Polymer-based implants

New natural polymers based on synthesis from renewable resources, improved recyclability based on retrosynthesis to reusable precursors, and molecular suicide switches to initiate biodegradation on demand are the exciting areas in polymer science. In the area of biomolecular materials, new materials for implants with improved durability and biocompatibility, light-harvesting materials based on biomimicry of photosynthetic systems, and biosensors for analysis and artificial enzymes for bioremediation will present the breakthrough opportunities. Finally, in the field of electronics and photonics, the new challenges are molecular switches, transistors, and other electronic components molecular photoad-dressable memory devices and ferroelectrics and ferromagnets based on nonmetals. 

Graft copolymerisation is a unique method which is used for modifying the properties of the base polymer. A varieties of new properties can be imparted to the base polymer by implanting a number of suitable polymers. Various methods used for grafting are (i) Chemical method, (ii) Radiation method, and... 

Heat is the means of terminal sterilization that is preferred by the regulatory authorities because of its relative simplicity and the high sterility assurance that it affords. However, a significant number of traditional pharmaceutical products and many recently developed biotechnology products are damaged by heat, as are many polymer-based medical devices and surgical implants for such products alternative sterilization processes must be adopted. Whilst radiation is a viable option for... 

To be useful as an implant, the polymer must hydrolyze to small, water soluble and toxicologically safe molecules and to be useful as a surface-eroding system, the hydrolysis must occur at much higher rates in the outer layers than it does in the bulk. Therefore, the successful development of such devices requires the selection of bonds that are capable of undergoing rapid hydrolysis. Two such bonds are anhydrides which are rapidly hydrolyzed to diacids even at the physiological pH of 7.4 and ortho esters which at pH 7.4 are slow to hydrolyze but which hydrolyze at increasingly rapid rates as the pH is lowered. Polymers based on both of these linkages are under intensive development and this chapter will cover, in depth, the development and current status of poly (ortho esters). 

Edelman, E., Brown, L. and Langer R., Quantification of insulin release from implantable polymer-based delivery systems and augmentation of therapeutic effect with simultaneous release of somatostatin, ]. Pharm. Sci., 85(12), 1271, 1996. 

Malmstrom, J.A., T.G. McNaughton, and K.W. Horch, Recording properties and biocompatibility of chronicaUy implanted polymer-based intrafascicular electrodes. Ann. Biomed. Eng., 1998, 26 1055-1064. 

Lawrence, S.M. et al.. Long-term biocompatibility of implanted polymer-based intrafascicular... 

Several implants are commercially available for total or partial disk replacements. For instance, currently, two polymer-based cervical and two lumbar disk prostheses approved by the FDA are being widely used for disk replacement applications [103]. The first artificial disk (DePuy Inc.), approved by the FDA in 2004, was based on a hard-on-soft technology, which employed a CoCrMo alloy in conjunction with UHMWPE. Alternatively, ProDisc-C (Eigure 19.3b,i), approved in 2006 and 2007 for both lumbar and cervical replacements, respectively, was based on similar types of polymer composites. More recently, Medtronic developed Bryan prosthesis using titanium alloys and PU polymer (Figure 19.3b,ii) [104]. 

Sinclair, R.G. and Gynn, G.M. (1972) Preparation and Evaluation of Glycolic and Lactic Acid-Based Polymers for Implant Devices Used in Management of Maxillofacial Trauma. II, Battelle Memorial Institute, Columbus, OH. Sinclair, R.G. (1973) Slow-release pesticide system. Polymers of lactic and glycolic acids as ecologically beneficial, cost-effective encapsulating materials. Environ. Sci. TechnoL, 7 (10), 955—956. Manninen, M.J. and Pohjonen, T. 

Once an SMP device is implanted within the body and fully activated, the device ceases to be shape-memory and should have the properties of a typical polymer-based device and are subject to all the same long-term performance concerns. Obviously, long-term biocompatibility and carcinogenicity are a concern of implantable polymeric materials however, mechanical properties of polymers with respect to water absorption and biodegradation will be discussed for the remainder of this chapter. 

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