Biomaterials Tutorial

Biomaterials for Cardiovascular Applications

Mª Cristina L. Martins
INEB, Porto, Portugal

A biomaterial can be defined as a material intended to interface with biological systems to evaluate, treat, augment or replace any tissue, organ or function of the body [1]. Many different biomaterials are used clinically as implants or devices for diagnosis or therapy. Cardiovascular (commonly blood-contacting) applications, one of the most important categories of implant materials, include the extracorporeal pump-oxygenator used in many surgical procedures, hollow  fiber hemodialyzers for treatment of kidney failure, catheters for blood access and blood vessel manipulation (e.g., angioplasty), heart assist devices, stents and permanently implanted devices to replace diseased heart valves (prosthetic heart valves) and arteries (vascular grafts) [2-5].  Table 1 has a more comprehensive list of cardiovascular applications for biomaterials.

Table 1

Biomaterials for cardiovascular applications are usually prepared using polymers because these are available in a wide variety of compositions with adequate physical and mechanical properties and can be easy manufactured into products with the desired shape [3,5,6]. However, some metals and ceramics are used in the blood stream. Physical and mechanical properties of the most common synthetic polymers used as cardiovascular biomaterials and the respective application are shown in Table 2 [2,4,5].

Table 2

One of the greatest problems encountered when foreign materials are inserted into blood is the rapid formation of thrombus (an aggregation of blood cells) that could adhere to the surface of the biomaterial (a local effect) or be detached and carried downstream eventually occluding a blood vessel (embolism, a systemic effect) [2,3,7,8]. 

Because many metals tend to be thrombogenic, they are most commonly applied in situations requiring considerable mechanical strength, such as in the struts of mechanical heart valves and as endovascular stents or when electrical conductivity is required as in pacemaker electrodes [5,7].

Many devices are only “safe” when anticoagulant drugs are used (e.g. oxygenators, mechanical heart valves, hemodialyzers). Such anticoagulant use has important implications for health, i.e., risk of internal bleeding2,7Nevertheless, for certain applications there are no devices available at the present that perform adequately, even when antithrombotic drugs are used, as in the case of small-diameter vascular grafts (<5 mm internal diameter) and prostheses for reconstruction of diseased veins [2,3,7,10,11].

In addition to thrombus formation, biomaterials often become colonized with bacteria that cause infections [12-16]. These microorganisms are extremely resistant to antibiotic therapy and infections cannot be fully resolved until the biomaterial is removed [16].

References

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12. Gottenbos B, van der Mei HC, Busscher HJ. Initial adhesion and surface growth of staphylococcus epidermidis and pseudomonas aeruginosa on biomedical polymers. J Biomat Mater Res 2000:50:208-214.
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14. Boelens JJ, Zaat SAJ, Meeldijk J, Dankert J. Subcutaneous abscess formation around catheters induced by viable and nonviable staphylococcus epidermidis as well as by small amounts of bacterial cell wall components. J Biomat Mater Res 2000:50;546-556.
15. Barton AJ, Sagers RD, Pitt WG. Bacterial adhesion to orthopedic implant polymers. J Biom Mater Res 1996:30;403-410.
16. Jawetz E, Melnick JL, Adelberg EA. Review of medical microbiology, 17th ed. Appleton & Lange; 1987.