(Invited) Electrochemical Soft Devices

Abstract

There has been a great deal of researches to add functionalities to soft materials with intent to develop intelligent device, as they are potential candidates for bio-mimic materials. Our group has been developing functional devices by introducing redox active species and groups to polymer gels. We have employed gels because they are known to show abrupt volume change, which can be used as a driving force of the devices. A variety of stimuli has been used as trigger of changes in volume of the gel, including light, temperature, pH, ionic strength, voltage/potential, and magnetic field. Electric potential has been one of the most widely used stimulus since it can easily be controlled. The devices we have developed can be operated at low potential of a few volts, which depend on the redox species used. Further, since they are mainly composed of polymer gels, they can easily be downsized. I will present some of our representative devices, electrochemical actuators and an electrochemical micro pump. The electrochemical actuator consists of two poly(acrylic acid) gel films which sandwich a polyimide film (5 mm x 25 mm x 12.5 μm thick or 1.5 mm x 3 mm x 12.5 μm thick) coated with Au layers on both sides. It is driven by changes in the volume of the gel induced by redox reaction of p-hydroquinone (QH2)/p-benzoquinone (BQ). The oxidation of QH2 results in decrease in pH, which gives rise to collapse of the gel, whereas the reduction of BQ causes expansion. The Au film electrodes on both sides of the actuator are used as two electrodes setup. Thus, when the oxidation takes place at one electrode, the reduction does on the other electrode at the opposite side at the same time. In a solution of 5 mM QH2 and 5 mM BQ containing 1 x 10-4 M NaClO4the actuator bent to left (anode) side at 1.3 V, and bent to right side after the reversal of the polarity (Fig. 1). The actuator showed repeated stretching/bending at least 400 cycles (Fig. 2). Further, it was able to actuate after 6 times of repeated drying/wetting of the gel. The electrochemical micropump consists of poly(acrylic acid) gel containing Cu2+ serving as a piston and a Au pipe (1 mm i.d. x 14 mm length) serving as cylinder. It discharges and charges liquid at a level of a few hundred of picoliter per second upon expansion and contraction of the gel induced by reduction and oxidation of the copper, respectively. The pump discharged and charged equal volumes of 3.5 ml water under potentiostatic control at –1.1 V vs. Ag/AgCl and at +0.8 V for 3 h each, showing excellent reversibility. Under galvanostatic control the volumes expelled and taken were nearly controlled with the charge at steady states with flow rates of 3 x 10 pl s–1 for the discharging and 1.6 x 102 pl s–1 for the charging. Figure 1