S ̂ootf

University o f Szeged, Faculty o f Pharmacy Department o f Pharmaceutical Technology

Director: Prof. Dr. habil. István Erős Ph.D., D. Sc.

Ph.D. Thesis

Quality Development of Semisolid Dermal DrugDelivery Systems

Dr. Erzsébet Csányi

Supervisor:Prof. Dr. habil. István Erős Ph.D., D. Sc.

Szeged2004

Publications related to the subject of thesis

Papers:I. Csányi, E., Erős, I.: Bildung und rheologische Eigenschaften der Avicel-Gele Teil. 1.

Pharmazie 38, 244-246 (1983) I.F.: 0.740

II. Csányi, E., Erős, I.: Bildung und rheologische Eigenschaften der Avicel-Gele Teil. 2.

Pharmazie 38, 328-330 (1983) I.F.: 0.740

III. Erős, I., Mórocz, M., Csányi, E., Selmeczi, B.: Wirkung der Emulsionsstruktur von

w/o-Cremes auf Konsistenz und Arzneimittelfreisetzung Pharm. Ind. 51, 1446-1449

(1989) IF.: 0.279

IV. Erős, I., Csóka, I., Csányi, E., Takács-Wormsdorff, T.: Examination of Drug Release

from Hydrogels Polym. Adv. Technol. 14, 1-7 (2003) I.F.: 1.019

Abstracts:1. Csányi, E., Erős, I., Csóka, I., Kövér, T., Cserne, A.: Relationship between rheological

and biopharmaceutical characteristics of disperse and coherent dosage forms. 3rd

Central European Symposium on Pharmaceutical Technology, Portoroz, Slovenia,

Pharm. Vestn. 50, 285-286 (1999)

2. Erős, I., Csányi, E., Sipos E., Blum, A.: Polimer térhálók reológiai vizsgálata.

Congressus Pharm. Hung. XI., Siófok, Hungary, Gyógyszerészet Különkiadás 39

(1999)

3. Csányi, E., Erős, I., Kövér, T., Csóka, I., Cserne, A.: Drug release from emulsions and

its determining factors. Symposium on Lipid and Surfactant Dispersed Systems,

Moscow, Russia, Abstracts 211-212I (1999)

4. Csányi, E., Erős, I., Sipos, E., Mile, T., Csóka, I.: Drug liberation from hydrogels. 60th

International Congress of FIP, Vienna, Austria, Abstracts 112 (2000)

5. Csányi, E., Erős, I., Makai, M., Csóka, I.: Emulsions as Controlled Drug Delivery

Systems. 6th European Congress of Pharmaceutical Sciences, EUFEPS 2000,

Budapest, Hungary, Eur. J. Pharm. Sci. 11 Suppl. 1, S40 (2000)I

6. Csányi, E., Erős, I., Makai, M., Csóka, I.: Study of structure and viscoelasticity of

polymer gels. 5th International Congress on Cosmetics and Household Chemicals,

Budapest, Hungary, Abstracts 64 (2002)

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7. Csányi, E., Erős, I., Csóka, I., Sipos, P., Fehér, A.: Drug liberation from emulsion drug

delivery systems, 8th Conference on Colloid Chemistry, Keszthely, Hungary,

Abstracts 75 (2002)

8. Csányi, E., Fehér, A., Erős, I.: Tenzidekből felépülő kolloid rendszerek a gyógy­

szertechnológiában. XIV. Országos Gyógyszertechnológiai Konferencia, Hévíz,

Hungary, Abstracts 39 (2002)

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Table of contant1. Introduction............................................................................................................................ 6

2. Literature survey.................................................................................................................... 6

2.1. Semisolid dermatological preparations........................................................................... 6

2.2. Creams............................................................................................................................ 7

2.2.1. Structure of creams.................................................................................................. 7

2.2.2. Stability of creams................................................................................................... 8

2.3. Hydrogels........................................................................................................................ 9

2.3.1. Theory of hydrogels................................................................................................. 9

2.3.2. New theory of hydrogels.........................................................................................10

2.3.3. Application of hydrogels.........................................................................................10

2.3.4. Hydrogels as drug delivery systems........................................................................10

2.4. Rheology and semisolid preparations............................................................................11

2.4.1. Rheological characterisation of semisolid preparations..........................................11

2.4.2. Rheological Measurements.....................................................................................12

2.5. Methods to investigate the drug release from topical preparations...............................13

3. Aims......................................................................................................................................15

4. Experimental work................................................................................................................15

4.1. Materials.........................................................................................................................15

4.1.1. Components of creams............................................................................................15

4.1.2. Components of hydrogels.......................................................................................16

4.1.3. Model drugs............................................................................................................16

4.2. Methods..........................................................................................................................16

4.2.1. Determination of contact angle...............................................................................16

4.2.2. Determination of droplet size..................................................................................17

4.2.3. Determination of cooling curves.............................................................................17

4.2.4. Rheological measurements.....................................................................................17

4.2.5. Investigation of spreadability..................................................................................18

4.2.6. Drug release measurements....................................................................................18

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5. Results and discussion 19

5.1. Investigation of w/o creams containing mixed emulsifier............................................19

5.1.1. Formation of cream structure................................................................................19

5.1.2. Investigation of structure stability.........................................................................24

5.1.3. Investigation of drug release from creams............................................................25

5.2. Investigation of w/o creams containing self-emulsifier base.......................................26

5.2.1. Formation of cream structure................................................................................26

5.2.2. Stability of cream structure...................................................................................30

5.2.3. Drug release from creams.......................................................................................31

5.3. Investigation of hydrogels.............................................................................................33

5.3.1. Rheological characteristics of hydrogels.............................................................. 33

5.3.2. Interaction between hydrogels and drugs..............................................................39

5.3.3. Relationship between drug release and structure of hydrogels...............................40

6. Summary...............................................................................................................................44

7. References............................................................................................................................47

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1. IntroductionSemisolid dosage forms are coherent systems with a particular structure. Creams and

hydrogels are preparations used widely in medicinal therapy and cosmetology. Today the

factual knowledge published in literature concerning the properties of these material systems

is almost confusingly immense and full of contradictions.

However, it seems that this extensive knowledge and experience is insufficient to

optimise the formulation and stability of creams and hydrogels. Another important task in this

field is to set the release kinetics of the incorporated drug. Some data and examination

methods necessary for the selection of the components and for the determination of the

optimum concentration are lacking. The components of the stability of these systems are not

known and the extensive exploration of the factors determining drug release is still to be

carried out. It is not accidental that no pharmacopoeia includes an official system of norms for

qualification - based on the structure and deformability - of semisolid dosage forms.

Manufacturers evaluate their products with their own norms, developed by themselves, thus

the qualification system is characteristic not only of the product but of the manufacturer as

well. The assessment of quality independently of the manufacturer demands uniform quality

improvement and control in investigating the structure of semisolid preparations, too. This

would ensure, by optimising formulation, the selection of components and the formulation of

a stable, attractive semisolid preparation with good applicability and the desired effect.

As regards the structure and drug release studies of creams and hydrogels in

pharmaceutical technology, it can be stated that this field is undergoing a dynamic

development. However, several detailed examinations are still to be performed before we can

plan the composition of these dosage forms, set the parameters of production technology and

determine the optimal values of their functional properties on due theoretical grounds.

2. Literature survey

2.1. Semisolid dermatological preparationsDermatological preparations are classified into liquid systems including solutions,

emulsions and suspensions, solid systems (powders) and semisolid formulations.

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Semisolid preparations are dosage forms, characterized by being spreadable in a

temperature range between room temperature and skin temperature. They are applied on the

skin as well as on mucous membranes (1). Semisolid preparations are considered colloidal

systems possessing a gel structure. Munzel definied semisolids as plastic gels for cutaneous

application (2). The plastic behaviour of these systems is characterised by the existence of a

yield stress after which the system begins to flow. The semisolid preparations can be

classified into four groups.

1. Ointments are water-free semisolid preparations. In general the acceptability of

this formulation to the patient is bad, because of the skin and the in vivo efficacy

is very low (3, 4).

2. Creams are ointments in which an aqueous phase is incorporated. In the classic

o/w creams, the water is immobilized in the cream by either of two ways. On the

one hand, water could be bound mechanically through capillary forces and

supportive gel structure. On the other hand, water could be fixed inside the

hydrophilic gel phase (5, 6, 7). In contrast, in classic w/o creams, the aqueous

phase is incorporated in the form of droplets inside the base similar to liquid

emulsions (8). Ambiphilic ointments are considered a transition between o/w

and w/o creams (9).

3. Gels are one-phase semisolid preparations consisting mainly of a liquid fixed by

a gel building substance. Gels are also differentiated into hydrophilic and

lipophilic gels according to the type of liquid used (10, 11, 12, 13, 14, 15, 16).

4. Pastes are also considered as semisolids, defined as ointments with high (over

40 %) powder content (17).

The selection of the appropriate formulation depends on the state of disease and the

desired effect (18). The selection of the formulation also depends on the skin type too (19).

2.2. Creams

2.2.1. Structure of creamsAlthough dermatological creams have been used for decades as drug carriers, until

recently their development was essentially empirical with only a limited understanding of the

underlying principles. With the recognition of the importance of the topical route in the

delivery of drugs for both local and systemic effects, interest has been renewed in

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investigating the microstructure of these complex formulations. Although it is now well

established that the structure of cream can markedly influence drug bioavailability, the

mechanisms are far from clear.

