ReviewPectins functionalized biomaterials; a new viable approach for biomedical applications: A review
Introduction
Polysaccharides (also known as polyholosides or polyosides or glycanes) are high molecular weight carbohydrates which on hydrolysis form various monosaccharides. These are naturally occurring polymers found in animals, plants and microbial worlds, where they act as a source of biological activities, as structural materials and as energy storage component. They are also considered as the polymeric anhydrides of simple sugars [1]. In polysaccharides, monosaccharides are linked through glycosidic linkage by condensation reaction [2], [3], [4], [5], [6], [7], [8]. d-Glucose is the most common monosaccharide present in polysaccharides. However, d-and l-galactose, d-xylose, l-arabinose, d-mannose, d-glucuronic, d-mannuronic acids, d-glucosamine, d-galactosamine, d-galacturonic, and amino uronic acids are also present in polysaccharides [1]. Polysaccharides differ from each other not only in the composition of the monosaccharide but also in nature of chain whether linear or branched, in molecular weight, in the type of glycosidic bond whether α or β and the linkage type (1→2, 1→3, 1→4, or 10→6) in alternate monosaccharide units. In nature, majority of carbohydrates are found as polysaccharides [1], [2], [9], [10].
Chemically, polysaccharides are classified into:
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Homopolysaccharide: These polysaccharides are also known as “homoglycanes”, on hydrolysis, yields same monosaccharide units i.e. cellulose, glycogen, etc. [1].
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Heteropolysaccharide: They are also known as “heteroglycanes”, on hydrolysis yields more than one type of monosacchride unis i.e. pectin, hyaluronic acid, condroitin, etc. [1].
On the basis of functional aspect, polysaccharides may be grouped as:
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Digestible polysaccharides: These are energy storage polysaccharides, present in seeds, tubers, rhizomes, stems and liver. They act as metabolic site of monosaccharides in animals and plants e.g. starch, inulin, glycogen, etc. [1].
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Indigestible polysaccharides: These are also known as structural polysaccharides. They provide protective or lubricative coating to cells, mechanical strength to plants and animals, e.g. pectin, cellulose, chitin, hyaluronic acid and chondroitin [1].
Pectin is a complex heteropolysaccharide and is the most abundant, multifunctional component of cell wall of all land plants [11]. It is a high functional value food ingredient widely used as gelling agent [12], [13]. It is normally produced during the initial stages of growth of primary cell wall and constitutes about one-third of dry substance of cell wall of some monocotyledonous and dicotyledonous plants. It makes about 35% of primary cell walls in dicots and non-graminaceous monocots, up to 5% of walls of woody tissues and 2–10% of grass and other commelinoid primary walls [14], [15]. Pectin is basically composed of α (1,4) linked d-galactrounic acid (GaIA) residues [16], [17], [18], [19] (Fig. 1a) and variety of neutral sugars like arabinose, galactose, rhamnose and lesser amounts of other sugars [20], [21], [22], [23]. The α (1,4) linked d-galacturonic acid can be acetylated and methyl esterified [20], [24]. GaIA comprises about 70% of pectin, all the pectin contain GaIA linked at O-1 and O-4 position. Pectins have a linear anionic backbone with regions having no side chains known as “smooth regions” and regions with non-ionic side chains known as “hairy regions” [25], [26].
Structural classes of pectin include:
Homogalacturonan (HG): HG is the major type of pectin in cell walls of plants having 65% of pectin [20]. It is partially methyl esterified at C-6 and O-acetylated at O-2 or O-3 [20], [27] (Fig. 1b).
Rhamnogalacturonan I (RG-I): It contains about 20–35% of pectin and having more complex structure than HG. It has upto 100 repeatings units of (1,2)-α-l-Rha-(1,4)-α-d-GalA (Fig. 1b) [28]. Large amount of rhamnose sugar is substituted at O-4 by neutral side chains including arabinogalactan I and II, arabinan and galactan, among which galactan and arabinan are more abundant [27].
Rhamnogalacturonan II (RG-II): It contains about 10% of pectin and is structurally more complex component (Fig. 1b). Although it is present in minor amount but it plays a central role in plant cell wall architecture [29], [30].
