Super-stretchable, transparent carbon nanotube-based capacitive strain sensors for human motion detection

Abstract

Realization of advanced bio-interactive electronic devices requires mechanically compliant sensors with the ability to detect extremely large strain. Here, we design a new multifunctional carbon nanotube (CNT) based capacitive strain sensors which can detect strains up to 300% with excellent durability even after thousands of cycles. The CNT-based strain gauge devices exhibit deterministic and linear capacitive response throughout the whole strain range with a gauge factor very close to the predicted value (strictly 1), representing the highest sensitivity value. The strain tests reveal the presented strain gauge with excellent dynamic sensing ability without overshoot or relaxation, and ultrafast response at sub-second scale. Coupling these superior sensing capabilities to the high transparency, physical robustness and flexibility, we believe the designed stretchable multifunctional CNT-based strain gauge may have various potential applications in human friendly and wearable smart electronics, subsequently demonstrated by our prototypical data glove and respiration monitor.

Figure 1. CNT-based transparent capacitive strain gauge.

(a) Schematic procedures of the fabrication of the strain gauges. (b) Operating mechanism of the Poisson strain gauge. (c) Optical picture of two as-prepared strain gauges in a petri dish. Arrows indicate the locations of the devices. (d) Transmittance spectra comparison of the neat CNT films and the corresponding CNT-based strain gauge in the wavelength range of 400–1000 nm.

Figure 2. Morphological and electrical characterizations of the CNT-based electrodes.

(a–b) SEM images of continuously grown CNT films with transmittance of 95% (a) and 90% (b) at 550 nm. Scale bars, 5 μm. (c) Sheet resistance of neat CNT films and CNT/PDMS composite films with different optical transmittance. (d) Relative changes in resistance of a CNT/PDMS composite film under progressively increasing strains.

Figure 3. Systematic strain tests on the CNT-based capacitive strain gauge devices.

(a) Relative changes in capacitance of a strain gauge made from CNT/Dragon skin, under progressively increasing strains from 1% to 300%. (b) Capacitive response of a strain gauge made from CNT/PDMS during both loading (red circles) and unloading (green squares) of a strain of 100%, as well as the linear fit (blue line). (c–d) Results of durability tests: (c) Valley and peak values of capacitance versus number of cycles; (d) Curves of step-and-hold tests for the labeled peak strains and cycles. (e) Capacitive response (red) to fast step-and-hold strains (black, precisely measured using a laser motion monitor). Inset: Close-up of the overshoot. (f) Relative changes in capacitance (red) in response to a series of random values of strain (black, precisely measured using a laser motion monitor). Inset: Average normalized deviations between the measured strains and capacitive response.

Figure 4. Demonstrations of using the strain gauges to detect human motion.

(a) A prototypical data glove. Upper: Still pictures while the finger was gradually folded (I–V) and then unfolded (VI–VIII). Lower: Corresponding capacitive responses. Arrow: An accidental disrupt of the copper wire. (b) Capacitive changes of a strain gauge bonded onto a balloon in response to the inflation and deflation of the balloon. Inset: Optical picture of the strain gauge bonded onto a balloon. (c) Capacitive changes of a device integrated with a bandage responding to the movements of human chest while breathing. Inset: Optical picture of the strain gauge bonded onto a bandage. Note: The strain gauges (indicated by arrows) appear black and non-uniform because the RTV silicone rubber used to bond them was black.