Neuroprosthetics

Neuroprosthetics are mechanical prosthetic devices that utilize electric signals to allow control over artificial body parts, or stimulate movement for damaged parts of the body. Essentially, they are artificial aids applied following damage to the brain or nervous system. There are different types of neuroprosthetics, ranging from bionic limbs to sensory devices in the eyes and ears. Several kinds of professionals are needed for a neuroprosthetic to come together, including engineers specializing in the medical field and scientists who focus on the study of the brain. While existing as a more recent field, neuroprosthetics continues to make advances in technology, as researches are now finding ways to develop a sense of touch in the prosthetics and provide wireless capabilities, allowing users to exert more control than ever before.

How it Works

A man demonstrates BCI technology

The basic definition of a neuroprosthetic is a device that aids damaged areas of the nervous system, by providing signals to the body in place of cells that no longer can. The brain, composed of neurons, sends electrical commands, or signals, throughout the body--for example, to the muscles and organs. It sends these signals through the spinal cord, and then the nerves. However, if the nerves, or spine, suffer damage, this signal can be interfered. In order to compensate the loss of connection, the electrical signals are utilized. Electrodes, capable of producing such electric signals, are placed in the body, and then transmit these pulses to which the surrounding neurons can respond to. One example of this technology is targeted muscle innervation (TMR). In this method, nerves that have been cut off, such as in amputees, are rewired to different parts of the body, such as those in an amputated arm being moved to the chest. Then, electrodes implanted at this location sense whenever the person attempts to move their arm, in turn sending signals to move an attached prosthetic limb. Another method is through neural interfacing, where micro-sized electrodes are placed directly in the brain to interpret its signals, and then translate these commands to a prosthetic. Furthermore, neuroprosthetics can be divided into two groups: output neural interfaces, those that take signals from the brain and cause action, or input neural interfaces, those that collect signals from the person's surroundings and send them to the brain.

More specifically, a motor neuroprosthetic that decodes brain signals, or thought, into action is called a Brain Computer Interface, or BCI. One way to measure brain signals is through Electroencephalography (EEG). Electrodes placed on the scalp, specifically to monitor the cortex of the brain, record the intricate electrical rhythms produced by neuron and glia cell interaction. Then, connections are drawn between specific actions and specific rhythms. Once these translations are fully understood, a computer can control a prosthetic based on what signals it detects. The five different frequency bands of EEG used to categorize brain activity are Delta, Theta, Mu (Alpha), Beta, and Gamma. Another method for the direct interpretation of brain signals is electrocorticography (ECoG), which actually places the electrodes on the surface of the cortex rather than the scalp. Advantages of ECoG over EEG include higher signal resolution, wider range of frequency, and less noise in the signal, leading to more specific interpretations of intended action. Finally, peripheral nerves provide a third option with less risk in surgery, as they are easier to expose.

General Prosthetic Types

Typically, prosthetics are thought of as replacements for missing limbs, namely the arms and legs. There are two types of arm prostheses: transradial and transhumal. As described in their names, the transradial prosthetic has an attachment point below the elbow, while transhumal prosthetic are used if the elbow joint is missing, attaching to the upper arm. Harnesses secure the prosthetic to the person's body. However, if the prosthetic is electric in nature, then there will also be a cable. If the prosthetic is specifically controlled by the brain only, then it is called a myoelectric prosthesis, with electrodes within the prosthetic arm attaching to the muscles in either the elbow or upper arm. This allows for movement in the prosthetic's elbow joint, and the opening and closing of the hand. Similarly, there are two types of leg prostheses: transtibial, which attach below the knee, and transfemoral, which provides an artificial knee joint and attaches on the upper leg. Electric leg prostheses can possess sensors that gather data as the person moves, and adjust accordingly.

Although limb prosthetics seem to be thought of more commonly, the cochlear implant in the ear is actually the oldest neuroprosthetic, first approved in the mid-1980s. This device is able to aid those who cannot hear with a hearing aid due to the fact that it connects directly to the auditory nerve. It is implanted near the outer part of the ear, and sends electric signals to an array of electrodes in cochlea after detecting sound. The electrodes are set up to transmit certain patterns to the auditory nerve, letting people hear some sound, although it is not quite clear and requires practice in interpretation. Scientists are working to improve its quality. Another type of lesser known neuroprosthetic is the artificial retina, just approved in 2013. Known as the Argus II, it utilizes glasses with a camera to detect images, which the processor converts into light and dark pixels, then turning them into electric signals. These signals travel to a sheet of electrodes that stimulate photoreceptor cells, which transmit them to the optic nerve and lastly the brain. Due to how it only deals with light and dark, those who use it do not obtain normal vision, instead seeing contrast of brightness. Since photoreceptors are necessary for the prosthetic to work, it has only been used on those who have experienced photoreceptor deterioration, but scientists are seeking to improve and provide the technology for those with other causes of damage.

