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  Issue 2 (2002)

Authors' Articles
Rolf Daniels*
Novel Gel Systems

*Institut fuer Pharmazeutische Technologie, Technische Universitaet Braunschweig, Mendelssohnstr.1, D-38106 Braunschweig

Prof. Dr. Rolf Daniels presented this paper at a symposium of the GD Gesellschaft für Dermopharmazie in Düsseldorf, October 17th 2001.


Gels play besides lotions and creams an important role as a vehicle for the topical treatment of the skin. Gels are from a physico-chemical point of view dispersed systems. They consist of at least two components: a solid, which form a three-dimensional network (matrix, texture) and a liquid which as a coherent medium is immobilized within the solid matrix. The IUPAC defines gels as "colloidal systems with a finite, usually rather small, yield stress".

Gels are used for dermocosmetic products due to their unique properties, e.g. their rheological behaviour. Moreover, the optical clarity and the non-greasy properties of hydro-alcoholic gels are appreciated. Lipophilic gels are also visually appealing and can be used as viscosity modifiers.

Lipophilic and hydophilic gels may be basically categorized according the following scheme (Fig. 1):

Figure 1: Categorization of lipophilic and hydophilic gels

Enlarged version

Covalently bonded polymer networks with completely disordered structures
Physically bonded polymer networks, predominantly disordered but containing ordered loci
Well-ordered gel meso-phases

Dermocosmetic products use almost exclusively type two and three. Important product classes are:

Clear or translucent hydrogels
Creamy hydro dispersion gels
Water-free oleo gels

Oleo gels
Oleo gels consist of a lipophilic liquid phase. Homologues of the lipid phase with higher molecular weight, organo-modified bentonits (Bentone®) and colloidal silica are mentioned in the literature to be possible as network forming excipients. The latter does not yield cosmetically acceptable oleo gels. In contrast, gels with Bentone® provide skin feel, but they are brownish opaque due to the intrinsic colour and the particle size of the gelling agent.

An innovative oleo gel (Versagel®) which can be used alternatively uses Ethylene/Propylene/Styrene Copolymer (and) Butylene/Ethylene/Styrene Copolymer (INCI) as the gelling agent (Fig.2). When formulated with liquid lipids, e.g. Mineral Oil Hydrogenated Polyisobutene, Isopropyl Palmitate oder Isohexadecane, gels with extraordinarily good clarity are formed. Their composition and optical appearance are close to those from gel candles, which have found their way into our living rooms as the more recent variant of the classical stearin candle (Fig. 3).

Figure 2: Chemical structure of Ethylene/Propylene/Styrene Copolymer (R1 = H) (and)
Butylene/Ethylene/Styrene Copolymer (R1 = CH3) (INCI)

Enlarged version

Figure 3: Versagel®: Optical appearance is close to those from gel candles

Versagel® hydrocarbon gels are in contrast to "usual" gels shear thickening. The dynamic viscosity increases with increasing shear stress, i.e. with increasing mechanical agitation. Obviously these systems behave like highly concentrated dispersions of the well solvated gelling agent. With increasing temperature the systems go through drastic viscosity drop in the range of the skin surface temperature as can be seen for example from a gel based on Isopropyl Palmitate (Fig. 4). This makes the application easier and favours a homogenous distribution on the skin.

Figure 4: Decrease of Viscosity of an Oleo Gel from Isopropyl Palmitate and Ethylene/Propylene/Styrene Copolymer (and) Butylene/Ethylene/Styrene Copolymer (INCI) (Versagel® MP) with increasing temperature

Enlarged version

Compared to the properties of the pure lipid phase, oleo gels show a higher substantivity. Thus, the transepidermal water loss (TEWL) is reduced for a longer period during a short-term test (Fig. 5).

Figure 5: TEWL Reduction by Isopropyl Myristate compared with an Oleo Gel from Isopropyl Palmitate and Ethylene/Propylene/Styrene Copolymer (and) Butylene/Ethylene/Styrene Copolymer (INCI) (Versagel® MP).

