The sensor body is a 30mm disc 5mm thick. Libre measures interstitial fluid, which is the fluid that surrounds the cells in your flesh. Interstitial fluid “seeps” out of blood capillaries and contains the nutrients that are present in your blood. To access the interstitial fluid the sensor has a filament 5mm long that is inserted into your skin using a spring loaded applicator. The sensor reads interstitial fluid for up to fourteen days so this means it has to be kept in place for that time. The surface of the sensor body in contact with your skin has an adhesive layer, so there is a 30mm adhesive disc holding the sensor to your skin.

Closeup picture of the reverse of a sensor. The filament is in the centre. Notice how the filament attached to a white plastic unit.

The sensor disc contains some electronics, a battery and an antenna. The electronics measures the voltage returned from the filament and the antenna detects and transmits NFC (Near Field Communications) signals from the reader or a suitable phone.

In Europe the sensor lasts fourteen days. In the US (mandated by the FDA) the sensor only lasts ten days, but this will soon change.

How Do You Apply It?

The sensor pack has three items: the applicator, the sensor and alcohol wipes. To prepare the sensor, you peal off the lid of the sensor pack, and unscrew the top of the applicator pack. You then insert the applicator into the sensor pack, aligning the appropriate tabs, and push the applicator so it firmly attaches to the sensor. This compresses a spring in the applicator and “arms” it with the sensor. Next, you select an area on the back of your arm (Abbott says that it can only be used in this location) and clean the area with the alcohol wipes. Since the sensor will be stuck to your skin, it makes sense to shave the area if it is hairy, not least to make it less painful when you remove the sensor. When the alcohol has dried, you pull the “armed” applicator out of the sensor pack and place it on your arm. Finally, you push the applicator until there is a click as it inserts the sensor. The sensor filament should now be inserted in your arm and the sensor attached to your skin, so you can gently remove the applicator.

Since the sensor is intended to measure interstitial fluid it does not need to be inserted in blood capillaries. Occasionally, the filament will hit a capillary and blood will leak out of the hole in the centre of the sensor soon after inserting. Such bleeds are temporary and do not affect the sensor.

Once inserted you can activate the sensor. To do this you bring the reader close to the sensor. In Europe, the reader will count down one hour before it will show any readings. In the US the reader will wait twelve hours (again, an FDA requirement) before showing any readings.

Removing the Sensor

When a sensor has finished you should remove it. First you have to remove any cover (Tegaderm or Rock Tape) that you have already applied and then you can pull off the sensor. Some people use body oil or moisteriser, some say they go for a long hot bath. But to be frank the simplest way is to prise the edges away from your skin and then yank it off.

The used sensor will have the filament exposed. This is not a ‘sharp’ per se, but a fine flexible wire. If you wish to put it into the sharps bin, then rather than putting the entire sensor in the bin you can push out the filament unit. When the sensor is applied it is the filament unit that is pushed towards your skin, rather than the entire sensor, and once the filament has entered your skin the applicator pushes the rest of the sensor over the sensor unit and onto your skin. This means that the filament unit is

To do this simply push a pencil in the central hole from the “outside” of the sensor (the non adhesive side). The filament unit will simply pop out and this can be put in the sharps bin.

Pre-soaking