## 2 Design and Fabrication of the Robot Fingertip Module  
A tactile sensor with multimodal sensing capabilities is required to measure the various tactile modalities of objects. Additionally, the sensor must possess sufficient sensitivity to be able to discriminate between various materials. Hence, in accordance with strategies proposed in previous studies [16; 17; 18], we designed a hybrid-type 3-axis force sensor that possesses the excellent sensitivity and repeatability characteristics of single-crystalline silicon and the mechanical flexibility of polymer-based elastomers. Furthermore, we designed a tactile sensor with multimodal sensing capabilities by adding a gold wire-based temperature sensor. By using such inorganic substances as sensing materials, the developed sensor possesses greater mechanical and chemical stability and exhibits superior repeatability characteristics and lower hysteresis errors compared to other sensors that use organic sensing materials (such as carbon-polymer mixed sensors). These characteristics are crucial as they are directly related to sensor durability. The sensing layer structure with the advantages mentioned above is as follows. The sensitivity of the 3-axis force sensor, which is based on the strain gauge method, is determined by the gauge factor of the material of which the gauge is comprised, with higher values resulting in greater sensitivity. In the case of single-crystalline silicon, of which our sensor was comprised, the gauge factor depends on the doping concentration (in our case, 9.0 \(\times\) 10\({}^{18}\) ions/cm\({}^{3}\)); generally, the value falls within the range of \(-\)120-180 [19; 20]. This value is rather high considering how the gauge factor is approximately 2-4 for metals [21] and approximately 6 for graphene [22]. Therefore, to measure 3-axis forces using single-crystalline silicon-based strain gauges, four U-shaped strain gauges were arranged at 90-degree intervals (N, E, S, W) to form a cell. Most importantly, the sensor was designed with a single cell as it is more suitable compared to array types when measuring roughness, a tactile modality that is necessary to discriminate the texture of objects. This is because roughness measurements require fast Fourier transform (FFT) analysis with high-speed sampling. In the case of the temperature sensor, gold was used as it produces stable sensor outputs according to thermal stimuli. However, since gold is also a metal, the sensor can react to strain changes in addition to thermal stimuli. As such, the sensor was designed with a thin gold wire with a serpentine shape to only react to temperature. In general, it is known that the elastic stretchability of a wire designed in a serpentine shape improves as the arc angle (in our case, 180\({}^{\circ}\)) increases [23]. The design of the sensing layer of the multimodal tactile sensor is illustrated in Figure 1a. However, a sensing layer designed with inorganic materials poses the concern of being vulnerable to impacts upon contact. Thus, this mechanical weakness was compensated by adopting a hybrid structure in which a polymer-based elastomer was wrapped around the sensing layer. There are various types of polymer-based elastomer materials, with the most notable examples including Ecoflex, Dragon Skin, and PDMS. In our design, we wrapped the sensing layer with PDMS (Polydimethylsiloxane, Sylgard 184, Dow Corning, Midland, MI, USA), as its physical stiffness and adhesion can be easily controlled and since it is easy to bond with other PDMS-based layers. The designed PDMS layers include a deformation layer, a bump layer, and an encapsulation layer. The deformation layer, which corresponds to an elastic sensing element, was located below the PI (Polyimide, 25 \(\upmu\)m, DuPont Kapton(r) substrate (sensing layer) and served to generate a strain proportional to the contact force. To improve the sensitivity of the sensor, we used a 50:1 ratio (base:curing agent) PDMS to result in a deformation layer with soft physical stiffness [24].