Pumps Under Pressure

9 December 2008 (Last Updated December 9th, 2008 18:30)

It may seem like a small part of a device, but choosing a suitable pump should always be of high importance. Without it, a device could fail and cause further risks for the patient. Nic Paton talks to Doug Wilkerson of DLW Consulting about exactly what manufacturers need to consider.

Pumps Under Pressure

Imagine the scenario. You're a manufacturer who has spent countless hours and substantial investment developing a new, state-of-the-art steriliser. It goes out to market but, rather than plaudits and contented customers, the feedback you're getting is that it's just not working as effectively or consistently as it should.

The problem, as you discover after many late nights and much pulling out of hair, is that you selected a pump with a maximum pressure of 43psi, which also happens to be the minimum pressure needed for your steriliser to function. This is all well and good until the pump's pressure drops below 43psi for some reason, preventing the steriliser from functioning.

"Medical designers will often purchase a pump for their product late in the design process and simply go for an off-the-shelf product."

This situation is sadly not that uncommon. While the past ten years have seen a raft of major technological breakthroughs in the design of pumps and pumping devices, engineers are often failing to keep abreast of changes and advances. Even worse, medical designers will often purchase a pump for their product late in the design process and simply go for an off-the-shelf product, rather than properly defining the operating parameters and requirements of their product.

Doug Wilkerson, principal at Illinois medical devices consultancy DLW Consulting, thinks it is imperative to decide on a pump earlier in the design process. "It could be disastrous to wait until the last minute [to choose] the pump because there are now so many different types and options available." he explains. "It is a bit like trying to have a human body without the heart."

Recent pump applications

The first thing designers need to recognise is the sheer range and variability of medical pumps on the market today, each offering its own advantages for particular applications.

The three most common types of pumps are diaphragm pumps, peristaltic pumps and linear pumps. Diaphragm pumps use inlet and outlet valves to create pressure or a vacuum and are primarily used for products such as blood analysers or life-support systems. Peristaltic pumps flex a tube, often sterilised, to induce liquid flow within a system, and are commonly used within blood transfusion devices or automatic liquid-feeding devices.

Linear pumps use linear displacement, often magnetic or pneumatic, to move the pump's diaphragm. They will normally operate very quietly and so are popular with in-room patient care equipment, such as nebulisers and automatic drug-delivery systems.

"It could be disastrous to wait until the last minute [to choose] the pump because there are now so many different types and options available."

Within this basic technology, cutting-edge developments when it comes to pump development include increased miniaturisation, more remote and wireless technology, improved portability and reduced noise. Examples of some of this latest R&D includes researchers at the University of Maryland, who in September 2008 succeeded in creating an array of tiny pumps, motors and turbines that could in time be developed for use with implantable medical devices.

In 2006, engineers at the University of Utah developed a micropump designed to move chemicals, blood or other samples through a card-sized "lab-on-a-chip" medical laboratory. This, they argued, could eventually have applications including delivering pain medication or other drugs through devices attached to the skin.

In the cardiac arena, scientists are currently testing a new generation of heart pumps designed to act as a permanent alternative to transplants for patients with congestive heart failure. An example of this is the VentrAssist made by Australian firm Ventricor. This left ventricular assist device has a single moving part, an impeller, and at just 10oz is ideal for use on children.

Another left ventricular assist device, the HeartMate made by Thoratec, was recently approved by the US FDA, which also approved the first pump to provide temporary support for patients suffering severe right-side heart failure.

In May 2008, researchers at the Fraunhofer Institute in Germany developed an innovative mini-peristaltic pump system. Micro-pump systems sometimes work only in one direction, bubbles can appear in the liquid, they can get clogged with small particles and have a limited output. The institute's system got round this by using lead-zirconate-titanate films, bending elements made of carbon-fibre-reinforced plastic plus a flexible tube that can change shape via the application of an electric field. This enables tiny quantities to be pumped accurately through the system.

Portable yet powerful

The growth of non-invasive and minimally invasive surgical procedures has created a greater requirement for more outpatient ambulatory care. This has led firms such as North Carolina's Hargraves Technology Corporation to develop new miniature and portable diaphragm pumps, in particular using solenoid valves.

"The growth of non-invasive and minimally invasive surgical procedures has created a greater requirement for more outpatient ambulatory care."

Manufacturers and designers have spent a lot of time and effort developing different types of pumps for different solutions, points out DLW's Wilkerson. Pumps will always be specified so that they work with specific densities of solution, pressures and operating temperatures. "The problem for device manufacturers is not that they are not aware of new technologies that are coming in that could make things better, but that cost and regulatory concerns can sometimes stand in the way of that organisation making that change," he explains. "There are always trade-offs that have to be made.

Another problem is that it would be almost impossible for vendors to specify something in such detail that manufacturers can determine its suitability just by looking." It is therefore critical that designers specify as far as they can the pumping system's tolerance to various performance requirements, including electrical power, temperature, flow rate, vacuum and pressure.

Pump failures are usually not the fault of the pumping system, but of the designers failing to communicate complete system requirements to the pump supplier. Working closely together, designers and suppliers can select the right pump to match a system's requirements.

"You have got to have a true understanding of your requirements," says Wilkerson. "Then there are also fundamental things to consider, such as whether it can be invasive or whether it will need to be non-invasive. Cost will obviously be a factor. Beyond this, there is also the question of the environment that the pump is going to be operating in. So, for example, whether it will be in operation outside or what sort of temperature it is going to operating in.

"Then, increasingly, designers and manufacturers look at the size of the pump and, finally, what does this do to the environment around it, so what amount of heat does it give off, how much electrical noise and so on. But what sometimes happens is that a pump comes on to the market and one customer proves it works well in a certain situation and then it just gets wrapped into that situation."

Testing for a care setting

Along with portability, this issue of the care environment in which the pump will be operating is fast becoming much more central for designers. For example, the SC920 series vacuum pump system from KNF Neuberger makes a point in its marketing of its "extremely quiet operation" and wireless remote control.

"Along with portability, this issue of the care environment in which the pump will be operating is fast becoming much more central for designers."

One of the difficult things for most engineers, given the day-to-day pressures they are working under and the focus they have to have on the project in front of them, is what Wilkerson describes as "taking the time to read to the horizon". But it is important that they try.

While organisations such as the FDA and the International Organisation for Standardisation (ISO) do publish regular reports and updates on products, there is certainly a need for more, good-quality information out there.

"One thing therefore that I think would be beneficial would be to have white papers for engineers on how good pump x is for, say, pumping blood is a high heat environment," Wilkerson adds. "Engineers would love those sorts of things.

"It is also important to understand regulatory requirements when choosing a pump. A good project team nowadays should also have someone assigned to regulatory issues.

"Sometimes it seems like everyone has already thought of everything that needs to be made in this area. But it is also inevitable that someone will wake up every day with another way to make a mousetrap."