Creams are coherent systems with a particular structure. The research of their structure

and stability has constituted a major field in pharmaceutical technology and cosmetics since

the 1960s. The researchers’ attention was focussed mainly on o/w creams. Various structure

theories were important milestones in o/w emulsion research. The complex emulsifier theory

is one, which should be mentioned here. It was Munzel and his research team who first raised

the fundamental question what causes the shape-retaining character of washable (o/w type)

creams (20). It was found that the complex emulsifier constituted by fatty alcohols or fatty

acids as well as by tensides with a high HLB value is responsible for the formation of the

coherent structure.

Barry combined optical and rheological methods to prove that in the case of three- or

four-component creams a structure similar to liquid crystal arrangement arises (21). This

research conception was adopted by Eccleston, Junginger and several other research teams,

who used electronoptical, DSC and X-ray diffraction methods to elucidate the coherent

structure of o/w creams and the various mechanisms of water binding (22, 23, 24, 25, 26).

The problems of cream structure were approached from the practical aspect by Boylan et

al (27). They conducted research in order to determine under what conditions of production

technology attractive appearance and stable emulsion structure can be achieved (melting,

temperature of mixing, cooling rate).

Compared to the intensive development of o/w creams, the research of w/o creams has

been pushed into the background in spite of the fact that this product group is also a major

representative of dermal medicinal therapy and cosmetology. There are relatively few

monographs, which deal with w/o emulsion systems (8, 28, 29, 30).

Only in recent years has the researchers’ attention been directed to w/o creams, which

have an aesthetically favourable, high water content and also excellent in vitro and in vivo

availability, not to mention pleasant use for the individual (31, 32, 33, 34, 35, 36, 37).

2.2.2. Stability of creamsEmulsion instability is a complex process, which involves several different mechanisms

contributing to the transformation of a uniformly dispersed emulsion into a totally phase-

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separated system. This process may, however, take weeks to develop and the detection of

early phenomena is necessary to remedy the situation in time. One of the best ways, to

evaluate emulsion stability is to determine the droplet size distribution frequently during the

first few weeks of storage. Changes in the size distribution during this time may indicate

future instability problems. The viscosity of the cream and the droplet size are key parameters

in determining the instability process (40). There are few literature data available concerning

the examination of the stability of w/o creams for product development on scientific bases.

2.3. Hydrogels

2.3.1. Theory of hydrogelsHydrogels are one of the upcoming classes of polymer-based controlled drug release

systems (41). Graham was the first, who introduced the term, hydrogel for hydrates of silicic

acid with gelatinous properties (42, 43). Since then it has been difficult for chemists,

physicists and the other researchers to reach a consensus as to what constitutes a gel. This was

already recognized in 1926 by Lloyd (44), who stated the follows.

The colloidal condition, the ’gel’, is one which it is easier to recognize than to define.

The phenomenological definition proposed in 1993 by Almdal et alias (45) states, that a

gel is a soft, solid-like material, which consists of at least two components, one of which is a

liquid present in abundance. Technically, gels are semisolid systems comprising small

amounts of solid, dispersed in relatively large amounts of liquid, yet possessing more solid­

like than liquid-like character (46). Generally, hydrogels are described as aqueous gels.

The up-to-date definition comes from Peppas (47). According to his states, hydrogels are

three-dimensional cross-linked hydrophilic, polymeric networks capable of imbibing large

amounts of water or biological fluids. The networks are composed of different polymers.

Characterization of the hydrogel network structure is a complex procedure because of the

many types of possible networks including, regular, irregular, loosely cross-linked, highly

cross-linked, and imperfect networks. Because of these variations in the network, only

average values for the cross-linking density and the molecular weight between cross-links can

be obtained experimentally or theoretically (48).

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2.3.2. New theory of hydrogelsOn the research field there is arising a new definition of hydrogels. Gupta and his

coworkers introduced this new definition (41). According to them, although the term

’hydrogel’ implies a material already swollen in water, in a true sense hydrogels are a cross-

linked network of hydrophilic polymer. They possess the ability to absorb large amounts of

water and swell, while maintaining their 3D structure (49). As Gupta writes, this definition

differentiates hydrogels from gels, which are polymeric networks already swollen to

equilibrium and the further addition of fluids results only in dilution of the polymeric

network. A hydrogel exhibits swelling in aqueous media for the same reasons. Thus, the

feature central to the functioning of a hydrogel is its inherent cross-linking. Hydrogel,

sometimes referred to as xerogel, is a more rigid form of gel.

2.3.3. Application of hydrogels

2.3.3.1. Hydrogels as natural living tissueThere are numerous applications of hydrogels in the medical sectors (50, 51, 52, 53, 54,

55). Hydrogels resemble natural living tissue more than any other class of synthetic

biomaterials. This is due to their high water content and soft consistency, which is similar to

natural tissue (56). The high water content of the materials contributes to their

biocompatibility. Thus, hydrogels can be used as contact lenses, membranes for biosensors,

linings for artificial hearts, and materials for artificial skin and drug delivery devices (47).

2.3.3.2. Environmentally responsive polymersHydrogels that change their swelling behaviour, network structure, permeability or

mechanical strength in response to different stimuli, such as, e.g. a change in the pH (57),

temperature (58), light or electric field (59, 60) are known as ’enviromentally responsive

polymers’, or ’smart hydrogels’ or ’intelligent hydrogels’. They have recently attracted

considerable interest in the field of drug delivery (61, 41) as a means of providing an on-off

release (62) by swelling and shrinking in response to the presence and absence, for example,

glucose (63) or antigens (64, 65). Various stimuli can serve for modulating drug delivery.

2.3.4. Hydrogels as drug delivery systemsHydrogels are used for both local treatment and systemic effects. Many different

administration routes have been explored, including, for example, cutaneous (66, 67, 68, 69)

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and subscutaneous (70, 71) delivery, buccal delivery (72), delivery to the periodontal pocket

(73, 74), esophagus (75), stomach (76), colon (77), rectum (78) and vagina (79).

In case of drug delivery systems, the drug release profile is one of the most important

factors. To take full advantage of the residence time, the drug should be released in adequate

amounts throughout the entire period. Most hydrogels used in pharmaceutical applications

consist of 0.2-10 % of polymer and the rest of water. The viscosity can be substantial owing

to the presence of the polymer, but the transport conditions for a small drug molecule can be

expected to be approximately the same as they are in water (80).

The polymer network is of little hindrance and the drug is likely to diffuse out of the gel

rather rapidly. There are several ways to achieve sustained release, e.g. by suspending the

drug in the gel (74, 79), by formulating the drug as micro (81) or nanospheres (82), by

distributing the drug to liposomes (83, 84 85) or surfactant aggregates (86) or by utilizing

interactions between the drug and the polymer (87).

2.4. Rheology and semisolid preparations

2.4.1. Rheological characterisation of semisolid preparationsRheology is the branch of science that deals with the flow and deformation of materials

under different conditions. If rheology is considered not only as an examination method but

also as the property of material systems, structure in the stricter sense is a decisive factor of

rheological properties. The relationship of structure and rheological properties is realised in

consistency.

Many processes, in the developing of semisolid pharmaceutical and cosmetic products,

such as new ingredient selections, formulation of preparations, material packaging, and shelf

storage are associated with a complex flow of materials. The application and acceptance of

these products are also dependent on the flow properties of the final product. Therefore,

rheological measurements, an important route to revealing the flow and deformation

behaviours of materials can not only improve efficiency in processing but can also help

formulators and end-users find pharmaceutical products to be optimal for their individual

needs.

In general, rheological measurements are performed for the following reasons in

semisolids (88):

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• to understand the fundamental nature of the system,

• for quality control of raw materials, final products, and manufacturing processes

such as mixing, pumping, packaging, and filling,

• to study the effect of different parameters such as formulation, storage time, and

the temperature on the quality and acceptance of a final product.

Semisolid products are the most difficult materials to characterize rheologically because

they combine both liquid and solid properties within the same material.

2.4.2. Rheological measurementsFlow curves, viscosity curves across a wide range of shear rate can provide important

information about the state picture of the system, storage stability, optimal conditions for

mixing, transferring, and end-user applications (89, 90). It also provides information

regarding the ways in which the structure changes to comply with the applied shear

indifferent conditions, such as storage, processing, and application (91). If the shear rate

changes during process, the internal structure of the sample will change and the change in

stress or viscosity can then be seen (92, 93, 94).

The yield value measurement is crucial for pharmaceutical products in determining not

only their shelf life but also ease of application for the end-user.

There are two ways to investigate the rheological parameters: rotational and oscillation

measurements. Rotational experiments provide information concerning the flow properties of

the system; dynamic oscillation testing is a tool to get information about the viscoelastic

behaviour of the semisolids. Experimentally, this can be accomplished by commanding a

small sinusoidal displacement or strain (within the linear viscoelastic range) on the semisolid

sample under a controlled frequency sweep. In general, the material can respond to this type

of deformation through two mechanisms, elastic energy storage and viscous energy

dissipation. Quantitatively, these responses can be represented as storage modulus (G’),

energy stored per unit volume, and loss modulus (G”), energy dissipated per unit deformation

rate per unit volume. Storage modulus is proportional to the extent of the elastic component

(contributed by cross-linking, entanglement, and/or aggregation) of the system, and loss

modulus (G”) is proportional to extent of the viscous component (contributed by the liquid­

like portion) of the system. This method is especially good for the evaluation of

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mucoadhesive preparations (95, 96, 97), dermal preparations with the good spreadability (98,

99).

Creep-recovery test is an alternative test for obtaining the viscoelastic properties of the

materials. A constant stress below yield stress is applied to the sample and the deformation is

monitored with time.

