Abstract
Ionic hydrogels with excellent flexibility and good conductivity have great potential in diverse electric devices. However, it remains challenging to improve the biocompatibility of ionic hydrogels. Here, natural and environment-friendly cellulose nanofiber hydrogels were prepared without adding any organic active materials. The cellulose hydrogel networks with free metal ions (Li+/Ca2+/K+) were obtained by soaking in ion aqueous solution of different concentrations in order to endow tunable conductivity, thus a new kind of transparent ionic hydrogels with both excellent sensing performance obtained by a simple, low-cost and harmless process. The free metal ions locked in the negatively charged nanocellulose network through electrostatic interaction provided adjustable conductivity and sensing performance. Hydrogels doped with 4 mol L−1 lithium ions exhibited the best sensing performance with the conductivity of 4.36 × 10–4 S cm−1, and the current response value was as high as 127%. It was worth noting that the strain-sensitive performance of calcium ions was generally excellent even at low temperatures (−30 °C). The antifreezing of the hydrogel improved its service under extreme environment. This kind of hydrogel has great application prospects in artificial intelligence products, such as human healthy monitoring equipment and soft robotics at subzero temperature.
Highlights
Retractable, wearable (Darabi et al 2017), flexible strain sensors (Huang et al 2018) are widely used in the field of electronic devices (Amjadi et al 2016), such as health monitoring (Wan et al 2017), humancomputer interaction (Zhao et al 2017)
The unoxidized cellulose, TEMPO-oxidized cellulose nanofibrils (TOCNs), H+-TOCN, Ca2+-TOCN were analyzed by infrared spectroscopy
The convenient and low-cost method of adding metal ions to the TOCN hydrogels to obtain the antifreezing, high-conductivity, and transparent piezoresistive I-skin was employed in this work
Summary
Retractable, wearable (Darabi et al 2017), flexible strain sensors (Huang et al 2018) are widely used in the field of electronic devices (Amjadi et al 2016), such as health monitoring (Wan et al 2017), humancomputer interaction (Zhao et al 2017). Since the surface of TOC contained a large amount of carboxylate groups, it was easy to obtain a transparent and uniform TOCN dispersion under the action of high-pressure mechanical shearing. These hydrogels were immersed in LiCl, CaCl2, and KCl aqueous solutions with various concentrations, respectively, to endow TOCN hydrogels with ionic conductive properties.
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