Quantifying the mechanical properties of human skin to optimise future microneedle device design

Abstract

Microneedle devices are a promising minimally invasive means of delivering drugs/vaccines across or into the skin. However, there is currently a diversity of microneedle designs and application methods that have, primarily, been intuitively developed by the research community. To enable the rational design of optimised microneedle devices, a greater understanding of human skin biomechanics under small deformations is required. This study aims to develop a representative stratified model of human skin, informed by in vivo data. A multilayer finite element model incorporating the epidermis, dermis and hypodermis was established. This was correlated with a series of in-vivo indentation measurements, and the Ogden material coefficients were optimised using a material parameter extraction algorithm. The finite element simulation was subsequently used to model microneedle application to human skin before penetration and was validated by comparing these predictions with the in-vivo measurements. Our model has provided an excellent tool to predict micron-scale human skin deformation in vivo and is currently being used to inform optimised microneedle designs.

Experimental study of needle–tissue interaction forces: Effect of needle geometries, insertion methods and tissue characteristics

“…Influence factors presented in this study can be considered and added to construct more accurate needle-tissue interaction force models in future. The research on needle geometries can assist to optimize needle design (Groves et al 2012) by using suitable diameter, needle tip, and bevel angle (see Table 3). The study of insertion methods is beneficial for robotically steering needles by means of choosing appropriate insertion velocity and operation strategy.…”

Microneedle characterisation: the need for universal acceptance criteria and GMP specifications when moving towards commercialisation

“…An issue with numerical simulations are the inherent assumptions and difficulties: the skin is a nonlinear material and so does not exhibit ideally elastic behaviour; some models ignore the skin deformation before piercing; changing boundary conditions as contact between microneedle and skin change over time; neglect of the effect of underlying tissues such as muscle and bone and skin layer thicknesses assumed to be identical for all subjects [78,79]. These assumptions and difficulties can and do lead to under or overpredictions and as a result can be unreliable.…”

“…Groves et al [79] undertook computer simulation analysis and then used experiemtnal data to refine the computer algorithms in order to eliminate as much error as possible. They completed a similar study to Chen et al [78] using the same model for simulation in order to predict skin deformation and enable the rational design of optimised MN devices.…”

On the Importance of 3D, Geometrically Accurate, and Subject-Specific Finite Element Analysis for Evaluation of in-Vivo Soft Tissue Loads

“…elsewhere [5,6,17,18]. The final FEA meshes consisted of approximately 100000 solid tetrahedral elements and 7000 shell elements for the skin layer.…”