A New Technology for Metering Fluids in Medical Devices

by Eric Pepe and George Halfinger, KNF Neuberger, Inc.

Introduction

The positive displacement metering pump is generally the first choice for providing precise and repeatable flow in many medical device fluid dispensing applications. Understanding the full range of established and newer metering pump technologies available allows for the selection of the best metering pump to meet the fluid handling needs of the application.

Metering Pump Designs

The pumping of fluids is common in many medical devices. Liquids ranging from test samples to various reagents and wash fluids must be transferred, dispensed or metered, depending on the application. As these devices are designed to use smaller and smaller volumes of fluids, the requirements for very accurate metering pumps become greater.

Fluid transfer is sometimes accomplished using either centrifugal or positive displacement pumps. Centrifugal pumps transfer energy to a fluid via a spinning impeller, converting the impeller energy to fluid pressure which moves the fluid. These types of pumps are very pressure and fluid dependant and are typically not utilized for metering due to their inability to maintain very accurate flows under changing inlet and discharge conditions. Their advantage lies in providing high flow rates at low pressures.

Positive displacement pumps operate by trapping a fixed volume of fluid and moving this fluid via gears, pistons, diaphragms, vanes or other devices. These pumps typically operate at lower speeds, are less sensitive to changes in discharge and suction conditions, and allow flow regulation by adjusting speed and displacement. These features have made the positive displacement pump the obvious choice for metering fluids.

The metering pump can thus be defined as a positive displacement device designed to provide a very precise and repeatable flow within a specified capacity range. Capacity can typically be adjusted within the turndown range of the pump, typically from 10:1to as high as 1000:1 on some pump models. Accuracy, repeatability, and linearity across the turndown range are the features that differentiate a metering pump from a typical positive displacement pump.

Positive displacement metering pumps are normally classified as rotary or reciprocating. Rotary pumps include gear, lobe, vane, and roller (peristaltic) pumps. Reciprocating pumps include diaphragm, piston, and bellows pumps.

Rotary style metering pumps

Gear type rotary style pumps use gear teeth, lobes, or vanes to trap a fixed volume of fluid and carry that fluid via a rotary motion from the inlet to the outlet of the pump. In a gear pump, for example, two meshing gears rotate in a closed cavity with a close clearance maintained between the gear teeth and the pump casing. Fluid is captured between each tooth and the casing at the inlet port and carried to the outlet port. By maintaining very accurate volumes between the teeth and low leakage rates between the tooth and casing, the gear pump can be a very accurate metering pump. Flow control is easily achieved by controlling the rotational speed of the gears. How accurately you control the speed of the motor often determines the flow accuracy of the pump. Lobe and vane pumps operate in a similar fashion, but substitute smooth lobes or vanes for gear teeth.

Rotary pumps that use gear or lobes have the advantages of low pulsation, continuous flow, are capable of producing high pressures and handling high viscosities, require no valves, and can handle shear sensitive fluids. Their disadvantages include rubbing and wearing surfaces, the requirement for dynamic seals which can wear out and leak, they are not self priming, cannot operate dry, and have limited chemical resistance due to the requirements of the gear materials and casing. Very accurate rotary metering pumps are typically expensive due to the high pump tolerances and motor requirements.

The roller or peristaltic type pump eliminates some of the disadvantages of the gear pump by using a set of rollers which squeeze a flexible tube in a circular pump housing.

Fluid is trapped in the section of the tube squeezed off by the rollers and forced through the tube as the rollers rotate. For the pump to provide accurate flow, the roller must squeeze the tube down completely to prevent re-circulation, placing high stresses on the tubing.

Peristaltic pumps can accurately meter very low flows down to fractions of a milliliter, have a seamless, sterilizable, flow-path in which the pump never contacts the fluid, require no valves, can handle some particulates and are easy to maintain. They are not typically suitable for high pressures and their major disadvantage is the tubing life. Selection of the tubing is a balance between choosing a flexible material with long life and materials with adequate chemical resistance. As tubing wears and looses its flexibility, the accuracy of the pump in metering applications also suffers.

