Surface-Exposed Hydroxyapatite Microparticles in Electrospun PLLA Scaffolds: Mechanical Reinforcement and Osteogenic Response

The addition of hydroxyapatite (HAP) to electrospun poly-L-lactide (PLLA) scaffolds promotes cell adhesion and differentiation but generally leads to a significant deterioration in mechanical properties due to particle agglomeration. Moreover, the encapsulation of HAP particles within a polymer layer makes them inaccessible to body fluids and cells, thereby limiting the bioactivity of the resulting composite scaffold. In this work, HAP microparticles with median size of 26.3 µm were used to obtain exposed HAP particles on the surface of electrospun PLLA fibers. SEM images and EDX maps revealed that individual particles, particularly the larger ones, were exposed from the polymer scaffold surface. The addition of HAP particles significantly altered the scaffold morphology and structure, increasing the fiber diameter and surface roughness by 2.8–4.1-fold, promoting the formation of fused fiber junctions, and inducing the appearance of semicrystalline PLLA domains. These structural changes significantly improved the mechanical properties of the scaffolds. Specifically, the tensile strength and Young’s modulus of the prepared scaffolds are increased by 2.3–3.8-fold following HAP incorporation. Compared with neat PLLA scaffolds, HAP-containing scaffolds exhibited 1.2–1.4-fold higher osteocalcin and osteopontin expression by human adipose-derived mesenchymal stromal cells (hADSCs). Compared to tissue culture plastic, the expressions of osteocalcin and osteopontin on the composite scaffolds were 7.1–7.9-fold and 2.8–3.0-fold higher, respectively.