Intelligent Hydrogel Strain Sensor Based On Phytic Acid

Flexible strain sensor,portable and comfortable to wear,can be used as a wearable device for real-time continuous monitoring of human activities,pulse,heartbeat and other physiological information,and can be applied to human health detection,medical diagnosis,electronic skin and other fields.Hydrogels have mechanical properties similar to human tissue and good biocompatibility.They are ideal materials for the preparation of a new generation of flexible strain sensors.At present,although obvious progress has been made in the development and preparation of hydrogel-based flexible strain sensors,they are still faced with such shortcomings as poor environmental adaptation,narrow working range,low sensitivity,long response time,nonlinear output signal and poor interface adhesion,which are difficult to meet the needs of practical applications.Therefore,in view of the above problems,this thesis prepared a series of conductive hydrogels based on phytic acid(PA)from the perspective of the structural design of hydrogels,and explored and optimized the preparation process parameters of hydrogels,in order to obtain a multifunctional strain sensor with excellent sensing performance.And these strain sensors are applied in wearable devices for human motion detection.The main content of this thesis includes the following three parts:(1)In view of the poor adaptability of the flexible strain sensor to the environment and the sensor failure in the low temperature(below 0℃)or high temperature environment,a kind of flame-retardant and anti-freeze hydrogel based on PA was prepared.Using glycerol(G)-water binary system as solvent,double network hydrogels were formed by cross-linking between PA and polypyrrole,and between silica nanoparticles and polyacrylamide.PA and glycerin make hydrogels have good flame retardant/frost resistance.At the same time,they can interact with other components of hydrogel,improve the crosslinking density of hydrogel network,and improve the mechanical properties of hydrogel.Furthermore,the strain sensor was constructed with hydrogel.The sensor showed good sensing stability at low temperature(-20℃)or high temperature(～600℃),with a strain sensing range of up to 500%and a small fluctuation in sensitivity.It is suitable for detecting human movement signals at extreme working temperatures as a wearable device.(2)In this part,PA was introduced into the polymer framework composed of polyvinyl alcohol(PVA)and chitosan(CS)to prepare a green hydrogel with excellent performance.Due to the dense network structure,hydrogels have excellent ductility and toughness(the highest fracture strain is 915±32.44%,and the highest fracture stress is 6.02±0.18 MPa).At the same time,the unique molecular structure of PA endows the hydrogel with excellent water retention(more than 90%of the original mass after 30 days of storage),and the hydrogel also has thermoplasticity and degradability,which can be recycled.The strain sensor based on this hydrogel has a wide linear sensing range(up to 900%)and a fast response time(50 ms).It is the first reported hydrogel based sensor with both wide linear and fast response.Compared with the previous work,the sensing performance has been greatly improved.At the same time,after thermoplastic and long-term storage,sensors still have excellent mechanical and sensing properties,and are expected to be used in green and environmentally friendly wearable devices.(3)In order to solve the problem of lack of adhesion and low sensitivity of the hydrogel constructed in the second part of work,boric acid(BA)was added to the hydrogel network system studied in the previous work,and a conductive hydrogel with good mechanical properties and high adhesion was prepared.The synergistic action of PA and BA enables the hydrogel to adhere to a variety of material surfaces(such as glass,silicone rubber,PTFE,stainless steel and skin)and show good repeatable adhesion.Among them,the hydrogel had a maximum adhesion strength of 527 k Pa and 103 k Pa on pig skin,exceeding many adhesives sold on the market.The hydrogel was assembled into a strain sensor.The sensor showed a wide linear sensing range(up to 1000%,R~2=0.996)and high sensitivity(GF=4.61),which solved the contradiction between high linearity and high sensitivity of existing hydrogel-based strain sensors.The development of the hydrogel is expected to be a new method to prepare wearable electronic devices with high adhesion and excellent sensing performance.