Polymers lactic/glycollic acid, hydrolysis

As pointed out by Heller (2), polymer erosion can be controlled by the following three types of mechanisms (1) water-soluble polymers insolubilized by hydrolytically unstable cross-links (2) water-insoluble polymers solubilized by hydrolysis, ionization, or protonation of pendant groups (3) hydrophobic polymers solubilized by backbone cleavage to small water soluble molecules. These mechanisms represent extreme cases the actual erosion may occur by a combination of mechanisms. In addition to poly (lactic acid), poly (glycolic acid), and lactic/glycolic acid copolymers, other commonly used bioerodible/biodegradable polymers include polyorthoesters, polycaprolactone, polyaminoacids, polyanhydrides, and half esters of methyl vinyl ether-maleic anhydride copolymers (3). 

The rate of hydrolysis of the partially ethoxylated polymer was retarded, although not to the extent calculated from theory (Fig. 25), suggesting some contribution to the rate of chain scission by an uncatalyzed process. End-capping poly (glycolic acid-co-lactic acid) has a similar effect on the rate of hydrolysis of this polyester (100). 

Biodegradation of the aliphatic polyesters occurs by bulk erosion. The lactide/gly-colide polymer chains are cleaved by random nonenzymatic hydrolysis to the monomeric lactic and glycolic acids and are eliminated from the body through the Krebs cycle, primarily as carbon dioxide and in urine. 

In general, absorbent sutures are composed of materials that are natural to mammals, such as catgut, and to materials that are either quite susceptible to hydrolysis and/or polymers derived from natural materials such as polyglactin, which is a copolymer of lactic and glycolic acid. Nonabsorbent sutures can be made from natural materials such as cotton, which is a plant material, polymers that range from being hydrophobic to hydrophilic, and steel. 

Incorporation of F-chain end-groups into biodegradable polymers can help modulate their biodegradation (e.g., the hydrolysis rate of polyesters) and drug release [68], Surface treatment of polymer for example, poly(lactic-co-glycolic acid)... 

Hydrolysis of the polymers can be affected by spontaneous degradation of the material from the surface or the bulk or by enzymatic hydrolysis. Copolymers of lactic acid and glycolic acid (PLGA) are by far the most common biocompatible polymers that undergo bulk erosion by... 

For much of the last century, scientists attempted to make useful plastics from hydroxy adds such as glycolic and lactic acids. Poly(glycolic acid) was first prepared in 1954, but was not commercially developed because of its poor thermal stability and ease of hydrolysis. It did not seem like a useful polymer. Approximately 20 years later it found use in medicine as the first synthetic suture material, useful because of its tendency to undergo hydrolysis. After the suture has served its function, the polymer biodegrades and the products are assimilated (Li and Vert 1995). Since then, suture materials, prosthetics, artificial skin, dental implants, and other surgical devices made from polymers and copolymers of hydroxy carboxylic acids have been commercialized (Edlund and Albertsson 2002). 

The polyester named Lactomer is an alternating copolymer of lactic acid and glycolic acid. Lactomer is used for absorbable suture material because stitches of Lactomer hydrolyze slowly over a two-week period and do not have to be removed. The hydrolysis products, lactic acid and glycolic acid, are normal metabolites and do not provoke an inflammatory response. Draw the stmeture of the Lactomer polymer. 

A biodegradable implant is being developed that consists of a steroid combined with a degradable polymer that gradually releases the corticosteroid as a polymer that imdergoes hydrolysis. The breakdown byproducts of this implant are glycolic acid and lactic acid, which are then... 

This results in non-Hnear release kinetics, and makes accurate control of drug release very difficult. Additionally, release kinetics almost invariably show a large initial burst that is undesirable with dmgs that have a narrow therapeutic index, and is also wasteful. Because polymer hydrolysis liberates lactic and glycolic acids, and that occurs throughout the bulk of the material, the internal pH can reach values as low as 1 to 2 [30] as a result, incorporated drugs that are acid-labile (e.g., DNA) will show severely compromised bioactivity. 

PDS is produced by polymerization of p-dioxanone. The polymer has unusually high flexibility and, unlike copolymers of lactic and glycolic acid, can be used to produce a variety of monofilament sutures. Since PDS is a polyester, like pLA and pGA, the polymer chains break down by hydrolysis. Currently, PDS is also used in orthopedic applications (Orthosorb ), as a fixation element for bone repair. 

In contrast, degradable polyesters are generally derived from aliphatic monomers including lactic acid, glycolic acid, -caprolactone, etc. In these materials, no aromatic structures are present in the main chain, enabling hydrolysis to occur more easily. Although produced synthetically, these polymers are often preferred over natural polymers, as their properties can be tailored and the resulting materials have more predictable lot-to-lot uniformity than their natural counterparts [9]. 

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