Pneumatics in Small Medical Devices
January 6, 2011
Withthe design of medical devices moving toward smaller sizes, a hospital-to-hometreatment model and reduced power consumption, electro-pneumatic device makersare seizing the opportunity by providing more potent, miniaturized controlsolutions. By using proportional control to precisely profile the delivery ofair, for example, pneumatic devices are solidifying their position as a lowcost technology ideal for a wide variety of handheld and portable medicaldevices.
"Thereis a lack of awareness of how pervasive the pneumatic solutions are in medicalapplications," says Ed Howe, president of Enfield Technologies. "Most peopledon't realize how much pneumatics is used in surgical and life supportequipment. In the artificial heart made by SynCardia, for example, basicallythe whole drive system is pneumatic and some of the components are similar towhat is being used in factory automation."
Riseof Proportional Control
Oneclear trend is the move to proportional control and electro-pneumatics for moreprecise control solutions. Howe says that with devices such as ventilators orrespirators, the operation of units in the past has been largely on and off,pushing a breath in and taking a breath out. But now medical devicemanufacturers are using pneumatics to profile the breath provided to thepatient to be more natural and assist in the recovery process.
"Weare working on a ventilator that will work with infants through adults, so thatwhen it's used in an ambulance, air care helicopter or within a hospital, thereisn't a need for two expensive devices," says Howe. "The unit uses aproportional servo valve with embedded electronics and software, and it has avery specialized shape to the orifice and poppet - all together these carefullycontrol the air flow."
Anotherapplication using proportional pneumatic controls is the process of growingcells either for biomedical research or transplants such as bone marrow cells.Grown in a Petri dish, the yields are verylow. But researchers discovered that putting the cells on a flexiblemembrane and pulsing the membrane, stresses and pulls the cell. It's notcertain how growth is encouraged but the obvious theory is that cells recognizethe motion and, surrounded by other cells, sense it is in a living host.
"Usinga very smooth and fast valve, we are able to replicate the heartbeat of thehost animal rather than just turning the valve on and off," says Howe. "We areeven able to perfectly replicate the heartbeat of a hummingbird. This is anexample of pneumatics bringing real benefits to the medical field by doingthings that couldn't be done before. In this case, we are substantiallyincreasing yields."
Growthof Pneumatics
Theunderlying theme to this transition is the use of more proportional pneumaticsand better use of control theory. Mechanical elements in the valves arebecoming more sophisticated, along with better design tools and mathematicalmodeling. Systems are using more sensors, electronics and advanced controls tomanage pneumatic systems and compressed gases, which has always been the realchallenge.
"Pneumaticsystems are able to profile the air and deliver it in a more natural way," saysHowe. "Along with the air pressure profiling, systems consume less air by onlyproviding what is needed and lower the wear on components, but the bigdevelopments are linked to therapeutic contribution."
Comparedto the past where respirators used large glass tubes with bellows and wereprimarily electric systems, respirators now use compressed air in a smallcylinder, or a small compressor and valves, to modulate a system where thedevices have been miniaturized.
Anotherproportional valve application is controlling the pressure of a ventilator thatis actually breathing for patients. These more sophisticated ventilators providepressure control or PEEP (positive end-expiratory pressure), which allows theventilator to be more effective in the transfer of oxygen into the nodules ofthe lungs that complete the transfer of oxygen into enriched blood.
"WithPEEP, the pressure control device makes the ventilator more effective by usingproportional controls to vary the pressure as the ventilator expands thelungs," says Paul Gant, sales manager for Clippard Instrument Lab.
Electro-pneumaticsystems are also being used in automated blood pressure monitoring deviceswhere a valve both inflates the cuff and controls the rate of deflation. Theelectronic valve is cycling on and off, and the pressure drops as the deviceexhausts the air in the cuff. Valves used on these more sophisticated bloodpressure monitoring devices not only recognize systolic and diastolicpressures, but actually measure the point at which the blood begins flowing.
"Whenthe cuff is inflated, the blood vessels are restricted and create atourniquet," says Gant. "When the pressure is slowly released, there is a pointat which some of the blood vessels release and blood starts to flow."
Sophisticateddevices use techniques to notice how quickly the blood flow recovers and usethat information to diagnose different maladies with the patient. The devicesuse very precise pressure control, versus a device which turns on and off, andprovide a gross control. The proportional valve allows the application toinexpensively add fine pressure control at the point where those blood vesselsare just beginning to open and blood is beginning to flow.
DesignersFocus on Size and Power
Inpneumatics for medical applications, the trends are mirroring the changes inthe medical devices themselves. One trend is portability and movement where inthe past the focus was more on in-hospital treatment. Now there is a clearshift to more out-of-the-hospital and home clinical treatment.
