Study on the preparation of tannic acid-enhanced hydrogels and their properties
Polyphenols from agriculture and forestry biomass have received wide attention in the field of hydrogels due to their wide sources,low toxicity and environmentally friendly.As a typical polyphenol of agroforestry biomass,tan-nic acid(TA)has the advantages of antimicrobial,antioxidant,anti-ultraviolet,good biocompatibility,etc.Hydrogel is a wet and soft material with a three-dimensional cross-linked network structure.Due to their high-water content and biological tissue-like properties,hydrogels show great promise for applications in human motion sensing,soft robotics,biomedicine,and electronic skin.However,traditional hydrogels have low mechanical properties and non-adhesive properties,which largely limit their applications.Therefore,tannic acid is used to improve the properties of hydrogels for different applications.In the gel network,pyrogallol and catechol groups on the tannic acid molecules would form physical cross-links with the polymer using hydrogen bonding,π-π interactions,hydrophobic interactions,etc.,thus enhancing its mechanical properties and functionality.The pyrogallol and catechol groups on the tannic acid also readily form hydrogen bonding interactions with the various materials,thus providing the hydrogel materials with adhesive properties.In this study,the author synthesized the polymeric monomer N-acryloyl-2-glycine(ACG)from glycine nature's simplest structured amino acid.By introducing tannic acid,phenolic hydroxyl groups formed hydrogen bonds with the carboxyl and imino groups on the polyglycine chain,creating physical cross-linking points and increasing the cross-linking density,and tannic acid-reinforced polyglycine hydrogels were successfully pre-pared.The mechanical performance,adhesive strength,electrical conductivity and strain sensing properties of the de-veloped hydrogels were examined.The results showed that,after the introduction of tannic acid,the hydrogel crosslinking network became denser and its stress gradually increased,while the toughness and adhesion strength showed a pattern of first increasing and then decreasing.When the mass ratio of TA/ACG was 1∶100,the as-prepared hydrogel had great mechanical performance,reaching the fracture strain of 162％,tensile strength of 196 kPa,tough-ness of 143 kJ/m3,electrical conductivity of 1.48 mS/cm,and adhesive property(adhesion strength to porcine skin of 17.68 kPa).Besides,the hydrogel also had the good adhesive strength to different substrates(metal,silicone,wood,glass).Therefore,the introduction of tannic acid effectively enhanced the hydrogel network and improved its mechanical and adhesive properties.The prepared tannic acid-enhanced hydrogel was assembled into a flexible resistive sensor,which has good sensitivity to the deformation of the hydrogel and achieves the transformation of motion sig-nals,such as fingers and elbows,into stable electrical signals.This study provides a promising strategy for fabricating a sustainable and self-adhesive bio-based flexible electronic sensor devices.
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