Preparation Of Protein-based Hydrogels And Their Applications In Flexible Sensors

Flexible and wearable devices can be seamlessly attached to skin for realizing real-time and continuous monitoring of human motions and physiological information,which have broad application prospects in the fields of medical diagnosis and kinematics analysis.Hydrogels are a kind of typical“soft and wet”materials with flexibility,tissue similarity and stretchability,exhibiting unique advantages in flexible sensing and have become promising candidates for the development of new-generation wearable sensors.At present,the design and development of hydrogel-based flexible sensors has achieved significant progress,but is still difficult to accommodate the diverse requirements in practical applications due to the weak interface adhesion between devices and skin,poor environmental adaptability,low transparency as well as unsatisfying recyclability and biocompatibility.In regard of the problems above,this paper systematically explored the feasibility of protein-based materials for enhancing the adhesiveness and toughness of hydrogels.Furthermore,a series of studies and discussions were carried out,focusing on improving the transparency,remoldability,temperature tolerance and biocompatibility of hydrogels,and the wearable sensing application of hydrogels was realized.The main research contents and results are as follows:(1)In view of the weak adhesion of most hydrogels,the feasibility of the basic structural unit of protein(amino acid molecules)in tackifying the hydrogels was explored.The double bond functionalized lysine(Lys MA),as a model of amino acids,was mixed with acrylamide to form adhesive and tough hydrogels via one-step free radical polymerization reaction under the thermal initiation system.On the one hand,due to the presence of amino and carboxyl groups in Lys MA molecules,hydrogels could firmly bond to various materials(metal,skin,plastic,glasses,silica rubber,etc.)through hydrogen bonds and metal complexation,and showed excellent repeatable adhesiveness.On the other hand,the introduction of Lys MA could form intermolecular hydrogen bonds in hydrogel network,and further enhanced the cross-linking density of hydrogels.Therefore,compared with polyacrylamide hydrogels,the Lys MA-modified polyacrylamide hydrogels possessed excellent mechanical strength.(2)In the above research,the effectiveness of the basic structural unit of protein in improving the adhesion of hydrogels was verified.Thus,in the second part of this paper,inspired by the structure and composition of the dermal layer of skin,water-soluble collagen was directly introduced into the lithium chloride-contained chemically cross-linking polyacrylamide network for designing and preparing a kind of protein-based hydrogel materials with excellent self-adhesion and temperature tolerance.Collagen played a key role in improving the stretchability of hydrogel,and the hydrogel exhibited good elasticity and skin-like modulus.In addition,the peeling measurements showed that the collagen had obvious effect in enhancing the adhesion of hydrogel,because the amino acid groups(amino group,carboxyl group,hydroxyl group,etc.)rich in collagen could form multiple physical interactions with various organic/inorganic materials,thus endowing the hydrogels with universal and repeatable adhesion.The hydrogel could be firmly attached to skin,which helped to avoid the occurrence of interface friction and detachment in the process of human movement,thus greatly improving the sensitivity and reliability of electrocardiography(ECG)signal acquisition and human movement tracking.Besides,long-term stable signal transmission could be achieved at ambient or low temperature.(3)In this part,hydrogels with high transparency,self-adhesion,anti-icing and anti-drying ability were prepared by combining the adhesion factor of hydrolyzed keratin with lithium chloride-containing chemically crosslinked polyacrylamide network.On the basis of effectively improving the adhesiveness,the low transparency of hydrogel caused by the intrinsic color of collagen in the previous part was solved by using hydrolyzed keratin.The prepared hydrogel had excellent transparency and the transmittance was over95%.Moreover,the transmittance was almost not affected by hydrolyzed keratin contents and thickness of hydrogels.Besides,hydrolyzed keratin is a kind of low-cost materials derived from waste products,such as wool,feathers,hoofs,horns,claws and so on,which has the value of large-scale industrialization application.The hydrogel-based sensors possessed the advantages of high sensitivity(GF=6.2),fast response(103 ms)and negligible hysteresis,which could be directly attached to skin for monitoring whole-body movements and served as a writing board to quickly recognize handwritten letters.Besides,the hydrogel sensors had excellent freeze resistance and anti-drying ability,and could maintain adhesion,flexibility and conductivity after storing in environmental conditions for 5 days or low temperature(-20°C)for 24 h,which were suitable for motion signal monitoring in a wide temperature range(-20°C to 50°C).(4)In the above three experiments,polyacrylamide was used as the skeleton of gel network,but the incomplete polymerization reaction would inevitably result in the residue of acrylamide monomer,thus affecting the biocompatibility of hydrogels.In addition,hydrogels lacked remoldability and could not be recycled after damage.Therefore,in this part,hydrolyzed keratin and sodium chloride were added into the polyvinyl alcohol(PVA)solution to prepare a fully physically crosslinked hydrogel with skin-matching performances,biocompatibility and remoldability through one-step freeze-thaw method.Based on the hydrogen bonding interaction between hydrolyzed keratin and PVA chains as well as the entanglement of PVA chains induced by salting-out effect,the hydrogel exhibited robust strength(1360 k Pa)and skin-like properties,for example,high toughness(3.45 MJ/m~3),low modulus(110 k Pa)and anti-fatigue-fracture performance(≥1000cylces).H&E staining tests showed that the hydrogel had good histocompatibility and could maintain the long-term safe integration with human skin.The hydrogel-based sensors displayed high strain-sensitivity,low detection limit,fast response time,and cycle durability,which were appropriate for the continuous monitoring of whole-body motions and accurate identification of motion status.Besides,the hydrogel exhibited excellent recyclability,and could be reprepared through the heating-cooling process without sacrificing the original mechanical,conductive and sensing properties.