The Design of Interventional Devices: An Engineer’s Perspective

Mr. Laroya is an award-winning medical device design engineer. He is a consultant with Crux Biomedical, recently acquired by Volcano Therapeutics, and provides device development consulting services to various start-up medical device companies. 

When we think about the amazing technology of today’s interventional devices and procedures, it is hard to believe that as far back as 1929, Dr. Werner Forssmann, a physician from Germany, placed himself under local anesthesia, and was able to pass a catheter into his own heart.¹ His brave act was at first frowned upon, but he later shared the 1956 Nobel Prize in Medicine with two other physicians for developing the procedure that many believe ushered in the methods for modern cardiac catheterization. The age of the interventional medical device had arrived.

Background

The general notion of human beings developing tools to aid in the completion of tasks is not a new concept. But what has changed over the last 20 years is the onset of new technological breakthroughs, such as newer engineered materials and more precise processes, which have enabled better, more complex tools to come to fruition. The medical device industry has benefited from these new technologies, which have enabled physicians to discover improved concepts for instruments and treatments, improving both procedural outcome and quality of life for patients worldwide. Not since the day when Dr. Christiaan Barnard utilized the mechanical perfusion machine to enable the first human-to-human heart transplant², have we seen such a convergence of physicians and tools for the advancement of cardiac procedures up to the levels that we see today. Yet, inasmuch as today’s interventionalist is a part of every device design, there was an engineer, or team of engineers, who helped make today’s devices a reality. How engineers go about bringing life to an interventional device can be described as part science, part “magic,” and generous amounts of innovation.

Thanks to the efforts of Dr. Sven-Ivar Seldinger in the early 1950’s³, the method for safe and reliable vascular access became a reality. The Seldinger technique opened the door for physicians to efficiently access blood vessels, allowing for direct access to vital organs without major surgery. With this new access procedure, physicians began to think of new ways to treat various disorders. Dr. Forssmann’s concept of the vascular catheter became the default vehicle by which everything from localized drugs to vascular stents and other implants made their way into patient anatomies. But as the procedures for vascular treatments evolved, the need for better performing catheters grew, in both scope and outcome. The simple plastic tube was no longer good enough to meet the needs of physicians with ever-growing skill-sets. Devices needed to be smaller in crossing profile, easier to advance, quicker to introduce and retract, more laterally flexible, and easier to visualize — all while maintaining hemostasis, sterility, tensile and compressive strength, and torque-ability, while being atraumatic. Skilled interventionalists, who were developing new ways to treat more vascular disorders, were now driving the need for modern catheter design. 

Engineering design challenges

Though we understand the methods and skills used every day in the cath lab, the technology that drives innovation in the design of new devices changes regularly, and we see the results of this change in the myriad of new products that become available. This is the nature and cycle of device design; what exists today as a device and procedure, tomorrow becomes the same device used in a different procedure, which then evolves into a next generation product, enabling the new procedure to become commonplace. So what then, from a designer’s point of view, are the critical points of interventional device design? 

Engineers commonly apply all of the tools available to them to advance the design of modern interventional devices. Everything from advanced physics, miniaturized mechanisms, engineering polymers, structural enhancements like braid and coil wires, electronics and intricate user controls, all have all been employed in successful devices. By combining a variety of these features in a device design, the engineer has the opportunity to “fine tune” the functionality of the design to meet the user needs. A good example of this is a guide catheter — an elongate tubular structure, having a proximal hub and a distal tip, and a working lumen by which drugs, implants, or other devices can be passed easily through. The considerations taken in the design of a guide catheter might include:

• What part of the anatomy will the device have to pass through?
• How tortuous is the required path of the device?
• What is the required maximum crossing profile of the device?
• How will the device be visualized?
• What will be passed through the device?

These questions can be used to identify some key aspects of the device design itself, such as:

• Device working length and flexibility to reach the required treatment site;
• Combinations of lateral flexibility and axial stiffness to maneuver through tortuous paths;
• Preset bends to accommodate device positioning and/or seating;
• As low a crossing profile as physically possible, to fit into required small diameter vessels and reach distal sites;
• The use of markers and components to enable ultrasonic and/or fluoroscopic visualization;
• The use of advanced materials such as PTFE⁴ and HDPE⁵ to provide the working lumen with lubricity, chemical resistance, and working lumen patency.

