CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 ORAL SUSTAINED RELEASE DRUG DELIVERY SYSTEMS The oral route is the most promising and convenient route of drug administration. Conventional immediate release system achieves as well as maintains the drug concentration within the therapeutically effective range, but one has to take such formulations several times a day. This results in significant fluctuations in plasma drug levels and also the frequency of administration leads to patient non-compliance. Recently, several technical advancements in the pharmaceutical field have led to the development of many novel drug delivery systems that could revolutionize the method of medication and provide a number of therapeutic benefits. The most important objectives of these new drug delivery systems are It should, on a single administration, release the active ingredient over an extended period of time. It should deliver the active entity in the required concentration at the site of action, minimizing or eliminating the side effects. The limitations of conventional immediate release systems are overcome by these novel sustained release systems. Oral sustained release

2 2 dosage forms have been successful for the past three decades due to their various advantages which includes, Reduced dosing frequency Better patient convenience and compliance Reduced gastrointestinal side effects and any other toxic effects. Less fluctuations in plasma drug level Reduction in the total dose Improved bioavailability This also helps to provide better acceptability for drug molecules (new or existing) with potential therapeutic advantages. The development process of sustained release system includes several physiological difficulties such as inability to restrain and localize sustained drug delivery system within the desired regions of the gastrointestinal tract and variability in the nature of the gastric emptying process. The sustained release systems are also not suited for certain drugs that have a narrow absorption window [1]. Gastro retentive dosage form is a novel drug delivery system which has emerged and gathered momentum in the research to overcome the above mentioned limitations of conventional sustained release systems. 1.2 GASTRO RETENTIVE DOSAGE FORM Recent scientific literature reveals that an increased interest in novel dosage forms that are retained in stomach for a prolonged and predictable period of time exists today in academic and industrial research groups. These are the most feasible approaches for achieving a prolonged and

3 3 predictable drug delivery in the gastrointestinal tract which can control the gastric residence time and they are collectively called gastro retentive dosage forms. Delivery of drugs at a specific region in gastrointestinal tract, the so called absorption window has necessitated the development of gastro retentive dosage forms. The attempts to develop gastro retentive drug delivery systems may be largely divided into two classes, those that rely on the natural physiology of the gastrointestinal tract and those that are designed to overcome it. Approaches such as size or floatation, which rely on delayed emptying from the stomach, depend on the normal physiological duration of the fed state of 4-8 hours. The main approaches that have been examined for gastroretentive dosage forms are, low density systems that cause buoyancy (Floating drug delivery system), high density which retains the dosage form in the stomach, raft forming systems, concomitant administration of drugs or excipients which slow the motility of the gastro intestinal tract, bioadhesion to gastric mucosa, swelling to a large size which prevent passage of dosage form through the pyloric sphincter [2] Floating Drug Delivery Systems Floating Drug Delivery Systems (FDDS) have a bulk density lower than gastric fluids thus, remain buoyant in the stomach for a prolonged period of time, without affecting the gastric emptying rate. While the system is floating on the gastric contents, the drug is released slowly and almost completely at a desired rate from the system. After the release of the drug, the residual system becomes liable to be emptied from the stomach. This results in an increase in the gastro retentive time, bioavailability and a better control of fluctuations in the plasma drug concentrations. Thus floating drug delivery system is a safe and efficient technology for drug delivery.

4 Need for Floating Drug Delivery System Drugs that act locally in the stomach e.g., Antacids, proton pump inhibitors for ulcers. Drugs particularly useful for the treatment of peptic ulcers caused by Helicobactor pylori e.g., Tetracycline, Amoxicillin. Drugs that are absorbed primarily in the stomach e.g., Albuterol. Drugs that are less soluble or are degraded in the alkaline intestinal ph e.g., Bromocriptine. Drugs that have a narrow window for absorption e.g., Riboflavin, Levodopa, p-amino benzoic acid. Drugs that are absorbed mainly from the proximal part of small intestine e.g., Diltiazem, Furosemide. Drugs that degrade in the colon e.g., Captopril, Metoprolol Advantages of Floating Drug Delivery System Improvement of bioavailability: Furosemide has poor bioavailability due to its restricted absorption only from the upper gastrointestinal tract. This was improved by formulating its floating dosage form. The floating system containing Furosemide exhibit 42.9% bioavailability as compared to 33.4% shown by commercial tablet and 27.5% shown by enteric coated tablets [3]. Reduction in plasma level fluctuations: The reduced plasma level fluctuations results from delayed gastric emptying, for

