Medical Device Link.

MATERIALS

Current limitations

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The use of polymers in medical devices has revolutionised the types of surgery that are now available to patients. Simple angioplasty, for example, in its most basic form involves a small incision in the femoral artery, the insertion of a long thin tube that is then placed into the affected area in the heart, and a balloon inflated with or without the deployment of a stent. This “long distance” approach has resulted in faster operation times, higher patient throughput and quicker recovery times. Minimally invasive techniques such as this are now commonly used throughout the world and are only possible with the use of highly specialised polymeric materials. As with all technologies, once techniques become commonplace the need for further improvement becomes ever more apparent.

Most high performance catheters used in interventional coronary, peripheral and neurovascular applications are made from materials such as nylon, polyurethane and polyether block amide (Pebax), or these can be blended with other polymers to meet the device’s performance requirements. The ends of these catheters are composed of tubes approximately 1 mm in diameter that are guided into the lesion area along guidewires. This method creates friction not only from the external surface, but also between the catheter and the guidewire. One of the challenges is to reduce the surface friction of these catheters to allow them to reach distal lesions and/or cross tight junctions.^1,2 There are several hydrophilic coatings on the market, but their use is limited because they easily delaminate or they expand on hydration, which increases the overall profile of the device. These types of coatings are unable to treat the inner lumen of catheters to reduce friction between the guidewire and the catheter. Most of these hydrophilic coatings involve placing an extra layer on the surface of the device to reduce friction. However, this is far from ideal because the improvements can be marginal at the expense of increased size and flexibility of the device and in nearly all cases the inner lumen remains untreated.

Current coating techniques

Figure 1: Coefficient of friction measurements of the surface modification on Pebax 70 D tubing at varying pH. The samples show a response to the change in environment with a reduction in friction occurring above pH 5. The samples were measured on an FTS 5000 friction tester and clamped between silicone pads at a set force. The force required to move the samples was measured and coefficient of friction calculated. (click image to enlarge)

There is a vast array of methods for adapting a surface. Dip coating techniques provide an easy approach to placing a layer of new polymer on the surface and are often used on single use devices that come into contact with the mucous membrane.³ For devices in contact with blood, a more permanent attachment to the surface is often required. Plasma, corona and ozone treatments are able to introduce new species to a surface and this approach can be enhanced by introducing a reactive gas into the atmosphere in which the technique is performed. Chemical vapour deposition can be used to achieve a surface that is tougher and more hard wearing for implants with heavy wear requirements. Many of the coating products available on the market are used for the outer surfaces only and use the dip coating technique coupled with ultraviolet (UV) irradiation to create a surface layer with low friction properties.^4,5 These techniques work well if the surface provides a stable attachment to the extra coating layer; however, it still involves adding extra thickness and often more rigidity to the device. These techniques are also not applicable to the inner lumen because many polymer materials contain UV stabilising compounds and consequently UV light cannot penetrate the already UV stabilised material.

An alternative technique is to chemically modify the surface. By introducing new chemical species to the surface it is possible to grow a new surface with the desired properties from the bulk material. This approach allows the new surface to form a molecular level coating, rather than relying on the viscosity of a traditional coating solution. By carefully selecting the new surface’s molecular composition, the surface can be modified to display the desired properties. The new surface can then be lubricous, antimicrobial, anticoagulant or even display active release properties.

Low friction technology

Figure 2: The tracking profile of the surface modification technology compared with that for leading coatings on the market. The samples are incorporated into PTCA catheter devices and pushed through a tortuous path mimicking the pathway into the coronary arteries. The surface modification shows it is as lubricous as the leading coating products. (click image to enlarge)

Novel technology has been developed specifically to address the problems of high surface friction. It provides surface modification by growing a polymer from the surface of the substrate at molecular level rather than a coating. It can be used to treat the outer surface and inner lumen of tubing without compromising the critical profiles of the device. The process involves an initial reactive step followed by the grafting of a new polymer onto the surface by flowing aqueous solutions over the surface. The friction characteristics of catheters treated optimally with this technique are equal to the lowest in class, recording friction coefficient in a range of 0.01 to 0.04. Furthermore, they are tuned to be at their highest performance at a pH of 7.4, which is the same as blood and other physiological fluids, as shown in Figure 1.

