Preparation of Porous Composite Scaffolds for Bone Tissue Engineering

Abstract

Tissue engineered scaffolds are porous structures whose purpose is to act as temporary platforms for formation of new tissue. For bone tissue engineering, a scaffold material needs to meet certain requirements in its morphology and mechanical and chemical properties. Bioactive glasses are stiff and strong materials and exhibit osteoconductivity and osteoinductivity, but have the drawback of being brittle and difficult to fabricate into porous structures. Biodegradable polymers, which can be processed into various morphologies, lack bioactivity and have low stiffness and strength. By combining materials from these two classes into a composite, they can be used to counter each other’s shortcomings. The aim of this work was the preparation and characterization of porous poly(butylene succinate) (PBSu)/phosphate glass fiber (GF) and PBSu/polylactide (PLA) fiber composite scaffolds. The scaffolds were prepared by first compounding the composites by twin-screw extrusion or compression moulding and then making them porous by supercritical carbon dioxide (scCO2) foaming. Twin-screw extrusion was used to compound PBSu/GF composites with cut fibers. The fibers in the extruded composites after the extrusion were shorter than 1 mm, and the residual fiber length was found to depend on the methods used to prepare the fibers for the extrusion. Extruded composites with different fiber length distributions were produced. Compression moulding was used to compound both PBSu/GF and PBSu/PLA fiber composites with continuous and unidirectional fibers. These were made by first preparing mats of unidirectional GF or PLA fibers and then compression moulding the mats together with PBSu films. Compression-moulded composites with different fiber contents were produced. The fiber contents of the extruded and compression-moulded PBSu/GF composites were studied with thermogravimetric analysis and calcination test. The extruded composites were found to have fiber contents less than the target values, while the compression-moulded PBSu/GF composites had fiber contents near the target values. The extruded and compression-moulded PBSu/GF and PBSu/PLA fiber composites as well as extruded neat PBSu were made porous with scCO2 foaming. The pores in foams made from extruded PBSu/GF composites were larger and their pore size distribution was broader compared to neat PBSu, while the foams made from compression-moulded PBSu/GF composites had smaller and less interconnected pores than neat PBSu. The PLA fibers were not well dispersed within the PBSu matrix and formed large formations within the PBSu matrix during the foaming. These PLA formations themselves were also porous, but had a smaller pore size than PBSu. During the 8-week hydrolysis test of the porous composites, neither PBSu nor PLA showed significant mass loss. The PBSu/GF composites showed mass loss according to the weight fraction and the length of the GF in them, shorter GF dissolving faster than longer ones. The release of the ionic GF dissolution products from the porous composites was incongruent and got slower as the hydrolysis time increased. The mechanical properties of the porous sample materials were tested after different hydrolysis times by compression testing. Of the porous composites, only the PBSu/GF composites with continuous GF were stronger and stiffer than porous neat PBSu. The composites with cut GF had the poorest mechanical properties. Both the fillers and the different porosities contributed to the mechanical properties. While all the materials had a yield strength comparable to trabecular bone, none of them were as stiff as trabecular bone.