Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a microminiaturized device that can make complex viscosity measurements — critical data for a wide variety of fields dealing with things that have to flow — on sample sizes as small as a few nanoliters. Currently a table-top prototype, the NIST believes this rheometer could be a particularly valuable tool for biotechnologists studying minute quantities of complex materials that must function in confined spaces.Viscosity, elasticity and how materials flow when subject to a force is the subject of rheology, and the measurements tell a lot about a complicated material like a gel. Is it more like a liquid or a solid? By how much and under what conditions? The popular toy Silly Putty™ is a classic example of complex viscoelasticity, bouncing better than a rubber ball under a sharp, sudden force but slumping into a puddle when left alone.One common way to make dynamic rheology measurements (how behavior changes with the speed or frequency of the applied force) is with a sizeable lab instrument that traps a test sample between a fixed plate and one that moves, and measures how much the thin layer of test material resists being deformed. Typical sample sizes are around a couple of millilitersInspired by a talk by an NIST scientist working on the design of novel nano positioning microelectromechanical systems (MEMS), team leader Kalman Migler and his colleagues began a collaboration to build a MEMS device that duplicated a classic sliding-plate dynamic rheometer — but in a space about one-twentieth the size of a postage stamp. The sample size of the MEMS rheometer is about 5 nanoliters. The MEMS rheometer inherently tests materials when they are confined in a very small space. For many biological applications where the material is meant to be used in a confined region like a blood vessel or the interior of a cell — or must be injected through a thin needle — understanding the flow characteristics of small amounts in a small space is more important than knowing how it behaves in bulk.NIST”s early prototype MEMS rheometers include only the core sliding plate mechanism on the MEMS chip, and rely on a microscope and high-speed cameras for the actual measurements. In a more polished version, according to the research team, the necessary sensors could be included on the chip and the entire instrument reduced to a handheld device for, e.g., quality control measurements on a plant floor. The NIST MEMS dynamic rheometer is described in a new paper in the journal Lab on a Chip.** G.F. Christopher, J.M. Yoo, N. Dagalakis, S.D. Hudson and K.B. Migler. Development of aMEMS based dynamic rheometer. Lab Chip, 2010, Advance Article. DOI: 10.1039/C005065B.
