Ultra-low fouling is a rating of a surface's ability to shed potential contamination. Surfaces are prone to contamination, which is a phenomenon known as fouling. Unwanted adsorbates caused by fouling change the properties of a surface, which is often counter-productive to the function of that surface. Consequently, a necessity for anti-fouling surfaces has arisen in many fields: blocked pipes inhibit factory productivity, biofouling increases fuel consumption on ships, medical devices must be kept sanitary, etc. Although chemical fouling inhibitors, metallic coatings, and cleaning processes can be used to reduce fouling, non-toxic surfaces with anti-fouling properties are ideal for fouling prevention. To be considered effective, an ultra-low fouling surface must be able to repel and withstand the accumulation of detrimental aggregates down to less than 5 ng/cm2. A recent surge of research has been conducted to create these surfaces in order to benefit the biological, nautical, mechanical, and medical fields.
Making ultra-low fouling surfaces High surface energies cause adsorption because a contaminated surface will have a smaller difference between the surface and bulk coordination numbers. This drives the surface to reach a lower, more favored, energy state. A low energy surface would then be desired to prevent adsorption. It would be convenient if the desired surface was already low energy, but in many cases-such as metals-this is not the case. One solution would be to layer the surface with a low surface energy polymer such as polydimethylsiloxane (PDMS). However, PDMS coating's hydrophobicity causes any adsorbed particles to increase the surface energy, easing adhesion and ultimately defeating the purpose. Oxidizing the PDMS surface does generate hydrophilic anti-fouling properties, but the low glass transition temperature allows for surface reconstruction via internal rearrangement: destroying hydrophilicity. In aqueous environments the alternative is to use high-energy hydrophilic coatings; whose chains become hydrated by the surrounding water and physically bar adsorbates. The most commonly used hydrophilic coating is polyethylene glycol (PEG) due to its low cost. On the other hand, PEG is highly susceptible to oxidation, which eventually destroys its hydrophilic properties. Hydrophilic surfaces are generally created one of two ways; the first being physisorption of an amphiphilic diblock co-polymer where the hydrophobic block adsorbs to the surface, leaving the hydrophilic block available for anti-fouling purposes. The second way is via surface initiated polymerization techniques which has been greatly influence by the development of controlled radical polymerization techniques such as atom transfer radical polymerization (ATRP). The physisorption results in mushroom regimes leaving much of the surface area of the hydrophilic polymer coiled up on itself while the grafting from approach results in highly ordered, tailorable, brush polymers. A film that is either too thick or too thin will adsorb particles onto the surface, therefore film thickness becomes an important parameter in the synthesis of ultra-low fouling surfaces. Film thickness is determined by three factors that can be tailored individually to produce the desired thickness: one being the length of the polymer chains, the second being the grafting density and the last being the solvent concentration during polymerization. The length of the chains is easily manipulated by varying the degree of polymerization by changing the ratio of initiator to monomer. The grafting density can be adjusted through varying the density of initiator on the surface. Film thickness can be theoretically calculated by the equation below;
L o = n l 5 / 3 Γ 1 / 3 {\displaystyle L_{o}=n\,l^{5/3}\,\Gamma ^{1/3}}
where L o {\displaystyle L_{o}} is the thickness of the brush, n {\displaystyle n} is the number of segments in the polymer chain, l {\displaystyle l} is the average length of the grafted polymer chains, and Γ {\displaystyle \Gamma } is the grafting density. If long polymer chains are used then a relatively sparse grafting density can be employed, but if the chains are short, a high grafting density is necessary. Furthermore, solvent concentration during polymerization affects both of these factors. Low concentration yields high-density short brush polymers, while high concentration results in low-density long polymers. Eventually, increasing solvent concentration creates a surface prone to fouling. Due to the eventual degradation of the polyethylene glycol (PEG) anti-fouling surfaces, new techniques employ zwitterionic polymers containing carboxybetaine or sulfobetaine due to their comparable hydration by water. Zwitterions can be used to solve the fouling complications that arise from using PDMS, for PDMS is readily functionalized by zwitterionic polymers such as Poly(carboxybetaine methacrylate) (pCBMA). This allows for a cheap, readily available substrate (PDMS) to be easily converted into an anti-fouling surface.
Testing methodology
Surface plasmon resonance sensors Surface plasmon resonance (SPR) sensors are thin film-refractometers that measure changes in refractive index occurring in the field of an electromagnetic wave supported by the optical structure of the sensor. SPRs are widely used in order to determine the refractive index of ultra-low fouling surfaces, an important determinant in their anti-fouling capabilities. Protein adsorption can be measured using an SPR by detecting the change in refractive index arising from molecular adsorption at the sensor chip surface. SPRs used in this type of experimentation have a detection limit of 0.3 ng/cm2 for nonspecific protein adsorption allowing for the identification of a surface which is capable of achieving ultralow fouling (<5 ng/cm2).
… excerpt ends here. Continue reading the full article.
