Surface mineralization of fibrous polyester scaffolds for bone tissue engineering purposes

Abstract

Despite the remarkable ability of bone to remodel and spontaneously regenerate, the body is incapable of self-healing in case of a critical sized bone defect caused by severe trauma or after a bone tumor resection. In such a case a surgical intervention is needed. Concerns regarding autogeneic and allogeneic bone tissue transplantation necessitate the search for synthetic bone tissue engineering alternatives. Such a bone substitute or scaffold has to be biocompatible, bioactive and biodegradable so that the implant material can integrate with the surrounding bone matrix, promote bone formation and finally be replaced by newly generated bone tissue. Additionally, in order to allow cell in-growth, vascularization and efficient transport of nutrients, oxygen, growth factors and metabolites, the scaffold should be porous with a high degree of interconnectivity. In this thesis, two biodegradable polyesters, poly(ε-caprolactone) (PCL) and poly(D,L-lactide) (PDLLA), were processed by means of electrospinning and 3D plotting. As such, four potential bone tissue engineering scaffolds were fabricated: electrospun PCL, electrospun PDLLA, 3D plotted PCL and a multiscale PCL scaffold fabricated by combining electrospinning and 3D plotting. In order to enhance the bioactivity of these scaffolds, a surface coating with an apatite was applied. The apatite structure is chemically comparable to the mineral phase found in natural bone and acts as an excellent cell support to maintain desirable cell-substrate interactions. Moreover, as bone apatite has carbonate content of 3 – 13 weight percent (wt%), and several studies showed that the cell adhesion, proliferation and metabolic activity of bone cells was increased on apatites with a high carbonate content, the incorporation of carbonate in the apatite lattice of the coating was of interest. In the first part of the thesis, the electrospun PCL and PDLLA scaffolds were coated with a thin and homogenous primer layer of calcium deficient hydroxyapatite (CDHAp). Coating was achieved in two steps: activation and nucleation. During the first step, reactive groups were created on the fiber surface. These served as nucleation points for apatite deposition during the second step in which the samples were alternately dipped in calcium and phosphate rich solutions. A cell study was conducted to determine whether the CDHAp coating enhanced cell viability. CDHAp coated electrospun PCL and PDLLA demonstrated to be a good substrates for cell attachment and growth of MC3T3-E1 cells. In the second part of the thesis, coating of the 3D PCL scaffold with hydroxyapatite with various carbonate contents was achieved. Analysis of the cell viability and adhesion of MC3T3-E1 cells showed that a hydroxyapatite coating with a carbonate content of at least 10 wt% carbonate improved the osteogenic cell response when compared to uncoated 3D plotted PCL scaffolds and scaffolds coated with a hydroxyapatite with a lower carbonate content. In the final part of the thesis the previously optimized protocols for the coating of electrospun and 3D plotted PCL were combined for the coating of the innovative multiscale scaffold with hydroxyapatite with a low and medium carbonate content. Seeding of MC3T3-E1 cells on these coated II scaffolds showed that the cells reached confluence after 1 day of culture and formed a monolayer after 7 days of culture. Moreover, the cells were able to infiltrate and colonize the interior of the scaffold