Piston for use as a syringe with specific dimensional ratio of a sealing structure

1. An injector for delivery of a pharmaceutical composition, the injector comprising:

a cylinder with a longitudinal axis and an inner wall;

a piston having a piston body and a deformable sealing element with a convex surface, which deformable sealing element abuts the inner wall of the cylinder at an abutting interface and seals an annular gap between the piston body and the inner wall of the cylinder, the abutting interface and the deformable sealing element having axial dimensions parallel with the longitudinal axis;

wherein a ratio between a largest axial dimension of the abutting interface and a largest axial dimension of the deformable sealing element is in the range between 0.01 and 0.4, a diameter of the deformable sealing element in a relaxed state is 3% to 20% larger than an inner diameter of the cylinder, the deformable sealing element has, or the piston and the deformable sealing element have, a Shore A hardness in the range of 50 to 90, and the injector is free of lubricants.

2. The injector according to claim 1 , wherein the piston is solid and has two or more of the deformable sealing elements.

3. The injector according to claim 1 , wherein the piston is symmetrical relative to a transverse plane.

4. The injector according to claim 1 , further comprising a piston rod having a tubular section for housing a hypodermic needle, the tubular section comprising:

a needle insertion end comprising an engagement device for engaging a complementary engagement device of the outlet or the hypodermic needle;

a needle protection end opposite the needle insertion end;

a device for actuating the piston;

wherein the piston rod has a length, which is equal to or larger than an operating length of the cylinder defined by a distance from an actuating end of the cylinder to an outlet end of the cylinder minus the dimension of the piston parallel with the longitudinal axis.

5. The injector according to claim 4 , wherein the piston is free of structure to connect to the piston rod, and wherein the device for actuating the piston is free of structure to connect to the piston.

6. The injector according to claim 4 , wherein the piston rod has an elastomeric material located so a tip of the hypodermic needle is sealed by the elastomeric material when the hypodermic needle is inserted into the tubular section.

7. The injector according to claim 1 , wherein the piston is free of pigments or dyes.

8. The injector according to claim 1 , wherein the piston body and the deformable sealing element comprise the same material.

9. The injector according to claim 1 , wherein the piston is made by injection molding.

10. The injector according to claim 9 , wherein the piston comprises a styrene block copolymer.

11. The injector according to claim 1 , wherein the cylinder has an inner diameter of up to 45 mm.

12. The injector according to claim 1 , wherein the cylinder has an inner diameter in the range of 2 mm to 10 mm.

13. The injector according to claim 1 , wherein the cylinder is prefilled with a pharmaceutical composition.

14. The injector according to claim 1 , wherein the cylinder is made from a polymeric material, a glass, a metal or a combination of polymeric materials, glasses or metals.

15. The injector according to claim 1 , wherein the deformable sealing element has, or the piston body and the deformable sealing elements have, a Shore A hardness in the range of 60 to 90.

16. The injector according to claim 9 , wherein the piston is made from a thermoplastic elastomer.