"Whatwe are seeing with medical devices is that they're getting smaller and areoften made to be ambulatory where they need to be disconnected from the wallfor the patient to be able to move around," says Randy Rieken, sales leader,Americas for Norgren Life Sciences.
Insome cases, there is a need for the patient to take the device home. A patientmight use a ventilator in the hospital, and the same device might be taken homeby the patient and used remotely. With the devices getting smaller and therequirements for pneumatics changing, components are getting smaller and powerconsumption is an important design issue. Audible noise levels are criticalbecause, if a patient takes an O2 concentrator or ventilator home, they don'twant to hear the valves actuating.
"Becausethe components are smaller, issues such as accuracy and reliability are evenmore important," says Rieken. "The flowcapabilities of valves, even though they are getting smaller, are often thesame as before. Medical device manufacturers are looking for higher performancecomponents in a smaller package."
Riekensays it used to be that miniature valves were 16 mm, and then 10 mm. But nowmany are 8 mm and there is work on valves in the 4 to 5 mm ranges. At somepoint as devices continue to get smaller and smaller, the actual technology maychange but now valves are still using the same technologies. The current goalis to miniaturize and optimize designs to be smaller but, in the future,technologies such as shape metal alloys and piezo actuator technology may cometo the forefront as devices get smaller.
Onearea where pneumatics continues to provide effective components for portabledevices is ventilator products. We may think of a big ventilator next to apatient's bed, but units are becoming smaller and some companies are targetingthe C-PAP market (devices that help patients with sleep apnea breathe properlywhile asleep), where the patient can wear the ventilator on their waist orcarry it in a small bag. With oxygen concentrators, devices often weigh fivepounds and less and the size of the pneumatics internally is a huge consideration.
"Thedesign focus in on size and power, and there are things you can do with power,"says Rieken. "We actually have a PWM (pulse width modulation) valve with aprocessor that can detect when the plunger is moving to optimize the power butmost design decisions are based on the size of the device and battery usage. Anoxygen concentrator may need a 12-hour battery life and, if there are fourvalves in the unit, they need to be extremely low power."
Onearea where suppliers are concentrating to provide greater value to customers isengineering expertise specific to applications. The device manufacturer comeswith requirements, schematics and a willingness to consider more highlyintegrated pneumatic modules. Often these modules include flow control,filtration, switching valves, fittings and safety relief valves allmanufactured specifically for the application and designed into a compact,optimized module that is fully assembled and tested.
Energy-EfficientPiezo Solutions
"Inhandheld instrumentation and medical devices, power budgets are beingscrutinized more than ever, as a result of a delicate balance between addedfunctionality and power consumption," says Richard McDonnell, piezo productsprogram manager for Parker Hannifin Corp.
Asportable or handheld analyzers continue to shrink in size and devicefunctionality continues to increase, customers are expecting next-generationproducts to operate longer between battery charges. In the past, instrumentengineers might have considered using ahit-and-hold circuit to reduce a valve's power consumption which uses ahigher voltage to open the valve and then a lower voltage to hold it open.Power consumption is decreased, but it is not as efficient as piezoelectricvalves.
ButMcDonnell says that option is no longer good enough. For example, one clientcited that in an application of two 0.5W valves, the valve's parasitic powerloss was second only to the instrument's cooling fans. New piezoelectricactuator technology enables the valves to operate in the 100 mW range and,coupled with negligible heat generation, self-latching function and the abilityto stay in position without power being applied, are reasons for growing interestin piezoelectric technology.
"Howthat relates in the world of pneumatics is to provide customers withmulti-function valves. It was that idea that led us to developing advancedpiezoelectric actuator technologies to augment or perhaps even replace solenoidsin the future," says McDonnell.
Parkerhas opted to develop its piezo technology around two types of actuators: a 25mm round, short stroke, low force RLP actuator and its ViVa, a family ofactuators with mechanical amplification for applications requiring largedisplacement and high force. Either type can be used as an independent actuatoror as an alternative to solenoid and voice coil type actuators.
McDonnellsays Parker's compliant actuator design increases the piezo displacement wellbeyond the traditional stack-type piezo, while generating more exploitableforces when compared with Bender-type piezo actuators.
"Because the ViVa actuator is inherentlyproportional, we can apply the technology in a variety of applications toprecisely profile the delivery of compressed air or gases while minimizingpower consumption," says McDonnell. He says that there is also significantinterest in energy recovery and ultra-low-power technology with energyharvesting to create energy independent systems.
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