Towards a tactile artifical hand

Abstract

Amputation of a hand is a life-changing event, and the loss of motor and sensory functions leads to disability and has devastating effects on the individual. What is normally performed using two hands must be solved with only one, and the loss also affects body balance and body posture. In addition, amputation of a hand has psychological effects, and has an influence on social life, participation, and identity. A hand prosthesis has an important role in reducing the negative effects of an amputation. However, rejection of the prosthesis is common, due to expectations that are too high to be fulfilled and limitations in technical solutions, with possible overuse of the existing hand as a consequence. The overall aim of this work has been to further develop and implement a non-invasive concept for sensory feedback in hand prostheses. Specific aims were to explore forearm amputees’ views of prosthesis use and perception of sensory feedback, to investigate the sensory qualities of phantom hand maps (PHMs) in amputees, to determine whether it is possible to learn to associate sensory stimuli on the skin of the forearm to specific fingers in healthy non-amputee volunteers, and lastly to evaluate a non-invasive sensory feedback system for a prosthetic hand in the everyday lives of forearm amputees.Initial findings indicated that today’s myoelectric hand prostheses allow the wearer to experience agency―the experience of controlling one’s own motor acts―but the lack of sensory feedback appears to limit achievement of a sense of body ownership of the prosthesis. A PHM on the residual limb is a phenomenon seen in many amputees, and when it is touched it generates a perception of touch on the hand that no longer exists. The neurobiological basis of the phenomenon is not fully understood, but it probably originates from plastic changes within the brain following amputation and also changes in peripheral nerves. We have demonstrated that the PHM has better discriminative sensibility than the corresponding skin area on the uninjured arm. Thus, the PHM is an ideal target for a non-invasive concept to achieve sensory feedback in prostheses. Given that not all amputees have a PHM, it was also found that it is possible to learn to associate stimuli on the skin of the forearm with specific fingers, i.e. it is possible to create a PHM. An evaluation of the non-invasive sensory feedback system based on the PHM in a prototype prosthesis during four weeks of use at home was also performed. The participants experienced the sensory feedback as being real, which gave a strong feeling of being complete, linked to body ownership. However, this was not verified by the quantitative measurements. This thesis shows that the PHM may be a possible target for non-invasive somatotopically matched sensory feedback systems in hand prostheses. The fact that it is possible to learn to associate sensory stimuli on the skin of the forearm to specific fingers is also promising for future development of sensory feedback systems, e.g. for congenital amputees or amputees who do not experience a PHM. The long-term goal is that this non-invasive concept for sensory feedback will be applicable to several types of hand prostheses, for various levels of amputation