Touting the advantages of lower costs, less trauma, and faster recovery times, minimally invasive procedures have replaced countless high-risk, open surgeries and paved the way for innovative new treatment options. And as the workhorses of these procedures, guidewires are tasked with the critical responsibility of helping to ensure the safe, accurate delivery of surgical instruments and medical devices such as stents, catheters, and balloons to the area of the anatomy requiring treatment.
|Guidewire photo courtesy of NeoMetrics Inc.|
"A guidewire is necessary in pretty much any minimally invasive application," comments Michael Brown, COO and vice president of contract guidewire manufacturer TechDevice Corp. (Watertown, MA). "Depending on whether the procedure is a vascular intervention or endosurgical application, as well as where the access point into the body is, the wire will go into the body through the access point and reach the area of the anatomy where the doctor will perform surgery. Then, a catheter or another system is delivered over the wire. Once the device is in place, the guidewire is removed." Guidewires are commonly employed in a variety of vascular applications, including coronary, peripheral, and neurological intervention, among many others.
To make certain that the delivery devices will fulfill their essential duty, guidewire designers must evaluate a host of design and material considerations in order to achieve the desired performance characteristics for a given application. "A lot of people think that anybody can make one of those wires," notes Mark Christensen, senior product manager, stimulation therapies, at guidewire specialist company Lake Region Medical (Chaska, MN). "But we've found that customers that have gone to cheaper competitors come back to us because you still need to have the proper quality, consistency, and delivery capabilities to do the job."
Guidewire Core Values
A standard guidewire typically consists of three basic components: a core wire, a coil, and a unique distal tip profile. But despite the simplicity of the device's design, factors such as the target location in the body and the device being delivered merit careful consideration during material selection.
Designers typically employ either stainless steel or nitinol for the core wire, which accounts for the majority of the guidewire. Dependable, durable stainless steel offers strength and support characteristics that accommodate delivery of larger, bulkier devices. Its inherent stiffness also enables the development of smaller-diameter wires that can be easily pushed into the body and through vessels.
Among the metal's main advantages is its good torque transmission. This characteristic is critical to operator control and successful navigation through the body. "The core wire affects the torquability of the device, and the torquability affects the steerability of the device," Brown says. "You want the tip of your guidewire to turn in a 1:1 ratio with the proximal end."
Torquability can also be further enhanced, however, through specialized processing capabilities, according to Christensen. He notes that the manner in which the material is handled and processed can alter characteristics of the material to better suit user requirements. Lake Region Medical, he adds, employs a proprietary processing technique that yields stainless-steel cores boasting better torquability than that of conventional stainless steel.
But as with any material, stainless steel has a significant drawback. "Being as stiff as it is, stainless steel is prone to kinking," states Dave Liebl, vice president, R&D, RA, QA, at NeoMetrics Inc. (Plymouth, MN), a guidewire design and development company. "So, imagine if you have to get a catheter over this guidewire and you're going through an area that is tortuous with twists and turns. If this stainless-steel wire bends, you can't really get it back to its original shape. Then, the catheter can't follow the wire. Once a guidewire is bent or prolapses, you're done, and stainless steel is prone to that problem."
For applications in which kink resistance is paramount, a nitinol core is optimal, instead. In addition, the superelastic material allows designers to shape the material and heat-treat it, resulting in shape retention. This shape-retaining feature of the material, Christensen says, can come in handy while delivering stiff catheters that need the guidance of the shaped wire to reach the procedure location. The tradeoff of the material's kink resistance and flexibility, however, is that it may not support heavier devices for a given diameter.
As with stainless steel, proprietary nitinol processing techniques can help to improve the torquability of that core material. "What's unique to NeoMetrics is that we do our own straightening of nitinol, which a lot of guidewire manufacturers don't do," Liebl comments. "That's really important because the straightening process imparts how well the guidewire torques, which is an essential property for navigating tortuous vessels. The tensile and straightness properties of the core wire have a lot to do with navigation."
In addition to weighing the benefits of nitinol versus those of stainless steel, engineers must also often decide upon a coating for the core wire. To enhance lubricity and reduce friction, the core wire typically undergoes a surface-modification process to apply a PTFE, silicone, or hydrophilic coating. Such a coating serves to ensure that the catheter can glide smoothly over the guidewire during delivery and that, in turn, the guidewire can be easily removed from the catheter after proper positioning.
Guidewire coating selection is determined by the application and the device that will be delivered over the wire, according to Liebl. "If we're talking about a polyurethane (PU) versus a polyethylene (PE) device, the surface modification may be very different because PUs are sticky and you're going to want maybe a hydrophilic coating, whereas the PE isn't as sticky and a PTFE coating may be sufficient."
While the guidewire core features a relatively standard construction, the biggest variation among the devices is in their tip. Influenced primarily by the application and physician preference, guidewire tip design directly affects the device's ability to safely navigate the tortuous vessels of the body. Achieving the desired tip design, however, is accomplished using a specialized grinding process on the distal end of the guidewire.
"The distal grind is important because that's where you're going to get your tip flexibility," Brown of TechDevice explains. "How much you grind the wire down or your grind profile will determine the characteristics of the tip."