Enlarged version

Furthermore, these oleo gels have a good suspension capability for fine particles and effectively reduce the sedimentation of solids, e.g. zinc oxide, at both normal and elevated temperature. Versagel® hydrocarbon gels are usually not used as a sole vehicle. In most cases they are used as an addititive to the lipid phase. In this case a broad variety of applications exist (can be realized) (Tab. 1).

Table 1: Potential Applications of Oleo Gels, which use Ethylene/Propylene/Styrene Copolymer (and) Butylene/Ethylene/Styrene Copolymer (INCI) as Gel Builder (Versagele®)

After-Sun Care Gels Gel Lotions and Creams
Baby Care Products Lip Balm

Baby Care Oily Gels

Lip Gloss
Body Care Lotions Shower Gel Products
Face Care Products Tanning Gels

Surfactant gels

Surfactant gels are lyotropic liquid crystals. They form by association of hydrated or solvated surfactant molecules. Depending on the structure and concentration of the surfactant differently arranged liquid crystalline phases may form (Fig. 6). Cubic and hexagonal liquid crystals behave clearly visco-elastic, showing the rheological behaviour of both solid and liquid. In common with most typical gels, they possess a substantial yield stress. Cubic surfactant gels react highly elastic upon mechanical agitation. As their oscillating frequency is within the acoustic range, they are called ringing gels. Lamellar liquid crystals are regularly arranged in one dimension and are thus more fluid than cubic or hexagonal phases. However, they are, although less pronounced, also visco-elastic.

Figure 6: Basic Structures of Surfactant Gels

Enlarged version

Surfactant gels seem to be interesting for topical use because they can potentially interact with the intercellular epidermal lipids. The colloidal structure of lamellar phases and the stratum corneum lipids is very similar. The interaction with the skin and the resulting penetration enhancing effect seems to be correlated with the high concentration of surfactants. This affects the structure of the epidermal lipids and thus increases the skin permeability. Lyotropic liquid crystals may also provide improved skin care benefits. Conversely, the irritation potential may also increase with increasing surfactant content. This requires a careful of benefits and risks..

Furthermore it must be considered, that the water content after dermal application will change. Within a few minutes the water content reduces dramatically. This will alter the state of surfactant gels according to their phase behaviour (Fig. 7). When, for example, a cubic phase is applied to the skin it turns into a lamellar phase and ends up as an inverted micellar system at a low water content. Whereby several multi-phase transitional states are also observed. This metamorphosis of the vehicle is known for a longer period, however, its influence on the interaction of the vehicle with the skin is only little investigated so far.

Figure 7: Alteration of the State of Surfactant Gels according to their Phase Behaviour

Enlarged version

Hydro-dispersion gels

Hydro-dispersion gels are dispersed systems consisting of a hydrophilic continuous phase and a lipophilic dispersed phase. Usually, hydro-dispersion gels are oil-in-water emulsions with a content of liquid lipid phase between 2 and 20 %. In contrast to classical emulsions, the required stabilization is not provided by the addition of traditional emulsifiers but by means of suitable macromolecules (Figure 8). This is stressed when such formulations in particular in the field of sun care products are called "emulsifier-free".

Apart from the interfacially stabilized hydro dispersion gels there are products on the market where relatively small amounts of lipid are solely stabilized by the viscosity enhancing effect of the added polymer. Such "quasi"-emulsions are also termed "Balm". The ingredient list of an appropriate formulation is given in Table 4.

Figure 8: Chemical Structures of important Emulsifiers for Hydro-dispersion Gels

Enlarged version

The physical stability and an adequate shelf life are obtained through finely dispersing the lipids. This measure and the yield value of the external phase reduce droplet mobility and thus effectively prevent creaming and coalescence of the oil droplets.

Even persons with sensitive skin tolerate well hydro-dispersion gels. Their application can be recommended on normal or seborrhoeic skin in particular when large amounts of alcohol are added for antimicrobial preservation.

The stabilizing action can be categorized into two classes : (1) Stabilization by enhanced viscosity (so-called "quasiemulsions") (2) interfacial stabilization. In practice, often either of these two mechanisms is responsible for the storage stability with one component being more dominant.

