Pneumatics: For engineers, the very term conjures up images of hissing
valves and actuators pushing parts through automated factories. For decades,
however, pneumatics have quietly played another role-that of medical workhorse.
Look around any hospital and you'll see why. Most patients' rooms are equipped with compressed air hookups. Every day, doctors, nurses, and technicians attach life-giving equipment to those hookups. Pneumatic components in those environments help deliver life-giving oxygen and medicine.
Surgeons also use pneumatics. Most operating rooms have wall outlets for compressed air, oxygen, nitrous oxide, and, sometimes, nitrogen. During surgery, pneumatic ventilators aid heavily anesthetized patients to breathe. And pneumatic tools help surgeons perform exacting operations.
Twelve years ago, pneumatics also powered one of the most heralded devices in medical history-Robert Jarvik's famed artificial heart. The heart, attached to a 375-lb electro-pneumatic drive, gave medical pioneer Barney Clark 112 days of added life.
For years, patients have used pneumatic systems at home. Oxygen concentrators provide relief for individuals with chronic lung ailments. Pneumatic cuffs help push blood through limbs with weak circulation. Orthopedic devices, such as pneumatic beds and casts, help injured patients get well quicker.
Even in the factory, pneumatics play a key medical role in the production of such commodities as syringes, rubber gloves, and x-ray film. Three examples illustrate the role of pneumatics in the medical industry. They take place in the hospital, the home, and finally, in an extraordinary application in a process plant.
High-flow ventilator caters to large-animal surgery
Surgery requirements of large animals are dramatically different from those of humans. A 1,500-lb horse in pulmonary failure, for example, could require four times as much oxygen as a person in the same condition.
Yet ventilators-the machines that breathe for heavily anesthetized patients-typically are designed for human needs. "In the past, veterinarians would not do surgery on acutely ill animals in respiratory distress," notes Bob Pearson, president and chief design engineer for Mallard Medical, an Irvine, CA-based manufacturer of ventilators. "They didn't have a machine to ventilate them, so they would euthanize them."
That solution didn't sit well with owners of prized farm animals, breeding stock, show horses, and race horses. They wanted to save their valuable animals.
Pearson worked with veterinarians at Oregon State University to provide a beter solution: a ventilator for large animals. Having tested ventilators on animals ranging from rattlesnakes to Limousine Bulls, Pearson knew the needs of the application. So he and his daughter, Rachel, designed a machine that offered higher oxygen flow rates, greater oxygen absorption in the blood, more control over respiration, and greater overall safety.
The end product, known as the Rachel Model 2800 Large Animal Anesthesia Ventilator, reportedly operates at the highest peak gas flow rate of any ventilator on the market. While human ventilators typically operate at peak flows of 100 liters per minute, Mallard's large-animal version can exceed 600 liters per minute. To achieve such flows, Pearson redesigned conventional medical venturis and used two, rather than one, in his ventilator. The tandem venturis enabled the machine to achieve flow rates 50% higher than any other large-animal ventilator on the market.
Use of the tandem venturis, however, meant that Mallard's machine had to operate at higher pressures than those of conventional ventilators. Whereas most ventilators operate at 50 psi, the Mallard unit operates at 100 psi. To facilitate that, Pearson employed a wide range of Minimatics pneumatic components from Clippard Instrument Laboratory, Inc., Cincinnati, all of which could withstand pressures to 250 psi. Among the parts: three-way valves, electronic valves, miniature pilot actuators, and a host of other valves, hoses, and fittings.
In addition to offering higher flow rates, the new machine also provides greater control over the delivery of anesthetics. With it, Pearson says, clinicians can select a much more exact anesthetic volume per breath to be delivered to the patient's lungs. This allows for more precise control over the depth of anesthesia. He accomplished that by employing an inverted bellows-a component normally used only on human ventilators.Vacuum relief. In the past, engineers didn't use inverted bellows on large animals ventilators because the larger size would provide too much "end expiratory" pressure on the patient's lung. Pearson alleviated that problem by applying a vacuum pump from PIAB Vacuum Products, Hingham, MA. Pulling a vacuum between the bellows and an external chamber helped eliminate the potential problem of high-end expiratory pressure against the lung.
Pearson, who has spent much of his career designing ventilators for newborn infants, adapted human ventilator technology in his new system. The ventilator features a microprocessor-based electronic control system, which incorporates the advantages of human intensive-care ventilators. The unit provides LED displays of selected respiratory variables, while reducing the of work of breathing for the patient, and also providing complete control of expiratory pressure.
Early versions of the 2800 Model were supplied to veterinarians at the 1992 Olympics in Barcelona, Spain. The surgeons used the system during arthroscopic knee surgery on two horses that performed days later in equestrian events. "Without the anesthesia ventilator, they couldn't have done the surgery. A horse simply won't tolerate insertion of a scope in its knee," Pearson explains.
Fieldbus manifolds help convert bovine blood for human use
Every few months, blood banks around the country send out a desperate flurry of public service messages, hoping to attract donors and rebuild their dangerously low blood supplies. Despite their best efforts, blood shortages continue. Public health officials worry that AIDS-related concerns have diminished the good intentions of donors.
