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Neuroprosthetics (also called neural prosthetics) is a discipline related to neuroscience and biomedical engineering concerned with developing neural prosthetics. They are sometimes contrasted with a brain–computer interface, which connects the brain to a computer rather than a device meant to replace missing biological functionality.
Neural prostheses are a series of devices that can substitute a motor, sensory or cognitive modality that might have been damaged as a result of an injury or a disease. provide an example of such devices. These devices substitute the functions performed by the eardrum and stapes while simulating the frequency analysis performed in the cochlea. A microphone on an external unit gathers the sound and processes it; the processed signal is then transferred to an implanted unit that stimulates the auditory nerve through a microelectrode array. Through the replacement or augmentation of damaged senses, these devices are intended to improve the quality of life for those with disabilities.
These implantable devices are also commonly used in animal experimentation as a tool to aid neuroscientists in developing a greater understanding of the Human brain and its functioning. By wirelessly monitoring the brain's electrical signals sent out by electrodes implanted in the subject's brain, the subject can be studied without the device affecting the results. Accurately probing and recording the electrical signals in the brain would help better understand the relationship among a local population of neurons that are responsible for a specific function.
Neural implants are designed to be as small as possible in order to be minimally invasive, particularly in areas surrounding the brain, eyes, or cochlea. These implants typically communicate with their prosthetic counterparts wirelessly. Additionally, power is currently received through wireless power transmission through the skin. The tissue surrounding the implant is usually highly sensitive to temperature rise, meaning that power consumption must be minimal in order to prevent tissue damage.
The neuroprosthetic currently undergoing the most widespread use is the cochlear implant, with over 736,900 in use worldwide .
Regarding the development of implanted in the brain, an early difficulty was reliably locating the electrodes, originally done by inserting the electrodes with needles and breaking off the needles at the desired depth. Recent systems utilize more advanced probes, such as those used in deep brain stimulation to alleviate the symptoms of Parkinson's disease. The problem with either approach is that the brain floats free in the skull while the probe does not, and relatively minor impacts, such as a low speed car accident, are potentially damaging. Some researchers, such as Kensall Wise at the University of Michigan, have proposed tethering 'electrodes to be mounted on the exterior surface of the brain' to the inner surface of the skull. However, even if successful, tethering would not resolve the problem in devices meant to be inserted deep into the brain, such as in the case of deep brain stimulation (DBS).
A visual prosthesis system consists of an external (or implantable) imaging system which acquires and processes the video. Power and data will be transmitted to the implant wirelessly by the external unit. The implant uses the received power/data to convert the digital data to an analog output which will be delivered to the nerve via micro electrodes.
Photoreceptors are the specialized neurons that convert photons into electrical signals. They are part of the retina, a multilayer neural structure about 200 μm thick that lines the back of the human eye. The processed signal is sent to the brain through the optical nerve. If any part of this pathway is damaged blindness can occur.
Blindness can result from damage to the optical pathway (cornea, aqueous humor, crystalline lens, and vitreous humour). This can happen as a result of accident or disease. The two most common retinal degenerative diseases that result in blindness secondary to photoreceptor loss is age related macular degeneration (AMD) and retinitis pigmentosa (RP).
The first clinical trial of a permanently implanted retinal prosthesis was a device with a passive microphotodiode array with 3500 elements.A. Y. Chow, V. Y. Chow, K. Packo, J. Pollack, G. Peyman, and R. Schuchard, "The artificial silicon retina microchip for the treatment of vision loss from retinitis pigmentosa," Arch.Ophthalmol., vol. 122, p. 460, 2004 This trial was implemented at Optobionics, Inc., in 2000. In 2002, Second Sight Medical Products, Inc. (Sylmar, CA) began a trial with a prototype epiretinal implant with 16 electrodes. The subjects were six individuals with bare light perception secondary to RP. The subjects demonstrated their ability to distinguish between three common objects (plate, cup, and knife) at levels statistically above chance. An active sub retinal device developed by Retina Implant GMbH (Reutlingen, Germany) began clinical trials in 2006. An IC with 1500 microphotodiodes was implanted under the retina. The microphotodiodes serve to modulate current pulses based on the amount of light incident on the photo diode.M. J. McMahon, A. Caspi, J. D.Dorn, K. H. McClure, M. Humayun, and R. Greenberg, "Spatial vision in blind subjects implanted with the second sight retinal prosthesis," presented at the ARVO Annu. Meeting, Ft. Lauderdale, FL, 2007.
