"A lot of people in the biosciences are making very complex designer fluids based on proteins where you might make only 10 milliliters at a time. Polypeptide hydrogels for drug delivery or tissue replacement, for example," Christopher explains. "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 by a 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. "With our device, if you gave me a milliliter of sample, I could give you back hundreds of tests," Christopher says.
Equally as important, he says, 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 Lab on a Chip.*
This is the NIST MEMS-based rheometer. The moving plate is controlled by resistance heating elements in the chevron-like structure at the top; expansion and contraction of the vanes causes the plate to move up and down. Central square where the sample would rest is approximately 500 micrometers across.
(Photo Credit: Christopher/NIST)
This is the NIST MEMS-based rheometer. The moving plate is controlled by resistance heating elements in the chevron-like structure at the top; expansion and contraction of the vanes causes the plate to move up and down. Central square where the sample would rest is approximately 500 micrometers across.
(Photo Credit: Christopher/NIST)
Source: National Institute of Standards and Technology (NIST)