A Massachusetts-based firm may have a solution for the problem. Biopure Corp., Cambridge, MA, has under development a patented human blood substitute made from bovine hemoglobin obtained as a byproduct from the meat-processing industry.
Biopure's plant uses large stainless steel tanks with 3- and 4-inch-diameter pipelines to transport the company's life-giving liquids. Fluids move from tank to tank for ultra-fine purification, separation, heating, cooling, stirring, mixing, and various other processes. The company employs pneumatic systems to turn the valves on or off, and to open or close liquid flow to a vessel.
A Bailey Infi-90 DCS distributed control system from Bailey Controls Co. provided electronic control for plant-wide processes. It delivers supervisory control and data acquisition over a 4-20 mA current loop. A Bailey proprietary fieldbus network monitors and controls signals from process instruments around the plant.
The process of purifying and finishing the blood consists of about nine steps performed in an clean environment. Some processes take a couple of hours; others last a week. Process integration. To integrate electronics and pneumatics at a multitude of input/ouput points around the factory, Biopure employed special fieldbus manifolds from Festo Corp., Hauppauge, NY. The manifolds help cut costs and labor time during set-up. "We decided to merge the pneumatics into the DCS by using a Festo FPC 405 PLC as a gateway to the DCS," says Tony Fenn, research and development engineer at Biopure. "We wanted to maintain the security and redundancy afforded by the Bailey DCS system, while also achieving appreciable savings in wiring labor time and cost, which the digital fieldbus offered."
Festo and Bailey developed the special DCS/fieldbus interface. Festo supplied the fieldbus valve manifold and assembled special stainless steel cabinets for the project.
"We have over 20 fieldbus control nodes housed in stainless steel cabinets," Fenn says. Each cabinet contains 26 or 52 solenoid-actuated pneumatic valves mounted on a modular valve manifold with integral fieldbus node, which pilots the larger process valves.
The primary selection criteria for the valves centered on flow capacity, in addition to the labor- and cost-saving design of the manifolds. The modular manifolds incorporate solid-state electronics, eliminating all of the discrete wiring associated with conventional manifolds.
"By using the fieldbus valve manifolds instead of the older, discrete wiring method, Biopure saved the time, labor, and the cost of connecting over 1,000 discrete I/O points," Fenn explains. The compact, modular valve manifold design also makes it easier to maintain, change, and upgrade the system as needed.
The new, expanded plant will have annual peak processing capabilities of more than 4,000 kg of hemoglobin. Says Carl Rausch, co-founder, chairman, and CEO of Biopure: "The potential product for human use, when approved, will begin to lessen the estimated 100 million-unit annual worldwide shortfall of human blood."
Poppet valve withstands concentrator's dry air
For sufferers of chronic lung diseases, dependable oxygen delivery is critical. Victims of emphysema, lung cancer, and AIDS, among others, typically breathe concentrated oxygen at home. For those people, continuous operation of concentrators is a must.
Manufacturers of such pneumatic medical systems face a difficult design constraint: The air that comes in contact with the valving must be dry or unlubricated. As a result, pneumatic components in those systems don't receive the same level of lubrication as industrial pneumatics.
Stickless valves. When engineers from Puritan-Bennett Corp. set out to design their new Companion 590 concentrator, one goal was to address the challenges of unlubricated air. Their focus: finding pneumatic valves that wouldn't stick when subjected to unlubricated air.
The solution: a diaphragm poppet valve made by Humphrey Products Co., Kalamazoo, MI. Humphrey's Mini-Mizer 250A, a double diaphragm poppet design, uses a rubber-to-metal seating surface that requires no lubrication. The poppet valves offer reliable operation for 40,000 hours or 20 million cycles, according to Humphrey engineers.
A Humphrey valve using the same concept proved to be a key element of the famed Utahdrive, which powered the Jarvik heart. "One of the reasons we selected that valve was because it was one of the original designs used in the air module for the Jarvik heart," notes Steven Clark, product support manager for Puritan-Bennett. "Here, it has to operate in the same kinds of very dry conditions."
This design is an alternative to spool and sleeve valves, which don't hold up as well in unlubricated environments. "Spool-type valves use a sliding mechanism inside which operates more effectively when lubricated," notes Mike Hammond, a sales manager for Humphrey. "But poppet valves use no sliding mechanism, so there's no chance of them sticking."
During operation, the double diaphragm poppet valve shifts open and closed, charging and purging the concentrator's molecular sieve tanks. Like all oxygen concentrators, the Companion 590 operates by drawing in room air, compressing it, and running it through molecular sieve beds. The sieve beds adsorb nitrogen from the air. As a result, users of the concentrators receive more than 90% pure oxygen through an attached mask.
To ensure such purity, Puritan-Bennett engineers use the poppet valves and one-third horsepower, oil-less air compressors from Gast Manufacturing Corp., Benton Harbor, MI, and Thomas Industries, Sheboygan, WI.
One key to continuous operation, however, is the poppet valve. Without it, the valves could succumb to lack of lubrication, humidity, or cigarette smoke. And, if a valve sticks, seriously ill patients don't receive the oxygen they need. "Our operating environments are unlike those in industrial pneumatics," Clark says. "Our air is very dry. And it's crucial for us to use a valve that we know won't stick."