Neural engineering – involving the design of brain implants connected with external technology based on restoration and augmentation of human function via direct interactions between the nervous system and artificial devices – is a rapidly evolving field. Currently, intensive research is underway to study the coding and processing of information in the sensory and motor systems so that it can be manipulated through interactions with engineering devices, including brain-computer interface (BCI) applications and neuroprosthetics.
Neural engineering devices are deployed for specific applications such as the post-injury regeneration of peripheral nerve, spinal cord tissue, and retinal tissue. For developing these devices, neural engineers need to have in-depth knowledge of how the nervous system functions and malfunctions. They decode chemical, electrical, magnetic, and optical signals responsible for extracellular field potentials and synaptic transmission in neural tissue.
Signal processing techniques and computational modelling are used to understand the properties of neural system activity. To process these signals, the voltages across neural membranes are translated into corresponding code, a process known as neural coding. This involves analysis of the movement and sensory phenomena by understanding how the brain encodes simple commands in the form of central pattern generators (CPGs), movement vectors, the cerebellar internal model, and somatotopic maps.
Correcting anomalies in central nervous system
Neuroscience and engineering have come together to investigate the peripheral and central nervous system function and find clinical solutions to problems created by brain damage or malfunction.
Recent years have seen tremendous advancement in biomedical technologies that can enhance or suppress the activity of the nervous system with the delivery of pharmaceutical agents, electrical signals, or other kinds of an energy stimulus to re-establish balance in the brain’s impaired regions. Advancements in technology have resulted in delivering and analysing these signals with increased sensitivity, biocompatibility, and viability in closed loops schemes in the brain. This way, new treatments, and clinical applications can be created to treat those suffering from neural damage of various kinds. Neuromodulator devices have been found useful in correcting nervous system dysfunction related to Parkinson’s disease, dystonia, tremor, Tourette’s, chronic pain, OCD, severe depression, and eventually epilepsy.
Neuromodulation, of late, has been drawing attention for treatments for varying defects because it focuses on treating highly specific regions of the brain only, contrasting that of systemic treatments that can have side effects on the body. Neuromodulator stimulators such as microelectrode arrays can stimulate as well as record brain function with enhanced improvements. These are emerging as adjustable and responsive delivery devices for drugs and other stimuli.
Neural interfaces and neuroprosthetics
Neural interfaces involve the study of neural systems intending to enhance or replace neuronal function with engineered devices that can selectively record from associated electronic circuits to collect information about the nervous system activity. They can stimulate specified regions to restore function or sensation of that tissue. The devices, such as microelectrode arrays can work by virtue of materials that match the mechanical properties of neural tissue in which they are placed. The key lies in managing the body’s response to foreign materials. Brain cells become light-sensitive with optical neural interfaces that involve optical recordings and optogenetics stimulation. Fibre optics can be implanted in the brain to stimulate and record this photon activity instead of electrodes.
Neuroprosthetics imply devices that can supplement or replace missing functions of the nervous system by stimulating the nervous system and recording its activity. Electrodes can integrate with prosthetic devices and signal them to perform the purpose intended by the transmitted signal. Sensory prostheses use artificial sensors to replace neural input that might be missing from biological sources. Engineers designing these devices are tasked with providing a long-term, safe, artificial interface with neuronal tissue.
Perhaps the most successful of these sensory prostheses is the cochlear implant, which has restored hearing abilities to the deaf. Also under development is visual prosthesis for restoring the visual capabilities of blind persons. Motor prosthetics revolve around functional electrical stimulation (FES) of the biological neural muscular system that can substitute for control mechanisms of the brain or spinal cord. FES is aimed at restoring motor processes such as standing, walking, and hand grasp. Smart prostheses can replace missing limbs by transplanting nerves from the stump of an amputee to muscles. They can interpret signals and then control the prosthetic limb.
Brain computer interface
BCI is a technique that establishes direct communication with the human nervous system to monitor and stimulate neural circuits, diagnose and treat intrinsic neurological dysfunction. The most popular method in this context is Deep Brain Stimulation (DBS), which is particularly effective in treating movement disorders with high-frequency stimulation of neural tissue to suppress tremors.
Augmentation of human neural systems or human enhancement using engineering techniques is a highly useful application of neuroengineering. DBS has been shown to enhance memory recall in patients using this treatment for neurological disorders. It can sculpt emotions and personalities as well as enhance motivation, reduce inhibitions of the patient, etc.
