Design of Bioinspired and Customized Low-Cost Hand Prostheses and Exoskeletons

Hand prostheses and exoskeletons have great potential for restoring lost functions and enhancing the capabilities of individuals with upper limb impairments. Even if such robotic solutions are beneficial for potential users, a high abandonment rate still occurs. In fact, cosmetic devices are often preferred over active ones for their lightness and reduced cost, despite they lack adequate usability and functionality. Moreover, robotic devices usually do not meet the unique needs and anthropometric characteristics of individual users, thus leading to discomfort and difficulties in activities of daily living. Overcoming these limitations is crucial to improve the quality of life of individuals who rely on these assistive technologies. Additive Manufacturing (AM) techniques offer promising avenues for developing customized hand prostheses and exoskeletons that mimic natural hand functions and aesthetics while reducing complexity and costs. The use of these fabrication techniques, especially Selective Laser Sintering (SLS) and Fused Deposition Modeling (FDM), allows tailoring devices to the specific anatomical and motion characteristics of each user, potentially reducing the abandonment rate and improving overall usability. In this context, the main objective of this PhD work is to develop a design methodology for personalized hand prostheses and exoskeletons, aiming to enhance both functional performance and user acceptance. This is intended to be pursued by filling the gap between user needs and device characteristics, integrating advanced digital design tools, anatomical data acquisition, and optimization strategies. A cost-effective design is proposed by implementing AM technologies, privileging personalization and comfort, to prospectively reduce costs and promote the future widespread use of such assistive devices. The method is specifically applied to the design of a hand prosthesis and of an exoskeleton, whose prototypes are developed and tested. In the first part of the thesis, an atlas of 1-DoF planar mechanisms for implementing finger flexion/extension in hand prostheses and exoskeletons, based on 6 links and 7 revolute joints (six-bar linkages), is described. The enumeration process systematically analyzes the topologies and uses three tests to filter out those with open loops, which are isomorphic with respect to others, and which are not compatible with the human finger kinematic structure. This filtering process leads to the identification of 14 resulting suitable solutions and one of them is selected to be adopted in the final devices designed in this thesis. The design process of both prosthetic and exoskeleton devices is indeed pursued based on the identified 1-DoF finger mechanism module as well as by taking into account anatomical characteristics of target users, obtained by means of 3D scanning, and joint trajectories, derived from optoelectronic motion capturing. %, and are produced by means of AM. AM technology is then employed for fabrication. Within the dimensional kinematic synthesis phase, a set of possible solutions for finger mechanisms is calculated, evaluating a multifactor performance index to maximize the similarity with human motion and minimize the reaction forces loading the mechanism components, for the prosthesis, and the human joints/segments, for the exoskeleton. An optimal mechanism is selected based on a sensitivity analysis, to guarantee tolerance to geometric parameter variations introduced by potential fabrication inaccuracies. The final solutions feature a bioinspired intra-finger coupling among joint flexion/extension trajectories and a suitable similarity with the human fingertip (TIP) motion for both devices. The optimal solution is used for the 3D modeling of robotic prototypes. A prosthetic hand prototype is designed and manufactured by using the SLS technology with Nylon 12 material. Both transmission and actuation systems are placed in the palm. The transmission system consists of a series of commercial sliders, pulleys and cable, whereas a rotative DC motor was used for the actuation. In particular, each finger is potentially independent, whereas the ring and little fingers are coupled to flex together. The prosthesis was evaluated through a series of preliminary functional tests to grasp object and perform different type of grips, by implementing a simple control strategy and analyzing the intra-finger couplings and the TIP errors of each finger separately. A good level of similarity was reached with both parameters, resulting in a RMSE_{TIP} below 8~mm. Similarly, a custom hand three-finger exoskeleton is designed and manufactured by using the SLS technology with Nylon 12 for the fingers and FDM technology for the multi-material dorsal structure, with Nylon 12 and TPU95A. The transmission and actuation system consists of two linear actuators, one for the thumb and one for the couple index-middle, and is housed on the top of the hand dorsum. The exoskeleton was evaluated in terms of kinematics, by analyzing the range of motion on a healthy selected user wearing the device. Also in this case, a good level of similarity was reached with a RMSE_{TIP} below 8~mm.