So -micro or nano? Several research teams have tried to answer this question by studying different polymers. Defu Lyu et al. prepared poly(amino acid) (PAA)-based composites with three types of HAp: short nanorods, long nanorods and submicron nanorods (with a PAA:HAp weight ratio of 6:4) . They showed that the size of the HAp particles had no significant effect on the thermal properties or biocompatibility, however, the composite with long nanorods showed the highest compressive strength parameters. Another example of research on this topic is our previous work , where we compared composites based on poly(butylene succinate-dilinoleic succinate) with HAp nanoflakes and HAp microwhiskers. Due to the shape of the particles, the addition of microparticles proved to be more advantageous in terms of mechanical properties. Other analyses, including biological and thermal analyses, showed no significant differences. Similar studies have been carried out for PLA and PCL. Francesco Lopresti et al. produced electrospun PLA/HAp mats with 10 and 20 wt% HAp addition. Two types of HAp were used in the studies: one consisting of nanoparticles and the other containing both micro-and nanofractions. The results showed the superiority of the homogeneous nanofiller in terms of mechanical properties and biocompatibility. Other studies investigated the basic mechanical and rheological properties for PLA/HAp filaments containing 19.5 % HAp and a 1% impact modifier. The same HAp was used in its micron-sized form and in a ground variant with particle sizes in the hundreds of nanometres. The addition of the smaller fraction resulted in a slightly higher melt viscosity and improved tensile strength . A more detailed analysis of the viscoelastic properties was carried out for PCL/HAp composites (with 10-40% nano-and microfillers) produced using a twin-screw compounder and compression moulding. The authors found no significant differences between the two types of additives In this work, the authors have aimed to extend the understanding of the effect of the filler particle size on the properties of the material. An in-depth structure-property analysis of solid composites based on PLA and PCL with the addition of nHAp and µHAp was carried out, taking into account composite morphology, thermal and rheological behaviour, as well as micro-and macro-mechanical properties. In order to obtain the most reliable results, the authors focused on the following aspects: (i) high purity materials without unnecessary additives were used in the research, (ii) the fillers were selected to ensure a similar chemical and crystallographic structure, (iii) a processing procedure was developed to obtain composites with the desired and comparable filler content and crystallinity (comparable for specific polymer types), (iv) the processing parameters used were as consistent as possible for all materials, and (v) where possible, the same research procedures were applied. The main novelty of this work lies in the application of the above complex approach to simultaneously: (i) study the effect of filler particle size (HAp) on the properties of polymers (PLA, PCL), (ii) investigate the effect of filler addition on composite properties, and (iii) compare the nature of composites with significantly different polymer matrices.