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. 2019 Sep 25;10(10):642.
doi: 10.3390/mi10100642.

Biomimetic Tactile Sensors with Bilayer Fingerprint Ridges Demonstrating Texture Recognition

Eunsuk Choi  1 , Onejae Sul  2 , Jusin Lee  3 , Hojun Seo  4 , Sunjin Kim  5 , Seongoh Yeom  6 , Gunwoo Ryu  7 , Heewon Yang  8 , Yoonsoo Shin  9 , Seung-Beck Lee  10   11
Affiliations

Biomimetic Tactile Sensors with Bilayer Fingerprint Ridges Demonstrating Texture Recognition

Eunsuk Choi et al. Micromachines (Basel). .

Abstract

In this article, we report on a biomimetic tactile sensor that has a surface kinetic interface (SKIN) that imitates human epidermal fingerprint ridges and the epidermis. The SKIN is composed of a bilayer polymer structure with different elastic moduli. We improved the tactile sensitivity of the SKIN by using a hard epidermal fingerprint ridge and a soft epidermal board. We also evaluated the effectiveness of the SKIN layer in shear transfer characteristics while varying the elasticity and geometrical factors of the epidermal fingerprint ridges and the epidermal board. The biomimetic tactile sensor with the SKIN layer showed a detection capability for surface structures under 100 μm with only 20-μm height differences. Our sensor could distinguish various textures that can be easily accessed in everyday life, demonstrating that the sensor may be used for texture recognition in future artificial and robotic fingers.

Keywords: biomimetic; fingerprint ridge; piezoelectric sensor; tactile sensor; texture discrimination.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Schematic illustration and shear detecting mechanism of (a) the human fingertip and (b) the biomimetic tactile sensor.
Figure 2
Figure 2
(a) Four types of surface kinetic interfaces (SKINs) with differing mechanical configurations and (b) their fabrication processes.
Figure 3
Figure 3
Images of a fabricated biomimetic tactile sensor with SKIN: (a) a false-color SEM image of the fabricated SKIN #3, which was composed of (yellow) SU-8 epidermal fingerprint ridges with a 200-μm-period and a 50-μm height, and a (blue) polydimethylsiloxane (PDMS) epidermal board with a 50-μm thickness; (b) an optical image of the fabricated flexible biomimetic tactile sensor with SKIN.
Figure 4
Figure 4
Optical image of the measurement set-up, where the inset shows an optical image of a SKIN and a hovering polyethylene terephthalate (PET) tip.
Figure 5
Figure 5
Measurement results of tactile sensor with or without epidermal fingerprint ridges (ERs) when a PET tip was scanned at 2.5 cm/s on the tactile sensor: (a) the polyvinylidene difluoride (PVDF) output signal of a tactile sensor with ridge structure; (b) the region of 0.3–0.4 s in (a); (c) the PVDF output signal of a tactile sensor without ridge structure; (d) the fast Fourier transform (FFT) results of (b,c).
Figure 6
Figure 6
FFT results of tactile sensors with four types of SKINs when a PET tip was scanned at 2.5 cm/s on the tactile sensor. The indicated magnitude values show the magnitude of peak frequency (fER) induced by a 200-μm period of ERs and a 2.5 cm/s scanning speed.
Figure 7
Figure 7
Dependence of the magnitude of fER on (a) the height (h) of the ER and (b) the thickness (t) of the epidermal board.
Figure 8
Figure 8
Surface period-detecting characteristics of biomimetic tactile sensor: the FFT results of the tactile sensor output induced by scanning the grating structures with (a) a 150-μm and (b) 625-μm period at 1–4 mm/s scanning speed, where the insets show cross-sectional SEM images of the grating structures (scale bars indicate 200 μm); (c) the FFT results of the tactile sensor output induced by scanning a 3D-printed structure at 2 mm/s (varying the contact depth), where the inset shows a cross-sectional SEM image of the 3D-printed structure (scale bar indicates 500 μm).
Figure 9
Figure 9
FFT results of tactile sensor output induced by scanning (a) a human finger and (b) a PDMS replica, where the insets show optical images of a human finger pad and the PDMS replica. Blue arrows indicate the fER, and green arrows indicate the peaks of ERs of the contact objects.
Figure 10
Figure 10
FFT results of tactile sensor output induced by scanning (at a 2-mm/s scanning speed) (a) a sheet of papyrus, (b) a sheet of printing paper, (c) cattle leather, and (d) polyurethane artificial leather, where insets show SEM and optical images of each contact object (scale bars indicate 500 μm).
Figure 11
Figure 11
FFT results of tactile sensor output induced by scanning (a) a hard fabric with tight weaves and (b) a soft fabric with hair-like structures, increasing the contact depth. Inset images show SEM and optical images of the two different fabrics (scale bars indicate 1 mm).
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