Polylactic acid (PLA) is a new type of bio-based and renewable biodegradable material, which is made from starch raw materials proposed by renewable plant resources (such as corn, cassava, etc.). The starch raw material is saccharified to obtain glucose and then fermented by glucose and certain strains to produce high-purity lactic acid, and then synthesize polylactic acid with a certain molecular weight by chemical synthesis. It has good biodegradability. After use, it can be completely degraded by microorganisms in nature under specific conditions and finally produces carbon dioxide and water without polluting the environment. This is very beneficial to protecting the environment and is recognized as an environmentally friendly material.
1- PLA degradation mechanism
The molecular structure of polylactic acid (PLA), in which the ester bond is easily hydrolyzed, can be degraded by microorganisms in the body or in the soil to produce lactic acid, and the final products of metabolism are water and carbon dioxide, so it will not cause toxic and side effects to the human body and is very safe to use. Therefore, polylactic acid has been used in many aspects, such as medicine and pharmacy, such as surgical sutures, drug-controlled release systems, and so on.
Due to the optical activity of lactic acid, there are three corresponding polylactic acids: PDLA, PLLA, and PDLLA (racemization).
PLLA and PDLA are partially crystalline polymers with good mechanical strength and are often used as medical sutures and surgical orthopedic materials. Drug-controlled release preparations often use PLLA and PDLLA but more often use PDLLA. The degradation product of PLLA, L-lactic acid, can be completely metabolized by the human body, so it is more competitive than D-PLA.
2- Degradation in vivo
The hydrolysis of PLA is a complex process, mainly including four phenomena: water absorption, breaking of ester bonds, diffusion of soluble oligomers, and decomposition of fragments.
The main way of degradation: is body erosion.
After the PLA material is immersed in an aqueous medium or implanted in the human body, the material absorbs water first. The aqueous medium penetrates into the polymer matrix, resulting in the relaxation of the polymer molecular chain, the initial hydrolysis of the ester bond, the decrease of the molecular weight, and the gradual degradation into oligomers. The terminal carboxyl groups of polylactic acid (produced by polymerization introduction and degradation) catalyze its hydrolysis. As the degradation progresses, the amount of terminal carboxyl groups increases, and the degradation rate accelerates, resulting in an autocatalytic phenomenon. The internal degradation is faster than the surface degradation, which is attributed to The degradation products with terminal carboxyl groups remaining in the sample, resulting in a self-accelerating effect.
As the degradation progresses, there will be more and more carboxyl groups inside the material to accelerate the degradation of the internal material, further increasing the difference between the inside and outside. When the inner material is completely transformed into a soluble oligomer and dissolved in an aqueous medium, a hollow structure with a surface composed of a polymer that is not fully degraded is formed. Further degradation causes oligomers to be hydrolyzed into small molecules, which are finally dissolved in aqueous media. The entire dissolution process is from a water-insoluble solid to a water-soluble substance. Macroscopically, the overall structure of the material is destroyed, the volume becomes smaller, gradually becomes fragments, and finally completely dissolved and absorbed or excreted by the human body; microscopically, the polymer macromolecular chain undergoes chemical decomposition, such as molecular weight becomes smaller, molecular chain breaks and When the side chain is broken, etc., it becomes a small water-soluble molecule and enters the body fluid, is engulfed by cells, and is transformed and metabolized.
3- In vitro degradation
The decomposition of polylactic acid has two stages: after being decomposed by a hydrolysis reaction, it is decomposed by microorganisms. In the natural environment, hydrolysis first occurs, and the unstable ester bonds on the main chain are hydrolyzed to form oligomers. Then, microorganisms enter the tissue and decompose it into carbon dioxide and water. Under the conditions of composting (high temperature and high humidity), the hydrolysis reaction can be easily completed, and the decomposition rate is relatively fast. In an environment where hydrolysis reaction is not easy to occur, the decomposition process is gradual. Microorganisms are ubiquitous in nature, and polylactic acid can be degraded by a variety of microorganisms, such as Fusarium candida, Penicillium, Humicola, and so on. Different bacteria degraded polylactic acid with different configurations in different ways. The results of the study showed that Fusarium candida and Penicillium could completely absorb D and L lactic acid, and some of them can also absorb soluble polylactic acid oligomers.
4- Influencing factors of degradation
1) pH value
Both acids and bases can catalyze the hydrolysis of PLA.
The degradation rate of polylactic acid under alkaline conditions>degradation rate under acidic conditions>degradation rate under neutral conditions.
2) Crystallinity
The degradation process is always from the amorphous area to the crystalline area. This is because the sub-segments of the crystalline area are packed tightly, and water is not easy to penetrate into it. It penetrates into the amorphous region first, resulting in the breakage of the ester bond. When most of the amorphous region has been degraded, it begins to degrade from the edge to the center of the crystalline region. During the hydrolysis process of the amorphous region, stereoregular low-molecular substances are generated, and the crystallinity increases, which delays further hydrolysis.
3) Molecular weight and molecular weight distribution
Molecular weight is inversely proportional to the degradation rate. The larger the molecular weight, the tighter the structure of the polymer, and the less likely it is to break the internal ester bond; moreover, the larger the molecular weight, the longer the chain segment obtained through degradation, which is not easily soluble in water, and the less water and positive hydrogen ions are produced. The pH value drops slowly, which is one of the reasons why its degradation rate is lower than that of low molecular-weight polylactic acid. For polymers with the same average molecular weight, the wider the molecular weight distribution, the faster the degradation rate. This is because the polymer with a smaller molecular weight decomposes first, and the pH value of the environment changes from neutral to acidic, thereby accelerating the degradation rate.
4) Effect of Stereoregularity
Under alkaline conditions, the degradation rate is PDLA (PLLA)<P (LDL)A<PDLLAPDLLA. Since the methyl group is in a syndiotactic or atactic state, the absorption rate of water is faster, so the degradation is faster; For PLLA and PDLA, the hydrolysis is divided into two stages: the first stage, water molecules diffuse into the amorphous region, and then hydrolysis occurs; the second stage is the hydrolysis of the crystalline region, which is relatively slow.
5) Enzyme
The main chain of polylactic acid contains ester bonds, which can be accelerated by esterase. Such as Rhizopus lipase, porcine pancreas lipase, and porcine liver shuttle base esterase.