In the case of PCL-based samples (Figure ), 20% of both HAp types increased the complex viscosity compared to the PCL matrix. The complex viscosity of PCL/20nHAp was higher than that of PCL/20µHAp and it also increased the non-Newtonian region in the low frequency range. These results may indicate that the nHAp has a slightly stronger particles interaction than µHAp. Both composites had a viscoelastic liquid character almost throughout the range of measured frequencies, with the crossover point (G'=G") at ~ 360 kPa and at frequencies of 100 and 110 rad/s whereas the PCL matrix was at 233 kPa and 110 rad/s (Figure ). This indicates that the PCL/20Hap composites formed the dispersions in the melt without any physical interactions. It is worth noting that the PCL matrix had a relatively high complex viscosity and its value only slightly decreased towards higher frequencies at the applied temperature of 120 °C . Morover, the addition of 20 % filler caused an increase in the complex viscosity, while the addition of 40 % of filler already caused many processing problems and the composites were not prepared.