Flexible tactile sensor for shear stress measurement using transferred sub-µm-thick Si piezoresistive cantilevers

Bendable tactile sensors that can be easily attached to curved surfaces are being widely studied for several applications. For example, human interfaces, such as input devices [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib1" id="fnref-jmm442032bib1"="">1</a>–<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib3" id="fnref-jmm442032bib3"="">3</a>] and motion monitoring devices attached to shoes [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib4" id="fnref-jmm442032bib4"="">4</a>] or to the whole body [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib5" id="fnref-jmm442032bib5"="">5</a>], have been reported. Especially in the field of robotics, flexible tactile sensors for covering the thin, curved surfaces of the finger-type structures of robotic hands are required for dexterous manipulation of objects [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib6" id="fnref-jmm442032bib6"="">6</a>, <a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib7" id="fnref-jmm442032bib7"="">7</a>]. To realize dexterous manipulation by robots, two functions are required of the tactile sensors: (1) shear stress measurement for slippage detection and (2) a thin and bendable structure to cover the thin, curved surfaces of the finger structures of robotic hands.

Currently, several sensing mechanisms designed to measure shear stresses have been proposed for robotic tactile sensors [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib8" id="fnref-jmm442032bib8"="">8</a>–<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib10" id="fnref-jmm442032bib10"="">10</a>]. Among these various sensing mechanisms for shear stress sensing, the Si piezoresistive force sensor is effective because it has high sensitivity, and it is compatible with micro-fabrication or complementary metal oxide semiconductor processing [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib11" id="fnref-jmm442032bib11"="">11</a>–<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib13" id="fnref-jmm442032bib13"="">13</a>]. However, most of the current Si piezoresistive tactile sensors are formed on a thick and rigid silicon base layer, so these sensors have difficulty in bending and attaching to curved surfaces. Attempts to miniaturize the sensor fabricated on a silicon on insulator (SOI) wafer to a millimeter-sized chip and to array these chips onto flexible wiring have been reported [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib14" id="fnref-jmm442032bib14"="">14</a>, <a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib15" id="fnref-jmm442032bib15"="">15</a>]. However, the thick base layer of the SOI wafers remained in these sensors, so it was still difficult to attach them to a thin rod, such as a finger (for example, a human thumb has a radius of 10 mm). To increase the bendability of the Si piezoresistive sensors, it is necessary to reduce the thickness of the sensor structure.

Here, we propose a flexible shear stress sensor composed of thin, standing Si piezoresistive cantilevers embedded in a flexible elastic material (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f1"="">1</a>(<em="">a</em>)). These thin cantilevers were fabricated by using a 290 nm thick device layer on the surface of an SOI wafer. By using an adhesion-based stamping transfer method [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib16" id="fnref-jmm442032bib16"="">16</a>], only the thin cantilevers were transferred from the SOI surface to a polydimethylsiloxane (PDMS) layer. Because the sub-µm-thick silicon layer is capable of bending without damage, the proposed shear stress sensor sheet is bendable and able to be attached to a thin rod. We designed the standing Si piezoresistive cantilevers in orthogonal directions, <em="">X</em> and <em="">Y</em>, and embedded them in the PDMS layer. The standing cantilevers in the PDMS layer follow the shear deformation of the PDMS layer (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f1"="">1</a>(<em="">b</em>)). Therefore, the <em="">X</em> and <em="">Y</em> directional components of the shear forces applied to the sensor surface can be detected by measuring the resistance changes of the arrayed standing Si piezoresistive cantilevers. In this paper, we demonstrate the sensing characteristics of the proposed flexible shear stress sensor sheet by attaching it to curved surfaces.

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Our proposed flexible shear stress sensor was composed of three layers embedded in PDMS (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f1"="">1</a>(<em="">a</em>)): (1) a 290 nm thick standing Si piezoresistive cantilever layer, (2) a Au/Parylene-C wiring layer and (3) a PDMS base layer. The cantilever layer and the wiring layer were integrated to a PDMS base layer as shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f2"="">2</a>. The cantilever layer and the wiring layer were positioned in the neutral plane of the sensor, so that the wirings were not stretched or torn by bending the sensor.