5 5 example, the bioavailability of Madopar as conventional tablet was found to be 60-70% whereas a hydrodynamically balanced system is 100% bioavailable. The difference in the bioavailability of standard and hydrodynamically balanced systems was due to the incomplete absorption. The study also reported that hydrodynamically balanced systems had reduced fluctuations in plasma drug levels [4]. Reduction in the variability in transit performance: Floating dosage forms with sustained release characteristics are useful in reducing the variability in transit performance, for example formulating Tacrine as hydrodynamically balanced systems reduced its gastrointestinal side effects in Alzeihmer s patients [5, 6]. Dose reductions: The recommended adult oral dose of Ranitidine is 150 mg twice daily or 300 mg once daily. A conventional dose of 150 mg can inhibit gastric acid secretion upto 5 hrs only. If 300 mg is administered as single dose then, it leads to plasma fluctuations. On formulating Ranitidine as floating drug delivery system, the dose has been reduced and a sustained action was also observed [7, 8]. Enhancement of therapeutic efficacy: Floating systems are particularly useful for acid soluble drugs that are poorly soluble or unstable in the alkaline intestinal fluids. Bromocriptine used in the treatment of Parkinson s disease have low absorption potential that can be improved by formulating it as hydrodynamically balanced systems and thus its therapeutic efficacy was found to be enhanced [9].

6 6 Eradication of Helicobacter pylori: Helicobacter pylori is responsible for chronic gastritis and peptic ulcers. Its eradication from patients requires high concentrations of drug to be maintained within gastric mucosa which could be achieved mainly by floating drug delivery system [10, 11] Limitations The floating drug delivery systems based on effervescent technique cannot be efficient in patients with achlorhydria. Bioadhesion technique may be less effective in the acidic environment and high turnover of mucus. Retention of high density systems in the pylorus part under the migrating waves of the stomach is questionable. This is not suited for drugs that are unstable in the strong acidic environment. These systems do not offer significant advantages over the conventional dosage forms for drugs that are absorbed uniformly throughout the gastrointestinal tract Classification of Floating Drug Delivery Systems types, Floating drug delivery systems can be classified majorly under two Effervescent systems Non effervescent systems

7 Effervescent System Effervescent systems include use of gas generating agents, carbonates (e.g. Sodium bicarbonate) and other organic acid (e.g. citric acid and tartaric acid) present in the formulation to produce carbon dioxide (CO 2 ) gas, thus reducing the density of system and making it float on the gastric fluid. An alternative is the incorporation of matrix containing portion of liquid, which produce gas that evaporate at body temperature. These effervescent systems are further classified into two types. Gas generating systems Volatile liquid/ Vacuum systems Gas generating systems (i) Intra Gastric Single Layer Floating Tablets or Hydrodynamically Balanced System Hydrodynamically balanced systems (HBS) are formulated by intimately mixing the carbon dioxide generating agents and the drug within the matrix tablet. These systems swell as well as entrap the gas generated in the hydrogel, leading the system to have a bulk density lower than gastric fluids and therefore remain floating in the stomach for a prolonged period. The drug is slowly released at a desired rate from the floating system and after the complete release the residual system is expelled from the stomach. This leads to an increase in the gastric residence time and a better control over fluctuation in plasma drug concentration. (ii) Multiple Unit type floating pills These systems consist of sustained release pills as seeds surrounded by double layers. The inner layer consists of effervescent agents while the outer layer is of swellable

8 8 Figure 1.1 Intragastric floating tablet membrane layer. When the system is immersed in dissolution medium at body temperature, it sinks at once and then forms swollen pills like balloons, which float as they have lower density. This lower density is due to generation and entrapment of carbon dioxide within the system. (a) (b) Figure 1.2 (a) Multiple-unit oral floating dosage system (b) Stages of floating mechanism

