Synthesis and performance evaluation of thin film PPy-PVDF multilayer electroactive polymer actuators

Sensors and Actuators A 165 (2011) 321–328

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Synthesis and performance evaluation of thin film PPy-PVDF multilayer electroactive polymer actuators Babita Gaihre a , Gursel Alici a,c,∗ , Geoffrey M. Spinks a,c , Julie M. Cairney b a b c

University of Wollongong, School of Mechanical, Materials and Mechatronic Engineering, NSW 2522, Australia The University of Sydney, Australian Centre for Microscopy & Microanalysis, NSW 2006, Australia ARC Centre of Excellence for Electromaterials Science, University of Wollongong, Innovation Campus, NSW 2522, Australia

a r t i c l e

i n f o

Article history: Received 1 June 2010 Received in revised form 23 September 2010 Accepted 16 October 2010 Available online 3 November 2010 Keywords: PVDF thin film Electroactive polymer actuators Performance characterization Microactuators Electromechanical property Electrochemical property

a b s t r a c t Bending-type microactuators less than 1 mm in length and comprising of two polypyrrole (PPy) layers separated by polyvinylidene fluoride (PVDF) membrane have previously been fabricated and was shown to operate both in air and aqueous media. The main limiting factor to increase the bending angle and to further miniaturise these actuators was the thickness of the commercially-available PVDF membrane used (∼110 ␮m). In this study, we have synthesised a porous PVDF thin film with a thickness of 32 ␮m using a spin coating technique, and electrochemically deposited PPy layers on both sides of this thin film to make ultra thin film polymer actuators. The electromechanical and electrochemical properties are investigated and compared with those of the thicker actuator system using the commercially-available PVDF and under identical conditions. The thin film shows very promising performance compared to its thicker counterpart. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Electro-active polymers (EAPs) have received considerable attention from both academia and industry because of their many potential applications such as in artificial muscles and sensors [1,2]. Conducting polymers, especially PPy, have gained particular attention due to properties such as biocompatibility, biodegradability, ease of synthesis, low actuation power, ability to work in liquid and air environments and stability with large volume change [2,3]. The latter is especially important for electroactive actuators, whose operation principle is based on the volume expansion and/or contraction generated by the movement of ions during an electro-chemical reaction [2]. While most EAP actuators have been synthesised to operate in liquid electrolytes for numerous biomedical uses [3,4], multi-layer EAP actuators prepared by separating two electroactive polymer (PPy) films by an insulating soft, porous film (PVDF) and containing the electrolyte within the internal pores can operate both in dry and wet environments. Such systems are expected to widen the range of applications [5]. These multi-layer conducting polymer actuators can be electrochemically oxidized and reduced in a continuous and reversible way, leading to either bending or linear movements [1–9].

∗ Corresponding author. Tel.: +61 242214145; fax: +61 242213101. E-mail address: [email protected] (G. Alici). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.10.009

Several investigations have been realised in the field of multilayer conducting polymer actuators that can operate in wet and dry environments [1–9]. However, little effort has been dedicated to the miniaturization of such actuators. In one of the earliest studies, Jager et al. [10] fabricated a serially-connected micromanipulator articulated with micro-sized polymer actuators to pick, move and place 100 ␮m glass beads that is very suitable for single-cell manipulation. However, this system could only operate in ‘specific aqueous media’, as the media provided the source of ions (electrolyte) required achieving actuation. Small scale EAP actuators are useful in numerous applications, including the micromanipulation of living cells, bioanalytical nanosystems, datastorage, lab on chip, microvalve, microswitch, microshutter, cantilever light modulators, micro-optical instrumentation, artificial muscles for macro/microrobotics and more [3,5,10,11]. With this in mind, there is an increasing need for microsized conducting polymer actuators, which can operate both in dry and wet media. Miniaturization of these actuators greatly improves their electromechanical properties by increasing speed, stress output etc. As these size-reduced actuators use thinner films, the effects of ion diffusion and RC time constant are reduced so that an increased speed of response compared to large-scale actuators is expected. In addition, small size actuators require less charge during switching and these actuators can be operated by a coin battery. Further, the power requirement for large size actuators to convert electrical

