Two types of hydroxyapatite were used as fillers: 1) acicular powder with an average particle size of 60 ± 10 nm, hereafter referred to as nHAp, and 2) hydroxyapatite powder with an average particle size of 10 ± 2 µm, hereafter referred to as µHAp. Both fillers had the chemical formula Ca10(PO4)6(OH)2 and were supplied by Sigma Aldrich. Figure illustrates the structural and morphological characteristics of the fillers used. The microhydroxyapatite particles have a distinctly rounded shape, although they are not compact and sometimes contain internal voids, as shown in Figure . These particles do not tend to agglomerate. The situation is different for nanoparticles (Figure ), which have a strong tendency to agglomerate and form more compact structures. The phenomenon of particle agglomeration in HAp nanoparticles is often discussed in the literature .