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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