Modern catheters have been designed to track tortuous arteries such as in the brain and cross tight lesions such as chronic total occlusions. This surface modification technique is suited to improving the performance characteristics of devices such as stent delivery systems and guidewires, as shown in Figure 2. During most coronary procedures, the first device to be placed into the artery is a guidewire. These long thin metal devices are able to navigate around the arterial system, especially the narrow and often occluded areas around the heart, with more ease than a catheter itself. Once these devices are in place, the surgeon will then guide the catheter along the guidewire to perform the angioplasty or to stent the region. To enable the surgeon to manoeuvre these devices precisely, the friction between them must be minimal. Large friction forces along the length of this device result in both devices moving together or excessive force needing to be applied. As stated, most currently available coatings can only coat the outer surface.

Figure 3: The surface modification technology employed on the internal surface of a Pebax catheter lumen. The results show a decrease in the forces required to push/pull other devices through the inner lumen. The samples show the percentage reduction of the force required to remove a guidewire through a lumen size of 0.72 mm while it is within a tortuous path. Each cycle is five repetitions of pushing and pulling interspersed with 45 minutes of flushing the tubing with saline solution. (click image to enlarge)

This novel surface modification technique is based on aqueous solutions and allows the inner surface of the tubing to be modified on a molecular level. This means there is virtually no change in the diameter of the inner lumen and the lubricous properties of the modification facilitate manipulation of the devices being used. Figure 3 shows the percentage reduction in the forces required to pass a coronary guidewire through a metre long tube with an inner lumen of 0.72 mm when placed within a tortuous path.

Figure 4: The contact angle images of water when placed onto a blank sample (91°, showing calculation graphics) and with the surface modification technology (23°). There is clearly a hydrophilic nature to the surface after treatment resulting in a significant reduction of the contact angle between the solid (Pebax) and liquid (water) phases. (click image to enlarge)

By changing the functional groups at the surface, it is possible to change the way in which the surface interacts with its surroundings. The contact angle is a measure of the wettability of a surface; a high contact angle (>90 degrees) shows hydrophobic surface properties and a low contact angle (<90 degrees) shows hydrophilic properties. Many of the polymers used in medical devices show high contact angle values with aqueous solutions.⁶ However, by changing the chemical groups at the interface that interact with the aqueous environment, the surface can be modified so that it no longer displays a high contact angle in the presence of water, as shown in Figure 4. These new hydrophilic properties help to form a surface that displays more fluid like properties and lower the surface friction of the tubing when it comes into contact with other materials.

Applications

The surface modification technique described here provides polymer surfaces with ultra low friction forces from a molecular level adaptation. The material that is chemically “grown” onto the surface is permanently attached using covalent bonds and applied using a simple and inexpensive aqueous process. These aqueous solutions can be used on internal and external lumen surfaces, which is beneficial when using multiple devices in the same surgical procedure. Many of the polymers used in current medical devices are able to undergo this type of lubricous surface modification and the devices listed below benefit in particular from the reduced friction force:

• percutaneous transluminal coronary angioplasty balloon catheters
• neurovascular catheters
• coronary stent delivery systems
• peripheral stent delivery systems
• vascular closure devices
• guidewires.
  

Sam Whitehouse is Research Scientist and Kadem Al-Lamee* is Managing Director at PolyBioMed Limited, Lombard Medical Technologies plc, Sheffield Technology Park, 60 Shirland Lane, Sheffield S9 3SP, UK, tel. +44 114 249 1322, e-mail: kallammee@polybiomed.com www.lombardmedical.com *To whom all correspondence should be directed.