Watson-Marlow Bredel’s 400 F/A matchbox-size peristaltic pumps are smaller and more cost effective than syringe pumps, enabling the designer to place them anywhere on an instrument, saving them space and reducing costs.

In the realm of medical motors, pumping is one of the more common applications. Because of the need for sealing and or motor isolation, and the variety of pumping methods available (see sidebar, Pumping Technologies), pumps are typically sold as packages, with the appropriate motor matched to the appropriate pump head.

To specify the right pump, designers need to be familiar with the different approaches available so that they can match the package to their needs. It also follows that they must thoroughly understand those needs beforehand (see sidebar Specifying Pumps).

Most pump suppliers sell to multiple markets, developing an array of products to serve different flow and power requirements, along with specific space constraints. The latter issue is becoming increasingly important in the medical arena as devices become more complex, but even more so due to the need for portability. Medical equipment frequently must be moved from one room to another, often on wheels, but sometimes hand carried. This creates a demand for compact devices, and, subsequently, compact components that go inside.

One pump supplier providing solutions to that challenge is Micropump of Vancouver, Wash., which offers micro annular gear pumps that measure as small as 13 mm x 68 mm. The units come in a low-pressure (Series ML) or high-performance (Series MH) configuration. Both feature high-precision rotors that provide tight flow rate control, even at differential pressures as high as 80 bar (1,160 psi). These rotors allow the pumps to dispense volumes as small as 0.25 microliters and handle flow rates from 0.15 ml/min to 300 ml/min, with accuracies within 1 percent.

The ability to custom manufacture a compact system helped the company when working on a pumping system that needed to be small enough to fit into a fully automated microbe detector. The detector tests for total microbial count, as well as pathogen specific detection, and allows users hands-free operation of up to 36 samples.

Micropump’s efforts focused on the sheath flow system, which is an adjustable flow rate delivery directly into the sample cell. Sheath flow serves to “pull” the individual microbes off the needle in single file, creating the sample core. The sample core passes through a laser beam where individual microbes absorb, scatter or transmit the laser light.

This application required that there be no cross contamination or environmental contamination of the sample.

Micropump offers a range of pumps, drives, controllers and accessories. Technologies include magnetically driven, miniature gear pumps; micro-annular pumps; multiple and single piston pumps; peristaltic pumps; vane and centrifugal pumps.

Because the Air Squared scroll compressors use true rotary motion, they can be dynamically balanced for nearly vibration-free operation. Air delivery is continuous, which virtually eliminates inlet or discharge pulsation and associated noise.

An increasingly popular pump for OEMs is the company’s new I-Drive electro-magnetically driven pump. The pumps are magnetically coupled and static sealed, not dynamically sealed, so they are leak free.

The I-Drive is also pulse-less; it uses gear tooth forming technology that keeps pulsations to a minimum to provide smooth, constant flow. It combines a brushless DC motor and an electronic controller into a single, unit and when integrated with a pumpheads delivers a pulse-less flow at a rate of up to 3.2 L/min. The pulse-less pumps are good for metering applications. The lack of pulsation delivers higher accuracy and more uniform, smooth, continuous flow that cannot be achieved by a reciprocating piston pump, says Michelle Havist, marketing coordinator for Micropump.

One of the more established means to eliminate contamination is the use of peristaltic pumps, such as those sold by Watson-Marlow Bredel, Wilmington, Mass. The company designs and develops peristaltic pumps for a wide variety of industries, including medical equipment, foodservice appliances, testing equipment, and machines for industrial processes.

Peristaltic pumps offer complete isolation between the pumping mechanism and the fluid. This type of pump head uses a series of rotating rollers to push fluid through flexible plastic tubing. The rollers squeeze the tubing and push the enclosed material forward. During operation there is at least one of the rollers in contact with the tubing at all times that eliminates the need for valves to prevent backflow.

The pumpheads are designed so that the tubing is easily inserted and removed for cleaning or replacement.

Another advantage to this pumping method is that its low-shear action reduces degradation of the fluid material inside the tubing. Many other pumping methods with high velocities or arduous flow paths can damage the fluid material by interactions with vanes, impellers, lobes, etc.

Scrolling along

Enlarge this picture The four stages of scroll compression in the Air Squared compressor. Source: Air Squared

Fluids aren’t the only materials moved by pumps in medicine or in industry. Often the medium that needs to be transported is a gas or simply air. In cases where the air or gas is compressed, the unit is considered a compressor.

Robert Shaffer, president of Air Squared, a manufacturer in Broomfield, Colo., has developed pump and compressor applications for a variety of industries, but one of his most well known applications was in the medical arena, however.

Because pumps play such a vital role in the quality of life of patients, Shaffer says that the devices have to be extremely accurate, with low noise, low vibration and long life. Shaffer points to the Inogen One, a portable oxygen concentrator from Inogen Corp., Goleta, Calif., as an example.

The unit, which recently won a medical design award, weighs less than 10 lbs. total and replaces traditional bulky stationary oxygen concentrators or cylinders. It nearly triples the time respiratory patients requiring supplemental oxygen can ambulate freely on battery power alone.

The scroll compressor delivers more than 80 percent volumetric efficiency, while drawing less than 40 W, and operates in the acoustic range below 40 dBA. The operating element of the scroll compressor is made up of two identical involutes that form right- and left-hand components. One scroll is indexed or phased 180 degrees with respect to the other to allow the scrolls to mesh. This indexing creates crescent shaped gas pockets, bounded by the involutes and base plates of both scrolls.

Micropump’s micro annular gear pumps measure as small as 13 mm by 68 mm and come in low pressure or high performance series’. They dispense volumes as small as 0.25 microliters and handle flow rates from 0.15 to 300 ml/min, with accuracies within +/- 1 percent.

