Bacteria are ubiquitous in all aspects of human life, and there are numerous ways of infection. For example, if gas containing bacteria is inhaled by the human body, the wound may become infected with bacteria after injury, medical equipment contaminated with bacteria is inevitably in contact during medical treatment, and contaminated and spoiled food is ingested by mistake. Faced with the invasion of bacteria, humans have never given up resistance throughout history, and have gradually developed many types of antibacterial agents and materials. Antibiotics refer to substances that can inhibit the growth and reproduction of certain microorganisms, such as bacteria, fungi, and viruses, and even kill them. Antibacterial materials refer to materials that use antibacterial agents as effective antibacterial ingredients.

1. Classification of Antibacterial Materials

The widely recognized classification method is to classify antibacterial materials into three categories: natural antibacterial materials, organic antibacterial materials, and inorganic antibacterial materials.

1.1 Natural antibacterial materials

Natural antibacterial materials mainly come from animals and plants, and active substances with antibacterial properties are obtained through extraction, separation, and purification. They were the first weapons used by humans to protect themselves from bacterial invasion. For example, ancient Egyptians used cloth soaked in plant juice to make mummies. Natural antibacterial materials have good biocompatibility, light pollution to the environment, and low toxicity. However, due to limited sources, high extraction costs, poor stability of the extract, and limited application due to its narrow antibacterial range, low antibacterial efficiency, and weak antibacterial effect. According to the different sources of extracts with antibacterial properties, natural antibacterial materials can be divided into three categories: plant derived, animal derived, and microbial derived.

Plant derived natural antibacterial materials are the earliest antibacterial materials put into use by humans. Our ancestors utilized the resources of nature in their daily lives, relying on their own experience and wisdom to obtain antibacterial substances from plants. Natural antibacterial materials from plants mainly include terpenoids and their derivatives, alkaloids, steroids, saponins, lignans, amino acids, etc.

Animal derived antibacterial materials mainly include sugars, amino acids, and peptides, among which chitosan and peptides are widely used in current research. Most researchers tend to use chitosan as a matrix and composite other antibacterial materials with strong antibacterial properties, such as nano silver and natural antibacterial materials, to enhance their antibacterial ability and broaden their application scope.

The existence of microbial antibacterial materials is mainly due to the inherent antibacterial properties of certain microorganisms or the antibacterial ability of their metabolites. Phages and probiotics are typical examples of their own antibacterial properties, with antibiotics and bacteriocins being the most common metabolites. The widely used microbial antibacterial materials nowadays are mainly antibiotics, and as bacteria develop resistance to antibiotics, humans need to constantly develop new antibacterial materials.

1.2 Organic antibacterial materials

Organic antibacterial materials can be divided into two categories based on the molecular structure types of their antibacterial active ingredients: low molecular antibacterial materials and polymer antibacterial materials. Organic low molecular weight antibacterial materials mainly include quaternary phosphates, quaternary ammonium salts, phenolic esters, biguanides, imidazoles, etc. Antibacterial molecules combine with anions on the surface of bacterial cell membranes, or react with thiol groups to denature bacterial proteins, in order to inhibit bacterial life activities. Quaternary ammonium salt low molecular weight antibacterial materials are the most widely used and extensively studied type in this category. Organic polymer antibacterial materials are formed by polymerization (including copolymerization and homopolymerization) or grafting of antibacterial functional group monomers to form antibacterial polymers. Organic antibacterial materials have a wide variety of types, a wide range of applications, significant antibacterial effects, and mature application technologies. However, the safety issues of some organic antibacterial materials with strong toxicity, poor heat resistance, and easy decomposition and volatilization are also worth pondering. Long term use of organic antibacterial materials can easily lead to bacterial resistance. In addition, the antibacterial effect gradually weakens or even completely disappears over time.

1.3 Inorganic Antibacterial Materials

Inorganic antibacterial materials are relatively new antibacterial materials, which have the advantages of broad-spectrum antibacterial and long-lasting antibacterial properties, as well as excellent high-temperature resistance. During use, the bacteria will not develop resistance like organic antibacterial materials. But some products have complex processes, high costs, and some also have disadvantages such as poor stability and short antibacterial time. According to the different mechanisms of antibacterial action, antibacterial materials can be divided into two categories: metal ion containing (metal or metal oxide) and photocatalytic metal oxide antibacterial materials.

