Polylactic acid/hydroxyapatite composite filaments: Balancing printability, mechanical integrity, and osteogenic performance for bone tissue engineering.

Abstract

Biomedical devices for bone tissue repair require materials with a balanced combination of printability, structural integrity, and biological performance. Their fabrication is increasingly shifting toward additive manufacturing, in which thermoplastic biodegradable polymers, particularly poly(lactic acid) (PLA), serve as matrices. In this study, PLA composite filaments with varying hydroxyapatite (HAp) contents (0, 5, 10, 20, and 30 wt%) were fabricated via a solvent-assisted non-solvent-induced precipitation method and evaluated for extrusion-based bone tissue engineering applications. HAp content significantly influenced the physicochemical, rheological, mechanical, and biological performance of the composites: increasing HAp loading reduced filler dispersion homogeneity and, at 30 wt%, pronounced agglomeration and microvoid formation. Thermal analysis demonstrated that the incorporation of HAp did not significantly affect the glass transition temperature of PLA, which remained nearly constant regardless of HAp content. However, the presence of HAp shifted the cold crystallization temperature to higher values, suggesting delayed cold crystallization during heating. Intermediate HAp incorporation (5-10 wt%) provided the most favourable balance between homogeneous filler dispersion, mechanical reinforcement, and osteoblastic response, with PLA-HAp10 showing enhanced ALP activity and osteogenic gene expression together with reliable filament formation and scaffold printability. In contrast, 20 wt% HAp approached the upper composition-processing threshold of the system, while 30 wt% HAp compromised structural homogeneity and extrusion reliability. These findings highlight the importance of controlling HAp loading to achieve a balanced combination of processability, structural integrity, and osteogenic performance in PLA-based biomaterials.