X-ray microtomograms of trabecular bone geometry across six orders of magnitude of animals’ body mass. (a) lesser dwarf shrew, (b) Arctic fox, (c) Przewlaski’s horse and (d) Asian elephant. Credit: M. Doube et al; Proc. R. Soc. B.
There’s a significant contingent of ceramists and other materials scientists and engineers who use information about natural bone structure to make advanced biomaterials and to create new engineering designs for nonbiological applications that require high-strength materials. With this in mind, I just finished reading a paper in the Proceedings of the Royal Society B from a group of researchers at Imperial College London and the Royal Veterinary College in Hertfordshire (U.K.) whose investigation of trabeculae structures in femurs in a wide range of animals is yielding some new insights for materials development.
Trabeculae are the beams, struts and plates of bone found in spongy bone tissue that create the irregular cavities that hold red bone marrow. To the uninformed, trabeculae can look like an irregular but tough network, but the osteocytes that form the trabeculae build them along the lines of stress to provide maximum strength for a particular type of animal (and can be reorganized if the direction of stress changes). The corollary in the mechanical world is the system of struts and braces that might be used to strengthen a building or the wing of an aircraft.
The researchers, led by Sandra Shefelbine, a professor in bioengineering at IC, looked at the femurs of 90 mammals and birds. One purpose was to look at how trabeculae varied from tiny animals, such as a shrew, to the largest of terrestrial mammals, such as an elephant. In terms of mass, the specimens ranged from 3 grams to 3400 kilograms. According to Shefelbine’s website, she has been interested in how bone structure can change — fairly quickly — when exposed to a variety of loads. Consider, for example, how quickly the bones of astronauts can change in the absence of gravitational loading.
Shefelbine is seeking answers to some fundamental questions about bone biomechanics. “Bone modeling [e.g., response to changing mechanical environment] and remodeling [e.g., repairing microdamage] can alter bone shape, density and structure,” she notes. “Despite the critical role of such mechanical adaptation, the underlying mechanisms remain undefined and many questions remain. For instance: What mechanical stimuli are bones responding to? Why are some bones more sensitive to loading than others? Can we use the adaptive response to prevent deformities from occurring and to strengthen weak bones?”
Her group used a combination of X-ray microtomography and finite element analysis to examine in each of the 90 specimens how trabecular scaling changes in relation to size, and how bone mechanics change in relation to the scaling. For materials scientists, here are the key findings (some of which I find a little counterintuitive, though I am happily surprised):
- From animal to animal, the bone volume fraction does not substantially scale with creature size;
- Although the bone volume fraction doesn’t increase greatly, the trabeculae in the femur of larger animals are thicker, farther apart;
- Also in larger animals, the trabeculae are less densely connected (the number per unit volume is considerably fewer than in small animals; and
- Finite element modeling explains that scaling doesn’t alter the bulk stiffness of trabecular bone, but probably mitigates strain on the scale of the osteocyctes.
Trabeculae in larger animals have higher elastic moduli than in small animals. Credit: M. Doube et al; Proc. R. Soc. B.
The authors suggest that the differences in how trabeculae grow in various animals might be “an interspecific manifestation of bone tissue’s drive to maintain mechanical homeostasis. It appears that changes in geometry are preferred over increased bone mass.”
They and other researchers note that this preference “may be an adaption that limits the physiological cost of producing, maintaining and moving more tissue.”
So, what are broader implications? If one is thinking about how to develop a “smart material” that could adapt to a changing environment, there’s a lesson in bones: The modelling and remodeling of trabeculae and surrounding internal structures seem to be a mass-efficient strategy for dealing with strain. Elephants don’t require thick, dense bones to support their loads. They just use their internal capacity to alter their bone structure.
“We can learn a lot from nature, such as how nature develops these strong, lightweight structures,” advises Shefelbine in a story in The Engineer. “We could adopt this in design — it could inform how people develop structural foams.” In particular, the researchers say, “This may represent a new approach to designing cellular solids for engineered structures of different scales.”
One final note. According to a news release from IC, the research team has created an open-source computer program (“BoneJ“) for examining the number, thickness and spacing of trabeculae as well as analyses of whole bones.