Generally, there are few if any governmental regulations about lubricants for non-implant medical devices. Nonetheless, if the device comes into contact with the patient, it is wise to stick to base oils and additives that are “generally recognized as safe.” Typically, synthetic hydrocarbon oils are preferred. Worse case scenario if ingested, they would act as a very fast-working laxative. Beyond this caveat, the best lubricant for a medical or any carefully engineered device is one that is designed for the application and the operating environment.

When choosing a lubricant, the first decision is, oil or grease. Generally, low starting torque requires oil. While the consistency of a grease may overpower available motive force and impair the operation, oils provide minimal drag, which make them more suitable than greases for small delicate mechanisms used in many precision instruments. For high-speed applications, where oil migration is not an issue, oils
also dissipate heat better than greases. Oils do have one major limitation:
they tend to creep. Lubricant migration can often be prevented with a “barrier film,” a very stable, nonwettable, fluorocarbon polymer with a surface energy well below the surface tension of most lubricating fluids. Supplied in a fast-evaporating solvent, the polymer film acts as a dam around the lubricated area to prevent oil migration — and potential contamination of nearby, sensitive components.

Greases are formulated by combining an oil with a gelling agent, usually a soap or a clay. Even with a grease, it is still the oil that lubricates. The force created by two mating parts — slide on slide, gear on gear, bearing against raceway, for example — squeezes oil out of the gellant into the interface to reduce friction and prevent wear. Greases are generally recommended for heavier loads, especially where vibration or shock loading is likely. They are also more resistant to water wash-out and serve as effective seals against contaminants and moisture. Even with low-power devices, don’t quickly rule out a grease. Greases offer an important stay-in-place capability that can reduce costs associated with manufacturing an air-tight oil reservoir. Light, lowshear greases — formulated with light oils, special gelling techniques, and adherence modifiers — can approach a semi-fluid state under shear. So for all except flea-powered devices, a custom-formulated, extra-light grease may provide a cost-advantage over oils. Like in health care, natural or synthetic is a lubrication decision too. Petroleum, a hydrocarbon, is nature’s most popular lubricating oil.

Synthetic lubricants, which include hydrocarbons, esters, polyglycols, silicones, and perfluoropolyethers (PFPEs), are man-made through controlled chemical processes. Usually, the operating temperature of the application is the arbiter. Petroleum becomes virtually intractable at sub-zero temperatures and begins to degrade at or before 100°C. In either case, it won’t lubricate. By contrast, synthetic hydrocarbons stay lubricious at -40°C synthetic esters, at -60°C and some PFPEs, at -90°C. At the other end of the thermometer, synthetic hydrocarbons function at 130°C, silicone, at 200°C and PFPE’s at 250°C or higher.

Even when temperature doesn’t mandate it, design engineers often choose a synthetic lubricant. Compared to petroleum, synthetic oils are more chemically homogeneous. They offer better thermooxidative stability. Oxidation not only depletes lubricant supply, it often leaves abrasive oxides in its wake — which hastens component failure. At elevated temperatures, even without the presence of oxygen, synthetic lubricants are also less volatile than petroleum-based products of equivalent viscosity.

Synthetic lubricants also offer better film strength than petroleum products. The “film” of lubricant on a sliding or rotating part is what reduces friction and prevents wear. If the film is weak and ruptures under load, wear is accelerated. In general, under similar operating conditions and viscosities, a synthetic lubricant will perform more consistently and for longer periods of time than a petroleum product.