Vital to the performance of a dialysis machine, the peristaltic pump controls the blood flow. Prevalent in the healthcare industry, peristaltic pumps are used in many critical applications such as dialysis, enteral feeding, and infusion therapy to safely and effectively transfer critical fluids to the patient. Within the pump, a tube is pinched closed or occluded by rollers, creating a pressurized flow that moves fluids through the tubing without risk of contamination. As the fluids only come into contact with the tubing, material choice is extremely important to peristaltic pump performance and patient safety.
In addition to being made of medical-grade, biocompatible material, peristaltic pump tubing must demonstrate elasticity in order to allow the ongoing occlusion necessary to move vital fluids. Typical elastomers used include silicone, polyvinyl chloride, ethylene propylene diene monomer, thermoplastic vulcanizates and polyurethane. When it comes to patient safety, nothing can be left to chance. With this in mind, there are several advanced modeling techniques emerging that can aid medical device manufacturers in determining optimal pump performance at the design phase—well before the device comes off the production line and is put into service, where the stakes are exponentially higher. Using this technology can help manufacturers to specify the right high-performance material and cost-effectively design enhanced medical devices.
Measuring Cause and Effect
As its primary function is to move liquid effectively, the most important facet of peristaltic pump operation is flow rate, or the amount of fluid that flows over a given time. In healthcare applications, the accuracy of flow rate allows medical care providers to precisely measure the amount of fluid dispensed or circulated. This can mean the difference between life and death when dispensing medication to a patient through an IV or circulating blood during dialysis. While traditional flow rate tests require measuring the volume of liquid over a given period of time, the results of these measurements only provide an average volumetric flow rate. However, there is no indication as to the impact of individual variables on the flow rate—variables such as pump design, fluid type and viscosity, tubing material and dimensions, occlusion percentage, and lastly, flow rate consistency over time.
In order to fully understand the various factors affecting flow rate accuracy, one must identify the interaction between a flexible solid, such as peristaltic pump tubing, and the fluid running through it—also known as fluid-structure interaction (FSI). Computer simulation of peristaltic pump tubing is widely considered a difficult FSI problem because there is a strong interaction between fluids and solid tubing that can be hard to predict, and liquid topography changes due to occlusion—pinching of the tube—making it extremely challenging to create an accurate model simulation. As a major consideration in medical device engineering, however, manufacturers and suppliers are attempting to better understand this complex cause and effect relationship between the peristaltic pump tubing and the fluid in order to design safe and effective medical devices.
Previous Performance Indicators
In the past, the most common way to determine peristaltic pump performance was to develop and test a prototype of the finished product and hope for the best. While ultimately an effective method, it is also a time-consuming and costly one when performance does not match expectations or safety standards. Advances in computing capability and modeling programs ultimately led to the use of computer simulation. Using available technology, computer-aided engineering (CAE) software programs were developed in an attempt to predict performance based on the creation of virtual medical devices.
However, conventional computational fluid dynamics programs use grids to discretize space, presenting some limitations when it comes to working with a solid-liquid interface, as in an integrated peristaltic pump system. The finite volume method, for example, requires constant re-meshing of the liquid domain with any geometry evolution, making it an ineffective option to measure the relationship between liquid (fluid being pumped) and solid (tubing), and its effect on flow rate accuracy.
Meanwhile, more recent coupled Eulerian-Langarian methods available in high-end finite element analysis (FEA) programs are unable to resolve the solid-liquid interface at or near the pinch due to its averaged nature in describing the FSI interaction. In fact, nearly all existing CAE simulations of peristaltic pumps are limited to non-pinching situations, making them less than ideal for peristaltic pump modeling.
Innovations in Pump Modeling
The good news is that innovations in CAE technology allow for better modeling systems that can create a virtual peristaltic pump, factoring in the elements that affect flow rate to accurately determine future performance. One example is called smoothed particle hydrodynamics (SPH). Using this technology, manufacturers are able to run a battery of tests using different tubing materials and dimensions without having to develop costly prototypes, allowing for the design and manufacture of peristaltic pumps with enhanced safety and performance, increasing speed to market with reduced investment.
