A Novel Hydrogel for Use in Wearable Strain Sensors

By Surbhi JainSep 6 2022Reviewed by Susha Cheriyedath, M.Sc.

In an article recently published in the journal Carbohydrate Polymers, researchers discussed the development of self-powered, high-performance strain sensors using carboxymethyl cellulose-assisted polyaniline and conductive hydrogels.

Study: Carboxymethyl cellulose assisted polyaniline in conductive hydrogels for high-performance self-powered strain sensors. Image Credit: Rattiya Thongdumhyu/Shutterstock.com

Background

Flexible supercapacitors (SCs), which have the substantial advantages of quick charging, high power density, and fire safety, have demonstrated considerable potential applications in wearable electronics. Significant work has recently been put towards creating conductive polymers or carbon-based compounds in hydrogel electrodes. The pure all-conducting polymers and carbon-based hydrogels, however, experience embrittlement when stretched.

Electrically conductive fillers were mixed with the non-conductive and elastic hydrogels to create innovative flexible electrodes, which were flexible mechanically but not electrically. Hydrogel electrodes with excellent electrochemical capabilities and mechanical strength have been developed via several attempts, but it is still difficult to achieve all the requirements in one electronic device.

With benefits including low cost, amazing flexibility, high capacitance, and biocompatibility, polyaniline (PANI) holds a lot of potential for SCs. Many initiatives have been made to create biopolymer templates for PANI in order to enhance dispersibility in gel systems. Sodium carboxymethyl cellulose (CMC) is one of the most inexpensive, easily accessible, and environmentally benign polysaccharides.

The CMC could be used to achieve the dual goals of electrochemical reinforcement and conducting polymer dispersion. In addition to increasing the adhesiveness and stretchability of the double-network PEI/PAAM hydrogel, the chemically cross-linking polyacrylamide (PAAM) networks combined with the hyper-branched polyethyleneimine (PEI) macromolecules also endow good solubility for many electrolytes. Less focus has been placed on adding electrically-conductive nanofillers to this hydrogel scaffold.

About the Study

In this study, the authors discussed the development of a PEI/PAAM hydrogel with an interpenetrating double network with the help of CMC and high mass PANI. The all-in-gel CMC-PANI0.8M/PEI/PAAM supercapacitor offered a maximum energy density of 58.82 μWh/cm² at a power density of 14.69 mW/cm², a high specific capacitance of 679 mF/cm², and enhanced capacitance retention of 98% after 5000 cycles with the optimal mass loading of PANI at 9.04 mg/cm². The proposed gadget could function well at extremely high temperatures between 30 and 70 °C and could survive severe bending and compressing deformations. For wearable strain sensors, the CMC-PANI0.8M/PEI/PAAM hydrogel demonstrated good sensitivity and steady electrical performance. The constructed self-powered sensing device could precisely track human activities by coupling the supercapacitor and strain sensor. The CMC-PANI0.8M/PEI/PAAM hydrogel's multifunctional performance was proficient in the field of flexible electronics.

The team polymerized PANI at high concentrations by using a modest amount of CMC as soft templates. To create varied CMC-PANI/PEI/PAAM hydrogels with electronic/ionic dual-conduction, the as-prepared CMC-PANI composites were added to PEI/PAAM networks together with a phosphoric acid (PA) electrolyte. The interactions, structure, and physiochemical characteristics of the produced PANI/PEI/PAAM hydrogels and CMC-PANI composites were investigated. To produce dense, interconnected conductive networks in the PEI/PAAM scaffolds and control the dispersion of high mass CMC-PANI composites, CMC was crucial.

The researchers proposed that the hydrogel electrode's high mass loading of CMC-PANI would enhance its cyclic stability and capacitance performance in flexible SCs. Hydrogel film could be equipped with great durability and high sensitivity as wearable sensors to track and distinguish human motions owing to electronic and ionic dual-conduction. It was predicted that a self-powered sensing system based on the hydrogels CMC-PANI/PEI/PAAM would be able to detect human activity without the need for external power sources.

Observations

Regular granules with a diameter of about 300 nm were present in the CMC-PANI0.3M complex, and some extra maize-like nanorods with dimensions of around 300 nm in diameter and 600–800 nm in length were present in the CMC-PANI0.6M and CMC-PANI0.8M complex. The average size of the CMC-PANI complexes was increased from 295±50 nm to 825±200 nm, according to dynamic light scattering. The shift of the N-H bands at 3150-3350 cm^-1 and the -CONH₂ bands at about 1650 cm^-1 indicated that the CMC-PANI created hydrogen bonds with the PEI/PAAM network. The nitrogen weight ratio for the CMC-PANI0.3M, CMC-PANI0.8M, and CMC-PANI0.6M was 9.21 wt%, 10.20 wt%, and 10.09 wt%, respectively, according to elemental analysis.

The areal loading of PANI was reported to be 4.21, 9.04, and 6.88 mg/cm², respectively, in the thin CMC-PANI0.3M/PEI/PAAM, CMC-PANI0.8M/PEI/PAAM, and, CMC-PANI0.6M/PEI/PAAM, electrodes. While the elongation at break of the CMC-PANI/PEI/PAAM hydrogels increased from 400% to 740% as the mass of PANI increased, the tensile stress was reduced to around 20 kPa. The CMC-PANI/PEI/PAAM hydrogel could withstand temperatures as low as -40 °C without freezing, while the hydrogel lacking PA displayed a broad endothermic peak at about -0.8 °C.

At current densities ranging from 0.5 to 5.0 mA/cm², the galvanostatic charge/discharge curves of three SCs showed good isosceles triangles. At a current density of 0.5 mA/cm², the CMC-PANI0.8M/PEI/PAAM SC displayed the highest areal specific capacitance of 679 mF/cm². A small amount of CMC helped build dense, dispersed high mass CMC-PANI nanocomposites and interconnected conductive networks inside the PEI/PAAM scaffolds. The varied conductive CMC-PANI/PEI/PAAM hydrogels showed excellent anti-freezing capabilities and great recoverability, stretchability, and conductivity.

The flexible SCs based on CMC-PANI0.8M/PEI/PAAM hydrogel electrode provided an exceptional areal energy density of 58.82 μWh/cm² at an exceptional areal power density of 14.69 mW/cm², and exceptional capacitance retention of 98% after 5000 cycles. This corresponded to an optimal areal-specific capacitance of as high as 679 mF/cm². These gadgets could function successfully in a temperature range from -30 °C to 70 °C.

Conclusions

In conclusion, this study elucidated that CMC-PANI/PEI/PAAM had high sensitivity and quick response as a strain/pressure sensor to detect diverse human activities owing to the benefit of ionic/electronic dual conduction. The flexible SCs and strain sensors were integrated to create an ultrasensitive self-powered sensing system that could immediately convert different mechanical motions to electric signals without the need for an external power source.

The authors mentioned that the development of wearable electronic devices can be aided by the large mass of CMC-PANI in flexible hydrogel electrodes created using a CMC template.

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References

Li, Y., Gong, Q., Han, L., et al. Carboxymethyl cellulose assisted polyaniline in conductive hydrogels for high-performance self-powered strain sensors. Carbohydrate Polymers, 120060 (2022). https://www.sciencedirect.com/science/article/abs/pii/S0144861722009651

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