Damage to the Central Nervous System (CNS) leads to total or partial locked-in states wherein a person can lose the ability to control voluntary movements like walking, hearing, or speaking. Advancement in Neuroscience, specifically Brain-Computer Interface, gives hope to such people that they may regain some of their lost abilities through the use of an external device, allowing them to live better lives. Research in this field is going on in full rigor and we may see some of these devices coming to the market in the next few years.
Brain implants are technological devices attached to the surface of the brain or the cortex of the brain to create a connection between the brain and an external electronic device. These implants stimulate or record activity from a single neuron or a network of neurons.
Deep Brain Stimulation
Brain implants are widely used in Deep Brain Stimulation, in which an electrode is surgically implanted in a patient’s brain and electrical impulses are then sent to the area of the brain where the electrode was placed to treat conditions like Parkinson’s disease, epilepsy, or obsessive-compulsive disorder. FDA approved DBS implants in 1997 to treat a wide variety of neurological disorders.
Vagus Nerve Stimulation
Since the vagus nerve connects the brain to the rest of the body, electrically stimulating it can be used to treat a wide variety of conditions, including depression that has not responded to other treatments, heart disease, obesity, migraines, and drug-resistant epilepsy.
Brain implants offer a wide range of applications in the field of Assistive Technology, thanks to neural prosthetics. They’ve been used to operate computer cursors, robotic limbs, virtual keyboards, and wheelchairs, all of which can be of great help to people with communication and mobility impairments. Neuroprosthetics also comprise visual and auditory prosthetics to treat complete blindness and deafness respectively. Research in this field, called Brain-Computer Interface, has progressed significantly and with astounding results.
Read more about Assistive Technology.
Devices intended for implantation into the brain must be carefully designed with the appropriate materials, coatings, and geometry to avoid causing any unintended side effects on the patient.
The electrodes are made of biocompatible and biostable materials that should neither harm the brain tissues nor degrade inside the brain. The materials usually used for fabricating are silicon, platinum, gold, tungsten, and stainless steel. More advanced devices use carbon fibers and hybrid nanomaterials for a more intricate design.
Geometry and Physical Characteristics
The geometry and physical characteristics of the electrode have a big impact on its long-term viability. To reduce the risk of damaging healthy brain tissue, the electrodes are designed to be as small as possible. The roughness of the electrode’s surface also has a significant impact on the effectiveness of the implant.
The signal quality of the recording and the reliability of the device are both heavily influenced by the material’s electrical properties, such as its impedance and its ability to transduce signals. The electrical impedance is inversely proportional to the electrode’s surface area. As a result, the impedance increases, and the signal quality decreases when the surface area of the electrode is reduced, as required by the geometry of the electrode.
Another challenge is achieving the desired mechanical properties. There should not be any mechanical mismatch between the electrode and the brain tissue interface to circumvent any tissue damage. Therefore, the electrode materials are chemically inert in the brain environment. As an additional issue, electrodes tend to be hard and rigid, which is at odds with the brain’s supple tissues. This mechanical mismatch is widely believed to be a major factor in the deterioration of the electrode-tissue interface.
Evolution of Brain-Machine Interface Neural Implants
Utah or Multielectrode Arrays
In 2004, Cyberkinetics Inc. created the intracortical multielectrode array, also known as the Utah array, which was the first implantable BMI device. These devices contain multiple microelectrodes that can record and stimulate the neurons in different cortical layers to which they are attached.
ECoG Grids were the second implantable technology used to decode motor intention in an ALS patient. These grids are very similar to Utah Arrays, with the exception that the electrodes are spaced further apart and can only reach the superficial layer of the cortex.
Probes and Wires
The next generation of electrodes, in the form of probes and wires, offers significantly greater flexibility, channels, and stability. These are soft, flexible, freely floating, and stretchable that are much more compatible with the brain tissue, and provide high-quality signals. Such implants include microwires, flexible thread-like (linear) electrode arrays, high-density multi-shank silicon probes, and ‘stentrodes’ that embed electrodes within mesh structures for intravascular delivery. These types of implants resolve the difference in mechanical properties between current hard and rigid electrode materials and soft and elastic brain tissue.
In addition to the above, novel surgical workflows are being investigated for the implantation of these devices as they cannot be implanted by conventional brain surgeries. Robots inspired by sewing machines, neurovascular surgery, and laser surgery are just a few examples of the many new procedures that have been developed.
Invasive technologies provide a high signal-to-noise ratio by eliminating volume conduction and subsequently lead to superior devices. But it also comes with its own adversities. The patients had to go through painful brain surgery to get these electrodes implanted. In addition to this, sometimes the electrodes can cause tissue damage and inflammation to the brain area where they are attached. Consequently, it becomes hard to validate surgery for implantation unless it is a severe medical emergency.
Many neurotech companies, like Neuralink, Paradromics, Blackrock Neurotech, and Synchron, are coming up with smaller electrodes designed to have minimal adverse effects on the brain and wide coverage. Recently, a budding neurotech company, Synchron, completed the first FDA-approved BCI implant on a paralyzed US patient. The novel part is that the implantation didn’t require open-brain surgery. Stent-electrode recording array, or Stentrode, is a stent-mounted electrode array that can be directly implanted via blood vessels, thus eliminating the need for open-brain surgery.
Many more such ground-breaking innovations, which would lessen the negative impact of implantation and improve signal quality and decoding efficiency, are on the horizon. This will pave the way for the development of BCI devices suitable for use in clinical settings.
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