“Shocking Development” – Electrically Conductive Silicone is now ready for Medical Device Applications
Silicone rubber has many physical properties which make it an attractive material for medical applications: excellent biocompatibility, flexibility, high chemical resistance, stability over a wide temperature range, and ease of manufacturing. Pure silicone is naturally an electrical insulator, which is normally desired for medical device applications. New innovative medical applications which require a flexible and biocompatible, but electrically conductive rubber however, are now in the development pipeline. Enabling these advances is a new class of conductive silicones, which include additives that allow electrical current to flow through the silicone.
Electrically conductive silicones have been used in automotive and industrial applications such as radio frequency shielding and sealing. Until now, their use in medical devices has long been hampered by the purity, cost, and processing challenges of the two traditional additives used to make silicone electrically conductive: carbon black and silver coated glass spheres. A third option, carbon nanotubes, has none of the drawbacks of these old standbys is opening new applications for conductive silicones. Let’s explain why carbon nanotubes are such an advance over the previous additives.
Carbon Black: Economical but Dirty
Carbon black is the most common additive for non-medical applications, and consists of a fine carbon powder that is mixed into the raw silicone material and is held in suspension. While it is both economical and readily available, it does present some challenges for medical device applications. The first of which is purity. Carbon black is not manufactured in the type of environments required for medical device materials. It looks and feels like coal dust…..because it is basically coal dust. In addition to purity challenges, it is a very “dirty” material that can leave a residue with anything it comes into contact with, effectively shutting this material out of applications which require contact with, or implantation in, the patient. Silicones containing carbon black have little hope of passing the USP Class VI testing that is often a critical requirement for using a material in medical applications.
Silver Coated Microspheres are Pricey and Delicate
Silver coated glass spheres have been introduced as an alternative to carbon black. As the description implies, this additive is comprised of microspheres that have been coated with a thin layer of silver. While this additive is much cleaner that carbon black, it has some unique challenges as well. Mixing and processing of this material must be done in a way which doesn’t crush the glass spheres. Ensuring the spheres remain in suspension and in a homogeneous mix throughout processing and part fabrication is also a concern. When it comes to liquid injection molding, care has to be taken to ensure that the spheres do not get filtered by the mold’s gates or get pulled out of suspension by thinning the material too much. In fact, this is why some commercial grades of this kind of conductive silicone are gumstock (HCR) meant for compression or transfer molding. The high cost of the coated spheres presents a further obstacle to using this additive for practical medical devices.
The Nano Advantage
The new additive on the block, carbon nanotubes, is the game changer which is now enabling silicone raw material manufacturers to offer electrically conductive silicones that are capable of meeting some of the basic material testing requirements (including USP Class VI) for medical devices. Due to their purity, size, and robustness, carbon nanotubes offer a pathway to success to many electrically conductive applications for medical devices.
The high purity of commercially available carbon nanotubes combined with their small size makes them an ideal additive for conductive silicone medical applications. Because they are so small (they are “nano” after all), a silicone material loaded with carbon nanotubes processes much like an unloaded material, eliminating the problems associated with silver coated microspheres. Because nanotubes are consistently available in high purity grades, devices made with this additive are likely to pass biocompatibility testing. While more expensive than carbon black, the cost of the nanotube-loaded material is not prohibitive for medical device applications.
Carbon nanotubes are price competitive with silver coated microspheres, and are as durable and stable in suspension as carbon black, while being much more pure. All these advantages will make nanotubes the additive which enables innovative conductive medical device applications.
ProMed has partnered with several companies experimenting with conductive silicone medical products in the R&D stage. Our team has processed more than 10 varieties of raw materials manufactured by 3 suppliers, and continues to work closely with our customers and raw material providers to explore applications and turn them into released products.