Biomechanics of the finger pad in response to torsion

Abstract

Surface skin deformation of the finger pad during partial slippage at finger–object interfaces elicits firing of the tactile sensory afferents. A torque around the contact normal is often present during object manipulation, which can cause partial rotational slippage. Until now, studies of surface skin deformation have used stimuli sliding rectilinearly and tangentially to the skin. Here, we study surface skin dynamics under pure torsion of the right index finger of seven adult participants (four males). A custom robotic platform stimulated the finger pad with a flat clean glass surface, controlling the normal forces and rotation speeds applied while monitoring the contact interface using optical imaging. We tested normal forces between 0.5 N and 10 N at a fixed angular velocity of 20° s −1 and angular velocities between 5° s −1 and 100° s −1 at a fixed normal force of 2 N. We observe the characteristic pattern by which partial slips develop, starting at the periphery of the contact and propagating towards its centre, and the resulting surface strains. The 20-fold range of normal forces and angular velocities used highlights the effect of those parameters on the resulting torque and skin strains. Increasing normal force increases the contact area, the generated torque, strains and the twist angle required to reach full slip. On the other hand, increasing angular velocity causes more loss of contact at the periphery and higher strain rates (although it has no impact on resulting strains after the full rotation). We also discuss the surprisingly large inter-individual variability in skin biomechanics, notably observed in the twist angle the stimulus needs to rotate before reaching full slip.

Open-source instrumented object to study dexterous object manipulation

“…However, we observed significant differences in the behavior of the skin across participants, with some participants presenting large skin deformations and others presenting very small deformations. Similar variability between participant finger pad skin behavior has been observed in passive studies (du Bois de Dunilac et al 2022, Wang and Hayward 2007). du Bois de Dunilac et al 2022 observed that the skin deformation rate varied significantly across participants, leading to significant differences in the timing of strains during the rotational load.…”

“…1E). The complete details of the strain computation can be found in Delhaye et al 2016 and du Bois de Dunilac et al 2022.…”

“…Indeed, imaging of the skin has shown that when the glass plate was displaced relative to the finger, the skin started slipping at the periphery of the contact, with the slip propagating inwards until all skin was slipping (André et al 2011, Delhaye et al 2014, Tada and Kanade 2004, Levesque and Hayward 2003). Analysis of the finger pad images allowed the measurement of how the different parts of the skin deform under these tangential and rotational loads (Delhaye et al 2016, du Bois de Dunilac et al 2022). These images highlighted patterns of compression and dilation at the edge of the propagating slip wavefront.…”

Role of arm reaching movement kinematics in friction perception at initial contact with smooth surfaces

“…For example, in situations when greater lateral forces cause a relatively large net, skin displacement occurs on a less slippery surface than with a more slippery surface. One alternative possibility to reduce ambiguity of contact forces and skin displacement in a real‐life situation requires combining the magnitude of skin displacement with information about the skin stretch pattern caused by the lateral movement (Ingvars et al., 2001, 2009; Seizova‐Cajic et al., 2014) or torsion in the finger pad (du Bois de Dunilac et al., 2023; Khamis et al., 2015; Loutit et al., 2023). As in virtual environments, tangential skin stretches within the contact area ranging between 0.25 and 0.75 mm lead to friction perception (Kamikawa & Okamura, 2018; Provancher & Sylvester, 2009; Santello et al., 2002; Suchoski et al., 2018).…”

Role of finger movement kinematics in friction perception at initial contact with smooth surfaces

“…For example, when a relatively larger net skin displacement occurs on a less slippery surface, caused by an overall larger lateral movement than with a more slippery surface. One of alternative possibilities to reduce ambiguity due to variability of contact kinematics in real life situation is that the magnitude of skin displacement must be viewed in the context of the skin stretch pattern caused by lateral movement or torsion in the finger pad (Khamis et al, 2015; Dunilac et al, 2023; Loutit et al, 2023). As in virtual environments, tangential skin stretches within the contact area ranging between 0.25 - 0.75 mm leads to friction perception (Santello et al, 2002; Provancher and Sylvester, 2009; Kamikawa and Okamura, 2018; Suchoski et al, 2018).…”