Footprints of a blood cell

Mar 28 2006

A, B, AB, or O? A vital question, since a blood transfusion with a bottle of the wrong blood type can have deadly results. Usually, blood type is determined with antibodies fixed in a gel, centrifugation, and an optical examination.

Researchers at the University of Vienna have now developed a new technique that is very simple and works without expensive antibodies. It is based on the blood-type-specific adsorption of red blood cells (erythrocytes) on molecularly embossed plastic surfaces.

Production of the analytical chips needed for this method is a simple and inexpensive process: quartz microbalances (tiny piezoelectric quartz crystals) are coated with a whisper-thin prepolymerized film of the plastic polyurethane. Erythrocytes of a specific blood type in liquid are placed on a slide and stick to its surface, forming the “stamp” that is pressed into the surface of the prepolymerized plastic. Once the polymer layer is cured, these erythrocytes are washed away. The stamped plastic surface is now left with a large number of tiny impressions. These reflect the form of the blood cells. If a sample of blood is then placed on the chip, the erythrocytes will preferentially settle into those impressions with a matching shape. The resulting increase in mass is very sensitively measured by the quartz microbalances: if the quartz crystals are hooked up to an alternating current, they begin to oscillate. The frequency of the oscillation depends on the mass of the crystal.

The shape and size of the erythrocytes are the same for all blood types, so how can they be differentiated by these indentations? “The outer form is not the deciding factor,” says Franz L. Dickert, “instead, it is the differences in the surfaces of the different blood types.” There are sugar-like molecular fragments on the surface of the cells that differentiate the blood types. Apparently it isn’t just an impression of the shape of the erythrocyte that is formed on the surface; there is also an “embossing” on the molecular level, which reflects the chemical characteristics of the sugar fragments.

“Despite a noticeable cross-sensitivity for the other blood types, determination of the blood type by the embossed plastic films is unambiguous,” says Dickert, “because the strongest sensor signal comes from the microbalance that carries the impressions corresponding to the blood type of the sample.”