Innovative Polymer Films Restrict Microbe Accumulation

Ultra-thin bacteria-resistant films made of polymers applied to medical devices and other surfaces are being used to control microbe accumulation, helping to reduce the spread of hospital-acquired infections.

Researchers at The Massachusetts Institute of Technology (MIT, Cambridge, USA) used poly[allylamine hydrochloride] (PAH) and poly[acrylic acid] (PAA), assembled over a range of conditions, to create the weak polyelectrolyte multilayer (PEM) thin films. The films, which are about 50 nanometers thick, are formed of alternating layers added at different pH (acidity) levels, which determines how stiff the material is when hydrated at near-neutral pH. Polymer films assembled at higher pH (up to 6) are stiffer because the polymer chains crosslink readily and the polymers do not swell too much; those added at lower, more acidic pH (down to 2.5) are more compliant. The researchers found that they could control the extent of bacterial adhesion to surfaces by manipulating the mechanical stiffness of the polymer films; they surmised that the effect could be explained by the relationship between the surfaces and tiny projections from the bacterial cell walls known as pili - stiffer surfaces may reinforce stronger, more stable bonds with the bacterial pili. The researchers are now working on elucidating this mechanism.

The same trend was found in experiments with three strains of bacteria: Staphylococcus epidermidis, commonly found on skin, and two types of Escherichia coli. The adhesion of the microbes correlated positively with the stiffness of the polymeric substrata, independently of the roughness, interaction energy, and charge density of the materials. The new films can be combined with other methods of repelling bacteria to boost their effectiveness, such as coating surfaces with antimicrobial chemicals or embedding nanoparticle metallic spikes into the surface, which disrupt the bacterial cell walls. The study was published ahead of print on May 2, 2008 in the online version of the journal Biomacromolecules

"For those bacteria that readily form biofilms, we have no delusions that we can prevent bacterial films from starting to form. However, if we can limit how much growth occurs, these existing methods can become much more effective,” said co-author Michael Rubner, Ph.D., director of MIT's Center for Materials Science and Engineering.

The films could also be used on implantable medical devices, such as stents and valves; another possible application for the films is to promote the growth of so-called "good bugs” by tuning the mechanical stiffness of the material on which these bacteria are cultured. Doing so, the films could be designed to stimulate the growth of desirable bacteria needed for scientific study, medical testing, or industrial uses such as making ethanol.


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