Reivindicaciones

1. Automatic device (1) for performing skin biopsies that comprises a casing (2) that houses a rod (3) at the distal end (3a) of which there is a circular blade (4),characterised in that it further comprises means for causing a rotation and simultaneous longitudinal advance of the rod (3) from a first retracted position in which the blade (4) of the rod (3) does not project from the distal end (2a) of the casing (2) to a second extended position in which the blade (4) of the rod (3) projects from the distal end (2a) of the casing, followed by a rotation and simultaneous longitudinal retraction of said rod (3) from this second extended position to the first retracted position. 2. Automatic device (1) of claim 1 wherein said means comprise: - means of movement coupled to the rod (3) that cause the rod (3) to perform the movements of longitudinal advance and retraction at the same time as it rotates; and - means of actuation that act automatically on the means of movement in such a way that the latter cause the longitudinal advance and retraction of the rod (3). 3. Automatic device (1) of claim 2 wherein the means of movement comprise two parallel cams (5, 6) that rotate in opposite directions on two diametrically opposed points of the proximal end (3b) of the rod (3). 4. Device (1) of claim 3 wherein the means of movement also comprise a first longitudinal spring (7) that pushes the rod (3) in the proximal direction forcing continuous contact between said cams (5, 6) and the proximal end (3b) of the rod (3). 5. Device (1) of any of the claims 3-4 wherein the means of actuation comprise an actuator (8) with a longitudinal track coupled to gear wheels (5a, 6a) of the cams (5, 6) via rack teeth (8a, 8b), with the actuator (8) being further coupled to a second longitudinal spring (9) in such a way that when the rod (3) is in the first retracted position the second spring (9) is compressed, and on being released, the second spring (9) causes a longitudinal movement of the actuator (8) that is transmitted via the rack teeth (8a) to the cams (5, 6), the rotation of which causes the advance and rotation of the rod (3) from the first retracted position to the second extended position and its subsequent retraction to return to the first retracted position. 6. Device (1) of claim 5 wherein the cams (5, 6) and the actuator (8) are designed in such a way that one full stroke of the actuator (8) causes a full cycle of advance and retraction of the rod (3) that starts and finishes in the first retracted position. 7. Device (1) of any of the claims 5-6 wherein the actuator (8) comprises a loader (8c) that projects outside the casing (2) in such a way that the user can manually move said actuator (8) to compress the second longitudinal spring (9). 8. Device (1) of claim 7 wherein the loader (8c) projects in a longitudinal direction through the proximal end (2b) of the casing (2). 9. Device (1) of any of the claims 7-8 wherein it further comprises a locking mechanism (10) that engages with the actuator (8) when this compresses the second longitudinal spring (9). 10. Device (1) of claim 9 wherein the locking mechanism (10) comprises two rocking latches (10) furnished with a pawl (10a) for engaging with the actuator (8). 11. Device (1) of any of the previous claims wherein the distal end (2a) of the casing (2) comprises a threaded part (11) that enables controlling the distance that the blade (4) projects from the casing (2) when the rod (3) is in the second extended position. 12. Device (1) of any of the previous claims wherein the blade (4) is fixed to the rod (3) in a detachable way. 13. Device (1) of claim 12 wherein the blade (4) is attached to the rod (3) by a screw. 14. Device (1) of any of the previous claims wherein the blade (4) has an internal taper that narrows in the distal direction to catch hold of the cutaneous sample. 15. Device (1) of any of the previous claims wherein in the second extended position, the blade (4) projects through the distal end (2a) of the casing by a length of between 2 mm and 6 mm.