Taking the measure of a man — or a clod of dirt

I used to help my grandfather in his garden, at least until he got tired of my help and sent me home.

In the garden he had a screened, wooden frame with quarter-inch mesh wire on it. Every day he would throw several shovels of dirt onto this frame. The small stuff would fall through, but the larger rocks and sticks would be retained to be hauled away and discarded. This was one way he constantly improved the texture of the soil in his garden.

It was an easy concept to grasp. Anything that could be broken into a piece smaller than a quarter of an inch would pass through. The smaller the mesh, the smaller the particles. But we need a little more precision than the words “small” and “large” provide. It would be nice if we could determine the size of the particles passing through. That can be done by making precise mesh.

Humans can communicate and work together only because of shared experiences. When we use common measurements, we assume that others have had experiences with those measurement units. If I tell you something is about six blocks away, we may not have the exact same image of a block in our minds, but we would be relatively close because we both have experience with something called a block before. If I tell the barber to take an inch off, it isn’t likely he or she will take off three inches. The more we share experiences, the easier it is to agree on what is to be done and how to do it. Lacking shared experiences, we find it hard to set common goals or agree on methods to use.

Most Americans measure length in inches, feet, yards or miles. But in science we almost exclusively use the metric system. This is because we need to communicate across language and cultural barriers, and having a shared system of measurements helps us work collectively. So when scientists talk about screens and mesh openings, we use the metric system to define the mesh size.

Here is a brief metric review. A meter is approximately a yard in length and is divided into 100 centimeters. A centimeter is something around a quarter of an inch, and it is divided into 10 millimeters. A millimeter is similar to a sixteenth of an inch. Can you envision a millimeter, subdivided 1,000 times? Those minute units are called micrometers. Most living cells are measured in these units. For example, red blood cells are usually about seven micrometers in diameter. Now imagine etching 1,000 lines in a micrometer. Those units are called Angstroms. Cell membranes, internal cell structures, viruses and even some proteins are measured in Angstroms. But you probably haven’t measured anything in micrometers or Angstroms so you may not have a shared experience to understand them.

Recently a team of engineers and chemists created a computer chip that works about like my grandfather’s garden screen, only with much smaller openings. Liquids flow across a series of glass-screen computer chips where big particles get stuck and pushed one direction and smaller particles pass through a slot in the bottom like a coin sorter. Each chip has a smaller and smaller slot until a single particle measured in Angstroms can be trapped.

They have dubbed their creation a Lab-on-a-chip because they are hoping such a chip could greatly enhance early virus detection and identification in patients. Once the particles have been sorted by size, they could be examined with an electron microscope to identify them. The chips also could be used as a research tool for virus purification. Publication of the Lab-on-chip paper has not occurred yet, but you can learn more about the work being done by looking up the authors, Aaron Hawkins and David Belnap.

Me though? I’ll always think of their computer chip as a mesh screen in Grandpa’s garden.

Gary McCallister is professor of biology at Mesa State College.


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