Viscoelastic materials like creams and hydrogels will exhibit an unlinear response to

strain and, due to their ability to partially recover structure by storing energy, will show -

after removing of the stress - a final deformation less than the initial deformation (88).

2.5. Methods to investigate the drug release from topical preparations

In vitro release tests can serve primarily as a quality control tool to ensure batch-to-batch

uniformity and screen experimental formulations during product development. In the field of

solid dosage forms, there are standardized and well-known test methods, but for semisolid

preparations, neither universal release testing procedure nor universal test conditions exist.

Release test methods are tailored to a formulation, so suitable test conditions are developed.

The drug release methods are summarized in Figure 1. (100).

Figure 1. Drug release models

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Generally, in the release test, a layer of the semisolid dosage form is placed in contact

with a reservoir chamber and diffusion of drug out from the semi solid sample into the

reservoir medium is followed. In most cases, the semisolid sample and the acceptor phase are

separated with a special membrane to keep the product and the acceptor medium

physicochemically distinct (101).

The diffusion coefficient is taken as a parameter to describe drug release. It can be

calculated from the Higuchi equations (102), but should be taken distinguish between

‘solution type’, in which the drug is completely dissolved in the semisolid preparation and

‘suspension type’, in which the drug is rather in suspended form.

Drug release from solution type of sample

Q = 2 • c (1)V n

Drug release from suspension type of sample, if cs << c0

Q = V2 ' D ' c o ' c s ■ t (2)

Where

Q = cumulative amount of the released drug,

co = initial concentration of the drug in the vehicle,

cs = solubility of the drug in the vehicle,

D = diffusion coefficient,

t = time of application.

The drug release from semisolid preparations is affected by a great number of other

factors to, e.g. the structure and the viscosity of samples, interactions between the drug and

vehicle (103, 104).

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3. AimsThe aim of my thesis was to investigate the effect exerted by important factors of dosage

form development on structure in the case of semisolid drug preparations and more precisely

w/o and ambiphilic creams and hydrogels, and also to study the interaction between drug

release and microstructure. The focus of my research was given by the possibility that the

formation of coherent systems and the changes of structure can be monitored successfully and

characterised quantitatively with rheological measurements.

The following issues were to be studied:

• selection of the rheological characteristics of the greatest importance with respect

to product development,

• exploration of factors influencing structure formation, studying the relationship

between these factors and the rheological parameters, and if possible, to describe

it with functions,

• description of the mechanical stability of the formed structure with rheological

characteristics,

• studying the kinetics of drug release,

• elucidation of the relationships between drug release and consistency.

4. Experimental work

4.1. Materials

4.1.1. Components of creams

Surfactants• Imwitor 780 K: partial glyceride of isostearinic acid, HLB=3.7.

• Cholesterol: cholest-5-en-3£-ol (Ph.Eur. 4th).

• Span 80: sorbitan oleate (Ph.Eur. 4th).

• Protegin: mixture of mineral oil, petrolatum, ozokerite and lanolin alcohol

(Degussa).

• Protegin X: mixture of mineral oil, petrolatum, ozokerite, glyceryl oleate and

lanolin alcohol (Degussa).

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• Protegin W: mixture of petrolatum, ozokerite, hydrogeneted castor oil, glyceryl

isostearate, polyglyceryl-3 oleate (Degussa).

• Protegin WX: mixture of petrolatum, ozokerite, hydrogeneted castor oil, glyceryl

isostearate, polyglyceryl-3 oleate. The difference between Protegin W and

Protegin WX is ratio of components (Degussa).

• Emulgator BTO: mixture of different non-ionic emulsifier, HLB= 8 (Degussa).

Lipophil phases• Miglyol Gel: gelation product of Miglyol 812 and bentonite (Degussa).

• Miglyol 812: mixture of the triglycerides of saturated fatty acids with a medium,

average carbon atom number of 8-12 (Hüls).

• Liquid paraffin: mixture of liquid hydrocarbons obtained from petroleum

(Ph.Hg.VII).

4.1.2. Components of hydrogels• Methylcellulose: (Ph.Hg.VII), hydroxyethylcellulose (Ph.Hg.VII), carboximethyl­

cellulose sodium (Ph.Hg.VII).

• Xanthan: anionic polysaccharide, produced by fermentation (Jungbuzlauer AG).

• Keltrol: Xanthan derivative with acetate and pyruvate substituents (CP Kelco).

• Carbopol 980 NF: polyacrylic acid polymer (BF Goodrich).

• Avicel RC 591: mixture of microcrystalline cellulose and carboximethylcellulose

sodium (FMC Corporation).

• Hostacerin PN 73: methacrylate-methacrylamide copolimer (Hüls).

4.1.3. Model drugs• Salicylic acid (Ph.Hg.VII.), Sulfadimidine (Ph.Hg.VII), Griseofulvin (Ph.Eur.4)

and Ephedrine hydrochloride (Ph.Hg.VII) were used.

4.2. Methods

4.2.1. Determination of contact angleA thin film was formed from the cream to be examined on a glass plate, the diameter of

the distilled water dropped on the film from a microburette was measured and the contact

angle of wetting was calculated on the basis of Weber’s and Wolfram’s relation (105).

16

V _ (l - cos ©)2 • (2 + cos ©) r3 3 • sin3 0 (3)

Where

V = volume of droplet,

r = radius of droplet,

0 = contact angle of wetting.

Wetting ability was calculated from 10 parallel measurements each time. The standard

deviation of the given data was in the range 2.1 - 3.8.

4.2.2. Determination of droplet sizeThe droplet size was determined microscopically, used heatable microscopic stage.

4.2.3. Determination of cooling curves1000 g of cream was mixed at a constant rate in a Stella Lux ointment mixing apparatus,

at an outside temperature of 22 ± 1.0 °C. The temperature of the cream was read every 5

minutes. The given data are averages calculated from three parallel measurements each time,

the standard deviation of the measurements was in the range 8.0 - 10.3.

4.2.4. Rheological measurementsTo investigate the rheological characteristics of samples containing drugs and without

drugs RheoStress 1 Rheometer, equipped with a cone-plate sensor (diameter of 35 mm) and a

cone angle of 1° was used. The thickness of the sample in the middle of the sensor was 0.048

mm. Samples were kept under saturated water vapor during measuring process. In oscillation

measurements, the linear viscoelastic region was at first determined by measuring the

complex modulus versus stress at a low (0.036 Hz) frequency, and then 2.5 Pa was chosen as

stress amplitude, which was found to be in the linear viscoelastic region in all cases.

The values of the storage (G’) and loss (G”) moduli were determined, and the elastic and

viscous nature of the samples were examined with a Creep recovery test. The flow and

viscosity curves were drawn in the whole range. The dependence of viscosity on shear time

and temperature was examined. The measurements were carried out both at 25 °C and at skin

temperature, at 32 °C. The relationship between drug release and viscosity was invariably

analysed using the values obtained at 32 °C.

17

The given data are averages calculated from three parallel measurements each time; the

standard deviation of the measurements was in the range 2.1 - 4.8.

4.2.5. Investigation of spreadabilitySpreadability was determined with an extensometric method, the essence of which is that

spreading resulting upon the effect of a vertical force of known size can be measured. In the

examinations a force of 10, 20, 50, 100, 200, 500 g was exerted on 1.0 g of cream sample

distributed evenly in a circle with a radius of 10 mm. The area of the resulting ointment spot

was determined.

The given data are averages calculated from five parallel measurements each time; the

standard deviation was in the range 0.98 - 1.2.

4.2.6. Drug release measurementsSeveral methods were used for the examination of drug release. In addition to the agar

plate method, membrane dialysis leading to equilibrium and flow-through membrane dialysis

were also performed. In the case of flow-through membrane dialysis, an autosampling system

containing vertically diffusion cells (Hanson Microette Autosampling System) was used. The

drug release profile was determined at 32 ± 0.5 °C using phosphate buffer of pH=5.4, special

cellulose-acetate membrane was used with a pore size of 0.45 pm. The measurements of the

drug released were carried out with a Heliosa Unicam UV-Vis Spectrometer.

In the case of the agar plate method the values given are the averages calculated from ten

parallel measurements each time; standard deviation was in the range 0.12 - 0.54. The results

of the examinations performed with the Hanson Microette Autosampling System are the

averages of six parallel measurements; standard deviation was in the range 2.1 - 3.2.

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5. Results and discussion

5.1. Investigation of w/o creams containing mixed emulsifier

5.1.1. Formation of cream structureW/o creams constitute an essential group of dermatological preparations and biocosmetic

skin treatment. In spite of their practical importance, literature contains rather few data

concerning the quality improvement of w/o creams, and this is especially true for w/o creams

with high water content. W/o creams with a high water content have an occlusive effect and

exert good hydration due to their high water content and, last but not least, they look very

attractive, which is also a major user aspect. However, the group of additives necessary to

achieve this high water content and the stability of the arising structure is questionable.

During my experimental work, stable compositions with an extremely high, 60-70 %

water content - unusual in w/o emulsions - could be formulated with the use of emulsifier

pairs and self-emulsifying bases, and these compositions were suitable for studying the

problem of structure formation and stability. The examined cream compositions are

summarised in Table 1. and Table 2.