9 9 Volatile liquid / Vacuum containing systems (i) Intra-gastric floating gastrointestinal drug delivery system These systems can be made to float in the stomach because of floatation chamber, which may be a vacuum or filled with air or a harmless gas, while drug reservoir is encapsulated inside a microporous compartment. Figure 1.3 Intragastric floating drug delivery device (ii) Inflatable gastrointestinal delivery systems The inflatable gastrointestinal delivery systems have an inflatable chamber, which contains liquid ether that gasifies at body temperature to cause the chamber to inflate in the stomach. These systems are fabricated by loading the inflatable chamber with a drug reservoir, which can be a drug impregnated polymeric matrix, then encapsulated in a gelatin capsule. After oral administration, the inflatable chamber automatically inflates, make the whole system float and retain the drug reservoir into the gastric fluid for a prolonged period.

10 10 Figure 1.4 Gastro-inflatable drug delivery device (iii) Intragastric osmotically controlled drug delivery system Intragastric osmotically controlled drug delivery system comprises of an osmotic pressure controlled drug delivery device and an inflatable floating support in a biodegradable capsule. In the stomach, the capsule quickly disintegrates to release the intragastric osmotically controlled drug delivery device. The inflatable support inside forms a deformable hollow polymeric bag that contains a liquid that gasifies at body temperature to inflate the bag. The osmotic pressure controlled drug delivery device consists of two components: drug reservoir compartment and an osmotically active compartment. The drug reservoir compartment is enclosed by a pressure responsive collapsible bag, which is impermeable to vapor and liquid and has a drug delivery orifice. The osmotically active compartment contains an osmotically active salt and is enclosed within a semipermeable housing. In the stomach, the water in the gastrointestinal fluid is continuously absorbed through the semipermeable membrane into osmotically active compartment to dissolve the osmotic salt, as a result an osmotic pressure is created which acts on the collapsible bag and in turn forces the bag reservoir compartment to reduce its volume and expels the drug solution through the delivery orifice.

11 11 Figure 1.5 Intragastric osmotic controlled drug delivery system Non effervescent systems The non effervescent floating drug delivery systems are based on mechanism of swelling of polymer in gastric fluid. The most commonly used excipients in non effervescent floating drug delivery systems are gel forming or highly swellable cellulose type hydrocolloids, polysaccharides and matrix forming material such as polycarbonate, polyacrylate, polymethacrylate, polystyrene as well as bioadhesive polymer such as chitosan and carbopol. (i) Single layer floating tablets Single layer floating tablets are formulated by intimate mixing of drug with the gel-forming hydrocolloid, which swells in contact with gastric fluid and maintain bulk density of less than unity. The air trapped by the swollen polymer confers buoyancy to these dosage forms.

12 12 (ii) Bilayer floating tablets A bilayer tablet contain two layers an immediate release layer which releases the initial dose from system and a sustained release layer which absorbs gastric fluid forming an impermeable colloidal gel barrier on its surface maintaining a bulk density of less than unity. Such a device remains buoyant in the stomach. (iii) Alginate beads Multi unit floating dosage forms are developed from freeze dried calcium alginate. Spherical beads of approximately 2.5 mm diameter can be prepared by dropping a sodium alginate solution into aqueous solution of calcium chloride, causing precipitation of calcium alginate leading to the formation of porous system, which can maintain a floating force for over 12 hours. As compared with solid beads, which gives a short residence time of 1 hour, these floating beads give a prolonged residence time of more than 5.5 hours. (iv) Hollow microspheres Hollow microspheres (microballons), loaded with drug in their outer polymer shells is another technique prepared by a novel emulsion solvent diffusion method. In this system the microballons formed floats continuously on the surface of acidic dissolution media containing surfactant for more than 12 hours Future Potential Based on the floating drug delivery technology, various pharmaceutical products may be launched to the market soon, which are under research at present. They reduction in the fluctuations of drug plasma

