A wearable Neuroprosthetics system to restore natural thermal sensations in upper limb amputees

A person with a hand amputation not only misses a tool necessary to perform manipulations, but also a sensory organ through which is possible to explore the external world in a wide range of different dimensions, such as texture, softness, size, and temperature. In order to replace the missing limb it is then necessary to restore the information flow in a bidirectional manner: from the brain to the hand, bringing motor commands (efferent) and vice-versa, delivering back the wide range of somatosensory information to the brain (afferent pathway). Conversely, the prostheses currently preferred by people with an upper limb amputation are either completely passive, in the form of cosmetic prostheses, or body-powered through a system of cables connected to a harness, in both cases the motor control is limited (or absent in the case of cosmetic solutions) and no sensory feedback is available. More complex robotic hand prostheses, such as myoelectrical, present the same lack of sensory restoration but have one or more degrees of freedom controlled by the activations of residual muscles. While robotic solutions may seem more appealing for the end user, the abandonment rate is unexpectedly higher than simpler devices, stressing the fact that the acceptability of hand prostheses is indeed influenced by the balance of the beforementioned bidirectional information flow. This thesis starts with the exploration of the concept of realistic sensory feedback and proposes a solution to improve the current state of the art for a modality that was not completely characterized before: temperature. Thermal sensations can bring additional information regarding the manipulated object over touch alone: besides distinguishing hot and cold objects temperature can help discriminate between material, recognize living beings, and detect wet surfaces. Starting from the hypothesis that touching in specific locations of the stump, known for remapping tactile sensations to the phantom hand, also thermal sensations would have been projected we investigated the types of sensations evoked and their prevalence. Tests performed proved the technique to work on 56% of the 26 subjects tested, demonstrating that applies to a wide range of trans-radial amputees. A wearable thermal display (WTD) paired with a specifically designed temperature sensor, capable of mimicking the thermal behavior of the human finger, was then developed. Experiments performed on subjects with amputation with the WTD proved the system to be effective in a real-time scenario, letting the user discriminate temperatures and materials. This study paves the way for a more comprehensive approach to the design of sensory feedback strategies, with multimodal stimulators capable of eliciting natural sensations. Moreover, in the appendix section, additional works will be presented. In particular a publication regarding the illusion of the wetness sensation through temperature stimulation, a tool to improve the pipeline of sensory stimulation experiments and a novel decoding algorithm for intuitive control of lower limb prostheses.