Key Considerations for Selecting a Medical Pump

Newcomers to medical device design may think that pressure and flow rate are sufficient parameters to consider when it comes to selecting a pump. While this may be the case in some industrial applications, medical device requirements are a bit more demanding. A thorough review of the selection criteria for a medical pump is required to ensure the correct pump is chosen for every application.

CONTINUOUS VS RECIPROCATING PUMPS

The most common pump used in medical instruments is the positive displacement pump, which physically displaces a volume of fluid to generate pressure. Some positive displacement pumps such as gear pumps and peristaltic pumps provide continuous flow, whereas syringe and diaphragm pumps (also known as reciprocating pumps) provide non-continuous flow. Because reciprocating pumps produce discrete dispenses of fluid, aspiration phases between dispenses are necessary.

Both continuous and reciprocating pumps offer advantages for medical instrumentation applications. Continuous pumps are best suited for applications requiring flow over extended periods of time. They usually produce pulsatile flow due to the nature of their mechanical design. Increasing downstream restriction may also impact the volumes delivered.

Reciprocating pumps, on the other hand, aspirate and dispense fluid over relatively short periods of time. They are known for their high-precision volumetric dispenses, rather than sustained flow rates. Reciprocating pumps include both fixed volume solenoid pumps and variable volume piston pumps. The term “fixed volume” indicates that these types of pumps aspirate a predetermined volume and dispense the same amount, whereas variable volume piston pumps can aspirate and dispense
volumes from nearly zero to their full stoke volume. For example, a 250 µL pump could aspirate 250 µL and then make five individual 50 µL dispenses, or ten 25 µL dispenses, and so on. Of course, the volume of the dispense or aspirate is limited by the volume of the pump (upper limit) and the resolution of the pump (lower limit).

With reciprocating piston and syringe pumps, the displaced volume is based solely on piston movement. Changes in downstream restrictions will not influence the volume dispensed because the delivery pressure will vary to meet the dispense speed until the system delivers the target volume.

As downstream restriction increases, the pressure generated by the pump will increase exponentially to meet the same flow rate. In some cases, the high pressure can lead to valves cracking open, fittings leaking, and the pump motor stalling. To avoid these failures, it is important to understand the restrictions and pressure ratings of the system surrounding the pump.