Table 1. W/o creams containing mixed emulsifiers

Type Component Quantity [%!1st series 2nd series

Lipophil phase Miglyol Gel 25 25 25 25 25 15 15 15 15 15Liquid paraffin 35 25 15 10 5 47 37 27 17 7

EmulsifierImwitor 780 K 5 5 5 5 5 3 3 3 3 3Cholesterol 5 5 5 5 5 - - - - -Span 80 - - - - - 5 5 5 5 5

Hydrophil phase Distilled water 30 40 50 55 60 30 40 50 60 70

Table 2. W/o creams containing self-emulsifying base

Base Base 50 % - Miglyol 812 50 % Base 50 % - Liquid paraffin 50 %Water contant [%] Water contant [%]

Protegin 30 40 50 60 30 40 50 60Protegin X 30 40 50 60 30 40 50 60Protegin W 30 40 50 60 30 40 50 60Protegin WX 30 40 50 60 30 40 50 60

19

Three surfactants were chosen for the preparation of the creams included in Table 1.

These components, their quantity and proportion were determined with wetting examinations

to be detailed later. Miglyol Gel, due to its paste consistency, was diluted with paraffin oil

before the emulsification of the aqueous phase.

Contact wetting and the contact angle of wetting calculated from this is supposed to play

a major role in the first phase of the formation of the w/o structure as in these systems the

emulsifier, or a great part of it, is found on the boundary surface of the oil and water phases.

The calculation of the contact angle of wetting and the associated properties were studied in

great detail. In Figure 2. the emulsifier concentration is plotted against the contact angle of

wetting.

Figure 2. Influence of emulsifier concentration on the wetting

Wetting was observed to show a considerable improvement in the concentration range of

0.5-3 %. The further increase in the emulsifier concentration resulted in a much smaller

change. The effect of the emulsifier pairs on wetting is summarised in Table 3. These values

served as the basis for determining the concentration or concentration proportion suitable for

producing emulsion creams.

20

Table 3. Effect of emulsifier pair on the contact angle of wetting

Emulsifier pair concentration[w/w %]

Contact angle [°]

Miglyol Gel without emulsifier 77.1

Cetostearyl alcohol

2.5

Imwitor 780 K

2.5 59.53.0 5.0 57.35.0 5.0 52.0

10.0 10.0 51.8

Span 80

2.5

Imwitor 780 K

2.5 45.53.0 5.0 42.75.0 3.0 30.0

10.0 10.0 34.32.5 2.5 50.0

Cholesterol 5.0 Imwitor 780 K 5.0 48.010.0 10.0 50.4

Our aim was to achieve as high a water content in the creams as possible, on the one

hand in order to perform an extensive study of structure formation, and on the other hand

because a higher water content will result in more attractive appearance and better applica­

bility for the user. This explains why the detailed investigation of the quantity of incorporated

water and the factors influencing this was carried out.

The relationship between wetting and the quantity of water, which can be emulsified,

was examined. This relationship is presented in Figure 3.

Figure 3. Relationship between contact angle and water absorbing capacity of creams

21

The quantity of incorporated water increased with the improvement of wetting that is

with the decrease of the contact angle of wetting. A linear regression was found to exist

between these two parameters.

Wa = W

Where

Wa = water absorbing capacity,

W0 = value extrapolated to 0,

m = slope,

0 = contact angle of wetting.

m -0 (4)

The rheological examination of the creams with the composition given in Table 1.

revealed that, irrespective of the emulsifier and the water content, they are structure viscous,

thixotropic systems with a yield value, as shown by the flow curves of the 2nd series

presented in Figure 4.

The analysis of the viscosity curves showed that the relationship between structural

viscosity and the shear rate gradient can be described with a power function. Figure 5.

presents the structural viscosity - shear rate gradient relationship of the cholesterol - Imwitor

780 K compositions with various water content.

Figure 4. Flow curves of w/o creams, 2nd series Figure 5. Viscosity curves of w/o creams, 2nd series

22

The concrete form of the n = f (D) function can be given for the curves; this relationship

is valid for the creams of the 1st series as well:

Where

n

nD

m

lgn = lgno - m • lgD

structural viscosity

viscosity extrapolated to 0,

shear rate,

slope.

(5)

Thixotropy and structural viscosity characteristically increased with water content for

both series. The greater the quantity of water emulsified in the inner phase, the higher the

parameters characterising consistency will be. This finding is also confirmed by the data in

Table 4. The values of some major rheological parameters are summarised in this table.

Spreadability was determined with extensometric measurements. The values of spreading

measured under a force of 200 g are indicated in the table.

Table 4. Rheological parameters of w/o creams

Watercontant

[%1

Yield value

[Pal

Initialviscosity

[Pa-sl

Equilibriumviscosity

[Pa-sl

Spreadability w=200 g

[cm2]30 11.1 293 1.59 26.240 27.7 423 2.24 22.1

1st series 50 74.8 847 3.11 15.655 135.7 1336 3.72 13.460 311.0 2330 6.98 10.930 - 16 0.61 54.440 8.3 114 1.06 46.6

2nd series 50 16.6 325 2.24 31.260 58.2 570 2.96 21.470 105.0 1010 7.37 10.2

The basic difference in the compositions of the 1st and 2nd series is that in addition to

Imwitor 780 K, the 1st series contained a solid emulsifier, cholesterol, while the 2nd series

23

had a liquid emulsifier, Span 80. In the case of systems with a lower water content the

significant structure-solidifying effect of the solid emulsifier can be observed clearly, this is

shown by the greater values of yield value and viscosity. This difference decreases in the case

of greater water content in consequence of the viscosity-increasing effect of the emulsified

water droplets.

5.1.2. Investigation of structure stabilityThe stability of the structure of water-containing creams is of great significance. The

stability of the structure was investigated by performing centrifugation experiments, by

measuring the evaporation of water and also by examining the resistance of consistency to

shearing force and increasing temperature.

Upon the centrifugation test of 5000 revolutions/minute for 10 minutes, the emulsion

structure of none of the compositions showed decomposition. Water did not separate even

from emulsion creams with a water content of 70 %.

When the evaporation of the water found in the inner phase was examined, it was found

that more or less water evaporated from all the compositions in spite of the fact that this was

obviously hindered by the continuous lipophilic phase surrounding the emulsified water

droplets. The mass decrease of the samples, the quantity of the evaporated water increased

linearly as the function of the square root of time. The evaporation rate calculated from these

linearized functions is summarised in Table 5. Evaporation had the same order of magnitude

in both series. The rate of evaporation increased considerably with the increase of the

emulsified water content.

Table 5. Water evaporation from w/o creams

Water contant [%]

Evaporation rate1st series 2nd series

30 1.97 1.6740 3.06 2.2550 3.88 3.0555 4.66 -60 5.17 3.8170 - 5.24

24

The extent of structure break upon mechanical impact (e.g. mixing, material transport)

was also investigated as an important parameter with respect to manufacture and application.

Figure 6. illustrates the extent of viscosity change occurring with increasing shear time, and it

is obvious from the figure that the decrease in viscosity was relatively small. Thus, it was

confirmed that favourable consistency does not become unfavourably soft or fluidic when

subjected to a longer mechanical treatment (e.g. drug dispersion). However, as regards

applicability (e.g. pressing out of the tube, distribution on the skin) it is very advantageous

that a small extent of structure break takes place upon the effect of a small initial force, and it

is not followed by considerable shearing with the increase of the mechanical effect. The

investigation of the 2nd series revealed curves with a similar shape as in the case of the

creams of the 1st series.

Figure 6. Change of structural viscosity of w/o creams, 1st series

5.1.3. Investigation of drug release from creamsIn terms of the applicability of medicinal creams, it is very important to monitor whether

the incorporated drug is released from the base used, and if so, with what kinetics.

Two poorly water-soluble drugs - salicylic acid and sulfadimidine - were chosen for the

drug release studies, used in a concentration of 2 %. The results obtained with water-free

samples are also included in the summary for the sake of comparison. The time course of drug

release shows that a linear relationship exists between the square root of dialysis time and the

quantity of the released drug. It is clear from the data of Table 6. that both the quantity of the

25

released drug and the rate of liberation increased definitely with water content. It is important

to compare drug release from water-free and water-containing creams. A characteristically

greater amount of drug was released from water-containing creams than from water-free

bases.

Thus not only will w/o creams are aesthetically better with the increase of water content,

but drug release will also be enhanced.

Table 6. Drug release from w/o creams during 8 hours

Water[%]

1st series 2nd seriesSalicylic acid

releaseSulfadimidine

releaseSalicylic acid

releaseSulfadimidine

releaseQuantity

[mg]Rate

[mg/Vh]Quantity

[mg]Rate

[mg/Vh]Quantity

[mg]Rate

[mg/Vh]Quantity

[mg]Rate

[mg/Vh]0 2.44 0.943 0.14 0.080 2.27 0.987 0.16 0.082

30 2.54 1.001 0.16 0.082 2.64 1.272 0.17 0.09140 2.61 1.182 0.23 0.119 2.77 1.418 0.18 0.09750 2.73 1.155 0.29 0.134 3.07 1.575 0.26 0.12455 2.88 1.220 0.32 0.148 - - - -60 2.97 2.271 0.36 0.149 3.69 1.617 0.30 0.13570 - - - - 3.98 1.724 0.38 0.205

5.2. Investigation of w/o creams containing self-emulsifier base

5.2.1. Formation of cream structureThe investigation of the creams containing mixed emulsifiers revealed that the

determination of the contact angle of wetting can be used for prediction during

preformulation. In the structure research of w/o creams the size and size distribution of

droplets emulsified in the inner phase is of great importance, as is the detection of the factors,

which exert an effect on the size distribution of the emulsified drops and on their structure

dependence.

This served as the reason for investigating the contact angle of wetting and its role in

structure formation in the case of self-emulsifying systems as well.

