Neuroprosthetics aim to at least partially restore functions of the nervous system such as hearing, vision, motor and cognitive functions that were lost due to illness or injury. The worldwide market for neuroprostheses was estimated at US-$ 5.28 billion in 2016 . For the period from 2017 to 2022, a growth rate of 12.4% is expected, which is among others due to the growing number of neurological patients and increasing technical innovations. In addition, the number of scientific publications on neuroprostheses is growing continuously. Most contributions come from the USA. Germany follows on the second place.
How to treat neurological diseases with medical high-tech
Medical technology can open up completely new treatment options. This is particularly true for difficult to treat neurological diseases. For the development of these innovative approaches, Germany plays a prominent role in Europe. The USA is leading the field globally.
In Germany alone, more than 11 million adults are affected by severe and profound hearing loss . Cochlear implants, a promising treatment option for this hearing disorder, were the first neuroprostheses to be approved for human use. They consist of an external and internal unit. The external unit comprises a microphone, a speech processor, a battery for the power supply and a transmitter for wireless transmission of data to the internal unit, which is responsible for the direct stimulation of the auditory nerve, bypassing the damaged structures. Fully implantable and therefore invisible from the outside system-on-chip devices are under development . However, cochlear implants can only be used if the auditory nerve is still functioning. If this is not the case, implants are now being used, which directly stimulate the auditory brainstem .
Restoring vision is a far greater challenge. A retinal prosthesis approved in Europe and the US is the Argus II (Second Sight Medical Products), which consists of an external video processing unit that converts visual information from an eye-mounted video camera into electrical signals. The triggering of visual perception in the brain is achieved by stimulating the remaining neurons of the retina with electrical signals from a second implanted unit. In Germany, the company Retina Implant AG has developed a subretinal implant. However, these systems cannot be used if there is extensive retinal damage. Currently, alternative solutions based on the stimulation of the visual cortex are being investigated.
Muscle paralysis may be caused by neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) or stroke, as well as injuries to the spine or brainstem. Motor neuroprostheses are still in their early stages of development. For example, brain activity was recorded and processed and used to control a computer cursor . Animal experiments have also shown that walking can be restored by direct stimulation of the spinal cord . The most common clinically used procedure for the treatment of neurological movement disorders is deep brain stimulation . Here, electrodes are implanted into the brain, which enable continuous high-frequency electrical stimulation of the surrounding neuronal tissue. Other examples of the applications of neurostimulation include the treatment of chronic pain , Parkinson's disease  and epilepsy .
Recently, a new approach for partially restoring neurological functions in paraplegic patients was developed by an interdisciplinary research team of engineers, scientists and physicians through a special training combining virtual reality, tactile sensors and an exoskeleton .
In recent years, the new research field "Bioelectronic Medicine" has emerged. Stimulation of the vagus nerve has so far been successful in the treatment of chronic inflammatory diseases such as rheumatoid arthritis  and Crohn's disease . It has also been demonstrated in animal experiments that modulating the carotid sinus nerve may be a new approach for the treatment of type 2 diabetes . The investors' interest in the technology is strong, as is evidenced by the completion of a US-$ 30 million financing round by SetPoint Medical (US). The US Food and Drug Administration has also approved a clinical trial with SetPoint Medical's bioelectronic medicine platform for the treatment of rheumatoid arthritis .
Opportunities and challenges
From a technological point of view, the medical technology involved here faces a number of challenges. For every implanted medical device, foreign body reactions in the patient must be minimized and a certain long-term stability has to be ensured. In addition to the use of special coating materials, the use of bioresorbable materials could be useful in the future, as has already been shown for pressure sensors in the brain of experimental animals . Furthermore, the trend is towards "closed-loop" systems that allow in contrast to the widespread "open-loop" systems, an automatic adjustment of stimulation of the nervous system based on certain measurement parameters, such as a brain-computer interface for the treatment of epilepsy . Why will human-technology interaction play an increasingly important role in the treatment of diseases in the future? In the field of neurological diseases, the answer is that not always sufficiently effective and safe active pharmaceutical ingredients or in some cases even no active pharmaceutical ingredients are available. Recently, Pfizer announced that it will stop research on drugs for the treatment of Alzheimer's and Parkinson's disease . In addition, neuroimplants are a promising treatment option, as neurological conditions are often based on alterations in functional networks, and implants - unlike systemic drug treatment - allow for targeted therapy in the affected networks.