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Determining if a Material Makes the Grade for Combination Products

Tue, 10/09/2007 - 4:26am

This presentation—the follow-up to an article that appeared in the May issue—reviews the physical characteristics of the materials used when designing and manufacturing a combination product. Characteristics including impact resistance, lubricity and wear resistance, and properties to withstand load are examined.

In MDT's May issue, a discussion of combination products introduced the challenges of working with more than one FDA agency during the review process. Learning the requirements of an additional agency may prompt manufacturers to become familiar with further aspects of pharmaceutical or device regulation. Often, manufacturers must consider a broad range of materials to meet agency specifications as well as the performance demands of the product.

Part 1 of this discussion covered healthcare-related considerations for material selection, including advanced material surfaces to reduce interactions between the drug/biologic and the device; biocompatibility; food contact compliance (FDA, EU); and product response to sterilization techniques. (Editor's Note: The first article can be viewed online here.) In this article, mechanical and physical considerations for combination product applications—impact resistance, lubricity and wear resistance, and properties to withstand load—are explored.
Impact Resistance for Protection and Long Life
For combination products, mechanical and physical performance of the device itself, its housing, or even its packaging, is often critical for accurate and reliable operation over the long term. For example, many devices and package formats require high impact resistance to protect contents, which can be mechanical parts such as gears and levers, electronics, or drugs and biologics. Device and package designers often reference notched Izod and instrumented impact tests to assess the impact resistance of a plastic material for a given design by measuring triaxial mode failure and energy absorption. Impact performance must be evaluated at the specific material (grade) level. Some sample performance ranges include unfilled polycarbonate (PC), well-known for its impact resistance, with a notched Izod performance (ASTM D256; Izod impact, notched, 23°C) in the range of 694 J/m; unfilled acrylonitrile-butadiene-styrene (ABS) at about 293 J/m; and PC/ABS blends with performance ranging up to ~550 J/m. Device and package design can play an important role in impact resistance, so it is always advisable to perform an impact study on the final application.

The surgical stapler, though not a combination device, clearly demonstrates the importance of adequate compressive strength in allowing the two handles/levers to withstand the stress of being compressed together to perform the stapling operation.



Pre-filled syringes, insulin injection pens, and inhalation devices all need some form of durable protection. The barrel in pre-filled syringes and insulin pens, or canisters in inhalation devices, may act as the drug holder but also the protective housing for the drug. In addition, injection and inhalation devices can have external housings to protect the mechanisms or further protect the drug containers held within.
Wear Resistance for Smooth Operation
In a growing number of combination products, such as pre-filled syringes, insulin injector pens, and metered dose inhalers, the drug/biologic is packaged in a device ready for administering by a doctor, a nurse, or the patient. At time of use, the drug/biologic is transferred from the storage vessel to the patient. In many cases, this transfer involves the use of components such as gears, levers, and plungers that benefit from wear resistance and/or lubricity to provide smooth operation for a single use, or, for repeat-use devices, consistent ease of operation over a device's life.

There are many variables to consider when designing devices or packages with material-on-material moving components. Important to the smooth and consistent movement of such components is refining the coefficient of friction (COF), defined as resistance to motion. A device might need more or less COF depending upon usage. COF can be influenced in several ways—for example by the finish of the contact surface, thus mold features that offer highly polished contact areas may be employed. Additionally, it is important to note that COF may change due to wear of the parts/materials, so consideration of wear resistance may be important and are discussed below.

In some cases, external lubricants are used to refine COF, however, they can leave residuals that may pose cleanliness issues. Also, while lubricants may reduce friction, not all lubricants are chemically compatible with engineering thermoplastics, so it is advised to test compatibility before applying a lubricant to a thermoplastic. A better choice may be specialty compounds that combine a lubricant such as silicone, aramid, or PTFE with a base material to make internally lubricated products that avoid the application of topical lubricants.

Wear resistance—a related consideration—is primarily a function of material hardness, contact pressure, and the friction between material surfaces in contact with each other. In general, a harder material wears better than a softer one, but that is not the only consideration, particularly if the harder material exhibits fatigue (or degradation from stress) when exposed to high temperatures. Contact pressure can be managed by changing part dimensions. However, pressure may change over time due to stress relaxation when stress levels exceed material limits at a given temperature. Temperature itself can also change contact pressure significantly. An additional consideration must be made when dissimilar materials are used in a mechanism as contact pressure can be severe due to differences in coefficients of thermal expansion, particularly if temperatures vary during use. Overall, materials, and hence device components, will respond to their environment, so designers must anticipate circumstances of use to select materials that can withstand the expected operating conditions.

