title: Multidirectional In Vivo Characterization of Skin Using Wiener Nonlinear Stochastic System Identification Techniques.
abstract: A triaxial force-sensitive microrobot was developed to dynamically perturb skin in multiple deformation modes, in vivo. Wiener static nonlinear identification was used to extract the linear dynamics and static nonlinearity of the force-displacement behavior of skin. Stochastic input forces were applied to the volar forearm and thenar eminence of the hand, producing probe tip perturbations in indentation and tangential extension. Wiener static nonlinear approaches reproduced the resulting displacements with variances accounted for (VAF) ranging 94-97%, indicating a good fit to the data. These approaches provided VAF improvements of 0.1-3.4% over linear models. Thenar eminence stiffness measures were approximately twice those measured on the forearm. Damping was shown to be significantly higher on the palm, whereas the perturbed mass typically was lower. Coefficients of variation (CVs) for nonlinear parameters were assessed within and across individuals. Individual CVs ranged from 2% to 11% for indentation and from 2% to 19% for extension. Stochastic perturbations with incrementally increasing mean amplitudes were applied to the same test areas. Differences between full-scale and incremental reduced-scale perturbations were investigated. Different incremental preloading schemes were investigated. However, no significant difference in parameters was found between different incremental preloading schemes. Incremental schemes provided depth-dependent estimates of stiffness and damping, ranging from 300 N/m and 2 Ns/m, respectively, at the surface to 5 kN/m and 50 Ns/m at greater depths. The device and techniques used in this research have potential applications in areas, such as evaluating skincare products, assessing skin hydration, or analyzing wound healing.
authors.affiliation: Auckland Bioengineering Institute, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand e-mail: mpar145@aucklanduni.ac.nz.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Parker
authors.affiliation: BioInstrumentation Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 e-mail: ljones@mit.edu.
authors.forename: Lynette A
authors.initials: LA
authors.lastname: Jones
authors.affiliation: BioInstrumentation Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 e-mail: ihunter@mit.edu.
authors.forename: Ian W
authors.initials: IW
authors.lastname: Hunter
authors.affiliation: Department of Engineering Science, Auckland Bioengineering Institute, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand e-mail: a.taberner@auckland.ac.nz.
authors.forename: A J
authors.initials: AJ
authors.lastname: Taberner
authors.affiliation: Department of Engineering Science, Auckland Bioengineering Institute, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand e-mail: martyn.nash@auckland.ac.nz.
authors.forename: M P
authors.initials: MP
authors.lastname: Nash
authors.affiliation: Department of Engineering Science, Auckland Bioengineering Institute, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand e-mail: p.nielsen@auckland.ac.nz.
authors.forename: P M F
authors.initials: PM
authors.lastname: Nielsen
mesh_terms: D016880:Anisotropy; D003198:Computer Simulation; D006244:Hardness / Q000502:physiology*; D006245:Hardness Tests / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D008954:Models, Biological; D015233:Models, Statistical; D017711:Nonlinear Dynamics; D010812:Physical Stimulation / Q000295:instrumentation / Q000379:methods*; D015203:Reproducibility of Results; D012371:Robotics / Q000295:instrumentation / Q000379:methods; D012680:Sensitivity and Specificity; D012879:Skin Physiological Phenomena; D013269:Stochastic Processes; D013314:Stress, Mechanical; D014783:Viscosity