The peristaltic pump is an ideal method for moving liquids from one place to another. This type of pump has several distinct advantages over its counterparts, including the fact that pump machinery doesn’t touch the fluid-in-transport. Instead, a peristaltic pump keeps fluid contained in the transport tubing. Peristaltic pumps are self-priming, able to prevent backflow, and can be used to accurately dispense a specific amount of liquid. This all makes them the right tool for many medical uses involving blood and bodily floods.
By: Jeremy Cook
How Does a Peristaltic Pump Work?
Peristaltic pumps operate on the principle of peristalsis. In this context, peristalsis takes the form of compressing flexible tubing in a wave motion. This action pushes and sucks fluids through the resulting dynamically sealed chambers formed in the tubing.
The same principle is used in human digestive tracts, where muscles contract the esophagus muscles to push food from one’s mouth to the stomach. Natural peristalsis can also be observed in a worm’s motion, where its external form oscillates in the same manner to allow it to travel from point A to point B.
In the case of a mechanical peristaltic pump, a motor rotates rollers over a length of tubing fixed against a curved outer circumferential surface. This incrementally transports the liquid at a flow rate directly proportional to the motor speed. Other variables at play in peristaltic pump parts include the number of rotors and the circumference of the rotor position with respect to the motor shaft. A larger circumference means a greater flow rate for a given rotational speed.
Arrow offers a selection of peristaltic pumps, along with a wide range of motor control components and associated sensors appropriate for different pumping applications. Pictured below is a Gravity Digital pump, with built-in controls that allow it to behave like an RC servo. Motor speed and direction – thus flow speed/direction – are controlled via the specialized RC PWM form. To illustrate the peristaltic pumping principle, I partially disassembled this device, revealing the three rollers that are driven by the unit’s gearmotor:
Image: Jeremy Cook | Peristaltic pump head attached to geared DC motor. While dust particles and other contaminants may be present on or in a pump, it will not interact with the tubing interior and the fluid itself.
Image: Jeremy Cook | Peristaltic pump head disassembled, with inner roller retainer removed. In operation, rollers sequentially compress the flexible tubing to create peristalsis.
As these three rollers are turned by the central shaft, they sequentially compress the tubing, transporting liquid without any direct contact with the pump itself. Disassembly of this particular peristaltic pump head is a matter of pushing a few tabs. To test its operation, I hooked the pump up to an Arduino Uno to send pulses, based on a heavily modified implementation of the Arduino servo sweep example found here. It’s able to self-prime, as expected, and was able to cycle water back and forth from the reservoir to the smaller measurement vial.
Image: Jeremy Cook
Image: Screencap
Over time, peristaltic pump tubing wear is a disadvantage of this type of pump. Another is the nature of the flow that’s inherently pulsed and not a steady stream. Pumps are typically designed for easy access to the tubing, which may need to be changed due to wear or to match up with the substance that you’re transporting. For example, a dispenser meant to deal with acidic liquid will require much different tubing than one meant to deal with blood or other biological fluids for medical applications.
Peristaltic Pumps for Medical Applications
Just as your body uses natural peristaltic pumping for digestion, artificial peristaltic pumps can be appropriate for dealing with medical, pharmaceutical, and biological fluid transportation needs. External infusion pumps, in particular, are extremely useful to deliver necessary fluids and medications to a patient in a controlled manner, based on the rotation of the motor.
In other situations, such as cardiopulmonary bypass and dialysis, a peristaltic pump may be used to transport bodily fluids from and to the body in a closed loop. The use of peristaltic pumps for blood transportation was pioneered in 1932 by then medical student and later renowned cardiovascular surgeon, Michael DeBakey. This technique was initially used for blood transfusions, and later for cardiopulmonary bypass procedures.
In any medical situation, pump operation must be monitored to ensure the patient receives the correct dosage of drugs and that the proper flow rate of bodily fluids is maintained. Modern electronics controls can make this easier for the end-user, but proper precautions must be taken to ensure accurate and precise readings, equipment response, and user warnings to medical staff. While peristaltic pumps are typically reliable and low maintenance, the patient’s life is ultimately at stake, leaving no margin for error.
Powering a Peristaltic Pump
Peristaltic pumps may be driven by a variety of motors including AC, DC, BLDC, and stepper motors. Gearboxes can be used in conjunction with the driving motors to reduce motor speeds and increase torque to the rollers, at the cost of a lowered top speed and flow rate.
Motor controls can be as simple as using a relay or an H-bridge setup, or they can take advantage of solid-state motor drivers for precise speed control, potentially reducing noise and vibration (valuable in medical applications). Stepper motors are especially interesting for precise fluid dispensing, as the angular motion can be controlled directly via the appropriate driver. Depending on feedback needs, a motor’s rotation can be sensed directly, and/or the resulting fluid flow can also be measured. Desired flow rates, plus sensor inputs, can be combined by processing hardware to ensure optimum and precise fluid delivery.
Broadly speaking, peristaltic pumping systems need to be properly calibrated, correlating the motor’s rotation — and ultimately the movement of the rollers — to a measured fluid flow. Such calibration may need to be performed when switching liquids, as fluid viscosity can affect flow rates.
Peristaltic Pump: An Excellent Option for Medical Applications and More
Whether you need to pump blood or other biological fluids for a patient, control equipment for pharmaceutical manufacturing, or move liquids in a wide variety of other industries, peristaltic pumps provide an excellent means of performing fluid transport. While the pumps themselves are typically very low maintenance, engineers need to be aware of tubing selection and age, and to select the appropriate supporting electronics to get the most out of a system.
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