Originally Published MDDI May 2004

Cover Story
 


Tiny gas- and liquid-moving components fit the bill in smaller medical devices.

William Leventon

Many medical devices are going home to do their test, treatment, and monitoring work. Designed to be worn or carried by patients outside the hospital or doctor's office, home-healthcare devices come in compact, travel-friendly packages.

To produce these and other portable packages, medical device firms need diminutive parts like those made by manufacturers that specialize in miniature pumps and valves. These manufacturers are turning out a variety of gas- and liquid-handling pumps and valves that fit in the palm of your hand. Though they have some size-related limitations, these tiny units are designed to perform the same functions as their larger counterparts.

Small as they are, the current lineup of miniature pumps and valves won't be small enough for some of tomorrow's handheld and wearable medical devices. So component manufacturers are evaluating new technologies that may be the keys to producing so-called subminiature gas- and liquid-moving mechanisms.

Small Solenoid

Today, medical device makers in the market for miniature valves have several options. For compact liquid-handling equipment, The Lee Co. (Westbrook, CT) offers the VHS, a solenoid valve measuring 0.22 in. in diameter by about 1 in. long. The tiny VHS may be the smallest production solenoid valve on the market, according to Ralph Buck, Lee's electrofluidic systems product manager.

Besides small size, the VHS features a plunger and coil that are balanced to get the best response time from a small package. As a result, Buck says, the valve can run at speeds of about 1000 Hz, enabling small quantities to be dispensed in a precise and repeatable manner.

The VHS can be found in drug-delivery systems and high-throughput screening equipment. With its small clearances, the valve wouldn't be the right choice for devices moving fluids with high particulate content. Moreover, Buck notes, the valve can be twice as expensive as its larger counterparts, because of the difficulty of making small parts. So if an application doesn't require a very small, high-speed solenoid, a less-costly valve may be a better choice.

Mouselike Measurements

Many space-constrained gas- and air-moving systems are home to a Mouse valve, also known as the EV electronic valve, from Clippard Instrument Laboratory Inc. (Cincinnati). Recently, Clippard introduced the EVP, a proportional control valve with mouselike dimensions, measuring 1.5 in. tall by 0.875 in. in diameter.

But the two valves have more in common than being small enough for a mouse hole. In both, the conventional sliding-spool mechanism for opening and closing is replaced by an element called a flexing spider. The flexing spider is a circular piece of metal surrounded by metal legs. When the valve is closed, the spider's legs press the body against a seat. But when magnetic force pulls on the spider, it flexes like a pie tin, snapping away from the seat to open the valve.

The flexing spider has a couple of advantages over a spool inside a valve, according to Paul Gant, Clippard's international sales manager. For example, the spider is lighter than a high-mass spool, so it requires less power to function. In addition, spools wear as they slide back and forth, while the flexing spider is a spring “that will basically last forever,” Gant maintains.

The EV flexing spider is also reliable, opening and closing valves with a simple movement of 0.007 in. “There's not much that can go wrong,” Gant says. “That's one of the things people in the medical industry like a lot.”

In the EVP, however, the spider doesn't just snap into fully open and closed positions. Its movement is controlled so the valve can be opened any partial amount. So if a respirator were equipped with an EVP, the valve opening could be adjusted based on the patient's size or ability to breathe.

With its small size and low power requirements, the EVP is suitable for a variety of battery-operated portable medical devices. Then again, so are a number of similar proportional valves on the market. But according to Gant, the high-mass parts in those other valves make their movements more expensive to control. As a result, competitive offerings cost at least twice as much as the EVP, he maintains.

One-Piece Body

Even smaller than the EVP is the tiny X-Valve made by Parker Hannifin's Pneutronics Div. (Hollis, NH). Measuring 7.9 mm wide, 9 mm high, and 24 mm long, the X-Valve features a so-called unitized design that incorporates all functional features in a molded PBT body. The single-piece body is the key to a valve design consisting of just five parts, compared to the 12 to 20 parts in a typical solenoid, says Mark Garland, Pneutronics' market development manager.

According to Garland, the X-Valve's low parts count boosts reliability by reducing leak points and stack-up tolerances. It also cuts manufacturing costs, resulting in a less expensive final product, notes Robert Howard, the division's sales and marketing manager.