Three measures of wear and friction are:
  • Wear Factors—volume wear at a specified speed-pressure-time against a steel wear ring counterface
  • COF-Static—the material's resistance to motion during start-up against the counterface
  • COF-Dynamic—the material's resistance to motion during test operation under specified pressure and velocity conditions

    A comparison of unfilled PC with PC lubricated with 20% PTFE highlights different wear and COF properties that may be achieved with the same base resin. These differences highlight the range of capability for engineering resins and specialty compounded solutions to meet the specific needs of a device and/or its components based on the wear exposure and type of movement those components will experience.
    Wear Factor (10-10in5 min/ft-lb-hr)
    • Unfilled PC: 2600
    • Modified PC (20% PTFE): 31
    COF-Static (10-10in5 min/ft-lb-hr)
    • Unfilled PC: 0.20
    • Modified PC (20% PTFE): 0.21
    COF-Dynamic (10-10in5 min/ft-lb-hr)
    • Unfilled PC: 0.46
    • Modified PC (20% PTFE): 0.25
    Wear Factor (10-10in5 min/ft-lb-hr)
    • Unfilled PC: 2600
    • Modified PC (20% PTFE): 31
    COF-Static (10-10in5 min/ft-lb-hr)
    • Unfilled PC: 0.20
    • Modified PC (20% PTFE): 0.21
    COF-Dynamic (10-10in5 min/ft-lb-hr)
    • Unfilled PC: 0.46
    • Modified PC (20% PTFE): 0.25
    Examples of wear resistance are gears inside an insulin injection pen or inhalation device. Each of these devices may be used many times before being disposed. As such, the patient is expecting consistent operation of and interaction with the device every time—not harder to operate or looser in feel, but the same. Consistency helps ensure patient confidence and thus compliant use of the device.
    Withstanding Load and Force
    Finally, consideration of mechanical and physical properties of materials is particularly important for components of devices or packaging that are subjected to load and/or compressive force to initiate or carry out an action. This may occur in levers, triggers, and plungers/barrels that facilitate the transfer of a drug/biologic to the patient. The ability to withstand axially directed pushing forces may require specialized materials, particularly as component parts shrink with miniaturization and force is concentrated in a smaller area.

    Polyetherimide (PEI) resins have inherently higher strength and stiffness than many other engineering resins and thus are often strong candidates. A comparison of compressive strength at yield (approximate values; at 23°C) of unfilled PEI versus four other unfilled polymers demonstrates this point:
  • PC:
    ~70 MPa
  • PPSU (polyphenylenesulfone):
    ~90 MPa
  • PES (polyethersulfone) and PSU (polysulfone):
    ~100 MPa
  • PEI:
    ~130 MPa

    Another helpful measurement is assessing stress/strain curves to characterize materials. Characteristics that can be evaluated from these curves include relationships between applied load, deformation, and stiffness or modulus from simple tension. They also indicate the maximum load a material can sustain without permanent damage. Other characteristics include the ultimate stretch a plastic can handle before failure, and the plastic's energy absorption determined by the area under the curve.

    It is important to note that plastics tend to creep (lose dimensional stability under continued load) more than metals when exposed to conditions outside the optimal range for the polymer. Therefore, a data sheet for a polymer is only a guide, and designers need to consider what the expected operating temperatures and duration/repetition of load will be.
    Conclusion
    As device sophistication increases to meet expectations of caregivers and patients for greater capability and ease of use, the need for improved durability, protection, resiliency, and consistency of use also increases. A parallel trend is the growing number of combination device and drugs/biologic products, whether one-time or repeat use. This, in turn, is driving the need for precision performance to ensure delivery of accurate quantities. Engineering thermoplastics and compounds have evolved to respond to these specialized needs and are in a strong position to support new requirements of today's combination devices.

    Online

    For additional information on the technologies and products discussed in this article, visit SABIC Innovative Plastics at www.geplastics.com.

    Clare Frissora is the market director of healthcare for SABIC Innovative Plastics. She is responsible for leading the business' global healthcare initiatives and marketing team. Frissora can be reached at 413-448-6391 or clare.frissora@sabic-ip.com.

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