Abstract
The complex mechanical properties of skin have been the subject of much study in recent years. Several experimental methods developed to measure the mechanical properties of skin in vivo, such as suction or torsion, are unable to measure skin’s anisotropic characteristics. An experiment characterising the mechanical properties of in vivo human skin using a novel force-sensitive micro-robot is presented. The micro-robot applied in-plane deformations to the anterior forearm and the posterior upper arm. The behaviour of the skin in each area is highly nonlinear, anisotropic, and viscoelastic. The response of the upper arm skin is very dependent on the orientation of the arm. A finite element model consisting of an Ogden strain energy function and quasi-linear viscoelasticity was developed to simulate the experiments. An orthogonal initial stress field, representing the in vivo skin tension, was used as an additional model parameter. The model simulated the experiments accurately with an error-of-fit of 17.5% for the anterior lower forearm area, 6.5% for the anterior upper forearm and 9.3% for the posterior upper arm. The maximum in vivo tension in each area determined by the model was 6.2 Nm−1 in the anterior lower forearm, 11.4 Nm−1 in anterior upper forearm and 5.6 Nm−1 in the posterior upper arm. The results also show that a finite element model with a neo-Hookean strain energy function cannot simulate the experiments with the same accuracy.
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Abbreviations
- F A , F B , F C :
-
Forces measured at each force transducer
- R X , R Y , R Z :
-
Reaction force components at probe tip
- w :
-
Distance from centroid to apex of triangle formed by the transducers
- h :
-
Perpendicular distance between probe tip and base of rigid frame
- W :
-
Strain energy
- μ, α:
-
Ogden function material parameters
- λ1, λ2, λ3 :
-
Principal stretches
- p :
-
Lagrange multiplier representing hydrostatic pressure
- J :
-
Volume ratio
- C 10 :
-
Neo-Hookean function material parameter
- T e :
-
Elastic stress component
- g R :
-
Reduced relaxation function
- \({\bar{{g}}_1^{\rm P} ,\tau_1^{\rm G}}\) :
-
Prony series parameters
- σ X , σ Y :
-
Initial pre-stress in X and Y directions
- \({R_i^{model}}\) :
-
Reaction force at probe tip calculated from model
- \({R_i^{\rm exp}}\) :
-
Reaction force at probe tip measured in experiment
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Flynn, C., Taberner, A. & Nielsen, P. Mechanical characterisation of in vivo human skin using a 3D force-sensitive micro-robot and finite element analysis. Biomech Model Mechanobiol 10, 27–38 (2011). https://doi.org/10.1007/s10237-010-0216-8
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DOI: https://doi.org/10.1007/s10237-010-0216-8