Titanium has a specific gravity of 4.43 g/, more than four times heavier than water. Drop a solid piece of the metal into the ocean and it sinks to the bottom—basic physics. Marine structures like ships and buoys have traditionally achieved buoyancy by enclosing large sealed cavities to bring the overall average density below that of water. But if the outer shell cracks and water floods in, that buoyancy is lost. Metal lattices (periodic grid structures) with open pores that allow liquid to pass through can achieve an apparent structural density far below that of water, yet because water seeps into the gaps, they cannot avoid sinking either.
In September 2026, an international research team centered at RMIT University's Center for Additive Manufacturing published a design approach that overcomes this constraint in the journal Advanced Materials. The composite material fills only the interior of hollow titanium alloy struts with closed-cell polyurethane foam, while leaving the external space of the lattice open so seawater can flow freely through it. The paper is titled "Breaking the Surface: Buoyant Metal-Polymer Open-Cell Hybrid Lattice Metamaterials" (DOI: 10.1002/adma.74641). The research team frames this as a demonstration of a buoyant metal-hybrid open-cell lattice metamaterial.
The study was led by RMIT University's Center for Additive Manufacturing in Melbourne, Australia, conducted as an international collaboration with France's Conservatoire National des Arts et Métiers (CNAM). Funding support came from the Australian Research Council (ARC) and RMIT University's School of Engineering. The research team includes lead author Dr. Jordan Noronha, project leader Distinguished Professor Ma Qian, Associate Professor Andrey Molotnikov, Distinguished Professor Milan Brandt, and Professor Martin Leary. RMIT University describes this result as the world's first demonstration of a buoyant metal-hybrid lattice metamaterial. However, it should be noted that all reported data comes from small 3D-printed test specimens and a single small buoy prototype tested in a laboratory water tank—not from data gathered from marine infrastructure deployed in actual ocean environments.
The Design Principle of "Skeletal Density" for Floating Without a Sealed Shell
In conventional buoyancy engineering, flotation calculations have been based on the apparent average density derived from the overall volume and mass of a structure. For a sealed hollow vessel, buoyancy is determined by the volume of liquid displaced by the trapped air. But in porous materials with an open exoskeleton, seawater flows directly into the lattice's openings. Space filled with water no longer generates buoyancy. As Dr. Noronha points out, metal lattice structures have the potential to reduce apparent geometric density to less than one-tenth that of water, yet once water infiltrates the interconnected open pores, the structure quickly sinks under gravity.
To address this constraint, the RMIT research team applied a concept called skeletal density. Skeletal density is the effective density obtained by excluding openings accessible to external water from the calculation, and dividing the mass of only the titanium walls and sealed internal foam channels that continuously repel water by the total volume they displace. Even if water quickly passes through the external geometric gaps, as long as the combined skeletal density of the regions that physically exclude water remains below the density of the surrounding liquid (approximately 0.997 g/ for fresh water), the structure as a whole continues to float.
Metal 3D printing technology was used for fabrication. The metal powder used was titanium alloy Ti-6Al-4V (true density 4.43 g/), and laser powder bed fusion was used to print a lattice skeleton in which fine hollow struts connect three-dimensionally. Expandable polyurethane foam was then injected into the internal channels of the hollow struts. The foam expands within the struts at a volume ratio of 1:9 to 1:12, forming a closed-cell structure with a density of 0.077–0.112 g/ that then cures. No resin was left in the external lattice openings, maintaining flow channels that allow water to pass through freely.
The research team fabricated four types of lattice specimens with internal channel inner diameters varying from 2.5 mm to 4.0 mm. The measured skeletal densities ranged from 0.80 to 1.27 g/. Compared with computer model predictions, the measured values were 7.4% ± 0.6% higher, attributable to surface roughness and minor geometric deviations characteristic of additive manufacturing. In flotation tests using fresh water (density approximately 0.997 g/), specimens whose calculated skeletal density fell below this threshold floated stably for more than two months with no visible air leakage or water ingress observed. Meanwhile, specimens exceeding the threshold sank promptly, confirming in a laboratory setting that theoretical skeletal density predictions hold up as a valid design guideline.
Comparing Specific Strength at Equivalent Density, and the Reality Behind 10.3 MPa
In marine floating structures, weight reduction and mechanical strength have always been competing demands. Corrosion-resistant stainless steel and high-density polyethylene (HDPE) are widely used, but steel is heavy and resin materials face limitations in rigidity and impact resistance.
In its announcement, RMIT University stated that at equivalent apparent density, this titanium-polymer composite lattice structure showed 70% greater strength than stainless steel and high-density polyethylene commonly used in marine applications. However, to accurately understand the context behind this figure, one must carefully distinguish between specific strength normalized by density and the absolute strength of the material itself.
In measurements taken with apparent bulk density aligned at approximately 0.27 g/, the titanium-polyurethane composite lattice recorded a compressive yield strength of 10.3 MPa. This corresponds to roughly 1.5 times the yield strength of a porous 316L stainless steel lattice reduced to the same bulk density, and roughly 1.9 times that of high-density polyethylene. In terms of withstanding high loads relative to low density, this porous structure demonstrates excellent compressive properties.
