Resorbable polymers

The high level of interest in the ring-opening polymerization (ROP) of cyclic esters (lactones) stems from the biocompatibility and biodegradability of their polymers. Resorbable aliphatic... 

Lazennec, J. Y., Madi, A., Rousseau, M. A., Roger, B. Saillant, G. (2006) Evaluation of the 96/4 PLDLLA polymer resorbable lumbar interbody cage in a long term animal model. European Spine Journal, 15, 1545-1553. 

Biodegradation is more connected to biological agents (enzymes or microbes) as dominant components in degradation (eg, disintegration, erosion, dissolution). In bioresorption, the degradation products of polymers resorb in the body (metabolized). Bioabsorption involves polymers that dissolve in biofluids and are eliminated without chain scission, such as poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (Buchanan, 2008). 

The concept of using biodegradable materials for implants which serve a temporary function is a relatively new one. This concept has gained acceptance as it has been realized that an implanted material does not have to be inert, but can be degraded and/or metabolized in vivo once its function has been accompHshed (12). Resorbable polymers have been utilized successfully in the manufacture of sutures, small bone fixation devices (13), and dmg dehvery systems (qv) (14). 

There has been recent interest in lactic acid polymers and copolymers. These materials are environmentally attractive in that renewal and cheap source materials such as potato waste and cheese whey may be used. Such materials have been used for some time in degradable and resorbable surgical sutures but recent efforts have been directed at food packaging applications. There is... 

Special mention must be made of poly(lactic acid), a biodegradable/bio-resorbable polyester, obtained from renewable resources through fermentation of com starch sugar. This polymer can compete with conventional thermoplastics such as PET for conventional textile fibers or engineering plastics applications. Hie first Dow-Cargill PLA manufacturing facility is scheduled to produce up to 140,000 tons of Nature Works PLA per year beginning in 200245 at an estimated price close to that of other thermoplastic resins U.S. l/kg.46 Other plants are planned to be built in the near future.45... 

Stjemdahl A, Wistrand AF, Albertsson A-C (2007) Industrial utilization of tin-initiated resorbable polymers synthesis on a large scale with a low amount of initiator residue. Biomacromolecules 8 937-940... 

The most common synthetic biodegradable polymers for suture material and their corresponding weight loss in aqueous solution are listed in Table 3.10. Of these, poly(glycolic acid), PGA, poly(lactic acid), PLA, and copolymers of these two polyesters are the most widely used for resorbable sutme material. PGA is a tough. 

Polymeric carriers are biodegradable or water-soluble polymer matrices, typically in the form of colloidal-sized particles (microspheres or nanospheres), rods, or films. The active agent is entrapped within but not chemically bonded to the matrix. The drug is released in a sustained fashion as the polymer is dissolved or degraded, eroded, and finally resorbed [24,30,58-62]. 

In contrast, there are substances like some nitrofurane derivatives for which the presence of particular molecular structures is the decisive condition. Thus, a nitrofurane derivative prepared by Casini and his co-workers (87) has shown bacteriostatic properties similar to classical low molecular preparations of nitrofurane, e.g. l-[5-(nitrofurfuryliden)amino]hydantoine. The polymeric substance shows an activity considerably longer than that of the reference substance if parenterally applied, whereas oral application gives no effect. This is easy to understand because, as already mentioned, polymers cannot be resorbed in the digestive tract. Here, the active polymeric substance [22] has been prepared by condensation of 5-nitrofuraldehyde with poly(acryloylhydrazide). 

Resulting poly(a-hydroxyacids) are important biomaterials used as resorbable sutures and prostheses [196]. The mechanism of polymerization is not well established. Polymerization may be initiated with Lewis acids (SbF3, ZnCl2, SnCl4) however, other typical cationic initiators (e.g, triethyloxonium or triphenylcarbenium salts) fail to initiate polymerization [197]. Thus, it is not clear whether polymerization proceeds by typical cationic mechanism or rather involves the coordination mechanism. The chain transfer to polymer resulting in transesterification was postulated [198,199] and confirmed later by detailed, 3C NMR studies of lactide copolymers [200]. 

Kim GY, Tyler BM, Tupper MM, et al. Resorbable polymer microchips releasing BCNU inhibit tumor growth in (he rat 9L flank model. J Control Release 2007 123 172-178. 

Several criteria define the ideal material for a cell transplantation matrix, (i) The material should be biocompatible, in the sense that it does not provoke a connective tissue response which will impair the function of the new tissue (ii) it should be resorbable, to leave a completely natural tissue replacement (this is important, because it could avoid some of the problems that occur in long-lasting polymers such as those used in breast implants) (iii) it should be processable into a variety of shapes and structures which retain their shape once implanted and (iv) the surface should interact with transplanted cells in a way which allows retention of differentiated cell function and which promotes cell growth if such growth is desired. 

Poly (e-caprolactone), poly lactides, and polyglycolides have quite unusual properties of biodegradability and biocompatibility. The majority of polymers used in the biomedical field to develop implants, sutures, and controlled drug-delivery systems are the aforesaid resorbable polyesters produced by ring-opening polymerization of cyclic (di)esters. 

The solubility of apatites is becoming more important as emphasis is being placed on biomaterials for regeneration of tissues (Hench 1998b). Where apatites are incorporated with resorbable polymers for tissue engineering applications, it will become necessary to match the solubility rate of the inorganic and organic components within the composite. 

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