Medical device manufacturers are continually under scrutiny to produce products that will perform without issues throughout the life span of the product. This requires careful choice and testing of product components. However, an often overlooked detail is the choice of materials for the various labels that are required during the manufacture of the device and over the course of its lifetime. There are many factors to consider in choosing pressure-sensitive label materials. These include the surface to which the label will be adhered and the possible environmental conditions to which it will be subjected.
One condition of particular importance is heat exposure. It is important for medical device manufacturers to understand the temperatures to which labeled components may be exposed, both during the manufacturing process and during product use, in order to determine the appropriate pressure-sensitive label substrate for each label application.
For most labeling applications, a standard polyester construction is going to fit the bill. However, there are circumstances where higher temperature resistance may be required. For example, many medical devices have electronic components that contain printed circuit boards (PCBs). These boards are individually labeled in order to (a) track them through the manufacturing process, and (b) trace them back should a board or device fail. The PCB manufacturing process is one of the harshest environments to which pressure-sensitive films can be subjected, with exposure to a variety of caustic chemicals and temperatures reaching up to 750°F (398°C). PCB labels must meet stringent standards of ANSI scannability after exposure to high heat and chemicals. In many instances, label materials must also be UL-recognized and perhaps also halogen-free and RoHS and REACH compliant, depending on where the end use items will be sold.
In addition to the requirements for printed circuit boards, materials used to create modern medical devices have changed. In particular, the use of ceramics has become popular. Ceramics are cured at significantly higher temperatures than other components and, therefore, may require a more heat resistant label substrate to ensure that appropriate labels remain affixed and legible, whether they are component tracking labels for the manufacturing line alone, or warning, compliance or nameplate labels that need to function for the life of the product.
There is a variety of label substrate choices available in order to meet required heat resistance levels.
Polyester is by far the most economical and widely used label substrate for medical devices and other durable goods. It is typically printable by numerous print technologies and is available in a variety of colors and finishes. Polyester offers good chemical resistance and will withstand intermittent temperatures up to roughly 350°F (176°C).
Heat stabilized polyester and polyethylene naphthalate (PEN) films will perform at slightly higher temperatures than regular polyester. Although somewhat more expensive, these films are ideal for applications where the heat resistance of polyester is not sufficient, such as automotive underhood, aerospace and certain electronics applications. PEN is generally the film of choice in the U.S., while heat stabilized polyester tends to be the film of choice in Europe and Asia. Finally, polyimide is by far the most robust and offers heat resistance up to 500°F (260°C) for prolonged periods and up to 750°F (398°C) intermittently.
Another factor that must be considered in connection with high temperature labeling films is printability. Labels for printed circuit boards and very small parts need to be printed with very high-density barcodes and alphanumeric information due to their small size, so printability of up to 600 dpi for consistent ANSI scanning is often required. The topcoat must really bite into and bond to the film so that ink adheres well, and lay-down of the topcoat must be smooth enough to minimize the risk of print voids. Otherwise, the abrasion and chemical exposure that occurs during manufacturing and use may cause labels to become unreadable, rendering them useless.
Finally, the choice of adhesive is crucial to ensuring that labels will adhere properly and remain in place for the required duration. Adhesive performance is measured by three basic tests:
- Shear measures the internal strength of the adhesive. A soft adhesive may slide, especially at high temperatures, whereas a high-shear adhesive will stay put.
- Tack measures the “quick stick” of the adhesive to the surface.
- Peel measures the force required to remove the adhesive from the surface.
For applications where elevated temperatures will be involved, a higher shear adhesive is generally required. A higher shear adhesive is less prone to adhesive flow, which could allow the label material to curl or be removed from the surface easily, neither of which would be desirable for any medical device label.
The importance of the label should never be overlooked. It may carry vital tracking information, warranty or maintenance information, or mandated safety / instructional information without which the manufacturer could be held liable should labels fall off or become illegible. It is essential for medical device manufacturers to work closely with suppliers to identify the requirements of each specific label application in order to choose the most appropriate label material, ensure long term traceability and limit liability.