Transcutaneous vs. implantable glucose sensors

A transcutaneous sensor is the most common type of glucose sensor. It has a very thin, flexible filament, that is, a soft wire, which goes through the skin. Only the wire stays in the tissue beneath the skin, while the body of the sensor sits on top, held in place by an adhesive patch. The name itself explains this, because "trans" means "through" and "cutaneous" means "skin". So it is a sensor that passes through the skin [1].

A transcutaneous sensor is simple to apply, without a doctor's help. You put it on yourself, at home, with the help of an automatic applicator, and after a certain number of days you replace it with a new one. In this way, you always have a convenient way to track your glucose, without the need for any special procedure. This is the type of sensor used by the vast majority of continuous monitoring systems available today [2].

An implantable sensor is a small cylinder, the size of a matchstick. It is inserted entirely under the skin, usually in the arm. The procedure is carried out by a doctor or another trained healthcare professional. Unlike the transcutaneous sensor, no part of it remains on the surface — everything is located under the skin. The sensor stays there for about a year [3].

Even though the sensor itself is hidden under the skin, you wear a small transmitter on top of it, resting on the skin and held in place by an adhesive patch. The transmitter sends your glucose values to an app on your phone. Because this transmitter, too, is worn stuck to the skin, moving to an implantable sensor does not spare you contact with adhesive altogether. The implantable sensor thus offers you a long-lasting solution, suited above all if you would rather not change the device often.

The main difference relates to where the sensor is located and how it is fitted. The transcutaneous sensor has only a thin filament that passes through the skin, while its body remains on the surface. You apply it yourself, at home, with an automatic applicator, without any cut. The implantable sensor is located entirely under the skin and is inserted by a healthcare professional, through a minor in-office procedure. The procedure is done with local anesthesia and a very small incision, of about 5 mm [4].

A second difference relates to how long it works and how the sensor is removed. The transcutaneous one is worn for 7-15 days, after which you peel it off yourself. The implantable sensor stays under the skin for about a year. It is removed by a healthcare professional as well, through a short procedure similar to the fitting. The two types also differ in availability and in cost, which are not the same from one country to another. In short, the transcutaneous sensor is convenient and is changed often by the patient, while the implantable one is a long-lasting solution, managed by medical staff, as the figure below shows [3].

Yes. A transcutaneous sensor contains a filament, that is, a very thin, flexible wire, which stays in the tissue beneath the skin, surrounded by the interstitial fluid (the fluid between the cells). It is this soft filament that measures the glucose concentration. It is important to know that this is not a needle that stays in the skin, but a soft and very thin wire [2].

A needle is indeed used during application, but it is only part of the applicator. The needle simply places the filament under the skin, after which it retracts automatically, leaving only the soft wire in place. For this reason, most people barely feel the sensor's presence once it is applied, and the wire under the skin is not bothersome in everyday life [5].

The implantable sensor is fitted during a short procedure, performed by a trained healthcare professional. First the skin is cleaned, usually on the back of the arm, then the area is numbed with a local anesthetic (lidocaine). A very small incision is made, about 5 mm, through which the sensor is placed under the skin with a special insertion device. At the end, the site is closed with adhesive tape or a dressing, without stitches (no suture) [4] [6].

The entire procedure takes only a few minutes and is carried out in an ordinary medical office. Fitting the sensor is easy to tolerate, because the area is numbed locally. You can think of it as a minor and quick procedure on the skin [7].

An implantable sensor can be fitted only by a qualified and trained healthcare professional, such as a doctor or a nurse (depending on the local healthcare system). This is an important distinction, because you cannot insert such a sensor by yourself. Fitting it requires a minor medical procedure and suitable conditions, which is why it is always done by specialized staff [4] [6].

This aspect sets the implantable sensor apart from the transcutaneous one. The transcutaneous sensor you apply yourself, at home, without medical help, whereas for the implantable one you need to go to the office. Choosing an implantable sensor therefore also involves close collaboration with the medical team, both for fitting it and for removing it [1].

In general, fitting an implantable sensor is not painful, because the area is numbed beforehand with a local anesthetic. During the procedure you may feel a sensation of pressure or some minor discomfort, but actual pain is absent or at most minimal. Most patients say that the sensitive moment is in fact only the small sting of the anesthetic injection, after which they feel almost nothing [3] [7].

After fitting, slight tenderness, soreness or a passing bruise may appear at the site. These sensations are usually mild and disappear within a few days. If the pain increases instead of easing, if the redness spreads, or if fever or discharge appears at the implant site, contact your medical team the same day. Talk to your doctor as well if the discomfort persists or worries you.

Yes. Even though the sensor itself sits under the skin, you will wear a small transmitter on top of it, held in place on the skin by an adhesive patch. This transmitter wirelessly powers the sensor and sends the estimated glucose values to an app on your phone. It is practically the only visible part of the system. The transmitter can be removed and put back on whenever needed.

This way, you can take it off to shower or to recharge it, without affecting the sensor under the skin. You should know, however, that while the transmitter is off, the sensor does not record glucose values [6]. Throughout that time you also receive no alarms for glucose that is too low or too high, so you go by how you feel and, if needed, by a reading taken with your glucometer.

Removing an implantable sensor is done through another short procedure, also performed by a healthcare professional. The area is numbed with a local anesthetic, a very small incision is made, the sensor is removed, and the site is closed with a dressing or adhesive tape. As with fitting, you cannot remove the sensor by yourself [4] [6]. When a sensor reaches the end of its working period, a new one can be fitted in its place, usually on the other arm.

Often, the two procedures are carried out during the same office visit, so that you can continue monitoring without long breaks. In this way, the switch from one sensor to another is simple and well organized. If the sensor stops working earlier than expected, or if a reaction appears at the implant site, let your medical team know, because early removal is also done in the office.