Study On Preparation And Stimuli-responsive Property Of Polyacrylamide Hydrogel Fiber Based On Microfluidic Spinning

Electro-responsive hydrogels can be used as bionic actuators due to the bending motion of hydrogels in an electric field.However,the practical application of traditional hydrogels is limited due to the slow electro-response rate and the poor mechanical property.Moreover,the hydrogels are mostly made into bulks,rods and films,which lack flexibility in the fabrication of artificial muscle actuators.Based on the above problems,researchers have done a lot of work and made great progress.The electro-response rate of hydrogels can be improved by adding additives with excellent electric properties such as graphene,or forming hydrogel fibers with smaller size.Besides,the mechanical properties of hydrogels can also be enhanced by forming nanocomposite hydrogels crosslinked by inorganic Clay.However,the hydrogel fibers with fast electro-response rate and excellent mechanical properties are rarely reported.Recently,microfluidic spinning is emerging as a promising method to generate continuous hydrogel fibers at the microscale.In the microchannel,the continuous fluid of pre-gel solution is sheathed by a second solution and then generate solidified fiber after either physical or chemical crosslinking or solidification.The pre-gel solutions are mainly prepared by polymer such as alginate,gelatin,and chitosan.Meanwhile,the formation of fibers depends on phase separation or ionic crosslinking,which results in the poor mechanical properties of hydrogel fibers.Besides,the pre-gel solutions embedded monomer such as N-isopropylacrylamide?NIPAM?are also utilized to generate hydrogel fibers.However,the monomers are photo-polymerized,which isn't suitable for the preparation of nanocomposite hydrogel fibers doped with graphene or crosslinked by Clay.In this paper,a new method to generate continuous hydrogel fiber by on-chip microfluidic spinning and off-chip free radical polymerization was developed with NIPAM as the model monomer.On this basis,graphene oxide?GO?and Clay were introduced into the hydrogel networktoformgrapheneoxide/poly?N-isopropylacrylamide?/Clay/sodiumalginate?GO/PNIPAM/Clay/SA?nanocomposite hydrogel fiber,which synchronously improved the electro-response rate and the mechanical property.In addition,based on the near infrared?NIR?light responsiveness of GO/PNIPAM/Clay/SA nanocomposite hydrogel fiber,a Y-shaped coaxial laminar flow microfluidic chip was designed and utilized to generate continuous bilayer GO/PNIPAM/Clay/SA-PNIPAM/Clay/SA?GNC-NC?hydrogel fiber.Thus,the NIR light actuation was introduced by converting isotropic volume shrinkage into anisotropic bending motion.The main contents of the paper are summarized as follows:Part one:A coaxial laminar flow microfluidic chip was designed and constructed.Continuous as-spun fiber was formed via microfluidic spinning by using gel precursor solution consisting of monomer,crosslinker,initiator,catalyst and SA as core fluid and CaCl₂ solution as sheath fluid.Then the as-spun fiber acted as the template for the free radical polymerization of monomer and the hydrogel fiber was prepared.The results show that preventing the outward diffusion of monomer from the core solution and the as-spun fiber is the key to success in the preparation of hydrogel fiber.On the one hand,SA was introduced into the core solution to increase the viscosity of the core solution and then reduce the monomer diffusion,and as well as to help the formation of continuous as-spun fibers by ionic crosslinking.On the other hand,an oil-soluble photo-initiator benzoin dimethyl ether?BDK?was introduced into the polymerization medium and photo-polymerized the monomer on the as-spun fiber surface to prevent the outward diffusion of monomer.Besides,the influences of CaCl₂ concentration in sheath solution and SA content in core solution on the morphologies of hydrogel fibers were investigated and the optimal ratio was determined:CaCl₂ concentration is 200 mM and SA content is 1wt.