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.


Supplementary Figures and Figure Captions
Maximum strain and square resistance as a function of the MWCNTs mass ratio. d) Optical photograph of an Instron mechanical tester (Model 5969). The inset shows a photograph of the tensile test with a stretching machine. To quantify the mechanical properties of strain sensors with different wrapping layers, tensile tests were conducted using the Instron mechanical tester. As shown in d, the strain sensor was fixed between two air jigs of the mechanical tester and then stretched uniaxially at a constant speed of 10 mm·min -1 until the wrapping layers ruptured.
3 Figure S2 a-e) SEM images of NTTFs with different MWCNT weight ratio (a-e: 8, 10, 12, 14 and 16 wt%). All the films were soaked in ethanol for 2 min and dried by nitrogen gun.

Figure S3
Stress-strain curves for TPE fiber and NTTF 5 @fiber sensor. The fabrication strain of the NTTF 5 @fiber sensor was 1600%, and the thickness of the NTTF was 800 nm.     Fiber strain sensor with a length ( l ) of 40 mm was connected to the iron ball (500 g) using two polyester threads. Bottom of the iron ball and ground surface below it was both stuck with a thin layer of double-sided adhesive (~0.05 mm). At the initial state, the sensor and one of the connecting thread was relaxed, while the other was tensioned. The iron ball was in the position right under the fiber sensor with a height of hx  . In the following step, by cutting off the tensioned thread, the iron ball dropped vertically due to its own gravity. At a critical state that the remaining connecting thread was just tensioned, and the TPE fiber was still relaxed, suppose that the iron ball had fallen by a height of h , so its real-time height at the critical state was as the iron ball continued falling down by a height of dx , the fiber sensor would be stretched gradually to a length a dx . During the process, the TPE fiber was under action of a resilience force caused by stretching ( F ) and a pulling force originating from gravity of the connecting iron ball ( G ), while the iron ball was under action of pulling force from the TPE fiber (equal to F ) and its own gravity. According to Newton's second law, acceleration of the iron ball would be Resistance change-time plot for more than 10000 stretch/release cycles, at 4.6 s for each cycle, with applied strain range from 5% to 30%.
14 Figure S12 Response of the NTTF 5 @fiber sensor unit to applied strain (50%) with a frequency of 0.2 Hz. The fabrication ( pre ) was 1600% and the thickness of NTTF was 800 nm.
Figure S13 SEM images of the NTTF n @fiber sensor encapsulated by the TPE film. The used NTTF 5 @fiber sensor was encapsulated by spray coating a thin (~10 nm) TPE film on a relaxed fiber sensor, which did not have significant effect on the sensors performance.  Male Sprague-Dawley rats aged 12 weeks (weight: 220 g ~ 250 g) were provided by Chongqing Daping hospital. Before the experiment, lab rats were fasted for 12 h, with water provided ad libitum. Pentobarbital was injected into the abdomen at a dose of 30 mg kg -1 to anesthetize the rats, and the room temperature was maintained at 22.5 ℃ . In a sterile environment, the epidermis was cut along the front of the tibia of rats. Then, one end of the encapsulated fiber strain sensor was fixed on the tibia and the other end was fixed on the metatarsal bone, and two Cu wire electrodes were connected on both ends of the fiber sensor.
A digital source meter was used to measure the real-time electric current during the test, realizing real-time quantitative assessment of tendon rehabilitation.