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Micro Rheometer is Latest Lab-on-a-Chip

Device can take much smaller sample measurements of complex fluids

Published: Wednesday, September 8, 2010 - 06:00

(NIST: Gaithersburg, MD) -- Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a microminiaturized device that can perform 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 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 can tell a lot about a complicated material such as 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 (i.e., how behavior changes with the speed or frequency of the applied force) is with a sizable 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 about a couple of milliliters, which doesn’t sound much, says polymer scientist Gordon Christopher, but for some researchers is quite a lot.

“Many people in the biosciences are making complex fluids based on proteins where you might make only 10 milliliters at a time,” explains Christopher. “Polypeptide hydrogels for drug delivery or tissue replacement, for example—their flow behaviors are very complicated, and you really need to understand them. But in a traditional rheometer, your sample for a single test is a large percentage of what you just spent two months making.”

Inspired by a talk from a NIST scientist working on the design of novel nano positioning micro-electromechanical 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. “With our device, if you gave me a milliliter of sample, I could give you back hundreds of tests,” Christopher says.

Equally as important, says Christopher, the MEMS rheometer inherently tests materials when they are confined in a small space. For many biological applications where the material is meant to be used in a confined region such as 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 hand-held device for uses such as quality control measurements on a plant floor. The NIST MEMS dynamic rheometer is described in a new paper, “Development of a MEMS-based dynamic rheometer,” by Gordon F. Christopher, Jae Myung Yoo, Nicholas Dagalakis, Steven D. Hudson, and Kalman B. Migler, published in the journal, Lab on a Chip.


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Founded in 1901, the National Institute of Standards and Technology (NIST) is a nonregulatory federal agency within the U.S. Department of Commerce. Headquartered in Gaithersburg, Maryland, NIST’s mission is to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life.