Flexible backing allows micro-scale needle arrays to conform to brain contours, enhancing high-resolution brain recording

Engineering researchers have invented an advanced brain-computer interface with flexible, moldable support and penetrating micro-needles. Adding a flexible backing to this type of brain-computer interface allows the device to conform more evenly to the complex curved surface of the brain and more evenly distribute the microneedles that pierce the cortex. The microneedles, which are 10 times thinner than human hair, protrude from the flexible backing, penetrate the surface of brain tissue without piercing surface venules, and evenly record signals from nearby nerve cells over a wide area of ​​the cortex.

This new brain-computer interface has so far been tested in rodents. The details were published online Feb. 25 in the journal Advanced Functional Materials. This work is led by a team from the lab of electrical engineering professor Shadi Dayeh at the University of California, San Diego, in collaboration with researchers from Boston University led by biomedical engineering professor Anna Devor.

This new brain-computer interface matches and surpasses the “Utah Array”, which is the existing gold standard for brain-computer interfaces with penetrating microneedles. The Utah Array has been shown to help stroke victims and people with spinal cord injuries. People with implanted Utah Arrays can use their thoughts to control robotic limbs and other devices to restore certain daily activities such as moving objects.

The new brain-computer interface support is flexible, conformable and reconfigurable, while the Utah Array has a hard and inflexible support. The flexibility and conformability of the new micro-needle array support promotes closer contact between the brain and the electrodes, resulting in better and more consistent recording of brain activity signals. Working with rodents as model species, the researchers demonstrated stable broadband recordings producing robust signals throughout the duration of implantation which lasted 196 days.

Additionally, the way soft-backed brain-computer interfaces are manufactured allows for larger sensing surfaces, which means a much larger area of ​​the brain’s surface can be monitored simultaneously. In the Advanced functional materials article, the researchers demonstrate that a penetrating microneedle array with 1,024 microneedles successfully recorded signals triggered by specific stimuli from the brains of rats. This represents ten times more micro-needles and ten times the area of ​​cerebral coverage, compared to current technologies.

Thinner and more transparent media

These soft-backed brain-computer interfaces are thinner and lighter than the traditional glass backings of these types of brain-computer interfaces. The researchers note in their Advanced Functional Materials paper that lightweight, flexible supports can reduce irritation to brain tissue that comes into contact with sensor arrays.

Flexible supports are also transparent. In the new paper, the researchers demonstrate that this transparency can be leveraged to perform basic neuroscience research involving animal models that would not otherwise be possible. The team, for example, demonstrated simultaneous electrical recording from arrays of penetrating microneedles as well as optogenetic photostimulation.

Double-sided lithographic production

The flexibility, larger microneedle array footprints, media reconfigurability and transparency of the new brain sensors are all due to the double-sided lithography approach used by the researchers.

Conceptually, starting with a rigid silicon wafer, the team’s fabrication process allows them to build microscopic circuits and devices on both sides of the rigid silicon wafer. On one side, a flexible and transparent film is added above the silicon wafer. In this film, a bilayer of titanium and gold traces are embedded so that the traces line up with where the needles will be made on the other side of the silicon wafer.

Working on the other side, after the flexible film has been added, all the silicon is etched, except for the thin, pointed, self-supporting silicon columns. These pointed columns of silicon are, in fact, the micro-needles, and their bases line up with the titanium-gold traces in the flexible layer that remains after the silicon has been etched. These titanium-gold traces are patterned via standard and scalable microfabrication techniques, allowing for scalable production with minimal manual labor. The manufacturing process offers the possibility of flexible array design and scalability to tens of thousands of microneedles.

Towards closed-loop systems

Looking to the future, penetrating microneedle arrays with wide spatial coverage will be needed to improve brain-machine interfaces to the point that they can be used in “closed-loop systems” that can help people very limited mobility. For example, this type of closed-loop system could provide a person using a robotic hand with real-time tactical feedback on objects gripped by the robotic hand.

The robotic hand’s touch sensors would detect an object’s hardness, texture, and weight. This information recorded by the sensors would be translated into electrical stimulation patterns that travel through wires outside the body to the brain-computer interface with penetrating micro-needles. These electrical signals would provide information directly to the person’s brain about the hardness, texture and weight of the object. In turn, the person would adjust their grip strength based on sensed information directly from the robotic arm.

This is just one example of the type of closed-loop system that might be possible once the penetrating micro-needle networks can be enlarged to conform to the brain and coordinate activity between “command” centers and feedback from the brain.

Previously, the Dayeh lab had invented and demonstrated the types of touch sensors that would be needed for this type of application, as shown in this video.

Path to commercialization

The advanced double-sided lithographic microfabrication processes described in this article are patented (US 10856764). Dayeh co-founded Precision Neurotek Inc. to translate technologies innovated in his lab to advance the state of the art in clinical practice and advance the fields of neuroscience and neurophysiology.

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