3D Printing Metamaterials, Part 2: Nanoprinting

Continuing in our series on metamaterials, we will be exploring nano 3D printing. Nanoprinting technology is most often dedicated to printing specialty nano-scale biomedical and electronic devices, usually for research purposes. In some cases, however, researchers are looking at how printing microscopic objects can lead to macroscopic changes in physical properties. 

One early project exploring these possibilities was developed by Lawrence Livermore National Laboratory (LLNL), who used a method of projection micro-stereolithography (PµSL) to produce a part capable of supporting 10,000 times its own weight. The crux of this profound power, as is with most of the metamaterials we are exploring in this series, is the geometry of the structure.

LLNL materials engineer Chris Spadaccini put it succinctly: “Our micro-architected materials have properties that are governed by their geometric layout at the microscale, as opposed to chemical composition.”

To produce microlattice structures, the LLNL team used a system that projects ultraviolet light from an LED onto a micromirror which bounces the light through a series of optical components that shrink the size of the beam and cast it onto a photopolymer bath. Testing a variety of lattice geometries, the researchers found that the stiffness and strength of microlattice structures was dependent on their density. 

Though the polymer resin was their base material, the team was able to create microlattices out of metal, ceramic and a polymer-ceramic hybrid by coating the printed objects in these metals and then burn out the polymer core using heat. The resulting objects were even stronger, while remaining extremely light. 

Since this initial research, LLNL was able to expand on it in a variety of ways. For instance, LLNL applied this same technology to a study of how metamaterials and 3D printing could be used to optimize the design of helmets. The lab compared traditional elastomer foam materials to metamaterials made up of 3D-printed polymer microlattices, determining that the 3D-printed polymers aged more slowly than traditional elastomers; however 3D-printed elastomers aged faster than non-printed elastomers. Other research explored copper-polymer composites that shrink when exposed to heat, macroporous gold structures for use in electrochemical reactors, and silicone memory foam.

Lawrence Livermore National Laboratory researcher Cheng Zhu and former Lab postdoc Wen Chen created inks made of gold and silver microparticles and after printing, the 3D parts were heated to allow the particles to coalesce into a gold-silver alloy. The parts were put into a chemical bath that removed the silver (a process called “dealloying”) to form porous gold within each beam or filament. Image courtesy of LLNL. While it’s safe to say that metamaterials made from nanoprinted parts are still at the R&D stages of development, the technology used to make them is already available commercially. Several companies manufacture nanoprinting systems, including Nanoscribe, Microlight3D, Boston Micro Fabrication, Cubicure, UpNano and Swiss Litho AG.

The versatility sample impressively illustrates the capabilities of Photonic Professional systems in 3D Microfabrication.

Even these nanoscale systems, however, can still go tinier. Researchers at the Max Planck Institute for the Science of Light have developed a method described as a precursor to atom-scale printing. The technology couples light to a single atom or to individual nano-particles within a parabolic mirror, allowing for the tailoring of lightwaves.  

The temporal and spatial distribution of a light and the polarization vector or direction of oscillation of an electric field can then be focused onto an object at scales smaller than the wavelength of light itself. The team already used this method to produce nanostructures with unique properties and believes that it might be possible to trap a single atom with laser beams in order to build structures with singular atomic precision.

3D printing at the nano and microscales may be useful for dealing with the miniature worlds of cells, but much of this research is meant to then apply at the macroscale. Therefore, various scientific teams are working to produce these nano-sized objects at a scale that can then be deployed outside of the lab. Virginia Tech researchers are examining how the behavior of architected materials changes when enlarged by seven orders of magnitude. Going beyond two-photon polymerization, the team created metal parts at tens of centimeters in size made up of nanoscale hollow tubes to demonstrate tensile elasticity 400 percent greater than counterparts without architected nanofeatures. 

LLNL itself has worked on scaling up nanoprinting via Large Area Projection Micro Stereolithography, which combines nanoscale printing with traditional stereolithography. Its inventor, Bryan Moran, described it this way:

“The LAPµSL system is conceptually similar to building a mosaic of tiles that then combine to make a much larger picture. Each one of the tiles has a lot of detail and they go together to form the picture that, in turn, has significantly more detail. “It’s a new instrument that can make larger-size parts very quickly and is more useful.”

Observing this technology’s development with the human eye, it would seem to be occurring at a glacial pace. However, to the researchers in the field and to something living at the nanoscale, the development of nanoprinting for the creation of architected materials is moving rather quickly.