While the compressor is in operation, one scroll remains fixed; the other is attached to an eccentric that is driven by an electric motor. As the moving scroll orbits around the fixed scroll, the pockets formed by the meshed scrolls follow the spiral toward the center and diminish in size. The compressor inlet is at the periphery of the scrolls. The entering gas is trapped in two diametrically opposed gas pockets and compressed as the pockets move toward the center, where the discharge port is located in the fixed scroll. No valves are needed because the discharge port is isolated from the inlet. This reduces noise and improves the durability of the unit.

Its pulse-dose delivery system delivers nine flow settings, varying the speed of the compressor from 1,500 rpm to 2,700 rpm to meet demand. The lithium-ion battery provides two to three hours of use on a 3-hour recharge. The compressor weighs 2 lbs., including its motor, and measures 4.5 in. x 3 in. x 3.3 in.

Other applications

The two-stage scroll vacuum pump from Air Squared features a miniaturized profile for use on portable medical and laboratory equipment.

A pump technology suitable for medical equipment may also be a good fit for other applications. Take, for example, the WOB-L piston pump technology developed by Thomas Products, Sheboygan, Wis.

The company’s 2688 WOB-L piston pump is used in everything from oxygen concentrators and blood analyzers to mail sorters and photocopiers. WOB-L piston pumps work by reduction of a volume by a fixed-crank driven piston within a cylinder. Single-stage and multi-stage models are available with flow rates of up to 300 l/min, vacuum down to 10 mbar, and pressure up to 30 bar. The WOB-L principle has a unitary piston and piston rod whereby the piston wobbles inside the cylinder as the crankshaft rotates.

The company says that the WOB-L piston and piston rod are a single item, usually a single casting. The piston rod is mounted to an eccentric bearing assembly, which in turn is mounted to the motor shaft to convert rotary energy from the motor into linear motion of the piston within the cylinder. The piston is sealed to the cylinder by a flanged disk cup, which forms both a seal and mechanical guide for the piston and which runs without lubrication in contact with a low-friction, surface-coated cylinder with high thermal conductivity. As the piston is driven up and down, air resistance on the upward stroke expands the disk’s seal on the piston against the cylinder wall to increase its efficiency, while compensating for the wobble action.

One of its inherent benefits is that its fails by gradually wear-out, rather than a experiencing a catastrophic failure mode. Long and useful life is a benefit that all pumps must have no matter the application. Although customers may come from a variety of industries and applications, they all share a common goal. As Thomas Pumps puts it, the customers want a solution that “improves the performance of their product while delivering extreme value.”

Specifying Pumps

To prepare for specifying the appropriate pump, product designers should first collect the following information:

• Specific and unique characteristics of the application.
• Size of the pump needed.
• Target flow rate in terms of both pressure and volume.
• Requirements for torque, speed and duty cycle.
• Available power supply voltage and current source.
• Available space inside the application, both from maximum diameter and length.
• Degree of shaft loading from a radial and axial loading perspective.
• Number of prototypes required and in what time frame.
• Target quantity required once in full production.
• Target cost for the pumping system.

Typical Medical Applications for Pumps

• Hemodialysis (acute and home care): Deaerating Pump, Transmembrane Pressure Pump.
• Microbe Detection: Sheath flow delivery.
• Surgical Instrument: Cooling and flushing .
• Laser Cooling: Heat transfer fluid circulation.
• X-ray Cooling: Heat transfer fluid circulation.
• Thermal Therapy.
• Patient Cleansing and Flushing.
• Equipment Sterilization.
• Blood & Urine Analyzer: Sample aspiration, dispense and wash.
• Immunoassay Analyzer: Wash and flush.
• Ultrasound Interface: Liquid delivery.
• Lipid Removal.

Source: Micropump Inc., Vancouver, Wash.

Pump Technologies

External-Gear Pumps
Consists of two or more rotating gears which mesh together. One of the gears is turned by a power source and drives the other gear(s). The spaces between the gear teeth carry the fluid from the inlet to the outlet. The gear mesh point prevents the fluid from returning to the inlet. Gear pumps are available in cavity and suction shoe styles, both of which provide pulseless delivery.

Micro-Annular Gear Pumps
Are rotary pumps that are constructed with an externally toothed internal rotor as well as with an annular tooth external rotor, which bear slightly eccentric to each other. During rotation of the rotors around their offset axis, the pumping chambers simultaneously increase on the induction side and decrease on the delivery side of the pump. A homogeneous flow rate is generated between the kidney-shaped inlet and outlet.

Peristaltic Pumps
Tubing is used as the pump chamber and may be single or multichanneled. The fluid confined in the tubing is displaced when two or more rollers squeeze the tubing against the walls of the pump housing. Changing the tubing size or ther rotor speed will vary the flow rate. Maximum pressure capability is limited to the tubing.

Multiple-Piston and Single-Piston Pumps
Multiple and single piston pumps use piston like rotor elements supported from swash plates inset into the pump endplate to move fluid through the pump.

Sliding-Vane Pumps
Vanes are contained in a rotor, rotating i n an eccentric housing. The fluid is trapped between the vanes and housing and is swept through the pump. It is similar to a flexible impeller pump, except the impeller blades or vanes are made of a rigid material.

Centrifugal Pumps
Consists of an impeller rotating within a casing. Liquid directed into the center of the rotating impeller is picked up by the impeller vanes and accelerated to a high velocity. When the liquid in the impeller is forced away from the center of the impeller, a reduced pressure is produced and consequently more liquid flows forward. There is no closed volume, like in a positive displacement pump, therefore, a steady flow through the impeller is produced. The pump basically increases the Bernoulli head of the flow between the eye and the exit of the pump.

Source: Micropump Inc., Vancouver, Wash.