Survey of Sustainable Wearable Strain Sensors Enabled by Biopolymers and Conductive Organic Polymers

Abstract

The field of wearable sensors has evolved with operating devices capable of measuring biomechanics and biometrics, and detecting speech. The transduction, being the conversion of the biosignal to a measurable and quantifiable electrical signal, is governed by a conductive organic polymer. Meanwhile, the conformality of skin to the substrate is quintessential. Both the substrate and the conductive polymer must work in concert to reversibly deform with the user's movements for motion tracking. While polydimethylsiloxane shows mechanical compliance as a sensor substrate, it is of environmental interest to replace it with sustainable and degradable alternatives. As both the bulk of the weight and area of the sensor consist of the substrate, using renewable and biodegradable materials for its preparation would be an important step toward improving the lifecycle of wearable sensors. This review highlights wearable resistive sensors that are prepared from naturally occurring polymers that are both sustainable and biodegradable. Conductive polythiophenes are also presented, as well as how they are integrated into the biopolymer for sensors showing mechanical compliance with skin. This polymer is highlighted because of its structural conformality, conductivity, and processability, ensuring it fulfils the requirements for its use in sensors without adversely affecting the overall sustainability and biodegradability of resistive sensors. Different sustainable resistive sensors are also presented, and their performance is compared to conventional sensors to illustrate the successful integration of the biosourced polymers into sensors without comprising the desired elasticity and sensitivity to movement. The current state-of-the-art in sustainable resistive sensors is presented, along with knowledge of how biopolymers from different fields can be leveraged in the rational design of the next generation of sustainable sensors that can potentially be composted after their use.

Keywords: PEDOT; biopolymers; resistive sensor; sustainable sensors; wearable sensors.

Figure 1

The potential use of representative biopolymers as sustainable wearable sensors that can be enabled through the conductive polymer PEDOT.

Figure 2

Elastic properties of chitosan films with different plasticizers. (A) The effect of the composition of the film on the maximum strength contingent on the chitosan/PVA weight ratio. Reproduced with Permission [36]. Copyright 2022, MDPI. (B) Stress–strain curves as a function of the degree of deacetylation (DD) of 85% and 95% of chitosan films with (CHFP) and without (CHF) sorbitol plasticizer. Reproduced with permission [30]. Copyright 2014, Elsevier B.V.

Figure 3

Elastic properties of cellulose hydrogels. (A) Stress–strain curves and the effect of the tensile strength and toughness of HPPL hydrogel with different concentrations of hydroxyethyl cellulose (HEC). Reproduced with permission [52]; copyright 2022, Springer Nature B.V. (B) Tensile stress–strain of PAAm, HAPAAm, HAPAAm/CNC and HAPAAm/CNC/PAni-0.5 (C). Photograph showing the stretchability of HAPAAm/CNC/PAni-0.5 near the ultimate strain (2400%) (polyacrylamide (PAAm); hydrophobic association polyacrylamide (HAPAAM); cellulose nanocrystals (CNC); polyaniline (PAni)). Reproduced with permission [53]. Copyright 2024, Royal Society of Chemistry (Great Britain).

Figure 4

Flexible hydrogels from silk fibroin. (A) Schematic diagram of the preparation of the oriented MXene–silk fibroin nanofiber (MSNF) hydrogel. (B) Mechanical property analysis of non-oriented and oriented hydrogel samples prepared with different silk fibroin nanofiber (SNF) mass fractions. Reproduced with permission [55]. Copyright 2024, Royal Society of Chemistry (Great Britain). (C) Tensile stress–strain curves of silk fibroin (SF), tannic acid (TA) and polypyrrole (PPy) blend (SF/TA@PPy). Reproduced with permission [56]. Copyright 2014, Elsevier B.V.

Figure 5

Reaction scheme for the synthesis of hydroxyl derivatives of ethylenedioxythiophene (2) and propylenedioxythiophene (3) from 3,4-dimethoxythiophene (1), along with their representative conductive polymer counterparts (4–6).

Figure 6

Mechanical properties of doped PEDOT in sustainable strain sensors fabricated by blending. (A) Stress–strain curves of cellulose nanofibers (CNF), PEDOT/PSS, and PEDOT/PSS/CNF50 aerogels. Reproduced with permission [85]. Copyright 2018, American Chemical Society. (B) Representative stress–strain curves of PVA/Gly-CNC/PVP/PEDOT with different glycerol weight ratios to glycerol (Gly) and PVA. Reproduced with permission [86]. Copyright 2021, American Chemical Society. (C) Stress–strain curves of PAVK/PANS, PAVK/PAN-CCNWs, and PAVK-PEDOT:PSS-PANC bilayer hydrogels. Reproduced with permission. Polyacrylamide (PAAm); poly(vinyl pyrrolidone) (PVP); poly(N-methylol acrylamide) (PNMA); carboxylated cellulose nano-whiskers (C-CNWs); carboxymethyl chitosan (CMCS); calcium chloride (CaCl₂); potassium chloride (KCl). Hydrogel composition: PAAm/PVP-KCl (PAVK) and PAAm/PANC-CNWs (PANC) [83]. Copyright 2022, MDPI. (D) Tensile stress–strain curves of CNC-PEDOT:PSS/PVA hydrogels before stretching and different healing times after stretching (insets show the digital images of the hydrogel during tensile tests). Reproduced with permission [84]. Copyright 2023, American Chemical Society.

Figure 7

Mechanical properties of sustainable strain sensors obtained through surface-coating with doped PEDOT. (A) Stress–strain curves of PEDOT:PSS-coated cotton fabrics with the mercerization of different amounts of graphene nanoparticles (GNPs) (0%, 10%, 20% and 30%). Reproduced with permission [87]. Copyright 2017, Elsevier B.V. (B) Stress–strain curves of pristine PLF, MWCNT@PLF, and PEDOT/MWCNT@PLF. Reproduced with permission [88]. Copyright 2024, Springer Nature B.V.

Figure 8

Macromovement detection with sustainable strain sensors with doped PEDOT. Response curves of strain sensors prepared from PEDOT:PSS films monitoring human movement: (A) finger-bending, (B) wrist-bending, (C) skin-wrinkling, (D) and walking. Reproduced with permission [89]. Copyright 2022, Elsevier B.V.

Figure 9

Micromovement/vibrational application of a sustainable polythiophene-based strain sensor. Response curves of on-skin sensors based on CMC-PEDOT:PSS film for respiratory humidity (A), drinking (B), and speaking the words hello (C), cellulose (D), conducting (E), and polymer (F). Reproduced with permission [51]. Copyright 2022, MDPI.