Research ArticleSENSORS

A hierarchically patterned, bioinspired e-skin able to detect the direction of applied pressure for robotics

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Science Robotics  21 Nov 2018:
Vol. 3, Issue 24, eaau6914
DOI: 10.1126/scirobotics.aau6914
  • Fig. 1 Human skin inspired e-skin.

    (A) Cross-section of the skin from fingertip depicting key sensory structures. The spinosum is a layer found between the dermis and the epidermis, forming interlocked microstructures (hills) responsible for tactile signal amplification (5). Four types of mechanoreceptors measure innocuous mechanical stimuli with different receptive field sizes and time scales (SA-I and SA-II are located near the surface of the skin and deeper, respectively; FA-I and FA-II measure low-frequency and high-frequency stimuli, respectively) (5, 6, 3538). Because of the 3D structure of the spinosum, the hills concentrate forces onto the mechanoreceptors differently depending on the direction of applied force. (B) Soft biomimetic e-skin. Black, CNT electrodes; blue, PU elastomer; gray, intermediate thin-film dielectric layer, which ensured electrical insulation of the capacitors. During assembly, the bottom and top electrodes were aligned perpendicularly so that an array of capacitors was formed, with each hill corresponding to 25 capacitors (1 capacitor on the top of the hill, 4 on the slopes, 4 in the corners, and 16 surrounding the hill).

  • Fig. 2 Biomimetic e-skin design concepts.

    (A) Previously proposed concepts for biomimetic e-skins with interlocked microstructures (cross-sectional view), including resistive (contact resistance based) (1316), piezoresistive (13, 17, 18), ferroelectric (17), triboelectric (19), and capacitive (20) sensing mechanisms. (B) Our proposed concept. The 3D hill structure allows for different deflection capabilities on the top and around of the hills, thus differentiating capacitive responses to a pressure event from different directions. Black lines are side views of electrodes. (C) Schematics showing the limitations associated with previously proposed concepts. R represents the physical variables that are measured in those concepts, for instance, electrical resistance. Although each of the previously proposed concepts used different mechanisms (resistance, piezoresistivity, etc.), they have in common a structure with two flat electrode layers [black lines as in (B)] and an elastic structure (interlocked hills, pyramids, and hairs are gray structures) in between, as illustrated by the cross-sectional views. The pixels in such structures react uniformly across the sensor to applied stimuli regardless of normal (top), shear (middle), or tilt (bottom) force. This is illustrated by the cross-sectional views and the top-view diagrams of the e-skin outputs for arrays of five-by-five sensing pixels, with different orange color intensities for different applied forces. Thus, it is not possible, at a given time, t, and based on the array output, to distinguish the various types of applied forces. (D) Schematics showing the advantages associated with our proposed design. With our design, it is possible to measure and discriminate in real time normal and shear forces and forces applied in various directions. The tilt force is a combination of normal and shear force. In the case of a robotic application, the data of a fraction of the 25 pixels could provide sufficient information (for instance, nine of them—one on the top of the hill, four on the sides, and four in the corners), but the 25 pixels show the significance of our concept.

  • Fig. 3 E-skin fabrication and appearance.

    (A) E-skin fabrication and assembly. The device consists of three layers, assembled by lamination: (i) a bottom 1-mm-thick PU layer with an array of hills (hill diameter, 1 mm; height, 200 μm), (ii) an intermediate 10-μm-thick PHB-PHV dielectric layer used as a spacer between the top and bottom electrodes, and (iii) a top 60-μm-thick PU layer with an array of pyramids. The electrodes were made of spray-coated and photolithography-patterned conducting CNTs embedded into the PU matrix (electrode width, 300 μm; separation distance between two electrodes, 50 μm). The construct was reinforced with tape at the sides, and no sliding of the layers was observed when shear force is applied. For our current sensor size, we observed that the use of tape was sufficient to stabilize the system for laboratory experiments. If we were to scale the sensor array, the proper adhesion between layers would need to be implemented to ensure mechanical stability. (B) Optical image of a fabricated e-skin and close-up view on the hills and electrodes (inset). (C) Optical image showing the CNT-PU interconnects for signal recording with LCR meter and SEM picture of the top e-skin layer with molded pyramids, showing CNT-PU and PU areas (inset).

  • Fig. 4 Response characteristics of the biomimetic e-skin, measured through successive single-pixel signal response acquisition.

