Influence of surface topographical modifications of titanium implants on bone cell responses

Gui, N 2018, Influence of surface topographical modifications of titanium implants on bone cell responses, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Influence of surface topographical modifications of titanium implants on bone cell responses
Author(s) Gui, N
Year 2018
Abstract Failures of orthopaedic implants often arise from poor osseointegration at the interface between the implant and the bone. Loosening of the implant is one of main reasons for implant failures. The surface of an orthopaedic implant plays an important role in determining bone cell functions such as cell morphology, adhesion, proliferation and differentiation. Consequently, a wide variety of surface topographical modifications have been developed to improve interactions with osteoblasts. In particular, engineering micro- or nano-structures on the implant surface holds promise to enhance bone cell functions. On the other hand, titanium (Ti) and its alloys have proved to be the materials of choice for orthopaedic implants, thanks to their outstanding biocompatibility, good corrosion resistance in bodily liquid, high strength-to-weight ratios and good mechanical properties. This thesis has addressed the role of surface topographical modifications at the sub-micro to micro scales in the regulation of bone cells. These surface topographical modifications include both random and ordered surface patterns on Ti surfaces. Surface characteristics including surface roughness, morphology and wettability have been evaluated using atomic force microscopy (AFM), surface profilometry, white light interferometry, focused ion beam/ scanning electron microscopy (SEM) and sessile-drop contact angle goniometry. Ordered groove arrays were fabricated using conventional micro/nano fabrication techniques including photolithography, reactive ion etching (RIE) and sputter coating. The biocompatibility of these surface patterns was evaluated by in vitro assessment using human fetal osteoblasts (hFOBs). Cell-surface topography interactions were studied using confocal microscopy and SEM. This thesis aims to provide critical experimental data for the future design of orthopaedic titanium implants for rapid osseointegration. The influence of submicron porous and smooth ultrafine-grained Ti-20Mo surfaces on osteoblast responses Research has shown that both interconnected porous titanium surfaces and dense ultrafine-grained titanium surfaces can enhance bone cell responses. However, it remains elusive as to which surface features are more effective in regulating cell responses. They represent two distinctly different directions in surface engineering of a functional implant. This study compares the in vitro osteoblast responses to ultrafine-grained (grain size: 100 nm), coarse-grained (grain size: 500 μm), fine-porous (pore size: 155 nm) and coarse-porous (pore size: 350 nm) surfaces of Ti-20Mo alloy. A large amount of original experimental data was produced for each type of surface in terms of surface topography, chemistry, wettability, cell morphology, attachment, growth and differentiation. This study concludes that the coarse-porous surfaces provide the optimum topographical environment for osteoblasts. Combining ultrafine grains with abundant grain boundaries is not as effective as porous surfaces to improve cell growth and osteogenic capacity. Furthermore, pore features including size and depth play a more important role in cell growth and osteogenic capacity than smooth surfaces. These findings reveal that the osteoblasts can discern the differences in pore size and depth and responded differently. Fabrication of titanium-coated microgrooves and their anisotropy in wettability The attainment of surface wettability is essential for the success of orthopaedic implants. Surface grooves are of particular importance to orthopaedic implants due to their similarities to collagen fibrils in geometry, which are the basic components of extracellular matrix (ECM, i.e., the cell living environment). Anisotropy in wettability can result from surface grooves. However, no information has been found on the anisotropy in wettability of Ti-coated grooves. Therefore, this study focuses on the fabrication of Ti-coated microgrooves with various groove widths (5-20 μm) and the subsequent characterisation of the resultant anisotropy in wettability was compared with the Wenzel and Cassie models. The results show that significant anisotropy in wettability was found among these Ti-coated microgrooves. In particular, the degree of anisotropy (Δθ) is elevated with increasing groove width from 5 μm to 20 μm on the sub-cellular scale. The Wenzel model can appropriately predict the contact angles measured along the grove direction while the Cassie model offers a better fit for the contact angles measured perpendicular to the groove direction. The anisotropy of wettability influenced osteoblast spreading. Consequently, osteoblasts preferred aligning, rather than perpendicular to along the groove direction. Osteoblast responses to titanium-coated microgrooves with sub-cellular scaled widths Ordered groove arrays have been widely used to modify orthopaedic implant surfaces due to their geometrical similarity with the groove-like collagen fibrils. In general, altering groove geometry has been proved to be an effective way of controlling osteoblast functions. However, the influence of groove geometry at the sub-cellular scale on osteoblast responses remains unclear. Groove width is crucial in regulating osteoblast functions. In this study, osteoblast responses to Ti-coated microgrooves with variable groove width on the sub-cellular scale from 5 μm to 20 μm were systematically investigated. The cell responses include cell morphology, cell-groove adhesion, the spatial arrangement of actin cytoskeleton, cell proliferation and osteoblastic capacity. Both FIB and SEM were used to investigate the osteoblast-groove adhesions, the first close-up study to understand how cells rest more comfortably on grooved surfaces. Full osteoblast-groove adhesion was achieved when the groove width reached 15 μm and beyond, while below 15 μm, the adhesion was gradually enhanced with increasing groove width. The cell spreading area and the cell width were found to be proportional to the groove width. However, the groove width over the range of 5-20 μm exert little influence on cell proliferation and cell differentiation compared to flat surfaces. Apart from the groove width, the groove geometry is another factor that can be tuned to facilitate cell adhesion. The favourable geometries for full osteoblast-groove adhesion include microgrooves with either vertical groove sidewalls (groove width: 15 μm, ridge width: 5 μm, groove depth: 2 μm) or slanted groove sidewalls (slope angle: 158.2°, groove width: 15 μm, groove open width: 25 μm, ridge width: 5 μm, groove depth: 2 μm). On this basis, the correlation between groove geometry and the osteoblast-groove adhesion has been redefined.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Metals and Alloy Materials
Keyword(s) Orthopaedic implants
Surface topographical modifications
Osteoblast responses
Cell-groove interface
Surface morphology
Surface wettability
Ultrafine grains
Submicron porous surface
Sub-cellular scaled microgroove
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Created: Thu, 20 Sep 2018, 09:05:23 EST by Adam Rivett
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