Mechanisms of Molecular Brush Inhibition of Protein Adsorption onto Stainless Steel Surface
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Protein-resistant ("non-fouling") surfaces are particularly important in many fields such as medical engineering, dentistry, pharmaceutical processes, bioprocessing, dairy and food manufacturing. Poly(ethylene glycol) (PEG) immobilized onto surfaces has been shown to confer high resistance to protein adsorption. The reasons for variable performance and optimal protein repellency of PEG layers have been the subject of much discussion; however there remains no general consensus on the molecular mechanisms underlying the protein resistance achieved with PEG coatings. The main objective of this study was to inhibit protein adsorption onto a stainless steel surface. This objective requires an exploration of the mechanisms of protein adsorption on a stainless steel surface and how these mechanisms are modified when a surface inhibits the adsorption of proteins. The stainless steel surface has been chosen as a substrate as it is a commonly used material in many relevant applications such as in the dairy industry, in food processing and in clinical uses. In order to elucidate the mechanisms of protein-PEG interactions the adsorption of lysozyme, β-casein, apo α-lactalbumin, holo α-lactalbumin and β- lactoglobulin onto various PEG-grafted surfaces was explored. The adsorption was conducted at room temperature and at 40 C. The modification of bare SS surfaces and adsorption kinetics of proteins on unmodified and modified surfaces (i.e. bare stainless steel and PEG surfaces) has been done in-situ and studied by means of a quartz crystal microbalance with dissipation sensing (QCM-D). The merit of the modification methods studied, compared to those of most published methods is that the process of modification is simple and easy, being done simply by passing a solution over the surface. The methods also do not involve any harmful or hazardous chemicals and thus are safe to be used even in food processing plants. The PEG coated surfaces prepared in this study were able to inhibit adsorption of β-casein, α-lactalbumin (calcium enriched) and lysozyme proteins especially; the lowest adsorption of these achieved as a percentage of that on bare stainless steel, β- casein, 45 %, holo α-lactalbumin, 11 % and lysozyme, 1 %. By contrast, and unexpectedly, PEG molecules enhanced the adsorption of apo α-lactalbumin (the form without calcium). It is suggested that the PEG to apo α- lactalbumin hydrophobic interaction plays a dominant role which leads to protein aggregation at the surface, for this latter observation. The results have shown that protein stability (i.e. whether it is a soft or a hard protein) greatly influenced the inhibition performance of PEG surfaces. It is apparently more difficult to prevent the adsorption of soft proteins than hard proteins. This appears to be because soft proteins tend to denature regardless of the surface properties (i.e. hydrophilic or hydrophobic) and attach more effectively in their unfolded state. The results also indicated that higher PEG grafting density is not necessarily reflected in better protein inhibition. At the end of the project, a novel method of surface modification was developed. In this method, stainless steel surfaces were modified by coating the surface with a protein layer (as a base) then followed by the attachment of PEG molecules. Interestingly, the method developed showed an excellent potential for preventing further protein adsorption at room and body temperatures. The adsorption of β-casein, lysozyme, holo α-lactalbumin and β-lactoglobulin on the SS-lysozyme- PEG surfaces was down to about 3, 1, 4 and 0.4 %, respectively compared to that on the bare surface. More interestingly and surprisingly also, there was almost zero adsorption on those surfaces of mixed protein and single protein solutions at the concentration found in milk. The method is believed to have the potential to be applied in the pharmaceutical industry, in the biosensor field and in artificial medical implants with some modifications perhaps to suit the application. The modelling results demonstrated negative free energy changes on adsorption, consistent with the studied proteins being thermodynamically favoured to adsorb on bare SS. The adsorption of proteins was an endothermic process. The proteins also showed large positive entropy changes on adsorption, indicating adsorption-induced denaturation mechanisms (especially apo α-lactalbumin protein). At high temperatures and concentrations, the adsorption was governed first by diffusion and later by surface kinetics, whereas under lower temperature (i.e. room temperature) and low concentration conditions (i.e. 0.1 g / L) the adsorption was able to be described solely by surface-reactions.