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energy to mechanical work is substantially higher in comparison with microactuators, yet, the yield is very low [3]. In our previous work, we demonstrated the synthesis of conducting polymer microactuators comprising of two conductive PPy layers separated by a passive PVDF layer, which acted as a cell separator and the storage reservoir of the electrolyte. In those actuators, one PPy layer acts as a counter electrode and another PPy layer acts as a working electrode so that they can operate in air [5] when a potential difference is applied to the two PPy layers. The main limiting factor in the actuator was the commercially available PVDF membrane layer, which was 110 ␮m thick, making the trilayer actuator 160 ␮m thick. The maximum tip deflection produced at 1 V was only 125 ␮m for 580 ␮m × 220 ␮m × 160 ␮m actuator (L × W × d). In this paper, we report on the reduction of PVDF layer thickness by using a spin coating technique with porogens to produce a thin, porous PVDF membrane. The membranes have been constructed into trilayer actuators by the deposition of PPy layer on both surfaces of the PVDF thin film. We have reduced the thickness of the PVDF layer to 32 ␮m and deposited 8 ␮m PPy layers on each side of the PVDF layer, providing an overall actuator thickness of 48 ␮m, which is 3.5 times thinner than our previously reported microactuator [5]. This smaller dimensions has a flow-on effect on the length and width of the microactuators in order to make microactuators with the dimensions of 200 ␮m × 50 ␮m × 48 ␮m (L × W × d) and beyond towards nano-sized conducting polymer actuators. It is our ultimate goal to reduce the small thickness of the dry polymer actuators towards fully miniaturized microactuators with a length below 1 ␮m. We have succeeded in reducing the thickness of the actuators. However, before proceeding towards reducing length and width, we have investigated performance of this new, thin actuator and compared its performance with our previous, thick actuator prepared under similar condition. The results show that tip displacement of thin actuator is higher than the displacement of the thick actuator under the same operation conditions. 2. Experimental 2.1. Chemicals and materials Polyvinylidene fluoride powder (PVDF, Aldrich), lithium triflouromethanesulfonimide (LiTFSi), salicylic acid (Ajax chemicals), propylene carbonated (Aldrich) and dimethyleformamide (DMF, Sigma–Aldrich) were used as received. PVDF membrane with thickness 110 ␮m was purchased from Millipore. Pyrrole (Merck) was distilled and stored under nitrogen at −20 ◦ C before use. 2.2. PVDF thin film preparation The PVDF film was prepared as follows: PVDF (1 g) powder and 0.1 g of salicylic acid as porogen were dissolved in 10 ml of DMF in room temperature by vigorous stirring overnight. The homogeneous solution was spin-coated (500 rpm, 20 s) onto a glass slide and dried at 50 ◦ C. The dried film was then peeled from the glass plate and heated at 200 ◦ C for 30 min to remove salicylic acid and solvent. A digital microscope (Leica DMEP) was used to observe the morphology and cross-sectional image of the PVDF film. 2.3. Gold sputter coating Gold was sputter coated (Magnetron sputter coater SC 100MS) on both sides of the PVDF film/membrane at 2 × 10−3 mBar pressure and 30 mA current. The coated layers of gold serve to provide a conductive surface onto which the PPy layers can be electrodeposited. The coating time for the thin film PVDF and PVDF membrane were

