Dynamic Cross-Linking Network Construction of Carboxymethyl Starch Enabling Temperature and Strain Bimodal Film Sensors

Abstract

Building stimulus-responsive units in the hydrogel coatings remains challenging for film sensors consisting of alternated layers of inert substrates and hydrogel coatings. An interesting film sensor with a carboxymethyl starch-based hydrogel coating was developed here. The cross-linking networks of carboxymethyl starch play the roles of structure-constructing units and stimulus-controlling units simultaneously, endowing the coatings with thermal sensing and strain sensing capabilities. The dynamic cross-links formed via the boronic ester bonds are temperature-sensitive, releasing or consuming additional acid ions with temperature alteration, and also as primary networks give the hydrogel strength and stretchability with the assistance of semi-penetrated polyacrylamide chains. Therefore, as-prepared flexible film sensors can be used to detect the periodic changes of human temperature and small-scale motion with multiple working modes, discriminating the physical states related to human health. Moreover, this kind of starch-based coating is degradable in a strongly alkaline solution and the inert substrate layer can protect the skin from erosion caused by direct hydrogel-skin contact, and thereby the film sensor is human- and environmentally friendly. This work also proposes a strategy of building temperature-sensitive units in the film sensor via regulating the chemical networks, instead of tuning physical structures.

Regulating Boronic Ester Bonds in Bilayer Hydrogels toward Fabricating Multistimuli-Triggered Actuators

“…It should be mentioned here that the bottom side of the bilayer sample is conductive because the ionization of borax retains large amounts of Na + in this layer. In addition, the ion concentration changes with altered temperature because B–O bonding/debonding releases or needs to consume additional borates. ,, This conveys that the electric conductivity of the CMC-H is temperature-dependent (Figure S1 of the SI). As expected, the CMC-H can be used to detect the temperature alteration as it is attached to the outer wall of a beaker (Figure a).…”

“…This structure is reversible, which depends on the environmental pH and temperature. , Increasing pH or decreasing temperature is conducive to cyclization, and accordingly, this dynamic bonding can be used as a structural design strategy for fabricating the pH- or temperature-triggered substrates to detect biomolecules or to deliver drug components. − The most attractive feature of B–O bonding is that the reactants such as borax are water-soluble, and boronic acid formed via the hydrolysis of borax acts as a Lewis acid to accept electron pairs, forming the complexes with Lewis bases. , In other words, B–O bonding easily occurs in aqueous solutions with water-soluble polyols or other substrates with electron-donating groups. Thus, B–O bonding has been widely used as a networking strategy to fabricate hydrogel materials. − The reported studies around the hydrogels with B–O bonds are mainly involved (a) as a structural unit to construct the secondary (weak) networks or cross-links for improving mechanical performance of hydrogels; , (b) as a responsive unit to endow the networks with reconstruction capability for realizing multistimulus responsiveness; − and (c) as a functional unit to fabricate ionic hydrogels for developing flexible devices. , The as-prepared dynamic hydrogels with B–O bonds show promising applications in the fields of sensors, biomedicine, and tissue engineering. , …”