The self-emulsifying substance itself is of paste consistency similarly to Miglyol-Gel,

therefore its dilution with a lipophilic phase is recommended by all means to facilitate

processibility. In accordance with our previous experiences, liquid paraffin and Miglyol 812

26

oil were used for dilution before the dispersion of water. The contact angle of wetting data of

undiluted and diluted bases are shown in Table 7.

Table 7. Effect of self-emulsifying base on the contact angle of wetting

Contact angle [°]

Base Basewithout oil

Base 50%Liquid paraffin

50 %Miglyol 812

50 %Protegin 91.6 85.9 52.7Protegin X 90.9 65.3 46.5Protegin W 85.9 63.3 45.2Protegin WX 84.1 60.5 44.0

It is evident that Protegins are poorly wetted with water; the value of the contact angle is

about 90°. Dilution with liquid paraffin did not enhance wetting substantially, while the use of

Miglyol 812 led to a considerable improvement. The importance of determining these values

in the preformulation is confirmed by Figure 7. The figure clearly shows that the arising

contact angle of wetting is in direct proportion with the water absorbing capacity of w/o

creams and with the water-retaining capacity of the emulsion system formed in this way.

The model compositions were chosen on the basis of the wetting behaviour and the

quantity of water, which can be absorbed.

Figure 7. Water absorbing and water-retaining capacity of w/o bases

27

The solidification characteristics of the gel structure formed from the melt constitute an

essential question as concerns the optimal production technology. The equation describing the

temperature-time dependence obtained by measuring the temperature during the mixing of the

melted mass was found to be suitable for determining this parameter. The slope of the line

obtained in this manner is the cooling rate value of the creams; these are shown in Table 8.

Table 8. Cooling rate of Protegin containing creams

Slopes of cooling curves with different water contantBase 30 % 40 % 50 % 60 %

Protegin 0.18 0.21 0.25 0.34Protegin X 0.19 0.21 0.22 0.24Protegin W 0.19 0.26 0.32 0.33Protegin WX 0.16 0.19 0.26 0.24

Table 9. Rheological parameters of Protegin-Miglyol 812-water system

Water Yield value Initial End SpreadabilityBase viscosity viscosity w=500 g

[%] [Pa] [Pa-s] [Pa-s] [cm2]30 5.5 49 3.42 51.6

Protegin 40 13.8 114 5.59 26.650 13.8 146 5.79 23.960 27.7 326 7.41 18.530 2.8 98 3.31 47.1

Protegin X 40 8.3 146 6.61 36.350 13.9 212 6.61 27.360 27.7 407 7.87 24.630 5.5 98 3.96 28.3

Protegin W 40 11.1 163 4.79 26.850 16.6 196 5.69 20.460 22.2 244 6.38 19.630 2.8 146 7.97 23.1

Protegin WX 40 11.1 212 8.66 21.250 16.6 261 8.84 19.860 27.7 407 9.69 18.1

28

The resulting creams have a small yield value and thixotropy. Consistency parameters

increase with the increase of water content (Table 9.) in line with the previously discussed

w/o creams containing mixed emulsifiers.

It seemed worthwhile examining how the size of the emulsified droplets changed with

water content. It was found that the droplets of emulsions with a water content of 30-40 %

exhibited polydisperse distribution. The obtained curves had similar shapes in the case of all

the three bases. There was an exponential relationship between the average droplet size of the

emulsified water droplets and the increase of water content in all the four cases. (Figure 8.)

Concentration of water [ w/w% ]

Figure 8. Relationship between water concentration of Protegin creams and droplet size

Not only structure formation (water absorption) depends on the wetting conditions on the

boundary surface but the resulting structure and properties also correspond to the properties of

the boundary surface. A relationship was found to exist between the contact angle of wetting

of the phases and the droplet size. (Figure 9.) The greater the wetting of the oil phase is, the

smaller the size of the resulting drops will be. The relationship between the droplet size and

the contact angle of wetting of the water-oil phase can be described with an exponential

equation in the case of creams containing the same amount of water.

29

Where

d = droplet size,

d0 = droplet size extrapolated to 0,

m = slope,

0 = contact angle of wetting.

d =d 0-em'& (6)

Figure 9. Relationship between contact angle of wetting of Protegin creams and droplet size

5.2.2. Stability of cream structureThe data of stability in time of the formed structure are summarised in Figure 10. The

investigations revealed that no macroscopic changes had occurred during storage for one

month, but a small extent of after-hardening was detected in the system with rheological

measurements. This after-hardening depends on the water content of the emulsion system and

on the wetting of the base. The increase of the yield value describing structure solidification is

slighter when the water content is higher or wetting is better.

30

TOÛ_

0)3TO>

■v0)>

P after after 1 PX after after 1 PW after after 1prep. month prep. month prep. month

Time [ after preparation and after 1 month ]

PWX after 1 after month prep.

Figure 10. Change of yield value of Protegin creams under storage

The heat stability examinations of the systems were performed. The values of activation

energy (Ea) calculated from these experiments (n = A ■ e~EaIRT) are summed up in Table 10.

The data clearly reveal that the increase of water content resulted in the decrease of the

activation energy that is in the increase of the heat stability of the system.

Table 10. Activation energy of Protegin creams

Base Activation energy with different water contant30 % 40 % 50 % 60 %

Protegin 43.5 48.5 33.4 31.6Protegin X 17.1 19.1 23.8 24.2Protegin W 21.5 27.5 25.4 17.6Protegin WX 20.8 17.5 16.7 14.2

5.2.3. Drug release from creamsIn the course of studying the interaction between applications - production it was

important to elucidate whether the size of the droplets emulsified in the inner phase has any

influence on the liberation of the suspended drug. Based on our examinations, it can be stated

that the quantity of the drug released increased characteristically with the size increase of the

emulsified droplets. (Figure 11.)

31

12

2

00 50 100 150 200 250 300 350 400 450

Average of droplet size [ pm ]

Figure 11. Relationship between droplet size and drug released from Protegin creams

In the case of w/o creams it seemed interesting to develop and to investigate a

transitional, ambiphilic cream base, in addition to the cream type containing mixed emulsifier

and self-emulsifying base. The composition of the ambiphilic cream base is shown in Table

11.

Table 11. Composition of ambiphilic creams

Series 1 2 3 4Components Quantity fw/w%lWhite petrolatum 47 45 42 40Cetostearyl alcohol 5 7 10 12Emulgator BTO 8 8 8 8

In the course of dilution with distilled water, the samples formed o/w creams with a

water content of 10 and 50 %, while w/o creams resulted when the cream base was diluted

with liquid paraffin at a liquid paraffin content of 10 and 50 %. The study of drug release

from these emulsion systems with a very specific behaviour seemed to be very interesting.

The suspended drug was salicylic acid. Figure 12. shows the values of drug release from

creams of the 3rd series formulated by diluting with water and liquid paraffin every 5 %. The

quantity of the released drug increased when diluted with water and decreased when diluted

with oil.

32

25.5

50 100 150 200 250 300 350 400Initial viscosity [ Pa - s ]

Figure 12. Effect of viscosity of ambiphilic creams on drug release

5.3. Investigation of hydrogels

5.3.1. Rheological characteristics of hydrogelsThe primary aim of our investigations of hydrogels was on the one hand to analyse the

relationship between polymeric concentration and consistency, and on the other hand to

investigate drug release from various polymeric matrixes. These matrixes were the aqueous

gels of polymers, which had very different chemical structures and build-up. Our

investigations were grouped along the following topics:

• the rheological nature of aqueous polymeric gels,

• common properties of network structures (matrixes) built up from polymers with

chemically different structures,

• the change of viscosity and other rheological constants with the concentration of

the network-forming material,

• the kinetics of drug release, what function describes drug liberation,

• how the process of drug liberation is influenced by the matrix structure,

• how liberation can be controlled by changing the structure of the matrix.

The hydrogels studied and the concentration values used are summed up in Table 12.

33

Table 12. Components of hydrogels

Polymers Concentration [%]Avicel RC 591 1.5; 3.0; 4.0; 5.0; 6.0; 7.0; 8.0; 9.0; 10.0Carboxymethylcellulose sodium 5.0; 6.0; 7.0; 8.0; 9.0; 10.0Carbopol 980 NF 0.1; 0.3; 0.5; 0.7; 0.9; 1.0; 1.1; 1.3; 1.5Hydroxyethylcellulose 5.0; 6.0; 7.0; 8.0; 9.0; 10.0Keltrol 1.0; 2.0; 3.0; 4.0; 5.0Methylcellulose 5.0; 6.0; 7.0; 8.0; 9.0; 10.0Xanthan 2.0; 2.5; 3.0; 4.0; 5.0

The dispersion medium was invariably distilled water. The gel series of Carbopol 980 NF

was produced with neutralization. The polymeric resin was swollen in water, then

trietholamine was added in a quantity sufficient to achieve a pH=6.5 - 7.0 in the colloidal

system.

The drugs were dispersed in a concentration of 1.0 % (dissolved or suspended) in the

gels, and the rheological and drug release examinations were performed the day after

preparation.

In order to describe the mechanical state of the network structure formed in the aqueous

medium, the flow and viscosity curves of the gels were plotted. The nature of hydrogels is a

disputed issue in the literature of pharmaceutical technology dealing with polymeric gels.

Polymeric hydrogels were defined by Dolz-Planas et al. as thixotropic systems (106). This

statement is in contradiction with the classic concept of thixotropy, according to which this

phenomenon is a time-dependent process of structure break and regeneration. In the case of

hydrogels, considerable structural changes take place upon the effect of mechanical forces,

but these depend on time as after the removing of the force immediate rearrangement occurs.

