**3.2 Drug delivery**

Drug delivery systems could transport and release drug molecules to a target position under the microenvironment, especially for poorly water-soluble drug molecules. Many polymers have been explored and investigated for drug delivery. Due to the outstanding biocompatibility and gelation properties, alginate has been received wide attraction and utilized in drug delivery applications. Alginate could be prepared excipient. Three forms of participation are exhibited, such as solid dosage, semisolid dosage, liquid dosage. Solid dosage form contains tablets and capsules, while, gels and buccal patches are the most common semisolid dosage. Liquid dosage usually includes emulsions and suspensions. Alginate could be decorated to adjust the properties for novel drug release. Due to the facility to bond with drug molecules and rapid gelation in a mild environment, alginate could be utilized to modulate the drug release properties. D.J. Mooney et al. found that the pore size of alginate gel is about 5–6 nm [46], which contributes to the diffusion and release of drug molecules.

Alginate could be utilized to design microcapsules for sustained release of the drug, which has attracted extraordinary attention by the reason of outstanding advantages, including high drug loading rate, satisfactory biodegradable, nonimmunogenic, and non-toxicity. Stimuli-responsive alginate-based microcapsules had shown a potential for targeted delivery and release drugs. Drug delivery system using magnetic nanoparticles was reported. Magnetic nanoparticles have the ability to control movement and aggregation at the target sites during the process of the external magnetic field. Therefore, the system could obtain a high local concentration at the target sites. It is demonstrated that magnetic reduction-responsive alginate-based microcapsules (MRAMCs) systems exhibited good magnetic targeted ability owing to the superparamagnetism of OA-Fe3O4 nanoparticles [47]. In addition, alginate microcapsules encapsulated glucocorticoid is a novel method for the treatment of osteoarthritis. *E. Lengert* et al. found that hollow silver alginate microcapsules could effectively deliver water-insoluble glucocorticoid betamethasone [48]. This system enhanced loading efficiency and sustained release, reducing the injury rate of intraarticular glucocorticoids delivery.

Alginate, as a well-known drug carrier, is utilized to prepare wound dressing. Antimicrobial agents or drugs are encapsulated into the hydrogel. Wound dressing provides a moist environment, and more importantly, drugs release to the wound and facilitate the healing. For example, alginate-based dressing encapsulated with vancomycin has a 44% drug release rate after 24 h, and the antimicrobial activity against various bacteria was confirmed [49]. Recently, double-membrane hydrogel formed with alginate and cellulose nanocrystals was developed. The results showed good release behaviors for complexing antibiotic drugs. The rapid drug release was completed by outer neat alginate hydrogel, meanwhile, the prolonged-release behavior corresponded to the inner hydrogel [50]. Another dual-drug delivery system, poly (D, L-lactic) (PDLLA) microspheres embedded in calcium alginate hydrogel beads, was developed by D.G. Zhong et al. [51]. The microspheres encapsulated glycyrrhetinic acid showed a sustained release, and hydrogel loaded with BSA exhibited a rapid release.

#### **Figure 3.**

*The synthesis of AlgPD-BTZ hydrogel and drug release properties. (a) Schematic of the synthesis process of AlgPD-BTZ hydrogel by the conjugation of dopamine to the alginate backbone, and subsequent oxidation of catechol groups for cross-linking; (b) pH sensitive cumulative drug release from AlgPD-BTZ hydrogel; (c) scheme shows pH sensitive boronic ester bond of BTZ with the polymeric catechol group and the drug release mechanism [52].*

pH-sensitive alginate carrier was investigated to enhance the efficacy, especially for localized drug delivery systems. The pH-sensitive reversible bond can control the drug delivery. C. H. Park and C. S. Kim prepared AlgPD-BTZ hydrogel using alginate-conjugated polydopamine as a building block polymer (**Figure 3**) [52]. The catechol group binds to the boronic acid group of BTZ drug, that covalent bond is a pH-sensitive reversible bond. Therefore, the system showed that BTZ was selectively released in cancer cells with a pH-dependent method. Sodium alginate could be used as a pH-sensitive bilayer coating on iron oxide nanoparticles by combining hydroxyapatite to deliver drug molecules. The higher encapsulation efficiency was detected, 93.03 ± 0.23% for curcumin and 98.78 ± 0.05% for 6-gingerol [53]. The electrostatic interactions of molecules could act as an adjustable gate to hold and release the drug molecules depending on the pH. The pH triggered drug-releasing mechanism play a virtual role in the releasing of tumor drug owing to the leaky vasculature [54].

