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Circuit-free cardiovascular monitoring via skin-interfaced nanophotonics

Torjus L. Steffensen, Arthur G. S. Torvund, Vegar Stubberud, Julia Lövgren, Nils K. Skjærvold, Martin R. Steinert, Angelos Xomalis

TL;DR

This work tackles the need for wireless, noninvasive cardiovascular monitoring by introducing OptoPatch, a circuit-free optical sensor that translates arterial pulsations into smartphone-visible diffraction color shifts using a skin-adhered nanophotonic meta-grating. The authors fabricate flexible PDMS films with nanoscale gratings, validate their optical-mechanical response in benchtop and phantom tests, and demonstrate in vivo measurements that recover arterial pulse waveforms with clinically meaningful features. In a five-participant study, the device achieves high fidelity to reference waveforms, reveals reflected waves and inflection points, and can detect tentative arrhythmic events, all with a simple, battery-free smartphone readout. The platform promises scalable, low-cost cardiovascular monitoring and rhythm screening for clinical and consumer applications, with potential extensions to respiration and broader biomechanical sensing.

Abstract

Continuous cardiovascular monitoring is essential for managing circulatory health and disease, yet most wearable sensors are constrained by reliance on electrical transduction and built-in electronics. We present a circuit-free, wholly optical approach using diffraction from a skin-interfaced nanostructured surface to detect minute skin strains from the arterial pulse. A smartphone camera records the shifting diffraction pattern in real time, removing the need for spectrometers or other optical hardware. In phantom and human studies, we recovered high-fidelity arterial pulse waves and detected benign arrhythmic events in close agreement with a clinical reference. Derived waveforms captured features linked to arterial stiffness, a key cardiovascular risk marker. Our approach uses battery-free, cost-effective, and disposable platforms enabling scalable monitoring for healthcare and broad consumer applications.

Circuit-free cardiovascular monitoring via skin-interfaced nanophotonics

TL;DR

This work tackles the need for wireless, noninvasive cardiovascular monitoring by introducing OptoPatch, a circuit-free optical sensor that translates arterial pulsations into smartphone-visible diffraction color shifts using a skin-adhered nanophotonic meta-grating. The authors fabricate flexible PDMS films with nanoscale gratings, validate their optical-mechanical response in benchtop and phantom tests, and demonstrate in vivo measurements that recover arterial pulse waveforms with clinically meaningful features. In a five-participant study, the device achieves high fidelity to reference waveforms, reveals reflected waves and inflection points, and can detect tentative arrhythmic events, all with a simple, battery-free smartphone readout. The platform promises scalable, low-cost cardiovascular monitoring and rhythm screening for clinical and consumer applications, with potential extensions to respiration and broader biomechanical sensing.

Abstract

Continuous cardiovascular monitoring is essential for managing circulatory health and disease, yet most wearable sensors are constrained by reliance on electrical transduction and built-in electronics. We present a circuit-free, wholly optical approach using diffraction from a skin-interfaced nanostructured surface to detect minute skin strains from the arterial pulse. A smartphone camera records the shifting diffraction pattern in real time, removing the need for spectrometers or other optical hardware. In phantom and human studies, we recovered high-fidelity arterial pulse waves and detected benign arrhythmic events in close agreement with a clinical reference. Derived waveforms captured features linked to arterial stiffness, a key cardiovascular risk marker. Our approach uses battery-free, cost-effective, and disposable platforms enabling scalable monitoring for healthcare and broad consumer applications.
Paper Structure (8 sections, 4 figures)

This paper contains 8 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Sensor concept
  3. Fabrication and characterization
  4. Benchtop quantification
  5. Phantom validation
  6. In vivo minimal contact cardiovascular monitoring
  7. Proof-of-concept in healthy volunteers
  8. Discussion and outlook

Figures (4)

  • Figure 1: Concept of circuit-free monitoring of arterial pulse waves via smartphone camera. A) OptoPatch, a skin-interfaced photonic sensor that translates tissue expansion to smartphone recording. B) Flexible nanoscale meta-grating made of a polydimethylsiloxane (PDMS) film interfacing the patient's skin directly. Inset shows a Scanning Electron Microscopy (SEM) image of the OptoPatch. C) Direct comparison of the OptoPatch with clinical blood pressure reference during an arrhythmic event. D-E) Working principle for measuring arterial expansion using OptoPatch. Insets show how (left) the artery expands in the systolic peak and (right) diffraction colors shift due to the applied strain with respect to the pixels of the observing camera.
  • Figure 2: Device fabrication and benchtop characterization. A) Images of the mold structures on Si and SEM and Atomic Force Microscopy (AFM) of the casted PDMS OptoPatch device. B) Goniometer configuration used for characterization. C) Observed (Obs. top) and simulated (Sim. bottom) color response at 0, 5, and 10% strain for samples with periodicity of 1700 (top) and 600 nm (bottom), respectively.
  • Figure 3: Phantom testing of the OptoPatch. A) A phantom module simulating the physiology of the human wrist. Diffracted colors from the OptoPatch were recorded with the aid of a digital microscope. Red line indicates the position of the artery. External pressure is applied simulating human pulsation with the aid of piezo pump and function generator. B-C) OptoPatch devices of different periodicities (P) were used to record pulsation. Scale bars correspond 1 mm. An equal area of 200$\times$200 pixels was used in all cases (scale difference due to microscope magnification). BG stands for background. Time series were obtained by tracking the average pixel values per channel within the ROI over time. The color channel with the clearest signal is shown (here, green).
  • Figure 4: Sample recordings from a small population. A) Video frame of OptoPatch on human skin with ROI insert and B) recordings from five healthy volunteers showing extracted OptoPatch data (top, blue line) alongside simultaneously recorded with continuous non-invasive blood pressure (NIBP) detector (Finapres finger clamp, bottom red line). C) Averaged waveforms and second derivatives for 10 sec continuous monitoring (dashed lines indicate the beginning of wave reflection). D) Benign arrhythmic events detected (indicated with black arrows). E) Normalized continuous wavelet transform (CWT) scalograms of the OptoPatch (top) and NIBP (bottom) showing clear evidence of an extrasystole event (indicated with white arrows).