Disability statistics suggest that 1 billion people (around 15% of the world’s population) are living with a physical disability and around 190 million adults have a major functional difficulty. In contrast, the number of amputees and individuals born without limbs who gain access to prosthetics to restore movement or function is very small.
The vast majority of those who are physically disabled have little to no access to modern prosthetics (artificial hands, arms or legs), nor do they have access to the skills or resources to fit them despite recent advances in prosthetics. In developing countries, the financial and logistical barriers to obtaining bionic limbs are especially high. The only option for many who are disadvantaged is to obtain prosthetics (artificial arms, hands and legs) that replace the limb but provide no voluntary movement.
The next step for many amputees is to obtain prosthetics that they operate myoelectrically. In this situation, a battery and electronic system are used to create muscle and nerve movement in the residual limb via sensors that are installed after the device is securely attached (using suction technology). For example, a prosthetic hand is controlled by translating muscle signals e.g. the contraction of wrist flexors and extensors to close and open the hand. UK-based Touch Bionics manufactures bionic hands with fingers that move individually in response to signals from electrodes placed over muscles in the upper arm. If muscle signals can’t be used to control the prosthesis, then different types of switches can be used. A greater number of tasks and more dexterity are achieved with the use of sensors and motorised controls.
Prosthetics that use myoelectric sensors are increasingly affordable. Companies like Open Bionics design their prosthetic hands to be ‘open source’ so they can be built from parts made on commercial 3D printers. Electronic signals can be delivered by relatively cheap, off-the-shelf chips. However, the user’s level of control is very reliant on the fit between the stump and the prosthetic. The sensors must sit on the right areas of skin.
In some cases, procedures may be undertaken to reconnect nerves (those that would normally be connected to arm muscles) to the pectoralis muscles. As the individual thinks about moving their arm or hand, they flex the pectoralis muscles and this movement is picked up by external electrodes that send a message to the prosthetic that the person is wearing. Similar techniques of targeted muscle re-innervation (TMR) are used to control the movement of leg prostheses. However, myoelectric sensors tend to work better in hands and arms than in legs because the movement of knees, feet and ankles is more autonomous and less consciously undertaken.
Current research is focused on either improving (1) the design of prosthetic limbs and socket technology or, (2) the design of new prosthetic systems that integrate with the body i.e. osseointegration that joins the prosthesis to bone. Unfortunately, there are few prostheticians even in large cities (practitioner numbers are not increasing) and multiple appointments are sometimes needed to customise prostheses. After an early consultation and a long wait, the stump has sometimes changed in shape or size due to weight gain and even the best-designed sockets can slip and cause discomfort and pain (especially on lower limbs). For this reason, osseointegration or bone anchored prostheses is a growing area of clinical practice.
Introduced in Sweden in the 1990s, osseointegration requires less follow-up appointments than standard prostheses and most people who have this procedure do enjoy a much improved quality of life. For transfemoral amputees (i.e. amputations that pass through the femoral artery), research has shown that bone-anchored prostheses deliver the best hip and pelvic motion when a person is walking, better perceptions of vibration, improved comfort while sitting and better overall functionality. However, phantom pain in the vicinity of missing limbs still continues after osseointegration. Stress is also felt where the osseointegrated prosthetic implant interfaces with residual bone.
Researchers are focused on any potential failures in the osseointegration that might come about due to stress in the vicinity of the bone integration. Understanding whether this stress increases or reduces when different prosthetic designs are used is a priority. Research is also ongoing to identify ways to resolve phantom pain – there is some evidence that neural sensory feedback e.g. sensations of knee motion and the sole of the foot touching the ground can decrease fatigue, increase walking speed and also reduce the phantom limb pain (Petrini et al, 2019, Nature Medicine, 25).
Delivery of genuine bionic mobility is no doubt the most desired outcome – a prosthetic limb that smoothly integrates with the person’s neuromuscular system and brain to enable movements such as flexing, bending and grasping.
Bionic mobility is achieved through the interaction of thought, action and response. For sustained movements (bionic mobility), microelectrodes are implanted in specific areas of the motor cortex to give the strong signals needed to control bionic limbs. A new electronic pathway connects the mechatronic limb with the brain and peripheral nerves are bypassed. Research in bionic mobility is advancing across different continents, but consumer access to thought-controlled mobility trials and related technologies is still limited.
Winners of the Bionic Mobility Challenge will deliver a ‘nextgen’ innovation with practical benefits for health consumers in one of the following areas:
Prosthetics design, manufacturing and/or their human integration (i.e. socket technologies or osseointegration processes)
Myoelectric technologies and sensory electric technologies that enable mobility
Bionic mobility enabled by a brain-machine interface (thought activated mobility) or personalised computational neuromuscularskeletal models that use finite element modelling, smart wearables and machine learning to achieve mobility
Brain-computer interfaces and technologies that lessen or eliminate ‘phantom limb pain’ and/or enable a sense of touch or pressure in bionic limbs to improve human mobility, balance and gait
Allied health services and practices that give those who are physically disabled a greater capacity to benefit from bionic limbs and related technologies.