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Medicine’s Fluid Dynamics

By Louise Elliott

Just as the general use of CFD in industry has grown rapidly over thepast few years, its use in the biomedical sphere has also foundincreasing application. This biomed-specific growth is due, in part, toresearch aimed at a greater understanding of human physiology and thenature of circulatory and respiratory diseases as well as a desire tohelp design medical devices better and faster. The “big three” CFDdevelopers—Fluent, ANSYS CFX, and CD-adapco—all report increased use ofboth software and consulting services in the biomedical field.

Fluent has been involved in pharmaceutical delivery, medical devices,and physiological studies for eight or nine years, reports AhmadHaidari, global industry director for Fluent’s healthcare division. Hesays that the company’s earliest medical work involved heart valve andblood pump design, but that the field has widened a great deal.
“For example,” notes Haidari, “laser eye surgery raises heat-transferquestions. Should the laser pulse be long and not have too manyrepetitions, or short and more frequent for the safest results? And nowpeople are using CFD for electrophoresis and labs on a chip, wherefluids are moved in micro-channels under electromechanical charges.”

Haidari finds that one of the most intriguing areas for CFD use is thestudy of human physiology in conjunction with medical imaging. “Thegraphical resolution of medical imaging has become excellent, and onecan now see physiological flow and the interaction of medicine anddevices on the body.” When the physician understands the extent of aproblem such as an aneurism by viewing a medical image, he can choosewhether to correct it with a stent, where to put it, and how to installit.
The same principles apply to respiratory system studies, Haidari says.Understanding what happens to airflow in the back of the mouth, forinstance, aids inhaler design for drug delivery. But the fastest growthhe has seen is in medical imaging. “If they have an MRI or CAT scan,users can save the image as an STL file, create an analysis domain fromthat, grid, and solve.”

Chris Reid, vice president and general manager of the fluids businessunit for ANSYS, reports growing use of CFX for heart, circulatory, andrespiratory system product development—such items as stents, bloodpumps, and inhalers. “Blood flow itself is a multiphase flow problem,”Reid says. “Modeling this has become easier with today’s software. Wecan model multiphase, nonlinear behavior in arterial walls withsoftware for fluid-structural interaction (FSI). An importantapplication area for FSI is medicine delivery in the respiratory tract.Designs for asthmatics require modeling the whole respiratory system.”

Reid says that CFD offers ways to model the behavior of a variety ofmedicine delivery methods. “For example, CFD can track particles tostudy respiratory drugs in the air flow,” he says. “Users can work withdifferent velocities, first in steady state, and then see what happensin the first few critical seconds after activating an asthma inhaler.It’s a complicated problem, but extensive tools now exist to simulatesuch problems.”

Materials present challenges in the medical arena, says Dennis Nagy,vice president of marketing and business development for CD-adapco.”Medical equipment by itself is similar in terms of analysis to otherequipment. The physiological issues, however, are more challenging. Ablood pump is still a pump, but the material properties of blood aredifferent from most fluids, and the application is more critical.”
Haidari of Fluent agrees. “It’s important to obtain accurateinformation about sheer fields in blood flow, for example, wherehigh-velocity flow causes a flow gradient in a product such as stent,or detects parts of the stent where blood flow may stop, such as goingaround a bend. CFD shows which regions are being sheered and at whatrate, and how that affects circulation.”

The material properties for tissue, Nagy says, are challenging, andCD-adapco has seen the use of software and consulting grow in thatarea. “Consulting—that is, running simulations for customers—accountsfor about 21 percent of our total business. Many such projects are inthe medical world, because typically medical equipment is developed bysmaller companies, or by small departments with small engineeringteams, in larger companies.”

He points out that CFD is being used extensively for research. “Oneresearcher with whom we work is studying smoke deposited in the lungsof smokers—to find out what the effects are on lung tissue, howparticles are dispersed or deposited, and what gets out of the lungs.”

Hengchu Cao is the manager of CAE for heart valves at EdwardsLifesciences, a medical equipment company in Irvine, CA, and a user ofSTAR-CD. He says, “CFD has really taken off with the availability ofhigh-performance computing—especially for problems with 3D complexityand needing 3D mesh. It helps as well to have advanced solvers that canmodel viscous flow of fluids like blood.”

He sees CFD in use by engineers dealing with ventometers, heart valves,catheters, a variety of diagnostic tools, as well as drug delivery anddiffusion in the cardiovascular system. “It is possible to study theseproducts with experimental methods such as ultrasound. However, theresolution of the instrumentation is limited, though improving. Thereal problem is that physical testing doesn’t enable what-if studies,where the performance of devices can be simulated in normal andabnormal physiology. With simulation, we can modify the physiology andstudy many phenomena that may occur in use. It enables true designoptimization.”

Dr. Rupak Banerjee, associate professor of biomedical engineering atthe University of Cincinnati, uses Fluent. He has performed studies ofthe cardiovascular system, drug delivery—with an emphasis on thermaland ultrasound, ablation, ocular, and spinal applications. “A majorbenefit of CFD simulation in biomedical studies is that it makes itpossible to reduce the number of animal experiments.”

He finds that it also assists in making treatments safer and moreprecise. “Such simulations help us to customize devices and make thempatient-specific. If a device needs to be implanted in a patient, itssize and placement can be worked out carefully in advance ofoperating.” In addition, CFD studies also make it possible to work outtherapies on a patient-specific basis. “If a physician wants to delivermedication to a tumor, for example, he can target the deliverydifferently from patient to patient,” says Banerjee.
Dan White is a senior CFD application engineer working for HeartWareInc. of Miramar, FL, and is a user of ANSYS CFX. He finds a very realadvantage of CFD simulation in reduction of design time. “Prototypesand physical testing can be cut down to just a few iterations. Andunlike physical testing, CFD lets engineers see every part of themachine—including inside, which we can’t do with physical testing.”

He adds that he finds CFD invaluable in any problem that requireslooking at flow. “In complex products with flow, CFD offers the bestreturn on investment.”

Reid of ANSYS points out, “Anywhere you have high-performance equipmentand health issues, you can run into heat-transfer issues. Theapplications for CFD in medicine are limitless.”

Contributing Editor Louise Elliott is a freelance writer based inCalifornia. Offer Louise your feedback on this article by clickinghere. Please reference “CFD and Biomed, February 2006″ in yourmessage.

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