Development And Preparation Of A New High Strength Intelligent Temperature-sensitive Hydrogel And Its Application In Wearables

The wearable sensors have gradually attracted widespread attention due to their flexible mechanical properties,lightweight quality,good ductility,excellent bending,easy to carry,easy to integrate and excellent stimulus response signals,which play an increasingly important role in human health monitoring,implantable medical devices,early diagnosis of diseases,artificial electronic skin,and human motion monitoring.The traditional electrical signal sensor is usually made of metal or semiconductor material,leading to poor flexibility and stretchability of the sensor,which seriously affects the application field of the sensor.Conventional flexible sensors are usually composed of flexible substrates and electronic sensors.The flexible substrates endow the sensors with good flexibility and stretchability,which greatly improves the brittleness of traditional sensors,and broadens the application of sensors in health monitoring,electronic skin and soft robotics.However,it is very difficult to make stable bonding between electronic sensors and flexible substrates,resulting in reduced sensitivity and longevity.Therefore,a new generation of wearable sensors combines conductive materials with flexible polymers to prepare flexible conductive materials.Currently,nanoparticles,nanowires,carbon materials,conductive polymers,and so on are used as conductive components.Flexible polymer materials mainly include hydrogel,polydimethylsiloxane(PDMS),polyimide,polystyrene,etc.In the past decade,researchers have developed various smart soft materials to fabricate flexible wearable sensors.Hydrogel material,which is mainly composed of biomacromolecular networks infiltrated with water,has similar physiological and mechanical properties to human skin(e.g.,flexibility,healable,and perception).Thus,their properties(e.g.,volume,optics,and conductivity)can dramatically change in response to external stimuli,such as pressure,temperature,and certain chemicals,making hydrogel material ideal to simulate the function of the human skin.Up to now,a variety of smart hydrogel materials have been developed for applications in wearable devices,human-machine interfaces and soft robots.However,the mechanical properties of traditional hydrogels are poor,which greatly limits the application of hydrogels in flexible electronic sensors.At present,dual-network structure,ring-slip structure,nano-recombination,supramolecular interaction and other methods have been used to enhance the mechanical properties of hydrogels.Although these methods greatly enhance the mechanical properties of gels,the prepared hydrogels only have a single function.Therefore,there is an urgent need to develop new high-strength functional intelligent hydrogels to expand their applications in health monitoring,implantable medical devices,early disease diagnosis,artificial electronic skin,human movement monitoring and other fields.Based on the above studies,poly(N-acryloylglycinamide)(PNAGA)with multiple hydrogen bonds was used as the main network of hydrogels,and various novel high-strength functionalized hydrogels were prepared by introducing other functional groups,which expanded the application of hydrogels in medical engineering.N-acryloylglycinamide(NAGA)has multiple hydrogen bonds,which makes the prepared PNAGA composite gel have selfhealing,thermoplasticity and strong mechanical properties.Therefore,in this paper,various functional composite hydrogels were obtained by combining various materials(eg.PNIPAm,gold nanorods and silver nanorods)with NAGA,which were used in the fields of simulating human temperature perception,wound treatment and monitoring,and bionic electronic skin.The main research contents are as follows:(1)In this work,we developed a novel PNIPAm/PNAGA double-network hydrogels by self-assembly cross-linking strategy that achieved a wide and adjustable dual temperaturesensitive with high stretchable and self-healable properties.PNIPAm was used as the first network and then self-assembled with NAGA by hydrogen bonding and polymerized to form a PNIPAm/PNAGA double-network hydrogel.Benefiting from the double-network structure and various hydrogen bond interactions between PNIPAm and PNAGA,the hydrogels showed wide dual temperature response behaviors of 0–32.5°C(LCST)and 32.5–65°C(UCST)and adjustable temperature response characteristics by controlling the ratio of monomers.In addition,the dual temperature-sensitive hydrogel exhibits extraordinary mechanical properties with a maximum tensile strength of 51.48 k Pa,elongation at break over 1400%,compressive stress over 1Mpa,Young’s modulus about 5.51 k Pa,and excellent healable properties of nearly100% temperature-sensitive repair rate,easily satisfying the needs of high stretchable and autonomous self-healable capability for electronic skins and wearable devices.To the best of our knowledge,this is the highest mechanical strength of reported PNIPAm-based dual temperature-sensitive hydrogels and simultaneously achieved the healable performance of dual temperature-sensitive hydrogels for the first time.The PNIPAm/PNAGA hydrogel displayed superior capability for simulating the behavior of human skin to monitor various ambient temperatures,such as human skin,hot and cold water,refrigerator,room temperature,and oven temperatures.This study provides double-network strategies to fabricate novel PNIPAm-based composition hydrogels.The proposed double-network hydrogel can simulate the temperature perception of human skin to monitor various ambient temperatures.The double-network PNIPAm/PNAGA hydrogel films were opaque at low temperature,whereas it could rapidly become transparent around 32°C and further change into opaque again at higher temperature.The obvious transparency variation of the hydrogel films under different temperatures provides a simple visualization approach to qualitatively evaluate of environmental temperature,which displays promising applications in the fields of electronic skin,wearable devices,bionics,and actuators.