Autoinjector

1. An autoinjector comprising:

a case;

a needle shroud slidably arranged in the case;

a chassis slidably arranged in the case, the chassis rotatably coupled to the needle shroud;

an outer plunger selectively engaged to the chassis;

an inner plunger selectively engaged to the outer plunger; and

a drive spring applying a biasing force to the outer plunger, the biasing force being applied to the inner plunger when the inner plunger is engaged to the outer plunger,

wherein axial movement of the needle shroud relative to the case causes rotation of the chassis relative to the needle shroud, and

wherein the rotation of the chassis causes the inner plunger to rotate relative to the outer plunger and disengage the outer plunger to remove the biasing force from the drive spring on the inner plunger.

2. The autoinjector according to claim 1 , further comprising:

a firing nut rotatably disposed on the chassis, the firing nut engaging the outer plunger when in a first angular position and disengaging the outer plunger when in a second angular position.

3. The autoinjector according to claim 2 , further comprising:

a trigger button coupled to the case and selectively engaging the firing nut, wherein axial movement of the trigger button relative to the case causes rotation of the firing nut from the first angular position to the second angular position.

4. The autoinjector according to claim 2 , wherein, when the firing nut is in the second angular position, the biasing force of the drive spring pushes the outer plunger in a distal direction relative to the case and pushes the chassis in a proximal direction relative to the case.

5. The autoinjector according to claim 1 , wherein the needle shroud has a first axial position in which a first portion of the needle shroud extends beyond a distal end of the case, a second axial position in which the needle shroud is retracted into the case, and a third axial position in which a second portion of the needle shroud longer than the first portion extends beyond the distal end of the case.

6. The autoinjector according to claim 5 , wherein, in the first axial position, resilient shroud latches on the needle shroud abut shoulders on the case.

7. The autoinjector according to claim 5 , wherein the outer plunger engages the shroud latches which deflect and do not engage the shoulders on the case when the needle shroud translates from the second axial position to the third axial position.

8. The autoinjector according to claim 1 , wherein the needle shroud includes a guide track adapted to engage a pin on the chassis.

9. The autoinjector according to claim 8 , wherein the pin moves from an angled portion to an axial portion of the guide track causing rotation of the chassis relative to the needle shroud when the needle shroud translates from a second axial position to a third axial position.

10. The autoinjector according to claim 9 , wherein the chassis includes a resilient clip adapted to engage a stop on a coupling carrier when the needle shroud is in the third axial position.

11. The autoinjector according to claim 1 , further comprising:

a syringe carrier slidably disposed in the case, the syringe carrier adapted to hold a syringe.

12. The autoinjector according to claim 11 , further comprising:

a coupling carrier coupled to the syringe carrier and selectively engaged to the inner plunger.

13. The autoinjector according to claim 12 , wherein the coupling carrier includes resilient arms adapted to releasably engage the inner plunger, and wherein the biasing force of the drive spring causes the inner plunger to deflect the resilient arms when a front stop on the syringe carrier abuts a shroud shoulder on the needle shroud.

14. The autoinjector according to claim 13 , wherein, when the resilient arms disengage the inner plunger, the inner plunger is adapted to push a stopper in the syringe.

15. The autoinjector according to claim 1 , further comprising:

a control spring axially biasing the needle shroud relative to the case.