Viscoelastic Characterization of the Primate Finger Pad In Vivo by Microstep Indentation and Three-Dimensional Finite Element Models for Tactile Sensation Studies

Abstract

When we touch an object, surface loads imposed on the skin are transmitted to thousands of specialized nerve endings (mechanoreceptors) embedded within the skin. These mechanoreceptors transduce the mechanical signals imposed on them into a neural code of the incident stimuli, enabling us to feel the object. To understand the mechanisms of tactile sensation, it is critical to understand the relationship between the applied surface loads, mechanical state at the mechanoreceptor locations, and transduced neural codes. In this paper, we characterize the bulk viscoelastic properties of the primate finger pad and show its relationship to the dynamic firing rate of SA-1 mechanoreceptors. Two threedimensional (3D) finite element viscoelastic models, a homogeneous and a multilayer model, of the primate fingertip are developed and calibrated with data from a series of force responses to micro-indentation experiments on primate finger pads. We test these models for validation by simulating indentation with a line load and comparing surface deflection with data in the literature (Srinivasan, 1989, "Surface Deflection of Primate Fingertip Under Line Load," J. Biomech., 22(4), pp. 343-349). We show that a multilayer model with an elastic epidermis and viscoelastic core predicts both the spatial and temporal biomechanical response of the primate finger pad. Finally, to show the utility of the model, ramp and hold indentation with a flat plate is simulated. The multilayer model predicts the strain energy density at a mechanoreceptor location would decay at the same rate as the average dynamic firing rate of SA-1 mechanoreceptors in response to flat plate indentation (previously observed by Srinivasan and LaMotte, 1991 "Encoding of Shape in the Responses of Cutaneous Mechanoreceptors," Information Processing in the Somatosensory System (Wenner-Gren International Symposium Series), O. Franzen and J. Westman, eds., Macmillan Press, London, UK), suggesting that the rate of adaptation of SA-1 mechanoreceptors is governed by the viscoelastic nature of its surrounding tissue.

Haptics model for human fingertips based on gaussian distribution

“…Three-dimensional (3D) models were also proposed under a flat load, a sharp wedge [11] and a line load [22]. For example, a 3D FE model was developed to predict the temporal force response of fingertip under both a line load and a cylinder load surface deflection [6].…”

“…A comprehensive understanding of human fingertip biomechanics is essential prior to full exploitation of its most potential in both clinical and industrial environment. Most existing studies concerning neural reaction properties under the fingertip skin only conduct experiments under a small indentation depth of 0~0.2 mm to 0.8~2.5 mm [6,7], while large deformations that may occur in the context of hand prostheses were not taken into account. Therefore, this study aims at achieving a quantitative characterisation of human fingertip's mechanical behaviour for the potential application of tactile rehabilita-tion in hand prostheses.…”

A model of human skin under large amplitude oscillatory shear

“…Since it is impossible to capture the full mechanical response of human skin in one single experiment, different types of experiments have been performed to partially capture this response. Most studies involved experiments on in vivo human skin, comprising uniaxial tensile (Mahmud et al, 2012;Delalleau et al, 2008b;Gambarotta et al, 2005), multiaxial tensile (Flynn et al, 2011(Flynn et al, , 2011(Flynn et al, , 2013Khatyr et al, 2004;Kvistedal and Nielsen, 2009), suction (Khatyr et al, 2006;Delalleau et al, 2008aDelalleau et al, , 2011Delalleau et al, , 2012Hendriks et al, 2003) and indentation (Groves et al, 2012;Delalleau et al, 2006;Kumar et al, 2015) experiments. Although it is preferable to measure skin in its natural environment, in vivo measurements have limitations for several reasons.…”

“…Other constitutive models described the mechanical behavior as linear elastic (Delalleau et al, 2006), nonlinear (hyper)elastic using Ogden (Groves et al, 2012(Groves et al, , 2013Jor et al, 2011;Li and Xiaoyu, 2016;Evans, 2009;Flynn et al, 2011Flynn et al, , 2011Karimi et al, 2015;Mahmud et al, 2012;Lapeer et al, 2010;Limbert, 2011), Mooney-Rivlin (Flynn et al, 2013;Hendriks et al, 2003), (extended) Neo-Hookean (Delalleau et al, 2008a(Delalleau et al, , 2011(Delalleau et al, , 2012, Kalman filters (Delalleau et al, 2008b), Tong-Fung (Kvistedal and Nielsen, 2009;Gambarotta et al, 2005) or viscoelastic (Lokshin and Lanir, 2009a;Khatyr et al, 2004;Holt et al, 2008;Bischoff, 2006;Karimi et al, 2016;Kumar et al, 2015). The necessity to capture a certain level of mechanical complexity with a constitutive model is dependent on the application.…”

Computationally Modeling the Impact of Measured Variance in Skin Mechanics on Neural Responses to Touch Stimuli

“…One might think to control force and then divide by the area of the tip to get "nominal pressure" and indeed this surface quantity, which Ge and Khalsa referred to as compressive stress, better correlates with firing rate than displacement or force [75]. [9][10][11]27,[77][78][79]. However, maximum compressive stress is invariant to rotations, unlike others such as maximum horizontal or vertical stress, which were excluded from analysis following past studies [27].…”