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Title: Controlled dissolution of CaP thin films via surface engineering
Author: Mutreja, Isha
Awarding Body: University of Ulster
Current Institution: Ulster University
Date of Award: 2012
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Calcium Phosphate (CaP) based bioactive coatings have an important role in biomedical applications, especially in the fields of orthopaedics and dentistry. This is mainly due to their chemical similarity with bone apatite which thereby enhances their ability to improve osteointegration. Bioactivity associated with CaP coatings is thought to be related to a timed sequence of dissolution and re-precipitation events that occurs at the interface with tissue. The capacity to control CaP thin film chemical physical properties, and thereby bioactivity, is crucial to the development of a surface that can undergo a dynamic interaction with cells. RF magnetron sputtering offers the potential to prepare thin coatings with well defined physico-chemical properties but in their as-deposited state they are limited in tenns of their crystallinity. These as-deposited CaP coatings are highly resorbable which is due to their amorphous nature, which thereby necessitates post-deposition treatments like thermal annealing. Other means of controlled dissolution of amorphous CaP coatings have significant advantages particular for their use on delicate substrates, such as polymers. Hence, the research presented in this thesis investigates the potential of surface engineering strategies to control the dissolution of very thin amorphous CaP coatings. Thin coatings of CaP were sputter deposited onto polymer materials and nanostructured titania surfaces and the dissolution rate of the coatings in the aqueous phase investigated. The bioactive nature of the as-deposited CaP coatings on two different types of substrates were tested in SBF and in vitro using U20s cells. Interaction of sputtered CaP species with polystyrene (PS) and polymethylmethacrylate (PMMA) resulted in localised damage. The nature of the damage, which has been reported here for the first time, was dependent on the polymer substrate type. The resultant structures were significantly different, with columnar topography observed on PS and pitted structures formed on PMMA. The resulting complex interactions resulted in the intermingling of CaP species with the polymer backbone. This interlocking of the bioactive species within the polymer thereby controls its dissolution in aqueous phase, ensuring the avai labil ity of CaP species over extended periods of time. When a thermal barrier layer of polycrystalline titanium was sputter deposited onto the polymers (PS in th is case, as coatings deposited on PMMA were not stable) followed by a CaP layer the surfaces showed no damage. However, the introduction of the metallic interlayer introduced interfacial mechanical stress, which affected the stability of the prepared coatings in the aqueous phase, causing the coating to delaminate in the liquid medium. In contrast to the polymer materials, where HA spuner deposition resulted in polymer damage and entrapment of CaP within the polymer matrix, titania nanotubes with two distinct pore-sizes (30 ± 7.5 nm and 128 ± 12.4 nm) were prepared by electrochemical anodization and a very thin layer of CaP was sputter coated on top of the prepared nanostructures. Dissolution studies performed in cell culture media over the period afability of21 days highlighted the potential of the nanotubes to prolong CaP dissolution when compared to their control counterparts on which the coating dissolved completely in less than 48h. The bioactivity of the as-deposited and partly dissolved samples of CaP coated polymers surfaces and titania nanostructures from the dissolution studies were assessed at two levels. The physical and chemical analysis of the CaP coated polymer surfaces obtained after incubation in SBF for 7 days suggested some apatite precipitation. The nature of precipitate differed significantly on the two polymer materials. Conversely, the CaP coated titania nanostructures showed no signs of apatite precipitation, but rather showed high levels of coating resorption, dependent on pore size. In vitro studies using U20s cells were used to determine the ability of different CaP coated surfaces to support cell growth and to induce cell differentiation over the period of21 days. The U20s's showed high levels of attachment and proliferation on all of the CaP coated surfaces with no signs of cell death. ALP activity measurements indicated high levels of cell differentiation on CaP coated PMMA surfaces in the polymer group and CaP coated small (17.7 ± 1.0 nm) pore size titania nanotubes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available