Interesting differences in the effect of the filler type can be observed after the addition of 40 % HAp. In the case of PLA/40µHAp, the material was so brittle that it was cracking in the pneumatic jaws, which necessitated further measurements using manual wedge grips. Despite the higher modulus of elasticity compared to previous materials, the other parameters deteriorated significantly. The aforementioned brittleness of the material is illustrated in Figures and'. It is interesting to note that individual HAp particles cannot be distinguished with the naked eye. In order to analyse this in more detail and to eliminate potential errors in the research, further fracture analysis was carried out using SE and BSE detectors. The results of this analysis are shown in Figure . It appears that in these samples the matrix-filler interaction network is so strong and the material so brittle that the boundaries between matrix and filler are blurred, making the particles difficult to detect with a standard SE detector. Interestingly, the PLA/40nHAp composite exhibits quite good mechanical properties, even outperforming the PLA/20µHAp composite in terms of elastic modulus and tensile strength.