Research Documented Fluid Suck-back

During the mid- to late-1950s, researchers within the Department of Defense (DoD) became cognizant of the undesirable phenomenon of fluid suck-back. The observation prompted researchers at Walter Reed Army Institute of Research along with engineers at the Medical Equipment Development Laboratory at Fort Totten (MEDL) with the task of remedying the problem. By 1959, the DoD developed a new multi-use nozzle jet injector (MUNJI) that was intended to be an improvement over previous models. The device was called the Automatic Jet Hypodermic Injection Apparatus, or more commonly known as the Ped-O-Jet.

Lt. Colonel Abram Benenson, who oversaw the use of jet injectors within the DoD, acknowledged the collaborative work that birthed the Ped-O-Jet in a 1959 paper:

“[S]ince July 1957, we [Benenson and Lt. Col. Robert Lindberg] have been responsible for the development of jet injectors for immunization. Our field and laboratory studies depended on the efforts and support of Captain Adrain D. Mandel and Mr Charles E. Buckler; engineering support was furnished by the Medical Equipment Development Laboratory, Fort Totten, New York, where the basic principle was improved into the current multidose jet injector” (emphasis added) (Benenson, 1959).

Aaron Ismach, the inventor of the Ped-O-Jet and a civilian engineer at MEDL, reported that previous MUNJIs of the 1950s allowed for fluid suck-back. In his December 14th, 1959 patent, Ismach stated, “unlike most earlier hypodermic jet injection guns, the instant invention is free from sucking fluid back from a patient either during or after the firing cycle is completed so that the danger of cross-infection is almost completely avoided” (Ismach, 1962). The Press-O-Jet would be one of the “earlier hypodermic jet injection guns” that Ismach was referencing.

Ismach’s patent specifically acknowledged the potential of fluid suck-back occurring when cocking the gun but states a ball check valve, located within the nozzle head, prevents this from happening. “The ball check valve 44 serves to prevent the entry of any air or suckback of any fluid during the loading cycle of the vaccine pump…” (Ismach, 1962).

From Ismach’s assertion the issue of fluid suck-back clearly rests upon the efficacy of the ball check valve. To gain an understanding of how Ismach improved upon the ball check valve, Jet Infectors compared the patents of the Press-O-Jet and Ped-O-Jet. One noticeable difference stands-out—the ball check valve on the Ped-O-Jet was spring-loaded to help keep it shutt, while the Press-O-Jet did not contain a spring.

The patent for the Press-O-Jet states, “Return flow of the inoculant through the bore 23 of plunger 22 is prevented by means of ball check valve 25.” Yet, the patent also notes that the ball check valve merely rests between two spaces without any reference of a spring: “A gasket 34 is interposed between the nose 33 and the end of plunger 22 and acts both as a seal and also as a retainer for the ball 25” (emphasis added) (Ziherl, 1958).

The diagram from the Press-O-Jet patent shows the ball check valve (number 25) resting between two partitions with no zigzagged line to indicate a spring.

Press-O-Jet ball check valve #25

(Ziherl, 1958)

Whereas the patent for the Ped-O-Jet clearly states the ball check valve is spring-loaded. “The ball check valve 44 serves to prevent the entry of any air or suckback of any fluid during the loading cycle of the vaccine pump, but the spring pressure on this valve 44 is light enough to be easily overcome during the firing or ejection cycle of the vaccine pump” (Ismach, 1962).

The diagram from the Ped-O-Jet patent shows the ball check valve (number 44) is retained by a spring, as indicated by the zigzagged line.

Ped-O-Jet Ball Check Outlet Valve #44

(Ismach, 1962)

The lack of spring-loaded ball check valve could be the reason the DoD had observed the Press-O-Jet to improperly work at times. “The failure of the ball check valve in the forward end of the plunger system to seat properly causes the gun to deliver less than the measured dosage and requires a metal lathe to gain access to the ball valve” (Anderson et al., 1958).

Even if the ball was properly seated within the counterbore the lack of resistance from a spring could account for the systemic fluid suck-back observed by Ismach.

The question now becomes did the spring-loaded ball check valve within the Ped-O-Jet prevent fluid suck-back?

As subsequent research reveals, the Ped-O-Jet still succumbed to fluid suck-back. Ismach’s assertions were nothing more than puffing statements.

