The Mechanical Stress Caused by Micro-Projection Arrays Penetrating the Skin for Vaccine Delivery

Abstract

The Nanopatch is a silicon array of micro-projections for epidermal and dermal delivery of vaccines, resulting in enhanced immunogenicity in comparison to intramuscular injection. Achieving this requires the fracture of skin superficial barriers and penetration to the targeted depth, reliant upon negotiating the complex non-linear elastic and failure properties of skin: a multilayer composite “biomaterial”. In this work, computational models of projection-skin mechanical interaction are developed and applied to investigate the mechanical stress generated to fracture skin. Our analytical results on a homogenous linear-elastic skin model suggest that the array projections exert an uneven force distribution on the skin surface, leading to a non-homogeneous stress across the loaded skin region. In addition, the creation of high localised tensile stress is sensitive to a precise trade-off between projection spacing and tip diameter. Numerical simulations are further performed using a layered hyper-elastic skin representation and compared with the analytical findings. The resulting deformation and stresses are significantly increased due to, respectively, the compliant top skin layers and their non-linear elastic properties. This underlines the importance of accounting for the stratified structure of the skin as well as the strain-hardening properties of its strata when assessing the achievement of failure criteria.