Successfully using liquid silicone rubber (LSR) and other advanced silicone technologies in medical devices often depends upon access to a deep and broad repository of research and test data, paired with specialized expertise in the multiple ways that material properties and processing parameters can — and do — interact. For example, a newer grade of LSR may process differently than an earlier generation, requiring a processing adjustment. Lowering processing temperature to accommodate the requirements of new wearable devices — whose sensors and batteries cannot withstand high heat — may extend cycle times, but also reduce surface defects and short shots. The direction of flow into the mold tool can affect the direction of shrinkage in the finished part, while the molding temperature in the cavity may influence mechanical properties such as modulus and tear strength. Extending in-mold cure times can help achieve sufficient cure to improve clean release to minimize scrap.

Sleep apnea masks are worn during the night while patients sleep. To provide patient comfort, silicone elastomer is used on the part of the device that is in contact with the skin; the material conforms to the face and is nonirritating and nonsensitizing to the skin.

The list of interactions is virtually endless, but the common option is data. Without a strong foundation of technical data that can be expanded upon and fine-tuned by silicone experts, choosing a material for a new part, and/or troubleshooting an existing part can require extensive rounds of trial and error that delay time to market. That is why it is important to partner with a silicone supplier that offers robust testing facilities and advanced equipment, including full design of experiment (DoE) capabilities.

In addition to the data itself, the scientists and application engineers who perform the testing and analyze the data play a critical role. Their training, experience and access to state-of-the-art equipment and proven methodologies are fundamental to finding and correctly using the right silicone solution for each medical device application.

Deriving the Data

Calibration of the moving die rheometer used for cure profile characterization of liquid silicone rubber (LSR) materials.

LSR is a popular material for medical device applications, thanks to its bio-compatibility, clarity, lack of taste and odor, and resistance to high temperatures (sterilizability) and chemicals. As a shear thinning material with low viscosity before curing, LSR flows well into the mold and forms flexible elastomeric parts after curing, making it well suited for components with complex geometries, such as needleless access valves, pump diaphragms or balloons, and high-precision parts like O-rings, stoppers, or gaskets. LSR is primarily processed using liquid injection molding equipment. Typically, LSRs are thermoset materials that can be further cured using a post-cure depending on the degree of cure achieved in the mold, adding to the versatility of how the material is processed.

As with other thermoset elastomeric materials, key physical properties of LSR include tensile modulus, elongation, tear strength, durometer, and density. Processing parameters focus on injection speed and pressure, cure temperature and time, cavity holding pressure, and recharge rate.

With all these variables and more in play, it can be challenging for medical device manufacturers to find the right material and determine the most favorable processing scenario for a given device component — or to identify the root cause of an issue. So, access to robust data is important. Data that is amassed over time from a range of tests and experiments — and is constantly refined with new results — helps to diagnose and resolve specific manufacturing problems. It offers a broad scientific foundation for establishing guidance in material selection and can help to identify unmet industry needs that drive the development of new medical-grade silicone technologies.

How is this data generated? Ideally, it is produced in a dedicated research and testing facility with a wide range of equipment. These should include a liquid injection molding machine and various mold tools representing different types of components, such as thin and thick parts, parts with torturous flow paths, and parts with high surface area. A press and delivery system that is typical for the healthcare industry is essential for accurately evaluating and adjusting processing parameters and correcting quality and productivity issues experienced by actual customers.

Equally important, the facility should be staffed by scientists and engineers trained in statistical methods and other necessary test methodologies and disciplines. This team should be capable of handling everything from the simplest test to a full DoE using statistical tools, such as two-level factorials and response surface mapping. Although, in many cases, the customer will not have the time or patience for an extensive evaluation, it can be critical for highly complicated issues — such as unexplained shot-to-shot variation — and for new material development and validation.

The data points, conclusions, and insights from all these different tests and experiments should be rigorously documented. In addition to expanding the materials database, they offer a source of continuing education for the staff, helping them become more knowledgeable and effective at assisting and training customers to use their products successfully.

Role of Properties and Processing in Performance

Stress-strain testing device used for physical characterization of liquid silicone rubber (LSR).

Deep and broad knowledge of LSR material properties and processing, supported by data, is essential in helping medical device customers achieve their product performance goals, which may include durability, precision, consistency, and operation reliability over device life-cycles. This knowledge includes an understanding of how material characteristics and processing conditions affect each other and the final part, which are particularly important in troubleshooting.

Because individual material properties and processing attributes do not exist in a vacuum, the research team must test out the specific factor or factors causing a problem and determine how a change may affect the outcome. Depending upon the complexity of the issue, this may mean systematically altering and then testing one variable at a time, or creating a matrix of variables with suggested high and low limits and a plan for how they need to be changed.

