Advances in Analytical Valve Technology

For today's medical and analytical equipment manufacturers, one of the most dynamic market sectors involves technology for diagnostics and pathology. Among the strongest design challenges is that of the ongoing miniaturization of the liquid-handling systems used for high-throughput, low–dead volume dispensing. The reasons are threefold: the desire for portability, the high cost of reagents, and the need for improved sensing accuracy requiring tighter control. To avoid waste, reagents must be dosed or dispensed over smaller flow paths, using ever-smaller tubing. For example, a new range of tubing used extensively in medical equipment is being supplied in sizes as small as 1-mm inner diameter (ID).

Given these design trends, it follows that the valves and other internal components that handle the dosing work must be reduced in size to accommodate smaller tubing. Flow orifices on valves used in medical and laboratory applications are shrinking down to dimensions of 0.6 mm or less. However, achieving high accuracy on very small volumes of aggressive media—over as many as 30 million cycles—requires more than putting an isolating diaphragm on a standard solenoid valve and machining a plastic body. A number of proven suppliers are acquiring extensive experience in molding both bodies and seals of these new-generation valves. With the help of modern materials, talented fluid-control engineers are transforming these seemingly ordinary devices into designs of remarkable technical sophistication.

A growing variety of material options for both valve bodies and elastomer seals is enabling manufacturers to more closely tailor performance requirements and costs to specific uses.

Along with the shift to 1-mm ID geometry, innovative actuating mechanisms are providing reliable, repetitive seal engagement in aggressive media. Until recently, isolating diaphragms were available solely on conventional spring plunger-type valves with 2-way (normally closed) function only. Proprietary flipper- or rocker-design actuating mechanisms can now be found on 3-way valves—with all three ports in the base—as well as on 2-way (normally open and normally closed) valves. These are being produced in a range of orifice sizes, from 0.6 up to 6 mm. Suppliers are also offering products that are much more specialized, featuring valves developed for specific medical applications or market niches.

At one time, most medical valves were made exclusively of polyphenylene sulfide (PPS) plastic bodies with fluoroelastomer (Viton) seals. Now, valves can be specified in a wide range of engineering plastics, with new perfluoroelastomer seal materials for analytical or technical applications.

Valve developments alone are only one aspect of component improvements for medical fluid-handling technology. Microdosing sensors can transform valves into complete dosing systems, with dosing capabilities ranging from 50 µl/hr to 5 µl/sec, with 2% full-scale (FS) accuracy. This flow range comprises piezoresistive devices for lower-volume dosing and standard solenoid pumps for higher-volume dispensing. Because the microsensors are so small, they can be integrated into the valve manifold or valve body with spacing less than 10 mm apart, allowing for 9-mm on-center dosing architecture for common 96- and 384-well requirements. Such systems can be designed to incorporate not just an on/off control, but also varying degrees of real-time flow diagnostics.

SORTING OUT SEALS

The new valves designed for high-speed dosing systems employ flexible rocker and flipper diaphragms to prevent fluids from contacting metal operating parts and help ensure optimum reliability and long life. These new designs and materials can provide complete fluid isolation, eliminating the metal-to-fluid contact that can cause abrasion, contamination, corrosion, or heat transference. The designs also offer significantly fewer moving parts and less overall movement, thereby reducing friction, heat buildup, and wear and tear.

Although elastomers such as Viton offer resistance to high temperatures and aggressive media, achieving the right balance between purity and mechanical performance is a recent breakthrough in elastomeric technology as applied to conventional plunger-style designs. In the past, highly pure elastomers combined with conventional designs suffered from poor mechanical properties, with a distinct decrease in reliability beginning at approximately 1 million cycles.

These deficiencies are being addressed by the advent of new seal materials such as the perfluoroelastomers (Simriz, Kalrez, Chemraz, and Zalak), which combine the chemical properties of Teflon (PTFE) with the mechanical properties of Viton.¹ Teflon is outstanding among fluoroplastics for its exceptional chemical, thermal, and dielectric properties, is almost universally resistant to chemicals, and, in virgin form, can withstand temperatures of between ­260º and 300ºC.

VALVE MATERIAL OPTIONS

Among other important characteristics, analytical valves must reliably transmit samples without introducing contamination from prior samples, or "carry-over." In addition, the requirement to flow a wide range of aggressive solvents and reagents make inertness a high priority for valve materials. Today, 2-way and 3-way, chemically inert solenoid valves are available with bodies of high-temperature crystalline resins (PEEK), polypropylene (PP), or polyvinylidenfluoride (PVDF) to ensure the broadest range of compatibility.

PEEK offers excellent high-temperature properties, high strength and stiffness, and both long- and short-term heat resistance. Alternatively, PVDF is a less costly, semicrystalline engineering thermoplastic with very good chemical resistance, excellent machinability, and versatility of application.

The wider choice of available materials means that device manufacturers now have the option of selecting lower-cost valve bodies and elastomer seals when the application warrants it. For example, a system might require Simriz seals for one area of the analyzer device, but it may turn out that, elsewhere in the analyzer, the fluid samples are dumped and purity is no longer as important, so lower-priced Viton seals would be adequate. By working with a supplier that offers a broad range of body materials and sealing options within its valve product line, device manufacturers can obtain maximum cost-effectiveness with the same inherent design benefits of optimum seal actuation. In other words, there's no need to buy an "over-engineered" valve.

