Oxygen saturation is among the most familiar numbers in medicine, but measuring it at the eye remains far from routine. Retinal and choroidal oxygenation can offer valuable clues about ocular and systemic disease, including retinal vascular occlusion and diabetic retinopathy, yet current ocular oxygen measurements typically depend on bulky, specialist equipment and are poorly suited to continuous monitoring.
Now, a team from Tianjin University, China, has developed a smart contact lens designed for non-invasive ocular blood oxygen saturation monitoring. Published in npj Flexible Electronics, the study introduces a flexible plasmonic nano-confinement nanowire photodetector integrated into a soft contact lens.
The system adapts the familiar principle of pulse oximetry to the ocular region. Red and near-infrared LEDs illuminate the closed eyelid, while the lens-mounted photodetector captures photoplethysmography signals from eyelid capillaries. The differing absorption of oxyhaemoglobin and deoxyhaemoglobin at 630 nm and 850 nm is then used to calculate SpO₂.
At the heart of the device is a highly ordered PEDOT:PSS nanowire array embedded with gold nanoparticles. This plasmonic nano-confinement architecture is intended to improve the performance of organic photodetectors, which can otherwise suffer from inefficient charge transport. The nanowires help guide the distribution of gold nanoparticles and support interfacial charge transport, while localized surface plasmon resonance enhances light absorption.
The study authors used nanoimprint technology to fabricate aligned PEDOT:PSS nanowires containing 20 nm gold nanoparticles. Structural characterization showed the nanowires were uniform, well ordered, and retained good film-forming properties after nanoparticle incorporation. Simulations and device testing suggested that 20 nm nanoparticles offered the best performance among the sizes tested, and the resulting photodetector achieved improved external quantum efficiency at both oximetry wavelengths compared with a pristine PEDOT:PSS control.
For a contact lens application, transparency and mechanical stability are essential. The device maintained transmittance above 90 percent across a spectral range of 300–900 nm, and retained more than 96 percent of its photocurrent after 1,000 bending cycles at a 5 mm bending radius. The lens system also incorporated a miniaturized backend circuit and wavelet-threshold denoising to reduce ambient and electronic noise while preserving physiological signals.
In initial finger-based testing, the developed sensor showed approximately 99 percent average agreement with a commercial finger-clip oximeter across the tested SpO₂ range. The PNC nanowire architecture improved test accuracy by 35 percent compared with sensors without the nanowire structure.
The researchers then evaluated the lens in anesthetized rabbits. The lens was placed on the conjunctiva, with red and near-infrared LEDs fixed to the eyelid, while a commercial oximeter on the leg provided a peripheral reference. Under 10, 20, and 60 percent oxygen conditions, eyelid SpO₂ changed in parallel with peripheral SpO₂, falling under hypoxia and rising under hyperoxia. At 20 and 60 percent oxygen, readings from the smart lens closely matched the peripheral oximeter, and no obvious redness, irritation, or visible tissue damage was observed after short-term wear.
The work is still in its early stages, and the authors note that validation covered an SpO₂ range of roughly 84–100 percent, not the severe hypoxaemia range near 70 percent, and that anesthetized animals minimized blinking and eye-motion artefacts. Future work, they say, will need to address improving lens stability, long-term tear-fluid exposure, creating a fully integrated wireless operation, and testing the device in more clinically relevant settings.
