An MD&DI September 1998 Column
Smaller, lighter, cheaper motors keep pace with industry demand.
In making motors for medical devices, striking the proper balance between size, cost, and performance can be brutal. A typical medical application might call for a motor small enough to drive an ultrasound probe inserted down a patient's throat, yet powerful enough to drive that ultrasound transducer from point to point with extreme precision. In infusion systems, motors are asked to meter out exact, almost infinitesimal amounts of drugs; yet they cannot be overly complex or else cost and size will be too great. Conductive materials can be used to optimize power consumption for portable devices, but the price paid for such gains must be negligible.
Today's miniature motors, such as these developed by Micro Mo (Clearwater, FL), deliver more power and performance in smaller packages than ever before.
A PERSONAL APPROACH
The challenges of coping with such a diverse—and potentially lucrative—marketplace are met in a surprisingly simple fashion: one customer at a time. After developing several core motor technologies, manufacturers tailor them to fit market segments, then tweak the performance of individual motors to meet specific application needs. This intensive customer focus sets the tone for the way R&D is practiced in the industry. "The motor world is more evolutionary than revolutionary," explains Steve O'Neil, vice president of advanced research and planning at Micro Mo Electronics (Clearwater, FL).
This evolution takes place daily in dozens of sites around the world. Last year, Micro Mo alone conducted more than 1600 customer support programs, most of which resulted in new or redesigned products being brought to market. RMB Roulemonts Miniatures SA (Bienne, Switzerland), another manufacturer of miniature motors, sets up teams of engineers who work with its customers' applications specialists. There's a lot at stake. RMB estimates that the annual global market for miniature motors could reach $100 million within five years.
"A company like ours, which is selling its product into the OEM market, has to have very good applications engineers who not only need to understand our own product but that of our customers," says Albert Birkicht, business unit manager at RMB. "They must have a very deep understanding of the application and of the necessary requirements to make that application successful. If we fail to understand the application properly, then our product will fail too, because we will not adapt it the right way."
Products with the widest appeal typically are adopted as part of standard product lines. Over time, motor companies may assemble families of products with hundreds of members. These motors then might be modified slightly to fit the needs of a new customer. Modifications may take the form of special voltage coils or gear ratios, custom connectors, or value-added services to make final product assembly easier and cheaper. The modified motor may also come with customized mounting holes and special lubricants or components designed to withstand autoclave, vacuum, or high- and low-temperature environments. "At last count, we were doing about 600 different 'special executions' just for the OEM medical community," O'Neil says.
Despite the many differences, motors in product families share a single trait: they each turn something. In medical instruments, that something may be the capstan in a heart recorder; a shaft in an arthroscopic tool; or the vane, cam, or paddle in a blood pump. In fact, pumping applications, according to RMB's Birkicht, account for most of the medical demand for miniature motors, where they are used, he says, "as part of analyzers to pump samples or as part of pumps for delivering medication."
Accomplishing these tasks can be done with surprisingly little force. Miniature electric motors are described as having fractional horsepower, often measured in quarters or tenths. But what they lack in brute force, they make up in versatility. These tiny units appear in a dizzying number of medical products, including high-speed surgical tools such as bone saws and drills, implantable infusion pumps, ophthalmic tools, syringe drives, positioning systems for endoscopic cameras, centrifuges, surgical robotics, prosthetics, ventilators, blood analyzers, and blood pumps.
THE RIGHT SIZE
Size is a premium and, in that regard, Micro Mo is among those leading the pack. Its latest creation spans just 1.9 mm in diameter—roughly the thickness of two sheets of typing paper. "Making such a product in a precise manner was no mean feat," O'Neil says. "No one else so far has been able to do it."
Such small motors open the door to new types of devices, ones that might fit easily into body cavities rather than having to be surgically inserted, he notes. Their availability ultimately could mean the elimination of expensive or dangerous procedures used to gain diagnostic information or to administer therapy. Tiny motors might even be inserted into blood vessels where they could chew through any plaque that was blocking arteries or aim ultrasound beams to analyze vessel walls.
The lightweight 3-mm Smoovy motor from RMB (Bienne, Switzerland) can be built into a microdosing infusion pump to deliver drugs in quantities of just a few picoliters.
But too much of a good thing can be bad. The pursuit of the very small, if unchecked, can put a manufacturer too far in front of its customers, whose design engineers have not kept up. "We have seen frequently that when people try to jump from whatever they used before to our 3-mm motor, they sometimes find that the jump is too big and they switch back to our 5-mm motor," Birkicht says.
Apart from size, there are a few other guiding lights for R&D. One is to optimize power consumption. Another is to make the motors lighter. A third is to improve dynamic performance. RMB's Smoovy motor succeeds on all three scores. Built into an infusion pump, the lightweight 3-mm-diam motor can microdose a drug in picoliter quantities.
FROM BRUSHED TO BRUSHLESS
Electric motors have been around for 150 years, but the biggest improvements in size and precision have followed recent advances in microelectronics. "One big advance has been electrical commutation and the ability to get rid of the brushes," says Pajhand Rianitalab, project administrator for the motion control division of Axsys Technologies (San Diego).
