Preparation and validation of a skin model for the evaluation of intradermal powder injection devices.

A very promising novel needle-free application method is epidermal powder immunisation, a method delivering particulate vaccines into the viable epidermis of human skin where a dense network of immunocompetent cells resides. These antigen-presenting cells (Langerhans cells) are able to recognise antigens, process them and present them to naïve T-cells and induce effective immune responses. Powder injection devices are being developed, and their evaluation is essential before applying them on live animals and individuals. An appropriate skin model will accelerate the development of such injection devices. Different films made from gelatin, silicon and agar were prepared and investigated as skin model candidates for the evaluation of powder injection devices. The mechanical properties of the skin model candidates were measured with an indentation method using a texture analyser, and the results were compared to the properties of human skin and pig skin. The indentation behaviour of the model films and the biological skin samples suggest that gelatin films plasticised with glycerol are very well suitable for a skin model. The mechanical properties of gelatin based films can be tailored by changing the glycerol content in the film making it even possible to simulate human skin with different mechanical properties as the mechanical properties depend on the individual, age, sex and site of injection. The stability of the gelatin films was also investigated under long-term storage. In addition, confocal laser scanning microscopy was used as a novel tool to determine the depths and size of fluorescently labelled particles in the gelatin model.