Recent Advances in Natural Functional Biopolymers and Their Applications of Electronic Skins and Flexible Strain Sensors

Abstract

In order to replace nonrenewable resources and decrease electronic waste disposal, there is a rapidly rising demand for the utilization of reproducible and degradable biopolymers in flexible electronics. Natural biopolymers have many remarkable characteristics, including light weight, excellent mechanical properties, biocompatibility, non-toxicity, low cost, etc. Thanks to these superior merits, natural functional biopolymers can be designed and optimized for the development of high-performance flexible electronic devices. Herein, we provide an insightful overview of the unique structures, properties and applications of biopolymers for electronic skins (e-skins) and flexible strain sensors. The relationships between properties and sensing performances of biopolymers-based sensors are also investigated. The functional design strategies and fabrication technologies for biopolymers-based flexible sensors are proposed. Furthermore, the research progresses of biopolymers-based sensors with various functions are described in detail. Finally, we provide some useful viewpoints and future prospects of developing biopolymers-based flexible sensors.

Keywords: biocompatible; electronic skins (e-skins); flexible strain sensors; functionality; natural biopolymers.

Figure 1

The advantages of three natural biopolymers and their composites for flexible sensors.

Figure 2

(a) Tunable and reversible turing-pattern microstructures of Cel-IL dynamic gel, (b) a piece of stretchable C-e-skin can attach and conform to a human external wrist, (c) current waveforms of C-e-skin can sense breathing (calm, deep, and rapid breathing, respectively) [49] Reproduced with permission from ref. [49]. Copyright 2020, Elsevier; (d) change in strain sensitivity of PPy/PVA (left), CNC_Fe/PVA (middle purple), CNC_APS/PVA (middle green), CNF_Fe/PVA (right red), and CNF_APS/PVA (right orange) before and after self-healing, (e) self-adhesive test of CNF_Fe/PVA on different substrates [50]. Reproduced with permission from ref. [50]. Copyright 2019, American Chemical Society.

Figure 3

(a) Schematics and photos of conformal attachment of AgNW/silk electrode, (b) ECG signals captured by electrodes attached to bump of the wrist. The nonconforming nature of the commercial electrode induces a large amount of noise in the signal. Reproduced with permission from [51]. Copyright 2018, American Chemical Society. Schematic illustration and simultaneous sensing performance of the combo e-skin sensor under various stimuli (c) exhaling stimulus and (d) finger pressing stimulus. Reproduced with permission from [52]. Copyright 2017, American Chemical Society.

Figure 4

(a) Fabrication process of microstructured fibroin adhesive (MSFA) and the schematic of an MSFA on the skin of the wrist, (b) scanning electron microscopy (SEM) images of bare (top) and fibroin (bottom) coated micropillars with a diameter of 80 μm and heights of 40 μm. Reproduced with permission from [58]. Copyright 2020, American Chemical Society; (c) the schematic illustration structure of pristine silk and carbonized silk, (d) multiple cycles of pressure response under 10 Pa–5 kPa, (e) the pressure response at high frequency and the response time within 16.6 ms, (f) the durability test of the pressure sensor over 10,000 loading-unloading cycles at a frequency of 0.5 Hz under an applied pressure of 2.5 kPa. Reproduced with permission from [59]. Copyright 2017, WILEY-VCH.

Figure 5

(a) Fabrication of sheath-core structured graphite/silk strain sensors through a dry-Meyer-rod-coating process, (b) photograph of the flexible strain sensor, (c) photograph of the strain sensor subjected to strain of 0% and 15%, (d) comparison of the sensitivity and elongation of sheath-core graphite/fiber strain sensors with different core fibers, including hair, silk, polypropylene (PP), and Spandex fibers. Reproduced with permission from [60]. Copyright 2016, American Chemical Society; (e) the response of the MoS₂/CSFs sensor for monitoring healthy adults’ wrist pulse, (f) the response curves for the sound signal when the volunteer spoke “silk” “as” “wearable” “sensor”, (g,h) electrochemical properties of MoS₂/CSFs as the anode in Li-ion batteries. Reproduced with permission from [61]. Copyright 2020, American Chemical Society.

Figure 6

(a) Overview of sensing applications of the SF/Mxene flexible sensor for the detection of various physiological human motions, (b) Photograph of the SF/Mxene flexible sensor attached onto the throat and response signals of deglutition motion, (c) the response of SF/Mxene flexible sensor in monitoring finger bending under different angles. Reproduced with permission from [63]. Copyright 2020, Elsevier; (d) schematic diagram and photograph images of the integrated strain sensor and the bio-TENG on a bandage, and the applications of the integrated devices to measure various human motions, (e) photograph images and outputs of the bio-TENG devices attached to a cell phone. Reproduced with permission from [64]. Copyright 2019, Elsevier.

