Features

Healthcare providers now, more than ever, make critical patient care decisions based on the test results obtained from medical diagnostic instrumentation. The accuracy, precision, and reliability of clinical chemistry and immunoassay analyzers not only rely on the inherent design and function of the instrument, but also on the design, function, and reliability of the multiple individual components integrated into the system.

Fig. 1 - Illustration of the CeramPump® pumping principle.

Since patient samples and reagents are typically in liquid form, critical functional components in diagnostic instrumentation are those responsible for fluidic handling and control. Accurately and reliably moving fluids throughout the system during the analytical process is the challenge of those involved with medical diagnostic equipment system design. These fluids are typically made up of samples (whole blood, serum, urine, etc.), reagents, buffers, and wash fluids. In addition, managing the removal of waste fluid generated as a by-product of process reactions and interactions is a fluidic challenge as well, as it contains a combination of different fluids with varying chemistries and physical characteristics including suspended solids, cell fragments, and possibly pathogens requiring special handling and disposal procedures.

The trend for fluidic systems used in medical diagnostic applications is the capability to deliver smaller and smaller volumes at higher and higher accuracies. Manufacturers of diagnostic instrumentation, for example, continue to develop clinical analyzers that can perform more and more tests using smaller and smaller samples. As a result, samples, reagents, buffers, wash, and waste fluids used in diagnostic instrumentation are often controlled within a range of few microliters. To accomplish this, a variety of pumping technologies have been employed including syringe, peristaltic tubing pumps, diaphragm pumps, and the valveless pumps, such as the CeramPump®.

Each technology has certain advantages with respect to accuracy, reliability, and cost. It should be noted that accuracy, precision, and reliability are fairly easy to measure given the availability of precision measurement instrumentation. However, assessment of cost can be a more subjective component to measure, as the actual purchase price of the component is seldom the actual the cost of integration. Design considerations and limitations, projected routine maintenance, and long-term reliability in the field can easily add to the components’ share of the overall instrument cost. When incorporating pumps into a medical instrument or system, size (footprint) and ease of mechanical and electrical integration are additional factors for consideration. This is especially true when retrofitting a fluid control component into an existing design, as well as updating or adding a new module to an existing system.

Valveless Piston Fluid Control

Fig. 2 - Dispense volume adjustment.

The valveless piston pump was a concept developed incorporating key features most often sought after by OEM medical instrument design engineers. The CeramPump from Fluid Metering Inc., for example, utilizes one moving part — a rotating and reciprocating ceramic piston — to accomplish both the pumping and valving functions to eliminate check valves typically present in all other reciprocating (syringe, diaphragm, bellows, and solenoid-operated piston) designs. The rotating and reciprocating ceramic piston moves within a precision mated ceramic liner to accurately move fluid in one direction through the pump.

The reciprocation action of the piston acts very similar to 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. However, what is unique about the CeramPump is that in addition to reciprocating, 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 ports (see Figure 1). As the piston rotates, the flat is alternately aligned with the inlet and outlet ports, 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 as shown in Figure 1.

Adjusting Displacement for Dispensing or Continuous Metering

The valveless piston pump can function as a dosing pump with discrete dispenses, as well as a metering pump for continuous fluid transfer. In both cases, the pump displacement controls the volume per stroke (or volume pumped per stroke) and is variable determined 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 beyond zero, with respect to the drive, the piston begins to reciprocate, and fluid is moved through the pump (see Figure 2). The greater the angle, the greater the displacement per cycle (also known as stroke). Adjustment is infinite between 0 and 100 percent of the rated maximum flow for a specific pump head. 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 precision and accuracy.

Typically for OEM instrumentation applications, pump head displacement is calibrated and locked in place at the factory to the customer’s specific requirements. This provides precision factory calibration while also saving time during system assembly. However, if customers prefer to calibrate the pump at their factory or in the field, or if multiple dispense volumes require varying calibration changes, pumps to meet these requirements are readily available as well.

The ‘No-Valve’ Design Advantage

Fig. 3 - Accuracy and precision.

The valveless design is this type of pump’s most significant feature. There are typically four check valves present in diaphragm, bellows, and traditional piston pumps. Even during normal operation, these will wear over time and will fail to seal properly, allowing backflow. As a result, accuracy drifts, and at a minimum, the pumps need recalibration. Eventually, the check valves need to be serviced. By eliminating check valves from the fluidic design, the associated maintenance and downtime is eliminated as well.

Syringe pumps, also very common for low-volume fluid control in medical instrumentation, use electronically actuated valves. The syringe port essentially functions alternately as both the inlet and outlet port of the pump. As the syringe fills, the port functions as the inlet port. As it dispenses, the port becomes the outlet port. Typically, for uninterrupted dispensing, syringes are paired so that while one syringe is dispensing the other is refilling. For this two-syringe system to operate, there are two to four valves per dispensing unit and two to four electronic valve actuators and electronics. Because the fluid travels in one direction through designated inlet and outlet ports, the valveless pump can accomplish uninterrupted dispensing with a single pump.

Sapphire-Hard Ceramic Internals

Fig. 4 - One design for all fluid control requirements in medical instrumentation.

In addition to the functional advantages of the valveless pump design, the piston and mated liner are made of sapphire-hard ceramics. These components are therefore both chemically and mechanically resistant to a broad range of fluids. In addition, the ceramic components are dimensionally stable and will not change shape or fatigue over time, which can be the case with pump designs relying on flexible tubing or elastomer diaphragms. These components ensure that the pump chamber remains dimensionally unchanged for millions of dispenses without accuracy drift or downtime for recalibration.

Drift-Free 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 with the desired or target value. Precision is the range or degree of variation from dispense to dispense. Having one moving part in the fluid path, eliminating valves, and utilization of ceramic internal components results in a pump that will maintain 0.5 percent drift-free precision (see Figure 3).

Design Versatility. The valveless piston pump is available in a wide variety of configurations to meet a broad range of medical, analytical, and laboratory automation instrumentation requirements. Typical fluid control functions include aspirating and dispensing, discrete dispensing, flush and wash, and waste fluid removal (see Figure 4).

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