Alginate is an unbranched, polyanionic polysaccharide composed of 1, 4-linked β-D- mannuronic (M) and α-L-guluronic (G) residues arranged in an irregular blockwise pattern of varying proportions of GG, MG, and MM blocks along the chain (Figure 1.2.). It is a natural biopolymer present in brown marine algae (Phaeophyceae) and also as an exopolysaccharide of bacteria such as Pseudomonas aeruginosa. The first scientific studies on the extraction of alginates from brown seaweed were done by British chemist E. C. Stanford in 1881. Commercially, alginate is extracted from brown algae including Laminaria hyperborea, Laminaria digitata, Laminaria japonica, Ascophyllum nodosum and Macrocystis pyrifera by treatment with dilute alkali solutions such as NaOH. The extract is filtered and alginate is precipitated by the addition of either sodium or calcium chloride to the filtrate. The alginate salt is subsequently transformed into alginic acid on treatment with dilute HCl (Lee et al., 2012). The composition, sequence and molecular weights of alginate vary with the source and species from which it is extracted.
Alginate is a cheap, environment friendly biopolymer that has favourable properties like biocompatibility, biodegradability, mucoadhesiveness, and hydrophilicity that makes it suitable for various biomedical applications. It is known to form a hydrogel in the presence of divalent actions whose affinity for alginate decreases in the following order: Pb > Cu > Cd > Ba > Sr > Ca > Co, Ni, Zn > Mn (Mørch et al., 2006). The cations interact ionically with uronic acid residues of G blocks, resulting in the formation of a three-dimensional network usually described as
‘egg-box’ model (Morris et al., 1978). In addition, alginate has large number of free hydroxyl and carboxyl groups that can be easily functionalized to tailor its properties such as solubility, hydrophobicity and physicochemical and biological characteristics.
Alginate Chapter 1
15 Table 1.2. Representative list of polymers used in drug delivery (Pillai and Panchagnula, 2001)
Classification Polymer Natural polymers
Protein-based
polymers Collagen, albumin, gelatin
Polysaccharides Agarose, alginate, carrageenan, hyaluronic acid, dextran, chitosan, cyclodextrins
Synthetic polymers Biodegradable
Polyesters Poly(lactic acid), poly(glycolic acid), poly(hydroxy butyrate), poly(ε-caprolactone), poly(β-malic acid), poly(dioxanones) Polyanhydrides Poly(sebacic acid), poly(adipic acid), poly(terphthalic acid) and
various copolymers
Polyamides Poly(imino carbonates), polyamino acids Phosphorous-based
polymers Polyphosphates, polyphosphonates, polyphosphazenes Others Poly(cyano acrylates), polyurethanes, polyortho esters,
polydihydropyrans, polyacetals Non-biodegradable
Cellulose derivatives Carboxymethyl cellulose, ethyl cellulose, cellulose acetate, cellulose acetate propionate, hydroxypropyl methyl cellulose Silicones Polydimethylsiloxane, colloidal silica
Acrylic polymers Polymethacrylates, poly(methyl methacrylate), poly hydro(ethyl- methacrylate)
Others Polyvinyl pyrrolidone, ethyl vinyl acetate, poloxamers, poloxamines
Alginate Chapter 1
16 Figure 1.2. Representative alginate structure: (a) chain conformation and (b) block distribution (Pawar and Edgar, 2012).
Biomedical applications of Alginate and its derivatives Drug delivery
Alginate gels have extensively been investigated for delivery of small chemical drugs.
Bouhadir et al. (2001) synthesized novel alginate hydrogel loaded with three antineoplastic agents’ viz. methotrexate, doxorubicin, and mitoxantrone for the simultaneous delivery of multiple drugs. Zhang et al. (2010) reported sustained release of theophylline from carbon nanotube (CNT)-incorporated alginate microspheres.The incorporation of CNT increased the mechanical stability of gels. Besides, alginate gels have also been used for the delivery of protein drugs. The proteins are incorporated under relatively mild conditions thereby minimizing denaturation and protected from degradation in the gels until their release (Chan et al. 2010). Recently, Möbus et al.
(2012) prepared Zn2+ cross-linked alginate microparticles via a simple one-step spray- drying process for controlled pulmonary delivery of BSA as a model protein.
