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University UK
Non-toxic coating technology to prevent microbial colonization
Bacteria adhere readily to surfaces for survival and propagation. Generally, this brings about the formation of an adherent layer (biofilm) composed of bacteria embedded in an organic matrix. The biofilm matrix is primarily a glycoprotein, “the exopolymer”, which is generated by the bacteria, although matter derived from the environment may also be present.
Usually, bacterial adhesion is promoted by the prior adsorption of organic material onto the surface (the conditioning film); for example, polysaccharides and proteins tend to be adsorbed strongly from aqueous solution by most surfaces. Once colonization has been achieved, formation and subsequent growth of the bacterial biofilm are largely independent of the substrate. A sequence of four phases is involved: (i) transport of bacteria to the surface; (ii) reversible attachment of bacteria to that surface-van der Waals interactions overcome repulsive electrostatic forces; (iii) specific interactions involving chemical bonding develop between the bacterium and the substrate, and (iv) colonization of the surface and formation of a bacterial biofilm.
Biocompatible synthetic materials and fouling-resistant coatings are of great significance in medicine, to industry and in the home. Several types of potentially fouling-resistant materials have been developed, and their performance in preventing the attachment of biopolymers and cells has been investigated. To suppress protein adsorption, molecular design considerations formulated by Harris, Andrade and Merrill have led to the use of structures that are based on the hydrophilic poly(ethyleneglycol) (PEG) backbone, whereas the groups of Marchant, Park and Davies have utilized biomimetic hydrophilic surfaces. Furthermore, the quest for readily accessible methods by which the attachment of microorganisms to surfaces may be prevented – as well as a full understanding of the underlying mechanisms of bioadhesion such that new anti-fouling strategies can be devised – has stimulated parallel research activities. Whilst certain properties of materials, such as surface free energy and roughness, are crucial in determining their in vivo biocompatibility, the relationships between the chemical functionality of a surface and the extent of bioadhesion are not understood fully.
Methods derived for passivating synthetic substrates often require aggressive reagents or conditions, which may be unsuitable for treating natural surfaces or those that are introduced into biological environments. Thus, in addition to biofouling-resistant surfaces that function through the incorporation of anti-microbial agents into the substrate (e.g. loading of the surface with bacteriocides), two surface-modification strategies have been tried, namely: (i) hydrophilic modification, and (ii) low-surface-energy approach.
It is axiomatic to both approaches that surface properties that minimize the initial adsorption processes would also make the substrate unattractive for the direct attachment of colonizing organisms. Based on “work of adhesion” considerations, hydrophilic coatings within the aqueous environment are believed to function by presenting a “non-stick” surface to bacteria and to other colonizing microorganisms. Interest in the low-surface-energy approach dates from the early 1980s when silicone lassoers were first tried as coating materials; the approach gained added credibility following the observation that gorgonian corals, which have low energy surfaces, are not susceptible to colonization by marine microbes.
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