Polymers, biodegradable

Polymers that slowly dissolve following implantation into the body have many potential medical uses. A wide variety of biodegradable polymers have been synthesized and characterized. This section reviews the characteristics of several families of biodegradable polymers. Readers interested in more details on the synthesis and chemistry of these classes of materials should consult the references provided, or general texts on biodegradable polymers [1], 

The market for biodegradable materials is limited to niche applications. General politically motivated goals in the past, like substituting commodity products for environmental reasons, provoked several costly industrial developments over many years. Finally, some of them proved imrealistic since the alternatives failed in properties as well as in processability and economics and sometimes also due to an undefined environmental outcome. 

This class of polymers is not described in this document because their production in the European Union currently does not represent a significant environmental impact. 

biodegradable products are developed for markets where biodegradability is considered a technical advantage like for instance  

Biodegradability does not depend on the origin of the raw materials but on the chemical structure. Thus, materials from renewable as well as from synthetic resources are on the market. While cellophane, starch and polyhydroxybutyrate have existed on the market for many years, newer developments include poly (L-lactide) as well as numerous fossil based biodegradable polymers, e.g. copolyesters. 

A legal situation recognising organised composting as one means of recycling and a standardised testing of the degradation behaviour are important preconditions for their successful development. 

Fully biodegradable polymers are completely converted into small molecules, such as carbon dioxide, water, minerals and biomass, by microorganisms and do not have any environmental impact 

Biodegradable and other naturally degradable polymers have been attracting attention from the view point of environmental protection. One of the most important issues for the tailoring of biodegradable polymers is the rate of degradation of the product. 

A number of plasticizers have been investigated for potential use in biodegradable polymers. Favorable compounds are citrate plasticizers. These are biodegradable esters. 

PLA has been extensively studied in medical implants, suture, and drug delivery systems due to its biodegradability. The synthesis of several mixed alcohol esters has been described in the literature 

PLA has been plasticized with four commercially available citrate plasticizers triethyl, tributyl, acetyltriethyl and acetyltributyl citrate 

The plasticizing effects on thermal and mechanical properties of PLA are satisfactory as the citrate esters produce flexible materials. 

The developments in biodegradable polymers has been reviewed extensively in recent years [36-43]. An increasing range of these polymers are 

Biodegradable polymers have been classified into three categories for 

Urea-formaldehyde resins are used to control the rate of delivery of fertiliser and pesticides. Their use permits nitrogen contents up to 30% in the fertiliser without burning the soil. 

2 Synthetic biodegradable polymers. Poly (8)-caprolactone), used in the manufacture of seeding pots, is susceptible to hydrolysis of the ester linkage by microbes. Other materials in this category include poly(tetramethylene adipate), block copolymers of poly(ethylene succinate) and poly(ethylene glycol), and block copolymers of poly(tetramethylene glycol) and poly(ethylene succinate). Copolymers based on lactic acid have also been synthesised. 

Unfortunately, the slow rate of the degradation processes [50], the additional cost of manufacturing these polymers, and the fact that they may produce materials that could leach from the landfill sites means they are not considered to be a feasible method of reducing the problem of solid-waste disposal [51]. 

Both synthetic polymers and natural polymers have been broadly studied as biodegradable polymeric biomaterials. Polymeric biomaterials based biodegradation entails cleavage of hydrolytically or enzymatically sensitive bonds in the polymer resulting in to polymer erosion [8]. Based on the means of degradation, polymeric biomaterials can be further divided into  

3 Natural Polysaccharides as Carriers and Scaffolds FOR Biomolecules and Cell Delivery in Tissue Engineering Applications 

Tissue engineering is the most reliable therapeutic approach that combines cells, biomaterials, and microenvironmental factors to stimulate differentiation signals into surgically transplantable formats and additionally encourage tissue repair 

4 Natural and Synthetic Polysaccharides for Wounds and Burns Dressing 

5 Present Research on the Blends of Natural and Synthetic Polymers as New Biomaterials 

Key words biodegradable polymer, biodegradation, classification of biodegradable polymers, mechanism of biodegradation, testing method, application. 

Biodegradation is the most acceptable means of solving these problems, as it is cost effective and eco-friendly. In many specific applications such as agriculture and biomedical use, biodegradation is the only means of degrading used polymers. Biodegradable polymers are therefore the only solution to the problem of landfill. 

There are a certain number of norms in existence, such as the American norm ASTM D-5488-94d, or the European harmoiuzed standard EN 13432, giving defiiutions for terms such as biodegradable, compostable and others which are widely used - and sometimes incorrectly - as an argument in favor of the use of so-called enviromnentally friendly materials. 

