Bioinspired Electronic Whisker Arrays by Pencil‐Drawn Paper for Adaptive Tactile Sensing

DOI: 10.1002/aelm.201600093 spatial mapping as well as detailed objects’ localization, orientation, and shape reconstruction. Figure 1a illustrates a cartoon cat with several long facial vibrissae drawn by a pencil. Under the inspiration of the biofunctions of vibrissae, we design a device that imitates the mammal’s tactile perception to detect the environment factors. The scheme of the target device, known as e-whisker, is shown in Figure 1b, which is simply fabricated using pencil trace by a graphite pencil that drawn on a piece of paper. Figure 1c demonstrates the paper covered with graphite trace, and Figure 1d shows the corresponding SEM image. We can see that the paper covered with a large number of graphite particles in Figure 1d and Figure S1e,f of the Supporting Information. For a comparison, we investigate the morphology of a blank paper on which it shows rough and porous surface composited with a great many cellulose fibers (Figure S1a, Supporting Information). It should be noted that the special rough structure of the paper surface could facilitate the covering graphite particles and flakes. As a more specifically experimental process to cover a jointed layer of graphite, a 2B pencil is scratched on the paper forward one direction, the pencil graphite particles or flakes can be exfoliated and coated around the cellulose fibers (see Figure S1b, Supporting Information). Repeatedly, the pencil is drawn on the paper for ten time, the graphite particles or flakes are strongly adhered on the paper and form a well layer of graphite chunks with a flat surface, as shown in Figure S1c of the Supporting Information. The Raman spectrum of the pencil trace shows the characteristic peaks of graphite in Figure 1e. The spectrum shows three pronounced peaks at 1350, 1573, and ≈2700 cm−1 of graphite, corresponding to the D, G, and 2D bands, respectively (see Figure 1e). The graphite pencil trace at outward and inward deflections is schematically illustrated in Figure 2a. It describes that the pencil trace on paper can be well conducted as a strain sensor in which resistance of the graphite varies with strains of paper deflection. Figure S2 of the Supporting Information shows changes in resistance have linear relations both with applied strains at tensile (outward) and compressive (inward) bending. The sensing properties of the sensor originate from the reversible microcontacts change in graphite flakes of the pencil-drawn paper.[13] Specifically, microcracks would appear in neighboring graphite flakes when applied a tensile strain, leading to a pronounced increase of resistance (see Figure 2b,d and Figure S2, Supporting Information); On the contrary, the neighboring graphite flakes could reconstruct overlaps when applied a compressive strain, resulting in an obvious decrease of resistance (see Figure 2c,e and Figure S2, Supporting Information). Based on strain sensing properties of the pencil-drawn paper, we design a cantilever structure using the pencil-on-paper Many mammals, e.g., cats, rats, and seals, relying on their long facial whiskers instead of their vision system, can easily find their way in narrow, dark or murky environments. The whiskers (or vibrissae) of these animals provide remarkable tactile units to sense the external environment. For example, cats are capable of discriminating all of an object’s spatial properties, including size, orientation and shape, according to tactile feedback from their whiskers. Nowadays, new approaches for developing artificial electronic whiskers (e-whiskers) to mimic mammalian vibrissal tactile perception are of great importance in advanced robotics,[1–4] artificial intelligence,[5] and human-machine interfaces.[6,7] Some recent advances in piezoelectric,[8–10] resistive,[11–13] and capacitive[14,15] devices have been reported to sense tactile signals adaptively. To data, e-whiskers have been outlined with binary contact sensor, capacitor microphone sensor, force/torque sensor, strain gauge, etc.[2] Among them, e-whiskers based on highly strain-sensitive nanostructured films have been recently demonstrated the capability of sensing the environmental factors such as strain (CNT-Ag nanoparticles)[6,7] and temperature (PEDOT:PSS-CNT).[7] However, the complex fabrication process and large expenditure of these reported e-whiskers would restrict their broad applications. Hence the facile and cost-effective approaches for e-whiskers are urgent to be explored. Pencil drawn paper, known as pencil-on-paper approach, illustrates an effective way of constructing graphitic strain sensors in a solvent-free or nonvacuum manner, with high gauge factor (GF), fast response, and good durability.[12,16–20] Moreover, personalized strain sensors can be easily fabricated only with a pencil, a pair of scissors, and a piece of paper in a few minutes. In addition to multiple advantages of the fabrication methods itself, paper-based devices also exhibit many virtues of lightweight, disposable, and easily-available characteristics.[19–22] Here, we present a versatile approach to fabricate bioinspired e-whisker arrays with a superior strain sensitivity (GF = 34), fast response (50 ms), and high durability/stability by using pencil-drawn paper. It demonstrates a good performance on adaptive tactile sensing/imaging, strain monitoring, 3D www.MaterialsViews.com www.advelectronicmat.de