Pectin is the main component of peel and pulp of several fruits [31], [32]. The peel of citrus fruits especially orange, lemon, grape fruit and lime, and apple pomace are rich source of pectin. Other plant sources of pectin are sunflower heads, potato, suger beet pulp, tomato and carrot [13], [33], [34], [35], [36]. Pectin from sugar beet pectin (SBP) has greater number of neutral sugar side chains i.e. hairy regions, and higher degree of acetylation [37]. Commercially, pectins are mostly extracted from citrus peel [38] and apple pomace [39], both are the by-products of juice manufacturing units. Apple pomace contains about 10–15% of pectin on dry matter basis, while citrus peel contains about 20–30% higher amount than apple pomace [40].
The degree of esterification (DE) or the number of methoxy groups (low (LM) or high (HM)) substituting the carboxylic acid (COOH) moiety on the galacturonic acid residues is often used to classify the different types of pectin. DE influences the gelation mechanism, processing conditions and properties of the pectin [20], [41], [42], [43].
HMP have DE greater than 42.9% [41], [42]. It is mainly used for canning applications and for gelation, it requires high amount of sugar and is very sensitive to acidity [44]. HMP forms gel at low pH values and at high concentration of soluble solids due to the presence of hydrogen bonding and hydrophobic interactions between the pectin chains [30], [44].
LMP have DE less than 42.9% [41], [42]. It is widely used in the food industry to form low sugar jams because does not require large amount of sugar for gelation. It shows less sensitivity toward acidity and requires Ca2+ ions to form gel [44]. It is generally formed by the de-esterification of HMP. Four types of agents i.e. acids, alkali, pectin methyl esterase and ammonia in alcohol or concentrated aqueous ammonia, are mainly used for the preparation of LMP from HMP [45].
Polysaccharide gums are complex hydrophilic natural polymers with many functional properties and are widely used as texture modifier, thickener, coating and gelling agent in different food and other products [46], [47]. Pectin has good biodegradability, biocompatibility and non-toxicity, making it a good biomaterial for various applications such as pharmaceutics, nutraceutics, foods packaging and cosmetics [48], [49]. Monovalent cation i.e. alkali metal salts of pectin are normally soluble in water while di- and trivalent cations are partially or completely insoluble in water. Dissolved pectin decomposes rapidly by de-esterification or depolymerization. The rate of decomposition depends on pH and temperature of solution. Maximum stability of pectin is at pH 4. Low pH and high temperature increase the rate of degradation due to hydrolysis of glycosidic linkage. At alkaline pH, pectin is rapidly de-esterified and degraded even at room temperature [50], [51].
Pectin based biomaterials have various applications in tissue engineering [52], wound dressing [53], gene transfer [54], drug delivery [55] and cancer targeting [56]. It is used as an emulsifier or thickener in cosmetics preparations i.e. creams and lotions, in anti-diarrheal preparations it is often used in combination with kaolin in oral formulations for drug delivery to colon [57]. HMP from citrus fruits and apple are hydrocolloids and used as texture enhancer [58], [59]. Other applications of pectin are; it is used as paper substitute, forming edible films, foams and plasticizers [60], [61], [62].
Gelation of HMP occurs in the presence of high concentration of sucrose contents (55–75%). So, the resulting products are not suitable for diabetic patients. It forms crumps in dissolved form, the crump formation seriously affects its dissolution [63]. Due to high water solubility of pectin, swelling, rapid hydration and erosion occurs. These factors reduce its ability to control efficient drug release in drug delivery to specific sites [64], [65]. As pectin is widely used as gelling agent in food products and other processing industries but due to commercial extraction methods, inconsistencies in chemical structure of pectin are present that limit its applications [66]. To overcome such drawbacks controlled chemical, enzymatic and physical modifications of pectin structure are carried out [67].
Section snippets
Modifications of pectin
Blends of pectin with different compounds and polymers can improve its properties. Nano-composite films with improved physical properties can be produced [68]. Azeredo [69] reported enhanced physical, mechanical, barrier, and antimicrobial properties of pectin/papaya puree-based films when essential oil nano-emulsions were added. Otoni et al. [70] prepared and characterized nanocomposite HMP or LMP/CSNP films with improved physical properties. Literature reports that the addition of halloysite
New trends in pectin biopolymer
Pectin can be blended with various polymers either natural or synthetic or with other compounds. The blending of pectin with other materials has widened its applications for future prospective (Table 1).
