Natural deep eutectic solvents as plasticizers for advancing sustainable strain sensors

Sustainability is driving the replacement of fossil fuel-derived materials with renewable alternatives. Transient electronics designed for short-term wear, such as resistive sensors, are poised to benefit from such initiatives by improving their environmental footprint. Their fabrication principally from renewable constituents with established degradability would increase their environmental circularity and eliminate their accumulation as electronic waste owing to their eventual degradation. This raises an important question: which ecologically benign materials can be integrated into resistive sensors for improving the circularity of such transient electronics without sacrificing sought-after metrics such as gauge factor sensitivity, stretchability, softness, and mechanical robustness? Combined soft and stretchable substrates fabricated from sustainable and degradable components, including chitosan and natural deep eutectic solvents (NADESs), were evaluated for use as wearable transient resistive sensors to resolve this question. The best composition of the stretchable self-standing films with NADESs fabricated with different carboxylic acids for resistive sensors was assessed. Mechanical testing revealed that the substrates plasticized exclusively with NADESs from malonic and lactic acids with choline chloride sustained elongations of ca. 160% strain. The elongation was reduced to ca. 90% when incorporating a conducting polymer (pPDS) into the rubber-like films. Functioning strain sensors for transducing mechanical deformation into reliable and consistent electrical signals were indeed possible by blending both NADESs and pPDS into the sustainable stretchable substrate. The NADES substrate with lactic acid was the most sensitive to strain-induced resistance changes with a gauge factor of 2.2. This was confirmed by accurately tracking biomechanical movements, including finger bending and arm extension/flexion. A biomechanical motion sensor can successfully be fabricated from sustainable and degradable components without compromising the gauge factor with the NADES fabricated from malonic acid, emerging as the best-performing strain sensor.