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Runni Wu1 K.W. Kwan1 Alfonso Ngan1

1, The University of Hong Kong, Hong Kong, , China

Small-scale soft actuators and stimuli-responsive materials have been the subject of intensive research because of the potential applications as artificial muscles in soft robots. Recently, NiOOH/Ni(OH)2 has been identified as a promising actuating material that can expand or contract as a redox reaction occurs between the oxyhydroxide and hydroxide states [1]. This material can be used as an active coating on a thin substrate to achieve a “skin-effect” actuator that flexes with large movements. However, the performance and durability of such a design are limited by the interfacial strength between the active and passive layers.

In this work, we demonstrate that, a new design of the skin-effect actuators, made by electrodepositing a few microns’ thick of the NiOOH/Ni(OH)2 active material on top of a gold-sputtered nano/micro-porous polycarbonate membrane, can exhibit unprecedented device strains with excellent stability and durability, due to the enhanced mechanical interlocking provided by the nano/micro pores on the passive substrate. For an actuator strip 15 mm long, 3 mm wide and 12 mm thick, a bending curvature as large as 1.3mm-1 about the width axis (i.e. a radius of curvature of 0.77 mm over the 15 mm length), with cumulative angular deflection > 1000° (i.e. 2.8 revolutions) at the free end, can be achieved in an alkaline electrolyte environment under a triggering voltage of less than 1 V. Under cyclic potential triggering, the actuation response is fast and stable. Although the intrinsic actuation strain of the NiOOH/Ni(OH)2 material is only about 0.2%, and actually, higher strains are not desirable as the elastic limit would be exceeded, the work density is high at 27 kJ/m3, which is comparable to mammalian muscles. The present results also demonstrate that the typical largest actuation curvature has a linear relationship with the thickness or plating time of the actuating layer, in good agreement with Stoney’s theory.

Under higher scanning frequencies of the triggering potential corresponding to actuating cycles shorter than 5 s, the motion of the actuator mimics well the muscle-like motion of a fishtail. The ultra-large deformation, low triggering voltages, and ease and low costs to fabricate, are significant merits of the present material system that justify its future developments in soft robots and other biomimetic devices.

Reference:

[1] K.W. Kwan, N.Y. Hau, S.P. Feng and A.H.W. Ngan, (2017), “Electrochemical actuation of nickel hydroxide/oxyhydroxide at sub-volt voltages”, Sensors and Actuators B: Chemical 248, 657-664.

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