By all the laws of physics, a metal structure full of holes should sink the moment it touches water. However, a team of engineers has created a structure that, despite everything, floats and remains afloat even after cracking, crushing, or tearing. A study published in the journal Advanced Materials describes the first-of-its-kind buoyant metal scaffold with open cells—a titanium mesh, which engineers call a lattice. This scaffold reliably floats in seawater and, for its weight, is stronger than common marine materials such as stainless steel and high-density plastic.
Metal scaffolds like this one, composed of tiny repeating elements arranged like a microscopic playground, are already used in aircraft parts, medical implants, and military equipment because they are strong and lightweight. But there’s a catch when it comes to water: the same open structure that makes these scaffolds useful also allows water to penetrate and carry the material to the bottom, no matter how lightweight it appears on paper. Engineers from RMIT University in Australia, working with a colleague in France, have solved this problem by combining titanium with a polyurethane foam core in a way no one has ever achieved before.
Their solution: 3D-printed titanium pillars, hollow inside like tiny tubes, into which expanding polyurethane foam is injected, sealing the internal channels while leaving the outer surface open and porous. This detail is important because the scaffold retains the surface texture that makes it functional while also becoming airtight enough to float.
Buoyancy boils down to a simple idea: an object floats if its overall density, including any water seeping in, remains lower than the density of the surrounding liquid. Previously, engineers relied on bulk density to predict whether a material would float, but this value changes as water penetrates the porous structure over time. Such a frame might appear light enough to float in the lab, then slowly take on water and sink as the air pockets fill.
The solution was a new measurement technique called frame density, which considers only the metal shell and any sealed filler within it, ignoring the open pores that typically allow water to pass through. By designing the titanium struts hollow and sealing these channels with foam, the team was able to calculate in advance how light the sealed areas would need to be for the structure to float.
Using laser 3D printing, the researchers created small titanium cubic frames with hollow struts and then injected expanding polyurethane foam through the channels until it filled all the passages and sealed them from the external environment. They created versions with hollow channels ranging from 2.5 to 4 millimeters wide inside each strut, intentionally designing some to float and others to sink to test whether their density formula could reliably predict the outcome.
Each structure was tested three times, immersed in freshwater and natural seawater from Port Phillip Bay, some for more than two months. The structures without foam core sank each time, as predicted, with noticeable air escaping as their supports filled with water. The foam-filled versions, built with a fairly low seal density, floated without leaks, precisely matching the formula’s predictions, although the actual samples were slightly denser than predicted. A separate two-week soak in seawater confirmed the titanium’s good resistance to corrosion, showing minimal weight loss or strength reduction.
Investigating what happens after the material is damaged yielded the most unexpected result. The researchers compressed foam-filled specimens under a compacting machine, stopping at several stages, from peak stress and initial cracks to complete structural failure, and then submerged each damaged specimen in water to test buoyancy. Separate laboratory tests, combined with computer modeling, showed that cracks invariably began where the foam entered the struts. However, buoyancy was maintained throughout all stages of damage, failing only when the compression became so severe that the sealed areas became denser than water. The foam remained sealed and continued to hold air even as the metal cracked, acting as a built-in backup float, essentially A significant improvement over a conventional sealed metal frame, which would allow water to instantly penetrate if a crack formed.
Structurally, the hybrid also performed well. When compared at the same density, it outperformed high-density polyethylene and 316L stainless steel, two materials commonly used in marine equipment, in terms of strength per unit weight, and the addition of foam had virtually no effect on the frame’s strength or rigidity compared to the unfilled version. To demonstrate that this wasn’t just a laboratory gimmick, the researchers built a small working buoy, roughly the size of a coffee mug, and tested it in real seawater under simulated turbulent current conditions. It floated stably and rocked in the current without external sealing or additional buoyancy aids.
This isn’t just one clever buoy design, but a formula that other engineers could apply in other applications. Since this approach only requires knowledge of the base material density, the researchers say the same design principle could potentially be adapted to other combinations, such as various metals combined with rubber, resin, or biodegradable materials for medical devices. This makes this project a model for creating floating metal structures capable of withstanding real-world damage at sea.
The published results were obtained on small, cuboid laboratory prototypes and one compact demonstration buoy approximately 100 millimeters tall, so it remains to be seen how the manufacturing process will perform on larger, more complex, real-world buoys. The testing periods were also short compared to real-world ocean use: the soaking period in freshwater lasted just over two months, and the corrosion test in seawater lasted only two weeks—significantly shorter than the years of exposure typically experienced by marine equipment.
The researchers themselves note that scaling up presents practical manufacturing challenges, including completely removing residual metal powder from the internal channels, achieving uniform foam filling of larger and more complex shapes, and reliably sealing each channel. The foam filling process was also conducted in open air rather than a vacuum, which the team says was a deliberate choice to simplify and scale the process, although this may not reflect all possible manufacturing conditions.
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