Newcomers to medical designs might think that a pressure and flow rate are enough to size a pump. Maybe in the industrial world. Medical devices, however, are a bit more demanding. So it's a good idea to review a medical pump's more involved selection criteria.

Continuous or metered?

There are generally two pump categories: continuous and metered flow. Continuous flow typically produces a steady rate for extended periods. This application is best suited by peristaltic or rotary pumps. These provide a constant pressure, although actual flow rates vary as downstream restrictions change or fluid properties (such as viscosity) change with time and temperature.

Metering pumps, on the other hand, dispense or aspirate precise amounts of fluid. These include variable volume and syringe pumps. Both are positive displacement designs.

The term “variable volume” refers to the dispensed volume versus the aspirated volume. Solenoid pumps are usually fixed-volume pumps. They aspirate a predetermined fixed volume and dispense the same amount. Syringe and stepper-motor pumps are not limited by fixed volumes. These can aspirate and dispense from zero to the full-stroke volume. For example, a 250 microliter pump could aspirate 250 microliters and then make five individual 50 microliter dispenses, or ten 25 microliter dispenses, and so on. Of course, the size of the dispense or aspirate is limited to the volume of the pump (upper limit) and the resolution of the pump (lower limit).

The dispense pressure compensates for changes in the system or fluid. If downstream resistance increases, delivery pressure increases until the system delivers the specific volume. It is also possible that pressure becomes so high a system component fails, typically by leaking or stalling the drive motor.

Many pumps also come with inlet and outlet valves. These may be simple check valves, or when a system needs greater accuracy, solenoid or actively controlled valves.

Don't neglect materials. After designing the flow circuit, judge the fluid compatibility with system components. This includes all the tubing, valves, and pumps. For relatively mild fluids, acrylic (PMMA) housings are quite suitable. More aggressive fluids may require engineering plastics such as PEEK.

Dispensing

Determine how the pump will be used and the volumes needed. Some applications may aspirate large volumes (drawn from a reservoir) and then dispense the fluid in smaller volumes. Other applications require a separate aspiration and dispense for each fluid. When multiple dispenses (or aspirations) are required, note the pump's resolution.

The dispense resolution in stepper-motor pumps is the smallest volume it can dispense. Several factors determine resolution, such as piston movement and diameter. Generally, the piston diameter is fixed and movement is determined by the stepper motor. It generates a particular linear displacement per rotation step (which is also influenced by the lead-screw pitch and the motor's angle/step). This resolution can be further modified by half or micro-stepping the motor.

Accuracy and precision requirements are critical to fluid circuit performance. Accuracy is usually expressed as a percentage of target volume. And repeatability is expressed in a C_V (coefficient of variation). If small C_Vs are required, the dispensed volume should be as close to the full stroke volume as possible. A pump spec might describe a fluid circuit that is to dispense 0.5 ml with 5% accuracy, a repeatability of 2%, and C_V of 0.04%.

Dispense speed also differentiates pumps. Generally the smaller the dispense resolution the slower the dispensed speed. A fine-pitch lead screw and small step angle can also lower the dispense speed or rate. When the application needs a higher speed, a larger dispense volume may be necessary.

Dispense speed is affected by system parameters such as the tubing that connects the pump to the fluid reservoir and dispense point. If the system uses small diameter tubing, the pump could draw entrained air out of solution during aspiration. A pump aspirating a viscous fluid through narrow tubing may reduce the pressure in the pump housing low enough to form bubbles in the liquid. These cases call for a larger diameter inlet tube to reduce the liquid restriction and thus the negative pressure (vacuum) needed to aspirate the liquid.

Problems can also occur with improperly sized outlet tubing. Too small a diameter may result in a troublesome pressure rise. A positive displacement pump will increase system pressure to overcome system resistance. If resistance is too high, it might be necessary to reduce the dispense speed. Otherwise, expect damage to the system by way of seal failures or tubing breaks.

Valves also play a crucial part in the speed and accuracy of the dispense. Valves must operate fast enough so they are open before the pump increases the system pressure (above a safe limit for the system). They must also close fast enough so that there is a crisp cutoff to the flow.

The valves must be compatible with the fluid and capable of handling the system pressure. Flow is also critical. If the valve is too restrictive, it may result in choked flow that in turn increases pressure during the dispense phase. The pump will displace a certain volume of fluid. If the downstream restriction is high, the pressure will rise so the fluid moves through the available opening. This is much like placing a finger over the end of a hose. The more area covered at the end of the hose, the higher the discharge pressure.

A valve too large generates a system volume problem. For example, a system that dispenses 1 microliter through a large valve with an internal volume of 100 microliters needs an initial prime. This may not be a problem, unless you are using an expensive or limited reagent.

In a nutshell, tubing and valving must be selected together so they are compatible on a system level (volume, flow rate, pressure capability, and chemical compatibility).

General characteristics

Pumps for medical applications come in a myriad of sizes. The application will have a significant influence on size and power consumption, especially as scientific equipment is taken out of the lab and into the field, and as instruments are required to perform a greater number of tests in the same space as older designs.

Pumps directly driven by stepper motors have size and weight advantages over conventional syringe pumps. Smaller motors required in direct-drive units also consume less power. This allows further space savings by reducing the pump size and power supply as well. Reducing power also leads to lower heat and less cooling, and in some cases, eliminating the cooling fans.

Maintenance should also be a deciding factor in instrument design. If a pump needs periodic maintenance, place it where it is easily accessible.

This may not be the most ideal fluidic location. A pump placed right next to the fluid supply has the advantage of short tubing runs, but is not readily accessible for service. Pumps placed where the technician can easily get to them for service may require significantly long lengths of tubing, often cutting down on system responsiveness.

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The Lee Co., www.theleeco.com

The selection guide in a nutshell

Here are few of the system characteristics a designer should gather before selecting a pump.

1. The application

   a. Continuous flow

   b. Metered or injection

2. Determine the chemical compatibility between fluid and wetted materials

3. Volumes in the fluid system

   a. Metered

   b. Dead volume

   c. Internal volume

4. Dispense characteristics

   a. Volume

   b. Accuracy

   c. Precision

   d. Repeatability

   e. CV

   f. Speed (aspirate and dispense)

   g. Timing

5. General properties

   a. Size

   b. Power consumption

   c. Life

   d. Maintenance

Fluid circuits 101

Although there are many designs to fluid circuits, they use a variation or combination of these basic forms.

Circuit 1 uses the pump to aspirate and dispense. The pump will pull liquid in and push it out, much like a turkey baster or eye dropper. Control and accuracy are limited.

Circuit 2 uses a valve to aspirate fluid from a reservoir and dispense it into many smaller volumes. This circuit uses a three-way valve to control fluid flow and direction. The valve provides a positive shut off to one leg of the circuit at a time. A three-way valve always has one leg open. When the valve is switching there is a brief period where all three ports are connected. If there is any residual pressure in the system it will result in unpredictable flow and adversely affect accuracy.

Circuit 3 uses separate two-way valves in place of the three-way valve. The advantages are greater control and hence greater accuracy. The inlet valve can be fully closed before the outlet valve is open. If there is residual pressure in the fluid reservoir, it is not transmitted to the outlet.