Date of Award

2017

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Craig R. Friedrich

Committee Member 1

Yoke Khin Yap

Committee Member 2

Chang Kyoung Choi

Committee Member 3

Tolou Shokuhfar

Abstract

It has been shown that nanotexturing the surface of otherwise smooth titanium orthopedic materials increases osteoblast proliferation in vitro, and the bone-implant contact area and pullout force in vivo. However, this prior work has not focused on the requirements for scale-up to industrial processes. This dissertation reports on titanium surface modifications by electrochemical anodization using a benign NH4F electrolyte, and a hybrid electrolyte also containing AgF, rather than hazardous hydrofluoric acid used elsewhere. Nanotube fabrication of Ti6Al4V foils, rods, thermal plasma sprayed commercial implants, and laser and e-beam melted powder materials was demonstrated.

It was found that the nanotextured morphology depends on electrolyte composition, and dimensional variation depends on anodization conditions using different NH4F and ethylene glycol electrolytes. The fluorine concentration was found to be the most influential factor affecting formation of porous nanostructures.

Recognizing the importance of packaged implant storage, the wetting behavior of nanotube surfaces was investigated. It was found that increased surface hydrophobicity due to aging in air can be restored by annealing, and the release of residual fluorine from the surface was measured. The kinetics of the amorphous to crystalline anatase transformation of nanotubes was quantified with isochronal and isothermal experiments by X-ray diffraction and transmission electron microscopy. The anatase phase transformation of TiO2 nanotubes was achieved in as little as 5 minutes at 350C, in contrast to reports of higher temperature and for hours.

The fluorine consumed by the formation of the nanotubes during anodization was analyzed and sources of fluorine consumption were identified. Fluorine from the electrolyte is removed and retained in the nanotubes and by the metal removed to form the nanotubes. A metric describing the fluorine removed from the electrolyte per anodized area was developed to help quality control in manufacturing scale-up.

A single-step anodization with controlled nanosilver deposition within and among the nanotubes, using a new hybrid electrolyte of NH4F and AgF was demonstrated. Successful fabrication of potentially antibacterial nanotubes on foils, rods and thermal plasma sprayed surfaces was demonstrated and nanosilver concentration was quantified. These new understandings led to improved manufacturing and storage technologies needed for regulatory approvals of nanotextured titanium surfaces for better orthopedic implants.

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