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In the design of wearable robots, the possibility of dynamically regulating the mechanical output impedance is crucial to achieve an efficient and safe human-robot interaction and to produce useful emergent dynamical behaviors. In this paper we propose a Variable Impedance Differential Actuator (VIDA) for wearable robotics applications. The system comprises two actuators (one being an impedance-controlled rotary Series Elastic Actuator) connected through a Harmonic Drive in a differential configuration used to separately control output position and mechanical impedance. Design choices regarding the overall architecture and the single components are presented and discussed. The mechanical structure also comprises a custom-made torsion spring designed after a CAD/FEM optimization. An electromechanical model of the system has been developed and a control strategy, based on the equilibrium point approach, is simulated to validate the performances of the system against system requirements. The actuation architecture allows to implement a control strategy where an equilibrium position and impedance field are simultaneously and independently regulated. This is possible still adopting very simple control laws: two controls for position and impedance regulation of the two input shafts.