Preparation Of PVA/MXene/PEDOT:PSS Hydrogel And Application In Flexible Strain Sensor

People’s life and health is an important platform of the"Four Orientations",so our state attaches great importance to the strategic decision of"Healthy China",and active health monitoring has become one of the cores of this decision.Flexible sensor can convert various vital signs signals,which including body temperature,blood pressure,heart rate,respiratory rate,ECG,EMG,EEG and other bioelectrical signals,into available electrical signals for real-time monitoring of human health parameters.Hydrogel has become a representative flexible sensor because of its good mechanical property,biocompatibility and high electrical conductivity.However,conventional hydrogel is easy to lose water at room temperature and lose its original properties due to freezing at zero degrees,resulting in device failure.Therefore,it is of great significance to design and develop a hydrogel with excellent mechanical property,high electrical conductivity and good antifreeze moisture in active health monitoring.Pure polyvinyl alcohol（PVA）hydrogel has attracted extensive attention because of its good biocompatibility and self-healing property.However,poor mechanical property and low electrical conductivity have hindered its further development.In order to improve these properties,the strategy of introducing conductive fillers into pure PVA hydrogel was reported.MXene with high metal conductivity,hydrophilicity and excellent mechanical property was used to prepare PVA/MXene hydrogel.However,the self-stacking characteristic of MXene nanosheets reduced the efficiency of ion conduction as well as weakened the mechanical property of the hydrogel.At the same time,the MXene-based hydrogel can be used as electromagnetic shielding material due to its internal water-rich environment and medium conductivity,so as to ensure that it is not interfered by electromagnetic waves when used as a flexible strain sensor,so that the sensor can operate normally for monitoring human health signals.Therefore,PVA/MXene/PEDOT:PSS（P/M/P）hydrogel was prepared by introducing poly3,4-ethylene dioxyethiophene/polystyrene sulfonate（PEDOT:PSS）,which has high conductivity and stability,to achieve both excellent mechanical property and high conductivity.In this paper,P/M/P hydrogel was prepared by chemical cross-linking method and P/M/P organohydrogel with good anti-freezing and moisturizing properties was prepared by solvent substitution method.The P/M/P hydrogel and P/M/P organohydrogel are used as flexible strain sensors for real-time monitoring of human health parameters,as follows:（1）To address the problem that PVA/MXene hydrogel cannot combine excellent mechanical property and high electrical conductivity,P/M/P hydrogel with both excellent mechanical property and high electrical conductivity was designed and synthesized.The mechanical property and electrical conductivity of hydrogel can be changed by regulating the content of MXene and PEDOT:PSS and a maximum fracture strain is 785%,a maximum tensile strength is 93.57 k Pa,and an electrical conductivity is 0.2109 S m^-1 can be obtained for the P/M/P hydrogel.Based on the dynamic cross-linking between PVA and tetrahydroxy borate ions and the hydrogen bonding between PVA,MXene and PEDOT:PSS,P/M/P hydrogel has self-healing property.The porous structure,high conductivity and water-rich environment of P/M/P hydrogel make it suitable for commercial electromagnetic interference（EMI）shielding.Meanwhile,P/M/P hydrogel has biocompatibility and biodegradability.With the synergy of these properties,P/M/P hydrogel has a wide strain range（0～763%）and high sensitivity（GF=2.31）wnen used as a flexible strain sensor,which can be used to monitor human movement.Based on this,the P/M/P hydrogel is applied to the joints of the human neck,and when the hydrogel is stretched and deformed while playing with the cell phone or looking at the computer,the resistance changes,and the signal of this change is captured and amplified by the designed circuit to give a warning sound to remind us to sit properly,which can help people to prevent cervical spine diseases.（2）To address the problem of poor durability caused by rapid water loss of P/M/P hydrogel at room temperature and poor anti-freezing performance at zero temperature,solvent replacement strategy was adopted to prepare P/M/P organohydrogel.By placing P/M/P organohydrogel and P/M/P hydrogel in a refrigerator at-20 ^oC for 8 h,it was found that P/M/P hydrogel would break after bending and twisting,while P/M/P organohydrogel still maintained good flexibility.The above results indicate that P/M/P organohydrogel are freeze-resistant at-20 ^oC,while P/M/P hydrogel undergoes freezing at the same temperature.The weight of P/M/P organohydrogel remained about 70%of the initial weight after being placed for 2 days at 25 ^oC and 35%humidity,while the weight of P/M/P hydrogel was only 19%.The above results prove that P/M/P organohydrogel has good anti-freezing and moisturizing properties.Good frost resistance and moisture retention enable P/M/P organohydrogel to be used as a flexible strain sensor with permanent stability at room temperature and can still maintain the original performance for human movement monitoring at low temperature.