Based On The PVA Microcrystalline The Asymmetric Distribution To Construct Intelligent Hydrogel Actuator

Hydrogels are a class of "soft" materials composed of polymer networks and large amounts of water,which have been widely used in various fields,such as agriculture,industry,biomedical,tissue engineering and etc.Among those,intelligent hydrogels as a particular species,has been paid great attentions over past decades.These artificial material similar to the biological tissues,can perform multiple actions with respect to such external stimuli as temperature,light,p H environment,electric fields,humidity and ion strength,etc.The hydrogel actuators are used in tissue engineering,cell purification,soft robotics and biomedicine.Hence,more and more researchers have devoted themselves into the construction strategies,including the bilayer structures,gradient structures,pattern structures,and anisotropic structures,etc.To introduce hetero-structures into the hydrogel matrix to fabricate the hydrogel actuators.Generally,the bilayer structure method is considered to be an effective and simple way.By modulating the polymer,crosslinking density and sensitive component in the doble layer matrix,the anisotropic swelling between bilayer structure and the resultant shape deformation and bearing load can be realized.Nevertheless,these intelligent devices usually suffer from the weak mechanical strength,leading to the failure of actuation.How to endow the devices with flexible responsiveness and high mechanical strength simultaneously is still challenging.Therefore,this paper aims to develop a new way to construct intelligent hydrogels,which can display diverse fast actuation behaviors and excellent mechanical strength.The innovative points of this work are as follows.（1）It is the first time that we employed the polyvinyl alcohol（PVA）microcrystalline into the hydrogel matrix to construct the bilayer hydrogel actuator.Benefitting from the asymmetric distribution of PVA microcrystal region in the bilayer structure,the as-fabricated hydrogel device can display flexible responsiveness and high mechanical strength.（2）Based on the synergistic effect of poly（N-isopropylacrylamide）（PNIPAm）network and PVA microcrystalline region,the muscle-like PNIPAm/PVA bilayer hydrogel actuator can be able to display high performances,such as self-strengthening,strainadaptive stiffening and controllable actuation.The main research contents and conclusions of this paper are summarized as follows.A novel strategy to fabricate bilayer hydrogel actuators based on the asymmetric distribution of crystalline regions across the bilayer structures was proposed.By employing PVA polymer chains into an alkali solvent-derived chitosan hydrogel matrix,chitosan/PVA hybrid bilayer hydrogels with both excellent responsive bending and mechanical properties were obtained as p H-controlled manipulators.In the design,the chitosan/PVA hydrogels upon treatment with freeze-thawing cycles were taken as the first monolayer,where excessive crystalline regions appeared.The original chitosan/PVA hydrogel as the second monolayer was then integrated into one bilayer device through the chemical-crosslinking of epichlorohydrin at the interface.The results showed that the resultant chitosan/PVA bilayer hydrogel actuator with a weight ratio of3:1 displayed better sensitivity upon exposure to stimuli.The actuation behaviors are strongly dependent on experimental parameters such as the p H,PVA content and the chemicalcrosslinking density.It is proposed that the driving force originates from the asymmetric distribution of crystalline regions,thus resulting in differential swelling ratios between the monolayers.In addition,programmable 3D shape transformations were achieved by using the bilayer hydrogel with designed 2D geometric patterns,and the tailored gripper-like hydrogel actuator can successfully capture and transport the cargo.Moreover,this actuation behavior can be erased and re-written on demand under certain conditions.The freeze-thawed PNIPAm/PVA hydrogel layer was assembled with the untreated PNIPAm/PVA hydrogel layer by means of photo-initiated free radical polymerization,and a bilayer PNIPAm/PVA hydrogel actuator with dual-responsiveness was obtained.The results showed that due to the highest thermal sensitivity of PNIPAm with the 0.5 wt% crosslinking agent,the as-prepared double-layered PNIPAm/PVA hydrogel device（weight ratio of 5:1）can display a rapid responsive behavior upon the temperature stimulus.Meanwhile,the actuation behavior can be readily controlled by varying the salt concentration due to the Hofmeister effect for PNIPAm polymer chains in aqueous solution.As a result,the hydrogel actuator can perform diverse programmable 3D transformations and cargo transportation with respect to the changes of ambient temperature and salt concentration.Moreover,it has been proved that PNIPAm hydrogel can show the self-toughening characteristic sunder cyclic loading,while PVA hydrogel can achieve an extremely high fatigue threshold by mechanical training through the alignment of PVA nanocrystalline domains into nanofibrils.Based on the synergistic effect between PNIPAm and PVA chains in semi-interpenetrating network,the bilayer PNIPAm/PVA hydrogel exhibits a quick self-strengthening behavior under the circumstance of repeated tensile stress.Summarily,taking advantage of this universal strategy,more attractive actuators derived from synthetic or natural polymers in combination with PVA are highly expected,which can be used as smart soft robots in various fields such as manipulators,grippers,and cantilever sensors.