Water is a vital component regulating the properties of topical formulations and their interaction with biological barriers, such as skin and mucosa. Changing the watercontent within the frame of the pharmaceutical triangle will have a huge impact on which type of formulation, such as a cream, ointment, gel, or lotion, is formed, as well as the physical properties of the formulation. The composition of a formulation, and the subsequent reformulation after application, will govern the features of the residual film. This will in turn affect the barrier properties of the underlying tissue and consequently the penetration of various substances across skin or mucosa.The primary aim of this thesis has been to provide further understanding on differences between traditional surfactant-based formulations and particle-stabilized, Pickering, formulations and how specific excipients, like alcohols, emollients, and thickeners can affect their physical and/or sensorial properties. The secondary aim has been to gain more knowledge on the role of water in topical formulations and how it affects the properties of the underlaying tissue on application.

By combining a portfolio of physicochemical techniques combined with sensory science, we have been able to identify differences between Pickering and surfactantstabilized formulations. Starch-based Pickering emulsions were perceived as less greasy and sticky than traditional creams, even at high oil content. Moreover, we were able develop a novel type of alcohol-based Pickering emulsion with combined moisturizing and antiseptic properties. We have also been able to link sensory attributes, evaluated by human volunteers, with physicochemical characterizations. Furthermore, the in vitro ForceBoard™ method was developed further and we evaluated its potential to be used as an ex vivo method using excised skin. In addition, we have shown that that the water gradient over a biological barrier has a general relevance with respect to drug absorption and should be considered not only in dermaldrug delivery but also for buccal and nasal drug delivery.

Tactile perception can be investigated through ex vivo friction measurements using a so-called ForceBoard™, providing objective assessments and savings in time and money, compared to a subjective human panel. In this work we aim to compare excised skin versus VitroSkin® as model substrates for tactile friction measurements. A further aim is to detect possible differences between traditional surfactant-based creams, and a particle-stabilized (Pickering) cream and investigate how the different substrates affect the results obtained. It was found that the difference in tactile friction between excised skin and VitroSkin® was small on untreated substrates. When topical creams were applied, the same trends were observed for both substrates, although the frictional variation over time relates to the difference in surface structure between the two substrates. The results also confirmed that there is a difference between starch-based Pickering formulations and surfactant-based creams after application, indicating that the latter is greasier than Pickering cream. It was also shown that the tactile friction of Pickering emulsions was consistently high even with high amounts of oil, indicating a non-greasy, and non-sticky formulation. The characteristics of starch-stabilized Pickering formulations make them promising candidates in the development of surfactant-free topical formulations with unique tactile properties.

Rising consumer demands for safer, more natural, and sustainable topical products have led to increased interest in finding alternative excipients, while retaining functionality and cosmetic appeal. Particle-stabilized Pickering creams have emerged as possible alternatives to replace traditional surfactant-stabilized creams and are thus one of the focuses in this study. The aim of this paper was to study relationships between sensorial characteristics and physical properties to understand how different excipients affect these aspects, comparing one starch particle–stabilized and three surfactant-stabilized formulations. A human panel was used to evaluate sensorial perception, while physical properties were deduced by rheology and tactile friction, together with in vivo and ex vivo skin hydration measurements.

The results show that sensorial attributes related to the application phase can be predicted with rheology, while afterfeel attributes can be predicted with tactile friction studies. Differences in rheological and sensory properties among surfactant-based creams could mainly be attributed to the type of emollients used, presence of thickeners and surfactant composition. Differences between surfactant-based creams and a Pickering cream were more evident in relation to the afterfeel perception. Presence of starch particles in the residual film on skin results in high tactile friction and low perception of residual coating, stickiness, greasiness, and slipperiness in sensorial afterfeel.

Formulations for buccal drug delivery often comprise polymers to facilitate mucoadhesion based on water sorption. The main objective of the current study was therefore to evaluate the effect of dehydration on drug uptake through oral mucosa. We have used diffusion cells with excised porcine mucosa to study uptake of three alternative drugs (i.e., Metronidazole, Benzydamine and Xylometazoline) together with polyethylene glycol (PEG) as the model polymer for adjusting water activity in the test solutions. Taking drug activity into account, we can conclude that addition of PEG results in a drug flux through mucosa that is about two times lower for Metronidazole and more than 40 times lower for Xylometazoline compared to that from a pure PBS-solution. However, for Benzydamine the uptake through mucosa was more or less the same, which could possibly be due to the high PEG-concentration (65 wt%) affecting the dissociation constant and thus the permeability. These results indicate that an increased water gradient may have the same limiting effect on permeability through oral mucosa as previously seen for skin. Thus, water gradient effects should be a factor to consider when developing buccal adhesive formulations.

Formulations for nasal drug delivery often rely on water sorption to adhere to the mucosa, which also causes a higher water gradient over the tissue and subsequent dehydration. The primary aim of this study was therefore to evaluate mucosal response to dehydration and resolve the hypothesis that mucoadhesion achieved through water sorption could also be a constraint for drug absorption via the nasal route. The effect of altering water activity of the vehicle on Xylometazoline HCl and Cr-EDTA uptake was studied separately using flow through diffusion cells and excised porcine mucosa. We have shown that a modest increase in the water gradient over mucosa induces a substantial decrease in drug uptake for both Xylometazoline HCl and Cr-EDTA. A similar result was obtained when comparing two different vehicles on the market; Nasoferm (Nordic Drugs, Sweden) and BLOX4 (Bioglan, Sweden). Mucoadhesion based on water sorption can slow down drug uptake in the nasal cavity. However, a clinical study is required to determine whether prolonged duration of the vehicle or preventing dehydration of the mucosa is the most important factor for improving bioavailability.

A novel technology based on using particles of biological origin was recently developed for stabilising emulsions. This technology, that utilises Pickering emulsions, was further employed to encapsulate different types of active substances. Advance development and refinement of the technology has been performed in order to suit different encapsulation purposes in areas within personal care, food and pharmaceuticals. Some areas where this technology has been tested include taste masking in food systems, encapsulation and protection of a cosmetic active, production of oil filled powder encapsulates, and on-skin performance.