  Emerging wireless IoT applications increasingly venture beyond over-the-air
communication, such as deep-tissue networking for implantable sensors,
air-water communication for ocean monitoring, and soil sensing. These
applications face the fundamental challenge of significant power loss due to
reflection at media interfaces. We present RF-Mediator, a programmable
metasurface system placed at media interfaces to virtually mask the presence of
the physical boundary. It is designed as a single-layer metasurface comprising
arrays of varactor-based elements. By tuning the bias voltage element-wise, the
surface mediates between media on both sides dynamically and beamforms towards
the endpoint to boost transmission through the interface, as if no media
interface existed. The control algorithm determines the surface configuration
by probing the search space efficiently. We fabricate the surface on a thin,
flexible substrate, and experiment with several cross-media setups. Extensive
evaluation shows that RF-Mediator provides a median power gain of 8 dB for
air-to-tissue links and up to 30 dB for cross-media backscatter links.
