Bioresorbable polymers aliphatic polyesters

Key words bioresorbable medical polymers, aliphatic polyesters, polyanhydrides, poly(ortho esters), polyphosphazenes, poly(amino acids), polyalkylcyanoacrylates, poly(propylene fumarate), poloxamers, poly(p-dioxanone), polyvinyl alcohol. 

PGS is a bioresorbable elastomeric polymer and extensively evaluated for various biomedical applications such as soft and hard tissue engineering and controlled drug delivery [8]. In a similar way, a number of aliphatic polyester elastomers for various biomedical applications were prepared from diacid monomers such as citric acid and a-ketoglutaric acid with aliphatic diols and triols using thermal polycondensation reactions [9-12]. 

A plethora of bioresorbable polymer materials are adopted for medical and pharmaceutical applications. Among them, aliphatic polyesters, polycarbonates, poly (amino acids), and polyphosphoesters are the main representatives. For the synthesis of NPs, aliphatic polyesters are the most adopted materials. Data on a huge number of different polyesters can be found in the literature, but this chapter focuses on the main polyesters poly (lactic acid) (PLA), poly(glycolic acid) (PGA), polycaprolactone (PCL), and their copolymers [4]. 

One of the major classes of synthetic bioresorbable polymers is that of aliphatic polyesters or poly(a-hydroxy acids). Poly(a-hydroxy acids) such as PGA, poly(lactic acid) (PLA) stereoisomers poly(L-lactic acid) (PLLA) and poly(D-lactic acid), and pol-y(lactic-co-glycolic acid) (PLGA) copolymers are the most widely used and most popular bioresorbable polymers since they received Food and Drug Administration (FDA) approval for clinical use in humans in different forms (eg, fibers for sutures, injectable forms) (Nair and Laurencin, 2007). These polymers are commonly used in regenerative medicine applications. An example is the InQu Bone Graft Extender Substitute (ISTO Technologies), an osteoconductive biosynthetic product used as bone graft substitute in the skeletal system to support new bone formation. The resorption rate of... 

The earliest synthetic bioresorbable polymers were based on linear aliphatic polyesters. More recently, many other classes of polymers have been explored to develop new resorbable systems for various biomedical applications. Today, a variety of polymer systems having tailored bioresorption profiles are being used or researched for specific biomedical applications. All these bioresorbable polymeric... 

Poly(ot-esters) are the earliest, most extensively studied, and commercially widely used bioresorbable polymers. Especially, the poly(a-hydroxy acids), including poly(glycolic acid) (PGA), stereoisomers of poly(lactic acid) (PLA) and their copolymers which have been extensively investigated for their degradation mechanisms and structure—property relationships [1—4]. Aliphatic polyesters based on poly(a-ester)s are discussed below. 

PolyhydroxyaUcanoates (PHAs), also known as bacterial polyesters , are linear aliphatic polyesters based on poly(o)-hydroxy acids). They are synthesized by bacteria as storage compounds for energy and carbon. PHAs exhibit a high degree of polymerization and command high molecular weights with values up to several million Daltons [28]. They are biocompatible, bioresorbable, and piezoelectric thermoplastic polymers. 

The most common polymer family to be attached to sohd inorganic materials is aliphatic polyesters, such as poly(E-caprolactone) (PCL). PCL is a thermoplastic, bioresorbable (via hydrolysis), and semicrystalline polymer that is synthesized by the ring-opening polymerization (ROP) of s-caprolactone monomer (Khan et al., 2010). As depicted in Fig. 3.2(e), this in situ polymerization scheme can be catalyzed by tin(II)2-ethylhexanoate(Sn(Oct)2) or boron trifluoride dimethyletherate(BF30(CH3)2)... 

MW and MW distribution are important factors in polymer degradation because of the autocatalytic character of aliphatic polyester hydrolysis. Vert et al. recommended the elimination of low MW products by the dissolution-precipitation method [23], a means to minimize degradation on processing. This procedure allowed the same authors to process totally bioresorbable composites composed of a semicrystalline PLA matrix reinforced by PGA fibers [129]. Leeslag et al. also showed that the purification of PLAioo led to a material more resistant to hydrolytic degradation [107]. Pitt et al. found that PLA50 films... 

Hie ester linkage of aliphatic and aliphatic-aromatic copolyesters can easily be cleaved by hydrolysis under alkaline, acid, or enzymatic catalysis. This feature makes polyesters very attractive for two related, but quite different, applications (i) bioresorbable, bioabsorbable, or bioerodible polymers and (ii) environmentally degradable and recyclable polymers. 

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