What we can't learn from nature

When attempting to design bio-inspired materials and structures, it is important to remember that Nature employs some strategies for optimising the use of load-bearing materials which we, as engineers, cannot replicate. A material in a biological structure, such as a bone or plant stem, experiences stresses which are higher than would be allowed in an engineering structure, even if we were using exactly the same material. There are several reasons for this: biological materials are subject to continuous inspection and repair; they are constructed in more optimal ways, especially in respect to fibre orientations, and they are allowed to operate at higher failure rates. In the present paper I discuss four particular examples of Nature's strategies and, in each case, I try to quantify their effect for the case of cortical bone. The overall conclusion is that, thanks to these four little tricks, Nature is able to operate bone at a stress which is about 17 times greater than the stress at which it could be used in an engineering component. Considering how a typical bone is loaded, this implies an increase in weight of about a factor of 10.