Bandage-Type Autocatalytic PdCl2‑Containing Film as Visual Hydrogen Sensor for Noninvasive Monitoring of Mg-Alloy Biodegradation.

We report a bandage-type film for visual hydrogen gas (H 2(g)) detection, designed to support noninvasive monitoring of Mg-alloy biodegradation. The sensor consists of PdCl2 dispersed in a PEG-40 hydrogenated castor oil matrix, which mitigates drying and enables patch-format use. Upon exposure to H 2(g), Pd2+ is reduced to Pd0, which catalyzes further H2 activation, producing an autocatalytic yellow-to-black transition that enables signal quantification by smartphone-based brightness analysis. The film showed no observable response in air and provided a practical linear range of 0-24.6% H 2(g) under controlled H 2(g) exposures. The device operates in the percent-level H 2(g) regime under controlled exposure conditions relevant to Mg-alloy degradation studies. Limits of detection determined at 10, 20, and 30 min were 3.20, 0.98, and 0.49% H 2(g), respectively. The formulation has improved storage robustness: the baseline color of the sensing film was maintained for up to 4 weeks in freezer storage, and measurable H2 reactivity by the sensing film was retained after up to 4 weeks of storage under ambient, refrigerated, and freezer conditions. Flow-rate testing at 100% H 2(g) showed that the response kinetics were independent of flow rate above 10 mL/min. Practical operation of the sensor was demonstrated by detecting H2 generated from Mg-alloy that was degraded in phosphate-buffered saline through a chicken-skin overlayer acting as a diffusion-barrier surrogate. This visually readable, autocatalytic bandage-type sensor provides a complementary approach for noninvasive monitoring of Mg-alloy biodegradation.