June 8, 1998

5 Min Read
Accurate assembly leads to safer syringes

Little Elm, TX-based medical device manufacturer Retractable Technologies Inc. has developed a product that addresses a major concern of healthcare workers--accidental "sticks" with contaminated needles. (see sidebar) The company says its VanishPoint(TM) is the first disposable syringe to feature a needle that automatically retracts after injection, greatly reducing the chance of an accidental needle stick.

Here's how it works: When a healthcare worker administers an injection, he or she continues to depress the plunger after the syringe is empty. This action releases a plug in the end of the plunger handle, knocking a friction ring off the spring-loaded needle hub or holder. The spring then opens, pulling the needle up inside the syringe barrel and removing it from the patient.

"The VanishPoint looks like and is used like a standard syringe," says Production Manager Jim Hoover, "but there is no risk of an accidental stick because the healthcare worker doesn't even see the needle after the injection."

To assemble the syringe, a stainless-steel needle, or cannula, must be bonded to a polypropylene hub. The appropriate adhesive for this process had to meet several requirements, including:

- The ability to deliver a strong bond between the dissimilar substrates

- UV-cure compatibility to ensure quick processing

- Low viscosity to accommodate an epoxy cavity with scallops on each side in the syringe design. The adhesive must migrate into this pocket within a few seconds and stick to the needle before being cured.

- A flourescent additive to enable automatic vision inspection through cameras. Workers inspect the adhesive under flourescent light, checking for migration out of the specified parameter limits.

The company chose Dymax Corp.'s (Torrington, CT) 1-20402 medical Ultra Light-Weld adhesive to meet its needs.

In addition to selecting the appropriate adhesive, Retractable Technologies recognized the importance of applying the correct amount of adhesive--every time. "The ability to regulate, document, and accurately repeat the amount of adhesive flowing into the hub's recess is critical," says Hoover.

Underapplication could result in the cannula separating from the hub during injection or retraction. Overapplication could result in the build-up of excess adhesive on the hub, which would prevent the needle from retracting.

During clinical trials, operators applied adhesive to one syringe at a time using air-powered benchtop dispensers from EFD Inc. (East Providence, RI). The dispensers proved accurate and reliable, however, the syringe manufacturer now needed a system capable of making ten deposits at once. And while output needed to be increased, accuracy and process control had to be maintained.

Retractable turned to EFD to design a semiautomated system to meet its expanding production needs. EFD recommended its Model 740V, an adjustable needle valve designed to make microdeposits of low- to medium-viscosity fluids. Deposit size, the company explained, could be determined by a combination of fluid pressure, flow control setting, tip size, and valve open time.

EFD also recommended pairing each valve with a dedicated, microprocessor-based VALVEMATE(TM) 7000 controller, to take full advantage of the 740V's precision dispensing capabilities. This pairing would ensure the required process control by allowing each valve's dispense-time setting to be adjusted in increments as small as 0.001 second--without stopping the production line.

Realizing the benefits the "one valve/one controller" approach could bring to its dispensing operation, Retractable Technologies designed a semi-automated tabletop assembly machine and had it fabricated by a local machine builder.

In the new system, ten valves are fixtured in a rack, with ten controllers mounted above them. A manifold at the rear of the machine distributes plant air to each valve/controller system, while pressurized tank reservoirs supply the adhesive.

The individual controllers make valve setup and deposit-size adjustment fast and easy. Once optimal settings are determined for the first pair, the company simply programs the remaining nine to produce the same results.

Retractable Technologies' new dispensing method is fast, efficient, and most important, accurate and repeatable, says Hoover. After placing a loaded work holder in position, the system operator simply lowers a bar to cycle the valves and make 10 identical deposits.

Hoover says the company's decision to go with dedicated controllers has been a tremendous help in maintaining process control. "They make it easy for us to set specific operating parameters for air pressure, fluid pressure, and dispense time, but still allow the operator to make minor adjustments within a narrow range of approved settings," he explains.

"The advantage of this approach," Hoover continues, "is that if something goes amiss and you are operating within the documented parameters, you know you need to look elsewhere to isolate the cause of the problem. It literally forces you to take the proper corrective action.

"Plus, with some automated systems, when you shut down at the end of the day and start up again the next morning, it's hard to duplicate the results you got the day before. With EFD valves and controllers, we never have that problem."

Design challenges

- Create a strong bond between dissimilar substrates

- Enable fast curing

- Use automatic inspection under flourescent light

- Increase production 10 fold

- Maintain accuracy and repeatability

- Ensure easy validation

For more information

Call 1-800-828-6344 x011 and enter the product code

Adhesive from Dymax Corp.: Product Code 4281

Dispensing systems, valves, and controllers from EFD: Product Code 4282

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