Planck: "Material Design Properties In Bone"

Understanding the difference between a collagen fibril and a collagen molecule (fibrils are composed of individual molecules), a little more on "structure" from Planck:

"HA crystals (see first illustration last post) inside the collagen fibrils grow primarily in gaps between the collagen molecules"

Thus, in the image (above) of the worm shaped collagen fibrils, possibly the blue spots are collagen molecules and the white stuff consists of HA crystals! Additionally, I'm speculating the yellowish-tan outside the fibrils represents "extra fibrillar mineralization", an important but different material to be discussed.

Planck has measured the typical HA crystal within the collagen fibril at at 15 to 150 nn. 150 nn relatively speaking in electron microscopy is relatively large.

Note that HA crystals occur both within the collagen fibrils and outside and thus "between" the fibrils. HA crystals outside the fibril are referred to as "extra fibrillar" and the relative size (fraction) of these is "less well understood" in terms of each individual crystal--"a matter of debate" says Planck which I'm assuming would be akin to the health care debate.

But, what do we know about extra fibrillar crystallization?

Described as "mineral containing blobs on the fibril's surface"--that's how they appear gazing into an electron microscope. But then, at even higher resolution imaginging "each collagen fibril is individually coated with extra fibrillar HA minerals with various shapes and sizes and part of these mineral formations are arranged with a period of 67nm, the same as that of the underlying micro structure of 'naked' collagen fibrils". I'm taking this important observation to mean that the "size" of the extra fibrullar crystals in toto is about the same as the size of a collagen fibril. Imagine then multiple fibrils where the size of the material (HA crystals) between the fibrils is approximatley the same size as the fibrils themselves. I'm taking it in terms of quantification that in mature cortical bone the ratio of collagen fibrils to extra fibrillar HA crysals is roughly stacked 1-1, i.e. fibril-crystal-fibril-crystal with each being roughly the same size.

With these structural understandings established, Planck goes on to indicate there have been several important, mostly recent, studies to determine the mechanical properties of individual collagen fibrils under stress, and how the shape, orientation, and size of the HA crystals (i.e. the mineralization) affects structure strength. That is to say they are studying precisely what I am currently questioning, which is the affect of the percentage of mineralization on bone strength and toughness:

How in heavens name do they study the weigh bearing ability of single fibrils? Computers. They simulate.It's called "Molecular Dynamics (MD) simulations"! "The properties of a single collagen molecule (as opposed to 'fibril') have been assessed." The year 2007: Bhomik et. al investigated how the presence of HA crystals influences the mechanical behavior of collagen molecules.

Then, this bomb: Broedling et al. 2007, showed that the strength and the toughness of HA crystals depend on the size and arrangement pattern of the crystals (paraphrased for clarity).

The last above, of course, implies that arrangement patterns may differ, and thus so may strength and toughness according to the particular arrangement. Presume the reader is starting to understand where this is going. Continue, next post.