Commutation, defined simply as the transfer of electrical power, can be accomplished conventionally through the use of brushes or electronically through "brushless" technology. Brushless motors have the advantage of being simpler, achieving power commutation electronically rather than mechanically. Brushless motors typically last longer because they have no brushes to wear out; they also don't need lubrication. What's more, brushes can slow a motor down, restricting its speed to no more than about 30,000 rpm, says Kirk Barker, director of marketing and sales for Neodyne (Tampa, FL), a designer and manufacturer of miniature motors. In contrast, brushless motors can reach up to 150,000 rpm or beyond, he says.
Brushless motors can be made smaller than brushed types and typically last a lot longer (Neodyne; Tampa, FL).
Most importantly, brushless motors can be built smaller than brushed types. Already, manufacturers have built brushless motors smaller than 2 mm in diameter; brushed motors generally cannot get smaller than 3 mm because the brushes take up a certain amount of space.
But brushed motors have an advantage in price. Comparing equivalent volumes of motors putting out 1/10 hp, for example, the cost for a brushless design is about $50; a brushed design costs only $30 to $40. Still, despite the emphasis on cost so prevalent in the medical device community nowadays, brushed devices are losing ground to their brushless cousins. The advantages of this more advanced technology eclipse the cost problem—and its appeal may grow even faster in the near future, thanks, in part, to further development in control electronics.
"A major advance has been in the use of high-accuracy position feedback using resolvers and other sensors," says Rianitalab of Axsys. "These things enable you to get better accuracy, which gives you more options in what you can do with the motors."
Encoders precisely control the function of brushless motors, allowing a pump, for example, to deliver an exact volume of drug. One form of encoding—optical encoding—typically combines a light-emitting diode (LED) secured to a disk mounted on the rotor and a stationary phototransistor that counts each pass of the LED. "One reason encoders are used is because they give binary output, just like a computer," Barker notes. "The light goes on; it goes off."
But electronics are not always needed. Brushless motors driven by electrical pulses can be designed to "step" through a series of actions such as pushing a plunger to squeeze out a precise quantity of drugs.
THE NEXT BIG THING
Having achieved sufficient miniaturization for most medical needs, some motor companies have begun to focus their R&D efforts on increasing operational efficiency. One such pursuit has yielded the slotless motor. As Neodyne's Barker explains, conventional brushless motors have slots into which copper wire is fitted. Electromagnetic fields are generated by this wire, causing the rotor to turn. Removing these slots reduces the size and weight of the motor while optimizing the effect of the electromagnetic field on the rotor. The result, Barker says, is a more efficient and cooler running motor. The slotless design also increases manufacturing efficiency by cutting die design time, raw material costs, and delivery cycles.
Other R&D efforts are targeting advances in materials sciences. Barker notes that the use of rare-earth materials, such as samarium cobalt to make the magnet, have already improved device performance. Still ahead lies the great—and largely unfulfilled—promise of superconductors. Research done at several institutions, including Lawrence Berkeley Laboratory (Berkeley, CA), has demonstrated the potential of using superconducting wire to carry electrical current more efficiently. This wire might be composed of "high-temperature" superconductors, which promise to convey hundreds of times the electrical current of conventional copper wire. Superconducting coils would be capable of generating much higher magnetic fields—and consequently higher speed and greater torque—than conventional electromagnets. The motors, being more energy efficient, might also be lighter and smaller.
But there's a problem. These so-called high-temperature superconductors still must be cooled to temperatures a scarce few degrees above absolute zero. As such, the advantages in size and weight are more than offset by the need for a cooling jacket.
With or without the glamour of superconducting materials, the miniature-motor industry may well be on the cusp of an unprecedented boom. Diagnostic and therapeutic devices are now being crafted to minimize trauma, both as a means for improving clinical outcome and reducing patient discomfort. Robotic devices under a surgeon's control are being made to snip tissues precisely and accurately. Cardiologists envision motor-driven devices that might be swallowed by patients to provide a backside look at the heart with ultrasound. These and similar systems will extend the market for miniaturized motors. Of course, just how far this miniaturization can go may be dictated by the practical laws of physics. "If you reduce volume, you reduce power," says RMB's Birkicht. "Also, the difficulties in application and assembly get worse and worse."
If ultrasmall motors ever do fill an important clinical need, they will likely do so as a large group. Several hundred or even thousands of tiny pumps might have to be integrated into a medical device to achieve the flow rate necessary for an infusion pump, for example. The advantage would be precision unlike anything possible today.
When industry wags talk about such possibilities, the subject of discussion is almost always MEMS (microelectromechanical systems). Gears made with MEMS technology are actually carved out of silicon in much the same way computer chips are fashioned. RMB already uses some MEMS gears in its 3-mm Smoovy. But Birkicht believes the time when whole motors are made from this technology is still far in the future. "There is right now a very limited infrastructure to support mass producing these pieces," he says.
As a result, the current size of motors could be the standard for the industry well into the future. As Micro Mo's O'Neil sums up, "It is very, very difficult to pack power and performance, not to mention quality, into a small package." Having recognized that size is relative—that today's level of miniaturization is sufficient for most medical device needs—motor manufacturers are starting to shift gears, redirecting their R&D efforts from the pursuit of ever smaller products to the development of products that run more efficiently and are less costly to make. Nonetheless, judging from the advances in the industry overall, motor suppliers will find ways to meet the demand for ever smaller and lighter designs for use in medical product applications.