If you or someone you know is suffering from diabetes, the blood glucose meter is a fixed device in your life, which is a good choice. They are cheap and rich, but they are actually quite high technology – at least in part.
The meter itself doesn’t look much, but it’s misleading. A battery, several components, a display, and enough controllers to do things, such as memorizing readings, seem to cover all this. Of course, you won’t be surprised that you can get the whole event “on the chip”. But it turns out that the real magic lies in the test strip. Getting a good reading from the test strip requires more measurement than you think. An ordinary electricity meter requires accurate current measurement to 10na. The reading must also be adjusted according to the temperature. This device is very complex for something that looks like disposable consumer goods.
Of course, there have been announcements about new technologies that do not require needles. So far, these have not really become popular for one reason or another, but of course this may change. For example, GlucoWatch G2 is a watch that can read blood sugar, but it obviously can’t cope with sweating.
Even a continuously monitored meter works in the same way as a cheap meter. As detailed by Dan Maloney of hackday a few years ago, the continuous blood glucose monitor leaves a tiny sensor under your skin to measure the fluid in your body, not necessarily blood. But sensors usually work the same way.
In this article, I just want to discuss the traditional instrument: insert the test strip, puncture the finger and let the test strip suck a little blood.
Two Different Kinds
No matter how you split it, the real magic is the test strip. However, the way the strips are measured may vary. By far, the most common method is to use amplifiers to read very small currents. However, you can also use accurate LEDs and photodiodes – which also requires very accurate current readings, additional chemicals and accurate time, so you rarely see optical based instruments today. The exception is that there are some test strips that you can read by looking at the color of the test strip. This is not very accurate, but it is cheap and convenient. For example, for most people, an accurate blood glucose reading is not necessarily more important than knowing whether it is normal, high or very high, so sometimes just looking at the color on the stripe is enough.
The usual test strip uses chemicals to react with blood samples. Using different chemical reactions or different numbers of electrodes, each strip type can be different. However, there are some common factors.
First, the test paper will use an enzyme to react with the glucose in the sample to produce something you can detect electronically. Glucose oxidase is the most common choice, although there are other similar enzymes, such as glucose dehydrogenase nicotinamide adenine dinucleotide or glucose dehydrogenase flavin adenine dinucleotide.
The following figure shows two different brands of slats with and without covers, which are used to hide all machining. Blood enters from the left, and there are contacts connected to the instrument on the right.
The internal structure of the top and bottom test strips has been exposed.
If you prick your finger with a needle several times a day, you will be very sensitive to the size of the needle. Over time, the blood glucose meter becomes more and more sensitive, so less and less blood is needed. Less blood means you can use thinner needles and may not need to set too deep. Any measure that can reduce the amount of blood required for each test will help sell the instrument to patients. Today’s state-of-the-art meters may require only 300 picolings of blood. A hungry mosquito needs about 10 microliters, so the amount of 300 microliters is really very small. Generally speaking, when you stretch out your finger, the action of capillaries will attract a known amount of blood into the strip.
A strip may have multiple units for different purposes. For example, a cell can simply detect that blood has reached this location, which means that the previous cell should provide a valid reading. Each battery has at least one pair of electrodes. In some cases, a battery will have more than two electrodes, in which case the instrument will apply a bias current to one electrode. This increases complexity, but can provide better results.
Some instruments choose to measure the total charge generated by chemical reactions. Others measure the charge at a specific time of the reaction and use it to infer the total charge.
By the way, test strips are all places to make money because they are disposable. Just like Radio Shack used to give away a flashlight when selling batteries, the blood glucose meter is usually given away for free or sold at a very low price, because you know you need to buy a stable supply of slats to make the blood glucose meter work.
Most instruments today do not require coding, but this was common in the past. Different batches of test strips have different characteristics, so you need to enter the code in the test strip into the instrument to obtain accurate readings. If you are a hard core, you can also get a calibration solution to test the accuracy of the instrument.
Some meters will use test strips to automatically transmit their codes to the meters. This may be achieved by some mechanism on the strip or in the container for the strip container. Although some test strips may have a unit dedicated to calibration, most test strips are now only produced according to strict tolerances, so coding automation can be easily achieved by using a single calibration constant or detecting one of a few constants by using a simple connection on the test strip.
High technology is everywhere
The technology behind these sensors is relatively new. The initial work can be traced back to the late 1950s, with practical sensors, similar to what you saw today in the early 1960s. However, over the past six years, the required blood volume, accuracy, and – of course – the whiz Bang function of all microcontrollers have improved. However, it is surprising that even after so many years, a better noninvasive method for measuring blood glucose has not been found.
If I let you go to a pharmacy to buy a nano ammeter, you may not want to go to the diabetes testing department. I’m a little surprised that we haven’t seen more meters – or the chips that drive them – intrude into other instruments, although we’ve seen at least one meter change the use of some electrochemical circuits. At the same time, we are still waiting for the noninvasive blood glucose monitoring system we keep hearing about.