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The design of the Si piezoresistive cantilever layer is based on our previous work [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib17" id="fnref-jmm442032bib17"="">17</a>] on a tactile sensor using a standing Si piezoresistive cantilever formed on a rigid SOI wafer. The Si piezoresistive cantilevers were arrayed in orthogonal directions and connected to Au/Si wiring with electrodes. An n-type Si piezoresistor was formed on the surface of a Si layer. A magnetic Ni layer was formed on the surface of the cantilever tips so that the cantilevers can be made to stand under a magnetic field [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib18" id="fnref-jmm442032bib18"="">18</a>]. Because the cantilever was covered with metal layers, except for its hinges, the deformation of the hinges of the cantilever can be detected by measuring the resistance change of the cantilever. This cantilever structure can detect its deformation in a uniform direction, as defined in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f1"="">1</a>(<em="">b</em>) [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib17" id="fnref-jmm442032bib17"="">17</a>]. The size of this cantilever was 300 µm in length, 100 µm in width and 290 nm in thickness. Its hinges were 100 µm in length and 30 µm in width.

The Au/Parylene-C wiring layer of the flexible sensor sheet has to be bendable and unbreakable. We found that a 50 nm thick Au layer patterned onto a 5 µm thick flexible Parylene-C layer is sufficiently strong for the wiring. The size of the Au/Parylene-C wiring layer was about 25 mm <b="">&#x00d7;</b> 25 mm. The Au wirings were 300 µm in width and 200 µm <b="">&#x00d7;</b> 200 µm sized holes were formed to their tips. The cantilevers connected to these holes. Around these contact positions, 2 mm <b="">&#x00d7;</b> 1.5 mm sized through-holes were patterned as alignments for the cantilevers and the PDMS base layer.

The size of the PDMS base layer was 40 mm <b="">&#x00d7;</b> 40 mm and its thickness was 1 mm. Alignment holes 1 mm <b="">&#x00d7;</b> 1 mm in size and 0.5 mm deep were fabricated with a 2 mm pitch. These alignment holes were used to align the cantilever layer and the Au/Parylene-C wiring layer to the PDMS base layer.

The proposed sensor was fabricated by integrating three layers. After the integration, the whole structure was embedded in PDMS.

First, the Si piezoresistive cantilevers were fabricated on the SOI wafer (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f3"="">3</a>); this fabrication process is similar to our previous work [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib17" id="fnref-jmm442032bib17"="">17</a>]. We used an SOI wafer, which has a 290 nm thick p-type top silicon layer, a 400 nm thick buried SiO<sub="">2</sub> layer and a 300 µm thick handling silicon layer. The n-type doped silicon layer was formed on the surface of this SOI wafer by doping with P<sub="">2</sub>SO<sub="">4</sub> (OCD T-1 P59230, Tokyo Ohka) using thermal diffusion [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib19" id="fnref-jmm442032bib19"="">19</a>]. Then, 200 nm/50 nm thick Ni/Au layers were deposited on the top Si layer. The Ni/Au/Si layers were patterned by wet and dry etching (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f3"="">3</a>(<em="">a</em>)). The fabricated Si piezoresistive cantilevers were released by using hydrofluoric acid vapor (HF 46%, Morita Chemical Industries), as shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f3"="">3</a>(<em="">b</em>). To remove the SiO<sub="">2</sub> layer, 10 µm square etching holes were formed with a 20 µm pitch to the cantilever surface, as shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f3"="">3</a>(<em="">c</em>). During the HF vapor etching, the SOI wafer was heated up to 30–40 °C to evaporate the H<sub="">2</sub>O created by the etching process and to prevent stiction between the Si piezoresistive cantilevers and the base Si layer [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib20" id="fnref-jmm442032bib20"="">20</a>]. A photograph of the fabricated Si piezoresistive cantilevers is shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f4"="">4</a>. The positions of the released cantilevers were fixed by microanchors, which were 50 µm in length and 2 µm in center width. These microanchors were arranged with a 100 µm pitch, and they connected the cantilevers to Si islands (figures <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f3"="">3</a>(<em="">c</em>) and <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f4"="">4</a>).