Metal ion (metal or metal oxide) type antibacterial materials refer to metal ions (mainly silver ions), but the stability of metal dispersions and the aggregation and particle size distribution of nanoparticles often pose challenges for researchers. The use of metal nanoparticles alone also poses potential hazards related to nanoscale, which may have adverse effects on humans and the environment, and silver based antibacterial materials are prone to oxidation and discoloration. In order to solve these problems, researchers actively carry out research on loaded metal antibacterial materials, focusing on seeking carriers with better performance. Metal ions are loaded onto substrates such as activated carbon, silica gel, and zeolite through ion exchange, adsorption, and melting methods. The antibacterial properties of metal ions are generally in the following order:

Ag+>Hg2+>Cu2+>Cd2+>Cr3+>Ni2+>Pb2+>Co2+>Zn2+>Fe3+.

Metal ions such as lead, mercury, and cadmium are rarely used due to their high toxicity and carcinogenicity; Metal ions such as copper and cobalt are limited in their applications due to their darker colors; The antibacterial properties of zinc ions and trivalent iron ions are limited, while precious metal silver ions have high bactericidal efficiency and play a crucial role in the research of inorganic antibacterial materials. Photocatalytic metal oxide antibacterial materials mainly include TiO2, which can kill bacteria under the action of ultraviolet light. The effective component of photocatalytic antibacterial materials is semiconductor compounds. The widely recognized theory in semiconductor photocatalytic antibacterial materials is that a class of materials with rutile like structures have broad application prospects due to their antibacterial and antifungal effects, strong disinfection ability, durability and stability, and will not cause secondary pollution.

2. Application of Antibacterial Materials

2.1 Application of Natural Antibacterial Materials

There are examples of natural antibacterial materials being applied to food packaging materials both domestically and internationally. Yang W et al. [16] fused cellulose nanocrystals and lignin nanocrystals with polylactic acid to prepare a film with antibacterial ability, which can significantly inhibit the growth of Pseudomonas syringae. It is expected to be applied to packaging materials for vegetables, fruits, and other types of food.

Gou Qiongyou applied three different antibacterial solution systems - capsaicin solution, chitosan solution, and capsaicin/chitosan composite solution - to cotton fabric, nylon fabric, and silk fabric, and investigated the antibacterial properties of the resulting fabrics. According to the analysis of experimental results, it is found that capsaicin antibacterial solution has little inhibitory effect on Staphylococcus aureus. Chitosan solution and capsaicin/chitosan composite solution can significantly inhibit this bacterial strain, and the antibacterial effect is best on cotton fabric, compared to silk fabric.

Chitosan has inhibitory effects on various bacterial strains such as Bacillus subtilis, Escherichia coli, and Staphylococcus aureus. Duran M et al. loaded lactic acid streptococcin, natamycin, and grape seed extracts onto chitosan coatings, and the resulting antibacterial materials can inhibit the growth and reproduction of thermophilic bacteria, molds, and yeast without affecting the brightness and redness of strawberries, thereby effectively extending the shelf life of strawberries.

Streptomycin, penicillin, aureomycin, erythromycin, and other antibiotics derived from microbial culture media or metabolites are widely used in medicine, benefiting countless patients and leaving a significant mark in the history of human antibacterial activity.

2.2 Application of organic antibacterial materials

Quaternary ammonium salt low molecules in organic antibacterial materials play important roles in various fields. In terms of packaging materials, there are mainly antibacterial packaging paper and antibacterial fibers. Zhang Wen prepared paper packaging materials with antibacterial properties using low molecular weight quaternary ammonium salts as additives. But with the widespread use of quaternary ammonium salts, bacteria have developed resistance, so researchers have developed new organic antibacterial materials - quaternary ammonium salts. The antibacterial activity of quaternary ammonium salts is about 100 times that of quaternary ammonium salts. The antibacterial material dodecyltriphenylphosphine bromide synthesized by Zhang Changjun has a much higher antibacterial efficacy than the antibacterial material 1227. Chen R et al. covalently grafted acrylic quaternary ammonium salts onto PU chains to prepare contact type antibacterial polyurethane (PU) films with high safety, which are expected to be applied in the fields of food and pharmaceutical packaging. Han Ruitao et al. successfully grafted the functional monomer N-hydroxymethylacrylamide onto microcrystalline cellulose using atomic transfer radical polymerization, and then halogenated N-H to N-Cl bonds, thereby obtaining organic high molecular antibacterial materials with antibacterial properties, and studied their antibacterial properties. This is an application example of polymer organic antibacterial materials.

