Compositional Control of Strontium-Substituted Hydroxyapatite Governs Bioactivity and Mechanical Response in Electrospun PCL/Gel Scaffolds

Introduction

Electrospun polymer/ceramic composites are widely investigated for bone tissue engineering; however, the role of strontium (Sr) substitution level within hydroxyapatite (HA) embedded in fibrous matrices remains insufficiently clarified. This study aimed to evaluate how different Sr substitution degrees modulate mineralization kinetics, mechanical reinforcement and cytocompatibility of electrospun poly(ε-caprolactone)/gelatin (PCL/Gel) scaffolds.

Methods

Electrospun PCL/Gel mats incorporating 10 wt% HA powders with different Sr contents were fabricated under identical processing conditions. Structural and chemical characterization was performed by ATR–FTIR and X-ray diffraction (XRD). Morphology and fibre uniformity were assessed by SEM. Surface wettability, swelling/degradation behaviour, and tensile properties were evaluated to link physicochemical features with mechanical response. In vitro bioactivity was examined in simulated body fluid (SBF), while cytocompatibility was assessed using direct and indirect MG-63 assays (WST-8, fluorescence imaging).

Results

All formulations produced uniform, bead-free fibres. Fibre diameter analysis confirmed a clear reduction in average diameter upon incorporation of apatite-based fillers, decreasing from approximately 240 nm (PCL/Gel) to 130 nm for Sr-containing composites. Sr substitution influenced apatite crystallinity and resulted in distinct performance profiles. HA-containing composites exhibited accelerated deposition of a calcium phosphate biomimetic layer compared with PCL/Gel, with mineralization kinetics depending on Sr content. Mechanical testing showed different stiffness values as a function of Sr/Ca ratio in the ceramic powders. Cytocompatibility assays confirmed maintained or improved cell viability across all composites, with Sr-containing scaffolds supporting favourable cellular responses.

Conclusion

These findings demonstrate that Sr substitution level acts as a compositional tuning parameter governing the balance between mineralization behaviour, mechanical performance and biological response in electrospun PCL/Gel scaffolds. Controlled Sr incorporation within HA provides a versatile strategy to optimize multifunctional fibrous constructs for bone regeneration applications.