The seminal experimental work towards the development of visual prostheses was done by cortical stimulation using a grid of large surface electrodes. In 1968 Giles Brindley implanted an 80 electrode device on the visual cortical surface of a 52-year-old blind woman. As a result of the stimulation the patient was able to see phosphenes in 40 different positions of the visual field.G. S. Brindley and W. S. Lewin, "The sensations produced by electrical stimulation of the visual cortex," J. Physiol., vol. 196, p. 479, 1968 This experiment showed that an implanted electrical stimulator device could restore some degree of vision. Recent efforts in visual cortex prosthesis have evaluated efficacy of visual cortex stimulation in a non-human primate. In this experiment after a training and mapping process the monkey is able to perform the same visual saccade task with both light and electrical stimulation.
The requirements for a high resolution retinal prosthesis should follow from the needs and desires of blind individuals who will benefit from the device. Interactions with these patients indicate that mobility without a cane, face recognition and reading are the main necessary enabling capabilities.Weiland JD, Humayun MS. 2008. Visual prosthesis. Proceedings of the IEEE 96:1076–84
The results and implications of fully functional visual prostheses are exciting. However, the challenges are grave. In order for a good quality image to be mapped in the retina a high number of micro-scale electrode arrays are needed. Also, the image quality is dependent on how much information can be sent over the wireless link. Also this high amount of information must be received and processed by the implant without much power dissipation which can damage the tissue. The size of the implant is also of great concern. Any implant would be preferred to be minimally invasive.
With this new technology, several scientists, including Karen Moxon at Drexel, John Chapin at SUNY, and Miguel Nicolelis at Duke University, started research on the design of a sophisticated visual prosthesis. Other scientists have disagreed with the focus of their research, arguing that the basic research and design of the densely populated microscopic wire was not sophisticated enough to proceed.
In contrast to traditional hearing aids that amplify sound and send it through the external ear, cochlear implants acquire and process the sound and convert it into electrical energy for subsequent delivery to the auditory nerve. The microphone of the CI system receives sound from the external environment and sends it to processor. The processor digitizes the sound and filters it into separate frequency bands that are sent to the appropriate tonotonic region in the cochlea that approximately corresponds to those frequencies.
In 1957, French researchers A. Djourno and C. Eyries, with the help of D. Kayser, provided the first detailed description of directly stimulating the auditory nerve in a human subject.John Niparko and B. W. Wilson, "History of cochlear implants," in Cochlear Implants:Principles and Practices. Philadelphia, PA: Lippincott Williams & Wilkins, 2000, pp. 103–08 The individuals described hearing chirping sounds during stimulation. In 1972, the first portable cochlear implant system in an adult was implanted at the House Ear Clinic. The U.S. Food and Drug Administration (FDA) formally approved the marketing of the House-3M cochlear implant in November 1984.W. F. House, Cochlear implants: My perspective
Improved performance in cochlear implants not only depends on understanding the physical and biophysical limitations of implant stimulation, but also on an understanding of the brain's pattern processing requirements. Modern signal processing represents the most important speech information while also providing the brain the pattern recognition information that it needs. Pattern recognition in the brain is more effective than algorithmic preprocessing at identifying important features in speech. A combination of engineering, signal processing, biophysics, and cognitive neuroscience was necessary to produce the right balance of technology to maximize the performance of auditory prosthesis.Fayad JN, Otto SR, Shannon RV, Brackmann DE. 2008. Cochlear and brainstern auditory prostheses "neural interface for hearing restoration: Cochlear and brain stem implants". Proceedings of the IEEE 96:1085–95
Cochlear implants have been also used to allow acquiring of spoken language development in congenitally deaf children, with remarkable success in early implantations (before 2–4 years of life have been reached).Kral A, O'Donoghue GM. Profound Deafness in Childhood. New England J Medicine 2010: 363; 1438–50 There have been about 80,000 children implanted worldwide.
