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Kim et al. Soft Sci 2024;4:12 https://dx.doi.org/10.20517/ss.2023.50 Page 7 of 11

Lastly, an example introduces a version of a dual-in-plane MA device [Figure 3D1] designed to detect
plosive speech patterns that generate a significant number of aerosols and droplets with intense turbulent
[10]
background flows . PIV was used to correlate the background flow physics with MA signals. Figure 3D2
illustrates the formation of a vortex ring from plosive sounds with the velocity vector field and vorticity
color contour. Figure 3D3 demonstrates the overall correlation between MA signals, droplet travel distance
induced by background flows, and associated biomechanics in the neck with respect to sound power. The
studies suggest that sternocleidomastoid (SCM), besides SN, is an ideal location for capturing plosive speech
sounds and decoupling them from other bio-signals. MA signals from these locations, using a Dual-in-
Plane (DiP) sensor, enhance machine learning-based classification of plosive sounds in various languages.

COUPLED MECHANICS IN HAPTIC SYSTEMS
On the other side of the spectrum, skin-integrated haptic technologies offer the capability to replicate
sensations in virtual reality (VR) and augmented reality (AR) settings. Similar to MA sensor technologies,
haptic interfaces need to softly laminate onto the curved skin surfaces to deliver sensory information
[27]
through programmable patterns of localized mechanical vibrations in the spatiotemporal domain .
Vibrations of the skin and the operating mechanisms of the actuator are closely linked with complexity due
to (i) viscoelastic properties of the skin altering the characteristics of vibrotactile actuators by adding
mechanical impedances and (ii) the vibrating direction and boundary conditions of vibrotactile sensors
affecting the dynamics of wave propagation in the skin and vice versa. Therefore, a quantitative description
of the mechanical effects is essential for designing such systems and understanding the fundamental aspects
of resulting tactile sensations.

In recent studies on skin-integrated haptic systems, the investigation of the coupled mechanics between the
skin and haptic actuators using computer vision methods has opened up the possibility of further
optimizing device designs and gaining a fundamental understanding of our sensory perceptions [8,28] .
Complex patterns of the skin deformation induced by an array of eccentric rotating mass (ERM) actuators
in a wireless haptic interface were quantified to investigate crosstalk in the system and the performance of
the actuators in terms of the activation of mechanoreceptors such as Meissner and Pacinian corpuscles
[Figure 4A1-3] . Strain distributions on skin phantoms induced by three representative vibrotactile
[8]
actuators, including ERMs, linear resonant actuators (LRAs), and vibrotactile linear actuators (e.g., tactors),
were explored and correlated with relevant perception studies to provide a fundamental understanding of
sensory perceptions [Figure 4B1-3] .
[28]
A wireless, lightweight, flexible haptic interface [Figure 4A1] was developed to deliver spatiotemporal
patterns of touch across large areas of the skin, controlled through smart devices in real-time . PTV and
[8]
3D-DIC were used to measure the wave propagation and 3D deformations resulting from structural
interactions between the actuators and the surrounding skin. The 3D colored contour map in Figure 4A2
illustrates the deformation in the out-of-plane direction at a representative instant during the actuator’s
vibration using 3D-DIC. Figure 4A3 displays the lateral component of wave propagation along the skin
induced by the operation of an actuator using PTV. The results reveal that the mechanism of an ERM
actuator shares some characteristics of Euler’s disk, and its induced wave propagation along the skin
exhibits Rayleigh waves. These mechanical findings further validate the design strategy of devices, showing
negligible crosstalk and delivering a strong, power-efficient perception.

Recently, the coupled mechanics of three representative types of vibro-tactile sensors, ERM, LRA, and tactor
actuators, were investigated and compared to evaluate their performance in delivering sensory perceptions
using 3D-DIC and the Triangular Cosserat Point Elements (TCPE) method for strain estimations . ERM
[28]