Syringes have a variety of medical and non-medical applications. In the field of medicine, syringes may be employed to withdraw blood, inject medicines, and, in association with other devices, to aid in biopsy procedures and bone-marrow transplant procedures. In the industrial field, syringes (commonly modified or unmodified medical syringes) may be employed, for example, to withdraw samples of fluids or gases from reactors, and to dispense adhesives or other fluid compositions. A medical syringe generally includes a barrel, commonly of plastic, and a plunger that is slideable within the barrel and which includes a piston that sealingly fits within the barrel. The plunger commonly is provided with an accessible finger grip at its proximal end for pushing and pulling the plunger into and out of the barrel, and the barrel itself may have opposing finger grips at its proximal end. Medical syringes have been made in a variety of sizes and configurations, and are well-known in the medical and industrial fields.
Syringe barrels often contain volume markings, and in a widely used commercial syringe, the barrel is made of a translucent plastic material so that the position of the piston at the end of the plunger can be viewed through the barrel walls. The volumetric markings carried by the barrel are then aligned with the distal end of the piston to provide volumetric readings.
It is often desirable to lock the plunger of a syringe at a given position within the barrel. For example, the soft-tissue biopsy procedure outlined in U.S. Pat. No. 3,882,849 requires the piston to be retracted to a certain point within the barrel to provide a partial vacuum within the barrel, and the piston should then be positively prevented from returning, under the influence of the pressure differential across it, toward its initial position. As another example, it may be desired to withdraw a particular amount of fluid from an organ, such as commonly is done in amniocentesis procedures, the fluid being retained in the syringe until it can be discharged into an appropriate receptacle. While the fluid is in the syringe, care should be taken that the plunger is not accidentally moved. It may be desirable to draw into each of a pair of syringes a particular volume of a reactive chemical; for example, one syringe may contain a reactive epoxide resin and another syringe may contain a particular volume of an amine "hardener" reactive with the resin. Since the relative volumes of the resin and hardener that are dispensed may be critical to the properties of the resulting resin, it is desirable that the plungers be prevented from moving within the barrels of the syringes until the resin and hardener are to be dispensed from the syringes.
U.S. Pat. No. 3,882,849 discloses a biopsy needle having a toothed plunger, the teeth of which can be engaged with a plate at the proximal end of the barrel at given positions of the plunger within the barrel. A somewhat similar syringe is shown in U.S. Pat. No. 3,938,505. Syringes of this type require significant additional manufacturing steps and expense, and the sharp edges of the teeth, such as those shown in U.S. Pat. No. 3,882,849, can catch, for example, on surgical gloves worn by doctors or nurses.