Fingerpad‐Inspired Multimodal Electronic Skin for Material Discrimination and Texture Recognition

Abstract Human skin plays a critical role in a person communicating with his or her environment through diverse activities such as touching or deforming an object. Various electronic skin (E‐skin) devices have been developed that show functional or geometrical superiority to human skin. However, research into stretchable E‐skin that can simultaneously distinguish materials and textures has not been established yet. Here, the first approach to achieving a stretchable multimodal device is reported, that operates on the basis of various electrical properties of piezoelectricity, triboelectricity, and piezoresistivity and that exceeds the capabilities of human tactile perception. The prepared E‐skin is composed of a wrinkle‐patterned silicon elastomer, hybrid nanomaterials of silver nanowires and zinc oxide nanowires, and a thin elastomeric dielectric layer covering the hybrid nanomaterials, where the dielectric layer exhibits high surface roughness mimicking human fingerprints. This versatile device can identify and distinguish not only mechanical stress from a single stimulus such as pressure, tensile strain, or vibration but also that from a combination of multiple stimuli. With simultaneous sensing and analysis of the integrated stimuli, the approach enables material discrimination and texture recognition for a biomimetic prosthesis when the multifunctional E‐skin is applied to a robotic hand.

. Schematic diagram of the fabrication processes

I.2. Sample Characterization
The sample morphologies were characterized by using field emission scanning electron microscopy (FESEM) (Hitachi S-4200), and optical microscopy (OM) (Axioplan Zeiss). The electrical properties of the strain sensor and pressure sensor, current-voltage (I-V) curves were measured using a Keithley S4200 instrument. The electrical voltage outputs were acquired by an oscilloscope (Tektronix, TBS2102). We measured signals from three operating mechanisms such as triboelectricity, piezoelectricity and piezoresistivity to detect different types of stimuli. For triboelectric and piezoelectric mechanisms, we conducted the measurements under open circuit conditions, which means that two electrodes of the device are connected directly to an oscilloscope to collect the output voltage changes. However, in case of the piezoresistive mechanism, we needed to measure the signals under a short circuit, so we additionally connected a reference resistance and a power source to characterize the voltage change of the device under voltage bias state. The tensile strain was applied by using a motion controller with a speed of 20 mm/s (Autonics Co., PMC-2HS 2axis Motion controller). To measure a degree of applied pressure, a mechanized z-axis stage (Future Science, 0.1 μm resolution) and a force gauge (Mark 10) were used ( Figure S2). The surface potential distribution and contact potential difference (CPD, V CPD ) was collected by Kelvin probe force microscope (KPFM) (Multi-Mode 8, Bruker) with a Pt coated tip. Accuracy of tactile classification was acquired by using the Confusion matrix. [2]

I.3. Statistical Analysis
Most of data were presented as mean ± standard deviation (SD), obtained from at least five independent experiments. Two-way analysis of variance (ANOVA) and Confusion matrix were used for statistical analysis. The statistical values of p < 0.01 were shown, indicating the reliability of this statistical method. The numbers of sample size (n) for each experiment were indicated in the figure legends. The statistical analysis was carried out using IBM SPSS® software.

II. Mechanisms of Thin Dielectric Formation along the PDMS Wrinkled.
The key point of the fabrication processes of this multimodal sensor is curing condition.
There are three factors of curing PDMS after the spray coating of diluted PDMS solution including temperature, time, and placed position. Especially, to successfully manipulate the thin dielectric along the surface of wrinkled PDMS substrate, the sample placed position during the curing is the most important part to adjust various applied fluidic forces in PDMS solution right after spray-coating step ( Figure S3). After the coating, gravitational, capillary, and viscous forces are dominant to reduce the roughness substrate with the PDMS solution. [3] In this mechanism, rough surface can be flattened by filling the trough part of the PDMS wrinkle under normal curing positon. For the case of upside-down curing position, however, the gravitational force is opposite to the previous case by inhibiting to fill the roughness of the wrinkled substrate, which results in coating thin film morphology of the PDMS dielectric layer. This thin shape of PDMS dielectric maintains large surface area of the sensor substrate, leading to enhanced electrical performance of the device (Figure S4).

III. Mechanisms and Importance of Friction Force during Dragging Events.
As shown in Figure S7, the shape and time scale of output voltages between triboelectric and piezoelectric mechanisms are different from each other. We can decouple the dragging motion into two step, touch and lateral movement. When our device touches the surface of a detecting object, the triboelectric voltage with a sharp narrow peak is generated by contact electrification. After that, the piezoelectric voltage peak with a sinusoidal wave appears due to the stick-slip behavior (adhesion) between the device and the object under the lateral movement. At the end, the detachment produces the negative signal of piezoelectric voltage.
The friction involves the effective factors of both an adhesion and a deformation component. [4] The adhesion component is directly connected to the notion of real area and surface energy of contact (sum of micro-structured contact areas). The deformation component is associated with the geometry and deformation of asperities that resist the relative motion of the contacting surfaces. In solid mechanics, surface roughness (i.e. geometric characteristics of surface topography at a small scale) of materials is main contributor to friction.

III. 1. Dragging Detection with the Modification of Chemical and Physical Morphology of PDMS Dielectric.
We also changed the dielectric morphology of the E-skin with chemical and physical conditions ( Figure S8). When the dielectric is tuned with trichloro(1H, 1H, 2H, 2Hperfluorooctyl)silane, the surface is replaced with Fluorine atom (F), which results in increasing electronegativity of the surface. In this reason, more electrons are collected on the dielectric surface, followed by increasing surface charge potential and generating more output voltage from triboelectricity. In case of adhesion, however, lower surface energy because of the fluorine atom lead to lower adhesion force, which affects to reduce friction force and output voltage of strain and vibration from dragging force. When the dielectric is flat with a fluorinated surface, because of lower surface area, the friction force and surface charge density apparently became lower. These result in poor voltage signal from the shear force.
Therefore, the E-skin with thin dielectric and high surface energy is the most adequate device for detecting the shear force and applying texture recognition.

III. 2. Preparation of various contacted objects for material/texture recognition application.
Various materials are tested to identify and classify the material and texture. Each sample were cut into 2.5 cmⅹ2.5 cm dimension and attached on slide glasses (2.5 cmⅹ2.5 cm) by using a double-sided tape except for human skin (Figure S9 and S10). To investigate the detailed texture property variation, we prepared diverse morphologies of PDMS surface by modifying surface energy, roughness, and modulus. The surface energy of PDMS is easily controlled by UVO and CF 3 -silane treatment [1] . As shown in Figure S11, roughness of PDMS is modulated by fabricating micropillar structure with a mold [5] and foam one with a sugar cube. [6] In addition, the mixing ratio of pre-polymer and crosslinking agent are chosen with 5:1, 10:1, 20:1, and 30:1, as modulus decreases and stickiness increases. The overall output electrical signals are varied with the material substance and surface properties (surface charge potential and adhesion force) of each material (Figure S12-S14).