Radiopacity
Many medical devices are often implanted with the assistance of fluoroscopy -a technique that uses X-rays to produce real-time images of a medical procedure. Radiopacity refers to the ability of an object to absorb X-rays and therefore produce a high contrast fluoroscopic image. One way to make an object radiopaque is by ensuring that its thickness is great enough to absorb sufficient radiation. However, the thickness that is required for this will depend on the material that it is made of and there is often a great desire to have medical implants be small and/or thin.
A parameter that characterizes the ability of a material to absorb X-rays is the attenuation coefficient, µ. The higher the value of µ the thinner a part can be and still produce an acceptable fluoroscopic image. For example, the attenuation coefficient for iron is 9.5 (1/cm) while that of tantalum is over six times as great at 59.4. Iron is the primary component of stainless steel which is commonly used for medical implants because of its mechanical properties and resistance to corrosion. Another material that is commonly used for peripheral stents and other vascular devices is the shape memory alloy Nitinol. Nitinol consists of roughly equal amounts of nickel and titanium. The attenuation coefficients for these materials are 13.4 and 3.5, respectively. Because of the significantly higher attenuation coefficient of tantalum, medical devices made out of stainless steel and Nitinol can be made much more radiopaque by applying a coating of tantalum.
Isoflux VisTa® Coating
The Isoflux VisTa® radiopaque coating was developed specifically for medical devices that are made out of poorly absorbing X-ray materials such as Nitinol. Our VisTa® coating consists of tantalum and is made by Isoflux’ proprietary sputter deposition process#. Because this sputter deposition process produces a uniform coating even on complex shaped parts (largely due to our use of cylindrical sputtering cathodes#), the entire device is seen in the X-ray image. This is a great advantage compared to the common approach of attaching to the device radiopaque markers that show up on the fluoroscope as tiny moving dots. Medical devices such as coronary stents, peripheral stents, vena cava filters, LAP anchors and guidewires can all benefit greatly from the VisTa® coating.
Structure
The VisTa® coating is deposited in such a way that it consists of individual columns of tantalum that adhere extremely well to the medical device. Figure 1 shows SEM images of the VisTa® coating. This unique structure allows the device to be flexed without risk of delamination of the tantalum. This is extremely important for vascular stents since they undergo high strains during implantation and also need to be able to flex freely as long as they are in the body. The porous columnar structure completely prevents the development of tensile stress since the columns are free to move independently of each other. In contrast, any radiopaque coating that is does not have porous columnar structure like VisTa® is likely to crack and delaminate when the device is flexed.
In addition to producing a flexible and durable coating, the VisTa® sputter deposition process also preserves all of the important thermal and mechanical properties of the substrate. For instance, the highly sensitive austenitic and martensitic phase transition temperatures, Af and Mf, for Nitinol are not changed by the process. Other PVD and CVD coating processes cause the temperature of the substrates to rise to levels above which the thermal properties are destroyed.
Other Benefits
The nanoporous structure of the VisTa® coating provides another highly valuable benefit. The fully open and connected pore space can be used to deliver drugs# (anti-restenosis, anti-inflammatory, anti-microbial, and others) from the surface of the implanted device without the need for polymer coatings. This eliminates the major medical concern about the long term presence of polymers in the body and greatly simplifies the process for designing drug delivery capability for the device#. Yet another benefit of the VisTa® coating is that the nanosize surface features may enhance the rate of cell growth. Fast reendothelialization of vascular implant surfaces is highly desirable because it minimizes the time the implant is exposed to blood flow and makes it less likely that biocompatibility problems will arise.
Although the VisTa® coating was developed for Nitinol devices it has been demonstrated successfully on a wide variety of other substrate materials such as stainless steel, cobalt-chromium and even polymers such as PEEK (polyether ether ketone).
For more information on the VisTa® radiopaque coating for medical devices, please see: