Synthetic Lubricants For Medical Devices

BRAD RICHARDSON
Regional Engineering Manager
Nye Lubricant Inc.
Fairhaven, Mass.

Synthetic lubricants can do much more than reduce friction and wear. They can lengthen operating life, broaden the working temperature range, cut noise, and keep mechanisms from sticking. In medical devices, they can also impart a clinically appropriate feel, seal against dust and the environment, and reduce design and production costs.

By taking a look at how synthetic lubricants helped in a handful of different types of medical design projects, engineers can get an idea of what these lubricants can do for them.

Damping grease: Just what the doctor ordered
Eli Lilly and Co., the drug maker headquartered in Indianapolis, took a great leap forward in insulin-delivery technology last year with its disposable, pocket-size, prefilled "insulin pens." The Humulin and Humalog pens (named for the insulin they inject) accommodate diabetics on unpredictable schedules and those who have trouble with syringes.

The Chicago-based design firm IDEO and lubrication experts from Nye helped Lilly select the damping grease used in the pen. The pens present a textbook study of how damping greases benefit mechanical devices.

Designers and manufacturers of micro-scopes, telescopes, binoculars, and other optical instruments have relied on damping greases for more than 50 years. They provide the "velvet feel," virtually silent, and backlash-free operation of instrument focusing threads.

What distinguishes a damping grease from other greases is its internal shear resistance. Damping greases contain highly viscous base oils — some as thick as honey — so objects must be pushed with some force to move through them. This shear resistance minimizes free-motion problems such as backlash and coasting. Damping grease is also tacky, so it adheres to moving parts. This prevents wear and damps noise because parts move within the grease rather than against each other. The viscosity of the base oil determines shear resistance and the subsequent "feel" or tactile quality of the device. The more viscous the oil, the greater the shear and the more force needed for movement. Stereo tuning knobs, for example, use a lighter grease. Coin-return mechanisms in vending machines, on the other hand, may require an ultraheavy grease. All in all, damping grease is one of the most cost-efficient ways to give mechanisms smooth, controlled, quiet motion and fine tolerances.

The insulin pens use damping grease to quiet the injection cycle. The pen plunger consists of a screw and pawls, which prevent the plunger from backing up. The grease absorbs the noise of the pawl fingers snapping over the saw teeth on the screw. More importantly, the grease makes the entire injection cycle a smooth, uniform motion.

In early tests without grease, the plunger shot forward too quickly when the clutch disengaged. This could have made inexperienced patients mistakenly think the injection cycle was complete.

"Mechanically, two stages make up the injection cycle: clutch disengagement followed by full depression of the plunger, which actually delivers the insulin," explains Andrew Burroughs, IDEO design engineer. "The damping compound eliminates the abrupt action when the clutch disengages and makes those two stages seamless, normalizing the injection force over the full travel of the plunger. It really takes any confusion out of the injection cycle."

Lilly also mandated stringent requirements on the grease's shear resistance. It had to mirror the resistance of the insulin through the needle during injection. The grease also needed a good viscosity index, that is, its viscosity had to remain relatively stable throughout the operating temperature range of 4 to 40°C. Finally, the grease had to be compatible with the polycarbonate and ABS pen materials. Rigorous testing on specially constructed rigs let designers zero in on NyoGel 774VH damping grease. It ensures the same smooth, quiet feel whether the plunger is extended, rotated, or depressed.

Two other grease-control issues arose once the pens went into production. High-speed, automated-dispensing equipment won't remove air bubbles that get into the 20-oz cartridge containing the grease. Trapped air can keep some parts from being lubricated, especially when each part only gets small amounts of grease. Nye designed equipment that removes air bubbles from each cartridge before shipment. This deaeration of the grease also helped minimize defective parts. Second, torque specifications for the pen called for tight tolerances on grease viscosity. If viscosity wanders outside the acceptable range, the plunger feels either too stiff or too loose. Consequently, Nye ensures the viscosity of each lot stays within a ±10% range.

