Microstructures like this one developed at Washington State University could be used in batteries, lightweight ultrastrong materials, catalytic converters, supercapacitors and biological scaffolds. Washington State University

The rapid rise of 3D printing has driven innovation in areas as diverse as manufacturing, bioengineering and food science. Now, researchers from Washington State University (WSU) have developed a method which can print metal structures with complex 3D architectures, controlling details down to the nanoscale and closely mimicking the architecture of natural bio-materials like wood and bone.

Their work is published in Science Advances.

While the printing of softer materials, like polymers, is more established, printing metals has been a formidable challenge for engineers. Existing techniques involve depositing powdered metal using powerful lasers, or depositing metal onto a polymer template and burning away the polymer scaffold.

Rahul Panat, who led the project, and his team instead used a technique which prints tiny microdroplets of water containing silver nanoparticles. As the droplets evaporate, the nanoparticles are left behind to form a metal structure, with precise control of the 3D shape. While this work only made use of silver nanoparticles, the technique could also be applied using other materials, such as ceramics or other metals.

After printing, the materials are heated to 200 °C causing the nanoparticles to fuse together, creating strong structures with features as small as 20 microns (around a third the width of a human hair). By controlling these heating conditions, the researchers could also control the size of pores in the material down to the nanometre scale.

This technique is likely to find other applications in batteries, supercapacitors and biological scaffolds.

“This technique can fill a lot of critical gaps for the realisation of these technologies,” Panet says.

The WSU team printed a variety of structures, including pillars and tiny accordion-like assemblies which were used as stretchable wires to connect micro-LEDs. This demonstrates the potential of this technique for making microelectronic devices, which could make their way into wearable or implantable electronics.

But the potential strength of these materials lies in the ability to control structure across several orders of magnitude, from the nanometre to the centimetre scale. This means that 3D lattices can be printed with hierarchical structure, meaning that the structural elements – the rods that connect the lattice – can themselves be made of lattices of even smaller rods. This hierarchy of structure is often seen in biological materials like bone, and can give a massive increase in the compressive strength of a material. In the future, we might see biological implants and artificial bones printed using this method, making them lighter and stronger than materials used today.