*3.2.2 Poly(lactic-co-glycolic acid)*

*Microencapsulation - Processes, Technologies and Industrial Applications*

versatile and bioactive polymer for its use in encapsulation.

distributed widely in connective, epithelial, and neural tissues.

in ocular and plastic surgery, and in tissue engineering.

*3.1.2.2 Hyaluronic acid*

**3.2 Synthetic polymers**

*3.2.1 Poly(Ɛ-caprolactone)*

also be improved when the primary amino group is protonated at low pH. The viscosity of chitosan solution increases with increasing the concentration of chitosan [5]. These properties and the ease with which it can be modified makes chitosan a

Hyaluronic acid (HA) is a nonsulfated glycosaminoglycan, comprising a relatively simple linear structure of alternating units of D-glucuronic acid and N-acetyl-D-glucosamine, linked via β-1,3- and β-1,4-glycosidic bonds. Hyaluronic acid is biodegradable, biocompatible, nontoxic, and non-immunogenic glycosaminoglycan

The cluster of differentiation (CD) protein CD44 is the main HA binding receptor. CD44 is involved in the interaction between HA and the surface of specific cells, in cell proliferation, in cellular adhesion processes (aggregation and migration), angiogenesis, in cell survival and endocytosis of HA. CD44 receptor is also overexpressed in many types of tumors and this overexpression is related to tumor invasion and tumor metastasis, which makes HA a promising candidate for intracellular delivery of imaging and anticancer agents exploiting a receptormediated active targeting strategy. HA also interacts with hyaluronan receptor for endocytosis (HARE), lymphatic vessel endothelium receptor-1 (LYVE-1), and intracellular adhesion molecule-1 (ICAM-1), serum-derived hyaluronan-associated protein (SHAP), Brevican and Neurocan (brain and nervous tissue-specific HA and proteoglycan binding proteins), hyaluronan-binding protein 1 (HABP1) and toll-like receptors (TLRs), and all of which have specific functions. This is known as receptor-ligand interaction, which can be exploited to achieve receptor-mediated active targeting strategy [6, 7]. HA has been bioconjugated with anticancer drugs, like paclitaxel, doxorubicin, cisplatin, etc. and anti-inflammatory drugs like methotrexate, dexamethasone, methylprednisolone, etc. to achieve receptor-mediated endocytosis [8]. HA polymer has also been used in the treatment of osteoarthritis,

Over the past 5–6 decades, biodegradable polymers have gained tremendous attention due to their growing applications in biomaterials, drug-delivery systems, tissue engineering, and medical devices. Chemists, biologists, physicians, and engineers have collaboratively made significant advancements in these applications. The most commonly used synthetic polymers in micro-/nanocapsules for drug-delivery applications are poly(Ɛ-caprolactone), poly(lactic-co-glycolic acid), and polyethylene glycol.

Poly(Ɛ-caprolactone) (PCL) is a semicrystalline aliphatic polyester with glass transition temperature and melting temperature of about −60 and 60°C, respectively [9]. PCL mixes well with other polymers to form blends that impart good physical and chemical properties to achieve desired properties like swelling, porosity, and stability in different media. Microcapsulation or nanocapsulation with PCL has many advantages like modulation of drug release, control of drug penetration/ permeation into the skin, and improve photochemical stability and pharmacological response. Due to its long degradation times, PCL has found many applications in tissue engineering and prolonged drug release. PCL is approved by the US Food and

**6**

Poly(lactic-co-glycolic acid) (PLGA) is the most extensively studied degradable polymer to date. PLGA is an aliphatic polyester and it undergoes hydrolysis in the body to produce biodegradable metabolite monomers such as lactic acid and glycolic acid. During metabolism in the body via the Krebs cycle, carbon dioxide, and water are removed and thereby toxicity is minimized [10]. PLGA is approved by the US FDA for use in drug-delivery systems due to its biodegradability with tissue and cells, drug biocompatibility, suitable biodegradation kinetics, mechanical properties, and ease of processing. Thus, PLGA based microcapsules and nanocapsules are the most viable candidates for drug-delivery systems, anticancer agents, bioimaging, and vaccine immunotherapy.