2 and 20 min, respectively. A four point probe (Jandel RM3-AR) was used to measure the surface resistivity of the gold coated film and the membrane. 2.4. Trilayer actuator preparation Thin actuator was synthesised using galvanostatic polymerization method, which involves electro-deposition of PPy layers on both sides of the gold-coated PVDF film (acting as a working electrode) by passing a constant current into solution containing 0.1 M pyrrole dissolved in 0.1 M LiTFSi/PC (with 1% water). The polymerization was done for 4 h at −33 ◦ C and a current density of 0.2 mA cm−2 . Stainless steel mesh was used for two counter electrodes and potentiostat/galvanostat (EG&G Princeton Applied Research Model 363) was used to generate the constant current. Before the start of the polymerization, the solution was degassed by passing nitrogen gas for 30 min followed by cooling for 30 min. Thick actuator was also synthesised using similar conditions to those described above except PVDF membrane (thickness 110 ␮m) was used instead of PVDF thin film (thickness 32 ␮m). After polymerization, the actuators were rinsed with acetone and soaked with the 0.1 M LiTFSI/PC solution. 2.5. Electromechanical and electrochemical responses of the actuators The electromechanical performance of the actuators was investigated using a non-contact laser displacement sensor (microepsilon, NCDT-1700-10) to measure the displacement of the actuator tip. Datalogger (e-corder, ED821) was an interface unit between the computer, the laser displacement sensor and the potentiostat, which recorded a voltage signal applied to the trilayer actuator, associated current drawn and displacement measured by the laser displacement. The actuator input voltage signals were provided by an external function generator and amplified using an eDAQ potentiostat (eDAQ, EA161) operating in a two-electrode mode. The blocking force was measured using a dual-mode lever arm system from Aurora Scientific Inc. (Model 300C-LR). The electrochemical response of the actuators was investigated using Potentiostat (eDAQ, EA 161). A three electrode cell was constructed using a platinum mesh, Ag/Ag+ and the actuator strip acting as a counter, non aqueous reference and working electrodes, respectively and 0.1 M LiTFSi/PC solution as electrolyte. The scan rate was fixed at 20 mV s−1 . 3. Results and discussion Spin coating was used to apply the uniform thin PVDF films to flat substrates. To increase the porosity of the film, salicylic acid was used as a porogen during preparation of the thin film PVDF. Salicylic acid melts at 158 ◦ C and decomposes at 183 ◦ C [12]. The thermal decomposition of the salicylic acid leads to formation of phenol and carbon dioxide as shown below: C6 H4 (COOH)(OH) → CO2 ↑ + C6 H5 OH The phenol produced is easily evaporated from the film below 200 ◦ C, leaving tiny pores in the film in which to store the electrolyte consisting of a salt (LiTFSI) and a solvent (PC). Fig. 1a and b shows digital micrographs of the PVDF films produced with or without the salicylic acid after heating at 200 ◦ C for 30 min. For the film containing salicylic acid, the pores are evenly distributed throughout the whole area. However, the pores in the PVDF film are larger in diameter and uneven and less in number, probably due to uncontrolled evaporation of the solvent. Fig. 1c is a cross-sectional image of the PVDF film showing that the thickness of the film is 32 ␮m.

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Fig. 2. The galvanostatic polymerization of PPy on the gold coated PVDF film.

Fig. 1. Surface morphology of PVDF film prepared without (a) and with (b) salicylic acid and cross-sectional image of PVDF film (c).

Both surfaces of the film/membrane were sputter coated with the gold to make them conductive. Care is required during preparation of the film and sputter coating so that no electrical contact (short-circuiting) occurs between the two gold layers. The coating times for the thin film PVDF and PVDF membrane were selected as 2 and 20 min, respectively. Two different times were selected because, for the PVDF membrane, 2 min coating was insufficient to obtain a sufficiently high conductivity and, for the PVDF thin film, more than 2 min coating created a short circuit, damaging the film. The surface resistivity of the gold coated PVDF thin film and the membrane were measured as 8.24 and 4.68 ohm per square, respectively. In the electrochemical synthesis, the polymerization is believed to occur through monomer oxidation and creation of cation radicals, followed by a coupling reaction as shown in Scheme 1. The reaction starts with the formation of monomer radical cation.