(2)In this work,a multifunction all hydrogel-based smart dressing based on three different functional hydrogels(PNAGA/Ag NW,PNAGA/Au NRs and PNAGA/PNIPAm)was developed for the first time by self-healing cross-linking strategy for real-time monitoring of wound temperature,strain and on-demand drug delivery.The PNAGA/Au NRs hydrogel was used as a substrate and photothermal conversion component on the underlayer of the dressing system,while PNAGA/PNIPAm and PNAGA/Ag NW hydrogels were used as temperature and strain sensing components for the upper layer,respectively.Three different functional hydrogels can easily integrated into a whole by reversible multiple hydrogen bonds between PNAGA gels for real-time monitoring of wound temperature,strain and on-demand drug delivery.PNAGA/PNIPAm hydrogel was used as temperature sensitive component for wound temperature monitoring to identify whether the wound is infected.The PNAGA/Ag NW hydrogel was used as stress sensing component to monitor the strain loaded on the wound and prevent secondary injury to the wound in the proliferation and modulation phases.In addition,three different functional hydrogels were used as drug carriers to achieve the localized ondemand drug delivery by temperature or near-infrared stimulation.In situ rat models studies showed that the smart dressings could achieve simultaneous monitoring of the temperature and mechanical strain on the wound for the wound status assessment,and on-demand drug delivery on the infection wound by temperature or near-infrared stimulation to inhibit bacterial growth and promote wound healing.Several core parameters of all-hydrogel dressing systems are compared with advanced monitoring systems prepared by other methods.Compared with the monitoring system prepared by other methods,the all hydrogel dressing systems sensor has more functions and can add more functional modules through self-healing cross-linking strategy.This self-healing cross-linking strategy provides a novel,facile and rapid idea for the preparation of integrated multifunctional biosystem sensors,and the resulting dressing provides a promising solution for the development of all hydrogel-based system integrating monitoring,diagnosis,and therapy.(3)In this work,a highly sensitive temperature and strain dual-sensing hydrogel electronic skin with self-adhesion,self-healing,decomposition,and superior mechanical properties was developed based on the one-step covalent crosslinking method enhanced by dynamic RS-Ag interactions for various simulation of the physicochemical and sensory functions of the skin.There are three dynamic reversible chemical bonds(hydrogen bonds,RS-Ag bonds,and S-S bonds)in this gel.Based on the synergistic action of multiple hydrogen bonds and RS-Ag bonds,the gel has superior mechanical properties and room temperature self-healing properties.The gel has a wide and sensitive temperature response thanks to the synergistic action of multiple hydrogen bonds,dynamic RS-Ag bonds and dynamic S-S bonds.In addition,the introduction of Ag NWs also converts temperature-sensitive signals into electrical signals,the introduction of S-S bonds also endows the gel the property of decomposition,and the exposed multiple hydrogen bonds give the gel adhesive property.The various properties of the prepared hydrogel were compared with other hydrogel e-skin sensors with dual-sensing of temperature and stress.This hydrogel showed excellent mechanical properties(maximum tensile strength of 0.35 MPa,elongation at break nearly 1800%,compressive stress over 4.43 MPa),excellent self-healing(self-healing rate reached 96.82%(stress),88.45%(temperature),and 73.89%(mechanical property)),self-adhesion(adhesion strength: 0.23 k Pa on pigskin)function and decomposition and removability under the reducing agent DTT(the molecular weight after decomposition was less than 700).Futhermore,the hydrogel could also achieve a highly sensitive sensing of temperature(TCR: 10.89)and stress(GF: 1.469)and displayed superior capability for simulation perception of the human skin to monitor touch,pressing and ambient temperature simultaneously.To the best of our knowledge,this is the most various simulation of human skin’s physicochemical properties and sensing capabilities in the reported hydrogels.This research provides new insights into the fabrication of hydrogels with excellent physicochemical properties and high-sensitive sensory functions for various simulation of human skin.The proposed conductive hydrogel displayed promising applications in the fields of wearable devices,personal health care,and human–machine interfaces.