Biomedical polyurethanes-based materials

Polyurethanes are one of the most versatile families of polymers. They can be prepared from a wide variety of materials exhibiting extremely different properties and therefore, a high variety of applications. This wide range of properties has attracted the attention of biomedical devices developers. They have been testing these polymers in several biomedical fields including pacemaker lead insulation, breast implants, heart valves, vascular prostheses and bioadhesives.Throughout this review, an overview of the synthesis and properties of polyurethanes, as well as some of their biomedical applications will be presented. Since the surface properties of a material are directly related to its performance in a biological environment, a description of some of the available methods for surface modification will be carried out.The surface characteristics of a material can be adapted to a specific application by surface modification techniques, without compromising the material bulk properties. Chemical structure (e.g. hydrophilicity) and morphology of a surface (e.g. topography) are the most important properties and are the ones that can be tailored. Biological, chemical or physical methods can be used to modify a surface. Radiation grafting of monomers, chemical modification, immobilization of biological molecules and silanization are some of the available and most used methods in surface modification of materials. Hydrophobic surfaces can support the adsorption of proteins from aqueous solution, and highly hydrophilic surfaces may inhibit protein adsorption by repulsing protein molecules. Plasma treatment and ultra-violet ligh thave been increasing interest in the surface modification field since these methods are able to change the surface energy of polymers. Although polyurethanes have excellent mechanical properties, chemical stability, and are easy to process, which make them a good candidate to be used in several biomedical applications, they usually are hydrophobic. Thus, according to the intended application they must be surface modified in order to be adapted for the desired propose.Polyurethanes have also been used as bioadhesives. Although suturing remains the most common method of closing wounds, sutures well-known limitations are motivating factors for both researchers and surgeons to seek new and most attractive methods to accomplish tissue continuity. Therefore, surgeons have thought of an alternative way, the use of medical bioadhesives. Topical skin adhesives are increasingly being used by health professionals in the fields of trauma, plastic and other surgeries, emergency medicine and pediatrics. The most used bioadhesives nowadays are the fibrin based adhesives and cyanoacrylates. However, both present serious drawbacks in their application from immunogenicity (fibrin) to toxic degradation products (cyanoacrylates). Other options are recently coming into light, and among the synthetic materials, urethane-based adhesives have been considered to be quite promising for this application. Polyurethanes are good candidates to be applied as bioadhesives due to the possibility of being biodegradable, biocompatible and, if synthesized in the form of prepolymers, have the capacity to react with amino groups present in the biological molecules. This reaction results in the formation of urea linkages and also on the promotion of adhesion since these covalent bonds will hold the tissue together. Several authors have synthesized and characterize biodegradable polyurethanes prepolymers to be applied as bioadhesives. Also, urethane-based materials have been applied as bioadhesives under the form of photocrosslinkable materials.