To aid in navigation, some commodity guidewires feature a preshaped J-tip design. This curved configuration lends itself to gentle travel through vessels, reducing the risk of trauma. It also helps to ensure that the wire travels down the designated route and doesn't stray into side branches of vessels.
Some physicians, in contrast, prefer the use of straight guidewires with shapeable tips. These designs put control into the physician's hands by enabling users to create their own desired tip shape. "If you make the tip shapeable, though, a tradeoff is that the tip might not retain that shape too well. As it runs into vessel walls, it might reshape or kink," Christensen cautions. "That's what it's all about: finding where you want to be in terms of tradeoffs for your particular device."
Performance tradeoffs are an unfortunate reality of guidewire tip design, he adds. In order to be able to drive through a thrombus blockage when accessing the coronary arteries, for instance, the guidewire requires a stiffer tip. But a stiffer tip runs the risk of causing vessel trauma. An especially stiff tip can dissect a vessel and scar the lining; the scarred area of the lining can increase the likelihood of the formation of a new thrombus in that location, Christensen notes. Likewise, better tip trackability can increase the opportunity for tip prolapse if the device encounters an obstruction.
All Wound Up
In addition to tip design and core material selection, engineers also need to consider the incorporation of a wound spring coil onto the guidewire. Joined by means of soldering, brazing, adhesive bonding, or welding, coils can run the full length of a guidewire or may be positioned just at the distal end of the device, depending on the end use.
Commodity guidewires, for example, often are equipped with a stainless-steel spring coil that surrounds the device from the proximal to distal end, Liebl of NeoMetrics states. This full-length coil design is suitable for commodity guidewires, he says, because these devices are used to gain access to a vessel and do not need to be steered.
Interventional guidewires, in contrast, typically feature a 3-cm coil at the distal end of the wire. Commonly engineered from platinum, palladium, or tungsten, the coil provides radiopacity so that physicians can view the tip under fluoroscopy. This capability enables users to track the location of the guidewire tip within the vasculature with ease.
While the coil can serve as a locator of sorts, its primary function is to reestablish the diameter of the wire after the grinding process without adding stiffness, according to Liebl. "If you think of the grinder as a high-tech pencil sharpener, you sharpen the distal end of that wire down to a certain diameter," he says. "For that wire then to work, you need to put something over that ground end to bring the diameter back to the same [thickness] as the rest of the wire. That's what a coil is for." Liebl likens the coil to a miniature slinky, adding that it allows for the creation of a floppy wire tip that follows the vessel's shape rather than causing trauma.
Although guidewire designers and developers have become fairly comfortable with common device configurations, expanding minimally invasive opportunities and the changing device needs that accompany them present a steady stream of new engineering challenges. Advances in neurovasculature procedures, for example, are forcing designers to explore alternative alloys that will allow for the development of stiffer, more-steerable, small-diameter guidewires with enhanced torque properties for navigating these tiny, tortuous vessels. Such procedures are also creating a demand for a tip configuration that provides 3-D positioning to access these tortuous vessels while retaining its shape and boasting kink resistance, according to Christensen.
The exploration of new frontiers in coronary intervention is spurring guidewire research and experimentation as well. Lake Region Medical, Christensen states, has vast experience in supplying interventional guidewires for coronary artery procedures on the stimulation side for cardiac implants. However, an emerging procedure implants pacing leads in the coronary sinus, which is on the venous side, rather than the arterial side, of the heart.
"That whole process started by using coronary interventional wires to help guide the pacing leads into the venous side," Christensen explains. "But they found that they needed greatly different characteristics. One wire goes into the coronary arterial side and you need completely different guidewire characteristics to go into the venous side, even though it's all coronary intervention."
Guidewire research is also in full swing to address chronic total occlusions, observes Liebl of NeoMetrics. Fully occluded vessels still often require bypass or open-heart surgery, representing what some refer to as the last bastion of cardiology, Liebl says. He adds that guidewire designers are currently investigating stiff and tapered tips as well as wires that deliver ultrasonic energy to cater to this unmet need.
In addition to guidewire R&D in the coronary and neurovascular intervention spaces, there has been a flurry of design activity surrounding smart guidewires. With several technologies already on the market and more in development, smart guidewires have generated significant interest because they incorporate sensors or an optical or electronic component on the end of the device.
"Smart guidewires can take hemodynamic measurements or some kind of physiologic measurement and provide [that data] back to the operator either for positioning or for providing feedback on the patient status," Christensen explains. "There is a need to do this in such a way that the guidewire retains all of its great characteristics for positioning and torque, yet still provides that signal back to the operator."
Liebl adds that smart guidewires are already enabling a technique called fractional flow reserve (FFR), in which a sensor affixed to the end of the guidewire measures pressure on both sides of an arterial stenosis. This technology provides insight into the severity of the lesion and can help physicians to determine whether or not a stent should be implanted. The relatively minor cost associated with adding the sensing technology to a guidewire can ultimately contribute to significant cost savings if it is deemed that a stent is not necessary, Liebl states.
"This is certainly an area that has gotten some play, but whether or not it will be adopted widely in the market has yet to be determined," he says. "But there's definitely a trend toward more intelligence in a guidewire. It's simply a question of how smart a guidewire can get while still being cost-effective."