b) The addition of 20 % of µHAp to the PLA matrix slightly increased the complex viscosity and the non-Newtonian region in the range low frequency range as an effect of matrix filling, but it also caused a large decrease in the complex viscosity in the frequencies above 1 rad/s (Figure ). On the other hand, PLA/20nHAp showed lower complex viscosity than PCL matrix in the whole range of frequencies without any significant increase in the non-Newtonian region. In addition, the |η  | of PLA/20nHAp was higher than that of PLA/20nHAp at frequencies above 10 rad/s. This confirmed our observation during processing, where the melt-mixed materials with 20% HAp were very liquid when leaving the extruder die and entering the injection mould. This could indicate matrix degradation at higher shear rates. The degradation of PLA during extrusion has already been discussed by Cruz et al . The authors compared PLA samples produced by solution casting and melt-mixing. The extrusion process reduced the complex viscosity by an order of magnitude. Degradation of the PLA matrix has been already reported for other PLA composites such as PLA/copper slag (CS) , PLA/layered double hydroxides (LDH) or PLA/biochar (BC) . On the other hand, Kaavessina et al. prepared PLA/HAp composites with 5, 10 and 20 % of filler by solvent casting followed by melt-mixing. They reported the increase of complex viscosity for all samples and G" > G'