Focused microwaves let 3D printers fuse circuits onto almost any surface

Researchers at Rice University presented a method using a device called Meta-NFS that selectively heats and sinters only freshly printed conductive ink within an area of under 200 micrometers.

• The heating raises the ink's temperature to above 160°C while keeping the surrounding substrate cool.
• While existing near-field microwave sintering devices have an energy transfer efficiency of around 8.5%, Meta-NFS pushed this up to 79.5%.
• The graphene interlayer absorbs up to 50% of the microwave energy, enabling far more localized heating than infrared laser methods.
• This allows the electrical resistivity of silver nanoparticle ink to be tuned across a range of more than 3 orders of magnitude, and enables spatial programming of function within a single print process.

Verification demonstrations were also conducted.

• Conductive microstructures were printed directly onto a living plant leaf, plastic, silicone, paper, and a bovine femur.
• On the bone surface, a wireless strain sensor was implemented to detect micro-deformations and transmit them wirelessly.
• Sensors were also printed on medical implant materials such as ultra-high molecular weight polyethylene, pointing toward real-time monitoring of wear and stress.

Unprotected circuits broke down in water in about 2.5 seconds, while silicone-encapsulated circuits maintained conductivity for over 300 seconds. The research team is expanding applications toward ingestible electronics, bionic devices for organ interfaces, and soft robotics.