Page 415 - Handbook of Biomechatronics

410 Graham Brooker

to filter paper before measuring glucose in the precipitate using copper sul-
fate and potassium chloride. A description of the more complex but
3
extremely sensitive Hagedorn-Jensen method that used only 0.1cm of
blood is described by Miller and Van-Slyke (1936). These tests remain
extremely complicated in their execution and were not amenable to
at-home testing.
In 1941, the Ames company introduced the first colorimetric strip test
(Clinitest) using the old copper sulfate reduction method. Shortly thereafter,
the company introduced the more accurate Clinistix test based on the enzy-
matic reaction of GOx. In this test, the enzyme generates hydrogen peroxide
(H 2 O 2 ) when it reacts with the glucose, which in turn reacts with horserad-
ish peroxidase to produce oxygen that then oxidizes orthotoluidine to pro-
duce a blue or purple color (Kirchhof et al., 2008). Whole blood contains
hemoglobin that will interfere with the measurement color of a test strip, so
to avoid staining the strip with red blood cells, an ethyl cellulose layer was
applied over the enzyme and dye-impregnated paper substrate. To maintain
stiffness, this was in turn attached to a plastic support (Bronzino, 2006).
Clinic-based colorimetric strips that doctors could use to measure their
patient’s blood sugar levels became available in the late 1960s. Plastic Dex-
trostix use enzymatic reaction process developed by the Ames company, as
described by Rennie et al. (1964) and could provide measurements in 1min.
Like earlier urine-based tests, the strip changed color based on the amount of
sugar in a drop of blood and its color was compared to a color chart. These
strips needed to be washed to remove the blood cell residue to allow this
comparison to be made. However, through various generations of products
the formulation of the strips was slowly improved to eliminate the washing
and wiping steps.
3.1 Automatic Glucose Concentration Measurement Using
Colorimetric Strips and Optical Reflectance Meters
Visually comparing the color of a strip to a chart did not provide a very accu-
rate measure of the glucose levels, particularly at the extremes of the blood-
glucose spectrum, so methods to provide this function automatically were
devised.
The first attempts at automation of the color identification process
involved the use of a lightweight battery operated reflectance meter in con-
junction with the Dextrostix. Using a stabilized light source, the meter mea-
sures the light reflected from the surface of the reacted reagent strip and
converts this to a reading on a calibrated scale.