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In the present scenario, the research is now being focused on the naturally occurring polymers that can gradually replace the existing synthetic polymers for the development of bio composites having applications in medical surgeries and human implants. With promising mechanical properties and bio compatibility with human tissues, poly lactic acid (PLA) is now being viewed as a future bio material. In order to examine the applicability of PLA in human implants, the current article sheds light on the synthesis of PLA and its various copolymers used to alter its physical and mechanical properties. In the latter half, various processes used for the fabrication of biomaterials are discussed in detail. Finally, biomaterials that are currently in use in the field of biomedical (Scaffolding, drug delivery, tissue engineering, medical implants, derma, cosmetics, medical surgeries, and human implants) are represented with respective advantages in the sphere of biomaterials.
Keywords: poly lactic acid, human implant, biomaterial, drug delivery, implantation, tissue engineering
Poly lactic acid (PLA) is a thermoplastic polymer derived from various natural resources such as corn starch, sugarcane, biomass, and other vegetable wastes by the process of fermentation. It was first discovered in the 1920s by Wallace Carothers and commercialized in the 1990s at a larger scale owing to its better physical, mechanical, and thermal characteristics. Today, it has made a decent place in various food processing, textile, agriculture, and cosmetic sectors [1,2,3]. Apart from these sectors, its presence can also be felt in mechanically driven plastic equipment used on daily basis. The scope of PLA further widens due to its biodegradable nature particularly, in the field of medicine and in instruments and surgery [4,5]. The medical devices made of PLA are increasing rapidly in numbers due to their desired physical and mechanical properties and will hopefully rise in near future. The most important role which is currently being played by PLA is in medical surgeries and is also the topic of discussion in the present articles. Due to many unfortunate reasons, medical surgeries are performed. These medical surgeries require pre-surgery medications and implant material or devices [6,7]. Due to its desirable characteristics such as compatibility with human tissues, biodegradability, stiffness, non-toxicity, durability, and ease to resorb, PLA is now being considered one of the best-suited biomaterials. However, its applications are restrained due to its low strength, low heat resistance, and difficulty in machining. These characteristics can be improved by bringing various physical and chemical changes in the PLA. For example, blending PLA with other copolymers can bring significant changes in the structure of PLA with improved strength, toughness, and thermal properties, which can be used as a bone implantable material [8]. The strength of PLA can also be improved by reinforcing it with carbon fiber or other synthetic fiber but at the cost of biodegradability and resorbability. Using natural fiber in place of synthetic fiber, the biodegradable nature of PLA can be maintained but decreases the mechanical performance when compared with synthetic fiber. Several research works have been carried out in the past dealing with the physical, mechanical, and compatibility nature of PLA to make it better and better over a period of time for its implementation in medical surgeries and implants [9].
Poly lactic acid in medical implants is being used in many forms such as film, sphere, hydrogels, foam, blends, fiber, particulates, capsules, etc. Its composite offers the ability to embrace surgical applications such as the malfunctioning of tissue or cell in in vitro or in vivo surgeries like the malfunctioning of tissue or cell in in vitro or in vivo surgeries. However, work is still to be done for its improved functioning, longevity, and durability, which cannot be accomplished without past background information and suggestive measures that need to be taken care of [10,11]. The present article, therefore, provides a detailed review of the synthesis of PLA, its behavior with copolymers upon blending, its fabrication for medical devices, and its current application in the medical field. The discussion carried out in this article will not only provide valuable evidence about PLA but also frame a comprehensive background to project PLA as a promising biomaterial. The article also discusses several issues regarding its compatibility with other polymers and with human tissues as a medical implant.
The starting substrate to produce PLA is lactic acid (LA), which is basically acidic in nature and at its core structure, the carbon atoms are present in an asymmetrical form. Lactic acid exists in two isomers viz: (i) levorotatory form (D or R (−) lactic acid) and (ii) dextrorotatory form (L or S (+) lactic acid) [12]. D-Lactic acid is extracted from the muscles of animals while L-LA is produced by the fermentation of sugar through the action of bacteria. Beans, peas, corn, sugar beet, soymilk, and potato are some of the chief sources of L-LA. In biomedical applications, L-LA is preferred over D-LA due to several disadvantages such as metabolism rate, synthesis, and lack of optical purity [13]. Apart from PLA, several chemicals, e.g., propylene glycol, acrylic acid, and acetaldehyde are also synthesized from LA [14]. Lactic acid does not possess any charge and, it is very small in molecular size which enables it to infuse in the lipid membranes of cells [15]. It can serve as an energy source and provide antioxidant characteristics that protect from cell damage upon reaching the core of the cell via a monocarboxylate transporter. A large portion of Lactic acid is processed chemically by fermentation in the presence of bacteria whereas a small part of LA is obtained by the process of hydrolysis [16]. Usually, the process of fermentation yields a racemic mixture of LA, L-LA, and D-LA with optical purity [17]. The percentage of L and D in the LA depends upon the strain selected for the process. The biological advantage of lactic acid is not one but many. It is biocompatible with human tissue, provides support to cell tissues, allows accelerating growth of cell generation, and gets absorbs easily if the need arises [18].
The production of LA is carried out either by hetero fermentation or homo fermentation in the presence of various catalysts. The isomers formed during the process depend on the enzymes used [19] as shown in Figure 1 . The input sources such as glucose, sugar, carbohydrates, etc. used in the production are very costly but can be brought down by using alternate sources such as agriculture waste, biomass residue, and waste collected from the food industry provided the chemical process and the enzymes used are different from the conventional ones [20].