DESIGN : Coronary Angioplasty Balloon Catheters: Designing for the Real World

Medical Device & Diagnostic Industry
| MDDI Article Index

Originally published January 1996

Robert Ndondo-Lay

Catheter design engineers in the medical device industry face some compelling questions and challenges. The catheter industry has achieved unprecedented successes, and yet it is experiencing some stagnation. The industry is chasing a moving target. Namely, designers have been unable to create a product that prevents restenosis, the recurrence of stenosis after a corrective procedure. Developing a product that can solve this problem is the target of catheter designers because such a product is what physicians are seeking. Compounding the challenge of achieving this elusive target are design requirements that cause a lengthy list of delays.

Medical device design engineers use a traditional design method (formalized by NASA during the early 1960s) called phased project planning. A task is divided into four phases: defining the product, testing the product's feasibility, building prototypes, and initiating pilot production. Much like a relay race, the baton of responsibility passes from product planners to designers to manufacturers and finally to marketers. This is still a viable design method. Most of the time, however, designers start to work before feasibility testing is completed. Manufacturing and marketing begin gearing up well before the design is finished. And each step presents a variety of challenges.

By looking at the existing process, the obstacles and challenges that confront the designers become more apparent, and some lessons can be drawn that enable companies to streamline their processes and maximize the potential marketability of their products.


In an industry sector influenced by ongoing technological change, particularly in percutaneous transluminal coronary angioplasty (PTCA), design engineers wrestle with new concepts and approaches through trial and error. In the case of designing a PTCA catheter, it seems as though it were only yesterday that Andreas Gruentzig demonstrated a new and radical approach to myocardial revascularization. First introduced in the late 1970s, his device was a double-lumen dilatation catheter with an inflatable, nondistensible oblong balloon that would inflate to a predetermined diameter. The device applied pressure circumferentially against a stenosis, with a clinical result that allowed blood to flow within the vessel. This year, this technology will be used to treat approximately 300,000 patients in the United States alone.

More than a dozen companies are now involved in some phase of designing, producing, or marketing PTCA devices for treating coronary disease. These companies offer new balloon technologies including ultra-low-profile systems, perfusion balloons, long balloons, and high-pressure balloons. Today, interventional cardiologists can choose from many PTCA catheters to dilate multivessel, distal, tortuous, and calcified left main lesions and lesions at bifurcations and trifurcations. PTCA catheters can also treat high-risk patients with poor ejection fractions and low cardiac outputs.


Competition among catheter manufacturers continues to increase as new contenders enter the market. Rod Canion, Compaq's co-founder and CEO, says, "Developing a new product is like jumping out of an airplane. One way or another you are going to get to the ground, so you'd better be sure your parachute works." In balloon angioplasty, the parachute--the balloon catheter--has been and still is working well. Now, however, many jumpers are using parachutes of the same design but in different colors. The color of the shaft has become the primary variant, and it is becoming more difficult to distinguish substantively one manufacturer's balloon catheter from another's.

Balloon catheters are divided into fixed-wire, over-the-wire, and monorail designs. Although small shaft size has been addressed by fixed-wire and monorail designs, during the past year companies have introduced wire balloon catheters that combine small shaft size, axial rigidity (promoting pushability), and lateral flexibility (promoting trackability). Because rigidity and flexibility are essential characteristics to the functionality of the catheter, the material became the most important modifiable ingredient in catheter design. Catheter material must be stiff enough to maintain structural and functional integrity and flexible enough to minimize discomfort and the chance of injury. Therefore, material selection becomes a key function of design engineers. A broad range of technical properties (modulus of elasticity, apparent flexural modulus, and Shore A hardness) from thousands of resins are commercially available. Although catheter manufacturers claim sole proprietary formulation of their respective resins, all catheters are made up of one of four materials: polyvinyl chloride (PVC), polyethylene (PE), polyolefin copolymer (POC), or polyethylene terephthalate (PET).


In many instances, catheter stiffness has been evaluated empirically, by feel. Companies build a number of prototypes to evaluate stiffness for a particular application and circulate prototypes to several physicians for assessment. Designers select physicians based on their clinical experience and their relationship with the company. An essential element of device innovation is factoring in the reactions and suggestions of practicing physicians and other providers who use or prescribe devices in real-life settings. Many companies are reluctant to involve outside physician advisers in the process of designing or specifying their products because of issues such as protecting proprietary information and compensating outside physicians appropriately.

