Shape-morphing brain sensor adheres to curved surfaces for ultrasound neurostimulation

Transcranial focused ultrasound, a non-invasive technique to stimulate specific areas of the brain using high-frequency sound waves, could be a promising treatment strategy for many neurological disorders. Most notably, it could help to treat drug-resistant epilepsy and other conditions associated with recurrent tremors.

Researchers at Sungkyunkwan University (SKKU), the Institute for Basic Science (IBS) and the Korea Institute of Science and Technology recently developed a new sensor that could be used to perform transcranial focused ultrasound on patients. This sensor, introduced in a paper published in Nature Electronics, adapts its shape and can closely adhere to cortical surfaces, allowing users to record neural signals and stimulate specific brain regions via low intensity ultrasound waves.

“Previous research on brain sensors that contact the brain surface struggled with accurately measuring brain signals due to the inability to conform tightly to the brain’s complex folds,” Donghee Son, supervising author for the study, told Tech Xplore.

“This limitation made it difficult to precisely analyze the entire brain surface and accurately diagnose brain lesions. While a brain sensor previously developed by Professor John A. Rogers and Professor Dae-Hyeong Kim addressed this issue to some extent due to its extremely thin form, it still faced challenges in achieving tight adhesion in regions with severe curvature.”

The sensor previously developed by Professors Rogers and Kim was found to collect more precise measurements on the brain’s surface. Despite its promise, this sensor presented various limitations, such as failing to adhere to surfaces of the brain that had a larger curvature, as well as the proneness to slipping from its original attachment point due to micro-motions in the brain and the flow of cerebral spinal fluid (CSF).

These observed challenges limit its potential use in medical settings, as they reduce its ability to consistently measure brain signals in target regions for prolonged periods of time. As part of their study, Son and his colleagues set out to develop a new sensor that could overcome these limitations, adhering well to curved brain surfaces and thus enabling the reliable collection of measurements for extended time periods.

“The new sensor we developed can tightly conform to highly curved brain regions and adhere firmly to the brain tissue,” said Son. “This strong adhesion allows for long-term and precise measurement of brain signals from targeted areas.”

The sensor developed by Son and his colleagues, dubbed ECoG, adheres securely to brain tissue without forming any voids. This can significantly reduce the noise originating from external mechanical movements.

“This characteristic is particularly important in enhancing the effectiveness of epilepsy treatment through low-intensity focused ultrasound (LIFU),” said Son. “While it is well-known that the ultrasound can help minimize epileptic activity, the variability in patient conditions and the differences between individuals have posed significant challenges for tailoring treatments to each patient.”

In recent years, many research groups have been trying to devise personalized ultrasound stimulation treatments for epilepsy and other neurological disorders. To shape treatments based on the needs of individual patients, however, they should be able to measure the patient’s brain waves in real-time while stimulating specific brain regions.

“Conventional brain surface-attached sensors struggled with this because the ultrasound-induced vibrations caused significant noise, making it difficult to monitor brain waves in real-time,” said Son.

“This limitation was a major obstacle in creating personalized treatment strategies. Our sensor drastically reduces noise, enabling successful treatment of epilepsy through personalized ultrasound stimulation.”

The shape-morphing and cortex-adhesive brain sensor developed by Son and his colleagues comprises three main layers. These include a hydrogel-based layer that can bond with tissue both physically and chemically, a self-healing polymer-based layer that can change its shape to match the shape of the surface below it, and a stretchable, ultrathin layer containing gold electrodes and interconnects.

“When the sensor is applied to the brain surface, the hydrogel layer undergoes a gelation process, initiating an instant, strong attachment to the brain tissue,” explained Son.

“Following this, the self-healing polymer substrate begins to deform, conforming to the brain’s curvature, increasing the contact area between the sensor and the tissue over time. Once the sensor has fully adhered to the contours of the brain, it is ready to operate.”

The sensor developed by this research team has several advantages over other brain sensors introduced in recent years. Firstly, it can attach to brain tissue securely while also adapting its shape to fit tightly onto brain surfaces, irrespective of their level of curvature.

By adapting to the shape of curved surfaces, the sensor minimizes the vibrations produced by external ultrasound simulation. This could allow doctors to precisely measure the waves in their patients’ brains both under normal conditions and during ultrasound simulation.

“We expect this technology will not only be applicable in epilepsy treatment but also in diagnosing and treating various brain disorders,” said Son. “The most critical aspect of our work is the combination of a tissue-adhesive technology that enables the sensor to adhere firmly to the surface of brain tissue and a shape-morphing technology that allows it to conform to the brain’s contours without creating voids.”

So far, the new sensor developed by Son and his colleagues has been tested on living and awake rodents. The findings collected were highly promising, as the team was able to precisely measure brain waves and control seizures in the animals.

The researchers eventually plan to scale the sensor, building on their design to create a high-density array. After it passes clinical trials, this upgraded sensor could diagnose and treat epilepsy or other neurological disorders while potentially paving the way for more effective prosthetic technologies.

“Our brain sensor is currently equipped with 16 electrode channels, which presents an area for improvement in terms of high-resolution brain signal mapping,” added Son.

“With this in mind, we plan to significantly increase the number of electrodes to enable more detailed and high-resolution brain signal analysis. Additionally, we aim to develop a minimally invasive method to implant the brain sensor on the surface of the brain, with the ultimate goal of applying it in clinical research.”

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