Ultrastretchable Fiber Sensor with High Sensitivity in Whole Workable Range for Wearable Electronics and Implantable Medicine

Abstract

Fast progress in material science has led to the development of flexible and stretchable wearable sensing electronics. However, mechanical mismatches between the devices and soft human tissue usually impact the sensing performance. An effective way to solve this problem is to develop mechanically superelastic and compatible sensors that have high sensitivity in whole workable strain range. Here, a buckled sheath-core fiber-based ultrastretchable sensor with enormous stain gauge enhancement is reported. Owing to its unique sheath and buckled microstructure on a multilayered carbon nanotube/thermal plastic elastomer composite, the fiber strain sensor has a large workable strain range (>1135%), fast response time (≈16 ms), high sensitivity (GF of 21.3 at 0-150%, and 34.22 at 200-1135%), and repeatability and stability (20 000 cycles load/unload test). These features endow the sensor with a strong ability to monitor both subtle and large muscle motions of the human body. Moreover, attaching the sensor to a rat tendon as an implantable device allowes quantitative evaluation of tendon injury rehabilitation.

Keywords: fibers; implantable devices; strain sensors; ultrastretchable materials; wearable sensors.

Figure 1

Preparation and characterization of the ultrastretchable fiber strain sensor. a) Schematic illustration of the fabrication procedure for the strain sensor. b) SEM image of the NTTF_n@fiber strain sensor with five layers of NTTFs. The fabrication of the strain sensor was 1600%. c) Optical images showing a fiber strain sensor at a 1100% strained state and relaxed state (Inset), respectively. d) Photograph of the MWCNT/TPE composite film (NTTF) with a filler load of 12 wt% on silica glass. The size of the film was 15 × 8 cm². e) SEM image of a 12 wt% MWCNT/TPE composite film treated with ethanol for 2 min. The inset shows a high‐resolution SEM image. f) Sheet resistance distribution of a 12 wt% MWCNT/TPE composite film measured over an area of 50 × 50 mm². g) Photograph of a MWCNT/TPE composite film (2.5 × 2.5 cm²) on a dandelion. h) SEM image showing the cross‐sectional micromorphology of a MWCNT/TPE composite film treated with ethanol for 2 min. The thickness of the film was 800 nm.

Figure 2

Performance and working mechanism characterization of the NTTF_n@fiber sensors. TPE cores (diameter: 2 mm) were prestretched to 0%, 20%, 50%, 100%, 300%, 1000%, and 1600% strain. Five layers (n = 5) of NTTF (The single‐layer thickness is 800 nm) were wrapped as the sheath and then released to obtain sheath–core fiber strain sensors with different ε_pre. a) Relative resistance change as a function of the tensile strain and linear fittings for a NTTF₅@fiber sensor without prestrain (ε_pre = 0) in fabrication. SEM images with different number on the right show the surface morphology of the NTTF₅@fiber sensor under different strain sensors. (1, 2, and 3 for strain = 0%, 200%, and 300%). b) Change of the resistance of NTTF₅@fiber sensors with different ε_pre during the first release process. c) Resistance change as a function of strain for NTTF₅@fibers with different ε_pre. d) SEM images of NTTF₅@fiber sensors with different ε_pre in the relaxed state. e) SEM images of the fiber sensor edge showing the height change of the buckles. f) Cross‐sectional image of the buckles of NTTF₅@fiber sensors at 0% strain. The fabrication strain was 500%. g) Schematic illustration of the section morphology and current path of NTTF₅@fiber sensors with different fabrication ε_pre. h) SEM images of the surface morphology of the NTTF₅@fiber sensor with different strain (ε). Fabrication of the strain sensor was 1600%.

Figure 3

Investigation of the influence of the NTTF thickness on the performance of NTTF_n@fiber sensors. a) Changes in the resistance of NTTF₅@fiber sensors with different thicknesses (h _s) of the single layer NTTF upon increasing tensile strain. The fabrication strain was 500%, and the number of the layers (n) was 5. b) Sensitivities of the NTTF₅@fiber sensors with different h _s. The values of the NTTF₅@fiber sensors sensitivities were calculated by piecewise fitting of the curves in (a). c–g) SEM images of the surface morphology of NTTF₅@fiber sensors with different h _s at 0% strain. h) Schematic illustration of the section morphology change of NTTF₅@fiber sensors with different h _s.

Figure 4

Sensing performance of NTTF_n@fiber sensors and real‐time muscular movement and human motion detection. a) Real‐time response of a sensor upon application of a quasitransient step strain from 0% to 5%. Inset: Enlarged Figure showing the response time. b) Resistance change‐time plot for more than 20 000 stretch/release cycles at 0.6 s for each cycle with an applied strain of 2%. The testing apparatus is shown in Figure S11a (Supporting information). c) Real‐time variation in the relative resistance under repetitive stretching from ε = 5% to ε = 20% at different frequencies. d–f) Response to motions of d) arm muscle with different gestures, e) the throat when drinking, and f) the knee joint. The NTTF_n@fiber sensor used was encapsulated by spray coating a thin (≈50 nm) TPE film on a relaxed fiber sensor, which did not have a significant effect on the sensors performance. Parameters of the tested strain sensor: ε_pre = 500%, n = 5, and h _s = 800 nm.

Figure 5

Quantitative assessment of tendon rehabilitation using a fiber strain sensor. a) Optical images showing the process of fixing the strain sensor to the hamstring of a lab rat with an injured leg. Left: The hamstring of a lab rat with an injured leg without sensors. Middle: The hamstring of a lab rat with an injured leg with fiber sensors and θ₃ = 90°. Right: θ₃ = 45°. θ₃ is defined as in (b). The used NTTF₅@fiber sensor was encapsulated by spray coating a thin (≈50 nm) TPE film on a relaxed fiber sensor, which did not have significant effect on the sensors performance. b) Schematic illustration of a strain sensor on the hamstring of a rat leg, and the angle of the tibia and metatarsus under the relaxed state was denoted as θ₁ and that the under stretching state was denoted θ₂. θ₃ (θ₃ = θ₂ – θ₁) was defined as the angle of the ankle joint. c) Relative resistance change of the strain sensor as a function of time when the rat was doing cyclic leg stretching exercises with different levels.