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O/W creams with crystalline gel structures are supposed to be four phase systems. Their dominant structural elements are the hydrophilic and lipophilic gel phases. The hydrophilic gel phases have - depending on the hydrophilic character of their polar groups - a very strong swelling capacity. Further, a dynamic equilibrium is maintained between the water interlamellarly inserted into the hydrophilic gel phase and the bulk water phase. The latter is mainly fixed mechanically by the hydrophilic gel phase. The lipophilic gel phases, however, which immobilize the dispersed phase, have no or only an extremely weak swelling capacity. By means of thermogravimetry (TG) a quantitative differentiation between water interlamellarly fixed in the hydrophilic gel phase and bulk water is possible. Using dynamic TG, the curves of water release and the results obtained are discussed. In the water-containing hydrophilic ointment DAB 8 and in the non-ionic hydrophilic ointment DAC the fractions of interlamellarly fixed water are substantially higher than those of the bulk water. In the investigated stearate creams, however, the fraction of the bulk water is much higher than the fraction of interlamellarly fixed water. Consequently the water release from these stearate creams is higher than that of the other O/W creams mentioned above. It is concluded that the ratio of interlamellarly fixed water and bulk water is an important criterion for the properties of such O/W creams and that TG can be suitably applied not only as in process control during product manufacturing but also as a useful tool in the development of O/W creams with a controlled water release.

O/W creams with crystalline gel structures (Water Containing Hydrophilic Ointment DAB 8, Non-ionic Hydrophilic Ointment DAC and a stearate cream) were investigated by means of small angle X-ray diffraction, differential scanning calorimetry, thermogravimetry and freeze fracture technique in combination with Transmission Electron Microscopy (TEM). It was found that these bases are building up two different colloidal gel structures, a hydrophilic gel phase and a lipophilic gel phase, each of them forming a coherent network. The hydrophilic gel phase is characterized by an interlamellar water layer; this interlamellarly fixed water is in a dynamic equilibrium with the bulk water phase. The lipophilic gel phase immobilizes mainly the inner (dispersed) phase. So the O/W creams presented here are supposed to be four-phase systems. The ratio of interlamellarly fixed water and bulk water, which may be determined easily by means of thermogravimetry, is found to be an important quality criterion for these O/W creams. The desired properties of water release of such a cream may be chosen by shifting the ratio of interlamellarly fixed water and bulk water.

The freezing and melting behavior of water in poly hydroxy ethyl methacrylate (pHEMA) hydrogels of different cross-linker and water contents was investigated in relation to the glass transition temperature (Tg) of the gels. After prolonged cooling at -15 degrees C a constant amount of 1.7 mol water per monomeric unit did not freeze, regardless of both the cross-linker and the water content of the gels. At this water content and temperature, pHEMA gels were below their Tg, and the water molecules were prevented from diffusing to the ice crystals formed in the gel. Therefore, the inability of part of the water in pHEMA gels to freeze is not a thermodynamic phenomenon but is caused by kinetic factors.

Hydrogels composed of poly(hydroxyethyl methacrylate) (pHEMA) and water were investigated using differential thermal analysis (DTA) and adiabatic calorimetry (AC). The results show that the crystallization of water in the gels is a very gradual process, leading to the development of a metastable, non-equilibrium state. They are not in agreement with models that assume the actual presence of thermodynamically different classes of water in the gels, based on the abnormal melting behaviour of this water. The results indicate that the internal structure of these gels can be described as that of an elastic solution, in which the water molecules are distributed continuously over all possible orientations to and interactions with the polymer.

Within the Center for Bio-Pharmaceutical Sciences the release characteristics of drugs form the major research object of the Division of Pharmaceutical Technology. Transdermal systems are being developed that can supply a drug during several days. Irritation of the skin may be avoided by using hydrogels. To enable long-term transdermal application also the colloidal structure of creams and ointments is investigated. As most drugs are to be taken orally, however, a research project was also started to make cheap and easy to produce controlled release tablets. The results with a microporous polypropylene polymer are promising.