    Pyramids were arranged in a 2D orthogonal grids, the pyramid width was a = 30 μm, and the separation distance between pyramids was b/a = 4 unless stated otherwise. (A to C) Sensor arrays of five-by-five capacitors, centered around one hill, were characterized by measuring the pressure response curves upon applied forces. Top: Cross-sectional views with forces shown in arrows and the locations of the PU-CNT electrodes shown in black. Middle: Top views of the relative changes in capacitance shown for 25 capacitors. Bottom: Measured pressure response curves for three capacitors (one located at the top of the hill and two located at the bottom, surrounding the hill). Dashed circles represent the locations of the hills. (A) Normal force (green arrows) with the relative change in capacitance calculated as ΔC/Cmin = (C700kPaCmin)/Cmin, where Cmin and C700kPa are the capacitances without and with applied pressure, respectively. The experiment was reproduced five times. (B) Applied shear force and a necessary normal force of 5 to 10 kPa (red) and ΔC/Cmin with ΔC = (C340kPaCmin). The patterns for normal and shear forces are distinct. The experiment was reproduced two times. (C) Applied tilt force (a combination of normal and shear force, dark blue) and ΔC/Cmin with ΔC = (C340kPaCmin). The measured pattern combines the characteristics of (A) and (B). The experiment was reproduced two times. (D) Similar to the various types of mechanoreceptors in human skin, capacitors had different pressure response curves (and therefore sensitivities) depending on their locations. Top: Response characteristics, for applied normal force, for two capacitors located at the top and at the bottom of the hills. Bottom: Normalized response curves for the two capacitors. The slopes were used to calculate the sensitivities in various pressure ranges. The experiment was reproduced five times. (E) 3D plots of the measured relative change in capacitance for a sensor array of nine-by-nine capacitors, where each color band corresponds to a variation ΔC/Cmin = 5%. Normal force applied on the entire array (left) and on the bottom left corner (right). The pixels (capacitors) located at the top of the hills have a measured standard deviation on ΔC/Cmin below 20%. Experiments were reproduced two times (left) and one time (right). (F) Device response at applied pressures in the range from 0 to 1800 kPa (normal force, b/a = 2, where a + b is the distance between the centers of two pyramids). The robustness of the sensor is illustrated by the unaltered pressure response curves after several runs at various pressures (inset). The experiment was reproduced two times. (G) The e-skin was sensitive enough to measure objects as small as a 1-mm-diameter plastic bead (15 mg, corresponding to less than 0.5 kPa). The bead was placed on the array (zone 2) and removed several times, and the pressure response signals were measured for capacitors located at the top (left), slope (middle), and bottom (right) of the hill. The experiment was reproduced one time. (H) Cycling test illustrating the stability of the pressure response over 30,000 cycles (b/a = 2). A small signal drift was measured, illustrated by the fact that C15kPa increased by 2.3% and C80kPa increased by 0.2% after 30,000 cycles. The experiment was reproduced two times.

  • Fig. 5 Optimization of the biomimetic e-skin.

    (A) Optimization of the geometry of the pyramids (size and b/a) for the deflection of the top membrane in zone 1, corresponding to the capacitors located on the slopes and at the bottom of the hills. Top left: Cross-sectional view of the top electrode layer in zone 1 with pyramids, with the geometry used for COMSOL simulation. A 1-kPa uniform pressure was applied (normal force). The initial and deformation patterns are shown in black and red, respectively. Top right: Cross-sectional view showing the e-skin structure with deformed top electrode layer. Bottom: The simulations were performed for a values of 10, 20, 30, 40, and 50 μm and b/a = 0.4, 0.8, 1.2, 1.6, 2, and 4 (a + b is the distance between the centers of two pyramids). (B) Corresponding results for total displacement (left) and stress (right). The distance evaluated for total displacement is indicated with blue arrows. The stress is evaluated at the point indicated with blue circle. The red circle identifies the best conditions (best-case scenario) for high sensitivity. The influence on ΔC/Cmin, when comparing worst-case scenario (a = 50 μm, b/a = 0.4) and best-case scenario (a = 10 μm, b/a = 4, applied pressure of 1 kPa), is ~4%. (C) Summary of the requirements for optimized e-skin, in terms of pyramid width and separation distance, in zones 1 and 2. On the basis of these requirements, the spiral grid gave a good combination of high sensitivity in zone 1 and both high Cmin and fast time response in zone 2. (D) The spatial organization of sunflower florets was used to design the positioning of the pyramids, resulting in enhanced e-skin performances. [Credit: A. Marcus, https://digitalprovocations.wordpress.com/2011/11/10/project-3_2a-sunflower/ (E) Left: Microscope image of the Si masks used to mold the PU top electrode layer, with pyramids arranged along orthogonal grid (top) and spiral grids (bottom). Right: Top electrode layer with pyramids to be positioned on the 1-mm-diameter hills shown at the same scale. (F) SEM image showing the PU top electrode layer with pyramids arranged along phyllotaxis spiral grid. The CNT-PU conducting electrodes appear as light gray stripes (darker gray stripes correspond to PU without CNTs between the electrodes). (G to J) Measured response characteristics of our e-skin, for arrays of five-by-five capacitors with orthogonal and spiral grids of pyramids (experiment reproduced two times each). Top: Cross-sectional view with normal force (green). Middle: ΔC/Cmin shown for the 25 capacitors of each array. Bottom: Measured pressure response curves for all 25 capacitors. (G) Thirty-micrometer-wide pyramids positioned along an orthogonal grid with b/a = 4; (H) same with b/a = 0.4; (I)30-μm-wide pyramids positioned along a phyllotaxis spiral grid (b/a = 0.4 and 4 at the spiral center and border, respectively); (J) same with 10-μm-wide pyramids.

