The stiffness-related properties (Figs. ) corresponded to the generally expected trends: (i) the EIT and HIT of all PLA-based materials (blue columns) were considerably higher in comparison with all PCL-based materials (orange columns) and (ii) the both fillers (nHAP and μHAP) increased EIT and HIT values. In addition, the result in Figs. documented that nHAP and μHAP influenced the stiffness-related properties of PLA and PCL differently: for PLA, the effect of nHAP and μHAP was comparable, while for PCL the increase was higher for nHAP than for μHAP. Perhaps the dispersion and/or interfacial adhesion of nHAP in PCL matrix was somewhat better than in the case of μHAP, even though the SEM micrographs did not show a big difference in morphology (Fig. ).