13 13 levels results from delayed gastric emptying. Drugs that have poor bioavailability because of their restricted absorption to the proximal part of small intestine can be delivered efficiently, thereby facilitating their absorption at that site. This contributes for improving their absolute bioavailability. Many studies are being carried out to explore the eradication of Halicobector pylori by using the narrow spectrum antibiotics using floating drug delivery systems. Buoyant delivery system considered as a beneficial strategy for the treatment of gastric and duodenal cancers. The floating concept can also be utilized in the development of various anti-reflux formulations. Controlled release system of drugs used to treat Parkinson s disease are under study based on the floating systems. 1.3 BILAYERED TABLETS A bilayered tablet is one made up of two separate layers, with each layer intended for a specific result, layers can be formulated to separate physically or chemically incompatible ingredients or to produce repeat action or to dissolve at different times or to deliver the product to different locations or to give different pharmacological effects [12]. The introduction of bilayered tablets into the pharmaceutical industry has enabled the development of predetermined release profile of active ingredients. Bilayered tablets are multiple compressed tablets where one layer of the multi-layered tablet or the outer one provides the initial dose. The second layer or the core tablet shall be retained in the stomach for a sustained release of drug. Various bilayer tablets have been reported with an entirely different blood level profile with or without several hours or longer delay before the second fraction is emptied. Several pharmaceutical companies are currently developing bilayered tablets, the development and production of bilayered tablet needs to be carried out on purpose-built tablet press to overcome common bilayer

14 14 problems, such as layer separation, insufficient hardness, inaccurate individual layer weight control, cross contamination between the layers and reduced yield. Advantages of bilayered tablets Separate physically or chemically incompatible ingredients. Lend themselves to certain special release profile products such as bilayered Produce repeat action or prolonged action products. Offer a wide variety of color combination that gives product identity. Disadvantages of bilayered tablets Poor wetting properties, slow dissolution properties or combination of these features may be difficult or impossible to formulate and manufacture as a tablet. Capping and lamination problems. 1.4 BASIC GASTROINTESTINAL TRACT ANATOMY AND PHYSIOLOGY The fundamental understanding of the anatomy and physiological characteristics of the human gastrointestinal tract is essential for the successful modulation of the gastrointestinal transit time of a drug delivery system to ensure maximal gastrointestinal absorption of drugs.

15 Anatomy of Gastrointestinal Tract The stomach is an organ with a capacity for storage and mixing. Anatomically the stomach is divided into 3 regions, namely, the fundus, the body and the antrum (pylorus). The fundus and the body regions are capable of displaying a large expansion to accommodate food without much increase in the intra-gastric pressure. The stomach line is devoid of villi but consists of a considerable number of gastric pits that contribute to the storage capacity of the stomach. The pylorus region is responsible for the mixing and grinding of gastric contents. Under fasting conditions the stomach is a collapsed bag with a residual volume of 50ml and contains a small amount of gastric fluid of ph 1-3 and air. The two main secretions, mucus and acid are produced by specialized cells in the stomach lining. Mucus is secreted by goblet cells and gastric acid by oxyntic (parietal) cells. The mucus spreads and covers the mucosal surface of the stomach as well as the rest of the gastrointestinal tract. The thickness of this mucus coating varies from one region of the gastrointestinal tract to another. The gastric absorption of most drugs is insignificant under physiological conditions. The limited surface area ( m 2 ) covered by a thick layer of mucus coating, the lack of villi on the mucosal surface and the short residence time of most drugs in the stomach are the physiological conditions responsible for the insignificant absorption of drugs in the stomach. The contents of stomach are emptied through the pylorus into the proximal duodenal region of the small intestine. The gastro-duodenal junction controls the unidirectional passage from the stomach to the duodenum, even though a duodeno-gastric reflex occurs in some animals. In the proximal duodenum the contents of the gall bladder (bile) and pancreas as well as some duodenal secretions including bicarbonate are emptied. The pancreatic juice

