## 2 Design and Fabrication of the Robot Fingertip Module
However, this soft physical property raised two issues. One was that PDMS with such soft physical properties is not able to immediately return to its original state as it is deformed to a greater degree. As this is directly related to the hysteresis error of the sensor, a thin deformation layer with a thickness of 700 um was used to limit its deformation. The other concern was that the adhesive strength of PDMS increases as the physical stiffness becomes softer. Notably, due to the strong adhesive strength of 50:1 PDMS, it is difficult to move a deformation layer comprised of this material to a specific position once it is fabricated. This is similar to the challenges involved with handling double-sided tape with both adhesive surfaces exposed. Therefore, by designing the deformation layer above the PI substrate (adhesion layer), it becomes easier to use as one side loses its adhesion. The bump layer is a layer with cuboid-shaped bumps (W; 2.5 mm \(\times\) L; 2.5 mm \(\times\) H; 1.0 mm) that serves to ensure the bump is positioned on the cell in a stable manner. In addition to concentrating the forces onto the four strain gauges that constitute the cell to increase sensitivity, the bump serves to determine the direction of the force by generating different strains on each strain gauge according to the direction of the force [25; 26]. Therefore, the bump has to transmit external forces to the strain gauges without deforming its original shape, and thus it is advantageous to fabricate bump with greater mechanical stiffness. However, if the bump is fabricated using metal or plastic materials to improve mechanical stiffness, it will be difficult to ensure the mechanical flexibility of the sensor and excessive concentration of strain could damage the semiconductor strain gauges. Above all, it is difficult to attach bumps made of such materials to the sensing layer. Taking such conditions into consideration, a bump layer constituted of 5:1 (base:curing agent) PDMS was placed on top of the sensing layer [27]. However, even this design could not ensure the stability of the sensor performance as the bump layer is not combined with the sensing layer. Thus, an encapsulation layer constituted of PDMS with the same ratio was placed in between the sensing and bump layers to serve as an adhesion layer with the bump layer and protect the sensing layer from external physical/chemical contact. This multimodal tactile sensor has dimensions of 12 mm \(\times\) 26 mm. The cross section of the sensor and the thickness details of each layer are shown in Figure 1b. To build a robot fingertip module that can discriminate the textures of objects based on the multimodal tactile sensor designed in this structure, we designed a robot fingertip (W: 16.6 mm \(\times\) H: 36.16 mm \(\times\) t: 18 mm) similar in size to a human fingertip. Aspects that were considered when implementing the module included a heater that can maintain the temperature of the module similar to human body homeostasis, artificial skin with human-like fingerprints, and a sensor signal processing circuit board (described in detail in Section 3.1) that can be embedded in the robot fingertip. To maintain the module temperature above room temperature, a heater comprised of an enameled copper wire with a diameter of 0.05 mm was wrapped (472 turns/8 layers) around a bobbin (inner diameter: 1.5 mm, height: 4 mm), which was positioned on the metallic fingertip frame. Figure 1: Conceptual diagram of a robot fingertip module with multimodal tactile sensing capability. **(a)** An optical image about the sensing layer of the multimodal tactile sensor (w/o bump). **(b)** Cross-sectional view of a multimodal tactile sensor and its dimension. **(c)** 3D exploded view of the developed robot fingertip module with fingerprinted silicone skin.