In addition, alginate hydrogels are an excellent candidate and have been studied for protein drug delivery. Protein incorporated into hydrogels is protected, which reduces the denaturation and avoids degradation. Alginate hydrogels containing adjusting factors of neovascularization, vascular endothelial growth factor (VEGF), exhibited a sustained release, within 7 days approximately 60% of the total VEGF [55]. The protein release rate can be controlled by altering the degradation rate of alginate hydrogel. Also, sodium alginate-bacterial cellulose hydrogels had shown promising potential for carrying protein-based drugs, lysozyme (LYZ), via electrostatic adsorption [56].

#### **3.3 Tissue engineering**

Tissue engineering, proposed by National Science Foundation in 1987, belongs to the field of biomedical engineering, which is a valuable approach and can be used to restore, maintain, enhance tissues and organs [57]. Tissue engineering involves the getting of seed cells, biological scaffold materials, preparation of tissue and organs, and its clinical application. The biological scaffold materials were utilized to encapsulate and support the propagation of cells. Afterward, these materials

were transferred into the body and the target tissues were eventually formed. The research principal field contains bone [58], cartilage [59], vascular [60], ocular tissues [61], skin [62], and other tissues [63].

Alginate, as a most commonly known biomaterial with outstanding properties of scaffold-forming, has been extensively investigated and developed to treat the loss or failure of organs in tissue engineering. It usually combines with other substances to form new alginate derivative materials with the methods of physical or chemical. The different features and functions are obtained, accompanied by improved properties for tissue engineering, such as mechanical strength, cell affinity, and gel-forming ability. Therefore, alginate and its composite have received much attention in tissue engineering.

#### *3.3.1 Bone*

Bone has the hierarchical structure forming with 70% of nano-hydroxyapatite (HA, Ca10(PO4)6(OH2)) and 30% of collagen by weight [64]. It is a rigid connective organ and plays a major role in the movement of organs, involving affording structural framework, mechanical support and protection, mineral storage, and homeostasis [65]. There are several ways to cause bone defects or fractures in daily life, involving sports injury, accident damage, osteoporosis, osteoarthritis, bone neoplasm, and so on. Bone tissue engineering, as an effective alternative treatment for restoring bone defects, has addressed much attention. Several therapies have been available to treat bone defects or loss, including autograft (bone from patient's own healthy tissue), allograft (bone from the human donor), and xenograft (from other species) [66]. The final goal of bone tissue engineering is to construct bone tissue that should have the same qualities of structurally, physical, and chemical features as natural bone tissue. Alginate has been extensively investigated to synthetic scaffolding materials for bone reconstruction and transfer cells for bone tissue engineering [67].

Sufficient mechanical strength of scaffold in bone tissue engineering is essential to support bone regeneration. To obtain enough mechanical properties, alginate composite scaffolds were prepared mixing other polymers or inorganic components, such as chitosan, collagen, and hydroxyapatite. Chitosan, as a most abundant cationic polysaccharide, is selected to form alginate-chitosan composites. Chitosan/alginate composite scaffolds are extensively investigated for bone tissue engineering. The rigid strength and structural stability are obtained. S. J. Florczyk et al. prepared a chitosan-alginate scaffold with enhanced compressive strength, 0.79 ~ 1.41 MPa [68]. It has the homogeneous pore structure, and the pore size depends on acetic acid and alginate concentration (**Figure 4**). The cell proliferation potential was also improved when the viscosity was below 300 Pas. The greatest defect closure (71.56 ± 19.74%) was observed at 16 weeks [69]. Chitosan/alginate scaffolds present advantages in stimulating osteogenesis and vascularization [70]. Collagen is usually utilized to prepare scaffolds because of its specific properties, such as inducing cell adhesion and degradation [71]. Collagen I, as the essential component of bone tissue's ECM, contributes to migrant and penetration of osteoblasts and vessels [71, 72]. Therefore collagen/alginate hybrid scaffolds have an effect on the osteogenic ability of osteoblasts. It could promote cell spreading, proliferation, and osteogenic differentiation. S. Sotome et al. demonstrated that hydroxyapatite/collagen-alginate could deliver recombinant human bone morphogenetic protein 2 (rh-BMP2) efficiently [73]. There is bone formation within 5 weeks after implantation, accompanied with no obvious deformation. Hydroxyapatite (HA) is the main component of bone. The structure of alginate combined HA composites is similar to the native extracellular matrix of bone.