16 16 contributes lipolytic, proteolytic and carbohydrate splitting enzymes. Several enzymes in the intestinal secretion, including leucin aminopeptidase, were actually found to reside on the striated border of the intestinal absorptive cells as integral parts of the microvilli of the brush border. After the transit through the small intestine, which has a length of approximately 3m, the contents are passed through the terminal ileum into the colon via a junction known as the ileocecal valve. The small intestine is a tubular viscous organ and has enormous number of villi on its mucosal surface that create a huge surface area (4500m 2 compared to only m 2 for the stomach). These villi are minute finger like projections of the mucosa and have a length of mm, depending upon the degree of distention the intestinal wall and the stage of contraction of smooth muscle fibers in their own interiors. The villi cover the entire surface of the mucosa, with numbers varying from 10 40mm -2. The villi are most numerous in the duodenum and proximal duodenum. It should be noted that there is a progressive decrease in the villi and so the surface area from the proximal to the distal region of the small intestine. The proximal small intestine is the region with the most efficient absorption, for maximal systemic bioavailability the drug should be targeted for delivering at the vicinity of this region. The colon lacks villi and its primary function is to store indigestible food residue. It also contains a variety of flora that are normal residents of the gastrointestinal tract and may degrade the contents of the colon Dynamics The gastrointestinal tract is always in the state of continuous motility. There are two modes of motility pattern, the digestive mode and the interdigestive mode involved in the digestion of food [13]. The interdigestive gastrointestinal motility is characterized by a cyclic pattern that originates in

17 17 the foregut and propagates to the terminal ileum and consists of four distinct phases. Phase I (Basal phase) is a period of no contraction and it lasts from 40 to 60 minutes. Phase II (Preburst phase) lasts for 40 to 60 minutes with intermittent action potential and contractions. As the phase progresses the intensity and frequency also increases gradually. Phase III (Burst phase) lasts for 4 to 6 minutes. It includes intense and regular contractions for short period. It is due to this wave that all the undigested material is swept out of the stomach down to the small intestine. It is also known as the housekeeper wave. Phase IV lasts for 0 to 5 minutes and occurs between phases III and I of 2 consecutive cycles. Figure 1.6 General structure of gastrointestinal tract

18 18 Figure 1.7 Structure and different regions of stomach A complete cycle of these four phases has an average duration of minutes. Certain disease states such as bacterial overgrowth, mental stress and diurnal variations, or their combinations, can influence the duration of each individual phase as well as the total cycle [14, 15]. Phase III as a house keeping role and serves to clear all undigested materials from the stomach to the small intestine. Any sustained release gastrointestinal drug delivery system designed to stay during the fasted state should be capable of resisting the house keeping action of phase III if one intend to prolong the gastrointestinal retention time. The bioadhesive properties added to the gastrointestinal drug delivery system must be capable of adhering to the mucosal membrane strongly enough to withstand the shear forces produced in this phase Gastrointestinal Transit The transit time of a gastrointestinal drug delivery system along the gastrointestinal tract is the most limiting physiological factor in the

19 19 development of a sustained release gastroretentive drug delivery system. The patterns of gastrointestinal transit depend on whether the person is in a fasted or fed state. In addition the physical state of the drug delivery system, either a solid or a liquid, also influences the transit time through the gastrointestinal tract. The surface area, length and transit time of different regions of gastrointestinal tract are given in Table 1.1. Table 1.1 ph and transit time of gastrointestinal tract Region Surface area (m 2 ) Length (m) Transit Time Digestible Fluid Solid ph GIT Varying Stomach min. 8 h Fasted: 1-3 Fed: 3-5 Small intestine Large intestine h 4-9 h h 3 h-3 days Factors Controlling Gastric Retention Time of a Dosage Form The gastric retention time of dosage form is controlled by several factors, which affect their efficacy as a gastroretentive system, they are Density: Gastro retentive time is a function of dosage form buoyancy that in turn depends on the density difference of dosage form in the environment.

20 20 Size of dosage form: Dosage form units with a diameter of more than 9.5 mm are reported to have an increased gastro retentive time. Shape of dosage form: Tetrahedron and ring shaped device with a flexural modulus of 48 and 22.5 kilo pounds per square inch are reported to have better gastro retentive time, 90% to 100% retention at 24 hours compared with other shapes. Single or multiple unit formulation: Multiple unit formulation show a more predictable release profile as impairing of performance due to failure of a little fraction of such units give insignificant influence compared to single unit systems. They allow co-administration of units with different release profiles or containing incompatible substances and permit a larger margin of safety against dosage form failure compared with single unit dosage forms. Fed or unfed state: Under fasting conditions, the gastrointestinal motility is characterized by periods of strong motor activity or the migrating myoelectric complex (MMC) that occurs every 1.5 to 2 hours. The MMC sweeps undigested material from the stomach and if the timing of administration of the formulation coincides with that of the MMC then the formulation shall also be swept to the small intestine reducing gastric transit time considerably. Nature of meal: Feeding of indigestible or fatty acid salts can change the motility pattern of the stomach to a fed state, thus decreasing the gastric emptying rate and prolonging drug release.