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Second, the Au/Parylene-C wiring layer was fabricated as shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f5"="">5</a>. A 5 µm thick Parylene-C and a 50 nm thick Au layer were deposited onto a glass wafer (Micro Cover Glass No. 5, Matsunami). Before the deposition, the glass wafer was coated with a detergent (micro-90 International Products Corporation) diluted to 2% with DI water and dried at room temperature to make it easy to peel off the Parylene-C. Then, the Au/Parylene-C layers were patterned by using the photolithography technique: the Au layer was masked with approximately 1.2 µm thick photoresist (OFPR800-23cp, Tokyo Ohka) and patterned by using a compound liquid of KI (50 g) and I<sub="">2</sub> (50 ml) diluted to 30% with DI water. After patterning the Au layer, the Au/Parylene-C layers were coated with approximately 5 µm thick photoresist (OFPR800-100cp, Tokyo Ohka), and the Parylene-C layer was patterned by using oxygen plasma etching with 50 W of power and 5 Pa O<sub="">2</sub> gas (FA-1, Samco) (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f5"="">5</a>(<em="">a</em>)). The fabricated Au/Parylene-C wiring layer was peeled off the glass wafer, turned upside-down and put onto another glass wafer (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f5"="">5</a>(<em="">b</em>)). Because the Au/Parylene-C wiring layer was attached to the glass wafer by the naturally occurring electrostatic force, the wiring layer was prevented from rolling up.

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Third, the PDMS base layer was fabricated by using an acrylic mold (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f6"="">6</a>). This mold was formed by engraving 1 mm <b="">&#x00d7;</b> 1 mm<b="">&#x00d7;</b> 0.5 mm bumps on the surface of an acrylic plate with an engraving machine (EGX-300, Roland DG). We covered the surface of the mold with a fluorinated release agent (No. 10-20, Blenny) to make it easy to peel the PDMS structure from this mold. Then, PDMS (Sylgard184, Dowcorning) was poured into this acrylic mold and cured by baking at 60 °C for an hour. The mixing ratio between the PDMS base resin and its hardening agent was 10:1 by weight. After curing, the PDMS base layer was peeled from the mold and attached to a 2.0 mm thick glass plate. This glass plate serves as a support for the PDMS base layer.

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The stamping transfer steps used to integrate the Si piezoresistive cantilever layer and the Au/Parylene-C wiring layer with the PDMS base layer are shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f7"="">7</a>. Stamping transfer is a method used to transfer the microstructures from the base wafer to another target wafer by using the difference between the adhesion forces [<a xmlns:xlink="http://www.w3.org/1999/xlink" class="cite" href="#jmm442032bib16" id="fnref-jmm442032bib16"="">16</a>]. In the case of figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f7"="">7</a>, the Au/Parylene-C wiring was attached to a glass plate with the weak static electrical force, and the cantilevers were anchored to the SOI wafer with micro anchors. Because of the adhesion forces existing between the Si piezoresistive cantilever layer, the Au/Parylene-C wiring layer and their bases are smaller than the adherence of the PDMS base layer, the devices can be transferred from their bases to the PDMS base layer by stamping the PDMS base layer onto them. Using this transfer method, the PDMS base layer was first stamped onto the Au/Parylene-C wiring on the glass wafer, and the Au/Parylene-C wiring layer was transferred to the PDMS base layer (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f7"="">7</a>(<em="">b</em>)). Then, the PDMS base layer with the Au/Parylene-C wiring layer was stamped onto the Si piezoresistive cantilevers fabricated on the SOI wafer with 2.0 kPa of pressure and transferred (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f7"="">7</a>(<em="">c</em>)). The cantilever layer and Au/Parylene-C wiring layer were integrated onto the PDMS base layer by using only the adherence force of the sticky surface of PDMS without chemical reactions. Photographs of the Au/Parylene-C wiring layer transferred onto the PDMS base layer are shown in figures <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f8"="">8</a>(<em="">a</em>) and (<em="">b</em>), and the Si piezoresistive cantilever layer transferred onto the PDMS base layer with the Au/Parylene-C wiring layer is shown in figures <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f8"="">8</a>(<em="">c</em>) and (<em="">d</em>). The yield of the transfer was over 90%. As shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f8"="">8</a>(<em="">d</em>), the cantilevers were attached to the PDMS base layer at the holes formed at the contact position of the Au/Parylene-C wiring and their tips were positioned to the alignment holes formed on the PDMS base layer. To make the electrical contact between the cantilever layer and the Au/Parylene-C wiring layer, we placed an electrical adhesive (Dotite D-753, Fujikura Kasei Co.) on the contacting position and cured it at 110 °C in an oven for 20 min.

