Spring-shaped stimuli-responsive hydrogel actuator with large deformation

This study describes a novel microfluidics-based method for compressive/expanding actuation of stimuli-responsive hydrogel microsprings with large deformations. A continuous flow of mixed alginate and poly(N-isopropylacrylamide-co-acrylic acid) pre-gel solution can spontaneously form a hydrogel microspring with a wide range of gradient pitches via buoyancy force. This technique enables fabrication of hydrogel microsprings using only simple capillaries and syringe pumps. The resulting microsprings can be patterned via laminar flow inside the capillary, which can contribute to large deformation. Single-layered hydrogel microsprings shrunk isotropically while maintaining the shape of the spring. Compressing stimuli-responsive microsprings can be done by patterning the shrinking part of the spring. Here, the degree of compression in the double-layered spring depends on the initial pitch. Furthermore, large axial expansion of microsprings can be achieved by shrinking part of a microspring. Our large compression/expansion stimuli-responsive hydrogel microsprings have immense potential to be applied in various microengineering products including soft actuators, chemical sensors, and medical applications.