Reciprocating Style Metering Pumps

Reciprocating pumps operate by displacing a fixed volume through the reciprocating motion of either a piston, a diaphragm, or a bellows. In the simplest example, a piston is drawn back in a closed chamber, creating a vacuum which draws in a fixed volume of fluid. The piston then moves forward and expels the fluid. In this way, by either controlling the stroke length of the piston or the piston stroking speed, accurate flow control can be achieved. The reciprocating motion can be supplied by a motor driven eccentric or a liner magnetic drive (solenoid).

Piston pumps typically require seals or close clearances around the piston to operate accurately. This introduces the problems of seal and piston wear, contamination of the pumped fluid by wear particles, and limitations on the material selections for optimum chemical resistance.

The simplest form of the piston pump is the syringe pump, which is designed to accurately meter up to the volume of one full stroke of the syringe. By accurately stepping the piston on a syringe pump, very accurate flow rates in microliters can be obtained. The major disadvantage of this type of pump is that once the syringe is empty, the refill period allows no flow from the pump. Thus a syringe pump is not suitable in continuous metering applications.

Diaphragm metering pumps eliminate some of the disadvantages of piston style pumps by replacing the piston with a flexible diaphragm. Because the diaphragm is sealed by clamping around the edge, the pump uses no dynamic seals which can wear, eliminating leakage or contamination of the pumped fluid.

Operation of Liquid Diaphragm Pumps

Diaphragm liquid pumps operate by means of an eccentric that moves a diaphragm up and down, inside a chamber. On the down stroke, liquid is drawn into the chamber through a non-return valve. The valve closes as soon as the diaphragm starts to move in the upwards direction and this movement then compresses the liquid and forces it out of the chamber through another non-return valve, thus producing flow (Figure 1). This pumping concept is equally effective handling liquid or gases with the typical pump design providing self-priming, mixed media pumping capability, as well as the ability to operate dry (without liquid) indefinitely. The final design is mechanically simple and permits the pump designer the ability to select the pump wetted parts to be in chemically inert materials, a major advantage of diaphragm style pumps.

Traditional Diaphragm Liquid Metering Pumps

Flow control is normally exercised by changing the pump stroke or alternatively by changing the speed of rotation of the motor. In the case of AC motor , rotation is normally 2800 to 3200 rpm, but with DC motors the range can be between approximately 1200 rpm and 4000 rpm. For relatively low flow rates, stepper motors have been used, which have speeds up to approximately 300 rpm. This is the conventional diaphragm metering pump that is reliable, self-priming, chemically resistant, and dispenses accurate volumes per stroke.

The main disadvantage of this type of metering pump is that the flow is discontinuous (there is no flow when the diaphragm is on the down stroke) and if the stepper motor is run at very low speeds in order to achieve relatively small volumes, there are long periods when no liquid is dispensed. In fact, during 50% of each revolution of the shaft, the pump is not delivering fluid.

The result is a pump output that produces flow delivery that approximates a square-wave form for solenoid or linear magnetic drive powered pumps (figure 2a) and the classical sine-wave form for reciprocating pumps that use an eccentric and are motor driven (Figure 2b).

Until now, the many potential advantages of diaphragm metering pumps have been offset by the disadvantage of inconsistent flow at very low flow rates. Recently, diaphragm metering pump engineers have addressed this problem by combining diaphragm pump technology with advanced stepper motor drive technology to produce near pulseless flow.

Electronically Controlled Diaphragm Metering Pump

The principle behind an electronically controller diaphragm metering pump involves the use of electronics to precisely control motor speed throughout a single stroke cycle to dramatically enhance performance. This allows the well established and robust design of the diaphragm pump to take full advantage of microprocessor controlled stepper motor technology to effectively compensate for the inherent oscillation in the pumped output.

