Cationic Cellulose Nanocrystals-Based Nanocomposite Hydrogels: Achieving 3D Printable Capacitive Sensors with High Transparency and Mechanical Strength.

Po-Cheng Lai,Sheng-Sheng Yu

doi:10.3390/polym13050688

Abstract

Hydrogel ionotronics are intriguing soft materials that have been applied in wearable electronics and artificial muscles. These applications often require the hydrogels to be tough, transparent, and 3D printable. Renewable materials like cellulose nanocrystals (CNCs) with tunable surface chemistry provide a means to prepare tough nanocomposite hydrogels. Here, we designed ink for 3D printable sensors with cationic cellulose nanocrystals (CCNCs) and zwitterionic hydrogels. CCNCs were first dispersed in an aqueous solution of monomers to prepare the ink with a reversible physical network. Subsequent photopolymerization and the introduction of Al3+ ion led to strong hydrogels with multiple physical cross-links. When compared to the hydrogels using conventional CNCs, CCNCs formed a stronger physical network in water that greatly reduced the concentration of nanocrystals needed for reinforcing and 3D printing. In addition, the low concentration of nanofillers enhanced the transparency of the hydrogels for wearable electronics. We then assembled the CCNC-reinforced nanocomposite hydrogels with stretchable dielectrics into capacitive sensors for the monitoring of various human activities. 3D printing further enabled a facile design of tactile sensors with enhanced sensitivity. By harnessing the surface chemistry of the nanocrystals, our nanocomposite hydrogels simultaneously achieved good mechanical strength, high transparency, and 3D printability.

Highlights

We only showed one design for grooved hydrogels for tactile sensors, we anticipate that further optimization for the surface pattern of the hydrogels by 3D printing can achieve robust sensors
We designed a 3D printable nanocomposite hydrogel with cationic cellulose nanocrystals (CCNCs) that for tactile sensors, we anticipate that further optimization for the surface pattern of the hydrogels by 3D printing can achieve robust sensors
The strong physical network of CCNCs and the polymers/nanocrystals association imparted the high toughness of the hydrogels even when the concentration of the filler was reduced

Summary

Introduction

Made of a material with high stretchability and high ionic conductivity, have received much attention for their applications in wearable sensors [1], soft robots [2], and artificial muscles [3]. Hydrogels may be assembled with dielectrics to provide changes of capacitance during mechanical deformation [5]. Transparent hydrogels further allow the transmittance of visible content [6], such as electroluminescence [7] inside the devices. Besides conducting polymers [8] and inorganic nanocomposites [9], hydrogels are popular materials for soft electronics that monitor various human activities. The polymeric networks inside the hydrogel ionotronics have to be tailored to achieve high mechanical strength, high ionic conductivity, and good transparency. The applications of hydrogel ionotronics may be expanded if additive manufacturing can be used to fabricate functional structures

Results

Discussion

Conclusion