%.The non-contact direct current?dc?electro-responsive behaviors of the PNIPAM/SA and polyacrylamide/sodium alginate?PAM/SA?hydrogel fibers in the electrolyte solution were studied.Based on the mechanism of osmotic pressure,the hydrogel fibers containing anionic polyelectrolyte SA both bend toward the cathode.However,when the temperature of electrolyte solution is higher than the volume phase transition temperature?VPTT?of the PNIPAM/SA hydrogel fiber,the PNIPAM/SA hydrogel fiber bends back to the middle position with the shrinkage in length,which is caused by the thermosensitivity of PNIPAM network.Part two:Continuous GO/PNIPAM/BIS/SA hydrogel fibers were prepared by microfluidic spinning and free radical polymerization using N,N-methylenebisacrylamide?BIS?as the crosslinker.The structures and morphologies of the hydrogel fibers were characterized by X-ray diffraction?XRD?,infrared spectroscopy?FTIR?and scanning electron microscope?SEM?.The influence of GO content on the mechanical properties,swelling properties,thermo-and electro-response properties of the hydrogel fibers were also investigated.The results show the addition of GO increases the crosslink density of the hydrogel network and decreases the pore size due to the hydrogen bonding interaction between GO and PNIPAM.However,it has little effect on the thermo-response of the hydrogel fibers.Besides,the introduction of GO not only enhances the mechanical properties of the hydrogel fibers,but also improves the electro-response rate.Part three:Continuous GO/PNIPAM/Clay/SA hydrogel fibers were prepared by microfluidic spinning and free radical polymerization using Clay as the crosslinker.The influence of GO on the crosslinking function of Clay in hydrogel network was investigated.The influences of GO and Clay contents on the mechanical properties,swelling properties and electro-response properties of the hydrogel fibers were also studied.The results show that the presence of GO causes a slight loss of Clay in the hydrogel fibers,but has little effect on the crosslinking function of Clay.Compared with the hydrogel fibers crosslinked by BIS,the nanocomposite hydrogel fibers crosslinked by Clay not only exhibit faster swelling rate and larger swelling ratio,but also exhibit excellent mechanical properties and fast electro-response rate.The hydrogel fiber can reach a 90°bending angle within 3 s in 0.1 M Na₂SO₄ solution under the voltage of 20 V.The electro-response rate of the hydrogel fiber can be improved by the increase of GO content,the decrease of fiber diameter or the decrease of Clay content in a certain range.Part four:A Y-shaped coaxial laminar flow microfluidic chip was designed and constructed,and continuous bilayer GNC-NC hydrogel fiber was prepared by microfluidic spinning and free radical polymerization.The NIR light actuation of the bilayer hydrogel fiber was investigated.The influence of the thickness ratio on the NIR light actuation of the bilayer hydrogel fiber was also studied.The results show the bilayer hydrogel fiber first bends toward the GNC hydrogel layer and then bends back to the NC hydrogel layer.Because GO possesses higher absorption and photothermal conversion for NIR light,the GNC hydrogel layer with a faster heating rate first reaches its VPTT and shrinks,causing the bilayer hydrogel fiber to bend toward the GNC hydrogel layer.Subsequently,heat conduction causes the temperature of the NC hydrogel layer to exceed its VPTT,and then the NC hydrogel layer shrinks.As the NC hydrogel layer shrinks faster than the GNC hydrogel layer,resulting in the change of bending direction of the bilayer hydrogel fiber.Part five:The graphene oxide/polyacrylamide/sodium alginate?GO/PAM/BIS/SA?hydrogel rods were prepared by free radical polymerization using BIS as the crosslinker.The role of GO in the hydrogel network and the effects of GO and BIS contents on the mechanical properties of hydrogel rods were investigated.Meanwhile,the GO/PAM/BIS/SA hydrogel fibers were prepared from acrylamide?AM?monomer and calcium alginate fiber as template by microfluidics spinning and free radical polymerization,and the mechanical properties,swelling properties and electro-response properties were studied.The results show GO cannot be used as the crosslinker for PAM network without BIS.However,the mechanical properties of GO/PAM/BIS/SA hydrogel rods can be significantly improved by adjusting the contents of GO and BIS.Besides,based on the mechanism of double network hydrogel,the hydrogel fiber crosslinked by BIS can also be highly stretchable with the elongation at break of 525%and the fracture stress of 393 kPa.Meanwhile,the hydrogel fiber exhibits fast electro-response and good swelling property.