Digital printing of selective and reversible ion optodes on fabrics: toward smart clothes for epidermal chemical sensing†
While wearable chemical sensors often rely on electrochemical techniques, optical chemical sensors coupled with a smartphone or a miniaturized camera represent an attractive approach to the monitoring of sweat composition. In this paper, we modify real sports fabrics such as polyester–spandex fabrics with rational combinations of sensing chemicals including a pH indicator, an ion exchanger, and an ionophore via one-step inkjet printing. Highly selective and fully reversible pH optodes as well as Na+- and K+-selective optodes are obtained only when the most hydrophobic sensing chemicals are used (e.g., sodium ionophore VIII vs. sodium ionophore VI). These sensors exhibit large color-based responses that can be readily identified by naked eye or analyzed via an iPhone app. Their dynamic ranges well cover the physiological sweat concentrations of the analytes. Compared to most other sensors created on garments, our fabric-based optodes are cost-effective, mass-reproducible by the digital printing technology currently used in the textile industry, and do not significantly compromise the essential properties of fabrics such as flexibility, stretchability, wickability, and breathability.