Holographic hydrogel bandage sensor for continual monitoring of wound healing†

Chronic wounds pose serious health and economic challenges. A low calcium (Ca²⁺) ion concentration during the early stage often indicates infections. Holographic hydrogel sensors offer label-free sensing platforms, providing real-time and continuous detections of analytes upon diffractive wavelength changes detectable by the naked eye or spectrophotometers, improving the Ca²⁺ ion concentration quantification accessibility. Herein, we present a holographic Ca²⁺ ion bandage sensor using carboxylate-containing hydrogels on polydimethylsiloxane (PDMS) substrates for real-time wound-healing assessment through smartphone readout. Simulations are conducted to investigate the effects of mechanical strength on sensitivity. The holographic Ca²⁺ ion sensor replays blueshifts of 35 nm (hue value change of 7) with 0–4 mmol L⁻¹ Ca²⁺ ions, changing colors from dark red to red within 7 minutes. It can accurately and stably (over 24 hours) measure Ca²⁺ ions when bent. The stiffness of PDMS was tuned to balance comfort and sensitivity. In point-of-care settings, holographic bandage sensors, comprising the holographic hydrogel sensor, a backing layer, and a dark cotton layer, can continuously monitor Ca²⁺ ions over 10 hours via a smartphone application using hue values. A guiding square in the application assists users in capturing pictures within the inherently narrow viewing angle range of 20–33°. This holographic Ca²⁺ ion bandage sensor facilitates personalized wound assessment through colorimetric interrogation via smartphone readout.