Secondly, the analyses provided insight into the effect of HAp addition on the properties of the composites. For PLA, the addition of 20% and 40% HAp resulted in markedly different rheological and mechanical properties. The PLA/20% HAp composites exhibited lower melt viscosity compared to pure PLA over a wide range of angular frequencies. In contrast, for the PLA/40% HAp composites, a strong network of polymerfiller interactions was formed, resulting in a significant increase in melt viscosity, which exceeded that of pure PLA. Furthermore, the increase in filler content corresponded to a proportional increase in the stiffness-related mechanical properties (modulus of elasticity measured in macro-and microscale, macroscale yield stress, and microindentation hardness), accompanied by a reduction in other strength parameters (tensile strength, elongation at break, impact strength). In the case of PCL, the addition of HAp resulted in an increase in melt viscosity, an increase in macro-and microscale elastic moduli (> 100%), accompanied with an increase in other stiffness-related properties (macroscale yield stress and microindentation hardness), and a decrease in the ultimate properties (stress and elongation at break). The addition of μHAP to PCL exhibited similar effects like the addition of nHAP, but the improvement of the properties was lower.