Of course, an LSR expert may already have an opinion about the likely cause and solution, based on previous test data, past experience, and familiarity with the particular material. This background helps the individual to suggest quick fixes, such as altering the mold temperature or the holding pressure or cure duration, which have worked in similar situations. An experienced professional's opinion can also save a great deal of time by helping determine which tests to run and in which order.

For example, LSR technologists know that part performance and quality are affected by how the material fills the mold tool. In turn, mold filling can be influenced by several factors: injection speed into the tool, rate of cure, shearing behavior, injection pressure, cushion, and length of holding time. The direction of flow can determine the direction of part shrinkage. To correct an issue with mold filling, the experts will systematically test for these factors. This can be done using the facility's injection molding machine or by instructing the customer to try the most promising approaches on-site.

Another processing factor, cure time, can impact material properties and, ultimately, part performance. Under-curing or over-curing the part in the mold can lead to changes in elongation, modulus, and other characteristics. Even when a material's formulation offers high compression set, understanding the proper processing and/or post-curing required to achieve that compression set is important.

LSR tends to adhere to the half of the mold with the larger surface area or with the higher temperature. Under-curing may cause adhesion to the mold, or tackiness, and can be corrected by extending cure time or temperature. As an added benefit, the additional curing can enhance part strength in the tool.

Processing temperature can present a dilemma — higher temperatures mean faster cycle times, but lower temperatures can reduce surface defects from scorch or flow lines in the part, minimize short shots, or lower the release of volatile organic compounds (VOCs). In one case, a Dow Corning customer needed a low-VOC material to manufacture an infant care product. In this instance, the application development team used test data and experience with the particular LSR grade as the basis for a material recommendation: combine a good cure in the mold with post-cure to minimize volatiles. By following this guidance, the customer was able to meet regulatory requirements.

Importance of Data in Material Selection

Tensile strength analysis on a stress-strain testing device. Used for liquid silicone rubber (LSR) characterization.

Troubleshooting is one important service offered by a research and testing facility. Another is assistance with material selection. Customers interested in using an LSR product for their medical device may need expert advice in choosing the most appropriate grade or durometer. Here again, expert knowledge based on material testing and troubleshooting can save the customer time by minimizing the need for multiple trials during the selection process.

Another Dow Corning customer with a new application was looking for an LSR with a higher durometer and wanted to avoid limitations of traditional materials, such as a narrow processing window and slower throughput due to higher viscosity. The company's technologists recommended a grade from the Dow Corning® QP1-2XX LSR family of two-part, platinum-catalyzed LSRs for healthcare, which offer a broader processing window and lower viscosity to support faster fill rates. The customer successfully trialed this grade at its plant, demonstrating the importance of performing and documenting extensive laboratory testing on new materials. It also spotlighted how lab data can serve as a guide until a history of actual application testing can be established for a new grade.

Data generated during product development testing is also valuable when a customer is switching to a newer generation of LSR materials with different properties and processing requirements. This information helps the customer adjust settings, temperature, cure time, and other factors to get the best results from the replacement material.

Development of New LSRs

In addition to directly supporting customer needs, a comprehensive research and testing center offers the experienced LSR supplier the opportunity to experiment with and validate new technologies. One area of focus for Dow Corning is developing new LSR materials suitable for overmolding onto lower-cost plastic substrates. The ability to overmold an elastomeric material onto a rigid substrate improves the aesthetics, design, ergonomics, and functionality of medical devices used by clinicians and consumers alike.

Now, market demand for cost reductions has become a key driver. Because LSRs typically cure at high temperatures (around 340 °F) that can melt or deform many lower cost substrates, the company is working to find newer technologies that cure effectively at lower temperatures. These lower-temperature curing materials are tested to understand the cure performance at lower temperatures to assess the feasibility of overmolding using thermoplastics historically unsuitable with traditional high-temperature cure silicones.

Similarly, strong market growth in wearable medical devices is driving demand for LSRs that can be processed at lower temperatures, which are required to protect sensitive batteries and sensors.

Enhancing Device Design and Production

The material chosen for a particular device component is only one element in a broader strategy for medical device design and manufacture, but it can have far-reaching implications affecting part life span, quality, performance, and more. LSR offers a set of distinctive and desirable attributes for many medical device applications. Choosing the best LSR material, enhancing part production, and solving issues becomes much easier when a supplier offers robust research and testing resources — namely, a comprehensive database built from rigorous, ongoing testing and a highly qualified and experienced staff. The combination of hard data and professional experience can be instrumental in the successful use of advanced silicone technologies.

This article was written by Eric Reynolds, Technical Service and Development Engineer, Dow Corning Medical Materials, Midland, MI. For more information, Click Here .