Superelastic Sheath-core Ionically Sensory Fiber With Reprogrammable Conformal Buckling

With the application of soft electronics,a variety of stretchable electronic sensors have been developed.Polymer gels with a three-dimensional network structure usually possess excellent properties such as high transparency,high stretchability,and high elasticity,which have been widely applied in the field of soft electronic devices.At present,a variety of hydrogel-based stretchable sensors have made great progress in biomedicine,robotics,human movement monitoring and other aspects.However,the further applications of hydrogels are greatly restricted by the problems of freezing at low temperatures as well as easy dehydration.As an alternative,ionogels with ionic liquids as the solvent show great advantages than hydrogel in terms of high conductivity and low volatility.On the other hand,compared with bulk sensors,fiber-based soft sensors have more sites for precepting external stimuli in various directions,and the high aspect ratio of fibers minimizes the negative impact of Poisson effect in practical sensing applications.Nevertheless,the currently reported ionogel-based fiber sensors are still suffering from the issue of low sensitivity.The inner layer of the finger is soft and conductive subcutaneous tissue,while the outer layer is a relatively hard and electrically insulated skin(under dry conditions).Besides,the outer layer has uneven buckled textures,which make the finger not only be immune to external stimuli,but also sensitive to external touches.Inspired by human fingers,in this thesis,we synthesized superelastic sheath-core ionically sensory fibers with reprogrammable conformal buckling.Here,fluoroelastomer(PVDF-HFP-TFE terpolymer)was used as the sheath layer,and poly2-methoxyethylacrylate-co-[(2-(methacryloyloxy)ethyl]trimethylammonium bis(trifluoromethanesulfonyl)amide(P(MEA-co-MTMA TFSI))ionogel as the core material.The hybrid fiber was prepared by dip-coating the ionogel fiber in the fluoroelastomer solution,and subsequent stretching leads to reprogrammable conformal buckling.The main contents of this research are as follows:(1)Preparation and properties of P(MEA-co-MTMA TFSI)ionogel fiber: the precursor of P(MEA-co-MTMA TFSI)ionogel was prepared by simple mixing the reactants,which was then injected into a PTFE tube,and the ionogel fiber was obtained after photo-curing.Under the optimal ratio of MTMA TFSI,the ionogel fiber shows the best mechanical performance with the maximum strain up to 1360% and fracture stress of around 1 MPa.Meanwhile,the fiber shows good ionic conductivity,adhesion,and excellent frost resistance.(2)Preparation of sheath-core ionically sensory fiber and exploration of the reason for conformal buckling: the sheath-core ionically sensory fiber with good uniformity was prepared by a dip-coating method.The fiber was then stretched to a certain strain and released to form a conformal buckled structure.The morphology of the buckles is strongly dependent on the pre-strain.As the applied pre-strain increases,the wavelength of the buckles decreases and the amplitude increases significantly.The formation of conformal buckling is attributed to the difference of elastic recovery between low-modulus ionogel core and high-modulus fluoroelastomer sheath.(3)Performance and application of sheath-core ionically sensory fiber with conformal buckling: the sheath-core ionically sensory fiber with conformal buckling shows excellent deformation recovery ability in cyclic stretching with unchanged buckled morphology.As the buckled fiber is stretched or recovered,the transparency of the fiber can be tuned,and the position of water droplet can be controlled.Moreover,as a strain sensor,the buckled fiber shows much higher sensitivity than smooth fiber,suggesting its promising role in future fiber-based soft electronics.