Stronger Than Steel, Lighter Than Water – 3D Printed Micro Trusses

A team at the Karlsruhe Institute of Technology in Germany, led by materials scientist Jens Bauer, have a new 3D printing material which has a higher strength-to-weight ratio than the toughest known engineering materials and boasts a density lower than that of water.

Taking the structure of human bone as their inspiration, Bauer and his team made the nanoparticle material with a form of 3D laser lithography by using alumina layers and a specific honeycomb internal structure.

Coated with a 50 nanometers thick layer of alumina, the tiny structures can withstand loading up to 280 megapascals, a unit used to measure pressure. That makes the KIT structures at least as strong as many forms of steel.

"This is the first experimental proof that such materials can exist," Bauer said.

Porous materials like those found in nature, materials like bone and wood, have long been used as the inspiration for material scientists seeking strength in various parts and structures, but until now, those complex micro-structures have proven difficult to replicate.

Bauer and his collaborators have applied the most recent developments in lasers and 3D printing technology to achieve a next-gen model for the future. Nanoscribe, a KIT adjunct, developed the technique called 3D laser lithography (which focuses a minute laser beam via special lenses) to create the tough structures. That polymer was then coated with an aluminum compound (alumina) to create a pattern which is very much like the internal structure of human bone.

While there are limitations to the technique (the current system can only create objects a few micrometers in size), Bauer foresees a time 3D printing might lead to a whole range of neo-lightweight materials.

"One of (Nanoscibe's) newer machines can make materials in the millimeter-range, but that's about it for now," Bauer says.

That's still too small to create materials suitable for real-world applications, but he considers the work of his team the first step toward a day when those limitations are slowly overcome.

"My work is about design, production and mechanical characterization of micro-architectures," Bauer says. "Miniaturized structural design optimized according to the loading situation enables benefits from mechanical size effects occurring in the nanometer scale. Cellular materials with both high strength and low density can be fabricated."

It comes down to this; enhancing the strength-to-weight ratio of a given material means either attempting to improve the strength or lower the density of the material, and preferably both at once. According to Bauer, the lightest solid materials have a density in the range of 1,000 kg/m3, and only cellular materials like "technical foams" can achieve considerably lower values. He says "cancellous bone" and other natural cellular solids have the benefit of an optimized architecture which provides huge benefits from size effects in material strength.

He calls the forms "micro-trusses and shell structures," artificially created cellular materials which can  exceed the strength-to-weight ratio of all natural and engineering materials with a density below 1 g/cm³.

Have you heard the news? 3D Printer World Expo is coming to Seattle.