Bio‐inspired Design of Submerged Hydrogel‐Actuated Polymer Microstructures Operating in Response to pH

Abstract

IO N Responsive and reversibly actuating surfaces have attracted signifi cant attention recently due to their promising applications as dynamic materials [ 1 ] that may enable microfl uidic mixing, [ 2 ] particle propulsion and fl uid transport, [ 3 ] capture and release systems, [ 4 ] and antifouling. [ 5 ] Analogs in nature serve as inspiration for the design of such advanced adaptive materials systems—microorganisms use fl agella for propulsion, [ 6 ] cilia line the human respiratory tract to sweep mucus from the lungs and prevent bacterial accumulation, [ 7 ] and echinoderms use pedicellariae for body cleaning and food capture. [ 8 ] Signifi cant characteristics of these biological systems include functionality in a fl uidic environment, controllable actuation direction or pattern, and the ability to translate chemical signals or stimulus into mechanical motion. Researchers have taken various approaches to fabricating biomimetic actuators, among which are biomorph actuators made using microelectromechanical systems (MEMS) technology, [ 9 ] magnetically actuated polydimethylsiloxane (PDMS) structures, [ 10 ] and artifi cial cilia or actuators made from responsive gel. [ 11 , 12 ] However, most fabricated actuators, such as MEMS or magnetically actuated PDMS posts, must be driven by an external force or fi eld and are not responsive to chemical stimuli. Actuating structures that have been made from responsive hydrogel are either low aspect ratio and their motion is not patternable, [ 11 ] or the movement is irreversible. [ 12 ] Microscale actuation systems which exhibit reversible chemo-mechanical response and control of actuation direction or pattern have proven diffi cult to achieve. Inspired by biological actuators, which can be broadly interpreted as composites consisting of an active “muscle” component coupled with a passive “bone” structure, we recently developed a hybrid actuation system in which a crosslinked polyacrylamide-based hydrogel, acting as an analog to muscle, drives the movement of embedded silicon [ 13 , 14 ] or polymer [ 15 ]