Assembly comprising a capsule of a ready-to-wear device for monitoring a body analyte, the capsule comprising a sensor configured to provide a measure of body analyte concentration, the sensor comprising a sensor surface and at least one needle attached to the sensor on the sensor surface, the capsule further comprising a patch supporting the sensor, the patch covering at least part of the surface of the sensor, part of the patch defining an aperture in which the needles of the sensor are arranged, the assembly being characterized in it comprises: - at least one first compressible spacer, with a first face opposite the patch and a second face opposite the first face, - a receptacle comprising a receiving surface; - a pierceable pouch arranged on the receiving surface at a distance from and opposite the needle, the assembly being such that when the needle is being pushed in the direction of the pouch, it pierces the pouch, the spacer being compressed, the needle coming back opposite and at a distance from the pouch in the absence of the spacer of any thrust, the spacer being decompressed, the sensor being movable relative to the receptacle between a first position in which the needle is at a distance from and facing the pouch, the spacer then being uncompressed, and a second position in which the pouch is pierced by the needle and in which the needle is in contact with a volume of the pouch, the spacer being compressed, the pouch comprising a volume of a sensor preparation solution. the capsule further comprising a patch supporting the sensor, the patch covering at least part of the surface of the sensor, part of the patch defining an aperture in which the needles of the sensor are arranged, the assembly being characterized in it comprises: - at least one first compressible spacer, with a first face opposite the patch and a second face opposite the first face, - a receptacle comprising a receiving surface; - a pierceable pouch arranged on the receiving surface at a distance from and opposite the needle, the assembly being such that when the needle is being pushed in the direction of the pouch, it pierces the pouch, the spacer being compressed, the needle coming back opposite and at a distance from the pouch in the absence of the spacer of any thrust, the spacer being decompressed, the sensor being movable relative to the receptacle between a first position in which the needle is at a distance from and facing the pouch, the spacer then being uncompressed, and a second position in which the pouch is pierced by the needle and in which the needle is in contact with a volume of the pouch, the spacer being compressed, the pouch comprising a volume of a sensor preparation solution. - at least one first compressible spacer, with a first face opposite the patch and a second face opposite the first face, - a receptacle comprising a receiving surface; - a pierceable pouch arranged on the receiving surface at a distance from and opposite the needle, the assembly being such that when the needle is being pushed in the direction of the pouch, it pierces the pouch, the spacer being compressed, the needle coming back opposite and at a distance from the pouch in the absence of the spacer of any thrust, the spacer being decompressed, the sensor being movable relative to the receptacle between a first position in which the needle is at a distance from and facing the pouch, the spacer then being uncompressed, and a second position in which the pouch is pierced by the needle and in which the needle is in contact with a volume of the pouch, the spacer being compressed, the pouch comprising a volume of a sensor preparation solution.