With its supersmall dimensions, the X-Valve can meet the size and weight requirements of compact medical devices. But this unit and tiny valves like it have a harder time meeting the pressure and flow needs of some machines. So Parker Hannifin supplemented its original 6- and 30-psi X-Valve product lineup with 50- and 100-psi models for higher-pressure applications. To boost the valve's pressure capacity, Parker Hannifin switched to an internal spring that can exert more force to maintain the seal. The company also reduced the size of the valve's orifice, which improves pressure-handling capability by cutting back on flow.

To accommodate higher flows, the firm introduced a larger variation of the X-Valve. Called the Ten-X, it measures 9.9 mm wide, 12 mm high, and 32 mm long. The Ten-X was developed specifically to meet the flow and pressure requirements of noninvasive blood pressure monitors.

Another variation of the X-Valve is designed for compatibility with a wide range of media used in medical instrumentation. Inside the new Liquid X, the diaphragm serves as a barrier that separates the flowing medium from the springs, actuator, and solenoid.

Though the two valves have several parts in common, the Liquid X looks nothing like the X-Valve, measuring 12 mm wide, 29 mm long, and 40 mm high. According to Garland, the 12-mm width of the Liquid X makes it a bit smaller than similar isolation valves currently on the market.

For its X-Valve products, Parker Hannifin also offers a pneumatic manifold called X-System, which doesn't require screws or seals for valve mounting. Instead, Howard explains, valves are press-fit into the molded-plastic manifold, resulting in a simpler and less expensive assembly process.

Made of thermoplastic polyurethane elastomer, the X-System costs less than conventional aluminum manifolds, according to Howard. The system's molded-plastic contruction also eliminates the need for complex and time-consuming manifold machining, an advantage in rapid prototyping. 

Syringe Pump Substitute

Like little valves, miniature pumps come in a variety of shapes and sizes. Measuring 1.5 in. on a side and 5 in. long, Lee's LPV variable-volume pumps were designed to replace syringe pumps. Featuring direct drive off a stepper motor, LPV units are smaller than most syringe pumps that include a rack and pinion, according to Buck.

What's more, he adds, LPV pumps are sealed, maintenance-free units. Since the pumps are small and don't require periodic maintenance, they can be placed in hard-to-access spots deep inside an instrument. They can also be positioned close to the fluid reservoir in order to reduce transport volume.

Unlike syringe pumps, LPV units require users to stick with a fixed stroke volume, Buck notes. Lee is confronting that limitation by offering varied sizes. Full-stroke volumes of the original LPV pump designs range from 50 to 750 µl. Recently, Lee introduced larger sizes with full-stroke volumes up to 3000 µl. 

Even smaller than the LPV is the NF5 from KNF Neuberger Inc. (Trenton, NJ). About half the size of the company's diminutive NF10 unit, the NF5 measures about 0.75 in. wide, 1 in. long, and 1 in. high. These dimensions may make it the smallest liquid-handling diaphragm pump on the market, says Eric Pepe, KNF's sales and marketing manager.

According to Pepe, the NF5 includes several key features aimed at reducing pump size while maintaining adequate fluid-handling capability. These include a patent-pending valve system and new diaphragm technology that produces more-even stress distribution and reliable fluid displacement.

Like all pumps of its kind, the NF5 requires a motor to move the diaphragm. Thus, pump life has been limited by the wear of the brushes in conventional brush-type motors. To increase the durability and life span of its pumps, KNF now matches them with brushless dc motors and higher-quality brush-style motors such as ironless core units. According to Pepe, this has helped KNF meet the demands of its medical customers, who want devices that last 10,000 or even 20,000 hours before they need maintenance.

Like larger diaphragm pumps, the NF5 can run dry and handle either liquid or air. The pump is also available in different plastic and elastomeric materials to meet different fluid compatibility needs and system requirements.

The NF5 is suitable for fluid-handling diagnostic devices such as blood and protein analyzers. The unit's small size lets designers put it close to the spot where pumping is required. Thus, Pepe says, users needn't run long tubes down to the system's base, where a larger pump might have to be located.

Small Alternatives

Another small, diaphragm-style pumping option was developed for glucose self-testing devices. Made by Hargraves Technology Corp. (Mooresville, NC), the CTS microdiaphragm pump measures 2 in. long by less than an inch wide. The CTS is designed to pump gases such as atmospheric air, as well as some liquids. But the pump's small size limits its flow and pressure capabilities, notes Bill Nissim, director of sales and marketing at Hargraves.