Polymers are frequently added to increase the stability of an emulsion by thickening and adding yield value to the continuous phase. Such formulations where no interfacial activity is involved in the stabilizing action of the polymer are obtained when solely polyacrylic acid is used as a stabilizer. In the literature, such preparations are termed "quasi"-emulsions. Appropriate products on the market, often named "balm", usually consist of considerably small amounts of lipids dispersed in a hydrogel.

The physical stability and an adequate shelf life are obtained through finely dispersing the lipids. This measure and the yield value of the external phase reduce droplet mobility and thus effectively prevent creaming and coalescence of the oil droplets.
The ingredient list of an appropriate formulation is given in Table 2.

Table 2:
Qualitative Formula of a Lipid-containing Sun Protection Gel ("Pseudo emulsion")

Phase Ingredient (INCI - Name)


Lipid phase Octyl Methoxy-
UV B-filter
  Butyl Methoxy-
UV A-filter
  Carbomer Gel builder
Lipid component,
Silicone Oil
Water phase Aqua Solvent
  NaOH or
  Propylene glycole Moisturizing agent,

In the production the components of the lipid phase are melted and the acrylate dispersed. At about 60 °C the lipid and the aqueous phase are put together and then homogenized. After cooling to 30 °C fragrance is added and once again homogenized. The physical stability of such a product corresponds without any problem the requirements of the KosmV to be able to waive a declaration of shelf life. Typically, storage at 40°C for 6 months and at room temperature for a period of at least 30 months is possible without essential loss in quality.

If products with an acceptable shelf life and an increased lipid content have to be formulated a more effective stabilizer is required. This can be achieved by using surface-active polymers, e.g. carbomer 1342 or hydroxypropyl methylcellulose, as primary emulsifiers. Such polymers form structured interfacial films, which effectively prevent the coalescence of oil drops. In this case, the increase of the viscosity of the external phase plays only a minor role for the stabilizing action.
Carbomer 1342, a frequently used derivative of poly acrylic acid, displays such properties.

Carbomer 1342 polymeric emulsifiers are copolymers of acrylic acid, modified by C10-30-alkyl acrylates, and crosslinked with allylpentaerithrol. The hydrophilic acrylic acids portion dominates the lipophilic alkyl acrylate portion. The molecular weight of the resulting giant molecule is 4 x 109. The substance swells 1000-fold after neutralization with an appropriate base but it does not dissolve.

In aqueous media with low electrolyte concentration carbomer polymeric emulsifiers form thick protective gel layers around each oil droplet with the hydrophobic alkyl chains anchored in the oil phase. This allows to emulsify up to 20 per cent oil with typical usage levels of only 0.1 to 0.3 per cent of the polymeric emulsifier. Upon contact with the electrolyte containing surface of the skin such emulsions become unstable because the protective gel layer deswells instantly. The oil phase is released and a thin oil film deposits on the skin.

High performance homogenizers should be used carefully to avoid mechanical degradation of the high molecular weight polymeric emulsifiers which would then decrease emulsion stability. Such preparations often exhibit a mean droplet diameter between 20 and 50 µm. However, this has no negative impact on the physical stability. If for aesthetic reasons a finely dispersed system (1 - 5 µm) is desired, the addition of an amphiphilic co-emulsifier, e.g. sorbitan monooleate, is recommended.

The increase of the viscosity of the external phase plays only a minor role for the stabilizing action.

Contrary to those hydrolipid dispersions with carbomer 1342 polymeric emulsifiers, preparations with HPMC as polymeric emulsifier are less sensitive to electrolytes. Therefore o/w emulsions with a normal saline solution as the external phase are still stable on storage. When applied on the skin, the mechanical stress may cause a partial breakdown of these emulsions and a thin oil film spreads on the skin, thus reducing the wettability of the skin. After the water evaporates, the emulsion remains partially on the skin and forms a flexible film where oil droplets are embedded into a polymer matrix.

