Digital Theses Archive


Tesi etd-03302020-115707

Type of thesis
Physical Human Machine Interfaces for Lower Limb Prostheses
Scientific disciplinary sector
Istituto di Biorobotica
Istituto di Biorobotica - BIOROBOTICS
relatore Prof.ssa MENCIASSI, ARIANNA
  • Nessuna parola chiave trovata
Exam session start date
This Thesis explores new SRH solutions with high compliance, back-drivability and variable stiffness, to allow for novel functionalities in systems in direct contact with humans, improving the interaction safety through the mechanical design approach. Among the several possible applications in this field, the pHMIs of limb prostheses, i.e., the prosthetic socket systems, have been found as a proper implementation for new SRH solutions because featured by the integration of soft and rigid structures which have to continuously interact with the human body. Indeed, the prosthesis stability is usually obtained by coupling a rigid outer prosthetic socket, guaranteeing appropriate load transmission, biomechanical stability and controllability of the artificial limb, and a soft inner cover, namely the liner, which protects the residual limb tissues and improves the prosthesis comfort. <br>Even if different socket and liner designs can be chosen by the prosthetist taking into account the patient’s features, the majority of the amputee population still deplores several issues, such as poor comfort and dermatological disorders, due to the poor or not-optimal physical interaction with the prosthesis. This is particularly relevant for lower limb prostheses since the residual limb tissues are continuously subjected to high and repetitive stresses during locomotion. Thus, there is a strong need for new smart solutions based on the hybridization of soft liner and rigid socket materials. <br>Based on these concerns, the main objective of this Thesis is the design and the development of a new smart pHMI for lower limb prostheses. This represents a typical robotic application in assistance modality. Other SRH solutions in cooperation and teleoperation modalities have been investigated as well as supporting demonstration of the several possible applications of the SRH design principles in the biomedical scenario.<br>The Thesis is organized in five Chapters, each targeting a step forward in the pathway towards the integration of smart technologies at the prosthetic interface, able to improve the patient’s comfort and the prosthesis fitting, thus resulting in an advanced pHMI. <br>Chapter 1 reports an in-depth analysis of the state-of-the-art concerning current prosthetic socket systems and the main parameters affecting such an interface (i.e., residual limb volume fluctuations, interfacial stresses applied on tissues, temperature and humidity changes within the socket). It also aims at sketching the motivations behind the project, pointing out the limitations and the open challenges in the described domains. <br>Chapter 2 deals with the design and the manufacturing of a specific prosthetic socket system made up of a sub-ischial rigid socket and a personalised soft liner. In this phase, the design was aimed to maximize the possible advantages of the structure in which the smart technologies should be integrated. In agreement with the scientific results reported in the literature, the sub-ischial design might overcome some limitations of previous socket solutions. Then, it was selected as the most advisable design for the new smart socket system pursued in this Thesis. Concerning the prosthetic liner, a personalised design able to perfectly match the residual limb shape could avoid the risk of localized skin tensions and stress concentrations on tissues. However, the current manufacturing techniques turned out to be very time consuming and cost-intensive. Then, a new approach starting directly from the scan of the patient’s residual limb and manufacturing the liner by the cryogenic CNC technique was investigated, thus resulting in a potential alternative of the traditional handmade methods. <br>The socket system described in Chapter 2 allowed for some improvements of the prosthetic interface, but only to a limited extent due to its passive behavior. Then, in Chapter 3 the design specifications for switching from a passive structure to a smart pHMI have been defined. A custom questionnaire was proposed to patients and a user-centered design was followed for the final device. Clinical studies on the main interfacial parameters were carried out; the quantitative results reported in Chapter 3 represent a new knowledge, previously missing in the literature, upon which the design of the new smart pHMI has been based. <br>Chapter 4 deals with the development of a residual limb simulator able to change its volume, resulting a key component for bench tests of the newly proposed socket prototypes. Then, the integration of a variable stiffness interface and the development of an actuated adjustable socket have been reported. The first solution relies on the layer jamming technology - implemented for the first time in the prosthetic socket field - enabling the system to change its shape and stiffness based on the user requirements. When performing dynamic tasks, the interface in the stiff mode featured mechanical properties similar to current socket solutions, thus guaranteeing the required stability. Meanwhile, in static positions, the user comfort was improved by the soft configuration, ensuring shape adaptation and pressure relief on tissues. Regarding the actuated adjustable socket, in order to compensate for limb volume fluctuations, the sub-ischial design was properly modified and divided into four symmetric struts, kept together by a tensioning system controlled by an electric motor and a reduction gear mechanism. The new design improved previous actuated cabled-panel solutions, able to compensate volume changes only in the specific limb areas covered by the panels and leading to not uniform stress distributions. Once a closed-loop control will be implemented as a future evolution, this prototype has the potential to address both the issues of volume compensations and stress monitoring. Indeed, data coming from a sensorised liner could be used to regulate the socket volume by setting a stress threshold at the interface. In this framework, the cryogenic CNC manufacturing technique opened up also a new route allowing for a wide range of sensors to be embedded into the soft liner by slightly modifying the design.<br>The conclusions of the new prosthetic pHMI project and the future perspectives have been sketched out in Chapter 5, by describing possible routes and strategies to address scientific and clinical issues still hampering the design of advanced prosthetic socket systems able to meet all the requirements reported by the amputee population. <br>Finally, in the same streamline of the prosthetic interface project but with different possible applications, at the end of the Thesis novel variable stiffness concepts based on the SRH approach are briefly presented (Other applications of the SRH approach Chapter). While passive Variable Stiffness Joints (VSJs) might have constitute a starting solution for safer robotic designs in the cooperation modality, fluidic VSAs were proposed to actuate the DoFs of a surgical manipulator for MIS (Minimally Invasive Surgery) procedures (teleoperation modality).<br>