Since the introduction of the CTS, Nissim adds, Hargraves has made some life-lengthening design changes to the tiny pump. For example, the pump motor now includes dual bearings that give it greater shaft stability. The company also developed a proprietary rubber diaphragm material that can withstand longer cycling than the old material.

In addition to a diaphragm pump that fits in a 1-in. cube, Rietschle Thomas (Sheboygan, WI) offers a coffee-can-shaped rotary-vane pump with a 16-mm diameter, as well as a 6-cu-in. design based on the company's proprietary WOB-L piston. Like KNF, Rietschle Thomas is matching its pumps with brushless dc motors, which can extend pump life well beyond 10,000 hours. Brushless dc motors also allow more-precise control of pump input and output than old-style brush motors, says Tim Stellmacher, the company's international sales manager. 

Though it wouldn't quite fit in a 1-in. cube, the T2-05 diaphragm pump developed by Parker Hannifin's T Squared Pumps unit occupies a small enough volume for most portable medical devices. The firm plans to introduce the pump, measuring about 0.94 in. high, 1.25 in. long, and 0.53 in. wide, later this year.

The T2-05 features a dynamic preloaded valve design, notes Thomas Theilmeier, business unit manager for T Squared Pumps. In this design, the valves have some preload that forces them into a closed position without the reverse airflow that normally closes such valves. The valve design helps the T2-05 get more flow out of a small package, Theilmeier says. Under normal circumstances, he expects the pump to operate at a maximum free-flow rate of about 500 cm3/min. Even higher flow rates are possible, he adds, but only at the cost of product life expectancy and efficiency. Other pumps offering a comparable flow rate are significantly larger, he claims.

Besides high flow rate, the T2-05 offers a low parts count, which helps keep its assembly costs low, Theilmeier adds. As a result, he expects the pump to be significantly less expensive than competing units.

Meeting Demands

Despite the range of miniature pump options on the market, some medical device makers aren't satisfied. Stellmacher hears from medical customers who want still-smaller pump packages—but without sacrificing performance. Pump manufacturers can accommodate these customers by optimizing pump geometries, he says. For instance, the manufacturer can tighten critical clearances to improve electrical efficiency. Or the bore can be enlarged to increase piston diameter and boost flow capability.

Medical equipment companies also want more-durable and -reliable pumps, Pepe reports. Reliability suffers when interior valves become clogged with debris. So pump makers are trying to come up with valve designs that will prevent debris from accumulating in critical areas.

At Lee, Buck never stops hearing from medical customers asking for finer pump resolution. So the company continues to introduce miniature pumps that can dispense smaller amounts per stepper motor increment.

What's Ahead

As for miniature valves, Lee's next-generation products will probably be unveiled by the end of the year. These new valves will be 25 to 50% smaller than the company's current offerings, Buck says. In addition, the new products will feature a smaller plunger than the current one. Since the smaller plunger will be easier to move, he notes, the new valves should be faster and less power hungry than Lee's existing products.

At Clippard, engineers have produced prototype EV valves that are 30 to 40% smaller than the current Mouse products. Despite the tolerance and assembly challenges of miniaturization, Gant believes the smaller products will be 10 to 20% less expensive than today's EV valve. This is possible due to new machines and methods for manufacturing small parts, Gant says.

By the end of the decade, Howard believes, new technology may allow the production of subminiature valves. Small enough to be mounted on PC boards, these valves won't include a coil or spring. Instead, they'll feature special materials that are moved by current flow.

A similar technology may also be the key to producing subminiature pumps. According to Nissim, Hargraves is looking at a motorless pump concept featuring a diaphragm that changes shape in response to changes in the amount of voltage supplied to it. With no motor, such a pump could be small enough to mount on a PC board, he says.

At present, there may not be much demand for pumps that small. But Hargraves plans to be ready if and when a market develops for such products. Nissim says, “We're playing to where the hockey puck will be, versus playing to where it is now.”

Conclusion

Many portable medical devices rely on miniature pumps and valves. Measured in inches and even millimeters, these tiny parts have helped designers shrink a variety of gas- and liquid-handling systems into compact units that can be carried or worn. Of course, the performance of such small components can't measure up to that of larger pumps and valves. But miniature-component manufacturers have found ways to boost crucial performance values such as flow rate and pressure capacity. In the coming years, innovative flow-handling technologies may make it possible to produce PC board–mountable pumps and valves for the portable medical equipment of the future. 

Copyright ©2004 Medical Device & Diagnostic Industry