It is a primary goal of the engineer to leverage both design skills and materials technology to make these ideal features a reality. Every material offers specific properties, which enable characteristics like lubricity or flexibility in the device. The engineer uses his skills in process design to manipulate the material so that it can be extruded, bonded, folded or wrapped, collapsed or expanded, to become part of the device itself. 

As much as the materials in a device affect how it performs, the design of the user interface can mean the difference between an easy procedure and one fraught with complications. In effect, the translation of the device design and use with the end user should be both transparent and intuitive. The ideal device is self-explanatory, ergonomic, robust, and reliable in use.

Materials

The types of advanced materials used in today’s interventional devices are not so different from those used in fighter jets and aerospace vehicles. Metals such as nickel-titanium and chromium cobalt are seen regularly in implants like stents and vascular staples. Engineered polymers such as PTFE, HDPE, and Vestamid⁶ are used on catheter shafts and device handles. Advanced, engineered fiber such as Kevlar⁷, once known only for its bullet-stopping prowess, is now used in steerable catheter mechanisms and valvuloplasty balloon catheters. And advanced adhesives and thermal bonding methods allow for very low profile device joints, which can withstand the tensile, compressive and torsional forces seen by a device during clinical use.

The engineer will evaluate a specific design requirement and determine what material will yield that characteristic in the design, while still being safe, robust, easy to manufacture, and reliable. A prime example is the design of an over-the-wire balloon catheter having an inner member shaft. The inner member shaft must be lubricious to allow the device to slide over the guidewire with very low friction, even around tight radius bends. The inner member shaft must also be able to handle the inflation pressures required by the device, which means that the material used for the shaft must be able to reliably bond to both the balloon and to the proximal hub, to handle these very high pressures. The inner member shaft must also provide adequate hoop strength, so that the shaft does not kink as the device is advanced around a tight bend radius. As simple as an inner member balloon shaft is, it is easy to see that the component is required to meet or exceed many design requirements. 

In some situations, the engineer can leverage certain characteristics of a material to reduce costs, increase strength and reliability, reduce profile, or improve visualization. For example, many devices employ radiopaque marker bands, typically made of platinum/10% iridium, to act as fluoroscopic markers on the product for location and positioning purposes. But in some cases, the attachment of a metal marker band can add unwanted profile, stiffness, or additional bonding requirements to the product. In the case of a radiopaque distal tip marker, the engineer can replace the metal marker band with an engineered polymer such as polyether block amide, commonly known as Pebax⁸, which can be loaded with 70% tungsten. The material can be thermally formed into a shaft or ring-shaped component, which can then be thermally bonded to the catheter tip; the thermal bond reduces the profile of the joint and strengthens the attachment, while the tungsten filler provides needed radiopacity. This method also has a positive effect on the cost of the device; a platinum/10% iridium marker band can cost $3.50 a part in quantity, while the Pebax ring loaded with 70% tungsten can cost $0.45 a piece for the same quantity. 

Sometimes, the use of a particular material can degrade the functionality of a device, or it can enable enhanced device functionality. In the case of a stent delivery catheter, for example, the inner surface of the stent delivery catheter may be lined with a lubricious material such as PTFE. PTFE, commonly known as Teflon, is an engineered fluoropolymer, which possesses the lowest coefficient of friction of any polymeric material. But the downside of PTFE is that it is so slippery that attaching it to any substrate is very challenging and can be very expensive. Also, PTFE can tear easily, and tends to be very notch-sensitive. This means that, as a liner for a stent delivery catheter, PTFE material requires that the stent itself, and anything else passing through the working lumen, be very smooth and atraumatic, or else risk tearing the catheter liner during delivery, causing the stent to become jammed within the delivery system lumen. In this case, the PTFE liner would be a challenge to use in this stent delivery device. If the implant possessed rough edges, sharp anchors or barbs, the engineer could leverage an engineered polymer such as nylon-12, commonly known as Vestamid, which is a very robust polyamide that also provides lubricity close to PTFE, while being tear-resistant and less expensive. This means that a catheter lined with Vestamid, or even made entirely of the material, can be more reliable, and also less expensive, than the PTFE-lined design, at least in this application. So it is easy to see how engineers balance material selection to design the optimum device.

