Their solution, published in Science Advances, uses a custom device that concentrates microwave energy into an area smaller than 200 micrometers (0.008 in) – heating only the newly deposited material to above 160 °C (320 °F) while everything around it stays cool.

The device is called a Meta-NFS, short for metamaterial-inspired near-field electromagnetic structure. Think of it as a magnifying glass for microwaves. It combines a split-ring resonator (a tiny loop that traps and amplifies electromagnetic energy) with a tapered tip that squeezes that energy into an almost impossibly small zone.