Mechanical metamaterials exhibit unusual mechanical properties such as negative elasticity, Poisson’s ratio, bulk modulus, and thermal expansion, which originate from the topological design of the material. Much research in this field has investigated the acoustic applications of these properties. Now, using a novel additive manufacturing technique with a biocompatible titanium alloy (Ti-6Al-4V), researchers at Delft University of Technology (TU Delft) in the Netherlands have created a pentamode mechanical metamaterial that is orders of magnitude stronger than the previous polymeric versions, making it of possible value in biomedical applications—a “meta-biomaterial.”

Pentamode metamaterials are almost incompressible like fluids, with very little shear resistance; they are sometimes called meta-fluids. The pentamode topology used in this research was first proposed by Milton and Cherkaev in 1995 (J. Eng. Mater. Technol. 117, 483–493 (1995)), but it was impossible to manufacture until 2012, when advanced additive manufacturing techniques became available.  The topology is a lattice structure based on the diamond unit cell; the struts of the cell are double-cone shaped—picture two cones joined at their openings (bases) with their pointed ends (apexes) facing in opposite directions. This structure results in some interesting properties: The base can be made as large as possible without changing the mechanical properties because all the stresses of the mechanical load are going through the apexes.

“This basically means you can decouple mechanical properties from the relative density of this material,” says Amir Zadpoor, Associate Professor of Biomaterials and Tissue Biomechanics (TU Delft). Most porous materials have a power law relationship between relative density and mechanical properties, but this relationship disappears in pentamode mechanical metamaterials.  “Decoupling means mechanical properties can be tuned independently, and that we may someday be able to produce a biomedical implant whose porosity is independent of its relative density,” says Reza Hedayati, a postdoctoral researcher in Zadpoor’s group. “Such an implant would have the required strength to substitute for bone while having the necessary permeability for cells and nutrients to penetrate and bone ingrowth to occur.”

The pentamode topology had been realized in polymers before, but Zadpoor and Hedayati wanted to make a metallic version for the increased strength required for biomedical applications. They chose the Ti-6Al-4V alloy because it has been approved for biomedical implants. Hedayati and colleague Sander Leeflang developed a novel selective laser melting process of the metallic powder in a three-dimensional (3D) printer to create the difficult double-cone lattice structure. A major challenge was determining the laser processing parameters required for optimal energy distribution. The energy density had to be high enough to create overlapping melt pools to yield a fully solid matrix material, while avoiding overheating, which could vaporize some of the alloying elements and make the matrix material brittle. After much experimentation, they developed a vector-based energy distribution strategy. A major criterion of the strategy was that no material point was melted more than three times.

“Laser speed, laser power, the hatching pattern, how you scan—all these determine the energy inputs, the microstructure of the material and the number of pores that you will get,” Hedayati says.

To determine the effects of geometry on this pentamode metamaterial, the researchers created eight samples using four different ratios of strut diameter at the base-to-strut length, along with two different laser beam energy densities (1200 mA and 1600 mA). Subsequent mechanical testing revealed that the mechanical properties of the metallic pentamode metamaterials were indeed independent of the relative density. This demonstration of decoupling is critical, but much research needs to be done before it might be used in biomechanical applications.

“The direction of our research is to rationally design materials to get the properties that you want not by developing a new polymer or alloy, but by putting existing materials in different arrangements in space,” Zadpoor says.  “Before this, you had to be a chemist developing a new polymer or new alloy to achieve some new properties. Now you don’t need to that. You just need very powerful models and 3D printing.”

“These researchers have developed a clever and efficient process to obtain true pentamodals via 3D printing. The experimental results also confirm that their pentamodal lattices break Ashby’s law, which places a rigid constraint between relative density and mechanical properties in cellular materials,” says Fabrizio Scarpa, Professor of Smart Materials and Structures at the University of Bristol, UK, who was not involved in this investigation. “The manufacturing process described in this work and the evidence of breaking Ashby’s law could have far-reaching consequences in the way we design metamaterials for a wide range of applications, from biomedical implants to phononic 3D crystals.”     

Read the article in Applied Physics Letters.