The Adhesive-Selection Process

Originally Published MDDI September/October 2003

ASSEMBLY TECHNOLOGY

Adhesive selection can be a difficult and time-consuming process. While adhesive suppliers provide typical industry data on a variety of substrates, such data are difficult to extrapolate to needle assembly applications, since the cannula-to-hub joint is unique.

Following a five-step process can help simplify adhesive selection.

Step 1: Select the Adhesive Chemistry. Needle manufacturers should review adhesive chemistries and cure methods that can meet the desired processing parameters.

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Figure 1. Common needle designs. (click to enlarge)

Step 2: Select the Adhesive Candidates. Manufacturers should review the available products within that chemistry. Viscosity, fluorescence, and cure speed are critical parameters.

Three common needle designs are presented in Figure 1: protruding cannula, recessed cannula, and seated cannula. With a protruding cannula, the adhesive's surface tension typically prevents it from flowing out of the core. Even low-viscosity adhesives will not migrate after prolonged delays before cure. With recessed and seated designs, however, the adhesive is more likely to flow beyond the end of the cannula until capillary action pulls the adhesive into the cannula's inner diameter. This obstructs the needle and limits fluid flow. Potential migration depends on adhesive viscosity and allotted flow time; generally, shorter flow times reduce migration in lower-viscosity adhesives.

Fluorescence is important for determining the adhesive's presence and location, and cure speed dictates exposure time.

Step 3: Review Relevant Strength and Retention Data. Needle manufacturers should review available performance data on needle pull strength, strength retention following sterilization, and thermal exposure.

Reviewing pull strength data enables manufacturers to ensure that the adhesive can meet the established specification minimums for common hub materials.

All disposable needle assemblies are sterile devices that must undergo some form of sterilization, such as ethylene oxide (EtO) or gamma exposure. The majority of light curing acrylics and cyanoacrylates exhibit nearly 100% strength retention following exposure to typical EtO and gamma cycles. In some cases, pull strength increases slightly with light-cure acrylic adhesives. Some devices may also be exposed to liquid sterilants or steam sterilization prior to use. Table I provides typical needle pull strength data for several light-cure acrylic adhesives on polycarbonate following sterilization exposure.

Most needle manufacturers perform accelerated aging tests on their assemblies to predict the life of the needle at ambient conditions. By heating the bonded assembly and correlating a time at an elevated temperature, the manufacturer can estimate an expected life. Such techniques assume that failure mechanisms do not change as temperature increases. The majority of light-cure acrylic needle-bonding adhesives exhibit a minimum 85% strength retention following 4 weeks of aging at 60°C on polycarbonate hubs with 22-gauge cannulae.

Step 4: Build Prototypes. Needle designers should screen final adhesive candidates on actual assemblies to confirm that the adhesive meets or exceeds all performance requirements. Since screening experiments are often conducted on large groups of adhesive candidates, the replicates per experimental run are often relatively low (commonly 5 to 15 observations per run). Prototype testing ensures that the manufacturer obtains results consistent with expectations, and often helps narrow the lead candidates group for final qualification testing.

Step 5: Qualify Lead Candidates. Once final adhesive candidates have been screened, manufacturers can be confident that the lead candidates will be qualified successfully. The lead candidates should then go through a full qualification test program.

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