2.3 Application of Inorganic Antibacterial Materials

Metal ion (metal or metal oxide) type inorganic antibacterial materials are usually sorted onto carriers and used as antibacterial materials. Silicate clay minerals are commonly used carriers. Based on the unique physical and chemical properties and morphological characteristics of the composite, they can play a role in fixing and dispersing inorganic nano antibacterial materials, improving the antibacterial performance of inorganic antibacterial materials. Therefore, they are widely used in biomedicine, sewage treatment, food packaging, and other fields. Shu used halloysite as a carrier to study the dispersion characteristics and antibacterial mechanism of zinc oxide and silver nanoparticles on the surface of halloysite. The large specific surface area of halloysite provides a loading point for zinc oxide and silver nanoparticles, alleviating the agglomeration phenomenon of nanoparticles and improving their antibacterial performance. Meanwhile, due to the hydrophilicity of halloysite, its surface zinc oxide and silver nanoparticles are more concentrated on the surface of Escherichia coli, generating more reactive oxygen species, thereby inhibiting the normal proliferation of bacteria. Disperse silver ions or silver metal elements in a carrier, usually consisting of apatite, calcium phosphate, zeolite, etc., and then add them to the glaze for porcelain manufacturing. Antibacterial micro components can exist in the glaze layer of ceramic surfaces for a long time, thus producing antibacterial ceramics. Silicon dioxide is often used as a carrier due to its large specific surface area, easy surface modification, and high thermal stability. There have been many studies and reports on the excellent antibacterial properties of Ag/SiO2 both domestically and internationally. It has the advantages of inorganic and organic antibacterial materials, and its antibacterial effect is obvious. Experimental results have shown that its antibacterial performance is superior to SiO2 without silver loading [27]. The uniformity of mixing between different material systems and the migration of effective components within the matrix are important factors affecting the performance of antibacterial materials.

In addition to silver based antibacterial materials, researchers have also used other metals or metal oxides such as zinc, titanium, magnesium, calcium, etc. as antibacterial components. Tang Xiaoning et al. [28] prepared zinc cerium antibacterial white carbon black with antibacterial properties and added it to ceramics to make antibacterial ceramics. During the research process, conventional methods such as SEM and XRD were used for structural characterization, and an optimized process route for preparation was obtained through experiments. The zinc cerium antibacterial white carbon black has an inhibitory effect on the tested bacterial strain Escherichia coli. Ye Junwei et al. summarized the development of nano magnesium oxide materials from three aspects: antibacterial mechanism, preparation of composite materials, and structural design. They believe that nano magnesium oxide antibacterial materials can compensate for the shortcomings of silver based antibacterial materials and photocatalytic antibacterial materials, thus becoming a research hotspot. Some researchers have also combined nano magnesium oxide with organic compounds to make the antibacterial time of the material more durable. In the field of antibacterial, zinc oxide, magnesium oxide, and calcium oxide are also widely studied metal oxides. Nano ZnO has abundant sources and does not change color or decompose at high temperatures. It has low economic costs and can control its morphology, making it widely practical.

Zhang Chongmiao's research group [5] independently designed and prepared TiO2/ZnO composite powders, and compared them with pure TiO2 and ZnO powders without composite. Escherichia coli was used as the test strain to test the antibacterial properties of these three oxide powder materials. Similar to silver loaded ceramics, coating titanium dioxide film on the surface of ceramics can also obtain antibacterial ceramics.

Inorganic antibacterial materials often have limited antibacterial performance when used alone. Therefore, researchers are committed to combining various types of antibacterial materials in order to utilize the synergistic effect between materials to obtain better antibacterial performance. Wang Xu et al. [32] used atom transfer radical polymerization to covalently bond and graft acrylic polymer onto the surface of TiO2 nanoparticles, followed by quaternization treatment to obtain polymer quaternary ammonium salts, and then loaded the modified material onto the surface of leather. The influence of modification on the antibacterial performance of nano TiO2 particles before and after modification was studied using various analytical and testing methods. The grafted nanoparticles are evenly distributed on the surface of leather fibers, giving leather products excellent antibacterial properties and greatly inhibiting the growth and proliferation of Staphylococcus aureus.

3. Conclusion

With the increasing awareness of human health protection, research on antibacterial materials is receiving more and more widespread attention. Because bacteria are ubiquitous, the research process of antibacterial materials is also never-ending. At present, a large number of natural, organic, and inorganic antibacterial materials, as well as composite antibacterial materials, have been applied in the fields of functional textiles, food preservation and storage, building materials (such as architectural coatings), and other material engineering fields such as antibacterial ceramics and antibacterial plastics, achieving gratifying results. At the same time, there is still a lot of exploration space for the accurate antibacterial mechanism of various antibacterial materials, the biocompatibility and environmental friendliness of antibacterial materials, the dispersibility, stability, migration of nanomaterials, and the stability research between composite materials. The future research on the properties of nanomaterials and the research on synergistic antibacterial materials will become the main development direction in the field of antibacterial.