Publicação
Nanocrystalline diamond coatings for biomedical applications
| Resumo: | The main objective of the present PhD thesis is the development of nanocrystalline diamond (NCD) films on silicon nitride (Si3N4) substrates for biomedical applications, like joint implants. NCD films were produced by hotfilament and microwave plasma chemical vapour deposition techniques (HFCVD and MWCVD, respectively). The HFCVD deposition parameters were optimized in order to obtain welladhered and good-quality NCD coatings on Si3N4 substrates. NCD films with ~15 nm of crystallite size with a growth rate of ~1 μm×h-1 are obtained using a set of conditions as follows: Ar/H2 gas ratio of 0.1, CH4/H2 ratio of 0.04, total gas pressure of 50 mbar, total mass flow of 50 ml×min-1, filament and substrate temperatures of 2200ºC and 750ºC, respectively. The surface roughness of the films also depends on the Ar/H2 ratio presenting values between 13-16 nm for Ar/H2 in the range 0-0.63. When the filament temperature is raised from 2200ºC to 2300ºC, the growth rate increases from ~1 μm×h-1 to a value of 1.6 μm×h-1 but the crystallite size doubles from 15nm to 28nm. Optimal deposition conditions in MWCVD result in NCD growth rates up to 3.0 μm×h-1 with low roughness (Ra ~ 22nm) and grain size of about 10 nm. NCD deposition on Si3N4 biocomposites (Si3N4-bioglass) was also carried out by HFCVD. To avoid blistering and guarantee high adhesion, the substrate temperature must be lower than the bioglass transition temperature (637ºC). The cooling rate has a further influence on the adhesion level. No delamination was verified for a slow cooling rate of 2ºC·min-1. Biotribological characterization of NCD coatings was assessed by self-mated reciprocating pin-on-flat experiments under an applied load of 45 N in simulated physiological fluids: Hanks’s balanced salt solution (HBSS) and dilute fetal bovine serum (FBS). Very low friction coefficient values are attained (0.01-0.02) in HBSS media, while FBS lubricated tests result in increased friction coefficients (0.06-0.09) due to protein attaching effect. Wear rates of k ~10-10 mm3·N-1m-1 in HBSS and k ~10-9 - 10-8 mm3·N-1m-1 for FBS are characteristic of very mild wear regimes. CF4 plasma etched substrates show increased adhesion relatively to polished ones and withstand 6 km of sliding distance without any evidence of film fracture but with slightly higher friction values. In vitro biological characterization was performed with osteoblastic (MG63 cell line and human bone marrow) and fibroblastic (L929 cell line and human fibroblasts from gingival tissue) cell type cultures, denoting no deleterious or cytotoxic responses. NCD coatings provide a suitable surface for cell attachment, spreading and proliferation and allowed reproduction of the typical features of the cell populations. Results demonstrate an improved human osteoblast proliferation and the stimulation of specific metabolic activities, like ALP activity and matrix mineralization. The high chemical resistance and unique mechanical properties of NCD associated with its nano-topography, which mimics nanometric features of bone tissue, anticipates a wide range of applications in the dental and orthopaedics biomedical fields. |
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| Autores principais: | Amaral, Margarida Isabel dos Santos |
| Assunto: | Nanocrystalline diamond Biomaterials Biomedical applications Coatings |
| Ano: | 2006 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Aveiro |
| Idioma: | inglês |
| Origem: | RIA - Repositório Institucional da Universidade de Aveiro |
| Resumo: | The main objective of the present PhD thesis is the development of nanocrystalline diamond (NCD) films on silicon nitride (Si3N4) substrates for biomedical applications, like joint implants. NCD films were produced by hotfilament and microwave plasma chemical vapour deposition techniques (HFCVD and MWCVD, respectively). The HFCVD deposition parameters were optimized in order to obtain welladhered and good-quality NCD coatings on Si3N4 substrates. NCD films with ~15 nm of crystallite size with a growth rate of ~1 μm×h-1 are obtained using a set of conditions as follows: Ar/H2 gas ratio of 0.1, CH4/H2 ratio of 0.04, total gas pressure of 50 mbar, total mass flow of 50 ml×min-1, filament and substrate temperatures of 2200ºC and 750ºC, respectively. The surface roughness of the films also depends on the Ar/H2 ratio presenting values between 13-16 nm for Ar/H2 in the range 0-0.63. When the filament temperature is raised from 2200ºC to 2300ºC, the growth rate increases from ~1 μm×h-1 to a value of 1.6 μm×h-1 but the crystallite size doubles from 15nm to 28nm. Optimal deposition conditions in MWCVD result in NCD growth rates up to 3.0 μm×h-1 with low roughness (Ra ~ 22nm) and grain size of about 10 nm. NCD deposition on Si3N4 biocomposites (Si3N4-bioglass) was also carried out by HFCVD. To avoid blistering and guarantee high adhesion, the substrate temperature must be lower than the bioglass transition temperature (637ºC). The cooling rate has a further influence on the adhesion level. No delamination was verified for a slow cooling rate of 2ºC·min-1. Biotribological characterization of NCD coatings was assessed by self-mated reciprocating pin-on-flat experiments under an applied load of 45 N in simulated physiological fluids: Hanks’s balanced salt solution (HBSS) and dilute fetal bovine serum (FBS). Very low friction coefficient values are attained (0.01-0.02) in HBSS media, while FBS lubricated tests result in increased friction coefficients (0.06-0.09) due to protein attaching effect. Wear rates of k ~10-10 mm3·N-1m-1 in HBSS and k ~10-9 - 10-8 mm3·N-1m-1 for FBS are characteristic of very mild wear regimes. CF4 plasma etched substrates show increased adhesion relatively to polished ones and withstand 6 km of sliding distance without any evidence of film fracture but with slightly higher friction values. In vitro biological characterization was performed with osteoblastic (MG63 cell line and human bone marrow) and fibroblastic (L929 cell line and human fibroblasts from gingival tissue) cell type cultures, denoting no deleterious or cytotoxic responses. NCD coatings provide a suitable surface for cell attachment, spreading and proliferation and allowed reproduction of the typical features of the cell populations. Results demonstrate an improved human osteoblast proliferation and the stimulation of specific metabolic activities, like ALP activity and matrix mineralization. The high chemical resistance and unique mechanical properties of NCD associated with its nano-topography, which mimics nanometric features of bone tissue, anticipates a wide range of applications in the dental and orthopaedics biomedical fields. |
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