Polymeric ionic liquids (PILs) are an emerging class of materials which have attracted considerable attention as solid-state electrolytes because they combine the attractive properties of ionic liquids with the mechanical features of polymers. This paper presents a new method for the synthesis and characterization of stretchable and highly ionically conducting membranes and their subsequent use in truly all-solid-state, flexible, and soft electroactive devices. Linear conductive PIL and reinforcing poly(ethylene oxide) (PEO) network are first intimately entangled during the synthesis of a semi-interpenetrating polymer network (semi-IPN). Polymerization kinetics, thermomechanical properties, as well as ionic conductivity measurements reveal that for the 60:40 wt ratio of PEO:PIL a true synergy of the properties of both polymer partners is achieved, with ionic conductivities up to 8.7 × 10−5 S cm−1 at 30 °C and elongations at break greater than 100%, being both superior to each partners taken separately. The performances of these semi-IPNs as central membranes in all-solid-state electrochemical microdevices, composed of three self-supported and flexible layers, namely poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/semi-IPN membrane/PEDOT:PSS, are successfully demonstrated. Their testing as liquid-free ionic actuators and liquid-free piezoionic sensors undoubtfully proves that electromechanical and mechanoelectrical responses of these all-solid-state microdevices can reach performances identical to that of “classical” ionic liquid-filled systems.