Table 1.

Natural compounds (polymers and natural extracts) used as components/additives in new polymeric materials together with their methods of preparation and applications.

Source and Compound		Obtaining Method	Mixing Method	Application
Plant Sources
Polysaccharides
	Cellulose; used as pulp, nanocrystals, nanofibers and fibers	Cellulose can be isolated using a combination of chemical and mechanical treatments like ultrasonication combined with chemical pretreatments, high shear homogenization coupled with acid hydrolysis and steam explosion, etc. [13].	Extrusion (for example in polypropylene composites [14]), reactive extrusion [15].	Reinforcement in polymer composites [14,16,17,18].
	Starch	Starch is extracted from seeds, roots and tubers, by wet grinding, washing, sieving and drying [19].	Extrusion, injection molding, film casting [20], reactive extrusion [15]. For incorporating starch in plastics, commercialized technologies were developed to overcome the moisture sensitivity and inferior mechanical properties of starch [21].	As a filler in biodegradable food packaging materials [22,23,24] or in plastic films can improve the biodegradability [25].
	Pectin	Extracted using acids and enzymes [26].	Extrusion (for example in polyvinyl alcohol composites) [27].	Antimicrobial packaging materials [28].
Proteins
	Soy Protein, hydrolyzed proteins (wheat gluten, wheat gliadin), zein, polypeptides	- Alkaline extraction followed by protein precipitation at isoelectric pH; - protein extraction with salt solution, followed by precipitation from a salt extract by ultrafiltration, diafiltration membranes or dilution in cold water (micellization) [29]; and - novel techniques, such as ultrasound assisted extraction, enzyme-assisted extraction in the form of proteases and/or carbohydrolases [29].	Extrusion foaming [30], reactive extrusion [15].	Reinforcement in polymer composites [31,32]. Polypeptides: Reinforcement in polymer composites [33]. Food packaging applications [34] or incorporated as a reinforcement in films with enhanced barrier properties [35] (zein). Mixing different proteins with polysaccharides is an effective way to improve barrier and mechanical properties of protein- polysaccharides films [36].
	Lignins	Industrially, lignin is isolated from cellulosic fibers by chemical treatment, which breaks down lignin–carbohydrate complexes. During this process, partial depolymerization of the complex lignin macromolecules occurs along with re-polymerization (condensation) which may alter the native lignin structure [37]. The paper pulping process (lignin extraction from lignocellulosic biomass) which produces industrial lignin as a byproduct [37] may include chemical methods [38], such as - Kraft process which uses a mixture of Na2S and NaOH (White Liquor) at high temperature (150–180 °C), - sulfite process which employs sulfite or bisulfite to digest biomass, - organosolv pretreatment of lignocellulose which involves a biomass extraction in a mixture of solvent (ethanol being the most common) and water under high pressure [39], - single pot soda cooking pre-treatment for extracting lignin and isolate cellulose nanofibrils simultaneously [13].	The methods of blending lignin with thermoplastic polymers (natural or synthetic - as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polymethylmethacrylate (PMMA), polyvinyl alcohol (PVA), ethylene-vinyl acetate copolymer (EVA), polyester, starch, and protein) include melt-blending (extrusion, compression, injection, and blow-molding) and solution mixing [40].	Lignin as reinforcer/fillers in thermoplastic polymers improved mechanical properties, decreased water absorption, antioxidant effect due to the phenols in the lignin structure [41], improved water resistance, and thermal stability of the natural polymers such as starch or proteins. [40].
