The methodology used for composite's manufacturing is based on melt mixing processes. On the one hand, this is a well-established approach derived from industrial techniques . On the other hand, during these processes, the molten polymer is subjected to significant thermal (elevated temperature) and mechanical (shear forces between the material, screws, and the extruder barrel) stresses. Our previous work demonstrates that such stresses in the system can lead to the cracking of apatite microfibres during processing. In this study, we attempt to quantify this phenomenon for the first time. For this purpose, we analysed the particle size distribution of the initial apatite fillers and then reassessed it in the final produced materials. The results of the quantitative analysis are presented in Figure . The example SEM images used for the calculations are provided in Supplementary Figure . The two powders used as a sources of hydroxyapatite are characterised by very different particle size distributions. The 'nano' form of hydroxyapatite (nHAp) contains grains no larger than 100 nm (d10 = 0.021 µm, d50 = 0.033 µm, d90 = 0.049 µm), whereas the 'micro' form of hydroxyapatite (µHAp) contains grains larger than 1 µm (d10 = 1.140 µm, d50 = 2.246 µm, d90 = 3.655 µm). Another important difference is the aspect ratio (AR) of the particles. The 'micro' form is characterised by much more elongated grains (AR greater than 3.0 -in most cases around 4.0), whereas the 'nano' type has much more spherical grains (AR below 1.7).