The concept of combining simultaneous electric-acoustic stimulation (EAS) for the purposes of better hearing was first described by C. von Ilberg and J. Kiefer, from the Universitätsklinik Frankfurt, Germany, in 1999.V. Ilberg C., Kiefer J., Tillein J., Pfennigdorff T., Hartmann R., Stürzebecher E., Klinke R. (1999). Electric-acoustic stimulation of the auditory system. ORL 61:334–40. That same year the first EAS patient was implanted. Since the early 2000s FDA has been involved in a clinical trial of device termed the "Hybrid" by Cochlear Corporation. This trial is aimed at examining the usefulness of cochlea implantation in patients with residual low-frequency hearing. The "Hybrid" utilizes a shorter electrode than the standard cochlea implant, since the electrode is shorter it stimulates the basil region of the cochlea and hence the high-frequency tonotopic region. In theory these devices would benefit patients with significant low-frequency residual hearing who have lost perception in the speech frequency range and hence have decreased discrimination scores.B. J. Gantz, C. Turner, and K. E. Gfeller, "Acoustic plus electric speech processing: Preliminary results of a multicenter clinical trial of the Iowa/Nucleus hybrid implant," Audiol. Neurotol., vol. 11 (suppl.), pp. 63–68, 2006, Vol 1
For producing sound see Speech synthesis.
In ancient times the electrogenic fish was used as a shocker to subside pain. Healers had developed specific and detailed techniques to exploit the generative qualities of the fish to treat various types of pain, including headache. Because of the awkwardness of using a living shock generator, a fair level of skill was required to deliver the therapy to the target for the proper amount of time. (Including keeping the fish alive as long as possible) Electro analgesia was the first deliberate application of electricity. By the nineteenth century, most western physicians were offering their patients electrotherapy delivered by portable generator.D. Fishlock, "Doctor volts electrotherapy," Inst. Elect. Eng. Rev., vol. 47, pp. 23–28, May 2001 In the mid-1960s, however, three things converged to ensure the future of electro stimulation.
The design options for electrodes include their size, shape, arrangement, number, and assignment of contacts and how the electrode is implanted. The design option for the pulse generator includes the power source, target anatomic placement location, current or voltage source, pulse rate, pulse width, and a number of independent channels. Programming options are very numerous (a four-contact electrode offers 50 functional bipolar combinations). The current devices use computerized equipment to find the best options for use. This reprogramming option compensates for postural changes, electrode migration, changes in pain location, and suboptimal electrode placement.North RB. 2008. Neural interface devices: Spinal cord stimulation technology. Proceedings of the IEEE 96:1108–19
The related procedure of sacral nerve stimulation is for the control of incontinence in able-bodied patients.Schmidt RA, Jonas A, Oleson KA, Janknegt RA, Hassouna MM, Siegel SW, van Kerrebroeck PE. Sacral nerve stimulation for treatment of refractory urinary urge incontinence. Sacral nerve study group. J Urol 1999 Aug;16(2):352–57.
To capture electrical signals from the brain, scientists have developed microelectrode arrays smaller than a square centimeter that can be implanted in the skull to record electrical activity, transducing recorded information through a thin cable. After decades of research in monkeys, neuroscientists have been able to decode neuronal signals into movements. Completing the translation, researchers have built interfaces that allow patients to move computer cursors, and they are beginning to build robotic limbs and exoskeletons that patients can control by thinking about movement.
The technology behind motor neuroprostheses is still in its infancy. Investigators and study participants continue to experiment with different ways of using the prostheses. Having a patient think about clenching a fist, for example, produces a different result than having him or her think about tapping a finger. The filters used in the prostheses are also being fine-tuned, and in the future, doctors hope to create an implant capable of transmitting signals from inside the skull , as opposed to through a cable.
Prior to these advancements, Philip Kennedy (Emory University and Georgia Tech) had an operable if somewhat primitive system which allowed an individual with paralysis to spell words by modulating their brain activity. Kennedy's device used two neurotrophic electrodes: the first was implanted in an intact motor cortical region (e.g. finger representation area) and was used to move a cursor among a group of letters. The second was implanted in a different motor region and was used to indicate the selection.
Developments continue in replacing lost arms with cybernetic replacements by using nerves normally connected to the pectoralis muscles. These arms allow a slightly limited range of motion, and reportedly are slated to feature sensors for detecting pressure and temperature.