	Polyphenols Plant Extracts Essential Oils	The most commonly applied methods for the extraction of polyphenols uses water in combination with organic solvents (acetone, ethanol, methanol, ethyl acetate) as per the type of polyphenols present in the plant [42].	Blending methods to circumvent the loss of the volatile compounds: - melt blending requires the addition of the active compound in a later stage of the mixing after the polymer is melted, low melting temperature and decreased mixing time [43]. - dispersion/dissolution of the polymer and all active components in a common solvent that is subsequently evaporated (solution casting technique)—method that can also be used as a coating technique by casting the dissolution onto the particular surface [43]. - novel method which involves electrospinning/electrospraying the polymer/active component solution—the advantage of faster solvent evaporation compared with the solution casting technique with the possibility to encapsulate volatile compounds into polymeric fibers/particles.	Plant extracts and essential oils [44,45] are mainly used as antioxidant and antibacterial agents due to the components present in essential oils (eugenol, eugenyl acetate, carvacrol, cinnamaldehyde, thymol, squalene, rosmarinic acid, tyrosol, β-caryophyllene [46]) and plant extracts (isoprenylflavones, flavonone phytoalexins, isoflavonoids, monomeric polyphenols, epicatechin, epicatechin gallate, epigallocatechin gallate, terpenes, alkaloids) [47]. The minimal inhibitory concentration of an antimicrobial agent is the lowest (i.e., minimal) concentration of the antimicrobial agent that inhibits a given bacterial isolate from multiplying and producing visible growth in the test system. For example, in ethanol, thyme, clove and tea tree essential oils had approximately 1, 12, 25 v/v % MIC against Staphylococcus aureus and 1, 3, 12 v/v % against Escherichia coli [48].
Animal Sources
Polysaccharides
	Chitin	Isolation of chitin from crustaceans, such as crayfish, crab, shrimp, and other organisms such as fungi [49], by deproteinization with alkaline treatment at high temperatures, and demineralization with dilute hydrochloric acid [50].	Chitin nanocrystals and nanofibers were added by melt-mixing as fillers into thermoplastic starch-based biocomposites [51]. Also, chitin nanofibers were added in molten PLA by extrusion [52].	Reinforcement in polymer composites [52,53].
	Chitosan	By chitin N-deacetylation [50,54].	Solvent blending [55,56], extrusion blending and reactive extrusion blending [57] as chitosan may be heated up to temperatures below its glass transition temperature without affecting its physicochemical properties [58].	Polymer composites (polyvinyl chloride, polyurethane) with antibacterial properties [59,60]. Reinforcement in polymer composites [54].
Proteins
	Silk/Wool		- In thermoplastics: melt mixing, single/twin screw extruder, and compression molding - In thermosets: vacuum assisted transfer molding, vacuum bag resin transfer molding and vacuum-assisted resin-infused repairing [12].	Reinforcement in polymer composites [10,61].
	Collagen/hyaluronic acid	Hyaluronic acid it is mainly produced via streptococcal fermentation. Recently the production of hyaluronic acid via recombinant systems was studied due to the avoidance of potential toxins [49]. - Collagen can be basically obtained from the slaughter of pork and beef by chemical hydrolysis and enzymatic hydrolysis [62].		Bioactive composite scaffolds for bone tissue engineering [63,64].
Mineral Source - Clays/Nanoclays
	Natural clays: e.g., montmorillonite, hectorite, sepiolite, laponite, saponite, bentonite, kaolinite,	Relatively simple techniques are used in industrial processing for separation and purification of natural clays: decomposition of carbonates, dissolution of (hydr)oxides, oxidation of organic material, dissolution of silica, dialysis, and fractionation. [65].	Polymer–nanoclay nanocomposites may be prepared by melt or solution blending, with partially exfoliated clays, in situ polymerization, and melt intercalation by conventional polymer extrusion process, microwave and ultrasound irradiation [66].	Nanoclays used as fillers in various polymer matrices enhancing mechanical properties of the polymer matrix [67]. In biomedical field: - nanoclays as fillers in chitosan poli e-caprolactone poly-ethylene glycol poly(2-hydroxyethyl methacrylate) for drug delivery applications, as reinforcements for PMMA composites for bone cement applications or implants with improved bioactivity and mechanical properties or incorporated to polysaccharide hydrogels that can support cell proliferation (chitosan, gellan gum) [68].