A durable nanomesh on-skin strain gauge for natural skin motion monitoring with minimum mechanical constraints

Abstract

Ultraconformable strain gauge can be applied directly to human skin for continuous motion activity monitoring, which has seen widespread application in interactive robotics, human motion detection, personal health monitoring, and therapeutics. However, the development of an on-skin strain gauge that can detect human body motions over a long period of time without disturbing the natural skin movements remains a challenge. Here, we present an ultrathin and durable nanomesh strain gauge for continuous motion activity monitoring that minimizes mechanical constraints on natural skin motions. The device is made from reinforced polyurethane-polydimethylsiloxane (PU-PDMS) nanomeshes and exhibits excellent sustainability, linearity, and durability with low hysteresis. Its thinness geometry and softness provide minimum mechanical interference on natural skin deformations. During speech, the nanomesh-attached face exhibits skin strain mapping comparable to that of a face without nanomeshes. We demonstrate long-term facial stain mapping during speech and the capability for real-time stable full-range body movement detection.

Fig. 1. Fabrication and characterization of PU-PDMS core-sheath nanomesh conductors.

(A to C) Schematic of the fabrication process. (D to F) Corresponding microscopic images of the (D) PU nanofiber sheet, (E) PU-PDMS core-sheath nanomesh, and (F) Au/PU-PDMS nanomesh conductor. (G) Strain-stress curves of the bare PU nanofiber sheet and PU-PDMS nanomeshes. (H) Comparison of the sheet resistances of the bare PU nanomesh conductor and PU-PDMS nanomesh conductor (N = 10); the inset SEM images show the distinctive junction configurations of both devices. (I) Comparison of the water content of two bottles (one is not covered, and the other one is covered by the device).

Fig. 2. Structural and electromechanical properties of three nanomesh strain gauges fabricated from different diluted PDMS solutions.

(A to C) Microscopic characterization: (A) 1/40, (B) 1/80, and (C) 1/160 (w/w of PDMS/hexane). (D) Comparison of the surface profiles. (E) Comparison of the area fractions. (F) Comparison of the probability on pore size diameter. (G) Dynamic stretching/releasing electrical performance of the 1/40 sample in the strain range of 0.1 to 60%. (H) Linear sensitivities in the range of 0 to 16%, 0 to 35%, and 0 to 60% strain for the 1/40, 1/80, and 1/160 nanomesh sensors, respectively.

Fig. 3. Device sustainability, durability, and long-term stability.

(A to C) Reliable and reversible electrical responses for 12 hours of continuous stretching under 40% strain. (D) Uniform and repeatable electrical responses under 30% strain at frequencies from 0.6 to 3.1 Hz. (E) Cyclic stretching/releasing for 5000 cycles at 60% strain; the insets show 0 to 30 and 4970 to 5000 cycles, respectively (frequency = 1 Hz). (F) Stable conductivity over more than 3 months of storage under ambient conditions (w/w ratio of PDMS/hexane: 1/160).

Fig. 4. Facial skin strain mapping during speech of “a,” “u,” and “o” with nanomesh sensors on the right side of the face and black markers on the left side of the face.

(A) Photograph of a face during speech of “a.” (B) Strain mapping of the right side of the face during speech of “a.” (C) Strain mapping of the left side of the face during speech of “a.” (D) Photograph of a face during speech of “u.” (E) Strain mapping of the right side of the face during speech of “u.” (F) Strain mapping of the left side of the face during speech of “u.” (G) Photograph of a face during speech of “o.” (H) Strain mapping of the right side of the face during speech of “o.” (I) Strain mapping of the left side of the face during speech of “o.” Photo credit (A, D, and G): Yan Wang; The University of Tokyo.