Intradermal injector

1. An injection device, comprising:

a housing;

a chamber within the housing, the chamber configured for containing a substance to be injected;

a needle having a delivery end, the needle being operatively associated with the chamber and configured to deliver the substance to an intradermal injection site, the needle configured to move between a retracted position and an extended position in which the needle is disposed for penetrating a patient and injecting the substance;

a resilient needle cap covering the delivery end in the retracted position, wherein the resilient needle cap is in a relaxed configuration when the needle is in the retracted position and a compressed configuration with the needle in the extended position, wherein the resilient needle cap moves from the compressed configuration toward the relaxed configuration after an injection thereby retracting the needle from the extended position towards the retracted position; and

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,

wherein the delivery end of the needle is within the collar cavity when the substance is delivered to the intradermal injection site.

2. The injection device of claim 1 , wherein the needle is disposed for piercing the resilient needle cap when the needle 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 delivery 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 resilient needle cap to retract the needle.

4. The injection device of claim 1 , wherein the collar cavity is configured to allow 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 needle pierces the resilient needle cap as the needle moves from the retracted position to the extended position.

6. The injection device of claim 1 , further comprising a circumferentially discontinuous collar surrounding the needle and defining a collar cavity,

wherein the circumferentially discontinuous collar is disposed on an exterior surface of the housing and radially spaced from and at least partially surrounding the needle such that a patient's skin can move into the collar cavity to properly position the needle for intradermal delivery of the substance to the injection site.

7. The injection device of claim 1 , further comprising a stopper sealing the chamber, the stopper separating the needle from the chamber in the retracted position, wherein the stopper is configured to be pierced by the needle with the needle in the extended position to place the chamber in fluid communication with the needle.

8. The injection device of claim 7 , wherein the chamber includes a first portion having a first diameter and a second portion having a second diameter, wherein the first diameter is larger than the second diameter.

9. The injection device of claim 8 , wherein the stopper seals the second portion and a plunger seals first portion.

10. The injection device of claim 9 , wherein a proximal end of the needle is within the second portion when the needle is in the retracted position.