## 2. Mechatronics-Twin Framework for Prosthetic Sockets  
As the interface between amputee stump and prosthesis, prosthetic socket constitutes the most important factor affecting the comfort level of prosthesis usage. The operational context is given by gait cycles that characterize the dynamic process of walking. Each gait cycle is defined as a temporal interval that starts with an initial ground contact by the heel of one foot, and ends when the contact is repeated for the same foot [2]. As a fundamental design requirement, an optimized socket should not only ensure efficient fitting, but also guarantee appropriate load transmission under the constraints of operation stability and temperature variations. Normally, a change of the socket design made on the shape and material (_Shape\(\&\)Material_), or the suspension (_Suspension_) for the adhesion between stump and socket, could result in changes to the socket operational conditions that are characterized by the intra-socket pressure (\(P\)), shear stress (\(\tau\)), temperature (\(T\)), and volume fluctuations (\(\Delta V\)) [3]. Such physical conditions determine then the relative movements between stump and socket (_Displacements_) and thereby the operational comfort and bio-mechanical properties (_Comfort\(\&\)Biomechanics_). Especially, among all these physical conditions, an optimized pressure (\(P\)) load condition implies often not only better controllability of prosthetic limb but also lower stump stress and fluctuation. Accordingly, a more precise perception of the pressure load conditions is considered to be a key factor for successful socket design [3; 4; 5]. With this paper, the perception of the intra-socket pressure load conditions is the primary focus, of which the same methodology could be used for the perception of other intra-socket physical conditions of interest. The _mechatronics-twin_ framework serves as an analytical replica for revealing the complex operational interplay of amputee, prosthetic device, and prosthetic socket. As shown in Figure 1, the framework is comprised of a set of virtual and physical services. These include: (1) Biomechanical modeling and simulation of overall gait dynamics; (2) FEA for basic analysis and virtualization of possible intra-socket load conditions; (3) 3D printing and prototyping of specific test stumps and sockets for physical tests; (4) physical testing by _Stewart platform_ with the test stumps and sockets; (5) sensor data acquisition and collection; and (6) operation condition analysis and anomaly detection. Based on some measured basic bio-mechanical properties and operational conditions of the amputee, these services together allow a systematic exploration of possible intra-socket operational conditions. For the overall methodology, the support can be either generic or specific. The generic cases are related to the simulation and testing of some predefined stumps and sockets, in combination with some well-defined in-socket pads for overall state space exploration and design guide. The specific cases are related to individual configurations and analysis together with more specific operational scenarios. It is useful for covering critical situations not solved by the generic cases. As a fundamental step, the specific configuration of a stump needs to be reconstructed by combining and processing images from several noninvasive measurement methods, including 3D scanning for geometry, Computerized Tomography (CT) scanning for the bony structures, and Magnetic Resonance Imaging (MRI) scanning for the stump soft tissue. To determine material properties of a specific amputee residual limb, a modern SWE (Shear Wave Elastography) method [6] is preferrable. The replication by the test stump is then based on a mixture of hyper-elastic silicone for a reasonable conformance with the material properties [7]. Moreover, the _Stewart platform_ is a robotic manipulator where the motion and force profiles can be programmed.