#### **Figure 4.**

*(A) SEM images of CA PEC scaffold pore structures made from CA solutions (4 wt % chitosan and 3.75 wt % alginate) with acetic acid concentrations of (a) 0.75 wt %, (b) 1.0 wt %, (c) 1.25 wt %, (d) 1.5 wt %, (e) 1.75 wt %, and (f) 2.0 wt %. The arrows indicate incomplete interconnects and scale bars are 100 μm. (B) Compressive Young's moduli of CA PEC scaffolds prepared from CA solutions (4 wt % chitosan and 3.75 wt % alginate) with varying acetic acid concentrations. \*the significant differences between the scaffold groups for the modulus measurements. (C) Influence of mixing temperature on viscosity of CA PEC solution (1.0 wt % acetic acid, 4 wt % chitosan, and 3.75 wt % alginate) at zero shear rate [68].*

The interconnected porous structures and high porosity promote good cell viability, proliferation rate, adhesion, maintenance of osteoblastic phenotype, and bone regeneration [74]. The results of Lin et al. showed that alginate/HA is the better composite porous scaffold with an average pore size of 150 μm and over 82% porosity [75]. In another report, the alginate/HA composite scaffolds have 84% porosity [76]. The uniform pore morphology is favorable to improve compressive strength and elastic modulus, which is proportional to the content of HA. The satisfactory scaffold should have excellent performance to promote bone tissue growth, such as mechanical strength, cell proliferation, and morphology [77]. N. Firouzi et al. found that the addition of HA in alginate-based hydrogel could reduce degradation rate to 41.5%, and significantly improve compressive modulus, reaching 294 ± 2.5 kPa [78]. Meanwhile, microencapsulated osteoblast-like cells showed more proliferation as well as metabolic activities when they were cultured in Alg-Gel Ph-nHA microcapsules during the culture period.

#### *3.3.2 Cartilage*

The articular cartilage is an organized and specialized tissue that is highly hydrated (up to 80%), aneural, devoid of blood or lymphatic vessels [79]. It is the connective tissue covering the surface of articulating bones, that provides

the lubrication and mechanical strength for body weight and movement. Other body organs are also made by cartilage, such as the ear, nose, bronchial tubes, and so on. Cartilage has a restricted self-repair and regenerative capacity because of lacking nerves and blood vessels. It cannot heal appropriately after the injury, which eventually causes osteoarthritis and cartilage damage. Therefore, repair or reconstruction of damaged cartilage is still a major challenge. Tissue engineering is the potential approach to solve damaged or degraded cartilage. It could mimic the structure and function of body cartilage tissue, stimulating cartilage growth and restoration at the damaged sites. The tissue engineering scaffold has the threedimensional structure for cell attachment/proliferation.

Alginate scaffolds have been proved to apply for the regeneration of cartilage tissue. It can induce redifferentiation of 2D culture-expanded dedifferentiated chondrocytes [80], therefore alginate could promote the growth of chondrocytes cells, restore damaged cartilage, and keep chondrocyte properties [81]. Chitosanalginate scaffold effectively promotes the culture of osteogenic and chondrogenic cells. A.E. Erickson et reported that alginate-chitosan + hydroxyapatite scaffolds displayed a defined, interconnected porous network structure [82]. The compressive modulus and stiffness increased with polymer content. After culturing with

#### **Figure 5.**

*Effect of HAp concentration on 6% CA scaffold properties. a, b visualization of HAp nanorods with TEM. Scale bars represent 200 nm, and 100 nm, respectively, in (a) and (b). c SEM images of scaffold pore morphology with varying HAp (HAp concentration increases from left to right). Scale bar represents 200 μm. d density of CA + HAp composite scaffolds (n = 8) (p* ≤ *0.05). (e) MG63 proliferation on CA + HAp composite scaffolds over a 2-week culture period (n = 4). \*statistically significant from all or specified conditions (p* ≤ *0.05). \*\*statistically significant from all conditions except 0.1% HAp (p* ≤ *0.05) [80].*