Coupling of two monomer radical cations produce a dimeric intermediate. Further coupling of the cations produce trimeric, oligomeric or polymeric cation radical intermediates [13]. During the electrochemical polymerization, due to removal of electrons from the double bonds, a positive charge is delocalized in the PPy chain. To maintain charge neutrality, negatively charged anions TFSI− are incorporated into the polymer to compensate these positive charges on the polymer backbone, as shown in Scheme 1. The lost electrons can reversibly be returned to the polymer by applying a more negative potential. The potential difference between the working and counter electrodes was continuously monitored during polymerization and the result is presented in Fig. 2. The decrease in oxidation potential with increasing time in the initial 30 min (2000 s) can be justified as monomers have comparably higher oxidation potential than dimmers, trimers, oligomer and polymers [13]. After 2000 s, the potential started showing repetitive oscillations between −2.4 and −2.5 V on average showing a constant trend. Fig. 3a is the cross-sectional image of thin actuator. There are three main layers in the structure: two outer black PPy layers (each 8 ␮m thick), which are the electroactive layers and an inner porous separator of PVDF film (32 ␮m thick) that holds the liquid electrolyte. The gold particles are not visible in this image. Fig. 3b and c illustrates the chemistry of oxidation and reduction of the PPy layers resulting in a bending motion like a bilayer structure. When more negative potential is applied, the polymer is reduced and the backbone is left neutral as the TFSI− ions leave the polymer. Similarly, under a positive potential, the polymer is oxidized and the TFSI− anions will move from the electrolyte to the positively charged PPy electrode in order to maintain the charge neutrality within the PPy layers. A number of parameters change when the oxidation level of the polymer is altered, including the length of the carbon–carbon bonds on the polymer backbone, the angles between adjacent monomer units, cis–trans isomerization, changes in the interaction between polymer chains and solvent, backbone folding [3,14], etc. However, the movement of ions in and out of the PPy chains is considered to be major factor that results in bending motion of the actuator [15,16]. So, in the trilayer structure as illustrated in Fig. 3c, the forward and backward movements can be achieved by applying switching between a positive potential difference and a negative potential difference. The performance of the bending actuators was evaluated by measuring their electromechanical and electrochemical responses. The samples for this evaluation were prepared by cutting rectangular shaped strips of dimension 1 mm × 5 mm using a sharp blade to avoid electrical contact between the two PPy layers. The laser sensor was focused 1 mm from the tip of the actuator and the tip displacements recorded at different input frequencies as a step voltage with amplitude of up to ±0.5 V square wave was applied. The tip displacements as a function of different voltages were also recorded. The period of the square wave was 200 s (frequency 0.005 Hz). The amplitude of the input voltages was restricted to limit the displace-

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Scheme 1. Electrochemical polymerization of PPy.

ment and maintain the actuator tip within the range of the laser sensor. Fig. 4 represents the steady state values of the actuator response under the input voltages ranging from ±0.01 to ±0.5 V square waves. The results indicate that the tip displacement is linearly proportional to the input voltage, which is expected because increased voltages mean more energy or charge is provided to move the ions

in and out of active polymer layers, hence, leading to the contraction and expansion of the relevant polymer layers [2]. It must be noted that, although the conductivity of gold in thin actuator is much lower, its bending displacement is relatively larger than that of thick actuator. One explanation to this finding is that the area moment of inertia of thin actuator is much smaller, creating a relatively smaller resistance to bending.

Fig. 3. Cross-sectional image of thin actuator (a), chemistry of oxidation and reduction of PPy (b) and contraction and expansion of the PPy chain leading to bending motion (c).

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Tip displacements of Thick and Thin Actuators 2.5

Displacement (mm)

Thin Actuator Thick Actuator 2

1.5

1 Fig. 6. Beam model for the conducting polymers considered in this study.

0.5

0

0

0.1

0.2

0.3

0.4

0.5

Input Voltage (volt) Fig. 4. Tip displacement of the actuators under different input voltages: thin actuator uses the thin film PVDF and thick actuator uses the thicker commerciallyavailable PVDF membrane.

