Dispatching Biocompatible Polymers toward Antimicrobial Applications

Abstract

Antimicrobial materials have become an essential part of various fields. In the past decades, various types of antimicrobial materials were developed and practically used. Based on the feedback from the clinical usage and market, the biocompatible materials have been very welcomed due to less side effects. This chapter provides a small and general review of biocompatible polymer materials and their applications in antimicrobial fields. This chapter could be divided into several parts: starting from the background introduction of microbial threats, the first section discusses the demands of biocompatible polymers for antimicrobial applications, then, the following sections would describe the basic knowledge of biocompatible polymers, including the definition, advantages, and typical examples, the next section reviewed and discussed some approaches to apply biocompatible polymers into antimicrobial applications.

1. Introduction

Microbial invasions and infections have been accompanying human beings for thousands of years, and have caused countless causalities [1, 2]. The battle against microbial threats had already begun before people acquired the understanding of microorganisms [3]. During this long-lasting fighting, though the concept was not initially established, people had invented and developed antimicrobial materials, which contain contents that have the functions of preventing microbial invasion and/or sterilizing the microbes [4, 5]. Up to date, various types of antimicrobial materials have been developed and many of them have been successfully applied to real usages [6, 7, 8, 9].

One major concern regarding the usage of antimicrobial materials is the side effects [10, 11]. For example, it was reported that some antimicrobial materials might cause damage to human bodies while killing the microbes [11]. Also, some antimicrobial materials, once abandoned or released, could lead to environmental and ecological issues [12]. Therefore, it has become a consensus that antimicrobial materials, no matter how potent they are, should be less collateral damaging. Under this demand, biocompatible polymers-based antimicrobial materials (i.e., biocompatible antimicrobial polymers) have become more favorable among customers and markets, and this type of material has become a new area for research [12].

This chapter provides a mini-review of biocompatible antimicrobial polymers, including the description of typical polymer materials, their usage in antimicrobial fields, the current challenges, and future perspectives.

2. Understanding the biocompatible polymers

2.1 Definition of biocompatible polymers

Admittedly, the terminology of biocompatible polymers is not well defined yet, and researchers prefer to create their own descriptions in the publications. However, so far, some consensuses regarding the definition of biocompatible polymers have been built and recognized among different researchers, which at least include the following:

1. The term “biocompatible polymer” refers to the material (or substance) with a polymeric structure, which consists of repeated sub-units that bond together [13]. This kind of material could be either obtained from nature or artificially synthesized.

2. The components of a given biocompatible polymer should not be intrinsically harmful to the target living recipients, including the contact sites (if any), local areas, and whole systems.

3. A biocompatible polymer, once applied to the recipients, should not cause any undesirable side effects. The material is supposed to provide appropriate host responses. In most applications, this requirement refers to both polymeric structures and the produced/released functional moieties and side products.

4. If the material remains on the target recipients for a longstanding period, this product should always be non-harmful until it is finally removed or fully reacted. Particularly for polymers, if they are degraded (either by design or not), the depolymerized components should also perform in the appropriate responses to the recipients.

2.2 Advantages of biocompatible polymers

It is easy to recognize that biocompatible polymers are attractive, but much more than the common opinions, biocompatible polymers have various advantages in different aspects. The following briefly describes some significant points:

Availability: unlike traditional polymers obtained from the synthesis of monomers, most biocompatible polymers (in the form of raw materials) could be obtained or extracted from natural products. For example, cellulose was rich in cotton, flax, and straw [14]; chitosan and chitin could be extracted in arthropod animals [15]; alginate could be obtained as a major product of brown algae [16]. The easy accessibility of biocompatible polymers reduces the costs, especially exempting the processing of monomer preparations and polymerizations.

Low hazard: as mentioned above, biocompatible polymers could leave fewer side effects; this was considered one of the most significant advantages. Within the antimicrobial application, the biocompatible polymers could be comfortably applied on humans without considering the toxicities or body exclusions, and there will be fewer concerns about byproducts during usage.

Biodegradability: for most traditional polymers, one critical shortage is collateral environmental pollution, as they are hard to decompose in the natural environment. However, many biocompatible polymers are biodegradable to microorganisms or medium, simplifying depolymerization’s artificial processing after usage [17]. In addition, the degraded components are also generally biocompatible and could be further decomposed and absorbed by the microorganisms [17]. Therefore, the applications of biocompatible polymers could reduce environmental concerns.

2.3 Representative examples of biocompatible polymers

After years of development, certain biocompatible polymers have been commercially used, and more candidates are being investigated in the research labs. Among various products, they could be divided into natural and synthetic biocompatible polymers. This part listed some typical examples.

Cellulose: Cellulose is a linear polysaccharide that consists of 1,4-D-glucose units (Figure 1) [14]. It is one of the earliest used biocompatible polymers, even before people established the concept of polymer materials. Cellulose exists abundantly in nature and serves as a major component of the cell walls of plants and wood. As a natural product, cellulose is nontoxic, non-polluting, and fully biodegradable, making itself popular nowadays [18]. Within the antimicrobial applications, cellulose is more likely to be considered and used either as raw materials or substrate for further modification.

Figure 1.

Structure of cellulose.

