title: Characterising the material properties at the interface between skin and a skin vaccination microprojection device.
abstract: The rapid emergence of micro-devices for biomedical applications over the past two decades has introduced new challenges for the materials used in the devices. Devices like microneedles and the Nanopatch, require sufficient strength to puncture skin often with sharp-slender micro-scale profiles, while maintaining mechanical integrity. For these technologies we sought to address two important questions: 1) On the scale at which the device operates, what forces are required to puncture the skin? And 2) What loads can the projections/microneedles withstand prior to failure. First, we used custom fabricated nanoindentation micro-probes to puncture skin at the micrometre scale, and show that puncture forces are ∼0.25-1.75mN for fresh mouse skin, in agreement with finite element simulations for our device. Then, we used two methods to perform strength tests of Nanopatch projections with varied aspect ratios. The first method used a nanoindenter to apply a force directly on the top or on the side of individual silicon projections (110μm in length, 10μm base radius), to measure the force of fracture. Our second method used an Instron to fracture full rows of projections and characterise a range of projection designs (with the method verified against previous nanoindentation experiments). Finally, we used Cryo-Scanning Electron Microscopy to visualise projections in situ in the skin to confirm the behaviour we quantified, qualitatively.
  
  
  
  Micro-device development has proliferated in the past decade, including devices that interact with tissues for biomedical outcomes. The field of microneedles for vaccine delivery to skin has opened new material challenges both in understanding tissue material properties and device material. In this work we characterise both the biomaterial properties of skin and the material properties of our microprojection vaccine delivery device. This study directly measures the micro-scale puncture properties of skin, whilst demonstrating clearly how these relate to device design. This will be of strong interest to those in the field of biomedical microdevices. This includes work in the field of wearable and semi-implantable devices, which will require clear understanding of tissue behaviour and material characterisation.
authors.affiliation: The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, QLD 4072, Australia; ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, The University of Queensland, Australia.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Crichton
authors.affiliation: The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, QLD 4072, Australia.
authors.forename: Cameron
authors.initials: C
authors.lastname: Archer-Jones
authors.affiliation: The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, QLD 4072, Australia.
authors.forename: Stefano
authors.initials: S
authors.lastname: Meliga
authors.affiliation: The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, QLD 4072, Australia.
authors.forename: Grant
authors.initials: G
authors.lastname: Edwards
authors.affiliation: The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, QLD 4072, Australia.
authors.forename: Darren
authors.initials: D
authors.lastname: Martin
authors.affiliation: The University of Queensland, School of Mechanical and Mining Engineering, QLD 4072, Australia.
authors.forename: Han
authors.initials: H
authors.lastname: Huang
authors.affiliation: The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, QLD 4072, Australia; ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, The University of Queensland, Australia; The University of Queensland, Diamantina Institute, Princess Alexandra Hospital, QLD 4102, Australia; The University of Queensland, Faculty of Medicine and Biomedical Sciences, Royal Brisbane and Women's Hospital, Herston, Queensland 4006, Australia. Electronic address: m.kendall@uq.edu.au.
authors.forename: Mark A F
authors.initials: MA
authors.lastname: Kendall
mesh_terms: D000818:Animals; D001672:Biocompatible Materials; D020285:Cryoelectron Microscopy; D008422:Materials Testing / Q000295:instrumentation* / Q000379:methods*; D008810:Mice, Inbred C57BL; D008845:Microinjections / Q000295:instrumentation*; D049329:Nanostructures / Q000737:chemistry; D011677:Punctures; D012825:Silicon / Q000737:chemistry; D012867:Skin / Q000276:immunology* / Q000648:ultrastructure; D014611:Vaccination / Q000295:instrumentation*
chemical_list: D001672:Biocompatible Materials; D012825:Silicon
keywords: Material strength; Material testing; Microneedles; Puncture; Skin