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After integrating the Au/Parylene-C wiring layer and the Si piezoresistive cantilever layer with the PDMS base layer, the whole structure was embedded in PDMS (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f9"="">9</a>). First, a 350 mT magnetic field was applied to the structure, as shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f9"="">9</a>(<em="">a</em>), to pull the Si piezoresistive cantilevers into the alignment holes. Since the cantilevers' tips were positioned to the alignment holes on the PDMS base layer, the cantilevers can be deformed easily by weak force caused by a magnetic field. The 1 µm thick Parylene-C was deposited on this structure to maintain the position of the cantilevers. Then, 10:1 PDMS was poured onto the structure and cured at room temperature for 24 h. After curing the PDMS, the sensor structure was peeled from the glass plate, which was used to maintain the position of the PDMS base layer during the transfer and embedding processes (figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f9"="">9</a>(<em="">b</em>)). Photographs of the fabricated flexible tactile sensor sheet and the standing cantilevers embedded in PDMS are shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f10"="">10</a>. The size of this sensor sheet was 40 mm <b="">&#x00d7;</b> 40 mm, and it was 2 mm in thickness. The resistances of the standing Si piezoresistors in the sensor sheet were 5.5 kΩ on average. Around one cycle of the fabrication process, approximately 9 of the 16 cantilevers were able to be embedded into the sensor sheet without damage.

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4.1. Evaluation of the bending durability

To evaluate the bending durability of the fabricated flexible tactile sensor, we attached the sensor to acrylic rods that varied from 10 mm to 50 mm in radius and measured the resistance of the standing cantilever.

The relationship between the radius of the rod and the resistance of the standing cantilever is shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f11"="">11</a>. According to this result, the cantilever resistance changed by 0.02 kΩ on average by bending from its initial resistance (4.54 kΩ). However, the resistance was kept constant by fixing the sensor onto a rod so that the applied shear stress could be detected by measuring the resistance changes of the cantilever after attaching the sensor to the rod.

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Additionally, this result shows that the cantilever did not fracture even when it was attached to a rod that was 10 mm in radius. Because the 10 mm radius rod is comparable in size to a human thumb, we confirmed that the bending durability of the fabricated sensor was guaranteed to cover a thin rod-type structure, such as the finger of a robotic hand, by transferring only the top Si layer of the SOI wafer to the PDMS and removing the rigid Si handling layer.

4.2. Shear stress sensing on a curved surface

We evaluated the sensing properties of the flexible tactile sensor by attaching the sensor to a curved surface and measuring the sensor response to the shear stresses applied along its surface.

An image of the experimental setup is shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f12"="">12</a>(<em="">a</em>). The sensor was attached to an acrylic half-rod with a radius of 30 mm and covered with a rigid PET film with a thickness of 0.1 mm. Weights of 0.1 N were connected to both ends of the PET film as an initial load to maintain the contact between the film and the sensor. Shear stresses were applied along the surface of the sensor by attaching additional weights to one end of the PET sheet by using a thin wire. In this experimental setup,1.8<b="">&#x00d7;</b>10<sup="">3</sup> to 1.8<b="">&#x00d7;</b>10<sup="">3</sup> Pa shear stresses were applied in the <em="">X</em> and <em="">Y</em> directions relative to the sensor surface (each direction is defined in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f12"="">12</a>(<em="">b</em>)). To change the direction of the applied shear stresses from <em="">X</em> to <em="">Y</em>, we took the sensor off the half-rod, rotated it 90° and reattached it to the half-rod.

