Preparation And Strain Sensing Properties Of Anti-freezing, Adhesive And Tough Hydrogels

Hydrogel is a soft material and a three-dimensional network polymer with high water content.It is widely concerned by scientists because of its excellent flexibility and stretchability.Conventional hydrogel materials inevitably freeze below 0 °C to lose stretchability and flexibility,which severely limits their practical application.In addition,the adhesion of the hydrogel can make the hydrogel directly adhere to the skin or clothing without other media,which broadens the application of the hydrogel in the field of medical bonding and strain sensing.Therefore,it is very meaningful to prepare an adhesive and tough hydrogel which can be used normally in a low temperature environment.In this paper,we successfully introduced casein into a polyacrylamide hydrogel system to prepare a PAAm/Casein hydrogel.The casein network formed by physical interaction between casein can form a hybrid network with the polyacrylamide network.Therefore,the hydrogel has good mechanical properties,in which the fracture stress reached 168 kPa and the fracture strain reached 1450% or more.The PAAm/Casein hydrogels exhibited excellent adhesion to a wide variety of materials including skin,metal,glass,plastics and rubber,with a maximum peel force of up to 356.25 N/m on titanium surfaces.Moreover,the addition of LiCl not only allowed the aggregation of casein,but also imparted excellent frost resistance and electrical conductivity to PAAm/Casein hydrogel.Through differential scanning calorimetry(DSC)test,the antifreeze temperature can reach-21 ?.At the same time,the conductivity measured by the conductivity test can reach 0.0753 S/cm.On this basis,we designed the PAAm/Casein hydrogel as a wearable strain sensor for monitoring physiological signals and human motions.The experimental results showed that the hydrogel strain sensor can be used not only to monitor macroscopic limb movements in knee joints,wrists and elbows,but also to monitor small vibrations such as vocalization and finger bending.At the same time,the excellent adhesion allowed the PAAm/Casein hydrogel strain sensor to adhere better to human skin and efficiently transmitted electrical signals during vigorous exercise or repeated deformation.PAAm/Casein hydrogel has combined toughness,adhesiveness,frost resistance and electrical conductivity,making it ideal for wearable strain sensors in low temperature environments.This protein-driven hydrogel strain sensor provides novel inspiration for the development of flexible wearable hydrogel devices.The PAAm/Casein hydrogels will also be promising materials for flexible electrodes,supercapacitors and wearable devices in low temperature environments.