The pregnancy test is one of the most common medical diagnostics tests. Although it looks simple, the pregnancy test has been extensively engineered. Every part of the pregnancy test, from the outer plastic shell to the paper strip with antibodies, has been designed either mechanically or chemically.
Engineering medical point-of-care diagnostics – the pregnancy test
We aim to create similar devices in the Bioinspired Technology Lab to detect diseases in water samples.
This article will explain how parts of the device work and how they are made.
History of pregnancies
Keep
Learning?
This content was taken from
University of York online class,
Engineering the Future: Designing the Amazing
View Course
There have been many types of pregnancy tests throughout history. The best way to diagnose pregnancy was until the early 1900s when it was possible to observe the symptoms closely, e.g. morning sickness. The first clinical biomarker for morning sickness was not discovered until the 1930s. This was the human Chorionic Gonadotrophin (hCG span>), a protein found in the urine of pregnant women.
Modern pregnancy tests were based on immunoassay, which is a method for detecting specific proteins using a reaction between an antigen protein and an antibody protein. This allows for the calculation of the amount hCG protein present in a sample. Initial tests required multiple tubes to filter the sample. The first home-testing kits were made available in the ’90s, but it wasn’t until 20 years later that the modern stick-style test was developed and commercialized.
What is a pregnancy test?
Modern pregnancy tests use a technology called a lateral flow gadget (LFD). This device is used to determine whether a woman’s egg count is high or low. The lateral flow device is made up of several types of paper with small pores (or holes) of various sizes. The paper’s porous structure allows liquids to pass through it. It is possible to engineer the paper’s pores and chemical properties to filter chemicals, proteins, or cells from liquids or to allow coated microbeads to flow from one paper to another.
Figure 1: A Clearblue pregnancy testing stick
Clearblue, the market leader for pregnancy tests, claims over 99% accuracy. There are many types of pregnancy tests available. These range from the qualitative ones that show lines of pregnancy to the integrated digital readouts that can accurately track the progress of your pregnancy in just a few weeks.
The components of a pregnancy test are broken down into multiple parts (see Figure 2 below).
Figure 2: The components of a pregnancy test stick. b) Absorbent pad. c) Foam place holder. Conjugate pad with Latex microbeads. e) A nitrocellulose membrane with an anti-body test line. f) desiccant tablet. g) Device outer housing.
The plastic casing is designed to hold the test strips components together and lock them in place. However, it does not compress the contents of the pores. The housing also houses a desiccant tablet (Figure 2 f), which keeps all components dry and allows liquid to flow through it.
Figure 2. b. The urine sample is absorbed into an absorbent pad. The absorbent pad performs two functions. First, it absorbs urine like a sponge and collects it for use. Second, the absorbent pad changes colour to bright pink because of the presence a moisture indicator. This tells the user that the sample has been collected. Figure 3. The pad acts as a filter, removing any bacteria or proteins from the urine. This leaves only the urine (mostly water), and the hCG protein.
After the urine is absorbed by the absorbent pad and filtered, the liquid is drawn onto a test strip. The sample will interact with the latex microbeads (Figure 2d). These are coated with an antibody that targets a specific part of the human chorionic gonadotropin (hCG)
The urine flows through the strips and mixes with the beads (Figure 3, Figure 3). If hCG was in the sample, the antibody binds to it.
Figure 3. Negative pregnancy test. The absorbent pad may be bright pink because of the moisture indicator.
The beads flow through the strip of paper and into the test strip. This strip was engineered from Nitrocellulose paper (Figure 2.e). A second antibody stripe is located half-way down the test strip. The second antibody binds to both hCG and a different area than the one attached to the latex beads. The beads will flow through the test strips, and the hCG attached on the latex beads also binds with the antibodies, stopping them flowing through the strip. As more beads are bound together, the test line’s colour will become darker due to the accumulation latex beads. If hCG is present, the vertical line drawn on the test strip will overlap with the horizontal line printed on it to indicate pregnancy. If no hCG has been detected, the horizontal printed stripe will show a negative sign (-), indicating that there is no pregnancy.
The control line in Figure 4 is also included in this pregnancy test. It indicates that the test was successful.
Figure 4. Wetted Nitrocellulose gave a negative result. Also, a Control line (circled) is present. This is because latex beads coated with IgG are bound to the anti IgG on the test strip. This indicates that the test was successful.
Other types of lateral flow tests
Lateral flow testing can be easily designed and engineered and used for a variety different tests and biomarkers.
- Testing crops for diseases, such as potato bum
- Determining health of livestock
- Use of urine samples to test for illegal drugs
- Early stages of cancer can be diagnosed by detecting very low levels of protein in the blood.
We hope you found this article interesting and that it has shown that even something seemingly simple can be complex.
