Study On Tough Hydrogels Folding Based On Asymmetry Of Elastic Modulus

In nature,biological systems change their shape in order to suit environmental conditions.In order to simulate biological functions,in recent years,artificial shape morphing systems have emerged and received wide attention.These systems are widely used in biomedical equipment,biomimetic systems and aerospace.Tough hydrogels,a new type of hydrogels with ultra-high mechanical toughness,which overcomes the fragility of traditional hydrogels,are an ideal choice for artificial shape morphing systems.First,the tough hydrogel matrix was exposed to Fe³⁺,which diffused into the matrix to form a second ionically crosslinked network.The use of trivalent Fe cations as strong ionic crosslinkers further enhances the deformation resistance of the gel network.As a result,highly stretchable tough hydrogels become stiff,which is similar to"freezing"of the hydrogel.Therefore,after exposure to Fe³⁺at concentrations ranging from of 0.01 M to 1 M,the Young's modulus increased up to 2.90 MPa with the increase of Fe³⁺concentration.Subsequently,the tough hydrogels were exposed to Fe³⁺solution,which afforded shape morphing of the toughened gel into various shapes by adjusting the stiffness and Young's modulus.Secondly,through a simple and reliable mechanochemical regulation strategy,Fe³⁺is locally applied to the hydrogel and is able to diffuse into pre-stretched tough hydrogel.The local stiffness is significantly increased by forming a trivalent ionically crosslinked network.By removing the applied tensile strain,the residual stress causes the anisotropically encoded pre-stretched tough hydrogels to deform into complex three-dimensional origami structures.Since the density of the secondary ion crosslinking is denser,the Fe³⁺ion patterned region of tough hydrogels becomes resistant to swelling.By application of 0.01 M Fe³⁺solution the swelling ability of the tough gel was greatly suppressed from about 830%to about 110%.This difference was utilized for asymmetric swelling of layered structures by mechanically strengthening the Ca-alginate/PAAm tough hydrogel site-specific by patterning with Fe³⁺ions.To study the thickness ratio effect,the gel strip is modified by Fe³⁺.As the ratio of the thickness of the iron-calcium layer increases,the time for the strip gel to swell to form a closed loop increases.When the iron-calcium layer thickness ratio is greater than about 0.8,the loop did not close due to insufficient energy.Hydrogel strips with the same size were patterned with different lengths of Fe³⁺ion patterns from 5 to 20 mm.Here,longer patterns caused faster and larger deformations.Base on above studies,Fe³⁺patterning was applied to encode two anisotropic elements,thus,achieving a dual responsive shape morphing tough hydrogel.Gel strips with different tensile strain?150%,200%,and 250%?were modified on one side with Fe³⁺.Subsequently,the stress was released and the hydrogels swollen.Due to the different swelling ability swelling induced bending of the gel strips was observed,for example at a tensile strain of 250%a bending angle of 620°was obtained.Larger tensile strain were able to cause even greater swelling induced deformation.The shape deformation caused by the stiffness mismatch is solvent-free,while the swelling-induced shape deformation is carried out in water,which means that the dual-responsive shape deformation system is can be triggered in both air and water,which may provide opportunities for further development of amphibious robots.