Jun.18, 2013
Using computer-optimized designs of soft and stiff polymers and 3D printer, researchers have created artificial bone, a composite that is 22 times more fracture-resistant than its strongest constituent material.
In a paper published online June 17 in Advanced Functional Materials, associate professor Markus Buehler of the Department of Civil and Environmental Engineering and co-authors shared their approach and results.
Bone is strong and tough because it is composed of its two constituent materials: soft collagen protein and stiff hydroxyapatite mineral. The structure of bone is known to be important to its load-bearing characteristics, but relatively little is known about this structure or the mechanism that govern deformation at the molecular scale.
Bones are arranged in complex hierarchical patterns with structure that varies at each level. The hierarchical structures that give natural composites their strength are self-assembled through electrochemical reactions, a process very difficult to replicate in the lab.
This photo shows the brick-and-mortar pattern of simulated bone and nacre against the backdrop of real nacre found in the inner shell of many molluscs. Credit: Graham Bratzel
Now researchers at MIT have developed an approach to print synthetic bone. Using a 3D printer capable of using two synthetic polymers in an optimized geometric pattern researchers produced samples of synthetic materials that have fracture behavior similar to bone.
"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 natural composites, such as bone and pearl, and fracture behavior of biomaterials.
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 simulates the mechanical properties of bone and nacre (also known as mother of pearl). This synthetic has a microscopic pattern that looks like a staggered brick-and-mortar wall: A soft black polymer works as the mortar, and a stiff blue polymer forms the bricks. Another composite simulates the mineral calcite, with an inverted brick-and-mortar pattern featuring soft bricks enclosed in stiff polymer cells. The third composite has a diamond pattern resembling snakeskin. This one was tailored specifically to improve upon one aspect of bone's ability to shift and spread damage.
Researchers put different composite designs to the test to see if they could withstand stress and fracture similarly to bone. As predicted, the bonelike material proved to be the most resistant to fracture.
"Most importantly, the experiments confirmed the computational prediction of the bonelike specimen exhibiting the largest fracture resistance," said graduate student Leon Dimas, who is the first author of the paper.
"And we managed to manufacture a composite with a fracture resistance more than 20 times larger than its strongest constituent."
Buehler hopes that eventually entire buildings might be printed with optimized materials that incorporate electrical circuits, plumbing and energy harvesting.
Buehler's co-authors are graduate students Leon Dimas and Graham Bratzel, and Ido Eylon of the 3D 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."
Posted in 3D Printing Applications
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