The preparation of HPMC stabilized emulsion can be performed using a rotor stator homogenizer, e.g. Ultra Turrax®, yielding a mean droplet size between 2 and 5 µm. Nanoemulsions with a mean diameter between 100 and 500 nm can be achieved by using high energy input from ultra sound or high-pressure homogenization (Fig. 9) The resulting emulsions are fluid and can be applied conveniently by spraying.

Figure 9: Transmission Electron Microscopy Pictures of Replica from Hydro-Dispersions Gels with Hypromellose

Preparation with
(A) Rotor-Stator-Homogenizor, (B) High-pressure Homogenizer

Enlarged version

Table 3: Example for a Hyproxypropyl Methylcellulose stabilized Hydro-dispersion Gel

Hydro-dispersion Gel

(% m/m)
Lipid phase Caprylic/Capric
Lipid component
  Triticum Vulgare

Lipid component
Water phase Aqua
Polymer Emulsifier

Another special feature of HPMC-stabilized emulsions is that they may be sterilized in an autoclave without substantial impact on their quality. This is due to the fact that they show a thermally reversible sol-gel transition (Fig. 10). Above 60° C, the external phase gels and immobilizes the dispersed oil drops. The droplets cannot collide and the rate of coalescence is almost negligible. Thus, formulators have the opportunity to formulate a preservative-free o/w emulsion, presumed that a recontamination proof packaging is used.

Figure 10: Thermally reversible Sol-gel Transition of Hyproxypropyl Methylcellulose Gel

(a) Room temperature (b) 70 °C

Enlarged version


Gels comprise interesting and multifarious product groups and the field of dermocosmetic products would be unthinkable without them. If you search "real" innovations you will be rather disappointed. But there are several new developments which enrich the market for the costumer too. These developments base on known systems including the latest scientific findings both in the field of physical chemistry and in the field of skin physiology.

Further readings

Daniels, R., Emulsionen, In: Raab, W. und Kindl, U., Pflegekosmetik, G. Fischer Verlag, Stuttgart; Govi Verlag, Frankfurt 1997.
Daniels, R., Neue Anwendungsformen bei Sonnenschutzmitteln, Apotheker Journal 19 (1997) 22 - 28.
Daniels, R., Polymerstabilisierte Submikron-Emulsionen als Arzneiträgersysteme, In: Müller, R.H. und Hildebrand, G.E. (Hrsg.) Pharmazeutische Technologie: Moderne Arzneiformen, 2. Aufl., Wissenschaftliche Verlagsgesellschaft, Stuttgart 1998.
Friberg S.E., Yang J., Emulsion stability, In: J. Sjöblom (Ed.), Emulsions and Emulsion Stability, Marcel Dekker, New York 1996.
IUPAC (International Union of Pure and Applied Chemistry), Division of Physical Chemistry, Manual of Symbols an Terminology for Physicochemical Quantities and Units, Appendix II part I, Butterworths, London 1972, S. 611
Klech C .M., Gels and Jellies, In: J. Swarbrick (Ed.), Encyclopedia of Pharmaceutical Technology, Vol. 6, Marcel Dekker, New York 1992.
Müller-Goyman C.C., Arzneimittel mit Flüssigkristallen, In: Müller, R.H. und Hildebrand, G.E. (Hrsg.) Pharmazeutische Technologie: Moderne Arzneiformen, 2. Aufl., Wissenschaftliche Verlagsgesellschaft, Stuttgart 1998.
Penreco, Product Bulletin Versagel® Hydrocarbon Gels, Dickinson (TX), 2000
Rimpler, S., Pharmazeutisch-technologische Charakterisierung von O/W-Emulsionen mit Methylhydroxypropylcellulose als Polymeremulgator, Dissertation Universität Regensburg, 1996
Schulz, M., Entwicklung tensidfreier Submikron-Emulsionen mit MHPC als Polymeremulgator, Dissertation Universität Regensburg, 1996.
Wittern K.P., Hautpflegemittel, In: Umbach W. (Ed.), Kosmetik, 2. Aufl.; Thieme, Stuttgart 1995.
Wollenweber C., Oschmann R., Daniels R., Hypromellose stabilisierte Emulsionen als Träger für homöopathische Urtinkturen. Pharm. Ind. 64 (2002) 81-88.



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