Figure 7

(a–c) Optical micrographs of NIH3T3 cells exposed to blank, 50% and 100% of hydrogel extract, (d) Viability of NIH3T3 cells after incubating with hydrogel extracted DMEM at different concentration, (e) weight loss of okara hydrogels during degradation time, pieces of okara cellulose hydrogels in the soil on the first day (f) and day 28 (g) in the biodegradable test. Reproduced with permission from [69]. Copyright 2019, Nature Publishing Group; (h) The DSC curves of the PAM, HPMC-g-AN/AM_0.6, HPMC-g-AN/AM_0.6-ZnCl_2–15%, and HPMC-g-AN/AM_0.6-ZnCl_2–25% hydrogel at the temperature range from 25 °C to 40 °C. Reproduced with permission from [70]. Copyright 2020, American Chemical Society; (i) illustration processing of TWF for flexible electronics application [73]. Reproduced with permission from ref. [73]. Copyright 2020, American Chemical Society.

Figure 8

(a) The TEMPO treatment process significantly imparts negative surface charge on the cellulose nanofibers for highly selective cation transport and photographs of the hydrated pristine BC. Reproduced with permission from [76]. Copyright 2018, American Chemical Society; (b) typical tensile stress/strain curves, (c) compressive stress/strain curves of BCs-PDESs and PDESs, (d) possible energy dissipation mechanism of the double polymer chain network under stretching. Reproduced with permission from [77]. Copyright 2020, American Chemical Society.

Figure 9

(a,b) Sensitivity of rGO/cellulose composite to temperature and humidity, (c) schematic for in-situ monitoring electrical resistance changes of rGO/cellulose composite film placed at a distance up to 100 mm from one’s nose, (d) dependence of R_rel on time for in-situ monitoring human breathing cycles of inhalation and exhalation. Reproduced with permission from [83]. Copyright 2018, Royal Society of Chemistry; (e) In-situ SEM images and the corresponding schematic illustration for the microstructure change of the strain sensor during the tension process, (f–h) photographs of different items onto the e-skin and the corresponding spatial pressure distribution mapping based on the resistance variation. Reproduced with permission from [84]. Copyright 2020, Royal Society of Chemistry; (i) optical images of the 3D-printed TOCNFs/Ti₃C₂ fabrics with woodpile, fishing net structures and other designed geometric structures. Reproduced with permission from [85]. Copyright 2019, WILEY-VCH; (j) compressive stress-strain curves of aerogels for CNCs-PEG aerogels in the dry state and in water, shape recovery percentage of aerogels of CNCs-PEG aerogels under different compressive strains: in the dry state and in water. Reproduced with permission from [86]. Copyright 2020, American Chemical Society.

Figure 10

(a) Fruit growths of cucumbers were being measured. LCR meters were used to measure the real-time resistance values of the sensors written on a cucumber. Reproduced with permission from [87]. Copyright 2017, WILEY-VCH; (b) optical microscopy images of a CS/CNT fiber at original, damaged and healed states to turn on or off a LED light bulb, schematic illustration of the healing process of a CS/CNT fiber exposed to water vapor, repeated healing and recovery of electrical properties for five cuts on the fiber. Reproduced with permission from [90]. Copyright 2018, Royal Society of Chemistry; (c) self-adhesion properties of CS/PAA/TA@CNC-60, hydrogels tightly adhered between skin and various substrate surfaces, including rubber, wood, plastic, PFTE, metal and glass, schematic illustration of lap shear test, representative curves of adhesion shear force versus displacement for hydrogels with various substrates (wood, copper and glass). Reproduced with permission from [94]. Copyright 2019, American Chemical Society; (d) stress-sensing models of PANI/BC/CH aerogel, the schematic diagram of the LED light was applied to test the electrical response of the PANI/BC/CH aerogel under a power supply of 2 V in a series circuit, response of the LED light varies with the applied pressure on the PANI/BC/CH aerogel, current-voltage (I-V) curves of PANI/BC/CH aerogel under different pressures, the curve of ΔR/R₀ versus time when PANI/BC/CH aerogel-based sensor was attached on the bottom of a shoes as a pedometer for walking. Reproduced with permission from [96]. Copyright 2019, WILEY-VCH.