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. 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).

The presentation will focus on a feedback 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 specific locations of the stump, known for remapping tactile sensations to the phantom hand, can also project thermal sensations, we investigated the types of sensations evoked and their prevalence. Tests performed proved the technique to work on 15 of the 27 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 amputees with the WTD proved the system to be effective in a real-time scenario, letting the user discriminate temperatures and materials. A subsequent set of experiments started from the full integration of the WTD in the socket of an upper limb prosthesis and the ATS on a robotic hand. The subject was then able to perform sensorimotor tasks with the first, up to our knowledge, bidirectional hand prosthesis with thermal feedback. The first important result is that the thermal projecting spot chosen for the experiment was stable up to 260 days after the first thermal mapping, proving how the technique is applicable to a real-world scenario. The subject was able to replicate all the tests performed with the non-integrated WTD, such as temperature discrimination and material classification, with the difference of having an active role during each experiment, voluntarily grasping and touching the objects of interest. Thanks to the active role of the subject enabled by the new integrated prosthesis, two more experiments were performed: bodily contact classification and a novel sensorimotor functional test. The bodily contact experiment investigated the ability of the subject to exploit thermal and tactile cues (the latter only mediated by the socket), highlighting the importance of thermal sensations in the experience of human contact. The last test is a modified version of the classical functional box & blocks test, with the added task of discriminating two different temperatures while moving the blocks from one side to the other of the board. The results presented in prove the feasibility of a non-invasive feedback technique for restoring thermal sensations, ready to be integrated with other both invasive and non-invasive feedback techniques, aiming for the first time not only to restore a small subset of hand functionalities, but recreate the full spectrum of human experience.