Latest DBS implantable devices
Boston Scientific, a US company, has developed Vercise directional DBS systems that alleviate symptoms of Parkinson’s disease (PD), a neurodegenerative disorder that causes muscle stiffness, slow movements, and tremors, making everyday tasks difficult to perform. DBS therapy delivers mild electrical stimulation to specific regions in the brain through implants powered by an implantable pulse generator (IPG) that sits inside the chest.
Therapies, in general, improve PD symptoms but do not slow or stop disease progression, making it imperative to have options to adjust treatments as the disease worsens. As the progression of PD necessitates a therapy that can evolve with the patient, deliver a solution that improves patient comfort and enable physicians to easily adjust treatment as a patient’s symptoms change or progress, Boston Scientific modelled its research on cochlear implants that regenerate hearing and teach users to identify different sounds, voices, and music. Its engineers aimed to create a similar product that would target specific locations and produce specific levels of stimulation in the brain. It resulted in the development of Vercise Primary Cell (PC) and Vercise Gevia DBS systems, which facilitate the control of the range, shape, position, and direction of electrical stimulation.
To give neurologists better control of the shape and size of the stimulation field, Vercise directional DBS systems use individually controlled electrodes on each lead, providing stimulation adaptable to impedance changes within the brain. The systems encompass the following mechanical, electrical, and software aspects which constantly change as technology advances:
Electrodes – Cartesia directional leads allow current to be delivered in any direction around the lead.
Software – STIMVIEW technology developed by Boston Scientific allows neurologists and physicians to view the stimulation field being created on a screen, allowing them to easily match the shape of the stimulation field with the target region in the brain.
As part of constant advancements, engineers upgrade the devices’ capabilities that involve making electronics and mechanical packaging smaller, improving power supplies, and adding software capabilities. Computer modeling and in vitro studies ensure upgrades work and produce efficient results.
These devices are designed to interface well with body tissue. As they last at least 15 years, they are capable of long-term biocompatibility. The engineers envision the systems to become smarter, more autonomous, and more natural as a neural prosthesis.
Similarly, a laboratory in Grenoble, France, has developed an implantable wireless device that enables tetraplegic patients to walk and control both arms using a BCI and exoskeleton. This technology promises to provide further mobility to individuals with severe motor disabilities.
A lesion causes tetraplegia on the spinal cord that prevents the nervous system from controlling all four limbs. Medical doctors, physicians, and researchers at Clinatec, the CEA laboratory operated within Grenoble university hospital, have developed a device to control a four-limb exoskeleton that records and decodes brain signals. The device, an implant, records brain activity in real-time, and those impulses are used to drive the exoskeleton.
The device was tested on a tetraplegic patient who was able to walk and control both arms using the neuroprosthetic, which recorded, transmitted, and decoded brain signals.
The major innovation out of the clinical study under the BCI Project at Clinatec is the device’s ability to provide continuous high-resolution recording of the brain’s electrical activity, the activity related to moving intention is transmitted in real-time wirelessly to a computer for decoding to control the movements of the exoskeleton’s four limbs. The implantable medical device, called WIMAGINE recorded electrical activity in the sensorimotor cortex. It was designed for semi-invasive implantation in the cranium to record electrocorticograms (ECoG) with the help of an array of 64 electrodes in contact with the dura mater (a membrane that surrounds the brain and part of the spinal cord).
Microelectronics experts at CEA-Leti designed electronic boards for the electrocorticogram acquisition and digitalization systems together with a remote power supply and wireless data-transfer systems via secure radio link to an external base station.
Optimising the use of exoskeleton
The WIMAGINE implantable device collects brain signals in the sensorimotor cortex emitted when an individual imagines moving. A tetraplegic patient can mentally control the exoskeleton for his movements. This device is an important step forward in helping people with severe motor disabilities become self-sufficient.
The recorded electrocorticograms are decoded in real-time to identify the deliberate movement the patient imagines; the decoded transmissions control the corresponding limb of an exoskeleton. Decoding electrocorticograms require the development of highly sophisticated algorithms based on artificial intelligence (AI) and machine learning (ML), and software to control the movements of the exoskeleton in real-time.
The WIMAGINE device also involved research engineers from CEA-List, which specializes in smart digital systems. The design specifically took into account the interaction of a quadriplegic person with the exoskeleton to be able to mobilise it safely.
Emerging neural engineering devices will open the door to new applications for use at home by patients in their everyday lives. Engineers worldwide are working on integrating new effectors, such as a wheelchair, and developing even robust and more precise algorithms to perform more complex movements, with the hope of performing tasks such as holding an object.
The long-term goal of biomedical devices is to identify fields in which the BCI could be used to create compensatory systems for various types of motor disabilities and give patients more independence in their everyday lives, for instance, by driving a wheelchair or controlling an articulated arm.
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