21 21 Caloric content: Gastro retentive time can be increased by 4 to 10 hours with a meal that is high in proteins and fats. Frequency of feed: The gastro retentive time can increase by over 400 minutes when successive meals are given compared with a single meal due to the low frequency of MMC. Age: Elderly people, especially those over 70, have a significantly longer gastro retentive time. Posture: Gastro retentive time can vary between supine and upright ambulatory states of the patient. Concomitant drug administration: Anti-cholinergic like Atropine, opiates like Codeine and prokinetic agents like Metoclopramide increase the gastroretentive time when concomitantly administered with the dosage forms. Biological factors: In Diabetes and Crohn s disease the gastroretentive time is longer. 1.5 OPTIMIZATION Trans-Pacific Partnership economic framework agreement has clearly defined that many studies must be conducted to develop a formulation. Design of experiments (DOE) has proven to be an effective tool for formulation scientists throughout the many stages of the formulation process. At every step of formulation development, DOE can aid in making intelligent decisions. These steps include excipient compatibility studies, process feasibility studies, formulation optimization, process optimization, scale-up and manufacturing process characterization. Lastly, the product and manufacturing process must be validated before it is on the market.

22 22 The word optimize is defined as, making as perfect, effective or functional as possible [16, 17]. Optimization may be interpreted as to find out the value of controllable independent variable, that gives the most desired value of dependent variables [18, 19]. The application of formulation optimization techniques is relatively new to the practice of pharmacy when used intelligently, with the common sense, these statistical methods will broaden the perspective of the formulation process [20, 21]. At the preformulation stage, before any experiment is conducted certain problem arises, it is often not known before hand which variable will significantly influence the response. Screening designs and ANOVA helps to solve this problem. A second serious complication may arise with new excipients and new process factor, for which qualitative or quantitative effects are not known and are unpredictable. The following questions must be answered before choosing any design of experiment. Which part of the factor space should be chosen for the experiments? Are these constraints to be put on the levels of the variables? The third complication is that formulated products, in particular dosage form has to confirm to several requirements, very often competing. The formulator has to trade off objectives and choose a compromise. A fourth problem is the lack of insight` to perform an adequate optimization studies. Above all in the performance of an optimization study, the formulation development scientist can also be a factor as personal variation.

23 23 A person with prior experience and knowledge in the optimization process is a prerequisite [22] Terms used in Optimization Variables These are the measurements, values, which are characteristics of the data. There are two types of variables; dependent variables and independent variables. Independent variables(x) are set in advance, which are not influenced by any other values e.g., Lubricants concentration, drug to polymer ratio, etc. Dependent variables(y) are the outcome variables, influenced by the independent variables e.g., hardness, dissolution rate, etc. Factor Factor is an assigned variable such as concentration, temperature, lubricant agent, drug to polymer ratio, polymer to polymer ratio or polymer grade. A factor can be qualitative or quantitative. A quantitative factor has a numerical value to it for example, concentration (1%, 2% so on), drug to polymer ratio (1:1, 1:2 etc). Qualitative factors are the factors, which are not numerical value, for example, the polymer grade, humidity condition, type of equipment, etc. these are discrete in nature. Levels The levels of a factor are the values or designation assigned to the factor. For e.g. in concentration (factor) 1 % will be one level, while 2% will be another level. Two different plasticizers are levels for grade factor. Usually levels are indicated as low, middle or high level. Normally for ease of calculation the numeric and discrete levels are converted to 1 (low level) and +1 (high level).the general formula for this conversion is

24 24 X - Average of the two levels Level = (1.1) Half the difference of the levels Where X is the numeric value Response Response is mostly interpreted as the out come of an experiment. It is the effect, which we are going to evaluate i.e. Disintegration time, duration of buoyancy, etc. Effect The effect of a factor is the change in response caused by varying the levels of the factor. This describes the relationship between various factors and levels. Interaction Interaction is also similar to effect, which gives the overall effect of two or more variables (factors) on a response. For example, the combined effect of lubricant (factor) and glidant (factor) on hardness (response) of a tablet. In the trial and error method, a lot of formulations have to be prepared to get a conclusion, which involves lots of money, time and energy. These can be minimized by the use of optimization technique.