The structure of the actuators is a laminated (composite) structure. Since there is a comparable difference between the moduli of elasticity of the PPy and PVDF [17], the equivalent width technique is used to expand the layer (its width) with the higher modulus of elasticity to bring the whole structure into a single material of the lower modulus of elasticity. To maintain the same flexural rigidity, the width of the layer with the higher modulus of elasticity is increased by n = E2 /E1 , E2 > E1 . It has been reported [17] that the moduli of elasticity of PPy and PVDF layers for the thick actuator used in this study are approximately 190 MPa and 117 MPa, respectively. As there is no data available yet for the modulus of elasticity of the thin PVDF, we assume that the modulus of elasticity of the thin PVDF is the same as that of thick PVDF. This follows that n = EPPy /EPVDF = 1.624, and the width of the PPy layers is increased by ‘n’ to 1.624 mm (the original width is 1 mm) for both actuators. The new cross section consisting of a single material, which is the PVDF, is presented in Fig. 5. For the thick actuator with the dimensions of t1 = 8 ␮m, and t2 = 110 ␮m and b¯ 1 = b × n = 1 × 1.624 mm, the area moment of inertia is calculated as Ithick = 201.51e − 6 mm4 . For thin actuator, the other dimensions are the same except t2 = 32 ␮m. The area moment of inertia is calculated as Ithin = 13.26e − 6 mm4 . The bending stress  in the actuators is calculated from: =

Mc I

internal bending moment under the same input voltage. The bending stresses for the thick and thin actuators are then proportional to c/I ratio, which are calculated as 312.65 and 1809.57 for the thick and thin actuators, respectively. This suggests that the thin actuator will potentially generate more tip displacement. Of course, this does not mean that the thin actuator will generate more strain, as elaborated in the next paragraph. Based on the actuation principle of the conducting polymer actuators considered in this study, they are analogous to a cantilever beam with a uniformly distributed load [18], as shown in Fig. 6. The maximum transverse deflection of such a beam is given by y(x) = qx2 /24EI(x2 + 6L2 − 4Lx) [19]. The maximum deflection occur at the tip, where x = L, qL4 8EI

ytip =

(2)

The expression for the strain ε is obtained by combining Eqs. (1 and 2) with the Hooke’s law for elastic materials  = εE; ε=

4h(L − x)2 ytip L4

for x = 0, ε =

4h ytip L2

(3)

where h is the total thickness of the actuators; hthin = 48 ␮m and hthick = 126 ␮m. The strains corresponding to the tip displacements in Fig. 4 are calculated from Eq. (3) for the thick and thin actuators, and are plotted as a function of the input voltage in Fig. 7. As the overall thickness of the thin actuator is much smaller than that of the thick actuator, the thin actuator strain is much smaller. Another explanation is based on the charge density, which states that the bending stress generated is proportional to the charge density (charge per volume) [20]. In fact, with reference to the charge data depicted in Figs. 8 and 10, the charge injected in thin actuator is smaller by 40% compared with thick actuator. However, the

(1) Strains of Thick and Thin Actuators

where the area moment of inertia I is for the new cross section, ‘c’ is the half of the total thickness, which are hthin = 48 ␮m and hthick = 126 ␮m, and M is the internal bending moment generated due to the electrochemical process. Because the active layers of both actuators are the same, they are expected to generate the same

0.04

Thin Actuator Thick Actuator

0.035

Strain (mm/mm)

0.03 0.025 0.02 0.015 0.01 0.005 0

Fig. 5. Laminated structure of a polymer actuator and its equivalent cross section using equivalent width technique. The mechanical properties of the equivalent section with a single material are equivalent to that of the original multi-material structure.

0

0.1

0.2 0.3 Input Voltage (volt)

0.4

0.5

Fig. 7. Strains corresponding to the actuator displacements in Fig. 4.

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Fig. 8. The tip displacement of the actuators (a) and charge transferred to the actuators (b) at various excitation frequencies under square wave voltage of ±0.4 V.

