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Five Material Considerations for Device Design

Tue, 05/10/2011 - 10:36am
Kenneth E. Breeding Jr.

The material selection process during the design of a medical device can be a significant factor in the success or failure of that product. Therefore, designers need to be aware of the most critical considerations to keep in mind in this development phase. This article provides the top five variables that must be identified prior to making the final material selections.

Development of functional injection-molded parts to meet stringent medical-device standards requires astute planning and design. At the same time, product development teams are challenged to bring new devices to market quickly and efficiently. To create a successful new medical device on time and within budget, it’s important for design engineers to have a thorough understanding of the intended use of the device, including its functional and aesthetic needs. In addition, early collaboration with material suppliers, OEMs, and molders can help design engineers identify the optimum material for a given application.

Cannula_System_Photo.jpg
Cannulae tubes require a material, such as copolyester, that is durable, clear and color-stable, and chemical resistant.

For example, a designer must consider that a system of cannulae tubes must be designed for quick identification of tube sizes based on color, provide sterile pathways during surgery, and ensure an unobstructed view of the instruments and the suture inside them. Given these needs, a material for the system must be selected that is clear and color-stable; is able to withstand exposure to solvents, lipids, and blood; and has a high durability to the forces applied during insertion and manipulation of the surgical instruments.

Materials vary widely. To ensure a successful device, it is essential to have an understanding of the advantages of specific materials for particular design applications, have an understanding of how the materials are processed, and be aware of recent advances of the materials.

In the material selection process, consider a material’s durability and chemical resistance, clarity and color-stability, temperature resistance, sustainable attributes, and ease of processing and secondary operations.

Durability and Chemical Resistance
Selecting a durable material for a medical device is crucial to reduce broken parts, part failure, and waste. Durable devices are better equipped to withstand the rigors of shipping, storage, and end-use in hospital or patient recovery environments. As a result, devices made from tough materials can result in reduced risk and liability for manufacturers.

Selecting a durable material will help ensure a device is less prone to stress cracking and functional deteriorations because it can successfully survive contact with blood; lipids; chemical agents, like isopropyl alcohol and other disinfectants; and bonding solvents.

If durability is required in the device, copolyesters and polycarbonate (PC) often rank high on the consideration list, while materials such as acrylics, TABS, and polypropolyene are less desirable.

Clarity and Color Stability
Transparency of materials in devices, such as cannulae, needles, and fluid bags, is valued in the healthcare field for conveying safety, quality, and peace of mind for both the medical professional and patient. The clarity and color of a device can be a functional necessity, such as to provide an unobstructed view to detect foreign substances, bubbles, clot formation, and fluid levels. In addition, if a medical device is not clear, is slightly discolored, or has color-shifted as a result of sterilization, clinicians, as well as patients, may question its sterility and safety.

Selecting a material that can maintain long-term clarity and color stability after sterilization provides greater application and sterilization flexibility for designers and engineers, as well as a higher aesthetic quality and more confident color-coding of parts.

If retention of highly accurate color in a device is critical, consider potential sterilization methods, such as ethylene oxide (EtO) or autoclave sterilization, which will typically have little to no effect on the color shift and variability of a device made with polymeric materials. Radiation sterilization can cause a significant shift to yellow in some materials, such as TABS, acrylics, and PC.

Temperature Resistance
Amorphous materials with a high glass transition temperature provide a series of benefits during processing and to the final device. For example, high glass transition temperature can typically result in an end product with high-heat resistance, as well as lower creep and slower physical aging. Higher glass transition materials also tend to be easier to process due to the lower cooling requirements to reach a solid state. In addition, products with high heat-resistance are better able to maintain the integrity of their attributes after sterilization. In fact, the glass transition temperature of PC is high enough to allow parts to survive limited autoclave cycles.

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NP Medical’s IVEX filters utilize Eastman Eastar DN004 copolyester because it offers durability, clarity and gamma resistance, and it is capable of solvent bonding to PVC tubing and ultrasonic welding.

Sustainable Attributes
There is growing interest from the healthcare market and consumers in sustainable design to support human health and minimize the industry’s impact on the environment. There are environmentally friendly materials for devices that do not leach or have potentially active agents, such as bisphenol-A or orthophthalate plasticizer, reducing concerns about toxicity issues. Utilizing durable materials can extend the lifecycle of devices, and there are material options that offer the potential for lightweighting and source reduction.

Copolyesters are considered to be biocompatible, nontoxic, environmentally friendly materials that fit well within the industry’s evolving green health strategies. Also, copolyesters and PC offer the potential for lightweighting, especially when the limiting design constraint is toughness.

Sustainability is an important element in the development of a device. Materials that provide faster injection-molding cycles and eliminate the need for annealing can save energy while also allowing for quicker processing time and reduced production steps.

Ease of Processing and Secondary Operations
Early and ongoing collaboration with a knowledgeable material supplier can help designers anticipate working with a material in the production phase, which can help minimize manufacturing disruptions and decrease scrap. When considering materials, processing characteristics that should be assessed include the material’s molding and cooling temperatures, flow, shrink characteristics, and tooling requirements.

Additionally, material suppliers can help oversee secondary operations, such as solvent and adhesive bonding, welding, post-molding, and cold forming to ensure efficient production of quality, aesthetically pleasing medical parts.

Conclusion
Thorough understanding of a device’s intended usealong with material evaluation and analysis approached collectively by the designer, OEM, and material suppliercan bring a safe, reliable product to the market in a time- and cost-effective manner.

Kenneth E. Breeding Jr., technical platform manager, medical devices at Eastman Chemical Company, works with medical OEMs and molders to drive innovation and new application development by using Eastman specialty medical-grade polymers. He can be reached at 423-229-4931 or kebreed@eastman.com.

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