We report a group of ultralight mechanical metamaterials that maintain a nearly linear scaling between stiffness and density spanning three orders of magnitude in density, over a variety of constituent materials. We use the term “mechanical metamaterials” to refer to materials with certain mechanical properties defined by their geometry rather than their composition. The materials described here are highly ordered, nearly isotropic, and have high structural connectivity within stretch-dominated, face-centered cubic (fcc) architectures. The ultralow-density regime is accessed by fabricating microlattices with critical features ranging from ~20 μm down to ~40 nm. The densities of samples produced in this work ranged from 0.87 kg/m³ to 468 kg/m³, corresponding to 0.025% to 20% relative density.

A stretch-dominated unit cell structure…is substantially more mechanically efficient—with a higher stiffness-to-weight ratio…than its bend-dominated counterpart. This is attributed to its struts carrying load under compression or tension rather than bending. A fundamental lattice building block of this type is the octet-truss unit cell… The cell has a regular octahedron as its core, surrounded by eight regular tetrahedra distributed on its faces. All the strut elements have identical aspect ratios, with 12 solid rods or hollow tubes connected at each node. The cubic symmetry of the cell’s fcc structure generates a material with nearly isotropic behavior. … On the macroscale, under uniaxial compressive loading, the relative compressive stiffness and yield strength of these structures theoretically show linear scaling relationships…

—Zheng et al.

Architecture of stretch-dominated and bend-dominated unit cells and lattices. (A) Mechanical response to compressive loading of a stretch-dominated octet-truss unit cell. (B) Octet-truss unit cells packed into a cubic microlattice. (C) SEM image of a stretch-dominated lattice material composed of a network of octet-truss unit cells. (D) Mechanical response to compressive loading of a bend-dominated tetrakaidecahedron unit cell. (E) Tetrakaidecahedron unit cell packed into a cubic bend-dominated lattice (Kelvin foam). (F) SEM image of a bend-dominated lattice composed of a network of tetrakaidecahedron unit cells. Zheng et al. Click to enlarge.

Most lightweight cellular materials have mechanical properties that degrade substantially with reduced density because their structural elements are more likely to bend under applied load. The team’s metamaterials, however, exhibit ultrastiff properties across more than three orders of magnitude in density.

These lightweight materials can withstand a load of at least 160,000 times their own weight. The key to this ultrahigh stiffness is that all the micro-structural elements in this material are designed to be over constrained and do not bend under applied load.

—Xiaoyu Zheng, lead author

The observed high stiffness is shown to be true with multiple constituent materials such as polymers, metals and ceramics, according to the research team’s findings.

Our micro-architected materials have properties that are governed by their geometric layout at the microscale, as opposed to chemical composition. We fabricated these materials with projection micro-stereolithography.

—Chris Spadaccini, corresponding author and leader of the joint research team

This additive micro-manufacturing process involves using a micro-mirror display chip to create high-fidelity 3D parts one layer at a time from photosensitive feedstock materials. It allows the team to rapidly generate materials with complex 3D micro-scale geometries that are otherwise challenging or in some cases, impossible to fabricate.

The team was able to build microlattices out of polymers, metals and ceramics. For example, they used polymer as a template to fabricate the microlattices, which were then coated with a thin-film of metal ranging from 200 to 500 nanometers thick. The polymer core was then thermally removed, leaving a hollow-tube metal strut, resulting in ultralight weight metal lattice materials.

The team repeated the process with polymer mircolattices, but instead of coating it with metal, ceramic was used to produce a thin-film coating about 50 nanometers thick. The density of this ceramic micro-architected material is similar to aerogel.

It’s among the lightest materials in the world. However, because of its micro-architected layout, it performs with four orders of magnitude higher stiffness than aerogel at a comparable density.

—Chris Spadaccini

Lastly, the team produced a third ultrastiff micro-architected material using a slightly different process. They loaded a polymer with ceramic nanoparticles to build a polymer-ceramic hybrid microlattice. The polymer was removed thermally, allowing the ceramic particles to densify into a solid. The new solid ceramic material also showed similar strength and stiffness properties.

The LLNL-MIT team’s new materials are 100 times stiffer than other ultra-lightweight lattice materials previously reported in academic journals.

An Ashby chart plotting compressive stiffness versus density for ultralight, ultrastiff mechanical metamaterials and other previously reported materials. Dotted lines indicate contours of constant stiffness-density ratio. Zheng et al. Click to enlarge.

In addition to Spadaccini, Fang, Zheng, the LLNL-MIT research team consisted of LLNL researchers (Todd Weisgraber; Maxim Shusteff; Joshua Deotte; Eric Duoss; Joshua Kuntz; Monika Biener; Julie Jackson; and Sergei Kucheyev); and MIT researchers (Howon Lee and Qi Ge).

The Department of Defense's Defense Advanced Research Projects Agency (DARPA) and Lawrence Livermore’s Laboratory Directed Research and Development (LDRD) program funded the team's research.

Resources

• Xiaoyu Zheng, Howon Lee, Todd H. Weisgraber, Maxim Shusteff, Joshua DeOtte, Eric B. Duoss, Joshua D. Kuntz, Monika M. Biener, Qi Ge, Julie A. Jackson, Sergei O. Kucheyev, Nicholas X. Fang, and Christopher M. Spadaccini (2014) “Ultralight, ultrastiff mechanical metamaterials,” Science 344 (6190), 1373-1377 doi: 10.1126/science.1252291