Fig. 9. Step response of the actuators at frequencies of 0.003 Hz of ±0.4 V input voltage.

thickness of thin actuator is much smaller (3.5 times thinner) leading to a much higher charge density of 26.45 C/m3 for thin actuator compared with 13.69 C/m3 for thick actuator. The maximum displacements of both actuators for various excitation frequencies with an input of ±0.4 V square waves were also investigated and compared. The results are presented in Fig. 8a. Fig. 8b shows the variation of the charge injected into the actuator with the square wave frequency. The decrease in displacement with increasing frequency is justifiable as Fig. 8a suggests that there is a decrease in the charge going or coming through the actuator with increasing frequency. Ion diffusion controls the rate of the electrochemical reactions, so that at higher frequencies there is less time for the diffusion of ions into or out of the polymer [21]. As a result of the reduced ion flux, the charge transferred is also smaller at higher frequencies leading to reduced bending motion. The tip displacement for thin actuator is higher than that of thick actuator at lower frequencies (0.003–0.007 Hz) even though the charge passing through the thick actuator is higher than that for thin actuator. However, above 0.007 Hz, the tip displacement for thin actuator is reduced greatly compared to thick actuator. With reference to the tip displacement versus time responses (bottom plots) in Fig. 9, the time constant (RC)1 of thin actuator is much higher than that

1 When these actuators are electrochemically stimulated, the electrodes (PPy) are charged and discharged like a capacitor and the solvated counter-ions move in and out of the electrodes to create charge neutrality. This electrochemical process can be mimicked with an equivalent circuit consisting of a total capacitance, and total resistance representing the resistance of the contacts, the polymer layers, the ions, and the electrolyte. Therefore, the time constant of this electrochemical process is

Fig. 10. The charge transferred to the actuators under a step potential of 0.4 V.

of thick actuator. This will show itself in displacement–frequency response as a sharply falling displacement response, as presented in the left plot of Fig. 8. A typical step response under a ±0.4 V step voltage input is depicted in Fig. 9. The charge passed through the actuators during the same step responses are shown in Fig. 10. A significantly faster rate of charging is observed for thick actuator compared to thin actuator. This is due to the fact that the gold-coated PVDF membrane was more conductive than the gold-coated PVDF thin

the RC of the system, where R is the total resistance and C is the total capacitance. This RC behaviour can be seen in the displacement responses in Fig. 9.

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Fig. 11. Force of the actuator at frequency of 0.005 Hz and input voltage of ±0.5 V.

film. Apart from this, the ion holding capacity of relatively thicker and more porous PVDF membrane must have contributed to the relatively faster rate of charging. Although the PVDF membrane showed a higher conductivity leading to faster charging of the corresponding actuator, the displacement was lower for thick actuator compared to thin actuator as shown in Fig. 8. The electrochemical efficiencies (EE) were calculated for both actuators using following formulation [22,23]: EE (%) =

charge consumed for complete reduction/oxidation × 100 charge required for complete reduction/oxidation (4)

Assuming that pyrrole ring:dopant ratio = 3, and each pyrrole ring consumes 2 electrons, the total number of electrons consumed by the pyrrole ring for polymerization = 6 and the total number of electrons consumed by the pyrrole ring for oxidation = 6 + 1 = 7. charge required for complete reduction/oxidation

Charge consumed for complete reduction/oxidation

in both cases probably due to the relatively small potential used (±0.4 V). The blocking forces at the tip of the 5 mm long actuator were also measured for an input voltage of ±0.5 V and the result is presented in Fig. 11. For both type of actuators, the EE values suggest that PPy is not fully oxidized. If the displacements for thin and thick actuators are compared, we find a plateau is reached for thicker one but not for thinner one. For thinner one, this behaviour is justifiable as a relatively low potential is used. However, in case of thicker actuator, the PPy layers cannot generate a relatively large strain under the same low voltage due to having a stiffer membrane in its structure. The electrochemical response of the actuators was observed by measuring the cyclic voltammogram and the result is presented in Fig. 12. In these studies, the trilayer actuator was immersed in an electrolyte bath and both PPy layers were simultaneously connected as the working electrode. A separate platinum mesh was used as auxiliary electrode and a Ag/Ag+ as a reference electrode.

