Tissue Adhesive, Conductive, and Injectable Cellulose Hydrogel Ink for On-Skin Direct Writing of Electronics

Abstract

Flexible and soft bioelectronics used on skin tissue have attracted attention for the monitoring of human health. In addition to typical metal-based rigid electronics, soft polymeric materials, particularly conductive hydrogels, have been actively developed to fabricate biocompatible electrical circuits with a mechanical modulus similar to biological tissues. Although such conductive hydrogels can be wearable or implantable in vivo without any tissue damage, there are still challenges to directly writing complex circuits on the skin due to its low tissue adhesion and heterogeneous mechanical properties. Herein, we report cellulose-based conductive hydrogel inks exhibiting strong tissue adhesion and injectability for further on-skin direct printing. The hydrogels consisting of carboxymethyl cellulose, tannic acid, and metal ions (e.g., HAuCl4) were crosslinked via multiple hydrogen bonds between the cellulose backbone and tannic acid and metal-phenol coordinate network. Owing to this reversible non-covalent crosslinking, the hydrogels showed self-healing properties and reversible conductivity under cyclic strain from 0 to 400%, as well as printability on the skin tissue. In particular, the on-skin electronic circuit printed using the hydrogel ink maintained a continuous electrical flow under skin deformation, such as bending and twisting, and at high relative humidity of 90%. These printable and conductive hydrogels are promising for implementing structurally complicated bioelectronics and wearable textiles.

Recent Advances and Developments in Injectable Conductive Polymer Gels for Bioelectronics

“…While all hydrogels exhibited a reversible conductivity under cyclic strains from 0 to 400%, those containing Au ions possessed the highest conductivity whereas those based on Fe ions presented the lowest conductivity. 62 2.1.3. Ionic Conductive Injectable Hydrogels.…”

“…These hydrogels were directly printed on skin tissue in diverse patterns, showcasing consistently stable conductive properties even when subjected to deformation through tension, bending, and twisting. 62 3.1.2. Muscle Tissue Engineering.…”

Stretchable Surface Electrode Arrays Using an Alginate/PEDOT:PSS-Based Conductive Hydrogel for Conformal Brain Interfacing

“…Herein, we developed stretchable surface electrode arrays using the newly proposed soft tough ionic conductive hydrogel (STICH) composites with a triple-network structure consisting of poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), alginate (Alg), polyacrylamide (PAAm), and carboxymethyl cellulose (CMC) for adaptive conformal brain interfacing ( Figure 1 ). Alg, CMC, and PAAm are most widely used for preparing hydrogel-based applications which feature low moduli, high biocompatibility, good processability, and good ionic conductivity via polyelectrolyte complexes (PECs) between polyanions and polycations [ 81 , 82 ]. In addition, compared to other hydrogels, Alg-based hydrogels have less dynamic change properties due to heat, so when used as implant electrodes later, significant changes in the adjusted mechanical properties are not expected [ 83 , 84 ].…”

Mechanical Characterization of Synthetic Gels for Creation of Surrogate Hands Subjected to Low-Velocity Impacts

“…Other synthetic gels include poly-hydroxyethyl methacrylate (PHEMA), used in pharmaceutical applications, and Polyurethanes (PU), which are used in biomedical applications due to their ability to act as a thermoplastic elastomer. Hydrogels have been studied extensively for drug release and tissue engineering [ 16 , 17 , 18 , 19 , 20 , 21 ] and for monitoring human health [ 22 ].…”