Sustainable cross-linked poly(glycerol–co–δ–valerolactone) urethane substrates and multipurpose transparent electrodes for wearable electronics

Abstract

Substrates form the backbone of most flexible electronic devices. This study reports sustainable substrates based on a new class of cross-linked poly(glycerol-co-δ-valerolactone) urethanes for flexible and stretchable electronic devices. A cost-effective method is described for preparing these substrates via thermal cross-linking polymerization of poly(glycerol-co-δ-valerolactone) triol with diisocyanates on a glass mold. The developed substrates display high flexibility, stretchability (∼673 %), transparency (∼90 %), thermal stability (∼300 °C), and degradability, essential for next-generation flexible devices. Using synthesized polymers as substrates, we develop stretchable transparent conducting electrodes (TCEs). An innovative fabrication technique involves applying a thin electrospun polyvinyl alcohol (PVA) nanofiber mat as wet film leveling agent to enhance the adhesion and even distribution of sprayed silver nanowires. Through heat and pressure-based nanowelding of silver nanowire junctions, we create TCEs with uniform conductivity, low sheet resistance (∼40 Ω sq−1), and good transparency (∼70 %). To demonstrate the versatility of stretchable TCEs, we fabricated flexible devices like capacitive sensors, curvature sensors, strain sensors, and heaters. The TCE strain sensor exhibits low creep and consistent performance from 5–45 % strain, maintaining signal stability for over 200 cycles at 10 kPa. The fabricated pressure sensor responds to pressures from 0.5–300 kPa with a maximum sensitivity of 2.43 kPa−1 and stability for at least 2600 cycles. The curvature sensor shows increased capacitance at curvatures up to 600 m−1. The flexible heater reaches 85 °C in under 10 s with 5.5 V and responds rapidly under 0–35 % strain. These devices effectively detect human motion, serving as wearable sensors and heaters in cold conditions, demonstrating real-life applicability.