Biodegradable, in order to qnalify a packaging material as biodegradable, EN 13432 2000 stipnlates that the following tests be carried out  

It is difficult to compare the results obtained on the basis of different norms, because unfortunately, the test conditions (in terms of humidity, temperature cycle, etc.) vary from one norm to another. 

A few general rules enable us to predict the evolution of a material s biodegradabihty. For example, an increase in factors such as hydrophobic nature, molar masses, crystallinity or size of crystalhne regions harms a substance s biodegradabihty. On the other hand, the presence of polysaccharides, say (in the case of blends) favors biodegradatioa 

The above views, which are based on experimental evidence (pages 53-55), are often in conflict with the beliefs of environmental campaigners. The popular view is that all synthetic polymers are non-biodegradable in the environment. The following excerpts from a controversial report sponsored by Greenpeace in 1990 entitled Breaking down the degradable plastics scam epitomise popular misconceptions  

Department of Plastics Engineering, University of Massachusetts at Lowell 

Plastics and the Environment, Edited by Anthony L. Andrady. ISBN 0 71-09520-6 2003 John Wiley Sons, Inc. 

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Certain bacterial species produce polymers of y-hydroxybutyric acid and other hydroxyalkanoic acids as storage polymers. These are biodegradable polymers with some desirable properties for manufacture of biodegradable packaging materials, and considerable effort is being devoted by ICI Ltd. and others to the development of bacterial fermentation processes to produce these polymers at a high molecular weight (66). 

There are numerous further appHcations for which maleic anhydride serves as a raw material. These appHcations prove the versatiHty of this molecule. The popular artificial sweetener aspartame [22839-47-0] is a dipeptide with one amino acid (l-aspartic acid [56-84-8]) which is produced from maleic anhydride as the starting material. Processes have been reported for production of poly(aspartic acid) [26063-13-8] (184—186) with appHcations for this biodegradable polymer aimed at detergent builders, water treatment, and poly(acryHc acid) [9003-01-4] replacement (184,187,188) (see Detergency). 

Biodegradable polymers and plastics are readily divided into three broad classifications (/) natural, (2) synthetic, and (J) modified natural. These classes may be further subdivided for ease of discussion, as follows (/) natural polymers (2) synthetic polymers may have carbon chain backbones or heteroatom chain backbones and (J) modified natural may be blends and grafts or involve chemical modifications, oxidation, esterification, etc. 

Heteroatom Chain Backbone Polymers. This class of polymers includes polyesters, which have been widely studied from the initial period of research on biodegradable polymers, polyamides, polyethers, polyacetals, and other condensation polymers. Their linkages are quite frequendy found in nature and these polymers are more likely to biodegrade than hydrocarbon-based polymers. 

G. Swift, Expectationsfor Biodegradation TestingMethods, International Biodegradable Polymer Workshop, Osaka, Japan, Nov. 1993. 

Germany, 1990 M. Vert, J. Feijin, A. Albeitsson, G. Scott, and E. ChieUini, eds.. Proceedings of the 2nd International Scientific Workshop on Biodegradable Polymers andPlastic, Monpelier, Prance, Nov. 25—27, 1991, The Royal Society of Chemistry, London, 1992. 

E. A. Dawes, Novel Biodegradable Microbial Poljmers, Proceedings of an Advanced Workshop on Neiv Biosjnthetic Biodegradable Polymers of Industrial Interest, Sitges, Spain, 1990, NATO ASI Series E Apphed Science, Vol. 186, Kluwer Academic Pubhshing, London, 1990. 

In Industrial Chemicals. Recendy, as some amino acids (eg, L-glutamic acid, L-lysine, glycine, DL-alanine, DL-methionine) have become less expensive chemical materials, they have been employed in various appHcation fields. Poly(amino acid)s are attracting attention as biodegradable polymers in connection with environmental protection (236). 

T. Endo and H. Kubota, Mbstracts of the International Symposium on Biodegradable Polymers, Oct. 1990, Tokyo, Japan, pp. 114—119 Y. Saotome, T. Miyazawa, andT. Endo, Chem. Lett. 21 (1991). 

In order to become useful dmg delivery devices, biodegradable polymers must be formable into desired shapes of appropriate size, have adequate dimensional stability and appropriate strength-loss characteristics, be completely biodegradable, and be sterilizahle (70). The polymers most often studied for biodegradable dmg delivery applications are carboxylic acid derivatives such as polyamides poly(a-hydroxy acids) such as poly(lactic acid) [26100-51-6] and poly(glycolic acid) [26124-68-5], cross-linked polyesters poly(orthoesters) poly anhydrides and poly(alkyl 2-cyanoacrylates). The relative stabiUty of hydrolytically labile linkages ia these polymers (70) is as follows ... 

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