Despatch Industries - Best Practices for Curing Medical-Device Adhesives

Thermally curing adhesive bonds remains an essential part of medical-device manufacturing because thermal curing provides a high degree of process repeatability.

Rolf Kragseth, ITW EAE, Despatch Industries
Seeking to develop a two-part polymer adhesive bonding step in their assembly process, a medical-device manufacturer in the northern United States performed extensive design of experiments (DOE) with multiple variables. The team sought to characterize the process and ensure the process met the appropriate internal, federal and international standards. The parts were sensitive to ambient humidity, so it was important to maintain tight environmental control during the thermal cure step. In addition, the manufacturer needed to use vacuum and heated centrifuges to eliminate bubbles and outgassing in the part.

While the parts seemed to bond well initially, accelerated age testing revealed that in many trials, bonding strength diminished over time — as had appearance and other measures of quality. The testing result indicated that ultra-precise temperature control was necessary to ensure proper finished goods. Outside of that precise process control window, quality was diminished. Lesson learned: Temperature matters.

How the curing process can strengthen or degrade a resulting bond is the focus of this article. The tradeoffs inherent with different types of adhesives and their curing methods will be discussed in an effort to establish curing best practices. In addition, the article will demonstrate that while irradiation curing is convenient, fast and allows for a simplified workflow, it often requires a post-curing thermal step. Moreover, irradiation sometimes has negative effects on the part.

Curing Alters and Strengthens Adhesive Bonds

Adhesives used in assembling medical devices must hold reliably under many difficult stresses and strains. These adhesives must perform flawlessly inside a most inhospitable environment: the human body. Curing is the process that helps ensure adhesives remain stable.

What is a Medical-Grade Adhesive? The term “adhesive” refers to the largest, most general category of binding chemicals. Adhesives generally include epoxy, acrylics, polyurethane, silicone and cyanoacrylates.

While the term “medical-grade adhesive” is not strictly defined or regulated by the Food and Drug Administration (FDA), it is generally thought of as health-compatible.[1] Producing medical-grade adhesives may include these processes:

• Manufacture in a controlled environment such as a cleanroom.

• Use of bio-compatible and potentially hypoallergenic materials to help minimize adverse effects upon skin contact. Materials may have been tested and cleared for potential toxicity or irritation responses.[2]

• Use of equipment validated for medical-grade uses.

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