Degradation mechanism, characteristics and application of biodegradable materialsDefinition and degradation mechanism of biodegradable materials
Biodegradable materials, also known as "green ecological materials", refer to materials that can be degraded under the action of soil microorganisms and enzymes. Specifically, it refers to polymer materials that can cause biodegradation under certain conditions under the action of natural microorganisms such as bacteria, molds, and algae.
The ideal biodegradable material is a polymer material that has excellent performance and can be completely decomposed by environmental microorganisms after being discarded, and finally converted into CO2 and H2O to become a component of the carbon cycle in nature.
The decomposition of biodegradable materials is mainly through the action of microorganisms. Therefore, the degradation mechanism of biodegradable materials is the process of digestion and absorption of materials by bacteria and molds.
First, microorganisms secrete hydrolase in vitro to bind to the surface of the material, and cut off the surface polymer chains by hydrolysis to generate small molecular weight compounds, and then the degraded products are taken into the body by the microorganisms, and undergo various metabolic routes to synthesize microorganisms or transform. The energy for microbial activities is finally converted into water and carbon dioxide. According to the chemical nature of its degradation, it can be divided into hydrolysis and enzymatic hydrolysis.
01 Hydrolysis mechanism
The degradation of the material is essentially a process in which the internal polymer segments break into low-molecular-weight oligomers under certain conditions, and finally decompose into monomers. The "corrosion" of a material refers to the process in which the water-soluble small molecular substances formed due to the breaking of the molecular chain leave the polymer material, resulting in the reduction of the mechanical properties of the material, and the complete disappearance of the material. The dissolution can be surface dissolution and overall dissolution.
If the degradation speed of the molecular chain is faster than the diffusion speed of water molecules in the material, the hydrolysis of the chain is limited to the surface of the material, and it is difficult to enter the inside of the material. This method belongs to surface corrosion or heterogeneous corrosion. When the diffusion rate of the material is faster than the hydrolysis rate of the polymer segment, the surface and internal degradation of the material will proceed at the same time, so it belongs to the overall dissolution.
02 Enzymatic hydrolysis mechanism
For polymers that are easily hydrolyzed, there may be both simple hydrolysis and enzymatic hydrolysis in the body. Lipase can promote the decomposition of polyester, while hydrolase can promote the degradation of easily hydrolyzed polymers. Lipase R.delemer lipase, Rhizopus arrhizus lipase, and Pseudomnas lipase are specific degrading enzymes of PCL. In the presence of these enzymes, the degradation rate of PCL is accelerated. Under normal circumstances, it takes 2-3 years for complete degradation, but in the presence of enzymes The complete degradation time is shortened to a few days.
Figure: Enzymatic oxidation mechanism
For some non-hydrolyzable polymers, the possible degradation mechanism is the enzymatic oxidation mechanism. Immunohistological studies have confirmed that the material is finally absorbed and metabolized in the body through the endocytosis of phagocytes.
After polymer biomaterials are implanted in the body, they will cause different degrees of acute inflammation locally. When the tissue is injured, the permeability of the surrounding blood vessels will change, and the multinucleated leukocytes will quickly move to the inflammation site, and the activated neutrophils Can make monocytes differentiate into macrophages. The metabolism of polymorphonuclear leukocytes and macrophages produces a large amount of peroxy anions (O2), and this unstable intermediate is converted into a stronger oxidant hydrogen peroxide.
The reduced coenzyme (NADPH) and oxidase in the body are involved in this conversion reaction, and the superoxide dismutase (SOD) plays a role in accelerating the conversion. Hydrogen peroxide may initiate the decomposition reaction of the polymer itself at the implantation site; at the same time, hydrogen peroxide can be further converted into hypochlorous acid under the action of muscle peroxidase (MPO). Hypochlorous acid is also a strong oxidant for biological materials, which can oxidize the amino groups in polyamide, polyurea, and polyurethane, and break the polymer chain, thereby achieving the effect of degradation.
Among the biodegradable materials, microorganisms such as bacteria, molds, fungi and actinomycetes play a major role in degradation. According to the form of their degradation, they can be divided into three types:
The physical action of organisms, the material is destroyed mechanically due to the growth of biological cells;
The biochemical action of organisms, the action of microorganisms on materials produces new substances;
With the direct action of enzymes, microbes erode some components of material products and then cause material decomposition or oxidative breakdown.
Features of biodegradable materials
Biodegradable materials have the following characteristics:
1. It can be treated together with garbage, or it can be made into compost to return to nature;
2. The volume is reduced due to degradation, which prolongs the service life of the landfill;
3. There is no problem that ordinary plastics need to be incinerated, which can suppress the emission of harmful gases such as dioxin;
4. It can reduce the harm to wild animals and plants caused by arbitrary discarding;
5. It is convenient for storage and transportation, as long as it is kept dry and does not need to be protected from light;
6. It has a wide range of applications, not only in agriculture and packaging industry, but also in medical industry.
Application of biodegradable materials
Biodegradable materials are new types of materials developed after the 1980s with the emergence of environmental and energy contradictions. They can partially replace general-purpose plastics. Medical, packaging, and fibers are the three popular application areas.
Among them, especially with the implementation of the "plastic ban", whether it is takeaway shops and tea shops are replacing plastic straws with paper or biodegradable straws, or large supermarkets generally replacing paid plastic bags with biodegradable materials. Plastic bags. Biodegradable products have entered the lives of the general public in more diverse forms.