Originally developed for astrophysics applications but recently gaining popularity with mechanical engineers, SPH uses meshless discretization for virtual modeling. SPH allows for the digital representation of space using a collection of points and their influencing domains, versus the conventional grid method. Unlike other commonly used discretization methods, SPH represents space with points that can move with materials, such as polymers or liquids, a process called the Lagrangian method (vs. Eulerian method typically used in simulating fluid, in which the material flows through a stationary domain represented by grids). For example, during tubing simulations, SPH can handle the extreme deformation of the material, common during liquid flow, while maintaining accurate solid-liquid interface resolution.
This advanced virtual model can then provide the occlusion (OC) percentage, or the percentage of tube that gets pinched beyond its nominal wall thickness (because pump tubings are produced from elastomeric materials, the tubing walls will displace under compression). This is vital in a peristaltic pump, as a significant OC percentage is necessary to ensure consistent flow rate and prevent back flow. It is widely known that the tubing material properties and geometry have a major impact on occlusion for a given pump design. Using the input from stress strain curves, a pump model accurately can predict the deformation of tube walls during pumping and therefore predict the impact of the tubing material and geometry on flow rate.
More accurate performance predictions using SPH with full occlusion provides a better understanding of liquid-solid interaction cause and effect within an integrated pump system. This, in turn, allows manufacturers to design enhanced peristaltic pumps with greater flow rate accuracy for increased performance and patient safety.
One example of how SPH may reveal additional variables that affect flow rate can be seen by examining the simple physical properties of a silicone tube in the context of a pump model. Initial findings from research using SPH, conducted by Saint-Gobain Performance Plastics’ numerical modeling group, suggest the common perception that flow rate would increase if the internal diameter (ID) was expanded is not necessarily the case. Rather, testing revealed that increasing ID of a silicone tube while decreasing wall thickness might result in reduced flow rate, presumably because the reduced wall thickness causes the reduction in the occlusion and the duration in which the pump can maintain the occlusion. While further testing is needed to fully understand how this could impact pump or tubing design, it provides early insight how the simulation tools might help pump and tube manufacturers better integrate the individual components into a complete system.
The Bottom Line
Advanced medical devices using peristaltic pumps, such as dialysis machines, can be life-saving. Flow-rate accuracy is the key to peristaltic pump performance and overall patient safety. However, in order to design peristaltic pumps with precise flow rate accuracy, design engineers must first understand the correlation between fluids and solids seen between pump tubing and medical fluids. It is vital that peristaltic pump tubing be able to withstand multiple occlusions over its life cycle. Rather than invest time and money to develop and test different components using a variety of materials and design specs, as pump modeling innovations continue, manufacturers can use CAE software programs to predict performance based on discretized models. While the fluid-structure interface presents problems for computer simulations using grid-based virtual space representation, other advanced methods now exist. Innovative meshless discretization, such as SPH, enables the coordinates of a virtual device to move with liquid and flexible solids, resulting in accurate performance predictions. This use of innovative pump modeling software provides unique insights into the operation of peristaltic pumping and allows for the development of optimally safe and effective peristaltic pumps while making the most effective use of manufacturers’ resources.
Robert D. Schwenker is a business manager in the Healthcare Markets unit of Saint-Gobain Performance Plastics, based in Austin, Texas. Schwenker has a deep understanding of materials and fluid systems used throughout the medical device industry with an emphasis on silicone extrusion and molded components. He earned a degree in Chemical Engineering from Cornell University and an MBA from the University of Texas. He has worked in various locations and business units within Saint-Gobain Performance Plastics. For the last 10 years, he has been an integral part of the company’s Healthcare Markets group in both new product development and business management roles. Aaron Updegrove is a marketing manager in Saint-Gobain’s healthcare unit, based out of Portage, Wis. With more than 18 years of experience in sales and marketing for companies that produce engineered materials and components, he brings a strong understanding of how those materials impact the performance of end-use applications. Updegrove earned a Mechanical Engineering degree from Marquette University. Since 2009, Updegrove has supported the Healthcare Markets group in both sales and marketing roles.