With a global increased emphasis on biotechnology and alternative, biologically-based alternative fuel development, more and more analytical work is being done utilizing fluids that contain biological material. whether the work involves blood cell counting or bacteria culturing (among many other life science applications), one commonality is often the need to transfer biological samples from one point to another. sometimes this is accomplished through manual means (e.g., with manual pipetting); however, high throughput is also a major driver that continues to push the development of more automated techniques.
The challenge many researchers face is how various pumping technologies negatively impact cellular material:
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Reciprocating Pumps – Two common styles of reciprocating pumps used in analytical-scale life science work are diaphragm pumps and piston / syringe pumps. Diaphragm pumps typically employ the use of check valves and a flexible diaphragm mounted to the motor shaft of a driving motor, and fluid moves in and out of this pump style through the “pulsing” action of the diaphragm. Piston / syringe pumps work through combining a positive displacement piston or a plunger with some type of rotary shear valve, with fluid movement occurring by the movement of the piston or plunger.
Both of these styles of reciprocating pumps create problems for cellular viability due to a moderately strong vacuum force and the shear forces to which the cells are exposed. These forces can cause cells to undergo lysis, thus dramatically reducing cellular viability and the opportunity to conduct longer-term testing. Additionally, these pumping technologies are more difficult to clean, increasing the possibility of sample carryover and cross-contamination. - Gear Pumps – Gear pumps work through the rapid rotation of two (or more) intermeshing gears. As the “driving” gear and the “driven” gear(s) interface with one another at high rotational speeds, fluid is moved along between the gears’ teeth. This style of pumping can create problems with biological samples due to the physical forces involved as the fluid transfers at high linear velocities. The gears’ teeth will often shear cellular material, rendering the sample or fluid being analyzed useless. Additionally, because fluid touches the mechanical portion of these pumps, it is difficult to avoid cross-contamination between samples.
One pumping technology that is finding increased popularity is Peristaltic Pumps. Peristaltic pumps transfer fluid using a series of rollers that facilitate the compression and expansion of soft-walled tubing. This pumping technology offers several advantages to help maintain cellular viability while also reducing sample-to-sample cross-contamination through several unique features:
- Low Vacuum Force – Peristaltic pumps typically work with soft-walled elastomeric tubing. This tubing is easily compressed but also returns to its initial shape quickly. Peristaltic pumps employ the use of rollers, which completely compress the flow path tubing as they rotate under a tubing bed. Once compressed, as the rollers continue their movement, the tubing expands back to its original shape, creating a low vacuum that helps pull fluid into the tubing before the next roller compresses the tubing once again. The small amount of vacuum created by the re-expansion of the tubing is sufficient to move fluid without typically damaging the cellular material.
- Low Shear Force – Peristaltic pumps maintain fairly consistent flow rates (except for the pulsation effect inherent in the pumps) and avoid having the fluid come into direct contact with the mechanical part of the pump. Both of these work to minimize the shear force the sample might experience and help lead to increased sample viability.
- Minimal Tubing Compression Points – Because the soft-walled peristaltic tubing is only fully compressed at finite points – with a majority of the tubing left open – the number of opportunities for biological material to become compressed and damaged is limited.
- Tubing-Only Flow Path – One of the unique design features of peristaltic pumps is that only the tubing comes into contact with the material being transferred – the material never comes into contact with the mechanical portion of the pump. This feature allows the tubing to be either cleaned and sterilized or replaced between analyses, virtually eliminating the possibility of sample-to-sample contamination.
An additional feature unique to most single-channel Ismatec® pumps is the actual design of the rollers and the tube “bed” against which the tubing is pressed by the rollers. While many pumps employ the use of flat surfaced rollers and tube beds, most of the Ismatec single-channel pumps have been designed with convex-shaped rollers and a tube bed with a variable level of concavity. As the rollers make initial contact with the tubing, only the center of the tube is compressed, allowing biological material to escape through the gap towards the tubing wall and avoid being damaged or destroyed. (See Figure 1 Below)
Figure 1 – Convex Rollers & Concave Tube Bed
Independent research has been conducted, comparing the impact of this roller / tube-bed design with other options, resulting in clear evidence of both increased cellular concentration (during incubation) and increased cellular viability. (See Charts 1 & 2 Below)
Chart 1 – Comparison of Cell Concentration Over Time
(black is “no pump”, red is Ismatec pump with convex rollers and concave tube bed, other colors are less-effective pump options)
Chart 2 – Comparison of Cell Viability
(black is “no pump”, red is Ismatec pump with convex rollers and concave tube bed, other colors are less-effective pump options)
There are disadvantages to peristaltic pumps that should be considered. Some of these disadvantages include the limited pressure differential against which the pumps may operate. Also, the tubing itself can create challenges, as the chemical compatibility of elastomeric tubing is not universal and the tubing can wear over time, creating flow inconsistencies and/or changes during its lifetime.
But perhaps most importantly, peristaltic pumps experience pulsation, a phenomenon inherent in their operation. Pulsatile flow can lead to effects such as “splashing” of fluid as it leaves the flow path tube as well as inconclusive real-time flow analysis due to constantly-changing flow rates inside the analysis chamber.
While these limitations to peristaltic pump technology can hinder the use of the technology in some applications, for many applications – especially those classified as life science where biological samples are involved – peristaltic pumps are the pumps of choice.
About the author: John Batts is a Technical Specialist and chromatography expert at IDEX Health & Science. He is also the author of the “All About Fittings Guide,” which you can download at idex-hs.com.