Senior research associate, Syngene International Ltd, Bengaluru, India
The relevance of Biodegradable polymers
The severe environmental problems including increased difficulties of waste disposal and the deepening threat of global warming (due to carbon dioxide released during incineration) caused by the non-biodegradability of polyethylene (used in packaging and agriculture field) have raised concerns all over the world. We must confront them to build a new society and economy free of plastic pollution in the 21st century. The use of biodegradable counterparts as an alternative to non-biodegradable polymers is therefore evoking considerable interest lately. In recent years, biodegradable polymers from renewable resources have attracted much interest for environmental and medical applications because of their desirable properties of biodegradability, biocompatibility and natural abundance. Polylactides (PLA) is the leading candidate among these biopolymers. Due to its high capital cost and slow degradation rate as compared to the waste accumulation rate, the focus of PLA has been mainly on the biomedical field. However, certain limitations of PLA such as low hydrophilicity and degradation rate, poor soft tissue compatibility, low thermal and physical properties, and lack of processability limit their wide utilization. These limitations of PLA along with its increased use in medicine generated more research interest for new materials by copolymerization of PLA with suitable monomers and/or polymers so that some of the problems associated with them can be solved for wider applications. Controlled solvation and degradation could be achieved by graft copolymerization of lactide into chitosan, an amino polysaccharide present in nature.
What is Biodegradation?
There are several definitions on biodegradation which depend on the field of application of the polymers (biomedical area or natural environment). Van der Zee and Seal reviewed all of the definitions found in different standards. The definition given by Albertsson and Karlsson appears to give a broad outlook on biodegradation. According to them, biodegradation can be defined as an event that takes place through the action of enzymes and/or chemical decomposition associated with living organisms and their secretion products. It is also necessary to consider abiotic reactions like photodegradation, oxidation and hydrolysis, which may alter the polymer before or during biodegradation because of environmental factors.
Synthetic Biodegradable polymers
Polymers produced from feedstocks derived from petrochemical or biological resources such as polyesters, polycaprolactone (PCL), polyamides, polyurethanes, polyureas, polyanhydrides, poly(vinyl alcohol) (PVA), poly(vinyl esters) etc. generally fall in this classification. The higher cost of production is the main barrier for the wide utilization of these polymers.
Polylactide: A dominating biopolymer
Polylactides (PLA) is considered as the most versatile material among biodegradable polymers because of its inherent biodegradability, biocompatibility and the easy availability from renewable agricultural sources. These attributes make them a leading candidate in biomedical and pharmaceutical industries as a resorbable implant material, wound closure, bone fixation devices and a vehicle for controlled drug delivery. It is also used as an environment friendly plastic, although their market is still limited due to its higher cost and slow degradation rate as compared to the waste accumulation rate. However, their clinical applications are sometimes affected by the high hydrophobic and consequent poor water uptake, which results in a slow hydrolytic degradation rate. Another potential disadvantage is the complications resulting from the accumulation of lactic acid produced in the process of PLA degradation and its poor processability. Ultra-high molecular weight is required for processing PLA into strong fibres. PLA is belonging to the group of thermally less stable polymers and has poor physical properties and high cost of production. Copolymerization of lactide with other comonomers and/or polymers is used to modify the properties of PLA and to control its degradation behaviour suitable for the specific applications in the field.
PLA based copolymers: A mini literature review
In 1932, Carothers demonstrated the synthesis of po]y(L-lactide) (PLLA) by ring-opening polymerization of lactides. Since that time PLA has been used as a bioabsorbable material in the medical and pharmaceutical fields. However, the application scope of PLA is limited because of the certain weaknesses mentioned above. Copolymerization of L-lactide with other monomers has been recognized as an important tool to modify PLLA suitable for specific applications in the field. The frequently employed comonomers are D-lactide, Meso-lactide, glycolide, caprolactone (6-CL) and trimethylene carbonate (TMC). Grijpma and Pennings have done a detailed study on the synthesis, thermal properties and hydrolytic degradation of these copolymers. Glycolide copolymers are much more hydrophilic than PLLA, while 8-CL and TMC reduce the glass transition temperature of the L-lactide copolymer. All comonomers decrease the crystallinity of the polylactide and provide optimal mechanical properties to the materials. Another approach for increasing the hydrophilicity and degradation rate of polylactide was the copolymerization with other polymers such as polyethylene glycol and chitosan. In 1998, Kim et al. prepared biodegradable nanospheres composed of methoxy poly(ethylene glycol) and D, L-lactide block copolymers as novel drug carriers. In 2001, Otsuka et al. gave a review article regarding the self-assembly of poly(ethylene glycol)-PLA block copolymers for biomedical applications. Albertsson et al. prepared a pH-sensitive physically cross-linked hydrogel by grafting D, L-lactic acid onto amino groups in chitosan without using a catalyst. Later Dutta et al. reported that the molecular mechanism of gelation involves interaction between chitosan and lactic acid. Yao et al. reported the in vitro fibroblast static cultivation on a cytocompatible poly (chitosan-g-L-lactic acid) film and the cell growth rate on the copolymer film was found to be much faster than that of the chitosan film. In another work, Liu et al. have reported the synthesis of a brush-like copolymer of polylactide grafted onto chitosan. Later, Wu et al. studied the amphiphilic properties of a graft copolymer of water-soluble chitosan and polylactide prepared by using triethylamine as a catalyst. In 2005, Peesan et al. prepared hexanoyl chitosan PLA blend films and in 2006, Wan et al. prepared a biodegradable polylactide/chitosan blend film. The biodegradability of these chitosan/polylactide graft copolymers was, however, not studied in these works.
It can be noted from the preceding discussions that the biodegradable polymers for short time applications have attracted much interest all over the world in different sectors such as surgery, pharmacology, agriculture and the environment. The reason behind this growing interest is the incompatibility of the polymeric waste with the environment where they are disposed of after the usage. The development of novel biodegradable polymers satisfying the requirement of degradability, compatibility with the disposed environment and the release of low-toxicity degradation products are the ultimate solution to these issues. An analysis of the existing biodegradable plastics indicates inadequacies in terms of either technology or cost of production especially in the case of applications in environmental pollution. So, there is a need to look for novel methods for introducing biodegradability in existing polymers for use in the environmental areas. Alternately, one can consider modifying suitably the natural polymers obtained from the renewable resources, by tailoring their properties by altering the composition and structure to solve some of the existing problems associated with them for the synthesize of new biodegradable polymers.
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About the author
Mr Sandesha Nayak is Senior research associate at Syngene International Ltd, Bengaluru, India from past four years.