total charge passed during synthesis total electrons consumed during polymerisation growth charge density × time of growth = 7 0.2 mA cm−2 × 4h = 411.42 mC cm−2 = 7 = 6.35/0.05 = 127 mC Cm−2 for thin actuator = 8.63/0.05 = 172.6 mC Cm−2 for thick actuator =

Putting the values in Eq. (4), the EE was found to be 30.8 and 41.9% for thin and thick actuators, respectively. These values indicate that the PPy has not been fully oxidized or reduced

The anodic current peak was found to be around 0.05–0.1 V in both cases as expected [24]. A broad cathodic current peak is also seen centred around −0.5 to −0.6 V. The anodic and cathodic peaks show that the PPy layers are electrochemically active in both actuators. The area of one cycle corresponds to the charge passed during the voltage cycle. The cycle area is larger for thick actuator compared to thin actuator, which corresponds to the thicker gold layer used in the former.

4. Conclusion

Fig. 12. Cyclic voltammetry of the trilayer actuators. The scan conditions are; Initial and final potentials −500 mV, upper potential 800 mV, lower potential −1000 mV and scan rate 20 mV/s.

In this study, thin film actuators were synthesised and their performance was evaluated using frequency and step response experiments and compared with the performance of a thick membrane actuator. A comparably thicker gold layer could be applied to the thick actuator without problems of shorting. The thicker gold layer makes it more conductive. Similarly, since membrane highly porous and about 3 times thicker than the PVDF film, these factors must have affected the conductivity of ions, making charge transfer faster in thick actuator than in the thin actuator. However, the tip displacement of thin actuator is larger compared to thick actuator under identical conditions and at low excitation

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frequencies. The thinner PVDF layer is less resistant to bending, so that a higher tip displacement results. The tip displacement of the thin actuator diminishes rapidly at higher excitation frequencies. Our further work involves increasing fabricating micro-sized dry-type conducting polymer actuators with the dimensions of 200 ␮m × 50 ␮m × 48 ␮m (L × W × d) and beyond towards nanosized conducting polymer actuators. The microfabrication work that is well ahead of the planned time will be reported in the very near future. Acknowledgements The authors would like to thank ARC Discovery Project (DP0878931) for financial support of this work. References [1] S.A. Wilson, R.P.J. Jourdain, Q. Zhang, R.A. Dorey, C.R. Bowen, M. Willander, Q.U. Wahab, M. Willander, S.M. Al-hilli, O. Nur, E. Quandt, C. Johansson, E. Pagounis, M. Kohl, J. Matovic, B. Samel, W. van der Wijngart, E.W.H. Jager, D. Carlsson, Z. Djinovic, M. Wegener, C. Moldovan, R. Iosub, E. Abad, M. Wendlandt, C. Rusu, K. Persson, New materials for ␮-scale sensors and actuators: an engineering review, Mater. Sci. Eng. R: Rep. 56 (2007) 1–129. [2] T.F. Otero, M.T. Cortes, A sensing muscle, Sens. Actuator B: Chem. 96 (2003) 152–156. [3] E. Smela, Conjugated polymer actuators for biomedical applications, Adv. Mater. 15 (2003) 481–494. [4] E.W.H. Jager, E. Smela, O. Inganas, I. Lundstrom, Polypyrrole micro actuators, Synth. Met. 102 (1999) 1309–1310. [5] G. Alici, V. Devaud, P. Renaud, G. Spinks, Conducting polymer microactuators operating in air, J. Micromech. Microeng. 19 (2009) 025017. ˜ [6] J.M. Sansinena, V. Olazábal, T.F. Otero, C.N. Polo da Fonseca, Marco-A. De Paoli, A solid state artificial muscle based on polypyrrole and a solid polymeric electrolyte working in air, Chem. Commun. 22 (1997) 2217–2218. [7] F. Vidal, C. Plesse, G. Palaprat, A. Kheddar, J. Citerin, D. Teyssie, C. Chevrot, Conducting IPN actuators: from polymer chemistry to actuator with linear actuation, Synth. Met. 156 (2006) 1299–1304. [8] M.T. Cortés, J.C. Moreno, Artificial muscles based on conducting polymers, ePolymers 041 (2003) 1–42. [9] A.S. Hutchison, T.W. Lewis, S.E. Moulton, G.M. Spinks, G.G. Wallace, Development of polypyrrole-based electromechanical actuators, Synth. Met. 113 (2000) 121–127. [10] E.W.H. Jager, O. Inganas, I. Lunstrom, Microrobots for micrometersize objects in aqueous media: potential tools for single cell manipulation, Science 288 (2000) 2335–2338. [11] Y. Fang, X. Tan, A novel diaphragm micropump actuated by conjugated polymer petals: fabrication, modeling, and experimental results, Sens. Actuator A Phys. 158 (2010) 121–131. [12] H.P. Zhang, P. Zhang, G.C. Li, Y.P. Wu, D.L. Sun, A porous poly(vinylidene fluoride) gel electrolyte for lithium ion batteries prepared by using salicylic acid as foaming agent, J. Power Sources 189 (2009) 594–598. [13] A.F. Diaz, J. Bargon, in: T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Dekker, New York, 1986. [14] L. Bay, T. Jacobsen, S. Skaarup, K. West, Mechanism of actuation in conducting polymers: osmotic expansion, J. Phys. Chem. 105 (2001) 8492–8497. [15] Q. Pei, O. Inganas, Electrochemical applications of the bending beam method. 1. Mass transport and volume changes in polypyrrole during redox, J. Phys. Chem. 96 (1992) 10507–10514. [16] Q. Pei, O. Inganaes, Electrochemical applications of the bending method 2. Electroshrinking and slow relaxation in polypyrrole, J. Phys. Chem. 97 (1993) 6034–6041.

