Graphene-based neural sensors are showing promise in the treatment of brain disorders and injuries.November 27, 2017
Neuroscience is one of the most exciting – but still largely unexplored – opportunities for graphene, the versatile 2D carbon allotrope that has already transformed other areas of biomedical engineering. In a significant step forward, a team of European engineers recently unveiled a novel, graphene-based brain implant with unprecedented resolution and signal-to-noise performance – a key requirement for the development of functional brain implants, neuromodulation therapies, and brain-machine interfaces that control prosthetic limbs and sensory systems.
Engineers have struggled to develop a more fool-proof communication interface between the brain and electronic medical devices. Current microelectrode technologies for recording brain activity present challenges with issues such as electrical performance, safety, and durability. Working under the auspices of the Graphene Flagship, Europe’s largest research consortium, the team developed a flexible graphene solution-gated field-effect transistor (FET) designed for safe implantation directly on the brain. With potential use in the early detection of neurological events such as epileptic seizures, the implant capitalizes on graphene’s remarkable chemical stability, physical flexibility, low intrinsic electronic noise, and high charge carrier mobilities to accurately map brain activity. Graphene is also prized for its biocompatibility, which the team verified in preclinical in vivo experiments with rats in which they observed no significant toxicity or inflammation.
“Graphene is one of the few materials that allows recording in a transistor configuration and simultaneously complies with all other requirements for neural probes,” said team member Benno Blaschke of the Technical University of Munich.
Comprising densely packed miniature sensors capable of detecting the highly localized electric fields that arise when neurons fire, the micro-transistor was used to record two types of brain waves in rats: the large signals generated by pre-epileptic activity and the smaller signals generated during sleep or when exposed to light, which are more consistent with typical brain activity. According to team leader Jose Antonio Garrido, director of the Catalan Institute for Neuroscience and Neurotechnology in Barcelona, the next step after these proof-of-concept studies in animals will be the first human clinical trial with graphene devices during intraoperative mapping of the brain.
Other researchers are also exploring graphene’s potential in neural implants. A group at the University of California, San Diego, has developed flexible porous-graphene-based neural electrode arrays for cortical microstimulation and sensing. And a team from Daegu Gyeongbuk Institute of Science and Technology in Korea have created a flexible electrode array based on gold, zinc oxide nanowires, and a gold-graphene mixture.
Head to Toe Health
Advanced neural implants have the potential to address a surprising array of human health problems. Cochlear and retinal implants treat the nerves essential to hearing and vision. Brain and spinal cord stimulators can mitigate symptoms of Parkinson’s disease and chronic pain. Vagal nerve stimulators can block seizures. And electrodes stationed in the brain and nervous system help with motor rehabilitation, control of artificial limbs, and real-time brain mapping during surgery.
The human nervous system is intricate and delicate. Many traditional neural stimulation systems use sharp metal microelectrodes with electrochemical properties that can damage tissue and degrade long-term stability of the implant. To avoid damage, neural devices must be minimally invasive and simple to implant surgically. They should operate efficiently over long service lifetimes to avoid the need for repeat surgeries or adjustments. Devices designed for neural recording should be able to detect signals from individual neurons over large areas at high spatial resolutions.
Neurostimulation devices require carefully established charge injection capacity to ensure they deliver the necessary stimulus without damaging tissues. Graphene is an attractive choice on many of these fronts, due to its biocompatibility with neural cells and the ease with which it interfaces with brain and nerve tissues. The ability to perform both recording and stimulation separately or in tandem in a single device is a major advantage for graphene over current single-function neural interface materials.
Despite the advantages of graphene for such delicate applications, the group encountered significant design and manufacturing hurdles. “Although graphene is ideally suited for flexible electronics, it was a great challenge to transfer our fabrication process from rigid substrates to flexible ones,” Blaschke said. “The next step is to optimize the wafer-scale fabrication process and improve device flexibility and stability.”
Garrido acknowledged the manufacture and scale-up challenges still ahead for the device.
“Mechanical compliance is an important requirement for safe neural probes and interfaces. Currently, the focus is on ultra-soft materials that can adapt conformally to the brain surface. Graphene neural interfaces have already shown great potential, but we have to improve on the yield and homogeneity of the device production to advance towards a real technology.”
Michael MacRae is an independent technical writer based in Portland, OR.
Read more about these types of devices on AABME.org.