SEM analysis of fractures in PCL-based materials confirmed the weak adhesion of HAp particles to the polymer matrix. Figures and' show whole particles in the fractured composite structure with no visible interaction with the matrix. In addition, as can be seen in the images, the µHAp particles cause local weakening/discontinuity in the material. Despite this, it was found that the addition of HAp significantly improved the modulus value by over 100 %. However, the elongation at break was significantly reduced and the tensile strength was slightly reduced. It is important to note that the PCL/20nHAp composite has very similar stress-strain characteristics to the pure polymer. Analysing images F and F' in Figure , it can be seen that agglomerates are particularly problematic in the context of matrix-filler interactions, while the HAp nanoparticles themselves show quite good affinity to PCL. This is also confirmed by the fact that the Young's modulus for PCL/20nHAp is about 0.59 GPa, while for PCL/20µHAp it is 0.53 GPa. The enhanced interactions between HAp nanoparticles and the PCL matrix, compared to microparticles, were discussed in greater detail earlier in the rheological analysis of the composites in Section 3. Another mechanical parameter considered in the evaluation of the macro-mechanical properties of the composites is impact strength. Measurements were performed separately for PLA and PCL-based materials, with parameters (hammer energy, notch) chosen to allow comparison of results. As expected, the addition of HAp significantly weakened the materials in terms of dynamic load transfer, a phenomenon widely discussed in the literature . Impact tests also showed that composites containing nHAp exhibited more desirable mechanical properties in this respect compared to those containing µHAp. The analysis also highlighted the nature of the polymer matrices tested: PCL, a polymer that is highly resistant to dynamic loads, and PLA, a brittle polymer that requires reinforcement in this respect.