The particle size distribution was estimated based on the approach described in our recent work . The composite samples were calcined at 500 °C for 24 h under static conditions, to remove the polymer fraction from the system while preventing HAp degradation. Particle size distribution was analysed from the collected SEM images. Each image was processed using ImageJ software. Images were converted to 8-bit grey scale, then smoothed and segmented. Segmentation was based on finding local maxima on the images with a prominence of at least 10 (ion a scale of 0-255). The background was subtracted to obtain the final images of the separated particles. Several thousand of objects (particle images) per sample were obtained and subjected to automated size and shape analysis. For each object the area, perimeter, major and minor ellipse axes, circularity, aspect ratio and roundness were analysed. The area of each object was converted to equivalent diameter and then analysed in relation to the shape parameters using a custom written macro (Python language).