A team led by RMIT University researchers has 3D printed a titanium lattice that floats and stays afloat even after severe damage–the first documented floating metal-hybrid lattice metamaterial.

Hollow, interconnected titanium struts filled with polyurethane foam let water pass through the lattice while the sealed struts held it up. Samples floated in freshwater for more than two months.

“Although metallic lattices can be incredibly light–with densities less than one-tenth the density of water–their open, interconnected spaces allow water to enter, causing them to sink,” said Jordan Noronha, Lead Researcher at RMIT University’s Centre for Additive Manufacturing.

“By filling only the hollow titanium struts with polyurethane foam, we created a structure that allows water to flow through it while remaining buoyant even after significant cracking and damage,” he said. “Tiny, sealed cells in the foam trap gas and prevent water from flooding the hollow struts.”

The team introduced a measurement called skeletal density, counting only water-excluding parts (titanium walls and sealed, foam-filled channels) rather than total volume.

“This gives engineers a simple design rule: if the skeletal density is lower than that of the surrounding liquid, the structure will float – even when water flows through all its external openings,” Noronha said.

At equal density, the lattice measured 70% stronger than stainless steel or high-density polyethylene. Two weeks in Port Phillip Bay seawater cost it 0.15% of its mass and under 1% of its strength, and a printed buoy stayed stable in tanks rotated to 45 degrees, without casing, coating or added flotation.

Ma Qian, Distinguished Professor and project leader, said the team would scale up parts and test long-term marine and deep-sea performance.

“By changing the material inside the titanium framework, we could tailor a similar structure for energy absorption, thermal management, vibration control and other applications,” he said.