Introduction: Electrospinning is a versatile fibre-based technique for scaffold production with smaller fibre diameter, and thus larger surface area to volume ratio compared to melt extrusion. Recent developments in direct writing melt-electrospinning makes use of the stable and predictable fibre deposition to fabricate scaffolds in the layer-by-layer manner[1],[2].
Synthetic materials including polymers and bioactive glass (BG) are most common for bone scaffold fabrication[3],[4]. Among the synthetic polymers, polycaprolactone (PCL), which is a FDA-approved aliphatic polyester, has been become a promising candidate for bone scaffold material due to its favourable rheological and viscoelastic properties[5]. Bioactive glass (BG) is a synthetic material known to bind seamlessly to bone and promote osteogenesis. Strontium-substituted BG derived from the original 45S5 BG by substituting 0-100% of the calcium component of the 45S5 formulation for strontium[6], and have shown superior osteogenic capacity to 45S5 BG[7]. We hypothesize that melt-electrospun PCL/SrBG composite scaffolds will provide a promising bone graft substitute, which will be osteoinductive, osteoconductive, capable of osteointegration and bioresorbable in a controlled rate. The scaffolds can be manufactured with a tailored structure according to specific defect site, and can be replaced by the body’s own newly regenerated bone over time.
Materials and Methods: The SrBG particles were ground down to < 6 μm prior to composite preparation with a micronizing mill to reduce the risk of needle blockage. The 50 wt% PCL/SrBG composites were prepared by incorporating fine SrBG particles into the PCL bulk using solvent precipitation technique. PCL/SrBG composite scaffolds were fabricated using a novel solvent assisted hybrid melt-electrospinning technique in the direct write mode. Light microscopy and scanning electron microscopy (SEM) were used to characterize surface topography and fibre diameter. Mechanical stiffness of PCL/SrBG fibres were assessed with atomic force microscopy. Cell attachment and morphology were assessed with confocal laser scanning microscopy and SEM . Cell proliferation and osteoblastic differentiation were assessed with PicoGreen and alkaline phosphatase assays.
Results and Discussion: Our hybrid melt-electrospinning system overcame the difficulties of PCL/SrBG composite melt-electrospinning associated with viscoelastic properties of the composite material. These composite scaffolds were produced in a layer-by-layer manner with 90° cross-hatched deposition with the fibre spacing of 1 mm. Examination by scanning electron microscopy showed enhanced micro structure on the surface of PCL/SrBG fibres, while the surface of PCL only fibres were smooth. The submicron structures generated during the hybrid melt-electrospinning process largely increased the surface area of composite scaffolds and created roughness ideal for cell attachment. The in vitro results showed good cell adherence onto both the PCL/SrBG and PCL only scaffolds. However, PCL/SrBG composite scaffolds showed significantly higher osteoblast differentiation compared to PCL only scaffolds.
Conclusion: We are the first to produce the composite PCL/SrBG (50 wt%) scaffolds via a novel hybrid melt-electrospinning technique. These composite scaffolds are promising bone graft substitutes, and may also lead to patient-specific and off-the-shelf solution for clinical treatment of critical-sized bone defects.
Dr Henrietta Cathey (Electron microscope probe analysis); Dr Charlotte Allen, Dr Sunny Hu and Mr. Mitchell De Bruyn (Inductively coupled plasma analysis); Dr Marie-Luise Wille (micro-CT scanning and analysis); Dr Christina Theodoropoulos (Confocal microscopy); Dr Tong Li (Atomic force microscopy); ARC Linkage for funding
References:
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