## 1 Introduction  
Biopolymers obtained from natural sources are usually biodegradable and their biocompatibility is excellent [1]. Natural biopolymers in protein and polysaccharide structures are widely used as backings,adhesives or matrix carriers in the compositions of dermal and transdermal delivery systems [2; 3; 4; 5]. Natural polysaccharides represent a group of polymers playing a fundamental role in controlling the mechanism and rate of drug release from a dosage form [2]. Alginic acid is a natural linear polysaccharide isolated from marine brown algae and from bacterial fermentation [6]. It is a block copolymer of two different monosaccharides, \(\beta\)-d-(1,4)-mannuronic acid and \(\alpha\)-l-(1,4)-glucuronic acid. Its salt, sodium alginate (Figure 1), is an anionic polymer, which plays an important role in the design of controlled drug delivery formulations. It is widely used in topical pharmaceutical products as a thickening and suspending, as well as stabilizing agent and as a wound dressing due to its excellent swelling properties and biocompatibility [6; 7]. The choice of polymers used in transdermal delivery systems has strong impact on drug release, permeability, elasticity, and wearing properties of the formulations. The bioadhesion strength of polymers with ionisable carboxyl groups, such as sodium alginate was found to be stronger than that of those with neutral or non-ionisable groups [8]. The gels prepared with sodium alginate to fabricate transdermal films have been found appropriate in terms of mechanical strength, flexibility, and bioadhesiveness of the films [5]. Transdermal delivery of drugs have numerous benefits including avoidance of hepatic first pass effect, maintenance of constant drug plasma level, and improved patient compliance to the treatment [10; 11]. However, transdermal transport of drugs is limited due to low permeability of the uppermost layer of the skin, _stratum corneum_. In order to overcome that barrier and to enhance the permeability of the drugs across the skin, permeation enhancers and co-solvents, which reversibly perturb the permeability of _stratum corneum_ or increase partition of drug to _stratum corneum_ are included in the transdermal formulation [12; 13; 14]. Terpenes are considered as effective class of chemical permeation enhancers and, they have been approved by the Food and Drug Administration (FDA) as generally regarded as safe (GRAS) [15; 16; 17]. Terpenes are derived from plant essential oils, have been shown to increase the percutaneous absorption of both hydrophilic and lipophilic drugs [12; 15; 18; 19]. It was reported that terpen enhance the permeation of drug across the skin mainly by disrupting the highly ordered intercellular chain packing of _stratum corneum_ lipids and thereby increasing drug diffusivity [20; 21]. It has been shown that the activity of terpen as permeation enhancer is primarily dependent on the lipophililicity and structure of both drugs and terpenes [22; 23; 24; 25]. In our study, four different types of terpen, nerolidol (amphiphilic sesquiterpene, logP: 5.32), eucalypt (oxygen containing monoterpene, logP: 2.82),  
Figure 1: Structural formula of sodium alginate [9]. terpinolene (hydrocarbon lipophilic monoterpene, logP: 4.52), and limonene (hydrocarbon lipophilic monoterpene, logP: 4.58) were selected. Metoclopramide (MTC) (Figure 2) is an antiemetic drug, effective in the treatment of nausea and vomiting associated with cancer therapy, migraine, pregnancy, _etc._[26]. The oral bioavailability of MTC is variable due to the hepatic first pass metabolism and oral dosage forms of MTC often get vomited out before systemic absorption occurs. Perneral and rectal administration routes both result in low patient compliance [27]. Therefore, the transdermal delivery of MTC seems to be an attractive alternative route. The aim of the present study was to develop transdermal films of MTC with sodium alginate as matrix agent and propylene glycol as plasticizer.