Interface

In the most basic sense, the engineer is challenged with matching human to machine, user to tool, and ultimately device to patient. The interventional device becomes the bridge between physician and patient, enabling the physician to access the treatment site while minimizing trauma to other parts of the patient anatomy. In some instances, the interventional device becomes an extension of the physician’s own hands, allowing positioning and placement of implants, localized delivery of medications, or clearing/extraction of emboli. The intuitiveness of the device design, combined with reliable user controls that are easy to understand, can enhance patient outcome while minimizing complications or procedural time. Along with this intuitive functionality, the device must be easy to visualize, such that the physician can interpret the position and location of the device during clinical use.

The distal end of the device, where the actual treatment activity typically occurs, must feel like a one-to-one extension of the device proximal end. Whether the proximal end of the device is a simple luer, or an intricate electro-mechanical handle, the user must feel confident that whatever input he introduces into the device results in a direct, predictable output at the distal end of the device. If the physician pushes forward or pulls back, he expects the distal tip to advance or retract. If the physician twists the handle, he expects the distal tip to rotate axially. And if the physician pulls back on a deployment knob, he expects an implant to deploy. In an ideal world, these functions happen seamlessly, every time. But the engineer must contend with real-world physical variables, such as friction, tortuosity, extended working length, tight design tolerances, and patient variation, to name a few. In the design of the interventional device, the engineer must take into account limitations of every component and each type of material used in the design. There is a constant balancing act between leveraging the positive aspects of a material with the materials limitations. As an example, if a catheter must traverse over a very long distance, such as a neurovascular micro-catheter, the engineer might elect to employ a shaft support component, such as stainless steel braid, which can enhance pushability while also enhancing kink-resistance. But braid support can limit the lateral flexibility of the micro-catheter in very tortuous distal vasculature, and so the engineer can integrate a stainless steel coil support component to just the distal portion of the catheter, which enhances distal lateral flexibility while adding hoop strength for even better kink-resistance. In addition, the engineer can design the device to employ a hydrophilic coating, which adds lubricity to the outer surface of the catheter where it interfaces with the vessel wall, reducing friction and vessel trauma while further enhancing axial and rotational one-to-one movement.

What the physician holds in his hand during the procedure, whether it is a luer hub or a handle, is typically his only interface with the portion of the device that sits at the treatment site. Therefore, he must feel confident that his inputs into the device result in a level of tactile precision that yield predictable actions at the working end. Any amount of lag, delay, looseness, or wind-up can result in mispositioning or misalignment. In severe cases, lack of tactile precision can result in vessel trauma or vessel/organ perforation. To minimize these effects on the device, the engineer can employ advanced design techniques such as shape setting, pre-loading, advanced construction methods, support structures and hybrid materials to make up for various design shortcomings. Shape setting and pre-loading can be in the form of designing a catheter with a pre-defined distal bend, which enables the physician to access an angled vessel, and seat the distal tip portion of the guide at the vessel ostium, to prevent it from backing out of the vessel when a secondary device is advanced through it. The engineer can employ advanced construction methods such as braid wire support to enhance axial and torsional control of the catheter, reducing or virtually eliminating lag associated with push/pull or rotational input by the physician. Examples of hybrid materials include shafts, which are extruded using two or more materials, such as a catheter shaft with an inner layer of HDPE and an outer layer of Vestamid; the HDPE provides lubricity to the inner lumen, while the Vestamid provides axial and torsional stiffness needed for one-to-one control.

