Apatite deposition on NaOH-treated HDPE, PEEK and UHMWPE films for sclera materials in artificial cornea implants.
PhD thesis, University of Glasgow.
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Corneal disease is the second most common cause of blindness in the word. It is estimated that 45 million people worldwide are bilaterally blind and 10 million are affected by corneal blindness. Corneal blindness mostly affects the population in the equatorial zone, due to the high exposure to UV light. Corneal grafting presents complications such as rejections and the lack of donor material and resources. Conventional cornea grafting (keratoplasty) is not advised for patients with bilateral corneal blindness or for those who suffer from a range of clinical problems including tear deficiency, chemical burns and uncontrollable intraocular pressure. At present, an artificial cornea, i.e. a keratoprosthesis (KRPO), is the only alternative to keratoplasty (corneal donor transplantation).
Cornea implants consist of a clear optic part and a surrounding ring known as the skirt, which needs to integrate with the sclera of the eye. Currently used skirt materials lead to poor tissue integration, a major failure of cornea implants. Better integration may be achieved when using a bioactive skirt material, which adapts to the metabolic activity of the cornea. For this purpose, high density polyethylene (HDPE), polyether ether ketone (PEEK) and ultra-high molecular weight polyethylene (UHMWPE) films provide interesting possible alternatives, if they can be rendered bioactive. This study investigated the potential of using surface modified polymer films to fabricate the skirt. To improve bioactivity of the materials a two-step treatment using chemical surface modification (immersion in NaOH) and formation of apatite layers from Simulated Body Fluid (SBF) was applied. The effectiveness of the different molarity of the NaOH on the formation of the bioactive layer was investigated. Results showed that with an increase in NaOH concentration the wettability improved but also some changes to the topography (increase/decrease of roughness) of the polymers were observed. Moreover, 10M NaOH treatments resulted in more rapid formation of the apatite layer when compared with a non-treated and lower molarity solution. As immersion time in SBF increased, further nucleation and growth produced a thicker apatite layer which can be expected to be highly bioactive. Interestingly, the apatite growth is dependent on both the concentration of NaOH solution and the structure of the polymer surface. It was concluded that hydroxyapatite layers were formed on HDPE, PEEK and UHMWPE films after they were incubated in 1.5 SBF, which promises to render such thin-film structures bioactive – a necessity if they are to be integrated into artificial cornea. The Ca/P molar ratio of the apatite deposited on the polymers increases with NaOH strength and SBF incubation time. The favourable effect of NaOH on apatite formation may at least partly be attributed to an increased wettability of the polymer films after such treatment, as well as to the modified topography. The apatite layer contained phosphate and carbonates ions, providing potentially good in vitro bioactivity on polymeric films. The inorganic layers are chemically stable as the calcium deposited on the films did not dissolve fully when immersed in water for one week. This demonstrates that polymer films can be rendered bioactive, using the described approach, hence providing potential materials suitable for artificial cornea implants.
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