Highly elastic Li2Gd4(MoO4)7: Ho3+/Yb3+ @PMMA/TPU flexible composite fibers for dual-mode encryption and motion tracking

High contrast is realized via switchable excitation in Ho 3+ /Yb 3+ co-doped molybdates. • Electrospun PMMA/TPU binary networks achieved fiber-welding for 400% stretchability. • Hydrophobic encapsulation with a large contact angle impedes moisture ingress. • Excellent thermal stability is verified by 90% retained emission intensity at 373 K. • Strain sensing and dual-mode encryption broaden application prospects for smart skin. Smart adaptive optoelectronic skins represent a critical foundation for next-generation wearable electronics, spurring extensive research into intelligent platforms that simultaneously harness mechanical compliance and excitation-dependent emissions to execute complex, multi-level security interactions. Pristine microcrystalline Li 2 Gd 4 (MoO 4 ) 7 : Ho 3+ /Yb 3+ phosphors display dual-mode emissions, yielding green under near-ultraviolet excitation and orange-red under 980 nm irradiation due to tailored energy transfer pathways, yet inherent structural brittleness completely prevents their direct deployment in flexible configurations. Overcoming this fundamental bottleneck relies on encapsulating these phosphors via electrospinning within a binary polymeric blend consisting of polymethyl methacrylate and thermoplastic polyurethane, yielding hybrid fibrous networks with exceptional stretchability up to 400% elongation driven by microscopic fiber-welded junctions optimizing tensile load distribution. Crucially, sophisticated multi-environmental modulation is manifested within these flexible scaffolds, where the photoluminescence intensity is dynamically governed by tensile strain due to shifting spatial densities, while this dense polymeric network maintains stable luminescence at 373 K and provides a superior hydrophobic barrier maintaining static water contact angles beyond 100°. Synergizing this strain-responsive sensing with the excitation-switchable dual-color outputs enables successful dual-functional integration of precise kinesiologic tracking and multi-level information cryptography, paving a promising way for self-supported smart skins and advanced optical information security.