Engineering researchers have developed an advanced brain-computer interface with a flexible and moldable backing and penetrating microneedles. Adding a flexible backing to this kind of brain-computer interface allows the device to conform more evenly to the brain’s complex curved surface and to distribute more uniformly the microneedles that pierce the cortex.

The microneedles, which are 10 times thinner than a human hair, protrude from the flexible backing and penetrate the surface of the brain tissue without piercing surface venules. They can then record signals from nearby nerve cells evenly across a wide area of the cortex.

This novel brain-computer interface has thus far been tested in rodents. The work is led by a team in the lab of electrical engineering professor Shadi Dayeh at the University of California, San Diego, together with researchers at Boston University led by biomedical engineering professor Anna Devor. The team reports this work in a paper in Advanced Functional Materials.

This new brain-computer interface is on a par with and outperforms the ‘Utah Array’, which is the existing gold standard for brain-computer interfaces with penetrating microneedles. The Utah Array has been demonstrated to help stroke victims and people with spinal cord injury. People with implanted Utah Arrays are able to use their thoughts to control robotic limbs and other devices in order to restore some everyday activities such as moving objects.

The backing of the new brain-computer interface is flexible, conformable and reconfigurable, while the Utah Array has a hard and inflexible backing. The flexibility and conformability of the backing of the novel microneedle-array favors closer contact between the brain and the electrodes, allowing for better and more uniform recording of brain-activity signals. Working with rodents as a model species, the researchers have demonstrated stable broadband recordings that produce robust signals for the duration of the implant, which lasted 196 days.

In addition, the way the soft-backed brain-computer interfaces are manufactured allows for larger sensing surfaces, which means that a significantly larger area of the brain surface can be monitored simultaneously. In this study, the researchers showed that a penetrating microneedle array with 1024 microneedles successfully recorded signals triggered by precise stimuli from the brains of rats. Compared to current technologies, this represents 10 times more microneedles and 10 times the area of brain coverage.

These soft-backed brain-computer interfaces are thinner and lighter than the glass backings of traditional brain-computer interfaces. In the paper, the researchers note that light, flexible backings may reduce irritation of the brain tissue that contacts the arrays of sensors.

The flexible backings are also transparent. The researchers demonstrated that this transparency can be leveraged to perform fundamental neuroscience research involving animal models that would not be possible otherwise. For example, the team demonstrated simultaneous electrical recording from arrays of penetrating micro-needles as well as optogenetic photostimulation.

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

Conceptually, starting with a rigid silicon wafer, the team's manufacturing process allows them to build microscopic circuits and devices on both sides of the wafer. On one side, they add a flexible, transparent, polyimide film. Within this film, they embed a bilayer of titanium and gold traces so that the traces line up with where the needles will be manufactured on the other side of the silicon wafer.

Working from the other side, after the flexible film has been added, the researchers etch away most of the wafer, leaving behind free-standing, thin, pointed columns of silicon. These pointed columns are, in fact, the microneedles, and their bases align with the titanium-gold traces within the flexible layer that remains after the silicon has been etched away. These titanium-gold traces are patterned via standard and scalable microfabrication techniques, allowing scalable production with minimal manual labor. The manufacturing process offers the possibility of flexible array design and scalability to tens of thousands of microneedles.

Looking to the future, penetrating microneedle arrays with large spatial coverage will be needed to improve brain-machine interfaces to the point where they can be used in ‘closed-loop systems’ that can help individuals with severely limited mobility. This kind of closed-loop system might, for example, offer a person using a robotic hand real-time tactical feedback on the objects the robotic hand is grasping.

Tactile sensors on the robotic hand would sense the hardness, texture and weight of an object. This information, recorded by sensors, would be translated into electrical stimulation patterns that travel through wires outside the body to the brain-computer interface with penetrating microneedles. 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 grasp strength based on the sensed information provided by the robotic hand.

This is just one example of the kind of closed-loop system that could be possible once penetrating microneedle arrays can be made larger to conform to the brain and coordinate activity across its ‘command’ and ‘feedback’ centers.

This story is adapted from material from the University of California, San Diego, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

Artist rendition of the flexible, conformable, transparent backing of the new brain-computer interface with penetrating microneedles. The smaller illustration at bottom left shows the Utah Arrays currently used as a brain-computer interface. Image: Shadi Dayeh/UC San Diego/SayoStudio.
Artist rendition of the flexible, conformable, transparent backing of the new brain-computer interface with penetrating microneedles. The smaller illustration at bottom left shows the Utah Arrays currently used as a brain-computer interface. Image: Shadi Dayeh/UC San Diego/SayoStudio.