Silicone Phase Behavior Resolves the Softness–Surface Functionality Trade-Off in Emerging Stretchable Electronics

The development of skin-like stretchable electronics is constrained by a persistent trade-off: ultrasoft elastomers such as Ecoflex and Dragon Skin offer mechanical compliance but resist surface modification, whereas stiffer silicones like polydimethylsiloxane (PDMS) enable plasma oxidation, surface modification, and device integration at the expense of stretchability. Here, we show that this trade-off originates from the phase behavior of low-molecular-weight species that soften the elastomer but also migrate to the surface, where they interfere with plasma oxidation and inhibit functionalization. Although removing these additives by solvent extraction typically sacrifices softness for surface functionality, we show that the commercial platinum-cured silicone elastomer Mold Star surprisingly becomes tougher and more extensible upon their removal, with dramatically improved surface reactivity. Mechanical testing, contact angle measurements, and atomic force microscopy demonstrate that native Mold Star contains a dispersed phase of low-molecular-weight species that weakens the network and passivates the surface. Extraction removes this phase, exposes the intrinsic nanoscale surface morphology, and enables robust plasma-induced modification and metal adhesion. These changes fundamentally alter interfacial performance: e-beam gold delaminates and electroless plating fails on native Mold Star, whereas extracted Mold Star supports adherent evaporated films and electroless Ni/Au coatings that remain conductive up to 80% strain. These findings establish that controlling the phase behavior of low-molecular-weight species is a powerful design principle for uniting softness and surface functionality in silicone elastomers, enabling the next generation of electronic skins and bio-integrated devices.