GETTING YOUR PORTS IN A ROW

If one takes a Simriz seal and combines it with a flipper- or rocker-style solenoid valve with an 0.8-mm orifice, the result is a miniature version of a well-proven actuating mechanism that operates an isolating diaphragm, separating the actuator and the coil from the fluid.

The flipper- and rocker-style valves characteristically provide fully swept cavities and minimal cross-contamination. They also largely eliminate friction and thus the risk of sticking, which contributes to their high reliability and long service life. Both designs can provide reliable, repetitive seal engagement in aggressive and contaminated media, without the "pumping" effect associated with conventional diaphragm valves when actuated. Available in 8-, 10- and 16-mm geometry designs for analytical, medical, and biotechnical systems with operating pressures from vacuum to more than 100 psig, they can be easily installed on a manifold since all the inlets are aligned on one side.

In the rocker version, a stainless-steel plate isolates the actuator from the electrical coil, ensuring no heating of the media and allowing removal of the coil while the fluidic valve body remains mounted to the manifold. These low–dead volume, low-carryover valves are especially effective in eliminating cross-contamination, thus making the system ideal for the control of critical fluids. Models can be specified in the standard port connections NPT 1/8, G1/8, or UNF 1/4­28, with barbed tube connections or in sub-base bodies combined with flying-lead or plug-style electrical connections. Leading valve manufacturers are also introducing products that are more easily integrated into customer designs. For example, all the ports can be located in the base for 2- or 3-way functionality. Seats can be machined directly onto the customer's manifold, without a valve body, or custom designed to meet the specific volume or flow requirements of the application.

By incorporating these features into their valve designs, manufacturers are able to meet the tight spacing requirements on the new generation of liquid-handling systems. Available in standard off-the-shelf configurations, the valves are a cost-effective solution for instruments that provide repetitive dispensing of sub-microliter to nanoliter liquid volumes into microplates 96 or 384 samples at a time.  

INTELLIGENT DISPENSING  

As high-throughput, microplate-based assays using 384 wells become more common, experienced valve manufacturers find their role changing: from component suppliers to providers of package solutions for new dispensing-equipment designs.

For example, one company recently introduced a new microsensor system capable of dispensing at rates ranging from 50 µl/hr to 5 µl/sec across a wide variety of fluids. The system uses a combination of a micro flow sensor and a 10-mm flipper solenoid valve with isolating diaphragm. The sensor is based on the measurement of pressure differential across an integrated fluidic restriction. A specialty coated, micromachined, fluid-interconnected silicon chip consisting of two piezoresistive pressure sensors forms the actual flow-sensor device.

Both the electrical and fluidic interfaces can be customized according to individual user needs. The sensor is mounted in a hybrid module with dedicated programmable electronics for primary signal conditioning and the measurement of pressure, temperature, and flow in both directions. The flow signal is corrected for the temperature dependence of the liquid viscosity using system software.

Based on the company's many years of experience in "smart" technology for process valves, this system was designed for liquid-dosing instruments in which sensor speed, small size, and consistent accuracy are of prime importance. The hybrid assembly permits the integration of the sensors and intelligent signal conditioning to be located very close to the point of use, thus increasing speed by reducing dead volume and allowing for a high density of measurement points. Flow rates up to 5 µl/sec FS can be achieved, with accuracy better than 1% FS of the flow rate over 20–50°C.

REAL-TIME FLOW DIAGNOSTICS

A number of different sensor technologies can be employed with standard, off-the-shelf analytical valves to provide flow diagnostics. Mechanical position indicators and LEDs offer a variety of different flow-control system solutions to indicate valve position or valve state. Real-time flow diagnostics are also available, which require a sensor to continually monitor the process.

Such "intelligent" valves are perhaps a year away from fully penetrating the marketplace. However, these valves—offering precise, fast dosing combined with real-time flow diagnostics—are available today for incorporation into the medical devices of tomorrow.  

CONCLUSION  

For device manufacturers, the good news is that a wide range of valves are available to suit almost any analytical application. As the medical device industry moves toward 1-mm-orifice geometries, there will be specific valve designs to handle just about any job.

The difficult news is that selecting the best valve is often a subjective experience, as opposed to an exact science. There can be a substantial degree of overlap in valve performance specifications, and more than one type of valve or sensor might be suitable for a given application. Fortunately, it is a fairly straightforward process to zero in on the handful of valves and systems that can potentially satisfy a particular set of requirements. From that point, a detailed analysis of capabilities—with assistance from an established supplier—can pinpoint the best-suited product.

REFERENCES  

 
Stephen J. Neil is national sales manager and Craig Occhiato is vice president of engineering at Burkert USA Medical Products Division (Irvine, CA). Sandro Schneider, PhD, is general manager of sensor systems and technology at Burkert Contromatic AG International (Hunenberg, Switzerland). The company provides microfluidic control systems and technology for the medical, analytical, and biotech industries.

Photo by Roni Ramos

Photo courtesy of Burkert USA