Chitin and chitosan: chitin and chitosan share a highly similar structure. Chitin is a linear polysaccharide that contains repeated N-acetyl-D-glucosamine units (Figure 2), and chitosan is a linear polysaccharide (Figure 3) composed of randomly distributed D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit) [15]. Chitin and chitosan are also rich in nature, but unlike cellulose, chitin and chitosan are more likely to be found in arthropod animals such as crabs, shrimps, and some insects [15]. It is interesting to notice that chitin and some sorts of chitosan could be fully or partially digested by certain types of enzymes and decomposed into monosaccharides and ammonia [19]. The digestibility of chitosan and chitin has become a huge advantage for antimicrobial applications, as they have great potential for in vivo usage [19].

Figure 2.

Structure of chitin.

Figure 3.

Structure of chitosan.

Alginic acid: alginic acid is a welcomed edible polysaccharide. Alginic acid has a copolymer structure of β-D-mannuronate block and α-L-guluronate block (Figure 4). In the natural environment, it often exists in the form of sodium salt (referred to as alginate) [16]. Thanks to its edibility, alginic acid is favorable in the food and drug industry and is becoming promising for antimicrobial applications.

Figure 4.

Structure of alginic acid.

Polyvinyl alcohol: polyvinyl alcohol (PVA) is a synthetic biocompatible polymer with the formula of [CH₂CH(OH)]_n. Though not invented as a biocompatible material, its biocompatibility was soon proved, and then, it started being investigated in biomedical engineering fields [20]. Unlike most polymers, PVA is water soluble, which could be fully carried forward in the case of antimicrobial application in aqueous conditions.

Of course, there are many more candidates than those listed above. To date, there is an increasing trend of research addressing biocompatible polymers [21], including obtaining novel materials, modifying current products, developing processing techniques, and exploring the markets.

3. Practicing biocompatible polymers in antimicrobial applications

3.1 Approaches toward antimicrobial applications

The intrinsic advantages of biocompatible polymers do not guarantee the feasibility of applications. Instead, the applications are largely associated with proper approaches. The following are some typical approaches developed for antimicrobial applications.

Direct processing: In terms of some biocompatible polymers, the raw materials could be extracted from natural products, and the raw materials could be directly manufactured to produce designed products without losing structure integrity, functionality, and biocompatibility. For example, chitosan with a high content of amino groups could be manufactured into thin films with antimicrobial functions due to the positively charged amino groups [22]. However, most biocompatible polymers in the form of raw materials do not have antimicrobial functions. Therefore, other approaches are needed.

Functional modification: For most biocompatible polymers, functional modification is the best approach to equip materials with antimicrobial abilities. A traditional and simple method is to physically blend polymers with antimicrobial moieties to obtain the antimicrobial mixtures, on which polymers could serve as the substrate materials to enhance the antimicrobial effects [23]. However, recent research has been more focused on the modifications of the polymer’s own structures, as these kinds of materials are more chemically stable. Notably, many biocompatible polymers have functional groups on their backbones or side chains. These groups are chemically reactive and could be transferred into an antimicrobial moiety. For example, alginic acid and cyclodextrin could be oxidized, and the hydroxyl groups were transferred into aldehyde groups, which present some practical antimicrobial effects [24, 25]. Another method is to use crosslinkers, such as chitosan or other amino-containing polymers; the amino groups could be linked with crosslinkers containing aldehyde groups [26], acyl chloride [27], or other groups so the antimicrobial moieties could be covalently bound onto the polymers.

Transformation: It was found that materials in different forms would present different characteristics. This clue could be useful for biocompatible polymers. Many researches indicated that the raw polymers could be made into some “new” forms, such as nanoparticles [28], hydrogels [29], and colloids [30]. With a different size scale, shape, and morphology, the material might present antimicrobial functions or become a good candidate for antimicrobial modifications.

3.2 Representative antimicrobial applications of biocompatible polymers

A general recognition of biocompatible polymers for antimicrobial applications is the low and controllable side effects on targeted recipients while eliminating microorganisms. But when it comes to different specific fields, biocompatible polymers could present unique advantages in various ways; the following are some leading examples.

Medical and clinical usages: medical products made of biocompatible polymers have been top-rated in the market. Biocompatible polymer-based products, such as bandages, blood bags, and gauze, could be smoothly applied to humans. While eliminating and inhibiting microorganisms, the products could cause less toxic or hazardous chemicals and leave less hurtful or uncomfortable sensing. There would be fewer concerns about the residues from the products after usage.

Food industry: traditional polymer materials have always been used in the food industry (such as food containers, food packages, and food processing instruments), but in recent years, biocompatible polymers have become more favored. One major advantage of biocompatible polymers is less generating and releasing poisons onto food, enhancing food safety [31]. In addition, biocompatible polymers with antimicrobial functions could protect the food from microbe-induced spoilage and improve the food quality [32].

Environment engineering: in this field, biocompatible polymers were adequately used to produce biocides while killing microorganisms; they would not cause pollution or collateral damage to the surrounding environment. In addition, biodegradable products could be processed by natural decomposers, leaving fewer problems in the after-usage stage [33].

4. Conclusion

As a novel class of polymeric materials, biocompatible polymers have various unique and irreplaceable advantages for antimicrobial applications. However, the basic information of biocompatible polymers was not well recognized yet. This chapter provided a brief summary about biocompatible polymers and usages in antimicrobial applications, including a comprehensible and understandable definition, a list of advantages and some typical examples. Based on these, the achieved and potential approaches for antimicrobial applications were discussed. This chapter could serve as a “getting started” brochure to introduce the concept of using biocompatible polymers for antimicrobial applications to the public.

Conflict of interest

The author declared no conflict of interest.