Block diagram of sensor system 

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... I NTRODUCTION There is a growing demand for a tactile sensor covering the whole surface of the robots that work in our daily life [1][2] in order to interact with humans and environment softly and safely based on tactile information [3]. Several robots having tactile sensors on their bodies are demonstrated until now for the purpose of elder care [4] or touch interaction [5]. The tactile sensor arrays implemented on those robots are low resolution, i.e. a few sensors on each body parts. A higher-resolution tactile sensor array is desirable to detect detail touch information. Various kinds of tactile sensors have been reported for that purpose, e.g., piezoelectricity [6], 1-bit touch sensors [7], multi-valued touch sensors [8], capacitance [9], resistance [10], and so on. The arrays of tactile sensors introduced above may suffer from wires from a huge number of sensors. In order to manage this problem, several approaches have been proposed. Telemetric skin [11] is an approach, in which tactile sensor chips are distributed within a silicone rubber body and both of signal transmission and power supply are based on inductive coupling. For more effective and high-speed communication, the two-dimensional signal transmission [12] is developed in after years, which uses microwaves propagating within a two-dimensional sheet instead of inductive coupling. In organic transistor matrixes [13], not only sensing elements but also wires are completed by printing technologies. The narrow flexible substrates with a serial bus enable us to change the network configuration easily in [14]. In this paper, we propose a new approach to manage the problem of wiring. We focused on human tactile sensors (i.e. mechanoreceptors) which a located densely underneath skins [15]. They are connected to the brain via axons, which are part of living neurons and assemble themselves in embryos. We got an idea from this process: If we can utilize axons as wires, it is possible to connect a huge amount of tactile sensors to a central computer thanks to the self-assembly feature of axons. In other words, the communication network is formed automatically in spite of its complexity. Besides, living axons are expected to be more extensive than electrical wires and more robust. While there are a lot of breakthroughs required to make this proposition practical, we believe it is worth considering and preparing it at the present stage. In employing this approach, it is reasonable to utilize mechanoreceptors as tactile sensors because they are well compatible with each other. There are several kinds of mechanoreceptors in the human skin. Two mechanoreceptors, Merkel cells and Meissner corpuscles, have relatively higher spatial resolution among them. While a Merkel cell is a single cell, a Meissner corpuscle has a complex structure, which consists of helical-shaped axons, flat Schwann cells, collagen fibers, etc. Even though the generation process of Meissner corpuscles would provide us with helpful information in implementing cell-based tactile sensors, the detailed forming mechanism is still unknown. This paper describes our concept of cell-based tactile sensors in Section II, surveys the literature of skins and Meissner corpuscles in Section III, discusses possible developing ways and hypotheses toward our goal based on the given knowledge in Section IV, and concludes in Section V. II. C ONCEPT OF NOVEL TACTILE SENSOR In this section, we propose two types of tactile sensor based on tissue engineering: the flat sensor and the sphere-head one (Fig. 1). The mechanoreceptor illustrated in Fig.1 is Meissner corpuscle. Both types consist of an outer layer and an inner structure including mechanoreceptors which is responsible for sensing and processing. The outer layer is an elastic artificial skin-like thin film which is responsible for absorbing interaction. Its outer and inner surfaces have the ridged texture like human finger-print. The inner structure consists of an artificially tissue-engineered axons, mechanoreceptor capsules, guide tubes, and growth solution, as shown in Fig.2. The guide tubes work as tunnels leading the axon terminals from parent fibers to prepared mechanoreceptor capsules. The growth solution provides inner pressure and keeps the mechanoreceptors living since organic components may decay easily. In order to supply growth solution frequently, we are thinking about using a two-way exchange system like human circulatory system, which is not illustrated here. This type of system may decrease the mobility but increase the stability of the tactile sensor. We intend to use tissue engineering merely for necessary components such as axons and/or inner components of mechanoreceptors, instead of all. The other component parts are appropriately produced by mechanical engineering methods. This conception is supported by some recent reports which showed the potentiality of the combination of cultured cell/tissue and particular mechanical material, such as tissue-engineered jellyfish [16] and biological machine [17]. The advantage of this method is the simplicity of producing, especially for outer components as ridged-layer, compared with pure biological method. Using mechanical engineering also costs lower and provides a high chance of mass-production. Meanwhile, the problem is that the high-accuracy of mechanical component production at micrometer level is required. The 3D printing technology which has capability of producing high-detailed tiny 3D objects is a solution for this issue. In recent years, the well-developed 3D printing technology somehow already surpasses micrometer level and may achieve nanometer level in model making [18]. The detailed developing or approaching methods are described in Section IV. Mechanoreceptors are distributed in couples along the inner ridges of capsule similar to those in human skins [19] and shown in Fig.3 with the sphere-head type as example. The operation is based on deformation of the contact surface outside ridged capsule. Depend on interaction, various areas of surface are deformed and the deformation of inner surface connected to mechanoreceptors is involved. Therefore, the axon inside mechanoreceptor is also distorted and then generates changes of ion charge. In other words, contact interactions are encoded onto electrical signals and transmitted to computing system via the axon. The computing system processes analysis of the signals and extracts interaction properties. A block diagram of the tissue-engineering-based tactile sensor is demonstrated in Fig. 4. The difference between the two types of tactile sensor is how they may be applied in robotics. The flat one is appropriate to being mounted on a flat surface or sensing one-directional pressure while the sphere-head one is appropriate to being mounted on a convex surface or sense multi-directional ...