  • Fig. 6 Experiments with e-skin mounted on a robot arm.

    (A) Experimental setup, with the e-skin sensor array mounted on an artificial hand and fixed on a gripper attached to the robot arm. The LCR meter recorded capacitance signals from the e-skin. Data were stored on a server and retrieved by the robot controller to be used in a closed-loop feedback scheme to control the movement of the robot arm in real time. (B) Typical test plate with holes [either eight holes, four holes, or no hole, as illustrated in (D) and (F) below]. The purple arrows show the consecutive movements executed by the robot arm, unless tactile feedback prevented the entire execution of the down movement. Inset: The e-skin, mounted on the artificial finger (in red), was exposed to either normal (green arrows, left) or shear (tangential force, dark red arrows, right) force. (C) Schematic of experiments where the e-skin was solely exposed to normal force, reproduced three times. Purple arrows show the movement executed vertically, unless normal force feedback (green arrow) was detected and prevented the entire execution of downward movement. (D) Experiments were performed with four different test plates with holes. The robot arm went consecutively from positions 1 to 8. For each experiment, the position of the robot arm in the z direction and the measured capacitance C were plotted as a function of time. Red arrows indicate the successful detection of normal force when the finger touched the plate (in the absence of a hole) and the corresponding close-loop control feedback movement upward. (E) Experiments where the e-skin was solely exposed to shear force, reproduced three times. A ping-pong ball (weight, 2.7 g) was positioned between the two artificial fingers. Purple arrows show the movement executed downward, unless shear (tangential) force feedback (dark red arrow) was detected and prevented the entire execution of downward movement. Because of the light weight, only a negligible tangential force contribution due to the weight (gravity) is initially present. (F) Experiments were performed with test plates as described in (D). Red arrows in the measured capacitance plots indicate the successful detection of shear force. When the ping-pong ball touched the table, we observed that it did not slide between the fingers because of the initial small normal force applied to hold it and the surface interaction between PU and the ball. Therefore, the tangential force was measured by the e-skin. This information feedback was used to program the next movement of the robotic arm. In this case, as soon as a tangential force was measured, the robot stopped its downward movement, went back up, and continued the experiment to the next position. (G) The capability to interact with fragile objects was demonstrated with fresh raspberries. The robot arm was preprogrammed to go downward unless normal force was detected. (H) Tactile feedback prevented flattening the raspberry. (I) Without tactile feedback, the fruit was irreversibly crushed.

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/3/24/eaau6914/DC1

    Fig. S1. Schematics of the position of the different capacitors or “pixels” around a hill.

    Fig. S2. Measured response characteristics of the biomimetic e-skin.

    Fig. S3. Experimental setup used to characterize the e-skin.

    Fig. S4. Response time of sensor.

    Fig. S5. Optimization of the separation distance d between the top and bottom electrodes of the capacitors in zone 2 (capacitors located at the top of the hills).

    Fig. S6. Optimization of the e-skin regarding the time response (zone 2, capacitors located at the top of the hills).

    Fig. S7. Experiments with e-skin mounted on a robot arm.

    Movie S1. The robot arm correctly interrupts its preprogrammed movement in downward direction as soon as shear force is detected and the ball touches the test plate at a location with no hole.

    Movie S2. The high sensitivity of the e-skin allows for interaction with fragile, deformable, and delicate objects such as a fresh raspberry (when tactile sensing is activated, the e-skin senses the contact with raspberry and the robot arm moves in upward direction without damaging the fruit).

    Movie S3. When tactile sensing is not activated, the fruit is crushed.

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Schematics of the position of the different capacitors or “pixels” around a hill.
    • Fig. S2. Measured response characteristics of the biomimetic e-skin.
    • Fig. S3. Experimental setup used to characterize the e-skin.
    • Fig. S4. Response time of sensor.
    • Fig. S5. Optimization of the separation distance d between the top and bottom electrodes of the capacitors in zone 2 (capacitors located at the top of the hills).
    • Fig. S6. Optimization of the e-skin regarding the time response (zone 2, capacitors located at the top of the hills).
    • Fig. S7. Experiments with e-skin mounted on a robot arm.
    • Legends for movies S1 to S3

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). The robot arm correctly interrupts its preprogrammed movement in downward direction as soon as shear force is detected and the ball touches the test plate at a location with no hole.
    • Movie S2 (.mp4 format). The high sensitivity of the e-skin allows for interaction with fragile, deformable, and delicate objects such as a fresh raspberry (when tactile sensing is activated, the e-skin senses the contact with raspberry and the robot arm moves in upward direction without damaging the fruit).
    • Movie S3 (.mp4 format). When tactile sensing is not activated, the fruit is crushed.

    Files in this Data Supplement:

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