No device works 100% every time, and so the engineer must take into account variables in the device itself, along with patient variation, to ensure that safety and functionality work in tandem with each device deployed. Where user input is key in the operation of the device function, the user interface must also provide a level of control that enables the physician to react to variations in device function or patient variability, as well as provide a means of bail-out should complications arise. The engineer takes these variables into account, and may elect to include design features, which are meant to enable changes to the procedure in situ. These features might include controls or components which can be disconnected to allow for safe device retraction, additional lumens or working channels which enable the introduction of ancillary support devices, and device controls which have a dual function such as a deployment knob that also acts as an emergency outer sheath removal control interface. Ultimately, the well-versed engineer determines the final interface design after first-hand use in hands-on animal studies and bench testing. A good user interface must “feel” right in the hands, generate confidence, and require minimal instruction and support during use.

Final comment

The designing of interventional devices has made huge strides since the early days of investigational interventions by innovative physicians. Device design engineers leverage the use of advanced materials, advanced design processes, and innovations in interventional procedures to drive the evolution of today’s devices. Contrary to popular belief, engineers don’t typically invent medical devices on their own. The nature of every engineer is to enable and develop a physician’s idea into something tangible and useful — to make something that is not only functional, but something that is better than what was originally conceived. What comes from an engineer’s hands and mind is, more often than not, the product of clinical insight combined with engineering design methods. The most typical question that an engineer gets is “can you make something that does this?” It is from experience that the engineer can “make” something, but it is from clinical insight that a physician is able to know what he wants that something to be, and how that something should function. 

A famous Columbian bicycle frame builder, Tinno Hincapie, once said, “In order to build a great frame, you have to understand how a bike moves, how it reacts, and you have to know what it’s like to suffer when riding a bike”.⁹ Ultimately, the seasoned engineer must take the device in question into real-world testing, under real-world clinical conditions, for a first-hand evaluation to see how it performs and where it falters. Other than first-hand testing, the coordinated interaction between the physician and the engineer is the most efficient way to get to that successful design, and in turn, experience successful procedures with great patient outcome — something to consider the next time you perform an interventional procedure, and come up with a great new idea. 

Gil Laroya can be contacted at gillaroya@comcast.net.

References

1. The Nobel Prize in Physiology or Medicine 1956. André F. Cournand, Werner Forssmann, Dickinson W. Richards. Available online at https://www.nobelprize.org/nobel_prizes/medicine/laureates/1956/forssmann.html. Accessed February 15, 2013.
2. Encyclopedia of World Biography. Christiaan Barnard. Available online at https://www.notablebiographies.com/Ba-Be/Barnard-Christiaan.html. Accessed February 15, 2013.
3. Greitz T. Sven-Ivar Seldinger. AJNR. 1999; 20: 1180-1181. Available online at  https://www.ajnr.org/content/20/6/1180.full. Accessed February 15, 2013.
4. PTFE and Polytetraflouroethylene is a registered trademark of DuPont Corporation. Available online at https://www2.dupont.com/Teflon_Industrial/en_US/products/product_by_name/teflon_ptfe/index.html. Accessed February 15, 2013.
5. HDPE and High-Density Polyethylene is a registered trademark of Exxon Mobile Chemical. Available online at https://www.exxonmobilchemical.com/Chem-English/brands/polyethylene-hdpe-resins.aspx?ln=productsservices. Accessed February 15, 2013.
6. Vestamid Nylon-12 is a registered trademark of Vestamid Corporation. Available online at https://www.vestamid.com/sites/dc/Downloadcenter/Evonik/Product/VESTAMID/en/brochures/VESTAMID%20L%20compounds%20characteristics.pdf. Accessed February 15, 2013.
7. Kevlar is a registered trademark of DuPont Corporation. Available online at https://www2.dupont.com/personal-protection/en-us/dpt/kevlar.html. Accessed February 15, 2013.
8. Pebax is a registered trade name of Arkema. Available online at https://www.pebax.com/sites/pebax/en/home.page. Accessed February 15, 2013.
9. Klaus. Steel is easy to love because it loves you back. Cycling Inquisition, November 2012. Available online at https://www.cyclinginquisition.com/2012/11/steel-is-easy-to-love-because-it-loves.html. Accessed February 15, 2013.