"Essentially, damping compounds are a cost-effective way of getting a lot of performance out of a very simple system," Burroughs says. "They provide elegant solutions to problems that are just about impossible to solve mechanically."

Wear insurance
Rolling and sliding can wear down gearsets without the right lubricant. At a minimum, the lubricant should keep gear teeth from wearing and permit power transfer with little heat and noise. Careful selection of the base oils, gellants, and additives also let gearing lubricants inhibit corrosion, damp noise, and limit or stop any free motion.

Autotrol Corp., Crystal Lake, Ill., for example, weighed such factors for lubricating a fractional-horsepower motor it designed for a Thrombelastograph (TEG) Coagulation Analyzer. The analyzer, developed at Haemo-scope Corp., Skokie, Ill., monitors blood's clotting ability during surgery and other medical procedures. Analyzing blood in real time — rather than sending it to a remote lab — is economical and improves patient care. Studies show TEG analysis can help reduce postoperative bleeding, shorten stays in the ICU, limit the need for blood transfusions, and perhaps head off additional exploratory surgery.

The TEG motor periodically turns a cylindrical cup containing a patient's blood. A pin is suspended in the blood by a torsion wire and monitored for motion. As clotting begins, the rotating cup transmits a torque to the immersed pin. Pin rotation is converted by a transducer to an electrical signal. Its strength is a function of clot strength: strong clots move the pin directly in phase with the cup motion; as the clot breaks down, less motion transfers to the pin. Software converts the output signal into a hemo-stasis profile which measures the rate of clot formation, the strength of a clot, and clot dissolution.

Autotrol engineers spec-ified Nye Rheolube 365F, a lithium soap-gelled grease. It has a high-film strength, a synthetic hydrocarbon base oil, and a useful temperature range of -45 to 125°C. Nye included an additive package to minimize friction and start-up torque.

"The grease gives the motor a long life, the first requirement for any gear lubricant," says Autotrol engineer Al Visin. "It's also compatible with plastic, brass, and steel, all of which are in this particular gearmotor."

Designers should note that greases can be made light enough to accommodate even sub-fractional-horsepower gearmotors. Some of these light greases flow under shear and return to a gel when static. Compared to gearing oils, which are often specified automatically for low-torque applications, the stay-in-place quality of a light, thixotropic grease can minimize the need for seals and associated machining.

Final testing uncovered the need for yet another lubricant. The shoulder of the drive assembly, which rotates the cup holding the blood, had to slide and turn a few degrees on the machined surface of the support frame. Though contact was light, the metal-on-metal sliding was a formula for friction and wear, which could compromise the rotation of the cup and the accuracy of TEG measurements.

Sliding parts, compared with rolling elements, such as ball or roller bearings, present a different challenge to lubricants. Ball bearings usually rotate at speeds high enough to build up a fluid film that separates the balls from the raceway. But the combination of low speeds and loads in sliding parts does not create a fluid film. Further, sliding parts are more likely to move backward, which diminishes fluid films even in ball bearings. These factors give sliding parts a higher potential for friction and wear. The analyzer thus needed a lubricant with good film strength, which is a function of the molecular structure and viscosity of the base oil.

Engineers tried a molybdenum disulfide spray, a lubricious material, without success. The solution turned out to be Rheolube 362HB, a wide-temperature grease designed for use on cams, sliding surfaces, small gear trains, and the mechanical linkages of switch gear. Comprised of a superrefined synthetic hydrocarbon oil, its additives improve adherence and lubricity.

Lubrication engineers prefer to be involved early in the design cycle, but the late addition of a grease to solve friction and wear problems underscores an important fact about lubricants: They're generally "retrofittable." If a performance issue crops up late in the game, lubricants can still solve a multitude of problems in the eleventh hour.

