Tiny brain sensor implanted without surgery dissolves after weeks

A tiny sensor can be injected through the skull with a needle to help monitor brain health before dissolving within weeks. These sensors have been tested in animals, and could one day enable minimally invasive human implants to monitor traumatic brain injuries or neurological conditions such as epilepsy.

“As far as I know, this is the first wireless sensor that can be used to monitor conditions within the body without requiring any surgery,” says Jules Magda at the University of Utah, who wasn’t involved in the work.

The sensor is a cube of soft hydrogel 2 millimetres wide, about the width of a rice grain. Jianfeng Zang at Huazhong University of Science and Technology in China and his colleagues created precisely spaced air columns throughout the hydrogel to make a structured “metagel” sensor. When an external source of ultrasound waves is applied to the sensor, the channels direct the reflection of the ultrasound. The sensor’s shape subtly deforms in response to changing conditions in the brain, such as pressure or temperature, which can be seen in the reflected ultrasound.

“No wires or electronics are needed,” says Zang. “It’s almost like the metagel is acting as a tiny acoustic mirror that changes its reflection in response to its environment.”

Zang and his colleagues showed that the metagel sensors can measure pressure, temperature, pH level and the flow rate of nearby blood vessels when injected into the brains of rats and pigs. They found comparable results to wired probes that are traditionally used to monitor brain health. Their experiments also found that the metagel breaks down into relatively harmless components, such as water and carbon dioxide, within four to five weeks.

To inject this sensor into the brain, a large-diameter needle is required. This could still cause some pain and discomfort, says Magda. He also pointed out the need to verify that the dissolved metagels are nontoxic.

The rats used in the experiment experienced very little brain tissue swelling or immune cell buildup after the sensor implantation and degradation, says Zang. But long-term testing in larger animals is still necessary to show that the metagels perform reliably and safely before the start of human clinical trials, he says.