Innovative Strategies for Establishing Physical and Functional Coupling with Prosthetic Limbs to Enhance Performance and Embodiment

The relationship between an amputee and their prosthesis is far from perfect. The literature indicates a tendency to abandon prosthetic devices and the reasons for this remain unchanged despite technological advancements. Issues with functionality and fit are among the primary culprits. Currently, people with amputations interact with their prostheses as they would with any other tool, treating it as something different and distinct from themselves rather than an integrated part of their body. This thesis work provides a description of instruments that best enhance the interface that is established between a prosthesis and those who control it: it is not only necessary to improve prosthetic technology, a method that was perhaps considered the sole approach in the early years of research in the field, but also to better prepare the receiving body to accommodate the artificial limb. The first strategy presented is the use of transcranial alternating current stimulation (tACS) to entrain the activity of the frontoparietal network, which is known to be involved in multisensory integration and embodiment processes. Results demonstrated that 40-Hz stimulation and consequent entrainment can increase embodiment indices measured by the rubber hand illusion paradigm. By transferring this technique to amputees, it will be possible to promote the embodiment of the artificial limb and thereby enhance its acceptance. In a more invasive approach, a case study of complete surgical stump remodelling for an upper limb amputee is presented. Cutting-edge interventions, including targeted muscle reinnervation, the agonist-antagonist myoneural interface, and osseointegration, allowed the stump to be functionalized and created a bridge connecting the amputee's body and the prosthesis. This anatomical and functional continuum lays the foundation for optimal integration of the prosthetic limb, creating a unique system where there is no longer a clear boundary between natural and artificial components. To conclude the part where either physiology or anatomy are modified to prepare the body to interface with the artificial limb, the patent for an implantable device is described. The inventors aim to use it to replace myoelectric control with a more intuitive method, potentially enhancing the acceptance of the artificial limb. This device involves a component surgically implanted in the stump, connected to the remaining muscles like a pulley, so that a linear shortening caused by muscle contraction results in the component rotation. This rotation is non-invasively tracked from outside the stump using rotary encoder technology and converted into a control signal for any robotic degree of freedom, such as those in limb prostheses. Shifting the focus to the improvement of the prosthetic devices, the topic of sensory feedback is explored. Along with more intuitive control, the lack of sensory feedback from the prosthesis and its resulting scarce embodiment are among the functional criticisms reported by subjects equipped with these devices. The absence of bidirectional communication, where not only does the amputee control the prosthesis with their motor intention, but the prosthesis also provides a useful flow of information in response to improve control, fuels the sense of estrangement that ultimately leads amputees to reject the artificial limb. The recovery of proprioception, which is the sense of position, movement, and force originating from the limb, is as important for efficient control as exteroception and is explored in a literature review. This review highlights the lack of a universally superior strategy, as methods such as vibrotactile or electrotactile stimulation each have their pros and cons. However, regardless of the method used, feedback enables better performance and potentially more benefits that should be explored in future studies. Expanding the range of investigated parameters will provide a more comprehensive evaluation of the amputee's overall experience with the device. Finally, recognizing the potential of vibrotactile stimulation, the last chapter reports the evaluation of an innovative method to encode and deliver two types of proprioceptive information simultaneously, taking advantage of the possibilities offered by linear actuator technology. The simultaneous delivery of position and force information using a single, compact device, easily integrable into a prosthetic limb, is shown to be feasible and beneficial for the recipients, who learn to use it with minimal training. The techniques introduced target various points within the sensorimotor circuitry that underpins the user's embodiment of the prosthetic limb and its proficient control. The body is adapted to transmit motor intentions intuitively and precisely to the prosthesis, which in turn provides continuous feedback information essential for its control. This multisensory congruency between motor output and sensory feedback fosters the sense of embodiment that grants the acceptance of the artificial limb as a part of the amputee’s new body.