High-Strength, Self-Adhesive, and Strain-Sensitive Chitosan/Poly(acrylic acid) Double-Network Nanocomposite Hydrogels Fabricated by Salt-Soaking Strategy for Flexible Sensors

Abstract

As a promising functional material, hydrogels have attracted extensive attention, especially in flexible wearable sensor fields, but it remains a great challenge to facilely integrate excellent mechanical properties, self-adhesion, and strain sensitivity into a single hydrogel. In this work, we present high in strength, stretchable, conformable, and self-adhesive chitosan/poly­(acrylic acid) double-network nanocomposite hydrogels for application in epidermal strain sensor via in situ polymerization of acrylic acid in chitosan acid aqueous solution with tannic acid-coated cellulose nanocrystal (TA@CNC) acting as nanofillers to reinforce tensile properties, followed by a soaking process in a saturated NaCl solution to cross-link chitosan chains. With addition of a small amount of TA@CNC, the double-network nanocomposite hydrogels became highly adhesive and mechanically compliant, which were critical factors for the development of conformable and resilient wearable epidermal sensors. The salt-soaking process was applied to cross-link chitosan chains by shielded electrostatic repulsions between positively charged amino groups, drastically enhancing the mechanical properties of the hydrogels. The obtained double-network nanocomposite hydrogels exhibited excellent tunable mechanical properties that could be conveniently tailored with fracture stress and fracture strain ranging from 0.39 to 1.2 MPa and 370 to 800%, respectively. Besides, the hydrogels could be tightly attached onto diverse substrates, including wood, glass, plastic, polytetrafluoroethylene, glass, metal, and skin, demonstrating high adhesion strength and compliant adhesion behavior. In addition, benefiting from the abundant free ions from strong electrolytes, the flexible hydrogel sensors demonstrated stable conductivity and strain sensitivity, which could monitor both large human motions and subtle motions. Furthermore, the antibacterial property originating from chitosan made the hydrogels suitable for wearable epidermal sensors. The facile soaking strategy proposed in this work would be promising in fabricating high-strength multifunctional conductive hydrogels used for wearable epidermal devices.

Highly sensitive, stretchable and durable strain sensors based on conductive double‐network polymer hydrogels

“…These results demonstrate that agar/PAAm-CNFs hydrogel sensor showed remarkable stability, which is comparable to previously reported hydrogel-based strain sensors. [6,37] Under ambient conditions, the water in hydrogels may volatilize in a long term leading to the upward drift of ΔR/R 0 , although silicone oil is coated on the hydrogel surface. In addition, as shown in Figure S11, the hydrogel strain sensors with other conductive nanofillers, CNTs and GNPs, also exhibited good stability under cyclic stretching.…”

“…In this study, the investigated CNF concentration (0.25 1 wt%) is assumed in the range of tunnelling junction or complete contact, which can be evidenced by the decrease of electrical resistance, for example, from 40 kΩ for the blank hydrogel to 30 kΩ for the hydrogel with 0.75 wt% CNFs. PAA-PEDOT:sulfonated lignin 7.0 3.01 100 [33] CS/PAA-TA@CNC 3.0 1.66 300 [37] PAAm-oxCNTs 3.39 1.54 700 [46] PVA/PVP-CNCs-Fe 3+ 0.478 0.61 200 [70] κ-Carrageenan/PAAm 0.23-0.63 0.83 b 1,000 [63] PAA/PANI 0.6-1.05 1.02 b 1,130 [36] PNIPAAm/PANI 3.9 2.20 120 [71] PVA/PEDOT:PSS 4.4 2.43 100 [34] Gelatin-silver nanowires 4.5 2.10 200 [41] Ethylenediamine-PT/rGO 7.98 3.89 52.8 [72] Abbreviations: CNFs, carbon nanofibers; CNT, carbon nanotube; GF, gauge factor; GO, graphene oxide; PAAm, polyacrylamide; PANI, polyaniline; PEDOT, poly(3,4-ethylenedioxythiophene); SEM, scanning electron microscope.…”

“…[31] To improve the conductivity of hydrogels, two strategies have been commonly used to date. The first is to introduce conductive polymers such as polypyrrole (PPy), [32] poly(3,4-ethylenedioxythiophene) (PEDOT), [33][34] polythiophene, [35] polyaniline (PANI) [13,36] or other polymers [37] into hydrogels. Considering the intrinsic hydrophobic property of most conductive polymers, high concentration of conductive materials may result in aggregation within the hydrogel matrices and, hence, the heterogeneity of the synthesized materials.…”

Enhancing Tissue Adhesion and Osteoblastic Differentiation of MC3T3‐E1 Cells on Poly(aryl ether ketone) by Chemically Anchored Hydroxyapatite Nanocomposite Hydrogel Coating

“…[32] The researches on PAA and other macromolecules forming double network adhesive hydrogels with reversible and strong adhesion (25 KPa) to multiple polar substrates and electrical conductivity also continuously emerge. [33][34][35] Except for that, it was reported that NHS achieves strong chemical adhesion, it has become a research hotspot in recent years, and then it achieves efficient peeling of NHS adhesion through disulfide bonds. [36] In fact, NHS functional groups can undergo cross-linking reactions with amino groups in proteins on the tissue surface to form chemical bonds.…”

Characterization and cytocompatibility of 3D porous biomimetic scaffold derived from rabbit nucleus pulposus tissue in vitro

“…These strategies have been developed mainly for the repair and regeneration of NP of the degenerated IVD [10][11][12]. Over the past few years, great efforts have been devoted to the design of innovative biomaterials for the repair/regeneration of nucleus pulposus in laboratory settings, such as bovine biomimetic NP scaffold [13], multi-polymers [14], injectable hydrogels [15][16][17][18], and cellular and cell-laden hydrogels [19,20]. However, scaffold-based NP repair without giving much regard to NP could not recover the complex microenvironment structure for cell attachment, proliferation, and migration, which might make the disc degenerative change continuously exacerbate.…”