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Biographies Dr. Babita Gaihre has done PhD in Bionanosystem engineering from Chonbuk National University, South Korea in 2008. Currently, she is working as associate research fellow in University of Wollongong. Her current field of interest is in micro and nano fabrication of actuators for various biomedical applications. Gursel Alici received the PhD degree in Robotics from the Department of Engineering Science, Oxford University, UK, in 1994. He is currently a professor at the University of Wollongong, NSW, Australia, where he is the discipline leader of Mechatronic Engineering. His current research interests include intelligent mechatronic systems involving mechanisms/serial/parallel robot manipulators, micro/nano robotic systems for medical applications, and modeling, analysis, characterization, control of conducting polymers as macro/micro/nano sized-actuators and sensors for robotic and bio-inspired applications. He is a technical editor of IEEE/ASME Transactions on Mechatronics, and a member of the Mechatronics National Panel formed by the Institution of Engineers, Australia. Gursel Alici who has published more than 160 refereed publications in his areas of research is also the recipient of the Outstanding Contributions to Teaching and Learning (OCTAL) award from the University of Wollongong in 2010. Geoffrey Spinks obtained his PhD from the University of Melbourne in 1990 for his work on the fracture behaviour of thermosetting polyesters. He has since worked at the University of Wollongong and is currently a professor in materials engineering. He maintains a research interest in mechanical properties of polymers and, in particular, in polymer actuators. He has published over 120 peer-reviewed papers and co-authored one book that is now in its third edition. He has received several awards from the Royal Australian Chemical Institute for research excellence. Julie M. Carney received PhD (Physical Metallurgy) in 2002 from University of New South Wales. She is currently working as senior lecturer in the The University of Sydney. Her research interests focus on the relationship between microstructure and properties of materials, with a particular emphasis on the application and development of new characterization technologies in electron microscopy, focused ion beam (FIB) technology and atom probe tomography. Current materials of interest include steels, non-ferrous engineering alloys (such as Ni-based superalloys and Ti alloys), nanocrystalline metals, hard coatings (including nanocomposites and thermal barrier coatings), and thin films (including ferroelectrics). She has produced over 75 publications including 4 book chapters, 55 journal articles and 1 provisional patent.