  • In 1977, CDC’s Hepatitis Laboratories Division conducted safety testing on the Ped-O-Jet. The researchers observed a drop of fluid remained on the injector nozzle after firing and would disappear back into the nozzle orifice within 3 to 5 seconds. The researchers concluded, “These manipulations causing disappearance of the fluid drop are common during clinical use of the jet injector” (CDC, 1977).
  • Weniger, Jones and Chen recalled CDC’s 1977 investigation of the Ped-O-Jet, stating, “After injections, they [CDC] observed fluid remaining on the Ped-O-Jet nozzle being sucked back into the device upon its cocking and refilling for the next injection (beyond the reach of alcohol swabbing or acetone swabbing).”
  • In 1994, the CDC retested the safety of the Ped-O-Jet. After artificially contaminating the underbelly of a shaved rabbit with Hepatitis B surface antigen (HBsAg), a sterile Ped-O-Jet was placed upon the site and administered an injection. The subsequent injection was fired into a vial and tested for HBsAg. The results found the ejected fluid of the next shot fired was positive for HBsAg in 19 out of 50 (38%) of the samples (Grabowsky et al., 1994). Cross-contamination of HBsAg from the skin surface to the ejectate of the subsequent shot was due to either fluid suck-back or retrograde flow.
  • The CDC collaborated with American Jet Injector Corporation and the University of Florida to test the safety of the Am-O-Jet, a MUNJI device. The Am-O-Jet was an identical design of the Ped-O-Jet. Within this study the researchers admit the check-valve had been redesigned; thus further implicating the inherent design faults of previous Ped-O-Jet models. It is noteworthy to add, the researchers found rates of contamination were significant with the Am-O-Jet (Sweat et al., 2000).

These reports document that jet injectors—including the most widely used jet injector, the Ped-O-Jet—allowed contaminates upon the nozzle orifice to be sucked into the internal fluid pathway and infect the next dosage to be fired. Despite attempts to resolve the problem with the ball check valve, fluid suck-back still occurred and has been continuously reported upon through-out the past 60-years.


  • (Anderson, Lindberg, & Hunter, 1958) Anderson EA, Lindberg RB, Hunter DH. Report of large-scale field trial of jet injection in immunization for influenza. JAMA 167:549–552, 1958.
  • (Benenson, 1959) Benenson AS. Mass immunization by jet injection. In: Proceedings of the International Symposium of Immunology, Opatija, Yugoslavia, 28 September—1 October 1959 (International Committee for Microbiological Standardization, Secton of the International Association of Microbiological Societies). Zagreb: Tiskara Izdavackog zavoda Jugoslavenske akademije; 1959;393–399 [Library of Congress QW 504 I60p 1959].
  • (CDC, 1977) CDC. DHEW Memorandum: Informal Quarterly Report of October-December 1977. From: Special Investigations Section (Petersen NJ, Bond WW, Carson LA) to: Deputy Director (Favero MS), Hepatitis Laboratories Division, Phoenix, AZ (unpublished).
  • (Grabowsky et al., 1994) Grabowsky M, Hadler SC, Chen RT, Bond WW, de Souza Brito G. Risk of transmission of hepatitis B virus or human immunodeficiency virus from jet injectors and from needles and syringes. Unpublished manuscript draft, dated January 3, 1994.
  • (Ismach, 1962) Ismach, Aaron. “Multi-dose jet injection device.” United States Patent 3,057,349. 9 October 1962.
  • (Sweat et al., 2000) Sweat JM, Abdy M, Weniger BG, Harrington R, Coyle B, Abuknesha RA, Gibbs EP. Safety testing of needle free, jet injection devices to detect contamination with blood and other tissue fluids. Ann NY Acad Sci 2000;916(31):681-682.
  • (Weniger, Jones & Chen) Weniger BG, Jones TS and RT Chen. “The Unintended Consequences of Vaccine Delivery Devices Used to Eradicate Smallpox: Lessons for Future Vaccination Methods.” National Center for Immunization & Respiratory Diseases. [Poster Presentation].
  • (Ziherl, 1958) Ziherl, Frank. “Multiple Injection Inoculator Instrument.” United States Patent 2821193. 28 January 1958.

© Jet Infectors, 2016 – 2021
Fair Use Notice (17 U.S.C. § 107)

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