OEMs often consider silicone because of its bioinertness and biocompatibility.
Selecting a Silicone Outsourcer
When working with silicone or any material, evaluation is a critical aspect of initial design for a medical device. A medical device manufacturer must consider multiple objectives when selecting a silicone material. They include physical properties, device aesthetics, regulatory support, production, and assembly concerns. As mentioned, an outsourcer can formulate a new material and outsource its manufacture or manufacture a preexisting silicone material.
Large corporations with extensive research and development departments may possess a competency in formulation development—vertical integration in R&D of sorts. Because this vertical integration is limited to R&D, the company might seek to outsource formulation scale-up and production. Smaller companies or large companies that limit R&D functions to device development will most likely pursue a pure evaluate-procure-and-use strategy. Reasons for both strategies have merit.
R&D vertical integration may reduce risk, but off-the-shelf material buying may reduce costs. For example, an OEM may choose to develop a material with novel properties that benefit its specific device, such as a high-tear-resistant material for a medical instrument grip. In another situation, a company looking to manufacture an implantable device may want to choose a preexisting silicone, because chances are that such a material has already gone through regulatory approval.
If a company chooses the vertical R&D approach and intends to outsource the by-products of these processes, several issues must first be addressed. An OEM must look at its partner's industry experience with an emphasis on its background with the particular scale of production. Many materials developed for use in specific medical devices are manufactured on a small scale, so selecting a materials outsourcing partner with the knowledge and willingness to work on that level is favorable.
An outsourcer's experience with the particular chemistry and manufacturing processes related to silicone material should also be considered. It is important to note that if an OEM chooses to have a vendor create a material the OEM has formulated, the OEM must be open to process and formulation changes suggested by the outsourcer. Research and development scale formulations and processes often differ radically from the final formulations and processes. Scaling up silicone formulations can produce different cured and uncured physical properties from those achieved in the lab. An OEM should note that some change is almost certain.
Silicones can be plagued with issues of contamination. They are sensitive to variation in equipment and raw material and need specific packaging configurations. Whether an OEM chooses a purchase-and-use relationship with a silicone materials supplier or opts for vertical R&D, the OEM should make sure that the silicone outsourcer understands the limitations associated with the material. Some of these issues may present themselves as downstream issues for vertical R&D. Industry experience, scale of manufacturing, and quality systems are critical to any outsourcing project. Manufacturers that choose to outsource must ensure that the contract manufacturer addresses the following areas: environment, procurements, and packaging.
Environment. A silicone manufacturer should have standard operating procedures (SOPs) covering the control of manufacturing environments, equipment cleaning, and equipment and facility maintenance. Silicones need to be manufactured in a clean, controlled environment, and equipment operators should wear appropriate attire. Equipment used in the manufacture of healthcare silicones should be appropriately cleaned and maintained to prevent contamination and to ensure that the equipment performs consistently from lot to lot.
Clean machinery prevents unwanted silicones from previous processes to enter the materials. Optically clear, high-refractive-index silicones can lose clarity instantly (and permanently) when mixed with other types of silicones. Some silicones will fail to cure when mixed with other types of silicones. Production delays and variable quality are results of inadequate machine cleaning procedures. Controlled environments can help prevent particulate contamination of the silicone. Silicones used in small-diameter tubing will reveal particulate-based defects easily.
Procurement. Selection of a materials partner needs to take into consideration the company's processes for feedstock procurement. SOPs should thoroughly define the processes by which a company approves vendors, raw materials, and capital equipment. In addition, SOPs should outline the processes used to qualify and test items, once procured. Silicone manufacturing can be sensitive to variations in raw materials and equipment. Incoming testing of water content is just one test that is critical to the evaluation of raw materials. Silicone polymers are sensitive to any water present during a reaction. For those who remember reaction stoichiometry from high school chemistry, for a reaction to reach a specific conclusion, it requires fairly precise amounts of those components.
Packaging. Once an OEM evaluates a company's experience, business interaction, and quality system, more- basic and practical issues need to be addressed. Issues such as the outsourcing partner's material packaging abilities can be important. Some medical devices require sterile material that comes from sterile packaging, so an outsourcing partner may need to have the ability to sterilize packaging before shipment or to package material in a configuration that allows sterilization.
An extension of the production scale issue is manifested in product packaging. Packaging configurations of the silicone material can affect the manufacturing of a medical device on the floor. For example, packaging material in side-by-side kits or tubes can reduce manufacturing time and increase ease in manufacturing medical devices, whereas packaging in bulk containers, such as drums or totes, can make use of the material more difficult.
One issue a medical device manufacturer should pay particular attention to during design and design transfer is cure inhibition. Cure inhibition caused by introduction of cure poisons can lead to unacceptable variations in the manufacturing process and, therefore, the finished product, as the cure mechanism of many silicone materials can be permanently inhibited or poisoned. Cure inhibition occurs when adjacent substrates, monolayers, or gases slow down or deactivate a cross-linking reaction.
Various poisons coming into contact with the silicone during the manufacturing process can cause cure inhibition.
Common materials that cause cure inhibition in silicones include the following:
• Amine, amide, or other UV-cured adhesives.
• Some elastomers, such as latex and natural rubber.
• Some chlorinated plastics.
• Certain tin- or sulfur-containing compounds.
• Various organic materials such as wood, leather, and clay.
During the initial design and design transfer process, medical device manufacturers should test candidate materials that may come into contact with the silicone when manufacturing.
Figure 1. (click to enlarge) A platinum cure process forms an ethylene bride cross-link.
Medical devices employing silicone most commonly have platinum cure systems because these systems do not create by-products. In addition, OEMs may prefer a platinum cure because it offers versatility on the production floor. Platinum cure systems are most commonly two-part systems, with each part containing different functional components. Generally, one part contains vinyl-functional silicones and a platinum catalyst, and the other part contains a vinyl-functional polymer, a hydride-functional (Si-H) cross-linker and cure inhibitor (see Figure 1). These cure inhibitors adjust the system's cure rate and are different from the poisons discussed here. The cure chemistry involves the direct addition of the Si-H functional cross-linker to the vinyl-functional polymer, forming an ethylene bridge cross-link. The material's cured mechanical consistency can be particularly sensitive to the final cross-link density. Severe cure inhibition can lead to complete cure elimination, while modest cure inhibition can result in a lower final durometer (i.e., the material will be softer than expected). Even weak cure inhibition can cause the silicone material to appear wet at the substrate interface.
Because silicone is a commonly used material in medical devices, there is a danger that OEMs may become complacent, thinking that there are no issues involved in incorporating silicone into their devices. OEMs need to be aware of the inherent weaknesses of silicone so that they do not use it inappropriately.
Understanding those issues will enable an OEM to carefully examine the outsourced process of silicone and maximize the benefit of outsourcing in this area. Although a materials outsourcer may have experience with silicone, manufacturers must ask their partners some simple questions to get the best results from silicone. The results are a greater understanding of the material, well-manufactured products, and a strong outsourcing partnership.
Alexander Kurnellas is lead technical writer and Stephen Bruner is marketing director at NuSil Technology LLC (Carpinteria, CA). Bruner can be reached at [email protected].