Dr. Todd Kuiken at Northwestern University and Rehabilitation Institute of Chicago has developed a method called targeted reinnervation for an amputee to control motorized prosthetic devices and to regain sensory feedback.
In 2002 a Multielectrode array of 100 , which now forms the sensor part of a Braingate, was implanted directly into the median nerve fibers of scientist Kevin Warwick. The recorded signals were used to control a robot arm developed by Warwick's colleague, Peter Kyberd and was able to mimic the actions of Warwick's own arm.Warwick, K, Gasson, M, Hutt, B, Goodhew, I, Kyberd, P, Andrews, B, Teddy, P and Shad, A:"The Application of Implant Technology for Cybernetic Systems", Archives of Neurology, 60(10), pp. 1369–73, 2003 Additionally, a form of sensory feedback was provided via the implant by passing small electrical currents into the nerve. This caused a contraction of the first lumbrical muscle of the hand and it was this movement that was perceived.
In June 2014, Juliano Pinto, a paraplegic athlete, performed the ceremonial first kick at the 2014 FIFA World Cup using a powered exoskeleton with a brain interface. 'We Did It!' Brain-Controlled 'Iron Man' Suit Kicks Off World Cup The exoskeleton was developed by the Walk Again Project at the laboratory of Miguel Nicolelis, funded by the government of Brazil. Nicolelis says that feedback from replacement limbs (for example, information about the pressure experienced by a prosthetic foot touching the ground) is necessary for balance. He has found that as long as people can see the limbs being controlled by a brain interface move at the same time as issuing the command to do so, with repeated use the brain will assimilate the externally powered limb and it will start to perceive it (in terms of position awareness and feedback) as part of the body. Brain-To-Brain Communication (audio interview with Dr. Miguel Nicolelis)
An AMI is composed of two muscles that originally shared an agonist-antagonist relationship. During the amputation surgery, these two muscles are mechanically linked together within the amputated stump. "On prosthetic control: A regenerative agonist-antagonist myoneural interface", Science Robotics, 31 May 2017 One AMI muscle pair can be created for each joint degree of freedom in a patient in order to establish control and sensation of multiple prosthetic joints. In preliminary testing of this new neural interface, patients with an AMI have demonstrated and reported greater control over the prosthesis. Additionally, more naturally reflexive behavior during stair walking was observed compared to subjects with a traditional amputation. "Proprioception from a neurally controlled lower-extremity prosthesis", Science Translational Medicine, 30 May 2018 An AMI can also be constructed through the combination of two devascularized muscle grafts. These muscle grafts (or flaps) are spare muscle that is denervated (detached from original nerves) and removed from one part of the body to be re-innervated by severed nerves found in the limb to be amputated. Through the use of regenerated muscle flaps, AMIs can be created for patients with muscle tissue that has experienced extreme atrophy or damage or for patients who are undergoing revision of an amputated limb for reasons such as neuroma pain, bone spurs, etc.
Once the I/O parameters are modeled mathematically, integrated circuits are designed to mimic the normal biologic signals. For the prosthetic to perform like normal tissue, it must process the input signals, a process known as transformation, in the same way as normal tissue.
Wireless controlling devices can be mounted outside of the skull and should be smaller than a pager.
Crossing the blood–brain barrier can introduce pathogens or other materials that may cause an immune response. The brain has its own immune system that acts differently from the immune system of the rest of the body.
A small, light weight device has been developed that allows constant recording of primate brain neurons at Stanford University.HermesC: Low-Power Wireless Neural Recording System for Freely Moving Primates Chestek, C.A.; Gilja, V.; Nuyujukian, P.; Kier, R.J.; Solzbacher, F.; Ryu, S.I.; Harrison, R.R.; Shenoy, K.V.; Neural Systems and Rehabilitation Engineering, IEEE Transactions on Volume 17, Issue 4, Aug. 2009, pp. 330–38. This technology also enables neuroscientists to study the brain outside of the controlled environment of a lab.
Methods of data transmission between neural prosthetics and external systems must be robust and secure. Wireless neural implants can have the same cybersecurity vulnerabilities as any other IT system, giving rise to the term neurosecurity. A neurosecurity breach can be considered a violation of medical privacy.
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