(458a) Studies of Electroosmotic Mobilities and Zeta Potentials in Plasma Polymerised Microchannel Surfaces

Miniaturisation has found applications in many biological, medical and pharmaceutical devices. Its advantages over macroscale systems include fast analysis time and lower sample consumption. These microfluidic devices usually require the use of electroosmotic flow (EOF) as the means of fluid transport, although variations and inconsistencies in EOF may interfere with device performance. Control of EOF is therefore crucial where one usually resorts to surface modification. In addition, control of non-specific adsorption of biomolecules onto device surfaces through surface modification is also an important issue, since any adsorbed molecules will cause non-uniform surface charge and thus EOF, depending on the nature and extent of adsorption. Physical adsorption and covalent grafting are usually employed as surface modification strategies. Both techniques have their unique advantages and disadvantages. Physical adsorption for example, despite its simpler application, often suffers from a lack of stability, and may alter system selectivity. Although covalently grafted surfaces are more advantageous in terms of stabilities compared to physically adsorbed ones, they often require complicated procedures and are time-consuming to implement. Difficulties in surface modification often arise due to the large variations in chip construction materials ranging from glass, fused silica, quartz, silicon, to a lack of functional group polymeric materials. Microchannel surface modification by the plasma polymerisation method may is an attractive alternative approach. Plasma polymerisation can be used to deposit thin films of various structures and functionalities. It offers several advantages over other techniques in its ability to modify almost any substrate without altering the bulk properties, has a large selection of monomers and a relatively simple one-step coating procedure. Plasma polymerisation of tetraethylene glycol dimethyl ether (tetraglyme) onto glass microfluidic channels and PTFE bases has been successfully demonstrated to a have polyethylene oxide (PEO)-like non-fouling characteristics (Salim et al. Lab Chip, Vol.7, 2007). X-ray photoelectron spectroscopy (XPS), direct enzyme-linked immunosorbent assays (ELISA) and fluorescent imaging were used to investigate the channel coatings. Successful plasma deposition of tetraglyme onto microchannel surfaces resulted in very low or almost no fibrinogen adsorption (0-0.1% Nitrogen). Water contact angle measurements and XPS studies revealed good stability of the tetraglyme coating upon rinsing. Finally, good shelf-storage of at least 3 months was demonstrated. This current study extends the investigation into the electroosmotic behaviour and zeta potential of this plasma polymerised tetraglyme (ppTG) surface subjected to various surrounding buffer solution pH. Streaming potential measurements on flat surfaces were used to corroborate the zeta potential values obtained from the EOF measurements using the Smoluchowski equation. Other types of plasma polymerised surfaces, i.e. cationic (plasma polymerised allylamine, ppAAm) and anionic (plasma polymerised acrylic acid, ppAAc) were also investigated. An assessment was then made between this plasma polymerisation technique (cationic ppAAm surface) and the conventionally used physical adsorption and covalent graft methods (i.e. the physically adsorbed cationic polymer polyethyleneimine (PEI), and covalent grafting of PEI onto pp.AAc surfaces using EDC/NHS chemistry respectively). Results indicate EOF suppression compared to uncoated microchannel surfaces, with isoelectric points of tetraglyme, acrylic acid and allylamine at ~ pH 3.8, 3.5 and 8.9 respectively. Comparisons between plasma polymerised allylamine and conventional PEI physical adsorption as well as covalently grafted PEI surfaces show comparable EOF properties. Plasma polymerised surfaces exhibit high stabilities, enabling EOF runs of more than 3 days without deterioration.The overall purpose of these studies serves to investigate the flow and surface properties of several plasma polymerised surfaces, and their viability towards EOF device applications for proteomic analysis.