Adhesives improve manufacturing processes

DN Staff

June 10, 1996

7 Min Read
Adhesives improve manufacturing processes

From expensive emergency equipment to a simple syringe, almost every medical device relies on adhesives for some aspect of its assembly. Here are a few examples that show how the right adhesive can help save time, money, and lives.

UV-cure adhesive ensures safer intubation

About 20 million people in the U.S. each year require respiratory life support in which a tube is inserted into the trachea to secure an open airway. Conventional technique using a laryngoscope to lift the tongue and visualize the tracheal opening is both difficult to learn and perform. The most common mistake, placing the tube into the esophagus, can cause death or brain damage from lack of oxygen.

One alternative is the TrachlightSTL from Laerdal Medical Corp., Wappingers Falls, NY. The Trachlight consists of a reusable handle and a tracheal lightwand that aid in placement of endotracheal (ET) tubes in patients.

First, the lightwand slides into a typical ET tube. As the tube is fed into the trachea, a small lightbulb at the end of the wand transilluminates soft tissues in the neck. The light shows the position of the ET tube and acts as a guide to direct the tube tip down the tracheal opening. A bright, distinct glow means the tube is in the trachea and not in the esophagus. Once the tube is properly in place, the wand is retracted while the tube remains in the patient.

A key component in the wand design is a small cap covering the end of the lightbulb. The cap serves two important purposes: retain the lamp on the end of the wand to prevent its loss in the trachea or a lung, and contain any glass fragments should the lamp break.

To hold the clear cap in place, Laerdal wanted a UV-cure adhesive that would bond to the PVC material. The company first specified Model 191M adhesive from Dymax Corp., Torrington, CT. Used on prototyptes and in early production, the adhesive was successful with Laerdal's single-patient-use product. But when extended-life testing exposed the adhesive to moisture for long periods of time, Laerdal found that 191M absorbed moisture, weakening the bond.

Turning to Dymax for help, Laerdal selected adhesive 1-20304 for its low tackiness, eliminating a common problem of UV-cure adhesives. But Laerdal also knew there was potential to get adhesive on the inside of the shaft when bonding the cap, and the company didn't want to leave any uncured adhesive. Using an adhesive that cures with visible light, such as Dymax's 1-20270, would solve the problem since visible light could pass through the white shaft and cure any adhesive on the inside.

Laerdal asked Dymax to create a special compound that combined the best properties of both adhesives--low tackiness and the ability to cure with visible light, and that absorbed very little moisture. The result: polyurethane Oligomer adhesive 1-20323.

Testing also showed that the company was using too little adhesive in the bond area. The company had been trying to cut time from its manafucturing process by applying less adhesive. Once the problem was recognized, Laerdal changed its procedure. Says Russell Mohberg, product designer at the company, "We knew that using too little adhesive could be a big mistake. The most important thing was to make sure the cap stayed on."

Laerdal also switched to the Greenspot high-intensity UV curing light from UV Source Inc., Torrance, CA, which cures the part from both sides. "Before we cured the part from one side and manually rotated it, which didn't give us very good consistency. Now, both the part and light are fixtured, and we cure from both sides with a light wand that is split in two," says Mohberg.

Though Laerdal doubled its adhesive application time by putting more on, it cut curing time in half by using the dual light guide. Time for the final manufacturing process is equal, says Mohberg, but consistency has gone way up. Most importantly, "testing results with the new adhesive have been outstanding," he adds. "We haven't had any caps come off. In fact, we've actually had problems testing the adhesive to failure because the other components failed before the bond did. It is well above our specifications now."

Improved seal eliminates syringe leaks

When International Medication Systems Limited (IMS), South El Monte, CA, discovered bonding problems on its line of pre-filled syringes, the company became concerned. The syringes, supplied to hospitals that allow patients to self-administer pain relief medication as needed, recorded above normal reject rates in in-plant quality testing. The testing involved putting the bonded parts through a constant 20 psi pressure test and a 12-lb pull test.

The bonding process involved joining a stainless-steel canula to a polypropylene syringe. The problem: a faulty seal between the adhesive and the polypropylene hub material, which could result in a leak during the syringe's end use. Initially, IMS believed the problem was caused by the UV-curing system. When this proved not to be the case, IMS turned to adhesive-producer Loctite Corp., Rocky Hill, CT, for help.

Technical staff at Loctite's testing laboratory analyzed samples of both failed and unassembled parts from IMS. The technicians found that using Loctite 3051 would meet all of the bonding requirements. They also suggested that corona or plasma treatment of the polypropylene would further increase pull strength of the device.

"We anticipated resolution of the bonding problem would take at least three weeks longer than it did, and could possibly impact our delivery schedule," says Dave Barry, director of regulatory at IMS. "But we solved the problem in five days because of quick turnaround by Loctite technical service and the results we achieved with their adhesive."

Most notable of those results: improved product quality. Nannette Monreal, director of quality, explains, "After we switched to Loctite 3051, we achieved acceptable results on both the pressure test at 20 psi and the pull test at 30 lbs. In a 100% inspection of the first 2,000 units off the line, there wasn't a single failure."

In fact, IMS was so happy with Loctite 3051's performance, the company is upgrading to Loctite adhesives in other medical device applications, as well.

Adhesive tapes offer durability and reliability

Whether for short-term laboratory environments or long-term monitoring applications, medical electrodes must withstand a range of tough conditions. To ensure its electrodes provide the necessary durability, Graphic Controls Corp., Buffalo, NY, relies on ad-hesive tapes from Avery Den-nison's Specialty Tape Div., Painesville, OH.

Graphic Controls starts with rolls of the chosen substrate slit to width from Avery. The rolls are first die cut into the appropriate shape. A stud and eyelet are crimped into each electrode, and a label is applied to the top for product identification. Gel is applied to a gel pad attached to the back of the electrode, which is then laminated onto a thermoform liner. The final product is packaged into a paper polyfoil strip substrate that helps maintain its shelf life.

The company uses Avery substrates for several of its products, including its ProTraceand MediTraceelectrodes. Graphic Control's most popular product, BiotacECG conductive adhesive electrodes with a solid gel, use Avery tapes as well.

The Biotac foam version features Med 632, a 1/32-inch-thick cross-linked polyethylene film coated on one side with a non-sensitizing acrylic adhesive. The adhesive is protected by a white, polyethylene-coated kraft release liner. This higher density foam provides greater strength and prevents the electrodes from lifting off during use.

The cloth version of the Biotac electrodes uses Med 5707. The white, spunlaced polyester, non-woven material coated on one side with a non-sensitizing porous acrylic adhesive provides higher user comfort and is both air and moisture permeable. The heavy coat weight of the acrylic adhesive makes the electrodes suitable for more demanding applications.

"We have tested products from other suppliers, but the quality is not as satisfactory as with Avery," says Patty Bartlett, senior planner/purchasing agent at Graphic Controls.

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