Introduction: The implantation of bone tissue scaffolds aims at regenerating bony tissues in the patients who are suffering with segmental bone defect. Whether or not the surgery is success depends on the osteogenicity, mechanical properties and the ability of revascularization of the bioscaffold implanted [1][2][3]. Our previous work demonstrated that appropriate amount of magnesium ions released could significantly enhance in vivo bone formation locally [4]. In this study, an engineered bone scaffold comprising of biodegradable polymer named polycaprolactone(PCL) and magnesium nano-oxide(MgO) particles has been developed in order to facilitate sustainable release of magnesium ions at effective dosage.
Methods: The scaffolds were fabricated by incorporating different proportions of magnesium nano-oxide particles into polycaprolactone. A total of 4 different amounts of MgO (0.1g, 0.4g, 0.6g and 0.8g, U-0.1, U-0.4, U-0.6 and U-0.8) were incorporated into 1g PCL, respectively. Compression testing based on the ASTM D695-08 was used to determine the bulk mechanical properties of the hybrids. Immersion tests were carried out to monitor the amounts of Mg2+ released from the scaffolds in simulated body fluid(SBF). The closed capsules containing the specimens in the solution were placed in an incubator for 5 different time points(1, 3, 7, 15 and 30 days). The Mg2+ were determined by using inductively-coupled plasma optical emission spectrometry (ICP-OES). Then hybrid U-0.4 were individually immersed for 40 days with daily refreshing of the SBF. Enhanced Green Fluorescent Protein Osteoblasts (eGFPOB) from GFP mice were cultured on the surface of the PCL and U-0.4 to determine growth and cytocompatibility for 1 or 3 days. The MTT assay was used to determine the cytotoxicity of the materials to murine cells for 1, 3 and 7 days.
Results: The compressive moduli of the MgO/PCL hybrids ranged from 358MPa to 1087MPa from 0.1g to 0.8g MgO(Fig. 1 a). However, the compressive modulus of pure PCL was found to be only 266MPa. The release rate of Mg2+ was found to increase steadily for U-0.4(Fig. 1 b). For the U-0.6 and U-0.8, significantly higher release rate could be found when compared to the U-0.4 especially at the early time points. Furthermore, the Mg2+ concentrations of U-0.4g immersion fluids were around 60ppm per day in SBF over 30 days(Fig. 1 c). When cultured with mouse osteoblasts, the cells exhibited higher spreading ratio and almost 100% of confluence on scaffold after 3 days(Fig. 2 a). The cell viability was also found about 90% higher on the hybrid throughout the whole period of culturing as compared with PCL(Fig. 2 b). The animal study will be undergoing in the next step.
Discussion: Studies have reported that materials with mechanical properties similar to cancellous bone may be not only beneficial in terms of reducing the risks of adjacent level fracture, but also reinforce the non-fracture osteoporotic bone without altering the stress distribution significantly under physiological loading [5][6]. In this study, the incorporation of MgO into the polycaprolactone matrix yields a dramatic improvement of compression modulus that is close to adult cancellous bone. Hence, the mechanical properties of these materials should meet the requirements of implantable bioscaffold needed. Magnesium ions are essential to human metabolism and naturally found in bone tissues [7][8]. Our previous work also demonstrated that 50ppm concentration of magnesium ions could significantly enhance in vivo bone formation locally [4]. With the steady release of magnesium ions, the scaffold U-0.4 hybrid materials seems promising for segmental bone defect implantation.
Significance: These economical and easily fabricated bio-scaffolds can be potentially applied to the treatment of segmental bone defect fixation, when their mechanical properties and release profile of magnesium ions are properly designed.
the Hong Kong Research Grant Council General Research Funds #772113 and #718913; The Science Technology and Innovation Committee of Shenzhen Municipality Basic Research Grants JCYJ20140414090541811
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