If you want a wearable sensor to be convenient and require no interaction on your part, perhaps the best place to “wear” it is inside your body. Continuous glucose meters (CGMs) insert a tiny needle under your skin in order to measure glucose levels around the clock. But there’s a problem with implants; your body’s defense systems eventually identify it as “other” and take measures to reject it such as inflammation and creating scar tissue.

We have written about Profusa‘s injectable biosensors before, but the company recently came out with some impressive news. Sensors that were injected in to volunteers are still working after more than four years. The sensors are made from a porous hydrogel — similar to materials used for soft contact lenses — and are sensitive to specific biomarker chemicals. The sensors fluoresce in the presence of the target chemical, emitting light in proportion with the amount of the chemical. Currently, the system has CE approval in Europe to measure oxygen levels in tissues. Measurements can be taken at any time by holding an external reader against the subject’s skin. The reader emits light than in turn excites the sensor, and the reader then measures the amount of light emitted by the sensor.

The company is working on sensors that are sensitive to other biochemicals, such as glucose and those measured in standard blood tests. The company also has some preliminary results that indicate that tracking changes in the oxygen levels within muscles could be used as an objective basis for determining physical fitness. It may even be possible to develop a single sensor that can detect multiple biomarkers at the same time, so that you can carry a blood lab with you at all times. A system could then issue an alert if one of the measures goes out of range. It could be an effective way to detect illness long before symptoms become noticeable, which could lead to early treatment that would be more effective and less expensive.