Seemingly trivial changes shown to affect the internal structure and the strength of bone could be used to make biomimetic engineering materials that could be fabricated with those structures using 3D printing. The same insights might also be useful in helping patients with bone diseases, such as osteoporosis. [Torres, A.M. et al. Proc. Natl. Acad. Sci. USA (2019); DOI: 10.1073/pnas.1905814116]

Researchers from Cornell, Purdue, and Case Western Reserve universities point out that for scientists studying osteoporosis X-ray imaging and densitometry are the key methods for analyzing bone structure and pinpointing weak spots but long-term fatigue life, the number of loading cycles bone can bear before it breaks is critical.

"The best way to understand the fatigue properties of a material is to think about a part in your car that breaks every so often, so you have to take it to the shop. Well, why did it break? It was clearly strong enough, because it worked for months, years, just fine. But after cycling and cycling and cycling, tens of millions of cycles, it breaks,” explains team leader Christopher Hernandez. "We've known about this property of materials for 150 years, and it's embedded in the design of everything we do. But not too many people had done this kind of study of the bone."

Bone comprises vertical plate-like struts that endow it with its strength when overloaded. Horizontal, rod-like struts seem to have little effect on strength. However, there are likely to be other features of the structure of bone that affect its characteristics and the team has used a new computer program to carry out a deep analysis of bone samples. It turns out that those horizontal rods are not simply a "cosmetic" feature of bone but affect fatigue life in a critical manner.

"If you load the bone just once, it’s all about how dense it is, and density is mostly determined by the plate-like struts," adds Hernandez. "But if you think about how many cycles of low-magnitude load something can take, these little sideways twiggy struts are what really matter. When people age, they lose these horizontal struts first, increasing the likelihood that the bone will break from multiple cyclic loads."

The team has 3D printed a biomimetic material based on their analyses of bone structure from urethane methacrylate polymer resin. With different print samples, they varied the thickness of the rods. With optimization, they were able to increase the fatigue life of their bone mimic by up to one hundredfold.

The optimized bone mimetic material could be useful in a wide range of engineering applications, particularly in aerospace where strength, endurance, and low density are vital.

David Bradley also writes at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.