Motion is essential to life and its control is a key issue in the design of artificial microrobots and micro-machines. A topical challenge in microscale locomotion is the development of synthetic materials that sustains self-propulsion by body shape deformation akin to microorganisms. Mechanical actuation also in nature is frequently caused by swelling and deswelling of hydrogels. In cases like the hydraulic opening and closing of a pine cone. Intrigued by these examples, an increasing interest has developed in artificial hydrogel actuator systems that exploit swelling/deswelling in response to an external stimulus. Mostly these systems utilize the volume phase transition of polyacrylamide derivatives and focus on the shape variance between end states in equilibrium. In this work the focus is on temperature responsive thin poly(<i>N</i>-isopropylacrylamide) microgel bodies that undergo bending and torsional motions upon swelling and deswelling. Very fast temperature jumps localized to the volume of the thin microgel body are achieved by near IR-light and heat has been engineered to enable temperature jumps. Because the heating is restricted to the inner volume of the small microgel, they also cool down quickly due to the fast heat transfer to the surrounding bath once the heating is ceased. Because swelling and shrinkage are diffusion controlled and cannot follow in time the fast temperature changes, the volume change can be effectuated out of equilibrium. As a consequence volumetric changes of anisometric gels tend to be inhomogeneous. The resulting stress results in shape deformations and the accumulation of elastic energy that is released with a delay. In principle, all these different effects can be well controlled. Mastering the nonequilibrium response provides control of the complex motion of a soft microrobot, which goes beyond what has been so far reported for hydrophilic microgels.

Non-reciprocal shape shifting microgels engine

255th National Meeting (2018)

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