1. Overview of biodegradable materials
(1) Definition and degradation mechanism
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 biodegrade under certain conditions under the action of bacteria, molds, algae and other natural microorganisms.
The ideal biodegradable material is a polymer material with excellent performance, which can be completely decomposed by environmental microorganisms after being discarded, and finally converted into CO2 and H2O, becoming an integral part 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, molds, etc. First, microorganisms secrete hydrolytic enzymes to bind with the surface of materials, cut off the polymer chains on the surface by hydrolysis, and generate small molecular weight compounds. Then, the degraded products are taken into the body by microorganisms, and through various metabolic routes, synthesize microbial substances or Converted into energy for microbial activity and ultimately into CO₂ and H₂O. According to the chemical nature of its degradation, it is divided into two types: hydrolysis and enzymatic hydrolysis.
(2) Classification and characteristics of biodegradable materials
Currently widely researched and used biodegradable materials include natural degradable polymer materials, microbially synthesized degradable polymer materials and artificially synthesized biodegradable materials.
1. Natural polymer materials
Natural polymer materials can be used to prepare biodegradable materials, which are a class of macromolecules extracted from organisms or directly obtained from the natural environment. They have good biocompatibility and degradability, but generally do not have sufficient mechanical properties and Processing performance, some materials can also cause heterogeneous immune reactions in the body, so more chemically modified derivatives or complexes with other materials are used in medicine.
According to the structure and composition, it can be divided into natural proteins, polysaccharides and their derivatives, as well as some biosynthetic polyesters. Typical proteins, polysaccharides and their derivatives include collagen, gelatin, cyclodextrin, starch, dextran, chitosan, hyaluronic acid, cellulose, alginic acid derivatives, chondroitin sulfate and heparin, etc. .
2. Degradable materials synthesized by microorganisms
Degradable materials synthesized by microorganisms (biosynthetic or biologically derived materials) mainly include poly-β-hydroxybutyrate (PHB), polyhydroxyvalerate, etc., which belong to polyhydroxyalkanoate (PHA). Among them, polyhydroxybutyric acid is a low-toxic material, which has been used in controlled release of drugs, sutures and artificial skin.
3. Synthetic biodegradable materials
Synthetic biodegradable materials are mostly aliphatic polyesters with ester groups introduced into their molecular structures, and their preparation methods mainly include condensation polymerization and ring-opening polymerization. However, high-molecular-weight polyesters can only be synthesized by ring-opening polymerization, because the polycondensation reaction is affected by the degree of reaction and the water produced during the reaction, and it is difficult to obtain high-molecular-weight products. The main products that have been developed so far are polylactic acid (PLA), polyhexyl ester (PCL), polyglycolic acid (PGA), polyglycolide, polyglycolide, polybutylene succinate (PBS), etc.
Biodegradable materials can be roughly divided into two categories according to their biodegradation process: one is completely biodegradable materials, such as natural polymer cellulose, artificially synthesized polycaprolactone, etc., and its decomposition mainly comes from: ①Due to the rapid microbial The growth leads to the physical collapse of the plastic structure; ② Various hydrolysis due to the biochemical action of microorganisms, enzyme catalysis or acid-base catalysis; ③ Free radical chain degradation caused by other various factors. The other type is biodegradable materials, such as blends of starch and polyethylene, whose decomposition is mainly due to the destruction of additives and the weakening of the polymer chain, so that the molecular weight of the polymer is degraded to the extent that microorganisms can digest it, and finally decomposed into carbon dioxide and water.
Biodegradable materials have the following characteristics:
①It can be disposed of together with garbage, or it can be made into compost and returned to nature;
②The volume is reduced due to degradation, prolonging the service life of the landfill;
③ There is no problem of ordinary plastics needing to be incinerated, and it can suppress the emission of harmful gases such as dioxin (the most terrible chemical substance created by humans, known as "the most toxic poison on earth");
④ It can reduce the harm of random discarding to wild animals and plants;
⑤The storage and transportation are convenient, as long as it is kept dry, it does not need to be protected from light.
2. Application fields of biodegradable materials
As a multifunctional high-tech material, biodegradable materials are widely used and have great market potential, involving many aspects of people's daily life. At present, biodegradable materials are mainly used in environmental protection and medical research and clinical fields.
(1) Applications in the field of environmental protection
The application of biodegradable materials in the field of environmental protection is mainly reflected in the following aspects:
1. Water resources and environment field
In recent years, the emergence of polyhexyl ester has opened up new avenues for the application of biodegradable materials in this field. The degradable materials used in the water environment can be degraded into low-molecular compounds under the action of enzymes secreted by microorganisms in the ocean after being discarded. The compounds eventually participate in the new generation of microorganisms and become carbon dioxide and water.
2. Food container and packaging industry
Among the packaging materials, the amount of disposable commodity packaging is very large, which is the main source of "white pollution". Therefore, food bags, packaging bags, garbage bags and various packaging made of biodegradable materials are favored .
3. Agriculture and forestry
Ideal agricultural materials would work synergistically with other biodegradable materials to transform them into soil-enhancing materials. Biodegradable materials are mainly used in agriculture as agricultural mulch and crop growth containers. Biodegradable materials can also be used as turf planting sheets, bags for composting, and controlled release materials for agricultural drugs.
(2) Applications in the medical field
In the medical field, biodegradable materials have been widely used in drug sustained-release coatings, surgical implant materials, anesthesia antagonist materials, drug release carriers, non-permanent implant devices and other medical supplies, and tissue repair substitutes, etc. The research field of tissue engineering is mainly reflected in the following aspects:
1. Surgical sutures
An ideal suture should have good adaptability, non-toxic, non-irritating in the body, and can be absorbed by the tissue after maintaining a certain strength in the body for a certain period of time. Its suture, knotting performance, and flexibility should meet the operational requirements. At present, most absorbable surgical sutures are made of chitin, chitosan, polylactic acid, and polyglycolic acid, which can be completely degraded and absorbed within 2 to 6 weeks. However, in orthopedic surgery, since the bone healing time is 3 to 6 months or even longer, the sutures of the soft tissues connected to the bones are required to have an appropriate degradation rate, and the sutures made of polyhexyl ester spinning can be very good satisfy this requirement.
2. Drug sustained release agent
The biodegradable polymers used for drug sustained-release agents include polylactic acid, polyhexyl ester copolymer, chitin, collagen, etc. These biodegradable materials are ideal carriers for long-term medication for patients with cancer, heart disease, and hypertension.
3. Bone fixation material
With the continuous development of material science and orthopedics, biodegradable materials have been widely used clinically in the field of orthopedics. Degradable polymer materials commonly used as orthopedic materials mainly include polylactic acid, chitin, etc., and antibiotic-polyanhydride slow-release agents have also been deeply studied. Through the molecular design of chitin, the use of tissue engineering methods for articular cartilage repair and reconstruction has become a new goal of chitin research and development programs.
4. Artificial skin
Artificial skin made of biodegradable polymer materials can be used in the treatment of burns, skin replacement, wound dressing and other occasions. Existing collagen, chitin, poly-L-leucine and other enzyme-catalyzed biodegradable materials.