Dual
Salt- and Thermoresponsive Programmable Bilayer Hydrogel Actuators
with Pseudo-Interpenetrating Double-Network Structures
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Abstract
Development of smart
soft actuators is highly important for fundamental research and industrial
applications but has proved to be extremely challenging. In this work,
we present a facile, one-pot, one-step method to prepare dual-responsive
bilayer hydrogels, consisting of a thermoresponsive poly(<i>N</i>-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly(3-(1-(4-vinylbenzyl)-1<i>H</i>-imidazol-3-ium-3-yl)propane-1-sulfonate) (polyVBIPS) layer.
Both polyNIPAM and polyVBIPS layers exhibit a completely opposite
swelling/shrinking behavior, where polyNIPAM shrinks (swells) but
polyVBIPS swells (shrinks) in salt solution (water) or at high (low)
temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS
bilayer hydrogels enable us to achieve fast and large-amplitude bidirectional
bending in response to temperatures, salt concentrations, and salt
types. Such bidirectional bending, bending orientation, and degree
can be reversibly, repeatedly, and precisely controlled by salt- or
temperature-induced cooperative swelling–shrinking properties
from both layers. Based on their fast, reversible, and bidirectional
bending behavior, we further design two conceptual hybrid hydrogel
actuators, serving as a six-arm gripper to capture, transport, and
release an object and an electrical circuit switch to turn on-and-off
a lamp. Different from the conventional two- or multistep methods
for preparation of bilayer hydrogels, our simple, one-pot, one-step
method and a new bilayer hydrogel system provide an innovative concept
to explore new hydrogel-based actuators through combining different
responsive materials that allow us to program different stimuli for
soft and intelligent materials applications