The hysteresis loop characteristic of thixotropy arises only in great concentrations. These

polymeric matrixes are rather characterized by the simultaneous occurrence of plastic

deformability and viscoelasticity.

Our investigations performed with a great number of chemically different polymers

confirm the supposition according to which matrixes of hydrogel nature are not thixotropic

systems. This is illustrated by Figure 12, in which the flow curves of the hydrogels of

carboxymethylcellulose sodium can be seen.

34

2000

Figure 13. Flow curves of carboxymethylcellusose sodium

The up-curves (continuous line) and the down-curves (dotted line) completely overlap even in

the case of extremely great concentrations. Similar curves were obtained in the case of the

other examined polymers, too. The gels of Avicel RC 591 are an exception, as they exhibit

considerable thixotropy. The rheological parameters of the hydrogels of Avicel RC 591 are

summarised in Table 13.

Table 13. Rheological parameters of Avicel hydrogels

Avicelconcentration

[%]

Initialviscosity

[Pa-s]

Equilibriumviscosity

[Pa-s]

B _ n - nln ^

1̂

M _ n - n 2 ln D

d 2^ 5

1.5 - 0.03 0 0.02 1.203 0.37 0.08 0 0.15 1.304 3.41 0.35 0 0.96 1.305 18.47 0.80 0.06 4.62 1.176 33.23 1.25 0.18 9.73 1.117 99.70 1.75 0.10 18.08 1.078 166.17 2.45 0.13 22.45 1.189 182.78 3.31 0.25 31.80 1.26

10 348.96 - 0.76 33.81 1.25n and are structural viscosity measured after t1=5 s and t2=300 s shear time.

n and are structural viscosity measured at D1=8.1 s-1 and D2=0.5 s-1 shear rate.

35

The M and B coefficients of thixotropy clearly demonstrate the existence of thixotropy.

The reason for the formation of the thixotropic structure is that the bulk of Avicel RC 591

contains microcrystalline cellulose, and this specific arrangement leads to the thixotropic

behaviour of gels. The ^25/^32 ratio confirms our concept according to which in order to

examine the relationship of rheological properties and drug release, it is indispensable to

perform rheological measurements at skin temperature in the case of hydrogels, too.

The viscosity curves (Figure 14, Figure 15.) illustrate the decrease of viscosity as a

function of the shear rate gradient. The figures clearly prove that a considerable structural

arrangement takes place in the matrixes upon the effect of shearing, however, this is not

thixotropy. (The up-curves and the down-curves overlap once again). The viscosity decrease

is very significant; it generally comprises two orders of magnitude.

Figure 14. Viscosity curves of Xanthan

36

100

10rao.

0.10.1

R 2 = 0 .9 9 8 3 1.5%

R 2 = 0 .9 9 8 6 1.3%

R 2 = 0 .9991 1.1%

R 2 = 0 .9 9 9 1.0%

R 2 = 0 .9 9 8 6 0.9% ---- ---------

R 2 = 0 .9 9 7 6 0.7%

R 2 = 0 .9 9 8 7 0.5%

R 2 = 0 .9 8 8 6 0.3%O

R 2 = 0 .9 8 0 6 0.1%

10Shear rate [1/s]

Figure 15. Viscosity curves of Carbopol 980 NF

100

1

1

The creep-recovery test of hydrogels is suitable for monitoring the viscoelastic properties,

which are very important in terms of application and production. The analysis of the shape of

the curves evidently demonstrates that the hydrogels under examination are viscoelastic

material systems. The value and shape of creep compliance (deformation) curve are

fundamentally important. Subjected to a constant stress, the strain of an ideal elastic material

would be constant and the material would return to the original shape when the stress was

removed. In contrast, an ideal viscous material would show a steady flow, producing a linear

response to stress with the inability to recover any of the imposed deformation. Viscoelastic

materials, like our hydrogels, exhibit a nonlinear response to strain and, due to their ability to

partially recover structure by storing energy, show a final deformation less than the initial

deformation. It was typical of the studied gel series that the elasticity properties of the gels

characteristically increased with the increase of the polymeric concentration. Two examples

for this are shown in Figure 16. and Figure 17. When the two figures are compared, the

elasticity-increasing effect of the polymer can be seen.

37

Figure 16. Creep-recovery test of 5 % carboxymethylcellulose sodium and 5 % methylcellulose hydrogel

Figure 17. Creep-recovery test of 10 % carboxymethylcellulose sodium and 10 % methylcellulosehydrogel

During our rheological measurements the elasticity modulus (G’) and plasticity modulus

(G”) of the hydrogels were determined, in this way the dependence of the elastic and plastic

properties of the sample on concentration can be determined. Our measurements were

performed each time within the linear viscoelastic region, at 2.5 Pa. Figure 18. presents a

typical state, in which the dependence of the G’ and G” values of the carboxymethylcellulose

sodium gel series on concentration is shown. In the figure we can see a cross-over in G’ and

38

G” at a concentration. Over this point, the elastic properties of the hydrogels become to be

dominant.

Figure 18. Viscoelastic behaviour of CMC Na hydrogels

5.3.2. Interaction between hydrogels and drugsInteractions, e.g. desolvation manifested in the form of viscosity change, association,

etc., can be easily determined with the rheological measurement technique. The flow curves

of hydrogels investigated in our experiment can be characterised by power function. The

power function character is confirmed by the flow curves. The changes induced by the drugs

are well illustrated by the values of exponent of power function. The value of the exponent

shows the hidration of the polymer chains and the interaction between the molecules and their

segments. If the value of the constant changes, a change has occured in the solvation of the

polymer chains and the forces holding the gel frame together have increase or decreased. The

data in the Table 14. show that the value of the exponent was characteristically decreased by

the four active agents. This decrease measurable, mainly in small polymer concentrations, can

be explaned by the fact that the active agents probably dehydrated the swollen polymer or

modified the charge of matrix and the result the forces holding the frame together decreased.

39

Table 14. Change of the slope of flow curves of hydrogels containing different drugs

Polymer Conc.[w/w%]

Withoutdrug

Salicylic acid Ephedrinehydrocloride

Sulfadimidine Griseofulvin

Hostacerin PN 73

0.5 0.378 - 0.144 0.396 0.2631.0 0.256 - 0.191 0.243 0.2421.5 0.208 0.141 0.216 0.217 0.2192.0 0.220 0.170 0.207 0.202 0.2122.5 0.209 0.205 0.193 0.194 0.197

Xanthan

2.0 0.182 0.054 0.069 0.160 0.0602.5 0.108 0.061 0.076 0.123 0.0593.0 0.111 0.087 0.107 0.178 0.0714.0 0.105 0.102 0.105 0.145 0.1275.0 0.116 0.107 0.105 0.148 0.116

Keltrol

1.0 0.093 0.040 0.058 0.100 0.0972.0 0.160 0.071 0.070 0.156 0.0893.0 0.128 0.093 0.064 0.123 0.1014.0 0.126 0.110 0.115 0.133 0.1105.0 0.120 0.107 0.110 0.125 0.107

5.3.3. Relationship between drug release and structure of hydrogelsThe process of drug release, the kinetic relations of the phenomenon and the factors

influencing liberation were investigated extensively. Based on these investigations, note­

worthy and characteristic statements can be made concerning the kinetics of drug release.

The time course of the release of suspended drugs (Griseofulvin, Sulfadimidine, salicylic

acid) can usually be described with the power function (Figure 19.) like relationship known

from literature and described first by Higuchi (102).

In the case of dissolved drugs (ephedrine hydrochloride, Figure 20.) the kinetics of the

process will be different.

40

Figure 19. Kinetics of release of suspended Griseofulvin from Xanthan hydrogels

Figure 20. Kinetics of release of dissolved Ephedrine hydrocloride from Xanthan

hydrogels

As it can be seen in the Figure 20, the process is a linear relationship:

Q =Q0 +m • t (7)

Where

Q = drug released under t time,

Q0 = drug released to 0 time,

m = slope,

t = time.

In theory Q0 is zero, in practice it is a negative value and its size depends on the

saturation time of the membrane.

This process can essentially be described with zero-order kinetics that is the rate of

liberation does not depend on concentration in a relatively long time interval. The reason for

this phenomenon is that the whole amount of the drug is present in a dissolved state that is

ready for liberation.

In addition to the dissolved or suspended state of the drug, the decisive factor of drug

release depends on the matrix structure. Gel viscosity is the physical parameter which best

describes the structure of the matrix. In our examined systems, the viscosity of the polymeric

gels and the quantity of the drug released during the maximum diffusion time (120 minutes)

was compared in each series.

41

The relationship between structure and liberation is the key to controlling and prog­

ramming drug release, therefore it needs detailed investigations. Higuchi (102), in his models

giving the quantitative description of the relationships between drug release and the structure

of polymeric matrixes, pays great attention to the morphology of the matrix, that is to the

volume and winding of the pores. Drug diffusion is increased or decreased by these factors.

These properties cannot be measured directly in the aqueous gels included in our study.

The relationship between structure (volume, morphology of the pores) and drug release

was approached in the following way. Viscosity was found to increase exponentially with the

concentration of the polymer in the studied polymeric matrix series:

n =n0-ek'c (8)

Where

n = viscosity,

n 0 = viscosity extrapolated to 0,

k = constant, characterise the hardening of structure,

c = polymer concentration.

Constant k expresses the hardening of the structure, it gives a quantitative description to

what extent the volume of the pores decreases and their winding increases with concentration.

This constant is going to be called structure formation constant hereafter.

