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