Automatic retracting hypodermic syringe

Claim 2

In a hypodermic syringe comprising a generally cylindrical barrel and a plunger reciprocable in said barrel, the improvement comprising: resilient means incorporated between said barrel and said plunger to provide and transmit a force to move said plunger axially in an outward direction for a limited distance; said resilient means including a piston head of hollow configuration, and capable of distortion in response to movement of said plunger in an inward direction relative to an inner end of said barrel to decrease the effective volume thereof, said piston head returning to relatively unstressed condition upon the release of pressure upon said plunger.

Claim 3

Structure in accordance with claim 1, including means for attaching the plunger to the piston head; said means including a projection from the inner heading of the plunger, which projection gives rise to a circular collar, and, adjacently and displaced downwardly, also gives rise to two opposing lugs, and where the piston head is of a composite structure with components of varying resiliency and thickness, including two headings with through bores, the bore of the outer heading allowing its being forced over the collar of the projection to make a liquid tight connection with the plunger, the other heading having around its periphery sealing rims for yielding liquid tight contact with the inner aspect of the barrel, and same other heading having at its outer aspect a partial slot extension of its bore, and immediately adjacent and displaced downwardly and opening centrally into the bore, two opposing notches, said notches receiving, when pressure upon the plunger causes the projection to enter the bore, and upon subsequent turning of the plunger, the lugs of the projection, to interlock both headings of the piston head with the plungers; said headings being connected by a circular bellows attached to the peripheral areas of said headings.

Claim 4

A hypodermic syringe as defined in claim 2, wherein said piston head is of a composite construction, with said components being of different thicknesses of resilient material to vary the firmness and resiliency of said components, and wherein the two opposing headings are of a greater thickness than the connecting Circular bellows, and wherein said bellows is fabricated with such that resilient force tends to maintain the opposing heads apart from each other in relatively unstressed condition.