Cylinder-piston unit suitable for storing injection solutions for a needle-free injector and method for the bubble-free automatic or manual filling of the cylinder-piston unit, also under atmospheric pressure

A method for filling a cylinder/piston unit (1) for a needle-free injector, the cylinder/piston unit (1) comprising a chamber (3) arranged in a cylinder (2), the chamber (3) for long-term and sterile storage of an injection solution (4), an end wall (5) with at least one nozzle bore or one outlet element (6), a pressure-stable outer cylinder (13), and a pressure-stable piston (7) arranged movably in the chamber (3) and blocking water vapor and oxygen, the chamber (3) formed with a first chamber (8) and a second, concentric chamber (9), wherein the cross section of the first chamber (8) is greater than the cross section of the second chamber (9), and each nozzle bore or each outlet element (6) is closed in a sterile manner by a membrane (26) supported by a closure cap (10) for blocking water vapor and oxygen and the membrane (26) opens at an overpressure and the piston (7) comprising a piston rod (21) and an inner piston body (18) and a flange (22), the piston (7) is surrounded by an outer compressible piston ring (17) for blocking water vapors and oxygen in a pressure stable and sterile closing manner, a transition area between the first chamber (8) and the second chamber (9) is formed as an interior annular surface (16) of cylinder (2), the cylinder (2) including annular pressure contours (B) and (C) and the outer cylinder (13) including annular pressure contour (A), said method comprising at least the following steps:

• filling the second chamber (9) with an injection solution (4), wherein the volume of the injection solution (4) is smaller than the volume of the second chamber (9);
• inserting the piston (7) with the outer compressible piston ring (17), so that the air flow passes by, into the first chamber (8) until the outer compressible piston ring (17) bears on the annular surface (16) in order to close the second chamber (9), wherein a gas cushion remains in the second chamber (9);
• rotating the piston/cylinder unit through about 180 degrees about the horizontal axis and waiting until the gas cushion in the second chamber (9) has risen fully upward to the at least the one nozzle bore or one outlet element (6);
• moving the inner piston body (18) with the flange (22) by means of the piston rod (21) out of the outer piston ring (17) into the second chamber (9), wherein the gas cushion is forced through the nozzle bore(s) or outlet element(s) (6) from the second chamber (9), and, the membrane (26) of the closure cap (10) lifts off from the nozzle bore(s) or outlet element(s) (6) and annular pressure contours A, B and C, and, after the emergence of the gas cushion, again closes the nozzle bore(s) or outlet element(s) (6) at the annular pressure contours A, B and C in a sterile manner.