Emulsion stability of cryogelated PVA-emulsion gel patches with controlled drug delivery

Emulsion gels represent a recent innovation in the domain of topical drug delivery systems. These emulsions are combined with an external gelling agent, thus offering numerous options for formulation and optimization. Emulsion gels facilitate the delivery of lipophilic drugs, in addition to enhanced physicochemical stability by the gel matrix.

In comparison to conventional semisolid and spreadable formulations, crosslinked emulsion gels have the potential as new long-term therapeutic patches for dermal application. Poly(vinyl alcohol) (PVA) has been found to be a versatile option, capable of functioning as an emulsifier and a gelling agent via cryogelation, while also enabling the reduction of formulated components to a minimum.

The present study investigates the role of the various oil phases within the emulsion in the context of emulsion stability with regard to freeze-thaw-cycling, emulsiongel formation as well as the ability to establish a controlled drug delivery system. An emulsion was prepared with a 5 % (w/w) solution of PVA 8-88 and medium-chain triglyceride (MCT), castor oil (CO) or rapeseed oil (RO) as the oil phase. Lidocaine was dissolved in the oils (c = 1 %) as a lipophilic active ingredient. To form the outer gel, the emulsion was combined with a 10 % (w/w) solution of PVA 56-98, generating an total oil content of 10 – 50 % (w/w). The resultant emulsion was then subjected to five freeze-thaw cycles (-20 °C to 20 °C) with conditions for cryogel production optimised in earlier studies. The obtained patches were characterised by oiling off measurement, tensile testing as well as in vitro release and permeation studies using Franz diffusion cells. Furthermore, the droplet sizes were measured by laser diffractometry after thermal de-crosslinking and Raman spectroscopy within the cross-linked emulsion gel patches.

These results provide important insights into the oil loading capacity of the hydrogel matrix during freeze-thaw-cycling. In principle, all the oil phases analysed demonstrate a decrease in Young's modulus with an increase in the patch oil content. Furthermore, an optimal oil loading capacity of 30 % (w/w) was determined. In addition, an examination of the droplet sizes within the patches with varying oil content reveals no substantial increase in the size distribution after freeze-thaw-cycling. With regard to the drug delivery capabilities, a controlled release and permeation rate can be observed using different oil phases. The release rate shows an increase depending on the used oil in the order of CO < MCT < RO. 

In summary, PVA-based emulsion gel patches offer a new promising alternative for dermal patches, characterized by their high mechanical strength and ability to control the delivery of a lipophilic drug.