Researchers develop method to design synthetic bone and quickly turn the design into reality using computer optimization and 3-D printing

June 17, 2013

Denise Brehm
Civil & Environmental Engineering

Researchers working to design new materials that are durable, lightweight and environmentally sustainable are increasingly looking to natural composites, like bone, for inspiration. Bone is strong and tough because its two constituent materials, soft collagen protein and stiff hydroxyapatite mineral, are arranged in complex hierarchical patterns that change at every scale of the composite, from the micro up to the macro. 

Markus Buehler and team designed and printed this simulated bone and nacre with a brick-and-mortar pattern, shown here with real nacre found in the inner shell of molluscs.  Photo / Graham Bratzel
Markus Buehler and team designed and printed this simulated bone and nacre with a brick-and-mortar pattern, shown here with real nacre found in the inner shell of molluscs. Photo / Graham Bratzel

While researchers have come up with hierarchical structures in the design of new materials, going from a computer model to the production of physical artifacts has been a persistent challenge. This is because the hierarchical structures that give natural composites their strength are self-assembled through electrochemical reactions, a process not easily replicated in the lab. 

Now researchers in CEE have developed an approach that allows them to turn their designs into reality. In just a few hours, they can move directly from a multiscale computer model of a synthetic material to the creation of physical samples. 

In a paper published online June 17 in Advanced Functional Materials, associate professor Markus Buehler and co-authors describe their approach. Using computer-optimized designs of soft and stiff polymers placed in geometric patterns that replicate nature’s own patterns, and a 3-D printer that prints with two polymers at once, the team produced samples of synthetic materials that have fracture behavior similar to bone. One of the synthetics is 22 times more fracture-resistant than its strongest constituent material, a feat achieved by altering its hierarchical design. 

Two are stronger than one

The collagen in bone is too soft and stretchy to serve as a structural material, and the mineral hydroxyapatite is brittle and prone to fracturing. Yet when the two combine, they form a remarkable composite capable of providing skeletal support for the human body. The hierarchical patterns help bone withstand fracturing by dissipating energy and distributing damage over a larger area, rather than letting the material fail at a single point. 

“The geometric patterns we used in the synthetic materials are based on those seen in natural materials like bone or nacre, but also include new designs that do not exist in nature,” says Buehler, who has done extensive research on the molecular structure and fracture behavior of biomaterials. His co-authors are graduate students Leon Dimas and Graham Bratzel, and Ido Eylon of the 3-D printer manufacturer Stratasys. “As engineers we are no longer limited to the natural patterns. We can design our own, which may perform even better than the ones that already exist.” 

The researchers created three synthetic composite materials, each of which is one-eighth inch thick and about 5-by-7 inches in size. The first sample s