The results of recent surveys suggest that more than half of all healthcare decisions are made using test results obtained from medical diagnostic instrumentation. These tests are routinely performed by diagnostic instrumentation at hospitals, diagnostic centers, and at a variety of “point-of-care” locations. The results of these tests greatly assist healthcare professionals in the detection and diagnosis of a broad range of health conditions and diseases.

Fig. 1 – Valveless piston pump functions.
Most medical diagnostic instrumentation is made up of clinical chemistry and immunoassay analyzers. Clinical chemistry tests quantitatively measure the chemical makeup of different kinds of bodily fluids, most common being blood components (whole blood, serum and plasma) and urine, although there are clinical tests for many other bodily fluids (amniotic, cerebrospinal, pericardial, and saliva, just to name a few) as well. A variety of different clinical chemistry tests exist to detect and quantitatively measure almost any type of chemical component in the body including blood glucose, hemoglobin, electrolytes, enzymes, hormones, cholesterol, lipids, proteins, and other metabolic substances. Immunoassays basically handle the same bodily fluids and are biochemical tests designed to detect the level of specific proteins, antibodies or antigens, in a sample. These specific substances are typically present or elevated as the result of an immunological reaction, manufactured by the body most often in response to the presence of a foreign protein or microorganism (bacteria, virus, etc.).

Precise, Accurate Micro-Volume Fluid Control

Both clinical chemistry and immuno-assay analyzers detect and aid in the diagnoses of patient health conditions, and design similarities result in having many functional components in common. Since samples, reagents, a variety of buffers, wash and waste fluids are in liquid form, there are many fluid control requirements and components that are essential to both systems. The performance of these components will have a direct impact on the accuracy of the instrument as well as the reliability of the information it yields. In addition, manufacturers of diagnostic instrumentation continue to strive to meet increasing demands to develop analyzers that can perform more and more tests using smaller and smaller samples. The challenge is to maintain precise fluid control at low dosing volumes, very often within a range of a few microliters. Equally important is to have a fluid control system that will maintain drift-free precision long-term, thus eliminating the need for recalibration, maintenance, costly service calls, and downtime.

Valveless Piston Pump Technology

Accurately moving fluids through diagnostic instruments is most often accomplished using a metering or dosing pump. Traditionally, a variety of fluid dispensing and control technologies have been used for fluid control in diagnostic instrumentation, including peristaltic, diaphragm, syringe, and solenoid piston pumps. Although these pump technologies have their merits, a valveless piston pump has had a significant impact for the task of precision micro-fluidic control of samples, reagents, buffers, wash, and waste fluids.

A valveless piston pump is a piston pump that eliminates valves and elastomer components typically associated with maintenance, accuracy, and calibration concerns. Its unique rotating and reciprocating piston design eliminates the need for valves by having one moving part accomplish all fluid control functions. In addition, all internal components are made from chemically–resistant, sapphire-hard ceramics. These ceramic components are dimensionally stable and will not distort, stretch or change shape over time.

Pump technologies that rely on elastomers typically require routine maintenance and recalibration over time (diaphragm and peristaltic). Internal pump components that define the volume of the pumping chamber made from elastomers will eventually stretch, fatigue, and lose memory. This basically translates to the elastomer component (diaphragm or tubing) not returning to its original dimension or shape. As a result, there is a gradual change the volume of the pumping chamber, which, in turn, causes variations in dosing volume and flow rate.

Syringe pumps, common in both medical diagnostic, as well as analytical instrumentation, basically eliminate the concerns relating to accuracy drift as the result of using elastomers to define the pumping chamber. However, syringe pumps have several moving parts in contact with the process fluid including single and/or multi-port valves. For applications requiring continuous dosing and high throughput, syringe pumps are typically configured in pairs. One pump is dispensing while the other pump of the pair is filling and vise-versa.

