Biomedical uses of polymers

The purpose of this chapter is to introduce a new class of polymers for both types of biomedical uses a polymer system in which the hydrolytic stability or instability is determined not by changes in the backbone structure, but by changes in the side groups attached to an unconventional macromolecular backbone. These polymers are polyphosphazenes, with the general molecular structure shown in structure 1. 

The biomedical uses of polyphosphazenes mentioned earlier involve chemistry that could in principle be carried out on a classical petrochemical-based polymer. However, in their bioerosion reactions, polyphosphazenes display a uniqueness that sets them apart. This uniqueness stems from the presence of the inorganic backbone, which in the presence of appropriate side groups is capable of undergoing facile hydrolysis to phosphate and ammonia. Phosphate can be metabolized, and ammonia is excreted. If the side groups released in this process are also metabolizable or excretable, the polymer can be eroded under hydrolytic conditions without the danger of a toxic response. Thus, poljnners of this tjT are candidates for use as erodible biostructural materials or sutures, or as matrices for the controlled delivery of drugs. Four examples will be given to illustrate the opportunities that exist. 

The use of polymers for biomedical applications has been widely accepted since the 1960 s (7), and specifically for controlled release applications since the 1970 s (2). The primary goal of this research was to create a controlled release matrix from polymers with pre-existing Food and Drug Administration (FDA) histories, which would be capable of releasing insoluble active agents, and upon exhaustion of the device, leave a stable, inert, removable skeleton. The application of such a matrix would be as an intracervical device which would prevent both conception and ascending infection. 

Starting from monomers Monomers in bulk or in solution are irradiated. Polymerization takes place as the first stage of reaction. The polymer chains are then cross-linked. It is frequent practice to add bifunctional monomers to increase the efficiency of cross-linking. Typically, this procedure is used for synthesis of wall-to-wall hydrogels or microspheres. For biomedical use of the formed gels, all non-reacted monomers and residues have to be extracted. 

The biomedical use of polyphosphazenes for drug delivery and controlled release systems still draws considerable attention. It has been demonstrated for the degradable polymer [NP(NHCH2C02Et)2]n that the rate of degradation increases by partially replacement of the ethyl glycinate groups by small amounts... 

Interest in biomedical applications of polymers dates back over 50 years. This interest is due in part to the fact that most biomaterials present in the human body are macromolecules (proteins, nucleic acids, etc.). When tissues or organs containing such materials need complete or partial replacement, it is logical to replace them with synthetic polymeric materials whenever natural replacement materials are not readily available. Although ceramics and metals can be used in certain cases, most biomedical applications require the use of some synthetic polymer or a modified natural macromolecule. Several recent books describe the range of biomedical applications of polymers (2-12). 

Other biomedical applications of polymers include sustained and controlled drug delivery formulations for implantation, transdermal and trans-cornealuses, intrauterine devices, etc. (6, 7). Major developments have been reported recently on the use of biomaterials for skin replacement (8), reconstruction of vocal cords (9), ophthalmic applications such as therapeutic contact lenses, artificial corneas, intraocular lenses, and vitreous implants (10), craniofacial, maxillofacial, and related replacements in reconstructive surgery (I), and neurostimulating and other electrical-stimulating electrodes (I). Orthopedic applications include artificial tendons (II), prostheses, long bone repair, and articular cartilage replacement (I). Finally, dental materials and implants (12,13) are also often considered as biomaterials. 

TABLE 1.1 Properties and Biomedical Uses of Some Common Addition Polymers ... 

Future biomedical applications of polymers will certainly profit from the design of new biomimetic polymers, which allow a specific interaction with living cells and tissues. Recent developments in this field not only demonstrate the advantages of these custom-designed polymers, but also highlight the still-existing limitations, like the difficulty to achieve cell selectivity or unspecific interaaions with biological fluids. Nevertheless, biomimetic polymers are already used in many clinical applications and can therefore be considered as one of the most important fields of current medical advances. 

The use of polymers in medicine is steadily growing. The synthetic and processing flexibility of polymers continue to permit polymers to be applied in a broad range of medical, biological, and implant applications. Creative polymer synthesis continues to expand the functionality and tunability of polymers for medical applications. There are now excellent biomedical polymers available to address general needs in medidne (the subject of this chapter). Opportunities that present themselves for enhanced or improved biomedical polymers are in the following areas ... 

Nanocomposites have potential applications in various industries namely automotive, packaging, aerospace, electronics, biomedical and defense. Various applications are briefly presented in Table 7.2. The first successful use of polymer/clay nanocomposites was in automotive industries. Reseacher at Toyata could reduce the weight of the automobile parts about 40% by using nanomaterials compared to the conventional... 

The biomedical use of bioresorbable polymers is beheved to have begun in the late 1960s with the approval of the first bioresorbable sutures. Since that time, numerous applications in the biomedical field have been made. Bioresorbable polymers are indeed playing a protagonist role in medical sciences today they are a key tool in a wide number of health-related technologies and routinely used medical devices and... 

A survey is made of developments in the use of polymers in biomedical applications, including vascular prostheses, orthopaedic implants, membranes for haemodialysis and haemofiltration, intraocular and contact lenses, controlled drug release, artificial skin and artificial pancreases. 19 refs. 

Over the past decade, the use of polymers for the administration of pharmaceutical and agricultural agents has increased dramatically. The controlled release technology has changed from being merely useful in research to having a significant impact in the biomedical industry. 

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