• Lyyra, Inari

Abstract

Replacing biological tissues requires materials with many properties, and a single material can rarely meet all the characteristics needed for a given application. Designing composites allows the creation of property combinations, enabling tailoring of the final material properties to meet the requirements of the foreseen application. This thesis explored biodegradable composites with a single polymer or a polymer blend matrix and bioactive glass (BaG) filler. Further, the influence of the composites’ composition on their initial properties and degradation was studied.<br/><br/>In this thesis, the matrices explored are polylactide (PLA), poly(butylene succinate) (PBSu) and their blends. Two ratios were studied for the blends: 75/25 PLA/PBSu and 50/50 PLA/PBSu. The studied fillers were a silicate BaG 13-93 (also denoted as Si-BaG), lithium, strontium, and boron-substituted 13-93 (13-93Li, 13- 93Sr and 13-93B, respectively) and a phosphate glass (P-BaG). In addition, the glass content in the composites varied between 10 and 50 wt%.<br/><br/>The degradation of the polymers during the processing of the blends and composites was assessed by measuring their molecular weight after processing. Si- BaG accelerated the molecular weight loss of PLA during processing, while P-BaG did not. In addition, the composites containing PBSu were less affected by Si-BaG during processing. The initial mechanical properties of the blends and composites were assessed by bending and shear testing. It was demonstrated that increasing the PBSu ratio increases the ductility of the materials. However, when the polymers were compounded with BaGs, their mechanical properties decreased, and they became more brittle, with higher filler loading causing further decreases.<br/><br/>Then, the hydrolytic degradation of the materials was studied either in tris(hydroxymethyl)aminomethane solution for 40 weeks or in phosphate-buffered saline for 24 weeks. Water absorption, mass loss, and changes in mechanical properties and microstructure were studied as a function of immersion time. While immersed in the buffer solution, the blends degraded faster than single polymers. Introducing BaG and increasing its content accelerated the hydrolytic degradation of the composites. However, despite the faster dissolution of the P-BaG particles, P-BaG dissolution did not accelerate the polymer degradation as much as Si-BaG. Furthermore, the degradation of the polymer matrix and the dissolution of the glass filler as such and in the composites were assessed. The dissolution mechanism of the BaGs was not affected by compounding. Still, the release of magnesium and calcium ions was found to be slowed down by the polymer matrices. In addition, single polymer matrices hindered the release of phosphorus ions but not the blend matrices.<br/><br/>Finally, the matrix and filler's cytocompatibility were evaluated separately using human adipose stromal cells (hASC) and human urothelial cells (hUC) to predict the behaviour of the composites with cells. In addition, the ion release results from the BaG particles alone and 13-93 in the composites were used to predict the ion release from the Li-, Sr- and B- substituted 13-93 composites. The PBSu-containing matrices, 13-93, 13-93Li and 13-93Sr extracts supported the viability and proliferation of hASCs, while PLA did not, and 13-93B reduced the hASC viability and changed their morphology. hUCs exhibited excellent viability and proliferation on all the matrices studied but did not tolerate the undiluted BaG extracts. Diluting the extracts increased the viability and proliferation of hUCs, and the hUCs cultured in 13-93Sr extract showed the most normal morphology. Combining the results mentioned above, the composites with a PBSu-containing matrix filled with 13-93Li or 13-93Sr particles were predicted to be the most beneficial for hASCs. The PLA+13-93Sr composite was predicted to have the best cytocompatibility with hUCs.<br/><br/>In conclusion, blending, BaG composition and loading affected the composites' initial properties, hydrolytic degradation and cytocompatibility. Composites with PLA/PBSu blend matrix enabled the unhindered release of potassium and phosphorus ions from the BaGs while not affecting the strength retention of the composites. The BaG composition affected the matrix's initial molecular weight and the composites' strength. The degradation rate of the matrix depended greatly on the water absorption induced by the BaG. P-BaG had a smaller effect on water absorption and matrix degradation than Si-BaG, although it was almost dissolved after 20 weeks in vitro. The cytocompatibility of the materials differed by cell type, and suitable composite compositions were predicted for both hASCs and hUCs.

Topics

• microstructure
• Magnesium
• Magnesium
• glass
• glass
• laser emission spectroscopy
• strength
• Strontium
• composite
• Potassium
• Boron
• Lithium
• Calcium
• molecular weight
• ductility
• Phosphorus
• polymer blend