A team from Cornell University has discovered that seemingly unimportant parts of the micro-structure of bone are actually critical to their ability to withstand long-term wear and tear (PNAS 10.1073/pnas.1905814116). This finding could lead not only to advances in studies of osteoporosis, but also to improvements in ultralightweight material design.
Doctors often use X-ray imaging to determine the density of bone and find weak spots before diagnosing osteoporosis. However, X-ray imaging only provides information about load-bearing capability and doesn’t give any information about the lifespan of the bone.
The research team, led by senior author Christopher Hernandez, was interested in bone fatigue and investigating how bone wears down over time because fatigue, the result of repeated load-bearing, has been neglected in previous studies of bone structure.
Bones have a complex internal structure formed by interconnected vertical plates and horizontal rod-like struts. When scientists talk about bone strength, they are referring to the strength of the vertical plates parallel to the direction of force. These bear the most weight, while the horizontal rods have previously been thought to be unimportant in load bearing. In this research, however, the team showed that it is in fact these horizontal rods that define the lifetime of the bone and its ability to resist fatigue.
After testing bones from donors under repeated weight loading, the researchers found that the amount of damage to the bone was not linked to bone density or other measures of microstructure. In an unexpected result, they found that it was actually the thickness of the horizontal rods in the bone, previously thought to be unimportant to weight-bearing, that was linked to the amount of damage caused.
“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,” Hernandez describes. “But if you think about how many cycles of low-magnitude load something can take, these little sideways twiggy struts are what really matter.” The researchers theorized that these horizontal rods act as sacrificial elements of bone structure: taking damage to protect the load-bearing vertical rods. As people age, the horizontal rods are lost, increasing the probability that the bone will break.
Using a 3D-printer, the researchers designed and manufactured structures with a similar micro-architecture to bone structure. By varying the thickness of the horizontal rods, they were able to increase the material’s lifetime by up to 100 times, showing that the effect is not specific to bone and can be generalized to other materials.
This discovery is important beyond studies of osteoporosis. Many modern materials also have a micro-architecture that allows them to be extremely strong as well as lightweight. If these materials are to be used in more durable devices such as vehicles, they will need to be able to withstand fatigue.
A typical strategy to improve efficiency when designing micro-architectured materials is to remove some of the struts perpendicular to the weight-bearing direction, as they don’t experience high loads. However, the research team has shown that this strategy reduces a material’s lifespan. This discovery could lead to more durable lightweight materials for a variety of applications, including the aerospace industry.
“If you want to make a durable device or a vehicle that is lightweight and will last a long time, then it really matters how many cycles of loading the part can take before it breaks” Hernandez explains. “The mathematical relationship we’ve derived in this study lets somebody who’s designing one of these lattice structures balance the needs for stiffness and strength under a single load with the needs for tolerating many, many lower-level load cycles.”