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 (F) 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 (F), 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 (F) 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 (F), 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 (F) 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 (F), 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 . <EOS>