Highly water-retentive thermoresponsive hydrogel-enhanced fiber spr sensor for wearable respiration monitoring

A water-retentive thermoresponsive hydrogel was designed for fiber SPR sensing. • Nano clay and polyhydroxy additives improved bound water and network stability. • The hydrogel exhibited a high thermo-optic coefficient of −5.727 × 10⁻⁴ °C⁻¹. • The fiber-optic SPR sensor exhibits a temperature sensitivity of 3.85 nm/°C. • The sensor enabled stable wearable monitoring of human respiration. Due to the excellent biocompatibility, flexibility and environmental responsiveness of hydrogels, the combination of responsive hydrogels with optical fiber sensors shows great potential for application in wearable devices. However, existing responsive hydrogels generally suffer from poor water-retention capacity, making them prone to dehydration-induced shrinkage or even cracking in air, which limits their ability to meet the requirements of wearable devices for long-term stable monitoring. This study designed a thermoresponsive hydrogel with high water-retention capacity. By reinforcing the hydrogel network crosslinking density with nanoclay and introducing highly hydrophilic materials to increase hydrogen-bond content, more bound water is formed, thereby enhancing the hydrogel’s water-retention capacity and thermal stability. Meanwhile, temperature variations induce reconstruction of water molecule configurations and the hydrogen-bond network, rendering the hydrogel’s refractive index highly sensitive to temperature changes. Experimental results show that the hydrogel exhibits excellent water retention over a temperature range of −10–100 °C and possesses a significant thermo-optic coefficient (−5.727 × 10⁻⁴ ℃⁻¹). When integrated with an optical fiber SPR sensor, it achieves high-sensitivity temperature detection (-3.85 nm/°C). Leveraging the hydrogel’s water-retention properties, the resulting optical fiber SPR temperature sensor is applied for long-term human respiratory monitoring. This study not only expands the application of hydrogels in the field of optical fiber sensing but also provides a new strategy for designing high-performance flexible optical fiber temperature sensors. Download: Download high-res image (238KB) Download: Download full-size image