It was also found during the experiments that within each series the increase of polymer

concentration was associated with the characteristic decrease of drug released (107). This

phenomenon can be described with a power function:

42

0 =0 0 * - g (9)

Where

0 = drug released,

00 = drug released extrapolated to 0,

c = concentration of polymer,

g = constant.

Where the absolute value of constant g expresses to what extent drug release decreases

with the increase of concentration that is with the decrease of the pore volume and with the

increase of winding. Henceforth this constant is going to be called drug release constant.

The relationship of the two constants is presented in Figure 21.

Figure 21. Relationship between matrix structure and drug release constant

In Figure 21. the k and g values of various matrixes containing the same drug were

compared. When this relationship is examined in the case of different drugs, the relation

between structure and drug release becomes clear. A linear regression was found to exist

between the two parameters. The fewer pores there were in the matrix and the more winding

they were (this is expressed by the greater value of k), the smaller the parameter characteristic

of drug release was.

43

k =k 0-m ■ g (10)

Where

k = characterise the hardening of structure,

k0 = value extrapolated to 0,

m = slope,

g = changing of drug release by concentration.

These functions help to plan polymeric matrixes. The greater the drug release constant

and the smaller the structure formation constant is in a given polymer matrix series, the

greater extent (the more rapid) the rate of drug release will have. On the other hand, if the

structure formation constant has a great value, prolonged release will become possible. The

values of these two constants should be determined and used as the basis to choose the

polymeric matrix most suitable for the given therapeutic aim and for the given drug.

6. SummaryIn my thesis parameters of great importance in the quality development and control of

w/o creams and ambiphilic creams as well as of hydrogels were investigated from among the

semisolid dosage forms with dermal application.

The factors responsible for the structure formation of creams were analysed syste­

matically. The factors with decisive influence on the structure and stability of w/o emulsion

creams were selected. My major findings are going to be summarised in the following.

• The contact angle of wetting plays a significant role in the structure formation of

w/o creams. The increase in the concentration of the emulsifier results in the

considerable decrease of the contact angle of wetting but only up to a given value,

over which the further increase in the surfactant concentration leads only to a

slight decrease.

• Both the amount of the inner phase, which can be incorporated, and the water-

retaining capacity of the already formed structure increase with the decrease of

the contact angle of wetting. A linear regression was found to exist between these

two parameters.

44

• A linear relationship was found between the contact angle of wetting and the

water amount, which can be incorporated in the creams. The determination of the

contact angle of wetting of the water-free base containing the emulsifier in

different concentrations before the incorporation of the inner phase is essential in

the improvement of w/o creams.

• An exponential relationship was found to exist between the contact angle of

wetting of the water-oil phase and the droplet size of the inner phase of the

emulsion cream.

• The great number of the examined w/o emulsion creams containing mixed

emulsifiers and self-emulsifying bases all were, irrespective of the water content,

thixotropic systems with a yield value. The rheological parameters tended to

increase with the water content. A power function relationship was found between

the structural viscosity of creams and the shear rate gradient.

• Viscosity and shear time functions can help to analyse the processibility and

material transport of the creams. The small extent of initial structure break

observed in the creams, which is not followed by significant viscosity decrease,

ensures good processibility.

• The changes occurring in the microstructure of creams during storage can be

monitored with rheological parameters.

• The value of the activation energy got during the investigation of the heat stability

of creams provides a possibility for rapid numerical comparison or evaluation.

• Cooling curves are suitable for determining the solidification characteristics of the

gel structure formed from the melt.

• Creams containing water in the inner phase also exhibit considerable evaporation

loss. The mass decrease of creams is in a linear relationship with the increase of

the mass percentage of the inner phase as the function of the square root of time.

• A relationship was found between the droplet size of the inner phase and the

quantity of the drug released. The increase in the size of the emulsified droplets

resulted in increasing the quantity of the drug released.

• The increase of the mass ratio of the inner phase increased the quantity and rate of

drug release.

45

• In the case of ambiphilic creams, the quantity of the released drug was increased

by the quantitative increase of the aqueous phase and decreased by the

quantitative increase of the oil phase.

Experiments were performed with a large number of hydrogels, during which the state of

the hydrogel matrixes built up from polymers was examined and the relationship between the

viscosity of hydrogels and drug release was studied.

• The hydrogels of the studied polymers, with the exception of Avicel RC 591 the

bulk of which has a microcrystalline structure, did not show thixotropic

behaviour.

• In the examined polymer matrix series an exponential relationship was found

between the viscosity of hydrogels and the concentration of the polymer. The

concept of the structure formation constant was introduced for the description of

the hardening of the structure.

• The hydrogels in our study were viscoelastic systems. The changes in the plastic

and elastic state of the gels can be monitored with the examination of the

concentration dependence of the G’ and G” values.

• The interaction resulting between the polymer matrix and the incorporated drug

was pointed out with the analysis of flow curves.

• A power function relationship can describe the decrease in drug release taking

place upon the effect of increasing the concentration of the polymer.

• The concept of drug release constant was introduced for the description of the

drug release decrease occurring with the increase of the polymer concentration in

the matrixes built up from various polymers.

• Based on our investigations performed with polymers it can be stated that during

the formulation of hydrogel-based semisolid preparations the structure formation

constant and the drug release constant should be determined and used for the

selection of the hydrogel best suited for the given therapeutic aim and for the

given drug.

46

7. References1. Ziegenmeyer, N.: Dermatika, Wissenschaftliche Verlagsgesellschaft mbH,

Stuttgart, 1992.

2. Münzel, K.: Pharm. Acta Helv. 28, 320 (1953)

3. Katrin, M. et alias: Eur. J. Pharm. Biopharm. 52, 103 (2001)

4. Osborne, D.W., Amann, A.H. (Eds.): Topical Drug Delivery Formulations, Vol.

42, Marcel Dekker, New York and Basel, 1990.

5. Junginger, H.: Pharm. Ind. 46, 758 (1984)

6. Junginger, H.:Deutsche Apoth. Zeit. 131, 1933 (1991)

7. Gurny, R., Teubuer, A.: Dermal and Transdermal Drug Delivery,

Wissenschaftliche Verlagsgesellschaft mbH, Stuttgart, 1993.

8. Junginger, H.: Systematik der Dermatika-Kolloidchemischer Aufbau, Chapter 8 in

Ziegenmeyer, N.: Dermatika, Wissenschaftliche Verlagsgesellschaft mbH,

Stuttgart, 1992.

9. Erős,I., Takács-Wormsdorff, T., Csányi, E.: Acta Pharm. Hung. 66, 119 (1996)

10. Bauer, K.H., Frömming, K.H., Führer, C.: Pharmazeutische Technologie, Gustav

Fisher Verlag, Stuttgart Jena Lübeck Ulm, 1997.

11. Katime, I., Novoa, R., Díaz de Apodaca, E., Mendizábal, E., Puig, J.: Polymer

Testing 18, 559 (1999)

12. Dreher, F., Walde, P., Walther, P., Wehrli, E.: J. Control. Release 45, 131 (1997)

13. Brinon, N. et alias: J. Control. Release 60, 67 (1999)

14. Shah, J.C., Sadhale, Y., Chilukuri, D.M.: Adv. Drug Deliv. Reviews 47, 229

(2001)

15. Luisi, P.L., Scartazzini, G., Haering, G., Schurtenberger, P.: Colloid Polym. Sci.

268, 356 (1990)

16. Al-Sayed, S., Enam, K., Ibrahim, H., Ibtisam, F.: Eur. J. Pharm. Biopharm. 53,

343 (2002)

17. Rácz, I., Selmeczi, B.: Gyógyszertechnológia, Medicina Könyvkiadó Rt,

Budapest, 2001.

18. Drake, L.A., Dinehart, S.M., Farmer, E.R.: J. Am. Dermatol. 35, 615 (1996)

47

19. Thoma, K.: Dermatika, Werbe-und Vertriebsgesellschaft Deutscher Apotheker

mbH, Frankfurt, 1983.

20. Münzel, K., Berneis, K.: Sci. Pharm. 37, 55 (1969)

21. Barry, B.W.: Rheol. Acta 10, 96 (1971)

22. Eccleston, G.M.: Pharm. Intern. 7, 63 (1986)

23. Marquardt, D., Sucker, M.: Eur. J. Pharm. Biopharm. 46, 115 (1998)

24. Marquardt D., Pedrussio, R., Herzog, B., Sucker, M.: Pharm. Dev. Technol. 2,

123 (1997)

25. Ketz, R.J. et alias.: Rheol. Acta 27, 531 (1988)

26. Erős, I. Kónya, M., Csóka, I.: Int. J. Pharm. 256, 75 (2003)

27. Boylan, J.C., Dekay, G.G., Banker, G.S.: J. Pharm. Sci. 51, 742 (1962)

28. Barry B.W.: Dermatological Formulation, Marcel Dekker, New York and Basel,

1983.

