Intradermal injector

1. An injection device, comprising:

a chamber configured for containing a substance to be injected;

a needle having a sharp distal end and a sharp proximal end, the needle being operatively associated with the chamber and configured to deliver the substance to an injection site, the needle having a retracted position and an extended position in which the needle is disposed for penetrating a patient at the injection site and injecting the substance;

a resilient needle cap covering the sharp distal end in the retracted position and disposed and configured such that the cap is resiliently deformed with the needle in the extended position and biasing the needle from the extended position towards the retracted position; and

a stopper sealing the chamber, the stopper covering the sharp proximal end and separating the needle from the chamber in the retracted position and disposed and configured such that the stopper is pierced by the needle with the needle in the extended position to place the chamber in fluid communication with the needle.

2. The injection device of claim 1 , wherein the needle is disposed for piercing the cap when moved from the retracted position to the extended position.

3. The injection device of claim 2 , further comprising:

a gas chamber containing a compressed gas; and

a plunger biased by the compressed gas with respect to the substance for forcing the substance through the sharp distal end for injecting of the substance, wherein the plunger and the gas chamber are associated such that once the plunger is moved to a predetermined position to inject a predetermined amount of the substance, the compressed gas is released from the gas chamber to allow the cap to retract the needle.

4. The injection device of claim 1 , further comprising a collar surrounding the needle and defining a collar cavity, the collar having a peripheral forward skin- contacting surface that surrounds and is radially spaced from the needle and injection site by an area that is sufficiently large to allow a patient's skin to move into the collar cavity to properly position the needle for delivery of the substance to the injection site to allow spread of the substance under the skin while inhibiting or preventing backpressure within the skin from forcing the substance out through the injection site.

5. The injection device of claim 1 , wherein the chamber includes a large diameter portion and a narrow diameter portion.

6. The injection device of claim 5 , wherein the stopper seals the narrow diameter portion and a plunger seals the large diameter portion.

7. The injection device of claim 5 , wherein the sharp proximal end of the needle is within the narrow diameter portion when the needle is in the retracted position.

8. The injection device of claim 1 , wherein the stopper is configured to be moved with respect to the injection device by a pressure of the substance.

9. The injection device of claim 8 , wherein a proximal end of the stopper is configured to be pierced by the needle after an initial movement of the stopper with respect to the injection device.

10. The injection device of claim 3 , further comprising a piston configured to at least partially seal the gas chamber.

11. The injection device of claim 10 , wherein the gas chamber is configured to be moveable with respect to the piston and the compressed gas is configured to be released from the gas chamber when the plunger associated with the gas chamber is in the predetermined position such that the seal between the piston and the gas chamber is disengaged.

12. The injection device of claim 1 , wherein the needle is fixed with respect to the chamber.

13. The injection device of claim 1 , wherein the sharp proximal end of the needle is within the chamber when the needle is in the retracted position.

14. The injection device of claim 1 , wherein the sharp distal end of the needle is configured to be at least partially within the stopper when the needle is in the retracted position.

15. An injection device, comprising:

a chamber configured to contain a substance to be injected;

a needle having a distal end and a proximal end, the needle being operatively associated with the chamber and configured to deliver the substance to an injection site, the needle configured to move from a retracted position to an extended position in which the needle is configured to inject the substance into a patient at the injection site;

a needle cap covering the distal end of the needle in the retracted position, the needle cap configured to be resiliently deformed when the needle is in the extended position to bias the needle towards the retracted position; and

a stopper sealing the chamber, the stopper configured to cover the proximal end of the needle and separate the needle from the chamber when the needle is in the retracted position, the stopper configured to be pierced by the needle when the needle moves from the retracted position to the extended position to place the chamber in fluid communication with the needle.

16. The injection device of claim 15 , wherein the chamber includes a large diameter portion and a narrow diameter portion.

17. The injection device of claim 16 , wherein the stopper seals the narrow diameter portion and a plunger seals the large diameter portion.

18. The injection device of claim 16 , wherein the proximal end of the needle is within the narrow diameter portion when the needle is in the retracted position.

19. The injection device of claim 15 , wherein the distal end of the needle is configured to be at least partially within the stopper when the needle is in the retracted position.

20. The injection device of claim 15 , wherein the stopper is configured to be moved with respect to the injection device by a pressure of the substance.

21. The injection device of claim 20 , wherein the proximal end of the stopper is configured to be pierced by the needle after an initial movement of the stopper with respect to the injection device.