Wound Dressings
Alginate-based products are most commonly employed in wound management.
Alginate dressings in dry form absorb wound exudates to form a gel which eliminates fibre entrapment in the wound, a major cause of discomfort for patient during dressing removal. Besides, the gel maintains a physiologically moist microenvironment conducive for rapid healing (Winter et al., 1962). Some examples of commercial
Alginate Chapter 1
17 alginate-based wound dressings include Kaltostat™ (dressing calcium/sodium alginate, ConvaTec), Kaltogelw (calcium/sodium alginate gelling fibre, ConvaTec), Seasorbw (calcium/sodium alginate gelling fibre, Coloplast) and Sorbsan™ (dressing calcium alginate, Maersk). Balakrishnan et al. (2006) incorporated dibutyryl cyclic adenosine monophosphate (DBcAMP), a regulator of human keratinocyte proliferation, into partially oxidized alginate wound dressing gel that accelerated the wound healing process due to sustained release of DBcAMP in a rat model. A complete re- epithelialization of full thickness wounds was observed within 10 days. Similarly, Rabbany et al. (2010) demonstrated the effectiveness of alginate gel releasing stromal cell-derived factor-1 in accelerating wound closure rates and reducing scar formation in pigs with acute surgical wounds. Further, Shalumon et al. (2011) developed sodium alginate/poly (vinyl alcohol)/nano ZnO composite nanofibres that were shown to have antibacterial effect against S aureus and E. coli due to the presence of ZnO, making them suitable as wound dressing material.
Cell culture substrates
In addition, alginate gels are also increasingly being used as either 2D or 3D mammalian cell culture systems by modifying the gels with synthetic peptides specific for cellular adhesion receptors. In this context, arginine-glycine-aspartic acid (RGD)- modified alginate gels have been most frequently used as in vitro cell culture substrates till now. Rowley et al. (1999) chemically conjugated RGD peptides to alginate and demonstrated enhancement in adhesion and proliferation of myoblasts cultured on alginate gels in comparison to non-modified alginate gels. The number of cells adherent to the gels and the growth rate were found to be strongly dependent on the bulk RGD density in the gels. Chueh et al. (2009) reported formation of alginate gel in a microfluidic device through UV-triggered release of caged calcium from DM- nitrophenTM. The gel was used as a 3D cell culture substrate for coculture of preosteoblasts (MC3T3-E1) and human umbilical vein endothelial cells demonstrating integration of 3D culture microenvironments into microfluidic systems. Furthermore, alginate gels have been also been used as 3D cell culture substrates for cancer cells and stem cells thereby opening up possibility to gain insights regarding cancer and stem cell biology (Fischbach et al., 2009; Huebsch et al., 2010).
Chitosan Chapter 1
18 Tissue regeneration
The alginate gels have also been used for the delivery of proteins or cells that can direct the regeneration or engineering of various tissues and organs in the body. These include delivery of angiogenic factors like vascular endothelial growth factor (VEGF) and basic fibroblast growth factor in SCID mice (Lee et al., 2003); bone regeneration factors like bone morphogenetic proteins BMP-2 and BMP-7 (Basmanav et al. 2008) and cell populations like primary chondrocytes and osteoblasts into mice (Alsberg et al., 2002);
mesenchymal stem cells for cartilage regeneration in large osteochondral defects (Ma et al. 2003) and also various growth factors and cells for regeneration of other tissues and organs including skeletal muscle, nerve, pancreas, and liver (Borselli et al., 2009; Prang et al., 2006; Calafiore et al., 2003; Dvir-Ginzberg et al., 2003). The alginate gels have also been used for sequential delivery of various growth factors based on their differential binding to various factors. For example, Ruvinov et al. (2011) showed sequential release of insulin-like growth factor-1 (IGF-1) followed by hepatocyte growth factor (HGF) from alginate-sulfate gels in a rat model of acute myocardial infarction.
Other applications of Alginate and its derivatives
Alginate is widely used in food industry as thickening agent, gelling agent, emulsifier and colloidal stabilizer. Furthermore, alginate has also been used in the removal of toxic heavy metals like uranium, lead, zinc, cadmium and nickelfrom industrial wastes by biosorption (Davis et al., 2003; Gok and Aytas, 2009).