“Compared with existing studies on wireless implantable sensors, our metagel sensor offers advantages specifically in regard to implant size, decoupled multiple signals and biodegradability,” the researchers wrote in a paper published in Nature on June 5.

“The injectable metagel ultrasound sensor we invented uses advanced acoustic metamaterial technology and is only 2×2×2 cubic millimetres in size, just like a sesame seed,” said Zang Jianfeng, corresponding author and a professor at Huazhong University of Science and Technology in Wuhan.

“Through an external ultrasound probe, it can wirelessly monitor changes in physiological parameters,” Zang said in a video released by the university.

When the researchers tested their gel sensor in rats and pigs, they found that it remained stable in the brain for up to a month, and completely degrades after four months.

The wireless and biodegradable design means that patients who have undergone cancer treatment or have a brain injury, and who need sensors placed into the skull for monitoring, would no longer need to undergo additional surgeries to remove them.

It would also prevent the risk of infection that can occur due to the openings left in the skin from wired clinical probes, which are used to transmit data, the paper said.

While development in the area has moved towards wireless sensors over wired sensors, scientists developing wireless probes have faced many challenges for clinical application, including implant size, biocompatibility and range of communication.

“Despite substantial efforts, current electronics-based research manages to tackle only a subset of these challenges,” the researchers wrote.

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To overcome these issues, the team designed a small, injectable wireless ultrasound sensor that naturally degrades in the body after a period of time.

“Implanted into intracranial space with a puncture needle, the metagel deforms in response to physiological environmental changes, causing peak frequency shifts of reflected ultrasound waves that can be wirelessly measured by an external ultrasound probe,” the team wrote.

Changes to the shape of the gel sensor, which features small holes or air columns, allows for an external ultrasound probe to capture the physical environment within the brain.

One gel sensor can independently measure a specific parameter such as temperature or pH based on the material it is made from, and multiple gels can be inserted at once to measure different parameters.

The gel sensors can be picked up by an external ultrasound probe more than 10cm away, “allowing thorough detection of brain tissue,” the team said.

The researchers tested the sensors in rats and pigs by injecting them alongside wired clinical sensors to compare measurement results.

When a rat was given an abdominal compression to simulate intracranial pressure, the team found that “the metagel outperformed the clinical [intracranial pressure monitoring] probe in terms of pressure resolution and time accuracy”.

They also found that the metagel registered temperature changes “akin to a commercial wired temperature probe”.

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“In addition we also detected traumatic brain injury in the rat using metagels, providing further insight into its potential for clinical application,” the team wrote.

In order to test the gels in a clinical model more closely resembling humans, the researchers also placed them into the brain of a pig alongside a clinical probe, as well as a device placed into the pig’s spine to measure and manipulate intracranial pressure.

The team found that the metagel not only accurately captured pressure changes “surpassing” the clinical probe in terms of precision and resolution, but it also measured changes in pressure caused by the pig breathing.

“By contrast, the clinical [intracranial pressure monitoring] probe could not measure such respiration-induced patterns,” the team wrote.

The team’s gel sensors begin to degrade after five weeks, however Zang said that they can be specifically designed to work for a longer period of time if needed.

Zang said that as their work focuses on the innovation of medical devices using soft materials, the team will discuss with clinicians to determine how their gel sensor could work in a clinical setting.

“Compared with existing commercial intracranial pressure sensors, ours can greatly alleviate the pain of patients, and we also hope that our research can contribute to people’s life and health,” Zang said.