The hydroxyapatite is a wide-recognized biocompatible material. The polymer matrix you noted (HA-LLDPE) is used as a biocompatible material too, mainly, for endoprotheses making. As your question, we were studying the opportunity of LLDPE application in heart valve as a reinforcing filler because of leaflets of artificial heart valve of design proposed should be made from relatively elastic material such as polyurethane (see, please, the papers "Strain-strength Characteristics of the Polymer Heart Valve Prosthesis // Russian Journal of Biomechanics.– 2006.– Vol.10, №.4.– P. 47-55"; "Nano- and Micro-Structured UHMWPE Composites Filled With Hydroxyapatite Irradiated by Nitrogen Ion Beams for Bio-Medical Applications // Russian Physics Journal. – 2014. – Vol. 56, № 10. – P. 1137–1143".
I think, the hydroxyapatite one can assume as a filler component of polymer composite heart because of capsulation in polymer matrix. But this conclusion should be approved by corresponding biological tests only.