Preparation And Electrically Programmable Adhesion Of Borate Polymer Hydrogel For Robot Anti-gravity Locomotion

Due to the strong demand for basic research and industrial/military applications,devices that can get rid of gravity constraints and move freely on vertical or inverted surfaces,such as climbing robots,have received constant attention.The participation of climbing robots make various dangerous and complex operations safe,convenient and efficient.However,the current climbing robots still have many deficiencies,such as the need for heavy power equipment,large energy consumption,weak controllability,and limited surface adaptability.Current robot climbing strategies mainly include:high-voltage electrostatic adsorption,magnetic adsorption,vacuum adsorption,and biomimetic methods that simulating the microstructure of the gecko’s sole.Theoritically,the climbing behavior of a robot depends on the interface interaction between the motion system and environmental objects,which is consistent with the key function of intrinsic adhesive polymers.However,intrinsic adhesive polymers are difficult to apply in the field of climbing robots due to the following issues:easy attaching and difficult detaching;reversible attaching/detaching cyclic transformation is a major challenge.In this study,an ion-conducting borate polymer hydrogel was designed.The borate bond was used to control the adhesion factor in the hydrogel:catechol groups.The pH of the hydrogel surface was programmed by water electrolysis,thus causing the reversibly cleavage and reformation of borate bond.Accordingly,the shielding and exposure of the adhesive factor catechol group can be controlled and the hydrogel can evolve from adhesive to non-adhesive reversibly in response to an electrical signal.The hydrogel can be further applied to the robot field,endowing the robot with anti-gravity locomotion ability.The main research contents are as follows:(1)Using the borate bond formation reaction between 5,5’,6,6’-tetrahydroxy-3,3,3’,3’-tetramethyl-1,1’-spirobisindan(TTS)and boric acid(BA)under alkaline condition,we prepared a cross-linking agent TB containing terminated with two boric acid groups.TBVA hydrogel was prepared by crosslinking water-soluble polyvinyl alcohol with TB to form a polymer network contains conductive ions and dynamic borate bond.The composition and structure of TB crosslinker and TBVA hydrogel were characterized by FT-IR,1H NMR,XPS and SEM.The dynamic cross-linked structure inside the hydrogel was verified by rheological analysis(temperature spectrum,strain spectrum and frequency spectrum);the mechanical strength and ductility of TBVA hydrogels with different cross-linking agent contents were studied through stress-strain test;evolutions of crack microstructure and mechanical properties.The self-healing ability of the TBVA hydrogel was analyzed.It could achieve 100%stress self-healing efficiency within 120 s.(2)The ionic conductivity of the TBVA hydrogel was measured by the electrochemical impedance spectroscopy,which reached a maximum value of 4.29 mS/cm.The hydrogel was alkaline inside(pH～8.0).The adhesion factor catechol group existed in the polymer network in the form of borate bond.The hydrogel has no adhesion to conductive substrates(such as:stainless steel,carbon paper,ITO Glass and aluminum).Under an electrical stimulation(1.5～6.0 V),the pH of the hydrogel surface contacting to the anode substrate is significantly reduced caused by the water electrolyzing,and the cleavage of borate bond induced the exposure of catechol groups.By simply switching the direction of the electric field,the hydrogel can reversibly attaching/detaching to conductive surfaces,with a minimum response time of 1s,and the adhesive strength can vary from～0.5 to～11 kPa.The influences of stimulation voltage,stimulation time and hydrogel size on the electro-adhesion performance were analyzed in detail.The mechanism of electronically controlled attaching/detaching of hydrogels was explored through FT-IR and electrochemistry.Based on the stimulus response characteristics of TBVA hydrogel,a three-legged robot capable of anti-gravity movement was designed to crawl on a vertical or inverted vertical stainless steel wall.The realization of the anti-gravity movement of the robot through the intrinsic adhesive material attaching/detaching has reference significance for the design of the new climbing robot.