Soft electrodes designed to perfectly match a person's brain surface may help advance neural interfaces for neurodegenerative disease monitoring and treatment, according to a new study led by Penn State researchers. Neural interfaces are powered by tiny sensors capable of tracking biophysical signals, known as bioelectrodes. These sensors are usually made from stiff materials in a one-size-fits-all design that struggles to match the brain's complex structure. The researchers have created a novel approach to 3D printing bioelectrodes that can stretch and morph to fit the minor differences that make every brain unique.
The team used software to simulate detailed brains based on MRI scans taken from 21 human patients, shaping a set of electrodes tailored for brains' specific structures before 3D printing the electrodes and models of the brains. In a paper published in Advanced Materials, they reported that their electrodes better fit the structure of the brain than traditional designs, while remaining effective and biologically compatible, even in tests done in rats.
The folds in the human brain are created through a process known as gyrification, where the cortical sheet on the outer wall of the brain bunches up into ridges, known as gyri, and grooves, known as sulci. This helps cells across the brain communicate at high speeds, and allows for a relatively large organ to fit compactly in the skull - a spread-out adult brain would be around 2,000 square centimeters, or about the size of two large pizzas.
Although the major cortical folds are consistent across individuals, the precise layout of the brain's gryi and sulci changes substantially from person to person, according to Tao Zhou, Wormley Family Early Career Professor, assistant professor of engineering science and mechanics and corresponding author on the paper. However, traditional bioelectrode designs don't take this into account.
"Each person has a different brain structure, depending on their height, weight, age, sex and more," said Zhou, who also holds an affiliation in biomedical engineering and the center for neural engineering at Penn State. "Despite this, we try to fit neural interfaces onto brains like they have identical structures. This motivated us to create electrodes that are tailored for each individual, based on the structure of their brain."
The electrodes are built mainly from a water-rich material known as hydrogel to better match with the soft tissues and patient-specific geometry of a brain. Furthermore, the team used a novel honeycomb-inspired structure that offers flexibility and strength, while remaining cost-effective and quick to print, according to Zhou.
"The honeycomb structure helps us significantly reduce the stiffness of the electrodes, without sacrificing their mechanical strength," Zhou said. "What's more, the structure helps us reduce the overall material used during fabrication, reducing production time, cost and environmental impact."
Production starts by taking an MRI scan of a patient's brain, which is used to conduct finite element analysis - a process that creates a detailed simulation of a person's neural structure. This analysis is then rendered as a 3D model of the patient's brain, where the team uses computer software to tailor a bioelectrode specifically morphed to fit the ridges and grooves of the cerebral cortex.
After shaping, the team 3D prints the hydrogel electrode using direct ink printing, a technique that can create electrodes capable of monitoring and transmitting brain signals over a relatively small surface. For this study, the team 3D printed models of 21 different participant brains, applying their electrodes and physically measuring how accurately the electrodes could fit the brain surface. Zhou explained how traditional fabrication approaches require specialized facilities like clean rooms, making them incredibly expensive to customize - 3D printing allows the team to personalize and manufacture electrodes much faster, for a fraction of the price.
