The initial design phase of a medical-device project doesn't effectively determine functions and features unless careful attention is paid to identifying optimum materials for the application. Without that, the design may take longer and prove more costly as it travels the path from concept to functional molded part.

Copolyesters, for example, are engineered to meet the performance demands of today's injection-molded medical applications while providing high gloss, low haze and transparency. Their chemical-resistance to solvents such as, lipids and isopropanol (IPA), and their toughness, which varies according to melt-flow rate, let these materials be tailored for the specific needs of the processor and end-user.

To determine whether copolyesters are right for the application, design engineers must evaluate a number of product, process, and manufacturing attributes as part of the material selection process. Some of these attributes are physical properties, bonding methods, sterilization methods, and processing methods. Some copolyester characteristics are shown in Table 1.

Know your options

Medical-device industry applications for copolyesters range from packaging to evacuated blood collection tubes to IV connectors. Clarity, color and property stability after sterilization — including EtO, gas plasma, e-beam, and gamma irradiation — make copolyesters the material of choice for applications such as safety syringes, blood separation devices, wound treatment applications, and rigid packaging. In addition, copolyesters are considered to be biocompatible, nontoxic, environmentally friendly materials, making them a good fit with the industry's evolving green strategies.

It is important to know that among copolyesters, there are great differences. Each brings certain advantages for particular design applications. As medical designers and materials suppliers determine design and material priorities in the early stages of development, they can identify which functions are needed and select the copolyester that is most suitable as illustrated in the following case studies.

Smith and Nephew

Smith and Nephew Endoscopy had specific reasons for selecting Eastar^™ MN005 copolyester for a single-use, disposable Clear-Trac Complete cannula system for use in arthroscopic surgery. The Clear-Trac system includes cannulae in nine sizes, which enable surgeons to find the best fit, depending on the size of the joint being treated and the thickness of the muscle around it. Each cannula is tinted so surgeons can distinguish between different cannulae during surgery. The tinting as a product-design feature is what led the OEM to choose the Eastar copolyester.

John Lipchitz, research and development project engineer for Smith and Nephew, says the company chose the material over others such as polycarbonate, for its color stability and clarity after undergoing gamma sterilization. The material's clarity provides surgeons with unobstructed views of the instruments and the sutures inside them, as well as the bone and soft tissue that surround the surgery sites.

Smith and Nephew also found the material easy to work with, even in difficult molds.

In the early stages of the project, the team reviewed the part design and determined that a single gate could be used to fill the 4.1-in. part with a nominal wall thickness of 0.040 in. Standard copolyester melt-flow rates are approximately 12 grams per 10 min. at 243°C to fill a single-cavity mold. A higher melt-flow resin was required for the multicavity tool, and the Eastar MN005 copolyester, with a melt-flow rate of approximately 23 grams per 10 min. at 243°C, was used to fill the multicavity mold and meet other fitness-for-use requirements.

Eastman's design services team provided a mold-filling simulation analysis to predict the success of filling the intricate shape of the Clear-Trac cannula medical devices. A mold-filling simulation analysis also can be used to determine if spots are present in the mold, which requires additional mold cooling to aid in part release and/or cycle time reduction. It also can be used to predict machine injection pressures required to fill mold cavities. The analysis alone can help determine if standard low melt-flow rate resins are able to fill a mold or if a higher melt-flow resin will be required.

Cardian BCT

The Trima Automated Blood Component Collection System separates blood into platelets, plasma, and red blood cells by a process known as aphaeresis and automatically harvests only the needed components and reinfuses the unneeded ones back into the donor.

The device is comprised of a plastic cassette that measures approximately 9.5 in. (24.13 cm) by 4.5 in. (11.43 cm) with a wall thickness of 0.080 in. (2 mm). The challenge was finding a material solution that was compatible with the cyclohexane solvent that adheres PVC tubing to the cassette. The company evaluated several materials and selected Eastar because of its solvent compatibility, as well as its ability to maintain clarity after EtO and gamma sterilization.

Production support critical

Supplier involvement during the production phase can help minimize manufacturing disruptions and decrease scrap. In addition, suppliers can help oversee secondary operations, such as bonding, welding, post-molding, and cold-forming, to ensure efficient production of quality, aesthetically pleasing medical parts. Also, certain materials provide faster injection-molding cycles. Under typical part design and cooling conditions, Tritan MX711 copolyester can reduce the cycle time by 20% over “heritage” medical-grade copolyesters. Tritan maintains excellent chemical resistance without the need for annealing, allowing for quicker processing time and reduced production steps.

The supplier's design engineering team also can provide certain review steps in the production process for additional support. For example, a part design review can assist in ensuring part strength (i.e., uniform wall thickness) as well as verify that unique part features such as snap fits, bosses and joints, are properly designed. A tooling review helps make certain that the type of feed system (hot or cold runner), gate design (hot or cold gates), cooling line arrangement, and venting and ejection systems are matched to adequately process the selected copolyester.

And a simulation of all phases of the molding process, including filling, packing, cooling and warping, helps optimize gate location and identify problem areas, such as air traps or weld lines. Simulation also helps ascertain fill-pressure requirements for the proposed part geometry and selected plastic, verification of the runner system dimensions, evaluation of the proposed cooling line layout, and other mold-filling attributes.

Thorough material evaluation, analysis, and simulation, when approached together by both the material supplier and the medical-device manufacturer will yield positive results in terms of product effectiveness and manufacturing costs.

Building on the strength of heritage

Tritan copolyester builds on the strengths of heritage copolyesters while providing additional advantages to product designers. Its chemical-resistant properties protect against blood, lipids, and chemical agents such as disinfectants. See Figure 1. One of the material's benefits is the minimal color shift after gamma sterilization so long-term clarity remains, providing greater application and sterilization flexibility for designers and engineers. See Figure 2.