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 Garcia et al. 2009);Table 2.1 Microbial biopolymers of industrial relevance.BiopolymerSources/micro‐organism producersChemical structureApplicationsReferencesXanthan gumBacteria from Xanthomonas genus.Backbone of repeating sub‐units, branched or not, consisting of 3–8 monosaccharides, such as D‐glucose, D‐mannose and D‐glucuronic acid. Molecular weight: up to 2000 kDa.Food Cosmetics PharmaceuticsJansson et al. (1975), Laws et al. (2001), Borges and Vendruscolo (2008), and Lopes et al. (2015)CurdlanBacteria from Agrobacterium (previously taxonomically classified as Alcaligenes), Rhizobium, Bacillus, and Cellulomonas genus.D‐glucose units linked by β‐(1➔3) glycosidic bounds. Molecular weight: up to 2000 kDa.Food Cosmetics PharmaceuticsTabernero et al. (2019)PullulanYeasts of Aureobasidium genus.Maltotriose trimer made up of α‐(1 → 6)‐linked (1 → 4)‐α‐D‐triglucosides. Molecular weight: up to 1000 kDa.Foods Cosmetics PharmaceuticsSingh et al. (2008)Gellan gumBacteria from Sphingomonas (formerly known as Pseudomonas elodea).Linear tetrasaccharide repeat unit consisting of (1➔4)‐L‐rhamnose‐(α‐1➔3)‐D‐glucose‐(β‐1➔4)‐D‐glucose‐(β‐1➔4)‐D‐glucose‐(β‐1➔). Molecular weight: ≈500 kDa.Food CosmeticsMorris et al. (2012)EmulsanAcinetobacter calcoaceticus and Acinetobacter venetianus.Polysaccharide backbone (three amino sugars in the ratio 1 : 1 : 1) with O‐acyl and N‐acyl fatty acid side chains (10–20 carbons representing 5–23% [w/w] of the polymer). The amino groups are either acetylated or covalently linked by an amide bond to 3‐hydroxybutyric acid. Molecular weight: ≈1000 kDa.Petrochemistry CosmeticsBelsky et al. (1979), Gorkovenko et al. (1997), and Panilaitis et al. (2007)AlginateAlgae (Laminaria and Ascophyllum) genus.α‐L‐guluronic and β‐D‐mannuronic acid residues linked by β‐(1–4) glycosidic bound. Molecular weight: 10–600 kDa.Food Pharmaceutics CosmeticsFAO (n.d.) http://www.fao.org/3/y4765e/y4765e07.htm and Liang et al. (2015)BotriospheranFilamentous fungus of Botryosphaeria rhodina species and some bacterial isolates.D‐glucose residues bonded by a β‐(1 → 3;1 → 6). Molecular weight: ≈4875 kDa.CosmeticsSelbmann et al. (2003) and Silva et al. (2008)Bacterial celluloseSpecies of nonpathogenic aerobic bacteria, mainly belonging to the genera Gluconacetobacter, Alcaligenes, Rhizobium, Agrobacterium, and Sarcina.β‐D‐glucopyranose units linked to each other by β‐(1➔4) glycosidic bonds.Food Pharmaceutics Cosmetics PaperIguchi et al. (2000), Belgacem and Gandini (2008), and Klemm et al. (2005)ChitosanFilamentous fungi from Mucorales genera and some bacteria.Molecular weight: 100–300 kDa.Food Pharmaceutics CosmeticsKaur et al. (2012), Campos‐Takaki et al. (2014), and Lecointe et al. (2019)Figure 2.2 Biopolymers and building blocks obtained by fermentation.Figure 2.3 Chemical structure of (a) poly(ε‐caprolactone) (PCL), (b) poly(lactic acid) (PLA), (c) poly(glycolic acid) (PGA), and (d) poly(glycolic–lactic
