Trilayer PPy Actuators

The trilayer PPy actuator is illustrated in Fig. 5.1. On both sides of the actuator are PPy layers. In the middle is an amorphous, porous pol3cvinyli-dene fluoride (PVDF) layer that serves both as a backing material and a reservoir for the electrolyte, which allows the actuator to work in air for a limited period of time (up to several hours). During the electrochemical deposition of the PPy layers, anions A in the electrolyte are introduced into the polymer matrix, which is a process called doping. When a voltage is ap- 

Trilayer PPy actuators were first reported by the Intelligent Polymer Research Institute at the University of Wollongong, Australia. A description of the fabrication process can be found in, e.g., [Wu et al. (2006)], and it is provided here for completeness. The PPy layers are electrochemically deposited on both sides of a gold-coated porous PVDF film. The PVDF film obtained from Sigma-Aldrich Co., has a thickness of 110 lam with pore size of 0.45 /Ltm. Gold layers of 100 nm are sputtered on both sides of the PVDF, 

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Trilayer PPy actuators have also been constructed in which PPy is electrodeposited on either side of a gold electrode using the large DBS ion on one side (cation-driven PPy) and a smaller benzenesulfonate on the other (anion-driven PPy), which bent quickly in a cooperative manner as the trilayer was oxidized or reduced in 1 M LiC104 [26]. A problem with any design in which the conducting-polymer remains attached to an inert metal... 

Fig. 5.1 Illustration of a trilayer PPy actuator (a) Working principle of a trilayer bender the sectional view of the trilayer structure (left) and bending upon application of a voltage (right) (b) observed large bending deformation. Reprinted from [Fang and Tan (2010)1 with permission from Elsevier, Copyright 2010.

Fig. 5.2 Fabrication of trilayer PPy actuator (a) The fabrication setup (b) a fabricated sample. Reprinted from [Fang and Tan (2010)].

Experiments were conducted on trilayer PPy actuators to validate the effectiveness of the redox level-dependent admittance model. The electrolyte used was tetrabutylammonium hexafluorophosphate (TBA+PFg) in the solvent propylene carbonate (PC). The samples were predoped with PFg during fabrication, and the nominal concentration Co in the absence of DC bias was estimated to be fOOO mol/m based on the deposition conditions. In experiments different values of Cq were achieved by applying appropriate DC biases. Sinusoidal voltages of amplitude 0.05 V and frequency 0.08 — 200 Hz were superimposed on the DC voltage, as perturbations, for the measurement of admittance (or equivalently, impedance) spectra. 

Fig. 5.16 Inner bending radius of trilayer PPy actuators under different DC voltages (a) Sample with dimensions 13 x 5 x 0.17 mm (b) (a) Sample with dimensions 33 x 6 x 0.17 mm . Reprinted from [Fang et al. (2008a)j.

Finally, trilayer PPy actuator strips PPy (3 x 20 mm ) have been implanted and wrapped around the right ventricle of a rat model to investigate whether the contraction of the PPy actuators could assist the contraction of the heart as an alternative to cardiac tissue engineering using stem cells. The authors claim that the trilayer PPy actuator strips were able to significantly augment right ventricular contraction (Ruhparwar et al. 2014). 

Conducting-polymer actuators have been developed for several applications by the Wollongong group [9]. These include the use of bUayer PPy actuators in a cochlear ear implant [85]. Hollow-tube actuators with helical wire interconnects have also been developed for use in an electronic Braille screen [39]. A trilayer actuator valve has been included in a combined sensor-actuator system to control a gas valve inlet for food... 

Bilayer and trilayer actuators Characterizations of Electrochemical cell Experimental procedure Materials Conducting polymers (CPs) Liquid electrolyte Open air Cyclic voltammetry Dibutyltin dilaurate Electronic conducting polymers (ECPs) Interpenetrating polymer network (IPN) Poly (3,4-ethylenedioxythiophene) (PEDOT) Polypyrrole (PPY) Actuation mechanism of Electrogeneration of Electropolymerization of pyrrole monomer Oxidation and reduction reaction of Polyvinylidene fluoride (PVDF) Solid polymer electrolyte (SPE) membrane Force characterizations IPNs Load curves and metrics PVDF membrane Strain characterizations... 

PPy doped with TFSl has also been used to create a fast bending trilayer actuator, based upon a gold-coated PVDF membrane operated in air, relying on high diffusion rates through the gel-like polymer, more so than the size of the polymer conductivity. Tip displacements peaked for certain actuation frequencies (between 4 and 90 Hz, depending... 

Wu Y, Alici G, Madden JDW, Spinks GM, Wallace GG (2007) Soft mechanical sensors through reverse actuation in polypyrrole. Adv Funct Mater 17 3216-3222 Zainudeen UL, Careem MA, Skaamp S (2007) Actuators based on pedot and PPy conducting polymer bilayers and trilayers. In Industrial and information systems, 2007. ICIIS 2007. International conference on, 9-11 Aug 2007. Peradeniya, pp 461-464 Zhou D, Wallace GG, Spinks GM, Liu L, Cowan R, Saunders E, Newbold C (2003) Actuators for the cochlear implant Synth Met 135-136 39-40... 

Trilayer actuators architecture have been proposed in 1996 by Baughman (1996) to allow their operation in air because die middle layer acts as an ion reservoir necessary for oxidation-reduction process. Several of diese macroactuator architectures have been realized using classical elecfrodeposition mediods of CP. This architecture has been downscaled to microsized structures for die first time in 2009 by G. Alici et al. (2009). They used die laser ablation to cut die PPy/PVDF/ PPy trilayer actuators into microbeam shapes. They were able to fabricate trilayer microactuators with a size of 850 x 250 x 170 pm using this technique. 

Some efforts have been reahzed to turn the microactuators working in air into various devices. It has been shown that an articulated finger (Jager et al. 2013a) can be built from thin film PVDF PPy trilayer actuators (Fig. 13). 

Fig. 13 Articulated finger built from thin-film PVDF PPy trilayer actuators with integrated contact pads...

PPy trilayers microactuators have been laser cut with lateral sizes down to 200 pm long and 50 pm wide. They have been made with a commercial PVDF membrane or spin-coated PVDF thin film, having a total actuator thickness of 126 and 48 pm, respectively. Since they are composed of a shghtly thicker PPy layer than the bilayer microactuators, their speeds are lower too, 0.1-0.6 Hz. The measured blocking force is up to 600 pN (see Gaihre et al. 2011b, 2012). 

Yao Q, Alici G, Spinks GA (2008) Feedback control of tri-layer polymer actuators to improve their positioning ability and speed of response. Sensors Actuators A Phys 144(1) 176-184 Zainudeen UL, Careem MA, Skaarup S (2008) PEDOT and PPy conducting polymer bdayer and trilayer actuators. Sensors Actuators B Chem 134(2) 467-470... 

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