*Alginate-Based Composite and Its Biomedical Applications DOI: http://dx.doi.org/10.5772/intechopen.99494*

chondrocyte-like (mesenchymal stem cells, MSC), the number of cells on 4% CHA scaffolds was significantly higher than the number of MSCs on 6% CHA scaffolds at day 10 (**Figure 5**) [80]. Hyaluronate, in addition, was explored to reinforce alginate to improve chondrocyte proliferation. It is demonstrated that alginate-hyaluronic acid hydrogel has stable physicochemical properties. C. Mahapatra et reported that alginate-hyaluronic acid-collagen hydrogel provided a binding motif for chondrocytes [83]. These gels effectively protected chondrogenic phenotypes after three weeks of cultivation. The results demonstrated the efficient expression of chondrogenic genes and the formation of cartilage ECMs. Another alginate hybrid gel made combining with polyacrylamide was confirmed to have remarkable mechanical strength, such as high toughness (up to 9000 J/m<sup>2</sup> ) and stretch ratio [84, 85]. It has

#### **Figure 6.**

*In vivo representative photomicrographs of tissue sections in the experimental groups stained with (A, B, C) H&E; (D, E, F) safranin O/fast green, (G, H, I) Gomori's trichrome stains, TB (J, K,L), and PAS (M, N,O). (A, D, G) control group demonstrates mostly fibrous tissue with limited tissue repair in the chondral region. (B, E, H) PAAm-Alg group shows fibrous tissue with few fibroblast and chondrocyte-like cells in the joint surface chondral regions. (E) Fibrous tissue in the deeper regions is still observed in some rats. (C, F, I) PAAm-Alg-NPTGF-β3 group reveals fibrous tissue with some fibrocartilage and hyaline cartilage groups in the chondral regions. PAAm-Alg and PAAm-Alg-NPTGF-β3 groups demonstrated higher (K, L) glycosaminoglycan deposition and (N, O) glycoprotein and proteoglycan contents compared to control group (the arrows indicate the margins of the defect area; scale bars: 100 μm) [87].*

also been proved that alginate-polyacrylamide showed good tribological behavior, ultra-low coefficient, and high wear-resistance [86]. After transplantation, chondrocytes displayed an organized distribution and superior integration with surrounding tissue (**Figure 6**) [87]. Alginate and its composites provide chondrogenic differentiation and cell proliferation, therefore, these hybrid gels can be used as cartilage implants.

#### *3.3.3 Liver tissue engineering*

Liver disease is a great threat to human health, and it is one of the major reasons for the increased mortality. There are about 2 million deaths all over the world every year [88]. Liver transplantation is the most effective way to solve this problem. Yet, this is a very difficult process because of lacking suitable liver donors. Liver tissue engineering is considered the best way to provide liver to meet the excessive requirement of the liver. It could improve the function of the liver and form a complete organ.

Alginate composites can be used for hepatocyte growth in liver tissue engineering. For example, R. Rajalekshmi et al. reported that fibrin (FIB) incorporated injectable alginate dialdehyde (ADA) - gelatin (G) hydrogel effectively supports growth, proliferation, and functions of hepatic cells [89]. The reason is that fibrin provides several cell adhesion pockets for cell attachment. Liver tissue engineering scaffold, extracellular matrix (ECM), were synthesized with oxidized alginate and galactosylated chitosan via Schiff base reaction. After being cultured in the scaffolds, the hepatocytes exhibited spheroidal morphology. And the multi-cellular aggregates and perfect integration were observed [90]. In addition, the formation of microcapsules is another method for liver tissue engineering. Microcapsules have the ability to encapsulate cells into microbeads. Subsequently, microbeads were covered by the semipermeable membrane to form microcapsules [91]. Microcapsules provide safety microenvironment for cells and protect the cell from interference. After encapsulated HepG2 cells into alginate-based microbeads, the proliferate and protein were clearly observed for at least 12 d [92]. These researches suggested that alginate is a potential candidate for LTE strategies.