High-speed, high-temp
Years ago, the steel in rolling-element bearings played a critical role in their lifespan. Sophisticated manufacturing processes have taken the quality of steel, alloys, and ceramics out of the equation. The issue now is often lubricant quality: How well and how long can it minimize friction and prevent bearing wear? Few devices in the health-care industry challenge a bearing lubricant more than the high-speed dental handpieces used to drill, shape, and polish teeth.

Dental handpieces contain a turbine with precision bearings. The steel or ceramic balls measure about 1 mm in diameter. They spin at up to 500,000 rpm, or more than 8,000 rps. Properly lubricated, the balls circle the raceway on a thin film of oil, which keeps them from directly touching the raceway. If the oil is not prop-erly filtered, though, the speeding balls are an accident waiting to happen. A ball hitting even a micro-scopic particle, will likely rupture the film, and scar the raceway, another ball, and it-self. Over time, these collisions create unwanted noise, accelerate wear, and shorten operating life.

To head off such troubles, lubrication experts recommend ultrafiltration of the oil, a process that can filter out all particles larger than 50, 25, 10, 5, or even 1 micron, if need be. (For scale, a grain of beach sand is about 600 microns.) In grease, ultrafiltration can keep out particles larger than 34 microns, and limit the density of particles between 10 and 34 microns to below1,000/cc.

Another challenge dental handpiece lubricants face is temperature. Sterilization protocols became more stringent in the early 1980s in response to the HIV virus, and dental clinics began autoclaving drills. Today, hand-pieces are flushed with a cleaner, dried, lubricated, and sterilized in an autoclaved after each patient. It takes a synthetic hydrocarbon oil to withstand the high-temperature steam and pressure of the autoclave procedure economically. There are several oils on the market, including Nye DHL-400 and DHL-600, tested to 150°C, specified by OEMs and sold under private label to dental practices.

Handpiece and bearing manufacturers have recently begun exploring the use of grease instead of oils in turbine bearings. Grease would eliminate the need for relubricating after each use. One manufacturer, in fact, has already introduced a "lube-free" handpiece though technically, it's not lube-free. With no lubricant at all, the bearing would last hardly a week before burnout. Rather, the handpiece uses a grease, so the bearing needs no relubri-cation during its life. While that's beneficial for maintenance, it raises questions about the life of precision bearings lubricated with grease. Even if the grease is formulated with a smooth, very fine grade of ultrafiltered gellant, the gellant is still a solid — which inevitably poses a threat to tiny, fast-moving, bearings. There are now efforts to develop greases specifically for dental handpieces, but the jury is still out on their long-term success. Grease certainly reduces in-office maintenance, but has yet to deliver as many cycles as oil-lubricated bearings. For now, dentists will need to either buy bearings more frequently or relubricate with oil after each use.

There are medical devices that use synthetic hydrocarbon oils to lubricate and clean bearings. For example, the OsteoPower Modular Handpiece System by OsteoMed Corp., Addison, Tex., relies on synthetic oil. The system is designed for dissecting and drilling small bones in the head, face, neck, and extremities. It introduced several innovations to small-bone surgical instruments when it debuted in 1996. Its housing is a polymer housing instead of aluminum or steel which insulates the surgeon's hand from heat generated by the electric motor, so doctors can work with it for extended periods. It is also the only motorized instrument of its kind to incorporate pressure-activated controls rather than mechanical levers on the handpiece. Finally, it's completely modular: one motor powers 11 attachments, including four kinds of saws, three contra-angle drills, straight drills, as well as a wire and pin driver.

DHL-600 lubricant is used on all the attachments. It is packaged in an aerosol container with a nozzle designed by Nye and is sprayed into the bearing prior to autoclaving.

"Our work-horse tool is the one-to-one straight drill, which is the only piece doctors should lubricate every time it's used," says Drew Hautt, OsteoPower's global product manager. "It's critical to clear out bone material and debris which can cause the bearing to fail. The lubricant's medical-grade filtering made it an obvious choice."