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  • Kayla Hui

The Future in Bionic Limbs

As of 2018, nearly 2 million people live without limb(s) within the United States, and by 2050, this figure is expected to double. Amputations often have devastating impacts on patients’ mental health and have created a demand for limbs that could mimic the original sensory limb functions. Current prosthetics have issues with attachment, as many prosthetics do not allow for high-level daily activity. This may cause the skin, nerves, muscles, and bones of the stump to become negatively impacted by chronic skin issues or fractures. Bionic limbs present a new frontier, as certain limbs provide better attachment through the use of osseointegrated implants. For this reason, bionic limbs have become a great source of innovation within the medical community, and many different designs have been presented.

Bionic limbs are artificial limbs that use an interface between the stump and an electronic device to provide motor control of the prosthesis and sensitive feedback. Some designs of bionic limbs may use sensors to receive signals from an individual’s muscles or nerves above the amputation area. Most bionic limbs have computers that are used to detect the muscle signals, while more advanced options have sensors implanted within the remaining muscles of the stump. For this reason, many bionic limbs are customized to the user’s specifications. Yet another type of bionic limb utilizes an osseointegrated implant, an implant that is inserted into the living bone and also protrudes from the skin of the stump to provide attachment for a bone-anchored prosthesis. However, a bone-anchored prosthetic does contain possible adverse effects such as breakage of the implant parts resulting in infection.

All these various designs of bionic limbs are classified into three categories, depending on the type of tissue that is interfaced and the type of implant which is used. The three types are nerve and muscle transferring, direct muscle interfacing, and direct nerve interfacing.

Nerve and muscle transferring use targeted muscular reinnervation, transferring the remaining nerves of the amputated stump to nearby muscles to amplify neural control signals. These neural control signals are registered by an electrode which translates them into the action of the prosthesis.

Direct muscle interfacing uses direct intramuscular implants to capture the control signal from residual muscular tissue. Intramuscular implant-based control utilizes small recording devices to record muscular contractions and integrate them into the socket. These muscle contractions then determine the movement of the prosthetic.

Direct nerve interfacing uses implantable peripheral neural interfaces, a connection between the peripheral nervous system and a computer interface, to interface the remaining nerves. This allows for control of the device and for the patient to feel sensation from the device using transformed electric signals from prosthetic sensors to stimulate the nerves in the stump to restore feeling. Through this phantom pain, pain from a body part that is no longer there has been shown to decrease.

There is also a difference between upper bionic limbs and lower bionic limbs since there are significant structural differences and functions. Bionic upper limbs are often focused on the control of the hands to allow for things such as picking objects up while bionic lower limbs focus on support and mobility to enable users to walk.

Bionic upper limbs have multiple different designs and often differ depending on the level of amputation. Currently, bionic hand dexterity is quite limited and is one of the sections for improvement. For example, the main function of a natural human finger is to open close, and a bionic finger mimics this action through a battery-powered motor moving the three joints of the bionic finger. Yet only one of the three joints can move independently, the other two joints automatically follow the first, restricting it to this fixed motion. However, natural fingers can spread out, twist, and perform a greater variety of functions. This is not accounting for a bionic thumb which is far more complex due to the greater capabilities of the natural thumb.

Bionic lower limbs use sensors, a microprocessor, an ankle, a foot component, and batteries to mimic the natural leg. The main purpose of the bionic leg is to replicate the basic mechanics of walking, which is extremely complicated. When walking, humans alternate between a stance phase, when the foot has contact with the ground, and a swing phase, when the foot is above the ground. There are also multiple sub-phases that the process of walking can be divided into. During these sub-phases, the body performs several critical functions while walking, such as shock absorption, arranging the parts of the leg into the correct position, the stretch and the recoil when transitioning between the two phases, and the muscle contraction to propel the body forward. Although the individual parts, muscles, tendons, and many others, often have mechanical equivalents, it is difficult to control and coordinate the multiple parts.

However, this is still a vast improvement from traditional prosthetics. Traditional prosthetics are often used for cosmetic reasons as they provide limited mobility. Bionic limbs can provide the user with more control and movement. As bionic limbs are typically electronically or mechanically powered, differing from the traditional prosthetic which relied on body power, it requires less effort from the user. Although bionic limbs still have a long way to go in terms of improvement, it is a critical innovation for providing those with disabilities with a better quality of life. Overall the variety of designs in bionic limbs show great promise for a variety of different solutions.

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