データを表で見る
| Compressive Yield Strength (MPa) | |
|---|---|
| Porous 316L Stainless Steel | 6.9 |
| Porous High-Density Polyethylene (HDPE) | 5.4 |
| Titanium-Polyurethane Composite Lattice | 10.3 |
As the chart shows, the titanium-polyurethane composite lattice clearly outperforms existing materials in yield strength per unit density within the low-density range.
However, the absolute yield stress value of 10.3 MPa itself is far smaller compared to bulk metal materials (for example, standard Ti-6Al-4V has a yield strength exceeding 800 MPa). This characteristic is not intended as a direct substitute for applications bearing enormous loads across broad surfaces, such as massive breakwaters or the primary structures of large vessels; rather, it targets areas requiring both extremely low specific gravity and moderate structural strength simultaneously, such as lightweight floating sensors or specialized buoys.
| Material/Structure | Bulk Density Compared () | Strength Metric and Measured Value | Relative Ratio to Composite Lattice | Test Conditions and Limitations |
|---|---|---|---|---|
| Titanium-Polyurethane Composite Lattice | ~0.27 | Compressive Yield Strength: 10.3 MPa | Baseline (1.0x) | Small 3D-printed specimen, static compression test |
| 316L Stainless Steel (Porous) | ~0.27 | Compressive Yield Strength: ~6.9 MPa | ~0.67x (Composite ~1.5x superior) | Benchmark comparison at equivalent density |
| High-Density Polyethylene (HDPE) | ~0.27 | Compressive Yield Strength: ~5.4 MPa | ~0.53x (Composite ~1.9x superior) | Benchmark comparison at equivalent density |
Initial Degradation in Seawater and Buoyancy Retention After Failure
Offshore structures are constantly exposed to both the mechanical impact of waves and chemical corrosion from salt. To evaluate the durability of the material, this study conducted immersion tests using natural seawater collected from Port Phillip Bay in Melbourne.
After two weeks of seawater immersion testing, the lattice's mass loss rate was limited to just 0.15%. The simultaneously measured decline in compressive yield strength and compressive strength was less than 1%. This suggests that the stable passive oxide film formed on the titanium surface showed strong resistance against initial seawater corrosion.
An even more striking result emerged from observing failure behavior under controlled mechanical compression. Conventional single-cavity floats lose all buoyancy the moment even a single crack appears in the outer shell, causing cascading water ingress. In contrast, the composite lattice in this study maintained buoyancy even after struts cracked, major node connections fractured, and an entire layer of the lattice was completely destroyed.
Buoyancy was lost only once the structure reached the densification stage, where the entire structure was crushed and the voids compressed. This occurred because the overall skeletal density exceeded the density of water as a result of the extreme volumetric compression. For specimens with 4.0 mm inner-diameter channels, while the theoretical model predicted a critical engineering strain for buoyancy loss of approximately 26%, the measured densification strain reached 37.2% ± 0.006%.
Dr. Noronha explains that the fine closed cells of the polyurethane foam trap gas internally, and even after the outer wall fails, they physically block water from infiltrating the interior of the struts—a key factor behind this damage tolerance. The distributed arrangement of countless microscopic bubbles served as a safety margin preventing localized failures from becoming catastrophic. The paper notes that the stress conditions applied during testing far exceeded the loading conditions expected under normal operational environments, confirming the material's resilience under extreme external forces.
Water Tank Testing of the Prototype Buoy and the Distance to Practical Application
After verifying the theory and fundamental properties, the research team fabricated a prototype buoy to demonstrate functionality. The additively manufactured buoy is small, standing approximately 100 mm tall and 85 mm wide. This prototype has no sealed outer casing, anti-fouling coating, or additional flotation elements.
When placed in a turbulent seawater tank simulating wave action, the prototype buoy maintained its posture and remained stably afloat even under the harsh flow conditions of continuous underwater rotation up to 45 degrees. The fact that waves could pass through the lattice openings, mitigating direct drag forces from the fluid, is thought to have also contributed to this stability.
Distinguished Professor Ma Qian, who led the project, identifies the next steps as manufacturing larger demonstration components and evaluating long-term behavior under real ocean conditions and deep-sea environments. The fact that the natural seawater immersion test was conducted over a short two-week period is insufficient to determine the service life of marine structures, which can span decades. Questions remain to be answered by future testing: how long-term biofouling affects water flow through openings and skeletal density, and whether the internal polyurethane foam, subjected to prolonged exposure to ultraviolet light and hydrostatic seawater pressure, might undergo hydrolysis or cell collapse.
Professor Qian also mentioned the possibility of replacing the material filling the titanium skeleton with functional resins other than polyurethane, or with phase-change materials, potentially expanding into other applications such as energy absorption, thermal management, and vibration damping. However, these are future proposals based on design philosophy, not facts that have been experimentally verified at this stage.
The combined manufacturing process of high-precision metal 3D printing and internal resin filling currently faces significant constraints in terms of manufacturing cost and production scale. It is not yet at a stage where it can be immediately applied to large-scale civil infrastructure such as harbor piers or large offshore platforms. Verification of scale-up continues, to determine whether the design principle demonstrated in a small laboratory water tank can be elevated into an industrial technology capable of withstanding harsh real-world ocean conditions.