Jonathan CJ Wei, Michael L Crichton and Mark AF Kendall*


Australian Institute for Bioengineering and Nanotechnology,
The University of Queensland, ST LUCIA QLD 4072, AUSTRALIA


Micro-scale medical devices have the potential to deliver vaccines more effectively than traditional needle and syringe administrations [1]. This class of technology are also being developed to provide new ways of diagnosing diseases; healthcare monitoring and measuring sporting performance etc. Focusing on drug delivery, our Nanopatch (Figure 1) is one of these examples that can deliver vaccines accurately into the cellular environment of the skin, where an abundance of immune cells resides. Unfortunately, such devices are often developed from animal models (e.g. mice), which are drastically different from humans. To facilitate the translation of devices, and therefore deliver vaccines more effectively, we proposed to understand the relationships in the anatomical and mechanical properties different species. This study will help facilitate the adjustment of those devices towards humans more effectively and more rapidly.

Figure 1: (a) 4x4 mm2 square Nanopatches (b) close-up of Nanopatch projections and (c) fractured cross-section showing Nanopatch projections inserted into ex-vivo human skin using Cryo-SEM. (d) Skin elastic moduli of species showing scale effects with indentation tip radius.

We quantified the skin layer thicknesses of mouse, rat, rabbit, pig and human skin with histology sectioning to study species of scales spanning from ~30 g to ~70 kg. Then, we measured the skin’s viscoelasticity and elasticity using indentation. The Ogden model was used to determine the elastic modulus from this data, while a two term Prony series characterised the viscoelastic properties with a load-relaxation experiment. We observed increase in skin layer thicknesses with species mass, and both viscoelastic/elastic properties of skin were mainly influenced by individual skin layers. We also observed an inverse-log relationship of elastic modulus and indenter tip size (Figure 1 (d)), which we hypothesised was due to cell sizes and stress distribution through the SC keratinocytes. Surprisingly, human skin showed significantly lower elastic moduli compared with other species. We developed a simple model, which suggested layer thicknesses/ratios affected the skin’s bulk elastic modulus. This also enabled the estimation of the skin’s Young’s modulus if the species mass was known. Finally, we will detail recommendations in the presentation on how this data can be used to optimise translational activities of medical devices. In conclusion, our study has shown the importance of scale and species on the mechanical studies of novel micro-medical devices. The relationships derived from this should facilitate the progression of these technologies from small to large animal models and humans.


This work is supported by the Australian Government Research Training Program Scholarship.

[1] Fernando, GJP et al. (2016), J Control. Release, 237:25-41.


Biographic Details: Mr Jonathan Wei, The University of Queensland, Australia

Phone: +61 7 3346 4193 Fax: +61 7 3346 4197 E-mail:

Research interests: soft tissues, biomechanics, mechanical characterisation, medical devices