Valve Basics for Medical Devices

The handling of gas and liquids is a key requirement in the design of many medical devices. Such devices are used in operating theaters, patient rooms, outpatient clinics and home care environments, as well as in laboratory and diagnostic suites. And in practically all of the devices that handle or deliver gases or fluids, the reliability and precision of valves and fluid controls are critical functional requirements. 
 
There are four major valve types that are most widely used in medical device applications:
 
1. Isolation valves: used to control the flow of neutral gases or aggressive liquids. Isolation valves use a sealing mechanism to separate the fluid from the actuator. This separation prevents liquids from coming into contact with the valve control surfaces and ensures that the sensitive fluid within the valve body remains pure.
 
Isolation valves use a variety of seal designs to achieve this separation, ranging from a simple diaphragm to more sophisticated flapper, bellows, lever or rocker seal designs. These sealing options make the valves versatile enough to handle and isolate virtually any type of liquid media while protecting it from any form of external contamination. In addition, these valves are designed to control and deliver very low internal fluid volumes and to be easy to clean for reuse. These technical advantages make isolation valves a primary choice for fluid handling in equipment used in research and diagnostic laboratories around the world.
 
2. Pinch valves: ensure the highest level of fluid purity and protection against cross-contamination because critical fluids are hermetically separated, carried within a flexible tube that is inserted through the body of the valve. The name “pinch valve” comes from the way that the valve actuator and body surround and compress, or “pinch,” the flexible tube to control fluid flow. When open, pinch valves ensure a consistent fluid flow throughout the tubing. Because of their hermetic separation and flow characteristics, pinch valves are ideal for handling all kinds of sensitive fluid media, including mixes, cultures, solutions, and even thicker slurries that contain high levels of organic or solid matter.
 
3. Proportional valves: designed to regulate gas pressures and flows with the highest accuracy and precision. They are equipped with a dedicated solenoid coil and an actuator. These controls respond precisely to the slightest changes in electrical input signals sent to the coil, which translate into small, proportional movements of the actuator. This adjustment of the flow path results in extremely accurate regulation of both gas flow and pressure. And, because the actuator is designed to minimize friction and the valve body is able to handle varying input pressures, proportional valves are capable of regulating a wide range of flow rates.
 
For example, proportional valves can manage rates as low as a few milliliters per minute for applications such as gas chromatography or up to 200 liters per minute for ventilators while consuming very little electrical power. In combination with electronic controllers and sensors, proportional valves can also be integrated into closed-loop regulation systems.
 
The speed, precision and flow-range capabilities of proportional valves make them an excellent choice for sensitive medical gas-handling tasks including anesthesia, where they are used to mix and deliver oxygen and other gas mixtures essential to surgery. The same capabilities make them essential for medical ventilators — both stationary and portable — since these devices must process and manage flow changes rapidly to adapt to the dynamic breathing volumes and patterns of different patients, from large adults to tiny infants. 
 
4. General-service valves: typically used to control inert gas flows and are widely used in medical device applications. Although these valves do not offer the design characteristics of pinch and isolation valves — e.g., physical or hermetic separation of valve controls from fluids — they can be built with biocompatible materials (such as o-rings, seals and valve bodies) that meet material compliance requirements for “wetted” surfaces. Therefore, they are quite versatile.
 
As a result, general-service valves, which are known for excellent stability and long cycle life under continuous duty, find their way into many critical medical device applications. Their versatility is aided by other options, including engineered-to-order configurations, cleanroom assembly and oxygen-clean assembly, that prepare general-service valves for use with ultrapure oxygen as well as high-purity, sterile or other critical fluids. 

Valves used in Medical Applications

Gas and fluid handling is a central requirement in medical treatment today, seen in applications such as:
 
Dialysis
One of the most prominent applications is dialysis, where general-service valves are used in both hemodialysis and peritoneal dialysis equipment.
 

• The hemodialysis process requires that blood flow through a series of dialyzing circuits, which expose the blood to physical filtration via diffusion and osmosis. This process transfers wastes to dialysate fluid. When used in high-volume hemodialysis equipment, flow-control valves must be able to deliver life of 5 million to 6 million cycles over a three- to four-year period of use. They must also be configurable, since a large hemodialysis machine can require 20 or 30 valves, enough to run three or four identical dialysis/filtration circuits simultaneously.
• Peritoneal dialysis, which relies on the lining of the peritoneal cavity as a natural filter for blood-borne impurities, also relies on the use of a series of circuits to continually draw out spent dialysis fluid and infuse new, clean fluid. Because this procedure is often done using portable equipment at home, equipment designs emphasize light weight, easy portability and setup, and low power consumption. One such application clusters 21 (10 mm) general-purpose valves, mounted on a printed circuit board that provides power and control signals, to regulate the input and output of dialysate through multiple circuits in the dialysis machine. 

 
Ventilation and Respiratory Therapy
Whether they are used for temporary periods for extended life support, ventilation and respiratory therapy equipment rely on proportional and general-service valves. Ventilators use proportional valves, such as ASCO’s Preciflow™ line from Emerson, to control the mix of oxygen and air delivered to patients or to support more advanced breathing features, such as variable positive end expiratory pressure (PEEP). PEEP is a key feature of intensive-care ventilators that can vary the level of pressure upon exhalation to assist patients who have weakened alveoli in the lungs. General-service valves in highest demand include those for gas-handling and -delivery functions, such as the ASCO Series 188, Series RB and Series 065.
 
Surgical Instruments
From a dentist’s office to a major surgical suite, gas and fluid handling is essential to patient care, comfort and safety during both major and minor surgical processes. Surgical facilities depend on proportional and general-service valves for gas mixing for patient anesthesia and for surgical processes including cardiac ablation and argon-plasma coagulation. More and more minimally invasive surgeries rely on gas insufflation, regulated using valves like ASCO Preciflow valves, to create or expand small cavities in the body to allow room for endoscopy or robotic surgery on joints, organs or other tissue. Surgical rinsing and suction demand the high-purity and hermetic separation characteristics offered by pinch valves, such as the ASCO Series 284 and 384 models. 
 
Therapeutic Devices
Inflatable deep-vein thrombosis (DVT) cuffs, whose pressure is regulated through a multi-station valve manifold based on the ASCO Series RB/090, inflate at intervals to exert gentle pressure on the legs of a recovering patient, helping to prevent the risk of blood clots. Similarly, pneumatically driven shock-wave therapy devices use carefully controlled alterations in air pressure, regulated by valves like ASCO’s Series 076, to change the “strength” of the shock wave being delivered to the affected tissues within the body, stimulating circulation and speeding healing. Both of these dynamic devices demand robust proportional valves to meet ever-changing pressure and flow requirements within their respective systems.