The FreeStyle Libre 3+ continuous glucose monitor system

[Doug Kerr]

Part 1.

I have diabetes. An important aspect of managing my therapy is to be aware of my blood glucose concentration.

A common way to do that is with a "finger stick" glucometer. A lancing device pokes a small blade into the finger. The small drop of capillary blood that emerges is picked up by a disposable test strip inserted in the instrument. An ingenious electrochemical process allows the instyrument to measure the glucose concentration in this venous blood sample, which value tracks closely with the glucose concentration in arterial blood, the metric that is actually of interest.

But there are disadvantages. A small one is the nuisance of doing this perhaps before every meal and perhaps also at other times of the day. But the best understanding of the state of control of the patient's diabetes comes from knowing how the blood glucose concentration varies over the entire day. It is just not practical for the patient to, for example, measure his blood glucose concentration with a finger stick instrument every hour, certainly not overnight.

These limitations are overcome by continuous glucose monitor (CGM) systems. These typically have a small disposable sensor fasted by adhesive to the patient's skin. A small probe extends into the flesh, where it observes the interstitial fluid. Again an ingenious electrochemical system allows the sensor to determine the glucose concentration in the interstitial fluid, which value tracks closely with the glucose concentration in arterial blood, the metric that is actually of interest. Such systems are made by several manufacturers.

For the past five years I have used a CGM system made by Abbott Laboratories, called the FreeStyle Libre 14 Day system. I have recently converted to a later generation product from the same company, the FreeStyle Libre 3+ system.

In the photos that follow, those that are my work carry the tag "dak" under them. The others are images found on the Internet, and are used here under the doctrine of fair use.

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The sensor of the system is about the diameter of a nickel coin and only slightly thicker. We see it here with a nickel:

dak​

We see the sensor as it would be seen in place on the patient. The recommended location is the back of the upper arm.

Here we see the opposite face of the sensor, the side that will be against the patient's skin, held by a disk of double sided adhesive tape with a very tenacious adhesive (not present here).

dak​

We see projecting from the sensor the probe, which is highly flexible. When the sensor is applied, this probe is surrounded by an open sided steel "introduction needle" with a sharp tip. This will allow the probe to be pushed into the flesh, which we could not do with the bare probe (it being too "limp" for that). We see the introduction needle here:

dak​

As soon as the sensor (and probe) is in place, that needle is extracted from around the probe and wholly out of the sensor.

[To be continued.]

 

[Doug Kerr]

Part 2.

The sensor of this system has an operating life of 15 days. The typical patient may chose to replace the sensor every two weeks (I do so, hopefully, every other Friday).

Each sensor as received by the patient from the supplier (typically a medical supply company or a pharmacy). is enclosed in the applicator, initially looking like this:

dak​

The patient unscrews the applicator cap (at the bottom in the illustration). We see the situation then here:

At the top we see the removed applicator cap. At the bottom we see the applicator proper. We see the sensor (with the double-sided adhesive disk) down in an orange cylinder. We see the projecting probe, now sheathed by the introduction needle.

The sensor can now be applied by placing the open end of the orange cylinder against the skin at the chosen location and pressing on the body of the applicator. We see that here:

The orange cylinder is thus forced to retreat into the applicator and eventually the sensor reaches the skin, to which is will stick by virtue of the adhesive disk. And by that time the probe (inside the steel needle) has penetrated the flesh.

When the orange cylinder has been fully pressed back into the applicator, it trip a spring-loaded mechanism that smartly withdraws the introducer needle from around the probe and in fact fully out of the sensor (through the hole we saw in the photo of the sensor from the back). The needle is now safely in the far reaches of the applicator, where its sharp end could not be accidentally contacted during the disposal of the applicator.

The patient now typically replaces the cap on the now-spent applicator and discards it.

[To be continued.]

 

[Doug Kerr]

Part 3.

The sensor at short intervals transmits its glucose readings via Bluetooth to either a dedicated reader device or to a smartphone equipped with an application for the purpose.

But after a new sensor has been emplaced, it must be "started" by the chosen receiving device. This process is conducted via the near field communication (NFC) system, by placing the properly-primed receiving device near the new sensor.

This having been done, there is a one-hour "hiatus" before the sensor begins transmitting glucose concentration data. This is primarily to allow the flesh region around the probe to stabilize, recovering from the affront of having a needle-sheathed probe inserted. This period also allow the sensor to conduct various self-calibration processes.

Then for the next 11 hours, any indication of the current glucose concentration on the reader or smartphone is
accompanied by an icon that in effect warns that the reading should not be used as the basis for, for example,choosing a glucose dosage before a meal. The implication is that during this period the system is still fully "calibrating" and so indications may not be "fully reliable".

The receiving device can display the most recent glucose concentration reading, can present the history of readings on different kinds of graphs, and can present various statistical measures of the glucose concentration history.

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For those interested in more detail about this system and its features, you may wish to see my detailed article on this system, available on The Pumpkin as listed here on the Pumpkin Index:

The Pumpkin

Index page of Doug Kerr's The Pumpkin

dougkerr.net

Best regards,

Doug

 

[Doug Kerr]

As a follow-on, while I did not here attempt to show the intricacies of the mechanism of the sensor applicator, I think that this shot of the "orange cylinder" (mentioned in my description) is rather fascinating:

Douglas A. Kerr: The orange cylinder 101​

It has ever so many detailed features, the roles of some of which are not yet known to me. Some of them seemingly only come into play during factory assembly.

An enormous amount of very clever engineering and design was involved in this system.

Best regards,

Doug

 

[Ted Cousins]

As a follow-on, while I did not here attempt to show the intricacies of the mechanism of the sensor applicator, I think that this shot of the "orange cylinder" (mentioned in my description) is rather fascinating:

View attachment 14111
Douglas A. Kerr: The orange cylinder 101​

It has ever so many detailed features, the roles of some of which are not yet known to me. Some of them seemingly only come into play during factory assembly.

An enormous amount of very clever engineering and design was involved in this system.

Best regards,

Doug

Many thanks Doug - as usual, an excellent review. Mary and I are both Type 2 diabetic and still using the old method.