Research On Self-healing Property Of Ultra-stretchable Ionic Hydrogel And Its Electrical Applications

Wearable electronic devices overcome the heavy and clumsy problems of traditional medical devices and gradually show the development trend of high integration,miniaturization,and low cost to achieve the direct or indirect connection between the human body and medical devices and networks.At present,wearable electronic devices in conventional use are often solid and rigid,which leads to a mismatch with the elastic coordination of human tissue.Although flexible wearable devices solve the problem of elastic coordination due to their mechanical compliance,ductility,and portability,they also have serious drawbacks,such as lower tensile elongation compared to human tissue,low transparency,performance failure under cyclic loading,and the risk of liquid metal leakage.In order to overcome those drawbacks,wearable ionic devices were achieved.In wearable ionic devices ionic touch panels have received much attention as an important form of human-machine interface.The existing ionic hydrogels have low tensile fracture elongation,which limits the potential applications of ionic touch panels.Moreover,ionic touch panels are unable to resist mechanical damage caused by friction,accidental scratches,or cuts,which can lead to device failure.In order to expand the application fields of ionic touch panels and to solve the durability problem of ionic touch panels during long-term use,there is a need to develop an ultra-stretchable self-healing ionic touch panel that can repair damage and restore its key properties like human skin.Firstly,the preparation and characterization of self-healing ionic hydrogels as the basis for self-healing ionic devices need to be considered.Poly-N,N-dimethylacrylamide（DMA）hydrogel is a commonly used hydrogel,and the synchronous interaction of Li⁺and Si O₂nanoparticles creates steric hindrance that prevents DMA from polymerizing into a hydrogel through a one-step in situ reaction.Inspired by the diatom mineralization process,hydrogels can be prepared by using pre-polymerized PDMA molecular chains to overcome the steric hindrance.Since the main components of the prepared hydrogel are PDMA,Si O₂nanoparticles,and Li Cl,it is referred to as DSL hydrogel for short.The elongation at break of 0.5DSL hydrogel(0.5 means that Li Cl concentration in the hydrogel is 0.5 mol·L^-1)can reach 4012%,and the strength can reach 36.61 k Pa.The mechanical properties of the hydrogel are significantly enhanced by the addition of Li⁺,which The main reason is that on the one hand,Li⁺can form lithium bonds with the carbonyl groups in the PDMA molecular chains and enhance the bonding strength between the molecular chains.On the other hand,during the synthesis process in the mixture of Si O₂ and Li Cl,the Si O₂ nanoparticles are assembled under the attraction of hydrated Li⁺spherical electric field,and the pre-polymerized PDMA molecules will be further polymerized with the assembled Li⁺and Si O₂ nanoparticles micelles as the template,and finally,the circular pore structure as shown in SEM morphology is formed.The hydrogel without the addition of Li⁺has a thin-walled hexagonal frozen tofu structure.The mechanical properties of 0.5 DSL hydrogels have excellent plasticity,flexibility,light transmission,electrical conductivity and thermal stability.Secondly,the self-healing properties of DSL hydrogels were characterized to determine the durability and service life of the self-healing ionic devices.In terms of mechanical self-healing,the scratch healing test of DSL hydrogel under 3D microscope and super-high magnification lens zoom 3D microscope showed that the shallow scratch of the hydrogel could be self-healed quickly at 25℃,25 k Pa contact pressure and 2 min healing time;the scratch width could be reduced from about 1250μm to 330μm and the scratch depth from 2537.1μm to 365.7μm after the repair,and the self-healing effect was evident.The self-healing efficiency of0.5 DSL hydrogel under the same healing conditions can reach 78.42%.The mechanical self-healing efficiency of DSL hydrogel increases with time,increases with increasing contact pressure,and increases with increasing healing temperature.The mechanism of mechanical self-healing in DSL hydrogels is chain diffusion of the polymer and the reconstruction of hydrogen bonds between carbonyl-silica hydroxyl groups driven by surface tension.For electrical self-healing,DSL hydrogels were subjected to a contact pressure of about 25 k Pa for15 s at 25°C.The electrical resistance recovered close to the initial value,and a slight increase in hydrogel resistance was observed during the pressure release,which was due to the viscoelasticity of the hydrogel resulting in the transmission distance of ionic conductivity in the hydrogel.Repetitive cut-off-electrical healing indicates the stability of DSL hydrogel electrical self-healing.LED-indicated active circuit healing tests show that DSL hydrogels can achieve self-healing of electronic circuits stably.Thirdly,surface capacitive touch panels,due to their simple design,continuous touch and low cost,are a popular form of ion touch panel construction.The DSL hydrogel is well suited for applications in the development of next-generation smart panels due to its hyperextension properties,surface adaptability,and self-healing properties.Among conventional touch panels,surface capacitive screens have a unique advantage in that their structure consists of only one conductor layer and four electrodes distributed at the four corners.One-dimensional touch panel touch current changes when the distance between touch points is equal,the touch current and touch distance are linear,its response delay is within 20 ms,the spatial resolution is 10^-4m,and it works even at a strain rate of 4000%.The 2D touch panel performs well in the process of fixed-point touch detection and continuous touch detection,and can work properly under 2D stretching and dynamic surfaces.But distortion due to uneven electric field distribution occurs at the edges.The 2D touch panel can work properly under 2D stretching and on dynamic surfaces.Also,it was demonstrated that the evaporation of water in the obtained DSL hydrogel did not affect the detection of the location of the touch points of the 2D touch panel.Finally,the skin touch panel was prepared.Before and after its assembly to the skin surface,since the skin is a conductor,which makes the parasitic capacitance of the system increase,the leakage current of the system increases,i.e.,the baseline current increases.After the skin touch panel is assembled to the arm,it exhibits the same touch pattern as the two-dimensional touch panel.A series of operations such as clicking,boxing,and moving can be achieved on the computer by the skin touch panel,and handwriting input can also be performed to achieve operations such as game control.As far as the self-healing performance of the touch panel is concerned,the severed 2D touch panel can be repaired repeatedly at the same location,and there is no significant change in the touch current after repair.After repairing the severed DSL hydrogel skin touch panel,the electrical properties were almost completely restored and did not change significantly with time and the numbers of touches.The self-developed 3D printing system can print conductive patterns on the surface of the hydrogel touch panel.The introduction of the conductive pattern can effectively prevent the pincushion distortion of the surface capacitive touch panel.The preparation method of the self-healing ionic hydrogel and design of surface capacitive touch panel in this project can be applied not only in human-computer interface,artificial intelligence,and mobile medic but also will facilitate the development of next-generation ultra-flexible,highly dynamic surface-adaptive,self-healing skin-like functional devices,especially for those application scenarios that require the human body to withstand extreme stretching,acceleration,and collision.It also provides design ideas for future applications such as ultra-flexible display and touch technology,optical stealth technology,and skin tissue replacement.