Polyurethane foam inside hollow titanium struts lets an open lattice float

Open-cell metals are already widely used in aerospace and biomedical parts that need to be both light and strong ^[1]. Water is the problem. Bulk density divides a lattice's mass by its whole envelope, open voids included, so a structure can sit well below water's density on paper and still sink once its connected pores flood ^[2].

Ma Qian, a materials engineer at RMIT University in Melbourne, co-led the work ^[16]. He worked with colleagues at RMIT's Centre for Additive Manufacturing and the Conservatoire National des Arts et Metiers in France, and they report a stricter figure in Advanced Materials ^[4]. Their "skeletal density" counts only the parts of a structure that keep water out. If it is lower than the liquid's density, the structure floats even with water running through every open pore ^[5].

The rule then says what to build. A hollow-strut lattice has millimetre or sub-millimetre channels inside each strut, and it is stronger than a solid-strut lattice of the same density ^[6]. Those channels can be filled without closing the open cells around them ^[7]. The team injected expandable polyurethane foam of 0.08 to 0.11 g/cm3 into a lattice of the titanium alloy Ti-6Al-4V ^[8]. That is roughly 8 to 11% of the density of the freshwater they measured ^[17]. The foam fills the struts completely, and its closed cells, about 10 to 200 micrometres across, exclude the water ^[9]. "The closed-cell PU foam limits water ingress into the internal strut channels while the external open-cell architecture remains permeable, so safeguarding marine buoyancy without notable density gain," Qian said ^[10].

The control is the strongest part of the design, because it isolates the foam. Unfilled lattices left in freshwater for more than two months all sank, whatever their strut diameter ^[11].

The thing this doesn't tell you is how the filled lattices fared in that same freshwater, although they were submerged alongside the unfilled ones ^[11]. The hybrids' reported flotation is in natural seawater over two weeks ^[13]. The team measured that seawater at 1.03 g/cm3, against 0.997 g/cm3 for their freshwater ^[12]. Seawater is about 3.3% denser ^[18], so it gives a floating lattice a slightly easier test.

Two weeks is also the length of the seawater durability record. Over that period the hybrids lost about 0.15% of their mass, which the researchers take as a sign of corrosion resistance ^[13]. That is a short exposure for a material whose first proposed uses are seafaring vehicles, lightweight floating structures and rescue equipment ^[15].

On strength, the foam-filled lattices showed higher specific strength than high-density polyethylene and 316L stainless steel, two materials commonly used at sea ^[14]. For a part that has to carry load and float, strength relative to weight is the comparison a designer would ask for first.

I think the skeletal-density rule will outlast this particular pairing of foam and alloy, because it applies to any permeable lattice whose internal volume can be sealed. Qian puts the claim more broadly. "The innovation opens a new design space for marine structures, vehicles and rescue equipment where we need strength, low weight and buoyancy all at the same time," Qian said ^[3].