Silicone rubber is an almost perfect biomedical material: In fact, stainless steel is probably the only material in which device makers place a higher level of trust.

However, It has only one glaring fault: it’s tacky to the touch and clings to any object or dust that comes in contact with it. When silicone rubber parts come in contact with other surfaces—including other silicone surfaces—they resist motion with a friction coefficient that exceeds 1.0.

One solution for this is to apply a thin dry lubricant to the surface of silicone parts such as Parylene. Historically, molders have opted for one of several formulations of Parylene, applied at room temperature to the exterior of tubes, seals, etc., via vapor deposition. This results in a film that is typically 2-20 microns thick, that reduces the surface friction of silicone rubber. Hardness of the film tends to increase the durometer values of the rubber substrate.

Parylene is stable to temperatures up to 125°C. However, it does not chemically bond to the rubber. When flexed or stretched, Parylene will crack—a problem that gets worse as film thickness increases . Thus , Parylene functions best as a low-friction coating when it is applied thin, but this reduces its abrasion resistance.

Silicone oil and other lubricants also have been used on silicone rubber, but have serious limitations. While oil is a highly effective lubricant, it is hard to contain and tends to migrate away from the rubber surface and contaminate everything nearby. It is particularly difficult to use in surgeries or clean-room environments because of this migration problem. Additionally, the cost of FDA-approved grades of lubricants is high, and they require costly labor to apply.

Unequal stretch

A fundamental problem with the Parylene/ silicone combination is that the elongation properties of the two materials are mismatched. Many formulations of silicone rubber can elongate 1,200%, whereas Parylene can only elongate only a relatively small amount. What happens is that when a coated silicone part—gasket, tube, or other shape—is stretched, flexed, or compressed, the coating cannot match the elongation of the silicone and cracks (Figure 1). When cracked, particles can dislodge and they can cause moving parts to bind against each other. This nullifies much of the benefit of the coating.

Another limitation of Parylene coatings involves how they are applied, by vapor deposition. Vapor deposition is a line-of-sight process in which the polymer falls onto and attaches itself to whatever portion of the substrate that it “sees.” Interior or shielded portions remain bare. Thus, the film cannot be easily applied to the inside diameter of tubing or the interior of blind cavities such as ports. Finally, Parylene cannot be made to be anti-microbial to kill biopathogens or pigmented to provide color identification.

New technology

Recently, a new type of coating was developed that eliminates the objections and limitations of both the Parylene materials and silicone lubricants. Known as Slick Sil®, it is a translucent coating that is chemically bonded to the silicone rubber surface. It reduces the coefficient of friction of silicone rubber parts, but does not crack as they stretch or elongate (Figure 2), regardless of how thick it is applied—even as much as 1 mil (.001 in./.025mm). Like Parylene, the new coating is typically very thin—13-16 microns—is free of VOCs, and is biocompatible (USP class VI). The resulting coefficient of friction of coated parts is 0.31, less than half that of bare silicone rubber and well within the range to eliminate the classic stick-slip associated with the silicone rubber. Durometer values of the silicone rubber are almost unchanged.

Unlike Parylene, the Slick Sil is chemically bonded to the silicone surface and can be tinted for color coding and also be made anti-microbial. Also, options exist as to where to apply the Slick Sil on a part. This includes inside long tubes or an entire part. This reduced-friction surface assists the motion of an enclosed shaft, without a lubricant (Figure 3). Parylene, because it is applied via vapor deposition, can only coat the area around an opening, not an entire tube or channel.

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