A Highly Sensitive, Ultra-Durable, Eco-Friendly Ionic Skin for Human Motion Monitoring

Abstract

Ionic conductive hydrogels have shown great potential in areas such as wearable devices and electronic skins. Aiming at the sensitivity and biodegradability of the traditional flexible hydrogel electronic skin, this paper developed an ionic skin (S-iSkin) based on edible starch-sodium alginate (starch-SA), which can convert the external strain stimulus into a voltage signal without an external power supply. As an excellent ion conductive polymer, S-iSkin exhibited good stretchability, low hydrophilicity and outstanding electrochemical and sensing properties. Driven by sodium ions, the ion charge transfer resistance of S-iSkin is reduced by 4 times, the capacitance value is increased by 2 times and its conductivity is increased by 7 times. Additionally, S-iSkin has excellent sensitivity and linearity (R² = 0.998), a long service life and good biocompatibility. Under the action of micro-stress, it can produce a voltage change ratio of 2.6 times, and its sensitivity is 52.04. The service life test showed that it can work stably for 2000 s and work more than 200 stress-voltage response cycles. These findings provide a foundation for the development of health monitoring systems and micro-stress sensing devices based on renewable biomass materials.

Keywords: ionic skin; motion monitoring; sensitivity; starch.

Figure 1

Schematic diagram of the synthetic preparation and working principle of S−iSkin: (a) Preparation process of S−iSkin. (b) Working principle of S−iSkin.

Figure 2

SEM image of S−iSkin: (a) the starch film. (b) the starch–SA film. (c) the ionic starch–SA film. (d) S−iSkin. (e) SEM image of cross section of the starch film. (f) SEM image of cross section of S−iSkin.

Figure 3

Experimental characterization of S−iSkin: (a) FT−IR curves, (b) XRD curves, (c) water contact angle, (d) the curve of stress−strain, (e) tensile strength, (f) elongation at break.

Figure 4

Electrochemical properties of S−iSkin: (a) Change curve of specific capacitance under CV test of S−iSkin. (b) The EIS curve of S−iSkin at 10⁵ Hz–10⁻² Hz. (c) Equivalent resistance (R_e) value of S−iSkin under EIS test. (d) Charge transfer resistance (R_ct) value of S−iSkin under EIS test. (e) Capacitance (C_dl) value of S−iSkin under EIS test. (f) The conductivity (σ) of S−iSkin under the EIS test. (g) The curve of specific capacitance and current density of S−iSkin under GCD test. (h) The energy density and current density curve of S−iSkin under GCD test. (i) The curve of power density and current density of S−iSkin under GCD test.

Figure 5

Sensing performance of S−iSkin and its application in human motion monitoring: (a) Changes in the response voltage of S−iSkin under different stresses. (b) The response voltage change rate of S−iSkin under different stresses. (c) The curve of the stress and response voltage change rate of S−iSkin. (d) The life test of S−iSkin. (e) The application of S−iSkin to human walking monitoring. (f) The application of S−iSkin to human running monitoring.