Pectin-cellulose based composite
Cellulose is the most abundant polysaccharide in the primary cell wall of plants. It plays the main structural and load-carrying roles [120]. It is made up of unbranched (1, 4)-linked β-d glucose chains with high degree of polymerization, about 2000–6000 units in primary cell walls and are tightly bonded by hydrogen bonding and van-der waals forces to form semicrystalline microfibrils [121], [122]. Lin et al. [78] investigated the interactions between pectin (having different contents of neutral
Pectin–whey protein based emulsion
Protein-stabilized emulsions are usually prone to instabilities by pH, ionic strength and temperature [194], [195]. Complexes and coacervates stable to pH, heat treatment and salt additions have been developed from various biopolymers [196]. Salminen [79] prepared stable emulsions by adsorbing whey protein isolates (WPI)–apple pectin complexes to the interface of an oil-in-water emulsion stabilized with whey proteins. At pH 4.5 the emulsion (without biopolymer complexes) was unstable but became
Pectin–chitosan based film
Non-biodegradable and renewable polymers (e.g. polysaccharides), when reinforced with nanostructures, have been used to produce novel, eco-friendly food packaging as alternatives to replace conventional packaging [70]. Films and coatings have been produced from several biopolymers, including polysaccharides [233], [234], [235], [236] and polypeptides [237]. Chitosan (CS), a widely available, biodegradable polysaccharide derived from chitin, has been reported to produce films, coatings, and
Pectin based PECs
A polyelectrolyte complex (PEC) is produced by ionic interactions among polycations and polyanions [259], [260], [261], [262], [263]. Different factors affect the structure, stability and the formation of PECs in which the charge densities, concentration, mixing ratio, kinds of functional groups, pH and ionic strength of solvent are involved [264], [265], [266], [267]. When density of positive charges on the polycations and negative charges on polyanions are not equal then PECs are
Pectin–starch based matrix
Hydrogels are hydrophilic in nature that absorb large amount of water and their increased applications in drug delivery systems depends on their ability to form a gel network in the swollen form which captures the drug and functions as a barrier to its discharge into the medium [302], [303], [304]. High amylase starch containing 70% of amylase contents has been successfully employed to prepare swellable hydrophilic matrices for controlled drug delivery [305], [306], [307], [308]. The successful
Pectin–ethylcellulose based powder
Colorectal cancer is the cancerous cell growth in rectum, colon, or cecum. Its treatment involves the delivery of 5-fluorouracil through the injection route. The drug can be formulated for release particularly in the colonic region [316]. Pectin is appropriate for use as colon specific drug delivery medium in the treatment of colon cancer as it is digested by microflora in colon and it has a strong ability to cooperate with intestinal mucosa to stop the biochemical processes for tumor growth,
Pectins–β-lactoglobulin based nanoparticles
Biopolymers are used for protection and delivery of bioactive compounds in food systems [322], [323]. They also replace fats in foods by improving the optical, rheological and sensorial properties of lipid molecules [324], [325]. When proteins and polysaccharides are combined they form electrostatic complexes [326], [327]. β-LG is the main globular protein present in whey [328]. It is easily available possessing interesting structural features, and wide range of applications in the food and
Pectin-PEO based fiber
Electrospinning is a simple and flexible fiber-forming technique that utilizes electrical instead of mechanical forces to draw fibers out of a polymer solution or melt [352], [353]. It produces continuous, uniform fibers with diameters reaching the nanometer scale [354] that produce a fibrous mat with increased porosity and available surface for chemical modifications and interactions. These nanofibers have a range of applications including membrane filtration [355], biosensing [356], tissue
Pectin–alginate based gel beads
The sorbent materials are derived from natural biomass, especially fungi [368], algae [369], humic substances from soil [370] and by products from industrial processes, such as wood and food industries, agriculture, fishery and textile manufacturing [371], [372]. Also some inorganic materials (zeolites, clays, etc.) are used for the same purpose [373]. Algae and fruits have been widely tested as sorbents for metal removal, including copper and cadmium [374], from polluted aqueous solutions. Few
Pectin–gelatin based hydrogel
Polymers derived from natural resources are used in the field of human healthcare such as drug delivery [384], wound care [385] and tissue engineering [386]. Most protein hydrogels are non-toxic, biocompatible, and biodegradable. Gelatin sponges are used for inducing hemostasis in bleeding wounds [387]. Recently, a lot of interest has been taken in fabricating in situ gellable, non-toxic hydrogels based on proteinaceous materials and polysaccharides [388], [389]. Under ambient conditions,
Conclusion and future trends
Pectin based biomaterials have various applications in biomedical field and food industry but due to high water solubility and high concentration of sucrose contents it is not suitable for efficient drug delivery to specific sites and for diabetic patients. Chemical modification of pectin can be carried out to reduce its water solubility. Thermal and the mechanical properties can be improved by fabrication of pectin with nano particles. It can also be blended with various polymers either
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