A tiny wireless implant powered by ultrasound waves could allow contactless monitoring of organ and tissue transplants. Engineers at the University of California, Berkeley worked with physicians from the University of California, San Diego, to develop a sensor smaller than a ladybug. The implanted sensor detects oxygen levels in tissue deep beneath the skin.

Like native tissue, implanted tissue from a donor requires a steady supply of oxygen. Traditional oxygen monitoring has been limited to the tissues closest to the surface of the body. Monitoring oxygen supply to deeper tissue, including organs, has required an MRI: a lengthy and costly diagnostic imaging procedure that doesn’t provide real-time data.

Devices that monitor blood oxygen levels in real-time, such as a finger pulse-oximeter, use infrared light which can’t penetrate far below the skin. Miniature implants powered by electromagnetic waves allow infrared sensors to take tissue readings; however, electromagnetic energy can damage delicate organ tissue, so the depth of these devices also is limited.

Ultrasound waves, in contrast, can pass through internal tissues without causing harm. UC Berkely engineers previously developed an ultrasonic implant that provides electrical stimulation therapy to the sciatic nerve from within the body. Building on this technology, the team incorporated an LED light, an oxygen-sensing film, and an optical detector within the tiny implant. The team also designed a miniscule circuit board that allows them to communicate with the device.

An external ultrasound transceiver that sits on the skin above the implant sends out sonic waves that power the device. This wireless acoustic link also transmits the sensor’s data back through the body. The sensor monitors oxygen in the tissue itself, rather than in the blood, giving a clearer picture of tissue health.

In a paper published in Nature: Biotechnology, researchers describe tests in which the implant accurately monitored tissue oxygenation in sheep. While further development of the technology is necessary for clinical application, the authors also stated that they believe the device could also monitor native tissue and tumors. The researchers also suggest that modifying the technology could lead to implants that measure pH, carbon dioxide, and other important biomarkers.