Researchers at the Terasaki Institute for Biomedical Innovation have created a soft, reusable contact lens that functions as a wearable sensor for detecting the neurotransmitter serotonin in tear fluid. The lens is made from a hydrogel matrix infused with flexible graphene and silver electrodes laid out in serpentine patterns, and coated with ferrocene to enhance serotonin binding. This construction preserves the lens’s transparency, flexibility, and comfort while enabling electrochemical sensing, and the materials are chosen to be biocompatible for extended wear.
This image illustrates the physical design of the serotonin-sensing contact lens described in the article, showing the electrode pattern that enables electrochemical sensing of neurotransmitters in tears
In laboratory tests using artificial tears spiked with known amounts of serotonin, the sensor demonstrated a detection limit of 72-trillionths of a mole per liter (72 fM), which is well below the typical serotonin concentration in human tears of about 15-billionths of a mole per liter (15 pM). The graphene‑ferrocene electrode system reliably distinguished serotonin from other molecules present in tears, confirming the platform’s sensitivity for the target biomarker. Such sensitivity is necessary because serotonin occurs only at trace levels in tear fluid, requiring a sensor that can bind the hormone selectively without interference from larger concentrations of salts, proteins, or other metabolites.
When the lenses were placed on the eyes of ten volunteers before and after they performed acute stress tasks—public speaking and mental arithmetic—serotonin levels in the collected tears fell as stress increased, indicating the sensor can track stress‑related biochemical changes. Additional experiments showed the lenses could withstand more than 28 days of repeated mechanical deformation without loss of function, and a version worn on an anesthetized pig’s eye detected serotonin at 50 and 100 nanomolar concentrations without causing irritation. A proof‑of‑concept wireless iteration incorporating a miniaturized NFC chip and stretchable antenna has been demonstrated, but the team notes that further optimization, safety testing, and validation are required before the technology could be used for continuous, noninvasive stress monitoring in everyday life. The researchers also suggest that measuring serotonin alongside cortisol—where cortisol reflects short‑term stress responses and serotonin provides insight into longer‑term states—could give a more complete picture of a person’s physiological condition, aligning with the long‑term goal of a comfortable wearable platform for continuous biochemical monitoring in everyday life.