Controllable Deformation Of Photolithographic Patterned Hydrogels

Smart deformations are widespread in nature and have attracted tremendous interest due to their promising applications in biomedical devices,soft electronics and soft robotics,etc.After years of research,scientists have realized controllable deformations including bending,folding,and twisting and other complex deformations by elaborately tuning the localized composition and thus internal stress.However,there stills remain many crucial points to be further developed in this filed.One is how to easily construct complex yet elaborate structures in active materials governed by previous studies.Another is to develop more novel deformation systems to meet the demands of different occasions.Here,through multi-step photolithography we fabricate composite hydrogels with in-plane and/or out-of-plane structural gradient and demonstrate the resulting controllable deformations under the specific stimuli,including planar-to-helical shape transformations and astonishing cooperative deformations.Besides,we provide a straightforward approach called pre-swelling to direct morphing structures of patterned hydrogels.The underlying mechanics are also investigated.Firstly,a patterned gel with a layered fibrous structure like bean pod is developed by muliti-step photolithography,which shows programmed deformations from a flat shape to a twisted helix.Through this way,the parameters of the helix can be deliberately tuned and different responsive polymers can be easily patterned in specific regions of composite gels,leading to multiple shape transformations under stimulations.Secondly,the near-infrared light(NIR)responsive gel is incorporated in non-responsive gel in a complicated and programmed structure by combining the photopolymerization and thermal polymerization together.Under partially or fully irradiation of NIR,the patterned hydrogels undergo partially or fully bending,folding,and twisting deformation respectively.Thirdly,nonswelling disc gels are periodically positioned in a high-swelling gel.During the swelling process,the compartmentalized high-swelling gel alternately bends upward or downward to relieve the in-plane compression,but the overall integrated structure remains flat.The synergy between the elastic mismatch and the geometric periodicity selects the outcome pattern.Both experiment and modeling show that various types of cooperative deformation can be achieved by tuning the pattern geometry and gel properties.Different responsive polymers can also be patterned in one composite gel.Under stimulation,reversible transformations between different cooperative deformations are realized.Finally,series of high-swelling disc gels were positioned in a non-swelling gel sheet;the swelling mismatch resulted in out-of-plain buckling of the disc gels.To locally control the buckling direction,masks with holes were used to guide site-specific swelling of the high-swelling gel under the holes,which built transient through-thickness gradient and thus directed the buckling during the subsequent unmasked swelling process.Therefore,various configurations of one identical patterned hydrogel can be well programmed by the preswelling step with different masks to encode the buckling directions of separate domains.In conclusion,by multi-step photolithography,we can construct both in-plane and/or out-of-plane gradient structures with different responsive polymers,which facilitates the control of internal stresses and 3D deformations of the gels.By changing the simulations or enforcing the preswelling step,one identical patterned hydrogel can undergo reversible shape transformation.These results should be applicable to other soft materials such as elastomers,shape memory polymers,and hybrid polymer-inorganic materials,and if the dimensions of patterns can be miniaturized to the micrometer-or nanometer-scale level,which might provide deformed sheet materials with versatile physical properties for specific applications in optical devices and soft robotics.