Design of halochromic cellulosic materials and smart textiles for continuous wearable optical monitoring of epidermal pH

The direct on-body monitoring of sweat pH and skin pH can provide vanguard information about a person’s health status and overall fitness or wellbeing. Sweat pH in particular is a readily accessible biomarker that may be determined with low-cost, non-invasive epidermal patches. With this in mind, we have created a diverse set of wearable halochromic materials, suitable for real-time and reversible monitoring of epidermal pH. A range of cellulosic materials in the form of nanocrystals, microcellulose particles, transparent cellulose films and paper were covalently functionalised with a 2-hydroxyethylsulfonyl azo pH indicator dye by vinylsulfonyl conjugation. Comparative analysis of the photo-physical and analytical properties was performed, and the apparent acid–base equilibrium constants — pKapp values of the immobilised indicator — ranging from 5.6 for cellulose paper to 6.9 for nanocellulose particles, cover the physiological range of epidermal sweat and skin pH. A polyester textile — typical of modern sports apparel — was subsequently functionalised with the biocompatible hydrogel containing the halochromic cellulose microparticles. The colour change of the smart textile was found to be fully reversible (RSD = 0.14%) and its pH response highly stable for over 280 min upon continuous exposure to pH buffers (signal change of 0.16%). The halochromic textile was characterised with custom-made low-power wearable optoelectronics combining a reflectometer and real-time data logger, and the system bench-marked against a calibrated laboratory glass pH electrode and meter. The wearable optoelectronic and halochromic textile system was able to continuously monitor pH in buffer solutions over the physiologically relevant range with an accuracy of 0.18 pH and precision of 0.05 pH units.