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The experimental results are shown in figures <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f12"="">12</a>(<em="">c</em>) and (<em="">d</em>). Figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f12"="">12</a>(<em="">d</em>) shows the relationships between the resistance changes of the two cantilevers and the shear stresses applied in the <em="">X</em> direction. The <em="">X</em>-cantilever responded linearly to the shear stresses applied in the <em="">X</em> direction (<em="">R</em><sup="">2</sup> = 0.974), which is the direction defined as the sensing direction of the <em="">X</em>-cantilever in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f1"="">1</a>(<em="">b</em>). Its sensitivity toward the <em="">X</em>-axis shear stresses was 2.7 <b="">&#x00d7;</b> 10<sup="">−6</sup> Pa<sup="">−1</sup>. In contrast, the responses of the <em="">Y</em>-cantilever to the <em="">X</em>-axis shear stresses were very small compared to its responses to the <em="">X</em>-axis shear stresses. The resistance changes of the cantilevers caused by applying <em="">Y</em>-axis shear stresses are shown in figure <a xmlns:xlink="http://www.w3.org/1999/xlink" href="#jmm442032f12"="">12</a>(<em="">d</em>). Similar to the case of <em="">X</em>-axis shear stress, the <em="">Y</em>-cantilever responds linearly to the shear stresses applied along the <em="">Y</em>-axis (<em="">R</em><sup="">2</sup> = 0.955), its sensing direction. Its sensitivity toward the <em="">Y</em>-axis shear stresses was 7.0 <b="">&#x00d7;</b> 10<sup="">−7</sup> Pa<sup="">−1</sup>, and its responses to the <em="">Y</em>-axis shear stresses were higher than the responses of the <em="">X</em>-cantilever to the <em="">Y</em>-axis shear stresses.

According to these results, we confirmed that each cantilever can independently detect the shear stress applied in its own sensing direction. Therefore, the <em="">X</em>- and <em="">Y</em>-axis components of the shear stress applied to our sensor surface can be detected by measuring the resistance changes of the <em="">X</em> and <em="">Y</em> cantilevers, and the magnitude and the applied direction of the shear stress can be detected by calculating the vectorial sums of the <em="">X</em> and <em="">Y</em> components of the shear stress.

In this experiment, the sensitivity of the <em="">X</em>-cantilever to its sensing direction was approximately 3.8 times higher than that of the <em="">Y</em>-cantilever. One reason for the difference in the sensitivity of the <em="">X</em> and <em="">Y</em> cantilevers is the experimental error caused by the misalignment of the PET film. In the experimental setup, we applied shear stresses to the sensor by pulling the end of the PET film which was attached to the sensor surface. Therefore, if the PET film's direction was misaligned, the shear stress applied to the cantilevers inside the sensor will become lower than the target value. We consider that when we measured the sensitivity to the <em="">Y</em>-axis shear stress, the PET film had been misaligned and the applied shear stress became lower than the target value.

A flexible tactile sensor sheet using orthogonally arrayed 290 nm thick standing Si piezoresistive cantilevers embedded in a PDMS layer was fabricated. The thin Si piezoresistive cantilevers were fabricated on the device layer of the SOI wafer and arranged on a 7 mm<b="">&#x00d7;</b> 7 mm area by using an adhesion-based stamping transfer. To evaluate the durability of the sensor, we had measured its bending durability and its sensing range. By using only the thin Si device layer as a sensing material, the bending durability of this sensor was made high enough to attach to a curved surface with a 10 mm radius, which is similar to the size of a human thumb, without damage. The sensing range of the sensor was −1.8 <b="">&#x00d7;</b> 10<sup="">3</sup> to 1.8 <b="">&#x00d7;</b> 10<sup="">3</sup> Pa and the average sensitivity of the sensor sheet attached to a half-rod with a 30 mm radius was 1.7 <b="">&#x00d7;</b> 10<sup="">−6</sup> Pa<sup="">−1</sup>. Each Si piezoresistive cantilever in the sensor sheet detected the shear stresses applied in its sensing direction. Therefore, the <em="">X</em>- and <em="">Y</em>-axis components of the shear stresses can be measured independently with the fabricated tactile sensor sheet. According to these results, we confirmed that our proposed flexible tactile sensor bends sufficiently to cover a finger-like surface and can measure shear stresses on curved surfaces. The proposed sensor could be a promising candidate for covering the complex curved surfaces of robotic hands. In the future, we believe that not only piezoresistive sensors but also other silicon-based devices, such as integrated circuits or photo-detectors, could be integrated into the flexible PDMS sheet by using the adhesion-based stamping transfer method to fabricate a flexible and multifunctional smart skin sensor.

This work was partly supported by the Kurata Memorial Hitachi Science and Technology Foundation. The photo-masks were made by using the University of Tokyo VLSI Design and Education Center's (VDEC) 8-inch EB write F5112+VD01 donated by ADVANTEST Corporation.