By a simple strategy of immersion in a CaCl 2 solution, carboxymethyl chitosan hydrogels exhibited ultralowtemperature freezing resistance below −50 °C. In addition, the introduction of electrolyte ions endowed the hydrogels with electrical conductivity, showing stable and reversible sensitivity to human activity, such as finger bending, pressing, and pharyngeal swallowing. The conductive carboxymethyl chitosan hydrogels could even be assembled into a two-dimensional integrated array of contact sensors, which successfully perceived the contour and pressure distribution of an object with a certain resolution. These transparent biological-based antifreezing conductive hydrogels are promising to find applications in integrated wearable sensing devices under extremely low temperature environments.
Through a simple strategy of immersion
in a mixed solution of water/ethylene glycol (EG)/lithium chloride
(LiCl), self-healing carboxymethyl chitosan (CA) hydrogels, that is,
CA/N-vinylpyrrolidone-EG-Li+ hydrogels
(CEH) with an ultra-low-temperature freezing resistance below −70
°C were fabricated. The introduction of electrolyte ions and
small-molecule polyol also made these hydrogels highly conductive
(0.8 S m–1) and imparted antidrying property to
them, showing stable and reversible sensitivity to finger-wrist bending
as well as 150 cycles of stretching. Such hydrogels also presented
highly efficient self-healing ability, with a stress–strain
healing efficiency of over 90%. Furthermore, the CEH-based sensors
maintained a stable sensing performance over a wide range of temperatures
below the freezing point (from −10 to −70 °C) and
exhibited stable sensitivity to temperatures with fast response and
no significant hysteresis. The present work is expected to provide
a simple and sustainable route for the preparation of multifunctional
antifreezing conductive hydrogels based on CA, leading to a wide range
of potential applications in soft sensor devices.
On
the basis of the original hydrogen bonding interaction and physical
entanglement, covalent cross-linking and ionic cross-linking were
additionally introduced to construct a carboxymethyl chitosan/allyl
glycidyl ether conductive hydrogel (CCH) through a one pot method
by a graft reaction, an addition reaction, and simple immersion, successively.
The multiple cross-linking networks significantly increased the strength
of CCHs and endowed them with ionic conductivity and an antifreezing
property at −40 °C, which showed stable, durable, and
reversible sensitivity to finger bending activity at subzero temperature.
The CCHs could even be assembled into a triboelectric nanogenerator
(TENG) to provide electric energy, which demonstrated stability against
temperature variation, multiple drawing, long-term storage, or large
quantities of contact-separation motion cycles. CCH-TENG can also
be used as a tactile sensor within the pressure range from 0.4 kPa
to higher than 8000 kPa. This work provided a simple route to fabricate
antifreezing conductive hydrogels based on carboxymethyl chitosan
and to find potential applications in soft sensor devices under a
low temperature environment.
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