Cellulose probably is the most abundant organic compound in the world which mostly produced by plants. It is the most structural component in herbal cells and tissues. Cellulose is a natural long chain polymer that plays an important role in human food cycle indirectly. This polymer has versatile uses in many industries such as veterinary foods, wood and paper, fibers and clothes, cosmetic and pharmaceutical industries as excipient. Cellulose has very semi-synthetic derivatives which is extensively used in pharmaceutical and cosmetic industries. Cellulose ethers and cellulose esters are two main groups of cellulose derivatives with different physicochemical and mechanical properties. These polymers are broadly used in the formulation of dosage forms and healthcare products. These compounds are playing important roles in different types of pharmaceuticals such as extended and delayed release coated dosage forms, extended and controlled release matrices, osmotic drug delivery systems, bioadhesives and mucoadhesives, compression tablets as compressibility enhancers, liquid dosage forms as thickening agents and stabilizers, granules and tablets as binders, semisolid preparations as gelling agents and many other applications. These polymeric materials have also been used as filler, taste masker, free-flowing agents and pressure sensitive adhesives in transdermal patches. Nowadays cellulose and cellulose based polymers have gained agreat popularity in pharmaceutical industries and become more and more important in this field owing to production of the new derivatives and finding new applications for existed compounds by pharmaceutical researchers.
Pure cellulose is available in different forms in the market with very different mechanical and pharmaceutical properties. The difference between various forms of cellulose is related to the shape, size and degree of crystallinity of their particles (fibrous or agglomerated). Microcrystalline cellulose (MCC) is the most known cellulose which extensively used in pharmaceutical industries. MC grades are multifunctional pharmaceutical excipients which can be used as compressibility enhancer, binder in wet and dry granulation processes, thickener and viscosity builder in liquid dosage forms and free-flowing agents in solid dosage forms. Mechanical properties of MCC grades are greatly influenced by their particle size and degree of crystallization. In recent years the new grades of MCC are prepared with improved pharmaceutical characteristics such as silisified MC (SMCC) and second generation MCC grades or MCC type I (MCC-II). These grades are prepared by co- processing of cellulose with other substances such as colloidal silicon dioxide or by special chemical procedures. Other types of available pure cellulose are powdered cellulose (PC) and low crystallinity powdered cellulose (LCPC).
Regenerated cellulose is one of the other forms of processed cellulose which produced by chemical processing on natural cellulose. In the first step, cellulose dissolves in alkali and carbon disulfide to make a solution called "viscose". Viscose reconverted to cellulose by passing through a bath of dilute sulfuric acid and sodium sulphate. Reconverted cellulose passed through several more baths for sulfur removing, bleaching and adding a plasticizer (glycerin) to form a transparent film called cellophane. Cellophane has several applications in pharmaceutical packaging due to its suitable characteristics such as good compatibility, durability, transparency and elasticity.
Cellulose ethers are high molecular weight compounds produced by replacing the hydrogen atoms of hydroxyl groups in the anhydroglucose units of cellulose with alkyl or substituted alkyl groups. The commercially important properties of cellulose ethers are determined by their molecular weights, chemical structure and distribution of the substituent groups, degree of substitution and molar substitution (where applicable). These properties generally include solubility, viscosity in solution, surface activity, thermoplastic film characteristics and stability against biodegradation, heat, hydrolysis and oxidation. Viscosity of cellulose
ether solutions is directly related with their molecular weights. Examples of mostly used cellulose ethers are: Methyl cellulose (MC), Ethyl cellulose (EC), Hydroxyethyl cellulose (HEC), Hydroxypropy1 cellulose (HPC), hydroxypropylmethyl cellulose (HPMC), carboxymethy1 cellulose (CMC) and sodium carboxymethy1 cellulose (NaCMC).
Cellulose esters are generally water insoluble polymers with good film forming characteristics. Cellulose esters are widely used in pharmaceutical controlled release preparations such as osmotic and enteric coated drug delivery systems. These polymers are often used with cellulose ethers concurrently for preparation of micro-porous delivery membranes. Cellulose esters categorized in organic and inorganic groups. Organic cellulose esters are more important in pharmaceutical industries. Various types of organic cellulose esters have been used in commercial products or in pharmaceutical investigations such as cellulose acetate (CA), cellulose acetate phthalate (CAP), Cellulose acetate butyrate (CAB), Cellulose acetate trimelitate (CAT), hydroxupropyimethyl cellulose phthalate (HPMCP) and so on (Heinämäki et al., 1994). The most available formulations in market which made by these polymers are enteric coated dosage forms which are usually produced applying acid resistant polymeric coats containing phthalate derivatives of cellulose esters especially cellulose acetate phthalate. Inorganic cellulose esters such as cellulose nitrate and cellulose sulphate are less important than organic cellulose esters in pharmaceutical industries. Cellulose nitrate or pyroxylin is a transparent compound with good film forming ability but rarely applied alone in pharmaceutical formulations due to its very low solubility in currently used pharmaceutical solvents as well as their very high flammability. The use of pure cellulose nitrate in drug formulations only limited to one topical anti-wart solution named collodion that made with 4%w/v concentration in diethvl ether/ethanol mixture as solvent. Cellulose nitrate/cellulose acetate mixture are also exploited to prepare micro-porous membrane filters used in pharmaceutical industries.