Highly Sensitive Skin‐Mountable Strain Gauges Based Entirely on Elastomers

TLDR

Quantifying naturally occurring strains in soft materials, such as those of the human body, requires strain gauges with mechanical compliance equal to or greater than the material. The study presents an all‑elastomer strain measurement device with a gauge factor up to 29 and a Young's modulus approaching that of human epidermis. The device employs thin CB‑PDMS strain gauges and thick CNT‑PDMS interconnects molded into a PDMS substrate, with serpentine interconnects ensuring that the electrical response depends almost entirely on CB‑PDMS strain, and the sheets can be laminated onto skin with minimal constraint. The integrated structures exhibit Young's moduli of 244 kPa, within the epidermis range, and measured wrist strains of 11.2–22.6 %.

Abstract

Abstract Quantifying naturally occurring strains in soft materials, such as those of the human body, requires strain gauges with equal or greater mechanical compliance. This manuscript reports materials and mechanics approaches are reported for an all‐elastomer strain measurement device with gauge factor as high as 29 and with Young's modulus that approaches that of the human epidermis. These systems use thin carbon‐black‐doped poly(dimethylsiloxane) (CB‐PDMS) for the strain gauges due to its high resistivity and strong dependence on strain, and thick carbon‐nanotube‐doped PDMS (CNT‐PDMS) for the interconnects due to its comparatively low resistivity and weak dependence on strain. Devices composed of molded, straight resistors of CB‐PDMS joined by serpentine‐shaped interconnects of CNT‐PDMS, both in a matrix substrate of PDMS, have electrical responses that depend almost entirely on the strain in the CB‐PDMS. Integrated structures of this type have Young's moduli of 244 kPa, which lies within the range of values for the human epidermis. Such sheets can be readily laminated on and form conformal contact to the human skin, with only modest mechanical constraints on natural motions. Strains measured in this mode on the wrist are between 11.2% and 22.6%.