Processing and microstructure characterization of a novel porous hierarchical TiO 2 structure
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J. Ringnalda FEI Company, Hillsboro, Oregon 97124 (Received 15 February 2008; accepted 3 November 2008)
We report on a novel biocompatible hierarchical TiO2 porous coating on the surface of Ti, processed via anodic oxidation. The coating consists of large (1–20 mm) pores on the microscale and nanotubes (50 nm diameter) on the nanoscale. This structure is exciting because of its potential application as a bioactive coating for Ti bone implants. Surface characterization of the coating showed nanotubes of relatively uniform diameter. The interface between TiO2 nanotubes and Ti, studied by transmission electron microscopy, was incoherent. The tubes were also somewhat interconnected. I. INTRODUCTION
Titanium (Ti) and Ti alloys are frequently used as bone implant materials because of their biocompatibility, superior corrosion resistance, and high strength-to-weight ratio. A major problem with Ti is that osseointegration via the natural oxide (TiO2) is a long process.1,2 As a result, buildup of fibrous soft tissue rather than hard bone can take place, which over time results in the loosening of the implant, causing discomfort and eventual failure.3 In an effort to improve the bioactivity of Ti implants and enhance bone growth a variety of techniques have been used, such as roughening of the Ti surface by grit blasting to improve mechanical fixation,4 applying chemical treatments to the Ti surface,5 and applying thin biocompatible coatings on the Ti surface.6 In this regard, porous coatings have been of significant interest in recent years. Early work in this area showed that microporous coatings with pore diameters of roughly 100 mm encourage bone in-growth resulting in strong mechanical fixation of the implant.7,8 On a much smaller length scale, there has also been significant interest in nanoporous coatings.3,9–16 This interest is motivated by the excellent biological performance exhibited by a variety of nanostructured coatings, such as nanoporous alumina16 and nanograined hydroxyapatite (HA), Al2O3, and TiO2.3,9 These nanoporous coatings also exhibit excellent bioactivity as well as long-term osteoblast function.15,16 Nanoporous TiO2 coatings, in the form of nanotubes, in particular, exhibit excellent osteoblast function in terms of cell adhesion and proliferation.3,9–11,15 The TiO2 coating has also been shown to form apatite in a simulated body fluid a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0221 J. Mater. Res., Vol. 24, No. 5, May 2009
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(SBF) environment.12,13 Popat et al.15 observed the deposition of bone matrix (primarily Ca and P) within the nanotube pores after a two week cell culture.15 These TiO2 coatings have a strong chemical bond to the Ti substrate, which provides an inherent advantage over conventional HA coatings where dissolution, cracking, and eventual delamination can occur.15,17 A hierarchical dual porous structure on Ti that encompasses the two types of desired functionality, i.e., (i) micr
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