One of the most important features for a system capable of working in uncertain and unstructured environments is reliability. Nowadays robots are excellent machines, but are still not able to interact with their surrounding environment as humans or animals do. Recent studies highlight the role played by impedance changes in the human arm during manipulation tasks. In particular the possibility to vary the stiffness of shoulder, elbow and wrist allows humans to interact easily with fast changing environments while rejecting unpredictable noise disturbances [1]. Several studies also showed how the capability of ‘co-contracting’ antagonistic muscles is required to interact with noisy/unpredictable environments. Starting from these premises we recently proposed novel design principles to build actuators with the ability to actively regulate the passive noise rejection (i.e. the ability to cancel the effect of disturbances without explicitly relying on feedback) [2]. In the present paper we implement these principles in the mechanical design of a novel actuator. The actuator is composed of two electric motors in agonist-antagonist configuration. The final design includes also four non-linear springs whose force-displacement characteristic has been customized on the specific application requirements. Validation of the proposed non-linear spring design has been conducted on a prototype and results are reported in this paper. Future works foreseen the integration of the proposed actuators on a two limbed robot with six artificial muscle, (three agonist-antagonist pairs) in a simple and bi-articular configuration.