29. Wellin-Berger, K., Neelissen, J., Bergenstahl, B.: Eur. J. Pharm. Sci. 14, 229

(2001)

30. Leopold, C.S., Lippold, B.C.: J. Pharm. Pharmacol. 47, 276 (1995)

31. Welin, B.K. Neelissen, J., Begenstahl, B.: Eur. J. Pharm. Sci. 14, 229 (2001)

32. Jui-Chen, T. et alias: Ind. J. Pharm. 188, 145 (1999)

33. Murakai, T. et alias: J. Pharm. Pharmacol. 50, 55 (1998)

34. Nook, T.H.: Br. J. Dermatol. 117, 243, (1987)

35. Refai, H., Müller, G.C.C.: Eur. J. Pharm. Biopharm. 54, 143 (2002)

36. Gorukani, L., Kim, K.H.: Int. J. Pharm. 192, 159 (1999)

37. Gao, A.Y., Li Wan Po, A.: Eur. J. Clin. Pharmacol. 46, 71 (1994)

38. Viegas, T.X. et alias: Int. J. Pharm. 219, 73 (2001)

39. Lalor, C.B., Flynn, G.L., Weiner, N.: J. Pharm. Sci. 83, 1525 (1994)

40. Tadros, T.F.: Colloid. Surf. Physicochem. Eng. Aspects 91, 89 (1994)

41. Gupta, P., Vermani, K., Gary, S.: Drug Discovery Today 7, 569 (2002)

42. Graham, T.: Philos. Trans. R. Soc. 151, 183 (1861)

43. Graham, T.: J. Chem. Soc. 17, 318 (1864)

44. Lloyd, D.J.: The problem of gel structure. In Colloid Chemistry, Chemical

Catalog Co, ed.: Alexander, J., New York, 1926.

48

45. Almdal, K., Dyre, J., Hvidt, S., Kramer, O.: Polymer Gels and Networks 1, 5

(1993)

46. Klech, C.M.: Gels and Jellies. In Encyclopedia of Pharmaceutical Technology,

ed.: Swarbrick, J. and Boylan, J.C., Marcel Dekker, New York, Basel, 1992.

47. Peppas, N.A., Bures, P., Leobandung, W., Ichikawa, H.: Eur. J. Pharm. Biopharm.

50, 7 (2000)

48. Peppas, N.A.: Hydrogels in Medicine and Pharmacy Vol. I., Fundamentals, CRC

Press Inc., 2000.

49. Tyle, P.: Specialised Drug Delivery Systems, Marcel Dekker, New York, 1990.

50. Hoffman, A.S.: Adv. Drug Deliv. Reviews 54, 3 (2002)

51. Kikuchi, A., Okano, T.: Adv. Drug Deliv. Reviews 54, 53 (2002)

52. Osada, Y., Okuzaki, H., Hori, H.: Nature, 355, 242 (1992)

53. B. Jeong, B., Choi, Y.K., Bae, Y.H., Zentner, G., Kim, S.W: J. Control. Release

62, 109 (1999)

54. Kim, S.W., Jacobs, H.A.: Drug. Dev. Ind. Pharm. 20, 575 (1994)

55. Suzuki, Y., Tanihara, Y., Nishimura, Y., Suzuki, K., Kakimaru, Y., Shimizu,

Y.J.:Biomed. Mater. Res. 42, 112 (1998)

56. hidrogél living tissue

57. Dong, L.C., Hoffman, A.S.: J. Control. Release 15, 141 (1991)

58. Bromberg, L.E., Ron, E.S.: Adv. Drug. Deliv. Reviews 31, 197 (1998)

59. Suzuki, A., Tanaka, T.: Nature 354, 291 (1990)

60. Kwon, I.C., Bae, Y.H., Kim, S.W.: Nature 354, 291 (1991)

61. Jeong, B., Gutowska, A.: Trends Biotechnol. 20, 305 (2002)

62. Yoshida, R., Sakai, K., Okano, T., Takurai, Y.: Adv. Drug. Deliv. Reviews 11, 85

(1993)

63. Kost, J. Langer, R.: Adv. Drug. Deliv. Reviews 46, 125 (2001)

64. Miyata, T., Uragomi, T., Nakamae, K.: Adv. Drug. Deliv. Reviews 54, 79 (2002)

65. Kim, Y. et alias: J. Control. Release 28, 143 (1994)

66. Di Colo, G., Carelli, V., Giannaccini, B., Serafini, M.F., Bottari, F.: J. Pharm. Sci.

69, 387 (1980)

67. Viegas, T.X., Hikal, A.H., Jones, A.B.: Int. J. Pharm. 152, 165 (1997)

49

68. Porzio, S. et. alias: Pharm. Research 37, 41 (1998)

69. Takahashi, A. et. alias: Int. J. Pharm. 246, 179 (2002)

70. Jeong, B., Bae, Y.H., Lee, D.S., Kim, S.W.: Nature 388, 860 (1997)

71. Barichello, J.M., Morishita, M., Takayama, K., Nagai, T.: Int. J. Pharm. 184, 189

(1999)

72. Kitano, M., Maitani, Y., Takayama, K., Nagai, T.: Int. J. Pharm. 174, 19 (1998)

73. Jones, D.S., Woolfson, A.D., Brown, A.F., Coulter, W.A., Mc Clelland, C., Irwin,

C.R.: J. Control. Release 67, 357 (2000)

74. Scherland, M., Brodin, A., Malmsten, M.: J. Colloid, Interface Sci. 229, 365

(2000)

75. Bourre, L., Thibaut, S., Briffaud, A., Lajat, Y., Patrice, T.: Pharmacological

Research 45, 159 (2002)

76. Lowman, A.M., Morishita, M., Kajita, M., Nagai, T., Peppas, N.A.: J. Pharm. Sci.

88, 933 (1999)

77. Blondsted, H., Kopecek, J.: Pharm. Res. 9, 1540 (1992)

78. Uchida, T., Sakakibara, S., Toida, Y., Nagareya, N., Yasuda, N., Matsuyama, L.:

Pharm. Pharmacol. Commun. 5, 523 (1999)

79. Veyries, M.L. et alias: Int. J. Pharm. 192, 183 (1999)

80. Upadrashta, S.M., Hagland, B.O., Sundelof, L.O.: J. Pharm. Sci. 82, 1094 (1993)

81. Albertsson, A.C., Carlfors, J., Sturesson, C.: J. Appl. Polym. Sci. 62, 695 (1996)

82. Desai, S.D., Blanchard, J.: Drug. Deliv. 7, 201 (2000)

83. Singh, R., Vyas, S.P.: J. Dermatol. Sci. 13, 107 (1996)

84. Bochot, A., Fattal, E., Grossiord, J.L., Puisieux, F., Couvreur , P.: Int. J. Pharm.

162, 119 (1998)

85. Paavola, A., Kilpelaine, I., Yliruusi, J. Rosenberger, P.: Int. J. Pharm. 199, 85

(2000)

86. Alvarez-Lorenzo, C., Concheiro, A.: Am. J. Drug. Deliv. 1, 77 (2003)

87. Sjöberg, H., Persson, S., Caram-Lelham, N.J.: Control. Release 59, 391 (1999)

88. Schramm, G.: A practical Approach to Rheology and Rheometry, Gebrueder

HAAKE GmbH, Karlsruhe, 1994.

50

89. Sanz Taberner, T., Martin-Villodre, A., Pla-Delfina, J.M., Herráez, V.: Int. J.

Pharm. 233, 43 (2002)

90. Zignani, M., Tabatabay, C., Gurny, R.: Adv. Drug. Deliv. Reviews 16, 51 (1995)

91. Wientjes, R.H.W., Jongschaap, R.J.J., Diuts, M.H.G., Mellema, J.: J. Rheol. 43,

375 (1999)

92. Togrul, H., Arslan, N.: Carbohydr. Polym. 54, 73 (2003)

93. Currie, P.K.: Int. J. Non-Linear Mechanics 39, 833 (2004)

94. Bak, H., Afoke, A., McLeod, A.J., Brown, R., Ayazi Shamlou, P., Dunnill, P.:

Chem. Engineering Sci. 57, 913 (2002)

95. Tamburic, S., Duncan, Q.M.: Eur. J. Pharm. Biopharm. 44, 159 (1997)

96. Barreiro-Iglesias, R., Alvarez-Lorenzo, C., Concheiro, A.: J. Contr. Release 77,

59 (2001)

97. Hagerström, H., Paulsson, M., Edsman, K.: Eur. J. Pharm. Sci. 9, 301 (2000)

98. Welin-Berger, K., Neelissen, J., Bergenstahl, B.: Eur. J. Pharm. Sci. 14, 229

(2001)

99. Tas, C., Özkan, Y., Savaser, A., Baykara, T.: Il Farmaco 58, 605 (2003)

100. Neurbert, R., Wohlrab, W.: Acta Pharm. Technol. 36, 197 (1990)

101. Flyn, G.L., Shah, V.P., Tenjarla, S.N.: Pharm. Res. 16, 1325 (1999)

102. Higuchi, W.I.: Pharm. Sci. 56, 315 (1967)

103. Fang, J.Y., Shen, K.L., Huang, Y.B., Wu, P.C., Tsai, Y.H.: Drug. Dev. Ind.

Pharm. 25, 17 (1999)

104. Fang, J.Y., Hwang, T.L., Leu, Y.L.: Int. J. Pharm. 250, 313 (2003)

105. Wolfram, E.: A kémia újabb eredményei, Kontakt nedvesedés, Akadémiai Kiadó,

Budapest, 1971.

106. Dolz-Planas, M., Roldand-Garcia, C., Herrarez, J.V., Belda-Maximino, R.:

Pharm.Sci. 80, 75 (1991)

107. Erős, I., Blum, A., Csányi, E., Csóka, I., Illyés, F., Takács-Wormsdorf, T.: Acta

Pharm. Hung. 73, 139 (2003)

51

Acknowledgements

I express my grateful thanks to Prof. Dr. István Erős, Head of the Department of Pharma­

ceutical Technology for providing the possibility to complete my work under his advice.

My sincere thanks go to Prof. Dr. István Erős for his valuable help in my practical work and

for giving me useful advice.

I express my grateful thanks to my co-author+s and colleagues for their co-operation and lot

of help.

52