Programmable Reversible Shape-morphing Hydrogels And Manipulation Of Their Far-from-equilibrium States

Hydrogels possess three-dimensional polymer network structure can absorb a lot of water yet not dissolve in water.They have shown remarkable potentials in targeted drug delivery,tissue engineering,electrical skins and soft robotics due to their various stimuli-responsiveness.Stimuli-responsive hydrogels are those can shrink or swell under appropriate environmental stimuli.Along with water loss or absorbent is the volume of contraction or expansion.Conventional hydrogels show isotropic volume change,which is clearly not to meet the requirements of many high value-added fields.In some particular type of application scenario,the shape of the hydrogel plays a crucial role.Therefore,it is necessary to develop a material and fabrication system that has an anisotropic deformation.At the same time,the existing deformation mode of hydrogel is monotonic,switching between two equilibrium shapes.It is also worth exploring if more complex shape-shifting mode can be achieved.Poly(N-isopropyl acrylamide)/polyvinyl(PNIPAM/PVA)hydrogels which own interpenetrating polymer network were synthesized.The temporary polymer chain orientation in the gel network was introduced by an external force,subsequently fixed by the polyvinyl alcohol crystallization through freezing-thawing.The oriented PNIPAM/PVA hydrogels showed anisotropic volume changes under external temperature signals.If the gel was programmed into a 3D shape during the fixing process,the subsequent temperature change could also cause anisotropic shape-shifting,rather than simple volume contraction and expansion.The relationship between programmed strain(degree of chain orientation)and anisotropic volume shrinkage was studied.Taking the three basic deformation modes of folding,twisting and spiral as examples,it is proved that the method of using polyvinyl alcohol crystals to fix the chain orientation is effective and stable.The physical crosslinking of polyvinyl alcohol can be dissolved by increasing the temperature,which gives the entire material system the ability to be reprogrammed.The orientation method proposed can introduce programmable and reversible complex deformations in the stimulus-responsive hydrogel,that is,two-way shape memory performance.Furthermore,in the process of studying the two-way deformation,we found that the gel has an unexpected deformation behavior.Researches on folding samples showed that,when the hydrogel deforms anisotropically from 90° to 55°,its angle would first shrink to 0° at a very fast speed,and then slowly increase to 55°,which is not a direct reduction as expected.During its temperature triggered morphing event,the programmed stress induces uneven water diffusion,which pushes the hydrogel off the equilibrium based natural pathway.The resulting geometric change enhances the diffusion contrast in return,creating a self-amplifying sequence that drives the system into a far-from-equilibrium(FFE)condition.Consequently,the hydrogel exhibits counterintuitive two opposite morphing events under one single stimulation,at a speed accelerated by more than one order magnitude.The conjecturing mechanism was proved through both experiments and theoretical simulations.The influence of different geometric structures on the over-steady state is explained and an environment sensitive actuator that can contract first and then relax under a stimulus signal was fabricated.Our discovery points to a future direction in creating FFE conditions to access otherwise unattainable morphing behaviors,with potential implications for many engineering applications including soft robotics and actuators.In order to prove the diversity of the FFE phenomenon,a polyacrylic acid/polyvinyl alcohol(PAA/PVA)gel is developed.Due to the presence of carboxyl groups in the gel,the system exhibits responsiveness to both pH and ionic strength.The shape of the gel after the orientation of the external force was fixed by the freezingthawing method.The results showed that the oriented PAA/PVA gel had significant anisotropy.For a gel of a specific shape,it owned programmable reversible complex deformation behavior.In the process of the transition between the two equilibrium states,there was still an FFE deformation caused by the difference in swelling.The influence of the geometric structure on the FFE shape-shifting was similar to that of the thermal response system,which proved that the mechanism of FFE shape-shifting was applicable to various stimuli-response systems.