It is difficult to tell whether this type of customer feedback provides an accurate or efficient design evaluation. In many cases, a potentially good design can be rejected because one physician responds with an unfavorable opinion. Sometimes two focus groups produce two entirely different evaluations. As a result, a third group may be consulted. Gut feelings permeate the decision-making process in real-world product design. In this sophisticated age of high-powered computer analysis, designers sometimes believe they can lay a consumer's psyche out on a spreadsheet. A plethora of market surveys, focus group data, and conjoint analyses are available detailing customer needs and wants. Nevertheless, designers sometimes know little about what a customer will actually buy, especially if they are developing a device that differs significantly from what already exists. Sometimes, designers are faced with knowing what the customer wants, but lack the technology, the financial backing, or the corporate support to design such a product. This predicament leads to designing products that differ only slightly or inconsequentially from what already exists on the market.


Catheter customers have reacted favorably to small shaft size. In the industry the current belief is the smaller the shaft, the better the catheter. However, substantially reducing the shaft size has limitations. Once companies reached a minimum catheter size, they shifted their focus to perception: change the color. In the catheter industry, black is perceived as small, blue as innovative, and green as environmentally conscious. Other catheter colors include purple, gold, and white. Red has yet to be seen, possibly out of fear that the catheter will look too much like blood, an indication of early contamination. So, color can have an impressive effect on the physician's perception of the device. Nevertheless, this focus on user perception is evidence that the industry is focusing more on features than on benefits. A product that provides a real, new benefit eludes designers.

Further, two traps result from concentrating on a feature such as a device's color. Certain features may be unnecessary to achieve a product's intended benefits and imposing them only adds design constraints. Another trap is that features can obscure a serious problem. A flawed feature may inadvertently render an intended benefit useless. For example, the color black gives the impression of a small profile. However, a small profile can hinder a designer or QA/QC engineer from seeing a potential problem such as a leak, foreign material, or material corrosion.


FDA Requirements. If all catheters on the market look alike and have essentially the same characteristics, why does it take such a long time to design a new one? Design engineers must provide FDA with data that indicate that biocompatibility testing has been completed and that the materials are safe for the intended use of the catheter. Medical device designers frequently ask raw materials suppliers to provide approval data to facilitate an FDA premarket approval application. Because FDA does not approve materials, it is the manufacturer's responsibility to show that a device incorporating a material is safe and effective for its intended use. Waiting for a supplier to provide the appropriate data increases the cost of designing the product.

Design Team. The time it takes to draw a blueprint or build a prototype is no longer the major cause of delay. Often, the design engineer is part of a development team. Many companies use a team approach to become more competitive, flexible, and responsive. Although companies have accepted the team concept in principle, sometimes these teams meet with mixed success. Working in teams can present a tremendous challenge for many design engineers who find themselves thrust into this dynamic, foreign environment. Managers dealing with these teams must know how to collaborate as team members and foster the work of others as team leaders. They must facilitate and lead high-performance teams to demonstrate excellent team efficiency, dynamics, and innovation. Leaders need excellent interpersonal skills to communicate effectively with their peers and with internal and external customers, so that the team can develop a competitive product that will meet customer needs.

Economic Factors. Like other medical device manufacturers, the makers of balloon angioplasty catheters face the pressures of managed-care industry demands: price and value. A new medical device generally carries a 26% added value. Catheter manufacturers generally mark up prices only 5%. In the future, balloon catheter design engineers will be under even more pressure to ensure that innovative technology is offered at affordable prices.


Medical device companies have made significant efforts to produce products that address many problems facing physicians who treat coronary disease. One problem remains unsolved: restenosis--a frequent consequence of angioplasty. The applicability of PTCA catheters has increased, enabling physicians to treat more-complex lesions. Concurrently, the rate of restenosis has continued to increase as well. Neither physicians nor device designers completely understand the pathophysiology of restenosis. It may be beneficial for design engineers to understand the problem's mechanism, since they have been called upon to design a product to address it.

Designers continue to explore new technologies to solve some of the major failings of balloon angioplasty. Ultimately, these new technologies should create a smoother lumen free of clefts and ridges. They should also enable plaque material to be removed from the lumen, leaving a larger posttherapy lumen than can now be obtained with balloon angioplasty. Some technologies that have been developed avoid the severe arterial stretch that disrupts the media and elastica. These solutions include directional atherectomy, laser therapy, use of stents, rotational angioplasty, ultrasound angioplasty, and other adjunctive therapies such as local drug delivery. Overall, however, physicians have experienced little success with these new technologies. The pioneering investigators significantly underestimated design complexity in terms of basic scientific research, engineering, and necessary clinical testing to achieve their goals. At the outset, the lay and medical communities had overly optimistic and unrealistic expectations for these new technologies, which has led patients and physicians to switch from one device to another.


