2.1 Introduction
In the beginning of the twentieth century, Hermann Staudinger formulated the hypothesis of the existence of very large macromolecules with high molecular weights (Mülhaupt 2004). This hypothesis was verified experimentally in the 1920s, when Theodor Svedberg and Lawrence Bragg proved that hemoglobin and cellulose consisted in macromolecules (Rånby 1995). The acceptance of the existence of macromolecules allowed the development of a myriad of polymeric materials, such as plastics, rubbers, paints, and varnishes, that are now part of our daily lives. In addition to intentional discoveries, such as nylon, polyesters, and isotactic polypropylene, there were also accidental discoveries, such as polyethylene and polytetrafluoroethylene. Today, new and interesting macromolecules are being created to obtain new mechanical, optical, and electrical properties.
The etymology of the word polymers comes from the Greek (poly meaning “many, several” and mers meaning “parts”), as the macromolecular architectures originate through the connection of several units of small molecules called monomers. Polymers are generally classified into synthetic and natural. Synthetic polymers are more frequent in our daily lives, being applied in many industrial sectors, agriculture, and services. These materials are of petrochemical origin (Figure 2.1), which represents about half of the chemical industry worldwide. The manufacture and transformation of petroleum into polymers guarantee the employment and support of millions of people, but also generate uncountable amounts of waste. It is estimated that 25 million tons of plastics are annually accumulated in the environment and due to the slow degradation rates, they remain unchanged for hundreds of years. The conversion of large pieces into smaller particles is an initial step of the degradation processes, which may cause contamination problems that directly impact the environment and health. In addition, polymers derived from fossil fuels put excessive pressure on nonrenewable energy sources (Andrady and Neal 2009; Rhodes 2018).
With sustainable development and due to the alarming pollution caused by nonbiodegradable materials, research institutes and companies from around the world have increasingly invested in the development and application of natural polymers to reach similar performances as those of synthetic origin.
Figure 2.1 Scheme of the productive chain of fossil‐based polymers.
Source: Based on Olivatto (2017).
The advancement of nanotechnology for the design of several biotechnological devices such as nanoparticles, nanofilms, and liposomes based on natural polymers has gained great scientific importance due to their ecofriendly properties and biocompatibility with living systems. The nanotechnological devices based on natural polymers have been highlighted particularly in the development of pharmaceutical systems for drug delivery, tissue engineering, and bioactivation mechanisms. The low or no‐toxicity and safety of natural polymers are essential characteristics for the use of these nanodevices in health, as will be discussed in this chapter.
2.2 Natural Polymers: Conceptualization, Classifications, and Physicochemical Characteristics
Natural polymers, also referred to as biopolymers, may have a natural occurrence (as is the case of chitin, cellulose, proteins, among others) or be produced by living organisms. The interest in such materials is progressively increasing due to their ecofriendly characteristics, such as low or nontoxicity, high biodegradability, and biocompatibility (Rendón‐Villalobos et al. 2016). Biopolymers are generally classified into geopolymers, phytopolymers (plant polymers), zoopolymers (animal polymers), and microbial polymers. It should be noted that the concept of biopolymers does not cover all ecofriendly characteristics of natural polymers (Ogaji et al. 2012).
In recent years, vegetable and microbial biopolymers have gained attention due to their versatility, ecofriendly characteristics, and the possibility of sustainable and large‐scale production. In addition to the environmental concern and the need to transition from the use of synthetic polymers to polymers from renewable resources, one must also consider geopolitical factors such as conflicts in the Middle East, Russia, and Venezuela, the main oil producers in the world (Mülhaupt 2012).
The most widely used polymers of plant and animal origins are cellulose and gelatin, respectively. Among microbial polymers, the most relevant today are the so‐called exopolysaccharides, such as pullulan, curdlan, bacterial cellulose, lasiodiplodan, xanthan gum, and gellan gum, among others, and also intracellular polymers, such as microbial polyesters, for example polyhydroxyalkanoates (PHAs). Among microbial polymers, exopolysaccharides (EPSs) or extracellular polymers have been extensively studied and used for the most varied applications. EPSs are produced by some microorganisms and are found attached to the surface of cells or excreted into the extracellular medium, in the form of biofilms or slimes (Sutherland 1998). These biomolecules are usually associated with mechanisms of population cellular communication called quorum sensing (QS) that gives the community protection against various types of aggression, such as the lack of nutrients, or the presence of antimicrobials or biocidal chemical agents (Pyrog 2001; Gao et al. 2012; Nwodo et al. 2012; Gupta et al. 2019). In (Table 2.1) shows some examples of exopolysaccharides.
Polymers that are synthesized by classic organic routes, using bio‐based molecules obtained by fermentation (building blocks) (Figure 2.2) as precursors, are also called biopolymers.
The most common examples of these polymers are poly (lactic acid) (PLA), poly (glycolic acid) (PGA), and poly (glycolic–lactic acid) (PGLA). This class of polymers has been widely used for biomedical applications, such as drug delivery in living organisms, fixators in surgeries (sutures, clips, bone pins) and special packaging. They consist of aliphatic polyesters that may be synthesized via esterification reactions (Figure 2.3), and the presence of ester moieties in the ensuing main backbone favors chemical, physical, and mainly biological degradation (Franchetti and Marconato 2006).
In addition to biodegradability, a hot topic due to the appeal for sustainable development and responsible use of polymers, biopolymers have been increasingly used due to their low toxicity and good biocompatibility. Nanotechnology has taken advantage of such properties for the development of tools for environmental, food, pharmaceutical, and medical applications. As previously mentioned, petroleum‐based polymers are often chemically, physically, or biologically degraded into toxic compounds that may compromise the health of living organisms. In sequence, some examples of the toxicity of relevant synthetic polymers are presented.
ε‐caprolactam (ε‐CAP) is a precursor of nylon‐6, widely used in industry for the production of carpets, clothing, and automotive equipment, systems, components, connectors, and as additive to plastic packaging. ε‐CAP waste can migrate from plastic packaging to food. Some toxicological studies indicate the possibility of ε‐CAP causing eye and skin inflammation, as well as irritation in the respiratory system. Hypotension, tachycardia, palpitations, rhinorrhea, nasal dryness, genitourinary, and reproductive effects such as disorders in menstrual and ovarian functions, and complications in childbirth may also occur, in addition to neurological and hematological problems (Bomfim et al. 2009);
Epoxy polymers of bisphenol‐A diglycidyl ether (DGEBA) and aliphatic polyamine co‐monomers: triethylenetetramine (TETA), 1‐(2‐aminoethyl)piperazine (AEP) and isophorone diamine (IPD) had their interactions with biological systems tested in vitro. Although the results show that DGEBA‐IPD and DGEBA‐AEP are hemocompatible and polymers based on the IPD system are not considered cytotoxic, protein adsorption tests showed that the surface of the polymers adsorbs human albumin (González