The purpose of our research is to make tactile feeling as a kind of sensation which can be recorded, reproduced, and transmitted, just like vision and auditory sensations.

We separate this goal into two parts. One is a tactile display, by which we could feel any kinds of cutaneous sensations. The other is a tactile sensor, by which we collect contact information from real objects.

Ultimately, we combine these two into one tactile information system. Though there have been many works on tactile sensors and displays, most of them were done separately and didn't consider making total system.

Tactile Display
Until now, we were working on tactile feeling display.

2D Array Electrode and Index Finger
Prototype 2D Array Electrode and Index Finger

Electric Mouse Electric Mouse(Magnified)
64 Electrodes on Optical Mouse Using Electric Mouse
Electric Mouse. Designed as a Braille. Each electrode is 1mm in diameter, stainless. 2mm pitch. 64 electrodes are placed to fit fingertip.
Load cell (force sensor) is located under the electrodes to measure finger pressure.
Electrical current is controlled as a function of this force.

``Tactile feeling'' is a sensation produced in our skin when we touch or rub something. If we generate tactile feeling in VR(Virtual Reality) space, we can not only experience more realistic presence, but also perform more difficult tasks.

Most previous works on tactile display are variations of pin-head type, or belt type displays, which vibrate vertically or horizontally on the skin. They are mechanical devices and have some difficulties. They are huge, difficult to fabricate, and not able to produce all kinds of tactile sensations.

Some other studies proposed to use electrical current as a stimulus. Though fabrication becomes much easier, most of them lacked principle and stayed ad-hoc system, because they didn't fully understand what was happening inside skin.

Our first approach was to consider mechanisms of tactile sensations. There are four types of mechanoreceptors in human skin and tactile sensation is basically a combination of these receptors' activity. Therefore, if we could stimulate these receptors separately, we could generate any sensations. We name these stimuli ``tactile primary colors'' after three primary colors (RGB) in vision, which stimulate different types of cone cells in human retina. Our main finding was that by using electrical stimulation, we could realize these primary colors.

We considered mechanisms of electrical stimulation. There are two processes in it. One is a map between current source distribution on skin surface (I(x,y)) and external potential along afferent nerves(V). The other is a map between this potential and membrane potential difference (Vm). Writing these maps mathematically, we derived activation probability of nerve as a function of current source distribution. Consequently, the problem of stimulating specific nerves is generalized to mini-max problem of this function.

We found out that by utilizing each receptor's geometry, we can separately stimulate each type of mechanoreceptors in two ways. One is to use arrayed electrodes instead of a single one. The other is to use an anodic current as a stimulus, as well as a cathodic current.

We also developed experimental system and tested these methods. In each stimulation mode, subject felt sensation of pressure, low frequency vibration, and high frequency vibration separately. These results implies that the designed stimuli succeeded in activating desired mechanoreceptors' axons.

As our display system uses array electrodes, making multi-position stimulation was achieved easily. By scanning, we succeeded in presenting sensation of movement (apparent movement).

Some practical problems arose. As skin impedance changes with time and place, and nerve activity largely depends on it, our generated sensation is not at all stable. Now we are making a system that utilizes electrodes as an impedance sensor. By using the information of the state of the skin, we will control input current dynamically to compensate for the variation of skin.

Tactile Sensor (Dummy Finger)
To develop tactile sensor that gathers contact information and sends it to the tactile display, we utilize the fact that our tactile display is capable of stimulating each type of mechanoreceptors separately. There are two approaches.

One is to get all precise data of an object that is related to touch; such as height, friction coefficient, elasticity, and so forth. Once the information is acquired, we simulate each receptor's behavior to calculate the tactile display's output.

The other approach is to make a tactile sensor that has artificial mechanoreceptors inside. These receptors gather information that real mechanoreceptors do. In this case, the sensor body should have the same mechanical dynamics as a real finger. Because of the resemblance with auditory dummy head, we name it ``dummy finger.''