## 1 Introduction  
Transfemoral (above knee) amputation is a surgical procedure performed to remove the lower limb above the knee joint when that limb has been severely damaged via trauma, disease, or congenital defect. Limb prostheses offer a solution to reduce the negative impact of physical disabilities due to the amputation. One key component of a transfemoral prosthesis is the prosthetic socket that serves as the interface between the residual limb and prosthesis. A suitable socket must ensure efficient fitting, appropriate load transmission, stability, and control. With an average prosthetic use of 10 h per day, the comfort from the socket is the most important factor among the artificial limb users. Nevertheless, the design of highly comfortable prosthetic sockets is a difficult task in practice. For example, around 35.3% of amputees reject their prostheses or show a rather low satisfaction level due to comfort issues [1]. Some typical socket-related issues include low bio-mechanical fitness, hampered dynamic control, unnecessary pain, and medical complications (e.g., skin lesions). One reason for this is that most of the operational properties are difficult to know a priori, to observe, and to quantify for socket design and optimization. The intra-socket operational conditions are inherently individual, resulting from very specific volume fluctuations, tissue evolvements, body dynamics, etc.  
This paper presents a novel _mechatronics-twin_ framework that addresses the above-mentioned challenge by serving as an analytical replica for revealing the complex operational interplay of amputee, prosthetic device, and prosthetic socket. The overall approach is characterized by an integration of (a) virtual behaviors based on a combination of advanced bio-mechanical modeling, simulation, and Finite Element Analysis (FEA); and (b) physical behaviors based on well-controlled motions of a six-degree-of-freedom parallel manipulator referred to as the _Stewart platform_. In particular, while the virtual behaviors are useful for establishing basic understanding of fundamental bio-mechanical operational conditions and interactions of individual amputees, the physical behaviors allow a refined investigation of such operational conditions and interactions that are otherwise difficult to observe or realize with the real amputees. Moreover, the framework also aims to constitute an important basis for successful deployment of next generation flexible wearable sensors inside bionic prosthetic sockets for real-time monitoring of dynamic load conditions. By reproducing the operational scenarios of concern, such virtual and physical behaviors allow the training and testing of data-driven algorithms for the sensor functions. With this paper, we also present the design of advanced analysis functions based on Deep Learning (DL) and Bayesian Inference (BI) algorithms for encoding and classifying the measured dynamic intra-socket operational conditions.  
The rest of this paper is structured into the following sections: Section 2 provides an overview of the _mechatronics-twin_ framework. Section 3 discusses related concepts and technologies in the domains of prosthetic design, operation perception, and analysis. Sections 4-6 elaborate the support for modeling, simulation, prototyping, dynamic testing, and analysis. An experimental evaluation of our perception system is presented in Section 8. In Section 9, we provide concluding remarks and discuss future work.