Circuit‐Free Cardiovascular Monitoring via Smartphone‐Readable Skin‐Interfaced Nanophotonic Films

ABSTRACT

Nanoscale mechano‐optical surfaces enable electronics‐free strain sensing, attractive for skin‐interfaced devices, yet reported implementations require laser/spectrometer interrogation, negating this advantage. Here, we report electrically passive mechanical transduction of arterial pulsation into diffractive colour shifts read by unmodified smartphone cameras, enabled by a dual‐function monolithic poly‐dimethylsiloxane (PDMS) film. Using large‐area double‐sided nanoimprinting, we achieve a strain‐sensitive nanophotonic surface on one face of the film and a bio‐inspired 3D structural adhesive on the other. We measure strain‐dependent optical response and reproduce it in color‐mixing optical simulations. In uniaxial cyclic loading tests, 2% strain produces a 9% RGB‐intensity modulation, stable over 1000 cycles. Further, 3D structuring improves adhesive shear strength by 65% on skin over flat PDMS. Hand‐held smartphone recordings in humans ( n = 13) resolve sub‐beat hemodynamics in agreement with clinical reference (per‐beat waveform ρ = 0.94 ± 0.03), exceeding established non‐invasive techniques such as active reflectance photoplethysmography (PPG), imaging PPG, and piezoelectric pulse‐force sensors in simultaneous recordings. Importantly, mechanical transduction at the elastomer–air interface presents an optical cardiovascular monitoring approach agnostic to dermal‐melanin, a known PPG confounder. Together, these advances establish camera‐readable mechanochromic elastomers as versatile materials platforms for wearable cardiovascular monitoring, point‐of‐care diagnostics, and electronics‐free human‐machine interfaces.