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Title: Atmospheric Pressure Plasma Modification of Biomaterials in Air and NH3/N2 gas mixtures
Author: Flynn, Cormac
Awarding Body: University of Ulster
Current Institution: Ulster University
Date of Award: 2013
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Abstract The use of surface modification techniques to enhance the biological response of biomaterials is well established. The work reported in this thesis focuses on the use of a dielectric barrier discharge (DBD) plasma reactor operating at atmospheric pressure to enhance the surface properties of polystyrene (PS), poly(ether)ether ketone (PEEK) and cellulose. The effects of the type of gaseous environment used on the efficacy of the functionalisation of the materials have been investigated. Two systems operating with a range of NH3/N2 containing environments have been used to obtain a nitrogenated polymer surface. The first so-called open system is capable of reducing the oxygen content in the discharge to ~8% while the closed system reduces this to ≤ 1%. Chemical changes to the surfaces were analysed using x-ray photoelectron spectroscopy (XPS) and time of flight-secondary ion mass spectrometry (TOF-SIMS) while surface roughness was determined by atomic force microscopy (AFM). Water contact angle (WCA), is then used to examine how these modifications to chemistry and topography are reflected in changes in wettability. It was found that the surface properties of PS, PEEK and cellulose could be radically altered using a DBD operating in air. XPS analysis indicated that this form of processing resulted primarily in oxygen contributions of 14%, on PS and 26% on PEEK. However, no increase in the oxygen content on cellulose was observed. ToF-SIMS indicated a uniform treatment despite operating in a filamentary discharge regime. Cleaning of the materials surfaces was observed using AFM and a significant drop in contact angle with partial relaxation resulting in a long living increase in wettability for both PS (CA from 88° to ~60°) and PEEK (CA from 82° to ~55°). Cellulose underwent cleaning and significant roughening of the surface resulting in a long living increase in wettability (CA from 33° to ~22°) with no subsequent relaxation observed. Operating the DBD plasma in an NH3/N2 gaseous discharge in the open system causes a reduced oxygen environment in which it is possible to functionalise the PS, PEEK and cellulose surfaces with nitrogen containing groups. The nitrogen is primarily present as amine groups, but amide and nitrite groups were also identified. Interestingly, the introduction of NH3/N2 also resulted in an increase in the surface oxygen content on PS and PEEK compared to treatment in air, an effect that has not reported previously. This condition also achieved a lower long living change in contact angle for PS (CA ~55°) and PEEK (CA ~34°). By comparison, cellulose initially became more hydrophilic but the wettability then recovered to become more hydrophobic (CA ~42°) than the pristine material, which again is an effect that has not been reported previously. The NH3/N2 treatment in the open DBD system did not result in etching of cellulose that was observed after treatment in air. XPS analysis indicated the creation of amine and amide groups which were identified as being uniformly distributed over the surface from ToF-SIMS data. Preliminary biological characterisation of cellulose DBD modified in NH3/N2 was carried out using U2-Os cells over a period of one week. However, no significant improvement in cell adhesion or growth was observed compared to a pristine control surface. Operation of the DBD discharge within a high purity NH3/N2 environment in the closed system again results in both nitrogenation and oxygenation of the surface of PS due to the creation of long life radicles on the surface which then react with oxygen upon exposure to ambient conditions post processing. This effect is not observed on cellulose. XPS and ToF-SIMS analyses suggest that the –CH2OH side chain on the cellulose molecules was the site of nitrogen functionalisation. Exposure of PS and cellulose to an atmospheric pressure diffuse (glow-like) discharge in a NH3/He environment in the closed system results in their further functionalisation due to the higher ion density that exists in a helium breakdown condition. The NH3/He discharge created the most crosslinking of the surface of the PS and resulted in a long living more wettable (CA~35°) surface compared to that for treatment in the other environments. In the case of cellulose, this form of discharge cleaned the surface and produced a smooth finish as a consequence of the low density of reactive species in the discharge. The results presented here offer a means to extend the effectiveness of atmospheric pressure DBD plasma processing of polymers by introducing reactive ammonia species which not only cause the creation of nitrogen functional groups but also increase oxygen functionality. The use of a glow discharge processing environment with reactive NH3 species has also been proven to be effective in providing enhanced surface functionality.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available