Abstract
We report multi-responsive and double-folding bilayer hydrogel sheet actuators, whose directional bending response is tuned by modulating the solvent quality and temperature and where locally crosslinked regions, induced by ionoprinting, enable the actuators to invert their bending axis. The sheets are made multi-responsive by combining two stimuli responsive gels that incur opposing and complementary swelling and shrinking responses to the same stimulus. The lower critical solution temperature (LCST) can be tuned to specific temperatures depending on the EtOH concentration, enabling the actuators to change direction isothermally. Higher EtOH concentrations cause upper critical solution temperature (UCST) behavior in the poly(N-isopropylacrylamide) (pNIPAAm) gel networks, which can induce an amplifying effect during bilayer bending. External ionoprints reliably and repeatedly invert the gel bilayer bending axis between water and EtOH. Placing the ionoprint at the gel/gel interface can lead to opposite shape conformations, but with no clear trend in the bending behavior. We hypothesize that this is due to the ionoprint passing through the neutral axis of the bilayer during shrinking in hot water. Finally, we demonstrate the ability of the actuators to achieve shapes unique to the specific external conditions towards developing more responsive and adaptive soft actuator devices.
Highlights
There has been intense interest and progress in engineering soft, shape-transforming materials, such as hydrogels, which can mimic the sensing and response mechanisms found in nature
We have demonstrated that the combination of multi-responsive hydrogel bilayers with localized crosslinking gradients induced by ionoprinting allows comprehensive control over the
We have demonstrated that the combination of multi-responsive hydrogel bilayers with localized crosslinking gradients induced by ionoprinting allows comprehensive control over the bending direction and axis of patterned thin sheets into complex, 3D shapes
Summary
There has been intense interest and progress in engineering soft, shape-transforming materials, such as hydrogels, which can mimic the sensing and response mechanisms found in nature. A number of strategies exist to create multi-responsive gel actuators by incorporating multiple penetrating polymer networks sensitive to different stimuli into one gel composite [18,27,40], incorporating bi-axial stress with crosslinking gradients [41] or by incorporating modular gel building blocks into 3D geometries [42]. The bilayer bending axis is determined by the location of stiff, highly crosslinked ionoprinted regions [21] This double curvature effect has been discussed previously as means of achieving complex hydrogel shape transformation [40] and demonstrated experimentally with PDMS bilayer sheets [45]. We demonstrate the use of these simple fabrication techniques to produce gel actuators that transform into unique 3D shapes with a rapid response time
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