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Small and soft, these sensors are designed to track body chemistry for months to years when inserted under the skin. (Credit: Profusa Inc.)

Personalized medicine is one step closer for patients, thanks to tiny, implantable sensors that could give an early warning of a person's developing health problems, indicate the most effective type of exercise for an individual athlete, or even help triage wounded soldiers.

Researchers solved this problem by fooling the body so it can't recognize their sensors as foreign objects. The sensors are smaller than a grain of rice and are made of a hydrogel scaffold that's as flexible as a contact lens. The researchers also made sure their sensor lacks any flat surfaces, which are a dead giveaway to cells that an object isn't natural. As a result, cells and capillaries grow into the sensor's porous structure without triggering the undesirable immune response.

The proprietary hydrogel is a formulation based on poly(2-hydroxyethyl methacrylate), a polymer used to make soft contact lenses. The researchers festoon the hydrogel scaffold with dye molecules that respond to the concentration of an analyte in the blood. The type of dye molecule attached to the hydrogel determines the analyte, such as oxygen, carbon dioxide, glucose or lactate, that a particular sensor can recognize.

A small detector held against the skin, or stuck to it as a patch, shines near-infrared light through the skin, causing the dye molecules to fluoresce more or less brightly depending on the concentration of the analyte. Although this fluorescent light is not visible to human eyes, it can be seen by the detector, which then wirelessly transmits the measurement to a computer or cell phone to record the change in brightness as the analyte concentration fluctuates over time.