25 Optimization Process Generally optimization process involves the following steps. Based on the previous knowledge or experience or from literature, the independent variables are determined and set in the beginning. Selection of a suitable model, based on the results of the factor, screening is done. The experiments are designed and conducted. The responses are analyzed by ANOVA, test on lack of fit, to get an empirical mathematical model for each individual response. The responses are screened, by using multiple criteria to get the values of independent variables Experimental Design Experimental design is a statistical design that prescribes or advises a set of combination of variables. The number and layout of these design points within the experimental region, depends on the number of effects that must be estimated. Depending on the number of factors, their levels, possible interactions and order of the model, various experimental designs are chosen. Each experiment can be represented as a point within the experimental domain, the point being defined by its co-ordinate (the value given to the variables) in the space.

26 Factorial design It is an experimental design, which uses dimensional factor space at the corner of the design space. Factorial designs are used in experiments where the effects of different factors or conditions on experimental results are to be elucidated. These are the designs of choice for simultaneous determination of the effect of several factors and their interaction. The simplest factorial design is the two-factorial design where two factors are considered each at the two levels, leads to the four experiments, which are situated in 2-dimensional factor space at the corners of a rectangle. If there are three factors, each at two levels, eight experiments are necessary which are situated at the corners of an orthogonal cube in a 3-dimensional space. The number of experiments is given by 2 n, where n is the number of factors. If the number of factors and levels are large, then the number of experiments needed to complete a factorial design is large. To reduce the number of experiments, fractional factorial design can be used (i.e., ½ or ¼ of the original number of experiments with full factorial design) The fitting of an empirical polynomial equation to the experimental result facilitates the optimization procedure. The general polynomial equation is as follows: Y = b 0 +b 1 X 1 +b 2 X 2 +b 3 X 3 +b 12 X 1 X 2 +b 23 X 2 X 3 +b 13 X 1 X 3 +b 123 X 1 X 2 X 3 (1.2) Where, Y is the response, X 1, X 2, X 3 are the levels (concentration) of the 1,2,3 factors, b 1, b 2, b 3, b 12, b 13, b 23, b 123 are the polynomial coefficients.

27 27 all factors is low). b 0 is the intercept (which represents the response when the level of Plackett-Burman design Plackett-Burman design is a special fractional factorial design with K = M 4 experiments, for screening of (K-1) variables, Where K is the number of variables and M is the number of levels Star design Star design is simply a 2 2 factorial design rotated over 45 0 angle in the space. A center point is usually added, which may be replicated to estimate the experimental error, so there will be three levels for each factor where quadratic effect can be measured, but the interaction effect cannot be measured as that in case of factorial design. In the star designs, 2k Factorial designs are rotated over 45 0 in (k-1) direction in k-dimensional space with a replicated center point. k is the number of factors in the design. This results in 2k + R c experiments, where R c is the replicates of the center point Central composite design A better design that combines the advantages of Factorial design or fractional factorial design and the star design, is the central composite design (CCD) developed by Box and Wilson. It is composed of 2 k Factorial design or 2 (k-p) Fractional factorial design. 2 * k star design This design enables the estimation of a full second order model. The equation for two factors is given by E (y) = B 0 + B 1 + B 2 X 2 + B 12 X 1 X 2 + B 11 X B 22 X 2 2 (1.3)

28 Validation of the Model The model is validated using ANOVA calculation, and then the estimation pure measurement error is done. The variance of these observations pooled over all to get an estimate of pure error of variance. The F- test on regression and lack of fit will be useful for judging descriptive properties of a model and the significance of model terms Predications using the Selected Model Once a model is selected and validated, the brute force method is applied for the predication of response. With the help of 3D-response surface or a 2D contour diagram, the predication is done using these graphs either by grid search or feasibility search methods Software for Designs and Optimization Many commercial software packages are available which are either dedicated to experimental design alone or are of a more general statistical type. Software s dedicated to experimental designs DESIGN EXPERT ECHIP MULTI-SIMPLEX NEMRODW Software for general statistical nature SAS MINITAB SYSTAT GRAPHPAD PRISM