Self-Healing, Self-Adhesive Silk Fibroin Conductive Hydrogel as a Flexible Strain Sensor

Abstract

Flexible and wearable hydrogel strain sensors have attracted tremendous attention for applications in human motion and physiological signal monitoring. However, it is still a great challenge to develop a hydrogel strain sensor with certain mechanical properties and tensile deformation capabilities, which can be in conformal contact with the target organ and also have self-healing properties, self-adhesive capability, biocompatibility, antibacterial properties, high strain sensitivity, and stable electrical performance. In this paper, an ionic conductive hydrogel (named PBST) is rationally designed by proportionally mixing polyvinyl alcohol (PVA), borax, silk fibroin (SF), and tannic acid (TA). SF can not only be a reinforcement to introduce an energy dissipation mechanism into the dynamically cross-linked hydrogel network to stabilize the non-Newtonian behavior of PVA and borax but it can also act as a cross-linking agent to combine with TA to reduce the dissociation of TA on the hydrogel network, improving the mechanical properties and viscoelasticity of the hydrogel. The combination of SF and TA can improve the self-healing ability of the hydrogel and realize the adjustable viscoelasticity of the hydrogel without sacrificing other properties. The obtained hydrogel has excellent stretchability (strain > 1000%) and shows good conformal contact with human skin. When the hydrogel is damaged by external strain, it can rapidly self-repair (mechanical and electrical properties) without external stimuli. It shows adhesiveness and repeatable adhesiveness to different materials (steel, wood, PTFE, glass, iron, and cotton fabric) and biological tissues (pigskin) and is easy to peel off without residue. The obtained PBST conductive hydrogel also has a wide strain-sensing range (>650%) and reliable stability. The hydrogel adhered to the skin surface can monitor large strain movements such as in finger joints, wrist joints, knee joints, and so on and detect swallowing, smiling, facial bulging and calming, and other micro-deformation behaviors. It can also distinguish physical signals such as light smile, big laugh, fast and slow breathing, and deep and shallow breathing. Therefore, the PBST conductive hydrogel material with multiple synergistic functions has great potential as a flexible wearable strain sensor. The PBST hydrogel has antibacterial properties and good biocompatibility at the same time, which provides a safety guarantee for it as a flexible wearable strain sensor. This work is expected to provide a new way for people to develop ideal wearable strain sensors.

Mechanism of Self-Healing Hydrogels and Application in Tissue Engineering

“…The dynamic equilibrium of borate ester bond reversibly formed or broken is affected by pH, heat, aqueous media and some biomolecules. For this reason, boronic acid can be applied to sensors that can selectively detect the biomolecules including saccharides, such as glucose [ 21 , 22 ], or can be used as a component in the drug delivery systems [ 23 , 24 ], and self-healing materials.…”

Soft Stretchable Conductive Carboxymethylcellulose Hydrogels for Wearable Sensors

“…From the aspect of fabrication cost, the use of natural polymers can also provide advantages for developing cheap and large-scale bioelectronics [ 25 ]. Nevertheless, the current hydrogels cannot be applied to tissue-interfacing electrodes or interconnects due to their low ionic conductivity, compared to those of metal or carbon materials [ 26 , 27 , 28 , 29 ]. Thus, a significant improvement of the electrical conductivity of those hydrogels is required for utilizing them as electrode and electrical circuit materials.…”

The Progress of Research into Flexible Sensors in the Field of Smart Wearables

“…Flexible sensors with the characteristics of flexibility, foldability, and wearability have become a vital breakthrough [ 1 ]. Using flexible sensors, researchers can convert the external force into electrical signals, perform signal processing, and use flexible sensors in wearable products to monitor human body indicators in real-time and accurately [ 2 , 3 , 4 , 5 ].…”