Oncology nurses, compounding pharmacists, and others who handle hazardous drugs, such as chemotherapy drugs, are put at risk of exposure. This is due to the antineoplastic agents with adhesive properties within these drugs that cause contamination of the workplace. Exposure to these agents has been linked to side effects ranging from skin irritation, birth defects, and miscarriage, to cancer in some cases.

Fig. 1 – Pressure equalization diagram showing the movement of liquids and air through the system.
To prevent hazardous drug exposure, healthcare practitioners use certain safety measures including wearing protective gowns, using double gloves, and working with the drugs under a biological safety cabinet. However drug aerosols, vapors, and liquids can easily escape regular syringes used in oncology practice and cause significant contamination of the workplace.

As part of the solution to combat hazardous drug exposure, a growing number of healthcare facilities are using single-use devices called Closed System Transfer Devices (CSTDs). According to the U.S. Centers for Disease Control and Prevention’s National Institute for Occupational Safety and Health Alert: Preventing Occupational Exposures to Antineoplastic and Other Hazardous Drugs in Health Care Settings, CSTDs “mechanically prohibit the transfer of environmental contaminants into the system and the escape of hazardous drug or vapor concentrations outside the system.”

CSTDs provide an added layer of protection to healthcare workers. However, there are various design approaches employed, making some devices more effective than others.

In considering the design of a CSTD, engineers have two main problems to solve. First, which design choices will allow the device to close off all routes of exposure and all potential routes of exposure and provide the highest level of safety? Second, what will make this device easier for healthcare practitioners to use?

Safety First: A Fully Closed System

To understand the design needs of a CSTD, one much first understand the challenges and risks that need to be addressed.

The direct routes of exposure include:

  • drug aerosols and vapors escaping from the system
  • drug residuals found on the connector pieces between the syringe and drug vial or IV bag
  • contamination of the syringe plunger

To eliminate these risks, a CSTD requires a full pressure equalization system, the ability to perform “dry connections,” and a syringe plunger designed to prevent its contamination by drug residuals.

Full Pressure Equalization

A full pressure equalization system is the essence of a closed system. When a syringe is used to withdraw liquids from a drug vial, there must be a store of sterile air to replace the volume of liquid that was removed. Conversely, when liquids are entered into the drug vial to reconstitute a powdered drug, there must be storage area for contaminated air that prevents the escape of hazardous drug aerosols and vapors.

There are three existing approaches to solving this problem. The first involves the use of a filtering system. When liquids are withdrawn, air is vented into the system to equalize the pressure, replacing the equivalent volume of liquids removed. While this is acceptable from a contamination standpoint, the reverse scenario becomes problematic.

When liquids are entered into a drug vial, there must be storage space for contaminated air. However, a vented system has contaminated air pass through the filter and out to the surrounding environment. While some of the contaminants are caught within the filtering system, hazardous vapor can still escape into the surrounding environment and may be inhaled or form condensate on surfaces such as the working area and the IV bags. These are then sent for administration to the wards and may contaminate the surrounding environment.

The second approach uses an external balloon to store sterile air that replaces the removed volumes of liquid, or as a place for contaminated air to be held to prevent over-pressurization of the system. This design is effective at solving the pressure equalization issue from a safety perspective. However, these systems can be cumbersome to use, as they require an initial injection of air into the external balloon.

The final approach involves building the pressure equalization system into the syringe barrel. This provides a selfcontained storage area for sterile or contaminated air. The syringe barrel must be fully encapsulated at the top to ensure that it is airtight. This also prevents accidental removal of the syringe plunger, which could cause major spills of hazardous drugs—a potential route of exposure.

Furthermore, there must be a double needle built into the syringe itself which allows the withdrawal or insertion of liquids through one needle, and the replacement of sterile air, or the removal of contaminated air through the smaller, parallel needle.

When it comes to a achieving the combination of safety and ease-of-use, the third option accomplishes both by creating a fully closed pressure equalization system with a sleek design, unencumbered by an external balloon, which is both bulky and requires additional steps for its use. (See Figure 1)

Dry Connections

Syringe adaptors must be able to create dry connections between the syringe and drug vials, IV infusion bags, and infusion tubing sets. To do this, most CSTDs utilize the double membrane locking technique. Each adaptor and syringe are equip with a rubber septum (membrane) docking port to create these safe, dry connections. (See Figure 2)

Fig. 2 – Connector pieces are used for the double membrane locking technique.
During the connecting process, the membrane of the syringe and the membrane of the adaptor are pressed together tightly, becoming one piece of rubber. The membranes remain locked together in this position throughout the drug transfer. The locked membranes are moved over the stationary syringe needle, which pierces through to create a fluid path. In some CSTD designs, the needles remain recessed within a “housing” area, out of reach, preventing needle sticks, which are a potential route of exposure.

While the double membrane locking technique is common in most CSTD designs, the way in which the lock is held in place differs. Some systems utilize a spring mechanism to hold the membranes in place. These simple springs provide poor control of membrane compression throughout the five steps in the connection process: pressing membranes; locking membrane; enabling movement of membranes; moving membranes over needle; and holding components connected. Since the spring is sensitive to unpredictable forces created by the needle friction with the membranes, performance irregularities and leaks can occur.

Alternate designs employ a solid locking of compressed membranes, set in a defined position. In this design, arms snap over the adaptor, locking the membranes in place.

In both cases, the double membrane locking mechanism should ideally allow the rubber membranes to wipe the needles clean and keep the connectors dry. If the design allows for completely dry connections, this should prevent infiltration of microbial elements in the drugs.

Another consideration with connections between syringes and adaptors is ease of use. Some systems come preassembled with connectors welded to the syringe body. This allows for a faster set up of the CSTD, and also prevents major spills, as disconnection between the syringe and connector pieces is impossible. This design provides for a safe and easy-to-use system.

Fig. 3 – Shown is the syringe with a metal rod plunger.
Other systems require the user to lock in the connector to the syringe with a twisting motion. These systems require longer set-up time, and can disconnect, causing spills which expose the user to the hazardous drugs.

Syringe Plunger Contamination

Multiple studies have been conducted in the past decade, which identify the syringe plunger as a major route of exposure. Most CSTD systems implement the use of a standard plastic syringe. These syringe plungers touch the inside wall of the syringe barrel with almost every use, exposing the person handling the syringe to residuals of the hazardous drug that infiltrate onto the surface of the plunger.

Newer, more advanced systems utilize a thin stainless steel plunger that is fully encased by a plastic lid at the top of the syringe barrel. The thin metal rod extends through from the top of the syringe barrel at a distance from the barrel walls. Utilizing a metal rod, sealed with an EPDM rubber O-ring allows the plunger to move axially, while the syringe itself remains airtight. The O-ring combined with the encased lid provides a double jacket protection for the syringe plunger, preventing any contamination by drug residuals. (See Figure 3)

While there are other considerations that must go into the design of a highly efficient, safe, and fully closed CSTD, these three elements provide a basis for CSTD design. Ultimately, the engineer must consider what will provide the highest level of safety combined with the greatest utility and ease-of-use for the healthcare practitioner.

This article was written by Marino Kriheli, Co-Founder, Equashield LLC, Port Washington, NY. For more information, Click Here .