While silicone is the obvious material of choice in many medical applications because of its excellent biocompatibility, its physical properties make it challenging to work with. Design engineers, especially those who work with medical devices, know the difficulties of connecting flexible silicone tubing to barbed fittings or mating molded silicone parts with complex geometries to hard plastic parts. The combined elements of its flexible consistency, its inability to expand or stretch without mechanical or chemical assistance, and the tacky surface of silicone rubber, make assembly with rigid parts difficult.

Fig. 1 – This image shows silicone tubing mounted on a barbed fitting. The tube was soaked in a swelling agent, which caused it to expand enough to easily accept the barbed fitting.
This challenge presents a need for innovative technologies that allow design engineers to design the most effective medical devices in the most efficient way possible. The solution to this challenge means that design engineers are able to design with minimal restrictions and use materials that maximize functionality for a completed device. That solution is treating the silicone with Volatile Methyl Siloxane-based swelling agents, like the MicroCare Medical Swellex™ Silicone Swelling Agent. Once these solvents are absorbed into the silicone rubber, regardless of the geometry, the silicone rubber will expand in size and once it has expanded enough to allow the silicone to easily accept the fitting or rigid part, the solvent evaporates and the silicone rubber returns to its original size. What remains is the silicone rubber component attached to another part with a secure fit.

A similar method for connecting silicone to fittings or other rigid parts is to use hexane solvents to swell silicone. However, hexane solvents are highly aggressive, possess a strong odor, are regulated as a volatile organic compound (VOC) by the environmental protection agency (EPA), and they can actually affect the physical properties of the silicone. Alternatively, the swelling agents that utilize Volatile Methyl Siloxane (VMS) technology are chemically similar to silicone tubing, which means two things: not only do they swell silicone more effectively, but they also ensure that the physical properties of the silicone remain intact. VMS fluids were introduced to the market in the early 1990s and were developed as a response to the call for high performance solvents that are also environmentally friendly. Unlike hexane solvents, VMS solvents do not contribute to ozone depletion in the upper atmosphere and also are not regulated as a smog producing VOC in the lower atmosphere. VMS solvents also make great candidates for use with medical devices because they are 100 percent volatile, meaning they evaporate completely, leaving no residue.

Fig. 2 – This image shows a comparison of treated vs. nontreated ends of silicone tubing.
Using VMS solvents to swell silicone ensures that two parts will be joined together tightly and securely; however, if the mated parts need to be separated, they could be — without damage to either part. With excellent materials compatibility, VMS solvents will not chemically soften or change the cosmetic or chemical properties of commonly used mating plastic components such as polycarbonate. Other solventbased swelling agents may actually attack or soften polycarbonate-based plastics, causing the plastic surface to physically bond to the silicone, and making it so that replacing either part or trying to detach one from the other is impossible.

Swelling silicone also allows manufacturers the option of connecting the same-size diameter tubing to different fittings with multiple diameters. Especially true in high-speed, high-assembly situations, silicone swelling is providing an easier and more efficient method of joining dissimilar parts than what was used in the past. VMS solvents, as with most new technology, cost more than other similar classes of solvents. However, when taking into account the environmental benefits, time savings, and quality assurance of the finished device, the cost of VMS technology is actually quite comparable to other options.

VMS solvents are providing an effective and much-needed solution to a familiar challenge of working with silicone. For design engineers and manufacturers, swelling silicone with VMS technology provides design flexibility, and an effective, efficient, and environmentally superior way to join silicone tubing or molded parts.

This technology was done by MicroCare Medical, New Britain, CT. For more information, Click Here .