Researchers at RMIT University have created the first buoyant metal-polymer open-cell hybrid lattice metamaterial that floats in water, addressing a fundamental incompatibility between porous metal architectures and buoyancy.
Published in Advanced Materials, the study introduces a hybrid architecture that combines 3D printed titanium lattices with injected polyurethane foam to achieve skeletal densities below 1.0 g/cm³. The breakthrough matters because it decouples buoyancy from the loss of external open-cell permeability.
Conducted at RMIT’s Centre for Additive Manufacturing (RCAM) with collaborators at the Conservatoire National des Arts et Métiers (CNAM) in Paris, the research rests on a reframing of how density should be calculated for porous structures immersed in liquid. Conventional density calculations for lattice structures include all void space, both internal channels and the open pores that connect to the surrounding environment.
That math breaks down the moment the structure is submerged, because water readily infiltrates those open pores, adding mass and sinking the lattice regardless of how light the base material is. Ti-6Al-4V, the titanium alloy used here, has an intrinsic density of 4.43 g/cm³, roughly 4.4 times heavier than water.
To get around this, the team developed what they call “skeletal density,” a metric that excludes externally accessible porosity and accounts only for the sealed mass and volume of the strut walls and their contents.
Having injected expandable polyurethane foam into the hollow internal channels of laser-printed titanium lattices, they sealed those internal channels while keeping the external lattice geometry fully open. The foam expands inside the struts at ratios between 1:9 and 1:12, producing a closed-cell PU infill with a density of just 0.077 to 0.112 g/cm³.
Flotation Tests Confirm the Density Threshold
The numbers held up in practice. Experimental skeletal densities were 7.4% ± 0.6% higher than the computer-modeled predictions, ranging from 0.80 to 1.27 g/cm³ across four lattice variants with internal channel diameters from 2.5 to 4.0 mm. Lattices designed with skeletal densities below 0.997 g/cm³ floated in freshwater for more than two months with no visible air leakage or water ingress. Those above the threshold sank, exactly as the framework predicted.
The hybrid lattices also outperformed conventional marine materials on a strength-per-unit-density basis. At matched bulk densities of around 0.27 g/cm³, the titanium-PU hybrid delivered a yield strength of 10.3 MPa, roughly 1.9 times the density-scaled strength of high-density polyethylene and 1.5 times that of 316L stainless steel.
After two weeks immersed in natural seawater sourced from Port Phillip Bay, the specimens lost only 0.15% of their mass, with yield and compressive strength dropping by less than 1%.
Perhaps the most striking result involved damage tolerance. The team subjected floating lattices to controlled compression and checked buoyancy at each stage of failure. The structures continued to float after reaching ultimate compressive strength, after initial node fracture, and even after complete layer fracture.
Buoyancy was lost as densification began, when volumetric compression physically compacted the lattice enough to push skeletal density above that of water. The predicted critical flotation threshold for the 4.0 mm variant was roughly 26% engineering strain, while the measured strain at densification was 37.2 ± 0.006%. The paper also notes that the tested stress states were substantially higher than those expected under typical service conditions.
To demonstrate application-level feasibility at a functional scale, the team fabricated a buoy prototype, 100 mm tall and 85 mm wide, and tested it in seawater under turbulent flow. It floated stably despite continuous rotation of up to 45 degrees about its central axis, without any external sealing or auxiliary buoyancy aids.
The skeletal density framework itself may prove as consequential as the specific titanium-PU combination. Because it depends only on intrinsic material density and channel geometry, it applies in principle to other alloys, other polymers, and other liquids, offering a general design rule for engineering buoyancy into structures that were previously assumed to sink.
Repurposing Hollow-Strut Lattices for Buoyancy
The hollow-strut lattice architecture at the center of this work did not originate as a buoyancy concept. The same RMIT lab introduced hollow-strut lattices in Ti-6Al-4V that combined two complementary lattice topologies into a single multi-topology design.
That structure was 50% stronger than cast magnesium alloy WE54 at comparable density, and it was optimized for load distribution, not flotation. The work solved a long-standing stress concentration problem by merging a second lattice topology into the overall structure to redistribute load at the nodes.
Interestingly, the hollow-strut architecture also left an internal hollow volume within each strut. In the new buoyancy work, those hollow channels become the injection pathway for polyurethane foam, allowing the researchers to seal the internal volume while preserving the lattice’s external open-cell geometry.
In other words, the structural architecture created the physical precondition for the buoyancy solution without having been designed for it.
Titled “Breaking the Surface: Buoyant Metal-Polymer Open-Cell Hybrid Lattice Metamaterials,” the study was conducted by Jordan Noronha, Joey Tallon, Raad Omar, Jason Dash, Andrey Molotnikov, Martin Leary, Milan Brandt, and Ma Qian.
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Featured image shows Dr Jordan Noronha holding a sample of the floating titanium. Photo via Sara Tan, RMIT University.
