Building ultralight lattices

The stiffness-to-weight ratio is an important figure of merit for the structural engineer. But to reach ultralight densities of 10 mg/cm³ or less, few material options exist: One must usually turn to aerogels or metal foams (see the article by John Banhart and Denis Weaire in Physics Today, July 2002, page 37). Architecture also matters, but both kinds of material consist of a disordered network of cells whose random distribution renders the gel or solid far less stiff than one might predict based on its parent constituents’ bulk properties. Now, Alan Jacobsen, Tobias Schaedler, William Carter (all at HRL Laboratories, Malibu, California), and their colleagues have developed a mechanically stiff, ultralow-density material based on a metal-lattice framework. The team starts with a liquid photopolymer that solidifies when exposed to UV light. Incident light passing through a patterned mask creates an interconnected array of polymer fibers, which the researchers coat with an ultrathin film of nickel–phosphorus. They then etch away the polymer template, leaving a hollowed-out network of metal tubes. The resulting structure, shown here atop a dandelion, can have a density as low as 0.9 mg/cm³, less than that of the lowest-density aerogel but with a stiffness nearly two orders of magnitude greater. Astonishingly, though made of brittle nickel, the microlattice recovers nearly completely—and repeatedly—after being squeezed to 50% of its original size. (T. A. Schaedler et al., Science 334, 962, 2011.)—R. Mark Wilson