [21]. Furthermore, internal screening effects may become more evident as concentration of ZnO increases which is a result of charge transfer between individual nanoparticles. This would result in reduced electrical output. A layer-by-layer polyelectrolyte coating procedure can be utilized to shield nanoparticles from each other and reduce screening. This procedure was not implemented due to reduce fabrication complexity.
The time varying output signal of the four sensors for three different excitation frequencies (100, 1000 and 2500 Hz) is shown in Fig. 10. Signal output for excitation and relaxation regimes were symmetrical for each sample throughout the tested frequency range. This is the expected output for piezoelectric sensors and also an indication of sensor and test setup robustness. Sensor hysteresis was analyzed from recorded oscilloscope data. The ZnO percentage had no significant impact on sensor hysteresis. On the other hand, time delay between excitation signal and sensor output were reduced across all samples as the frequency was increased. For all samples, a peak-to-peak time delay of 7.1–8.3, 0.84-0.88, and 0.37-
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4. Conclusion
Fig. 9. All samples exhibit repeatable and predictable performance with increasing frequency without any need for signal amplification.
0.375 ms were observed at 100, 1000, and 2500 Hz, respectively. Note that this time delay includes the response time of the amplifier and shaker. Hence, actual time delay of the sensors would be lower.
A disposable and novel mechanically flexible piezoelectric sensor based on inkjet printed G-CMC interdigitated electrodes (IDEs), PDMS coated paper substrates, and a thin PDMS/ZnO piezoelectric polymer composite was introduced. The constituent carbon electronics, natural fiber substrate and biocompatible polymer transducer are composed of nontoxic materials and the method of fabrication did not generate any hazardous by-products. The simple fabricated piezoelectric G-CMC sensors were tested without signal amplification for direct contact force response and low amplitude vibrations. A repetitive 6.3 N impact force, at very low frequencies (1–2.37 Hz), produced up 541 mVp-p . A further study of low-amplitude vibrations over the frequency range of 50 Hz to 2.5 kHz produced voltage outputs from 25 mVp-p to 452 mVp-p . All devices exhibited robust behaviour due to mechanical stress and did not lose functionality upon bending strain or pressure. Considering the small magnitude of the stimulus in all cases, it can be concluded that the proposed device has the capability of providing accurate signal output as a sensor for both low and high frequency applications. Furthermore, proposed sensors demonstrated remarkable mechanical robustness considering delicate substrates they were fabricated on.
Fig. 10. Excitation and relaxation output for all samples were symmetrical for all tested frequencies. Uneven excitation/relaxation output may indicate artifact inherent to the test setup or instability of device structure.
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The proposed sensor was designed and fabricated around the fundamental principle of a disposable electronic device created from environmentally benign materials and fabrication processes that could be incinerated or disposed of in a landfill without introducing additional toxic or hazardous by-products. The paper substrate provides the majority of the support for the device while the PDMS layer provides the non-absorbent surface required for inkjet deposition. PDMS is non-toxic to living beings and known to be degradable overtime [8]. Zinc oxide is also known to be nontoxic and not a major environmental concern in low quantities [4]. Consequently, these types of flexible sensors can be embedded in a variety of wearable short-lifecycle products, intelligent packaging, or distributed in an environment for short term monitoring applications. The same materials and fabrication methods can be scaled for larger or smaller sized sensors, or utilized in different applications by modifying the direct deposited transducer. For example, a similar pressure sensor can be fabricated by printing a simple electrode block of G-CMC thin film instead of an IDE. The PDMS/ZnO composite would be prepared and spin coated as explained in this study. A second layer of solid electrode block would then be inkjet deposited directly on the PDMS/ZnO composite, effectively sandwiching it between two graphene electrodes. Hence, substrate preparation, electrode fabrication, and chloroform based PDMS/ZnO polymer composite methods introduced in this paper are transferable for fabrication of sensors with different geometries and material compositions. CRediT authorship contribution statement Dogan Sinar: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. George K. Knopf: Conceptualization, Resources, Writing - review & editing, Funding acquisition. Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgement This work has been supported, in part, by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (Number: RGPIN/05858-2014) and Western’s Strategic Support Bridge Grant. Portions of this research were carried out with the support of the staff at Western’s Biotron Facility and Surface Science Western (SSW). References ¨ Consumer Technology Sales and [1] Consumer Technology Association, US ¨ 2019. Forecast, 2013 -2018/Mintel, [2] F. Yan, E. Parrott, B. Ung, E. Pickwell-MacPherson, Solvent doping of PEDOT/PSS: effect on terahertz optoelectronic properties and utilization in terahertz devices, J. Phys. Chem. C 119 (2015) 6813–6818, http://dx.doi.org/ 10.1021/acs.jpcc.5b00465. [3] D. Sinar, G.K. Knopf, Cyclic liquid-phase exfoliation of electrically conductive graphene-derivative inks, IEEE Trans. Nanotechnol. 17 (2018) 1020–1028, http://dx.doi.org/10.1109/TNANO.2018.2849264. [4] E. Hobbs, M. Keplinger, J. Calandra, Toxicity of polydimethylsiloxanes in certain environmental systems, Environ. Res. 10 (1975) 397–406, http://dx. doi.org/10.1016/0013-9351(75)90035-3. [5] S. Ertel, B. Ratner, A. Kaul, M. Schway, T. Horbett, In vitro study of the intrinsic toxicity of synthetic surfaces to cells, J. Biomed. Mater. Res. 28 (1994) 667–675, http://dx.doi.org/10.1002/jbm.820280603. [6] R.J. Watts, S. Kong, C.S. Haling, L. Gearhart, C.L. Frye, B.W. Vigon, Fate and effects of polydimethylsiloxanes on pilot and bench-top activated sludge reactors and anaerobic/aerobic digesters, Water Res. 29 (10) (1995) 2405–2411, http://dx.doi.org/10.1016/0043-1354(95)00067-U. [7] R.G. Lehmann, J.R. Miller, G.E. Kozerski, Degradation of silicone polymer in a field soil under natural conditions, Chemosphere 41 (5) (2000) 743–749, http://dx.doi.org/10.1016/S0045-6535(99)00430-0.
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Biographies Dogan Sinar is a Post-doctoral Research Associate in the Department of Mechanical and Materials Engineering at The University of Western Ontario, Canada. His research interests include synthesis of functional materials, nanomaterials, inkjet deposition methods, laser material processing of thin films, and sensor design. He received his B.Sc. in Mechatronics Engineering from Sabanci University, Turkey, and M.Eng. and pH.D. in Mechanical and Materials Engineering from The University of Western Ontario. His past industrial work experience includes technical and managerial positions for several automotive companies including the Ford Motor Company. George K. Knopf is a Professor in the Department of Mechanical and Materials Engineering at the University of Western Ontario, London, Canada. His research interests include bioelectronics, biosensors, laser materials processing, and flexible optical sheets. Dr. Knopf’s current work involves the development of conductive graphene-derivative inks and novel fabrication processes for printing electronic and optoelectronic circuitry on a variety of mechanically flexible surfaces. Applications involve flexible bioelectronic sensors, wearable electronics and soft polymer actuators. Over the past two decades he has acted as a technical reviewer for numerous academic journals, conferences, and granting agencies and has co-chaired several international conferences. In addition, he has recently co-authored a SPIE E-Book on Biofunctionalized Photoelectric Transducers for Sensing and Actuation (2017) and a CRC Press book Light Driven Micromachines (2018).