For these applications, one valveless piston pump can replace two syringe pumps and their corresponding valves, actuators, and driver boards. In addition, the primary seal of the valveless piston pump provides ultra-tight clearance between the ceramic piston and the mated ceramic liner. This capillary seal is capable of handling pressures up to 100 psi and eliminates the need for plunger o-rings and tips. (See Figure 1)

Pump Basics

This type of pump uses one moving part to accomplish both the pumping and valving functions thereby eliminate check valves which are present in all other reciprocating (syringe, diaphragm, bellows, solenoid piston) designs. It uses a unique rotating and reciprocating ceramic piston, moving within a precision mated ceramic liner to accurately pump fluid in one direction without allowing any backflow. The reciprocating action of the piston acts very similar to that of a standard piston pump. As the piston moves back, it draws fluid into the pump chamber. As it moves forward, fluid is pushed out of the pump.

Fig. 2 – Illustration of valveless pumping principle.
However, in addition to reciprocation, the piston also simultaneously and continuously rotates in one direction. The piston is designed with a flat cut into the end closest to the inlet and outlet port. (See Figure 2) As the piston rotates, the flat is alternately aligned with the inlet and outlet port, essentially functioning as a valve. At no time are the inlet and outlet ports interconnected, and therefore the need for check valves is eliminated. One complete synchronous rotation and reciprocation is required for each suction and discharge cycle.

Adjustment of Dispense Volume

The piston displacement (or volume pumped per stroke) is variable and controlled by the angle of the pump head relative to the drive. When the pump angle is zero, the pump head is in straight alignment with the drive, the flow is zero. In this situation, there is no reciprocation and the piston is only rotating.

As the angle of the pump head increases above zero in either direction with respect to the drive, the piston reciprocates, and fluid is moved through the pump. The greater the angle, the greater the displacement per cycle (volume per stroke). Adjustment is infinite between zero and 100 percent and a flow rate indicator provides for accurate and simple linear calibration. The pump is designed so that at any angle and flow rate, the piston always bottoms for maximum bubble clearance. This is especially important at very small dispenses and flow rates, as the presence of even a minute bubble will significantly affect accuracy.

Key Features

Design: The pump has only one moving part in the fluid path which functionally replaces multiple check valves present in diaphragm, bellows, and traditional piston pumps, as well as multiple valves, actuators, and drivers found in syringe designs. Even during normal operation, these will wear over time and not seal properly allowing backflow. As a result, accuracy drifts and minimally the pumps will require recalibration. Eventually, valves need to be serviced.

Ceramic Internals: This pump uses sapphire-hard ceramics for both the piston and mated liner. These components are dimensionally stable in that they will not change shape or dimension over time. Therefore, the pumping chamber remains stable for millions of dispenses without downtime or recalibration.

Accuracy and Precision: Consistency in dispensing can be measured by monitoring both the accuracy and precision of the dispenses. Accuracy is a comparison of the average value of the dispense volume compared to the desired or target value. Precision is the range or degree of variation from dispense to dispense. Ideally, metering and dosing should be both precise and accurate.

Fig. 3 – Examples of compact OEM single and duplex pumps.
Versatile Design: Pumps are available in a wide variety of configurations to meet a broad range application requirements from small compact designs for OEM medical, diagnostic, and environmental instrumentation, to fully programmable laboratory dispensers and rugged hazardous duty process pumps. Dual pump configurations are even available, ideal for proportional mixing, dilution, and dual channel production dispensing. (See Figure 3)

Additional Medical Applications

The applications where the valveless piston technology is used are numerous, ranging from micro-volume reagent fluid control in OEM clinical diagnostic instrumentation, hemodialysis machines, and automated immunoassay processing to precision production dispensing of cyclohexanone and other solvents used in the manufacture of disposable medical component kits (I.V. tubing sets). Valveless piston pumps are also used extensively for preparation of multi-well microtiter plates used in diagnostic instrumentation; filling of pharmaceutical vials, ampules and syringes; dispensing monomers used for disposable contact lens manufacturing; dispensing anticoagulants for coating of drug eluting stents; dispensing electrolytes used to make button cell batteries for hearing aids, and much more.

This article was written by Herb Werner, Marketing Manager, Fluid Metering, Inc., Syosset, NY. For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/55595-165.

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Medical Design Briefs Magazine

This article first appeared in the November, 2015 issue of Medical Design Briefs Magazine.

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