Construction Of Multifunctional Triple-network Hydrogel For Field Sensors

The existing wireless sensor for farmland environment detection in China has high energy consumption and high cost,so it is urgent to develop a new sensor with low cost and low energy consumption.This new type of sensor can change with the change of environment,then produce mechanical energy,then convert mechanical energy into electric energy,and transmit the electric signal to the processor port after amplification,so as to realize real-time monitoring of farmland environment.Polymer hydrogels can react mechanically to external stimuli,which can be used as driving materials of wireless sensor for environmental detection.And polymer hydrogels can be used as biomaterials because of their excellent biocompatibility and elasticity similar to biological tissues.However,the traditional chemical crosslinking hydrogels do not have the elasticity and toughness of biological tissues,and usually show brittleness and poor mechanical properties,which limits their application.Several new types of polymer hydrogels developed in recent years have excellent mechanical properties,but they do not have self-healing properties.Therefore,the design and preparation of wireless sensor hydrogels with high mechanical properties,self-healing properties and environmental responsiveness have important theoretical significance and broad application prospects.This work focuses on improving the strength and toughness of hydrogels,increasing the self-healing ability,low swelling,pH responsivity,antimicrobial and conductivity of hydrogels.Four parts are designed as follows.?1?A novel design strategy for triple-network structure hydrogels with high-strength,tough and self-healing properties:In this chapter,in order to make the mechanical strength of the hydrogel reach the required mechanical strength of the sensor,we prepared a novel triple-network?TN?structure hydrogel composed of polyacrylic acid?PAA?,agar and polyvinyl alcohol?PVA?.The obtained TN hydrogel has a superior mechanical property as well as self-healing property feature.In the TN hydrogel system,PAA-Fe³⁺polymer mainly providing hydrogel self-healing properties by the ionic coordinates between-COO^-and Fe³⁺as the first network,while,agar and PVA polymers gel mainly providing hydrogel high mechanical properties as the second and third network.The structure of TN the hydrogel can be controlled by adjusting the content of Fe³⁺and composition in the three polymers.When 1.5 wt%Fe³⁺in the TN hydrogels,the PAA/Agar/PVA TN hydrogels possess high mechanical strength,the fracture strain and fracture stress were 497%and 450 kPa,respectively,and the compression strength reached up to 1337 kPa under the compression deformation of 80%.Besides,the TN hydrogels exhibited excellent self-healing ability,when healed 72%at tensile stress and84%at tensile strain after 24 h without adding any solvent or additive.?2?Preparation of pH-responsive PAA-g-QCE/PVA hydrogel with high strength,self-healing and antimicrobial properties:Based on the previous chapter,this chapter endows the hydrogels with antimicrobial and pH-responsiveness,and improves the adaptability of hydrogels in natural environment.Novel antibacterial hydrogels with superior mechanical and self-healing properties are developed via the UV-initiated copolymerization of polyacrylic acid?PAA?-grafted quaternized cellulose?QCE?and polyvinyl alcohol?PVA?.The QCE functioned as an antibacterial agent,resulting in excellent antibacterial capability?antibacterial rate>93%?.The hydrogels are thus protected from microbial attack in natural environments,prolonging their lifetime.The PVA functioned as a physical cross-linker,resulting in superior mechanical properties.At PVA and QCE contents of 8%and 1.5%,respectively,the strain and stress at break of hydrogel were 465.37%and 1.13 MPa,respectively.The hydrogel maintained good self-healing properties owing to ionic bonding between the ferric ions and carboxylic groups,and hydrogen bonding between the PVA molecules.The hydrogel was responsive to pH;its water-holding ability could be controlled by changing the pH.The material is simply prepared and used.Hydrogels with such excellent properties could be applied in various biomedical fields.?3?Construction of pH-responsive PAA/Gela/PVA controllable swelling high strength self-healing triple-network hydrogel:This chapter focuses on the swelling properties and toughening mechanism of triple-network hydrogels.A polyacrylic acid?PAA?/gelatin?Gela?/polyvinyl alcohol?PVA?hydrogel was prepared by copolymerization,cooling,and freezing/thawing methods.This triple-network?TN?structure hydrogel displayed superior mechanical properties,a low swelling ratio and self-healing properties.The superior mechanical properties are attributed to the triple helix association of Gela and PVA crystallites by reversible hydrogen bonding.The characterization results indicated that the fracture stress and strain were 808 kPa and370%respectively,while the compression strength could reach 4443 kPa and the compressive moduli?up to 39 MPa?under the deformation of 90%.The hydrogen bonding in the PVA,contribute to maintain and improve the self-healing ability of the hydrogels.Every hydrogels exhibited a higher swelling ratio under alkaline conditions,the swelling ratios of PAA,PAA/PVA and PAA/Gela hydrogels are 27.71,12.30 and9.09,respectively.The PAA/Gela/PVA TN hydrogel showed the lowest swelling ratio?6.57?among these hydrogels.And the swelling rate of hydrogel can be controlled by changing the contents of three polymers.These results indicate that the novel TN hydrogels possess good environmental adaptability and have potential applications in the biomedical engineering and sensor field.?4?Preparation of PAA/CMC/PVA high-strength conductive hydrogels and pH-driven sensing simulation experiment:This chapter focuses on the feasibility of hydrogels in pH-driven sensor.Based on the previous three chapters,carboxymethyl cellulose?CMC?/PAA/PVA triple-network high-strength hydrogels were prepared.When the content of PVA was 8 wt%,the fracture stress and strain of the three-network hydrogels were 357.04 kPa and 300.67%,respectively.The compressive strength of the triple-network hydrogels could reach 2012.13 kPa,and the modulus of elasticity was169.05.kPa.The swelling experiment proved that the hydrogel had a good response to pH.NaCl was added to the hydrogel to give the hydrogel electrical conductivity,and the hydrogel had a certain mechanical response.Finally,conductive hydrogels are simulated to detect environmental pH.When the environmental pH changes,the shape and volume of the hydrogels will change.The mechanical energy generated by the volume changes triggers the switch lever and emits electrical signals.Real-time monitoring of environmental pH can be achieved.