Biodegradable synthetic polymers have been widely used in biomedical applications, such as in gene delivery, tissue engineering, diagnosis, medical devices, antibacterial/antifouling biomaterials, and as drug carriers. Their utility can be attributed to several factors, including their chemical and mechanical properties, smart responsiveness to diverse external stimuli and their improved processibility, functionality and degradability over other materials. In this article, we will discuss the key features of biodegradable polymers in biomedical applications from a sustainability perspective.
Biodegradable polymers in biomedical applications
Biodegradable polymers are petrochemical or bio-based compounds that can be degraded by microorganisms through the process of biodeterioration, bio-fragmentation, and assimilation. Due to their excellent biodegradability and biocompatibility, these polymers can be exploited for a number of biomedical applications. An example of this is tissue engineering. In the tissue engineering process, a biodegradable porous scaffold plays a crucial role. Scaffolds not only provide a tissue template, but also support cell attachment, proliferation and differentiation. One promising candidate for tissue engineering applications is aliphatic polyester, which can form stable porous structures that do not dissolve or melt in vitro and can undergo degradation rapidly through hydrolysis of the ester groups in their backbones in aqueous solutions or in vivo. Biodegradable polymers are also of great interest in the field of drug delivery. A biodegradable drug carrier can bring its payload to a specific site in the body and then degrade to release the drug in a controllable manner. Meanwhile, by-products of the biodegradation process are eliminated from the body via natural metabolic pathways. In addition, releasing the drug at a specific target site decreases the toxicity of the drug to healthy cells due to less off-target non-specificity. Synthetic biodegradable polymers also have great potential in gene delivery applications. Of non-viral gene-delivery vectors, cationic synthetic biodegradable polymers have gained the largest interest for use as an alternative to viral vectors due to their high chemical versatility, biocompatibility and low immunogenicity. The biodegradable features of synthetic biodegradable polymers can be utilized to unpack and release therapeutic genes into target cells as well as lower their accumulation in treated cells.
The degradation process of biodegradable polymers and their environmental impact
Traditional synthetic polymers have become a serious environmental hazard as they are typically disposed of through landfilling, recycling and incineration. The long degradation period of plastic waste, the increasing scarcity of landfill sites and the air pollution contributed by incineration have made people turn to biodegradable polymers due to their degradable nature, which is critical for global waste management. The biodegradation process is typically carried out by the enzymatic work of microorganisms, thereby forming metabolic products such as methane, water, biomass and carbon dioxide. This occurs through several steps. Firstly, extracellular enzymes and abiotic agents depolymerize long-chain polymers into oligomers through hydrolysis, oxidation and photo-degradation. Next, oligomers are bio-assimilated by microorganisms and then mineralized, followed by either aerobic degradation or anaerobic degradation. Aerobic degradation occurs in the presence of oxygen, producing CO2, H2O, biomass and residue, while anaerobic degradation takes place in the absence of oxygen, producing the above components along with methane. The non-toxic end products from biodegradable polymers minimize the stress from shrinking landfill availability and plastic pollution. Additionally, many biodegradable polymers, such as polylactic acid (PLA) and polyhydroxy alkanoates (PHA), are synthesized from renewable resources, thereby reducing the dependency on petroleum supply during manufacturing and decreasing greenhouse gas emission into the atmosphere.
One category of biodegradable polymers whose degradation mechanisms have been thoroughly characterized are hydrogels, which are three-dimensional crosslinked polymeric networks with high water absorbability that can be used for diverse drug delivery applications. Hydrogels can be degraded in the body via several mechanisms, such as hydrolytic degradation, pH change, enzymatic degradation and photodegradation. Upon desirable physiological environmental changes, hydrogels are degraded into biocompatible, non-toxic materials that are eliminated through the body’s metabolic system. Another well-characterized synthetic biodegradable polymer is PLA. These polymers possess hydrolytically labile chemical bonds in their polymer backbone, which can be degraded via hydrolysis into lactic acid, which can be rapidly eliminated from the body without toxicity to the body.
Future perspective of biodegradable polymers in biomedical industry
The future aspect of biodegradable polymers seems promising. However, there are still several challenges to overcome in order to achieve widespread use of biodegradable polymers in the fields of biomedicine and waste management, including the design and development of diverse and more sophisticated polymers to suit various applications, high manufacturing costs and economic burden for patients. Nevertheless, it is clear that biodegradable polymers will open up new possibilities in the effort to establish a better environment devoid of hazardous substances and products, as well as push the envelope of the development and applications of polymer-based biomedical products.
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