Prosthetists and the Scientists Needed for Neuroprosthetics

A prosthetist-orothist watches landmine survivors practice walking using prosthetic limbs

In order to create functioning neuroprosthetics, several types of scientists are needed. As suitable to its name, a prosthetic requires a prosthetist. A prosthetist is a professional whose job consists of assessing a patient, designing a method of treatment, and creating, fitting, and implementing a prosthetic to fit the patient's needs. Oftentimes prosthetists will be referred to even before an injured person receives an amputation, for their input and expertise on how to approach the process and the effects afterward. They continue to work with the patient during the transition, monitoring their progress, and finding ways to adjust and improve the device as needed. Prosthetists can specialize in specific types of prosthetics, certain conditions, or particular age groups. Once the decision has been made for the patient to receive a prosthetic, the prosthetist forms a mold of the part of the limb remaining, to make what is called a socket. Following this, they design the structure, choose the materials, and complete its production, staying in communication in case the device needs fixing later on. However, in order to create a neuroprosthetic, prosthetists must work hand in hand with neuroscientists, scientists who deal with the brain and how it influences a person's actions and cognitive functions. Neuroscientists can operate in many areas, including affective, behavioral, cellular, clinical, cognitive, computational, cultural, molecular, social, and systems neuroscience; it also includes neurolinguistics and neurophysiology.

Today, one the main branches of neuroscience, and a profession that works in the field of neuroprosthetics, is neuroengineering, where engineering techniques are utilized to study, replace, repair, and enhance the nervous system. However, it is not the only professional field where engineering is put to use in designing neuroprosthetics. Biomedical engineers also play a significant role in neuroprosthetic formation, especially with the new advancements of technology. With their knowledge in both engineering and the medical field, these engineers provide a wealth of essential skills in creating devices that must meld the two, such as neuroprosthetics. Just like prosthetists, biomedical engineers are consulted when using such a kind of equipment, as well as asked to repair damages. They, too, can specialize in designing certain devices, or be part of a research team that collaborates with other medical occupations to develop new technology. This technology can then be applied towards working together with the human body.

Newest Developments

The field of neuroprosthetics is rapidly advancing. One of the recent developments is the sophistication of prosthetic joints. Devices are now able to move individual fingers by having the patient flex another part of their arm, as well as bend these mechanical fingers at more than twenty-four points. The next step in this technology, is providing a sense of touch. Touch is key in picking up objects; amputees who have used prosthetics without it have stated that they have broken dishes and dropped objects, as it is hard to tell how much pressure is needed. This capability was first realized in 2014, when Dennis Aabo Sørensen became the first person to use a reported prosthetic hand with a sense of touch. With electrodes implanted in his arm and touch receptors installed in the prosthetic to automatically stimulate his nerves, Dennis can now differentiate between soft and hard objects, determine shape, and control the amount of force used to hold items. While the device is still in its early stages and will take time before it's ready to be applied clinically, researches are hopeful for future improvements and how they could positively impact the lives of those using prosthetics.

Another area of new neuroprosthetic developments is their utilization in restoring motor functions. Scientists, having success in 2015 in allowing rats to walk again, seek to apply this technology to humans. Previous attempts have had to deal with the problem of the electrode implant irritating the spinal cord, and lasting only short-term. This new method uses material that is stretchy and flexible, and instead is located on the surface of the brain or the protective layer in the spinal cord called the dura mater--earlier devices had been planted beneath. This has greatly reduced the risk of damage to the spinal cord, and increased the time the device can be embedded. Designed with similar mechanical properties as living tissue, the device sends electric signals to stimulate movement. With this as the leading example, researches and scientists in the field seek to further improve this branch of neuroprosthetics, including wireless devices that could help people affected by paralysis, epilepsy, neuromuscular disorders, and spinal cord injury.

Video

A man who lost a hand demonstrates neuroprosthetic technology.