Elastic modulus and viscoelastic properties of full thickness skin characterised at micro scales

The recent emergence of micro-devices for vaccine delivery into upper layers of the skin holds potential for increased immune responses using physical means to target abundant immune cell populations. A challenge in doing this has been a limited understanding of the skin elastic properties at the micro scale (i.e. on the order of a cell diameter; similar to 10 mu m). Here, we quantify skin's elastic properties at a micro-scale by fabricating customised probes of scales from sub- to super-cellular (0.5 mu m-20 mu m radius). We then probe full thickness skin; first with force-relaxation experiments and subsequently by elastic indentations. We find that skin's viscoelastic response is scale-independent: consistently a similar to 40% decrease in normalised force over the first second, followed by further 10% reduction over 10 s. Using Prony series and Hertzian contact analyses, we determined the strain-rate independent elastic moduli of the skin. A high scale dependency was found: the smallest probe encountered the highest elastic modulus (similar to 30 MPa), whereas the 20 mu m radius probe was lowest (below 1 MPa). We propose that this may be a result of the load distribution in skin facilitated by the hard corneocytes in the outermost skin layers, and softer living cell layers below. (C) 2012 Elsevier Ltd. All rights reserved.