Hydroxyapatite crystals tend to have large length to width ratios (more needle like than equant). That’s great for forming framework structures (eg HAp+proteins for forming bones and teeth or think why beavers build dams with sticks rather than stones). I wouldn’t expect structure formers to be desirable for things where homogeneity/isotropic properties are desired. That’s probably why they mention ‘microspheres’.
Materials scientists have been trying to produce microcrystalline HAp that approach spherical form for bio applications for many years (less likely to do unpleasant things like form clots - again that structure building problem). Unfortunately, those attempts are trying to defeat a fundamental physical characteristic of the substance. At a macroscopic scale growth rates of different crystallographic faces, & consequently shapes, can be modified by ‘poisoning’ some faces vs others. At a microscopic scale, that’s impractical so materials people rely on things like ‘homogenous nucleation’ schemes followed by quickly killing further growth or, more crudely, by milling. I’ve never seen any of those things work for HAp (but I haven’t messed w the topic in 20 yrs so maybe somebody pulled off a miracle).
I wouldn’t be at all surprised if a HAp filler felt hard or got lumpy. I’m guessing that it is used to prolong the filler’s lifespan but related to that I’d worry about long term effects cuz the HAp will last a lot longer than the organic part of the filler.
The good thing about HAp is that it is quite bio compatible, a variety of elements/groups can be substituted into its structure thereby allowing some tuning of characteristics, and it can be metabolized.
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