Natural, Biocompatible, and 3D-Printable Gelatin Eutectogels Reinforced with Tannic Acid-Coated Cellulose Nanocrystals for Sensitive Strain Sensors

Abstract

Ionic gels from eutectic mixtures are attracting extensive interest in bioelectronics owing to their nonvolatile nature, low cost, and inherently high ionic conductivity. Biodegradable electronics made of biopolymers envisage a promising future in this field, but unfortunately, they often feature poor mechanics. Herein, we explored tannic acid-decorated cellulose nanocrystals (TA@CNC) as dynamic nanofillers of biocompatible eutectogels based on gelatin and a eutectic mixture composed of choline chloride and ethylene glycol (ethaline). Small concentrations of TA@CNC (up to 1-2 wt %) allow increasing by two-fold the strength (30 kPa) and stretchability (180%) of the eutectogels while improving their ionic conductivity (105 mS·m-1). The reversible physical network of the protein and multiple hydrogen bonding interactions with tannic acid endow these eutectogels with good self-adhesiveness, suitable gel-to-sol transition for 3D printing, and recyclability. We further used the cellulose nanocomposite eutectogels as skin-conformal electrodes for monitoring different motions of the human body with excellent sensitivity in the open air thanks to the low volatility of ethaline. All in all, these results demonstrate a facile strategy to boost the properties of biopolymer eutectogels using inexpensive and renewable raw materials as rigid nanoreinforcers.