In this concept, the Diaphragm Metering Pump, not unlike traditional diaphragm liquid metering pumps, is an oscillating positive displacement pump. An eccentric converts the rotary motion of the drive shaft into an oscillating movement of a connecting rod, which in turn transmits its motion to the diaphragm. In combination with inlet and exhaust valves, this diaphragm motion produces the pumping or metering action.

The diaphragm metering pump is driven by a stepper motor drive. It turns at maximum speed for the suction (down) stroke, and then controls the speed of the diaphragm during the delivery (up) stroke. The speed of the motor is pre-set in the motor controller processor based on the relationship of the rotary motion of the pump eccentric to the linear speed of the diaphragm surface. When the rotary motion of the eccentric translates to a small change in the speed of the diaphragm, the motor runs at a faster speed. As the rotary movement of the eccentric produces a higher diaphragm speed, the motor slows. This "sine-wave compensation" produces a pump output that is as smooth and continuous as possible over each motor revolution (Figure 2c).

For example, when the pump with sine-wave compensation is operated at low to moderate flowrates, the ‘pumping-time’ lost to the suction stroke is much less than 1% of the total pump delivery time. This produces a smooth quasi-continuous delivery and is the decisive advantage opposite traditional diaphragm metering pumps with linear magnetic drives (Figure 2a) or eccentric-driven diaphragm metering pumps with conventional motors (Figure 2b). This unique approach is accomplished with the use of a motor position feedback that monitors the rotary position of the stepping motor. This position sensing, along with the inherent ability of the stepper motor design to resolve each revolution of the motor into many small steps, makes it possible to control the angular speed within a single revolution, (sine-wave compensation). With this method, the slightest change in speed of rotation is recognized. Using the microprocessor controller also permits the pump to be connected and controlled by a wide range of external digital and analog control signals as well as providing pump operational data outputs.

KNF's Model FEM 1.08 Liquid Metering Pump
with control board

Materials of Construction

Because liquid metering pumps may be used with aggressive media, or where chemical inertness is a concern, all pump parts that come into contact with the liquid (e.g., valves, gaskets, diaphragms, and head material), should be readily available in corrosion-resistant material. The use of PTFE, PVDF, FFPM or other combinations of materials, normally used in diaphragm pump applications are generally acceptable for most applications.

Applications

This ability to provide the chemical inertness, durability, and mixed media pumping in a nearly continuous, precise flow is ideal for a range of applications in the medical field. In in-vitro diagnostic (IVD) devices, the delivery of reagents (especially the newer, more chemically active materials) can be accomplished reliably with feedback control from the main system control. Clinical analyzers and flow cytometry applications can benefit from the ability of the pump to provide precise, nearly continuous and controlled delivery of sample transport media (sheath fluids, etc.). Histology and other microscopy preparation applications can use the high precision delivery of staining materials and other sample preparation materials. In high throughput diagnostic screening systems, the pumps can provide reliable delivery of washing fluids for assay microplates. The ability to provide reliable delivery of fluid can also be particularly valuable in the field of molecular biology, where multiple fluid dosing streams are necessary for sequential or parallel synthesis or processing is frequently required.

As with any new technology, the full range of applications is yet to be defined. As more experience and knowledge is gained in the use of the product in the field, more potentially valuable applications will be discovered.

Conclusion

A wide variety of methods are available to accurately meter fluids in medical device applications. Each has its own advantages and disadvantages depending on the application. Recent advances in pump designs have combined the advantages of traditional diaphragm liquid metering pumps (e.g., reliability, self-priming, easy to service, small size, over-pressure protection, corrosion-resistance) with electronically controlled stepper motor drive technology, broadening the fluid metering options available to the designer. With flow rates between 0.08 ml/min and 80ml/min, accurate dosing is now achievable with diaphragm liquid metering pumps.