(226z) Fundamentals and Applications of Hydrogel Actuation By Electric Fields Towards Soft Robotic Components

Authors: 
Morales, D., North Carolina State University
Palleau, E., Universite de Toulouse
Bharti, B., North Carolina State University
Dickey, M. D., North Carolina State University
Velev, O. D., North Carolina State University

We present a novel class of  "walking" gel prototypes of soft robotic devices comprised of cationic and anionic gel legs, which bend in response to the redistribution of mobile ions between the gel network and solution in an external field.  Stimuli responsive hydrogels could be key components in the next generation of soft matter actuating and sensing devices. Furthermore, they could serve as “smart” biocompatible materials for mimicking dynamic structures found in nature since they transduce chemical energy into mechanical motion without the use of external mechanical input. We present our work manipulating the internal hydrogel stress on both the molecular and macro level utilizing electrical fields. Electrical fields provide excellent control over induced osmotic pressure and electrostatic interactions between the fixed charges within gel networks for preprogrammed motion. The magnitude of bending is a function of  the induced osmotic pressure difference which is governed by the interaction between the mobile ions and fixed charge groups of the hydrogel. The sign of the fixed charges on the polyelectrolyte network determines the direction of bending under an applied external field.  Therefore, we combined two oppositely charged gels to promote the motion of the legs in two directions. We characterized the electro-actuated response of the hydrogels as a function of charge density and external salt concentration. The unidirectional gel walker motion on flat elastomer substrates exemplifies a simple way to move and manipulate soft matter devices and robots in liquid media. Our current work aims to control the bending of thermo responsive gels by embedding inside them chain-like structures of latex colloidal particles assembled directionally using electric fields.  When an external AC field is applied across a colloidal suspension, the particles align into chains due to  the acquired dipoles.  These chains can serve analogously to an endoskeleton within a gel matrix.  As the gel shrinks in response to heat, the organized particle chains determine the final structure as opposed to the isotropic shrinking that would be observed in the absence of chain alignment. This approach will result in complex gel/particle composites whose shape change can be programmed internally using dielectrophoretic structuring. Thus, we are developing simple soft matter actuator devices and robotic components utilizing conventional polymers and external stimuli, which may serve as active components where conventional, stiff materials are inadequate.