Measuring fluid shear
[in special issue: Emerging Trends in Oral Care. The Biofilm Revolution]
Scientific American, supplement Emerging Trends in Oral Care. The Biofilm Revolution, .
Very little is known about the material properties of dental biofilms. Unlike conventional materials like plastics, which can be molded into uniform test pieces, biofilms are nonuniform, microscopically small, and attached to surfaces. Removal from the surface will inevitably disrupt the sample, and it is difficult to reproduce in the lab the varying and complex physical forces existing in the mouth, so testing remains a challenge.
In our laboratory at the Center for Biofilm Engineering at Montana State University, we have developed methods for testing the material properties of biofilms using fluid shear as the deforming force. By measuring the deformation to biofilms caused by long- and short-term exposure to elevated fluid shear, we found that various pure and mixed-species aerobic and anaerobic biofilms grown in glass flow cells were in fact viscous fluids that behaved elastically over short loading time periods (seconds or less), but could flow like viscous fluids when the load was sustained. Also, biofilms grown at higher shear were more firmly attached and cohesively stronger than those grown at lower shear.
This has a number of implications. Because the mouth has an incredibly wide range of shear and normal stresses, we might expect that the biofilms will also exhibit a wide range of cohesive and adhesive strengths depending on the local growth environment in the mouth. The material properties of dental plaque will also likely change with time. As calcification occurs, the plaque will be expected to become more rigid and solid-like and behave less like a fluid. In this case, instead of flowing it may fracture in response to an applied physical force. Also, because biofilms can flow, albeit slowly, it is likely that the action of chewing or movement of the tongue may actually smear biofilm from one place to another. By looking at biofilms from a materials standpoint and refining our methods, we can begin to design new technologies to address their control.
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