Design And Application Of Multifunctional Poly（Vinyl Alcohol）-based Composite Hydrogel Electronic/Ionic Skin Sensor

As the interface between the human body and the surrounding environment as well as the largest organ in the human body,the skin integrates various receptors that can recognize complex stimuli in the external environment.Based on the concept of bionics,as an important part of artificial intelligence,electronic skin and ionic skin have received more and more attention and been widely used in health monitoring,robots,implantable prostheses and human-machine interfaces.It is worth noting that no matter which field it is used in,flexible biomimetic skin needs to respond to electrical signals converted from physical stimuli to achieve the sensing effect upon dynamic deformation.At present,the hydrogel based electronic and ionic skin strain sensors still have some limitations,such as poor long-time self-healing ability,low frost resistance,water losing,lack of adhesion,low conductivity at low temperature,and direction unrecognizing,etc.Therefore,in this work,we have prepared a series of multi-functional polymer based biomimetic skins and applied them to flexible strain sensor.1.We reported a direction-aware and ultrafast self-healing polyvinyl alcohol（PVA）/4-formylbenzoboric acid（Bn）/polyethylenimine（PEI）/Ti₃C₂T_x MXene（PBPM）dual network hydrogel.The hydrogel was synthesized via cross-linking PVA and PEI with Bn to form a polymer network and then incorporating MXene into the polymer network.The PBPM hydrogel shows an ultrafast self-healing ability（self-healing time～0.06 s）and the introduction of highly conductive MXene effectively improves its electrical conductivity and thus enhances the sensitivity of the hydrogel with rapid response performance（signal response time～0.12 s）.Importantly,an electronic skin strain sensor assembled by the hydrogel can not only detect the movements in different parts of the mold person body but also specifically identify the directions of the movements such as head-down/up and wrist-down/up.Such a direction recognition ability should be an important feature for intelligentizing the electronic skin for practical applications.This work solves the existing problems of hydrogel-based strain sensors such as long self-healing time and inability to distinguish direction.2.We demonstrated that a dual network organohydrogel with excellent ultra-low temperature anti-freezing,moisture retaining and fatigue resistant features was fabricated by doping conductive ZnSO₄ into the double network hydrogel of polyvinyl alcohol-polyacrylamide（PVA-PAM）with subsequent immersing in a mixed solvent of ethylene glycol（EG）and H₂O.It displays a high conductivity(0.44S m^-1)and exceptional moisture retaining ability with more than 99.3%retention of the initial weight after storage for 31 days at ambient temperature.When employed as ionic skins,the PPZE organohydrogel shows fast signal response time（～0.2 s）and excellent fatigue resistant features,and importantly,highlights its merits with superior anti-freezing ability,good flexibility and sensing capability with a high sensitivity at-50℃.This work makes up for the defect of conductive network fracture caused by separation of conductive materials in hydrogel-based electronic skin strain sensor under large strain.3.We proposed a method of fabricating self-adhesive,highly sensitive,ultra-low temperature anti-freezing,durable and stable moisturizing hydrogel-based flexible ionotronics skin strain sensor.The hydrogel（PPCM）was mainly composed of polyvinyl alcohol（PVA）and polyacrylamide（PAM）as its basic network framework.By adding CaCl₂ and conductive material MXene,the hydrogel exhibits an excellent self-adhesion and anti-freeze moisture performance,and its signal response performance is improved.As a bionic skin strain sensor,it displays an ultra-low temperature anti-freezing performance（-50℃）and a durable water retention stability.The water in the hydrogel can still be kept in a stable state after 70 days at room temperature,thus ensuring the long-term stability of the flexible strain sensor.In addition,the highly sensitive signal response（under 200%tensile strain,GF=3.0）and self-adhesive performance greatly expand its application as a flexible sensor.This kind of polymer based hydrogel bionic skin sensor with both electronic and ionic conduction not only overcomes the defects of single sensors such as electronic or ionic skin sensor,but also inherits the excellent performances of both.This work provides a new idea and scheme for artificial skin,health detection and human-machine interface,and promotes the further development of intelligent flexible bionic skin sensor.