Medical devices are not subjected to the same type of randomized, statistical efficacy studies as pharmaceuticals. It is difficult to fully and objectively assess the efficacy of new noninvasive diagnostic techniques when the training and experience of the physician could affect the assessment as much as the details of the device itself could. New devices have an inherent element of risk associated with them, and the professional consumers of medical device technology generally understand the trade-offs between risks and potential benefits.

Often it is not a therapeutic risk failure but rather a legal one that companies must grapple with. In the hospital setting, the physician is the likely cause of a device failure when, for example, he or she exceeds the balloon's maximum rate burst pressure, which can be fatal to the patient.

In general, however, products face greater risk in the courtroom than in the design studio or operating room. The limitations imposed by the risk of patent infringement are major challenges for companies to overcome. Patent control inhibits creativity, which is the essential ingredient of the product development process. Legal feedback very often arrives after the design is completed, instead of early on when it would be most useful. Sometimes it is difficult, if not impossible, for an engineer to know whether a patent for an innovative device has already been filed by another inventor. It takes an average of two years from the date of filing to receive a patent. Companies should encourage their inventors to file petitions as early as possible in the design phase. This enables the company to make a single claim and change or rephrase it later. Another option is to take advantage of the delays in the development process, in the patent process, and in FDA approval, and file for the patent later. Usually, inventors choose the first tactic when they need protection from infringers, who easily enter a rapidly growing market. The latter option is more appropriate for applicants unsure about a patent's economic viability. If the market demands it, the inventor can file a special petition, speeding the start of examination and preventing others from filing a similar design.


Innovation is an element of product design that no company can ignore if it wants to remain viable in the 1990s, as competition increases among the major players. Design engineers are responsible for the preponderance of medical device innovations. A few key safeguards are required not only to maintain the innovative spirit but to guarantee its future:

* Listen carefully to your customers. Although there are still many unanswered questions about treating specific lesions, physicians can indicate whether they want a product that eliminates a lesion completely or simply provides relief. It is impossible to design a catheter that can do everything, so this feedback can assist designers in speculating on market trends.

* Focus quickly on marketable ideas. Design engineers can exploit technical opportunities by synchronizing development with the latest market trends.

* Integrate user feedback early in the design process to assess the technical and market viability of the concept. The accuracy and efficiency of the feedback approach may be subtle and difficult to quantify, but it can nonetheless provide a great deal of perceived competitive power.

* Run an extensive patent search by carefully reviewing prior art and dissecting existing patent literature to ensure the noninfringeability of the concept. Do not forget to file for a patent if the concept itself appears innovative and patentable.

To ensure the safety and efficacy of a device, designers must strive to build a strategic capability from early concept to product launch. Designers must respect FDA clinical requirements and the complexity of the problem; however, they must equally consider the factors that determine which features and benefits should guide the final design choice. This exercise and attitude will expedite FDA device clearance regardless of the claims submitted to be reviewed. FDA's constrictive regulatory process was not put in place to hamper industry efforts to introduce new, potentially lifesaving technologies, but rather to ensure that devices offer the safety and benefits promised by the manufacturer. This is the challenge and opportunity for the design engineer.


Currier J and Faxon D, "Restenosis after PTCA: Have We Been Aiming at the Wrong Target?" J Am College Cardiology, 25(2):516­517, 1995.

Floram S, The Existential Pleasures of Engineering, New York, Sam Martin Press, Griffen Books, Preface xi, 1976.

George B, "Reinventing the Environment for Innovation," Med Dev Diag Indust, 17(2):24­32, 1995.

King S, "The Role of New Technology in Balloon Angioplasty,"J Am Heart Assoc, 2(5):74­77, 1992.

Levy R, "Health Care Reform," Med Dev Diag Indust, 16(11):36­40, 1994.

Litvack F, and Eigter N, "Coronary Laser Angioplasty: Tribulations, Trials and Directions," Coronary Arterial Disease, 3:533­ 536, 1992.

Neel G, "FDA and Materials: The Myth of Approval," Med Plast Biomat, 1(2):53, 1994.

Schael G, "Measuring Stiffness of Materials for Catheter Design," Med Plast Biomat, 1(1):19, 1994.

Serruys T, Interventional Cardiology, Philadelphia, Current Medicine, pp 1.7­1.9, 1994.

Smith P, and Reinertsen D, Developing Products in Half the Time, New York, Reinhold, pp 63­72, 90­91, 124­131, 1991.

Uttal B, "Speeding New Ideas to Market," Fortune, 15(5):62­66, 1987.

Robert Ndondo-Lay is senior design engineer for Medtronic Interventional Vascular Systems (San Diego, CA).

500 characters remaining