Development of Device for Measurement of Skin Potential by Grasping of the Device

Recently, Mental Work-Load (MWL), owing to the use of mobile phones and tablet PCs, has become a major social problem. Although an appropriate level of MWL positively affects human health, long-term excessive MWL can cause fatigue, which adversely affects concentration and thus leads to human errors and consequent health hazards. Evaluation of MWL of workers is critical for preventing and reducing such effects. Autonomic nerve activity is closely related to physical and physiological stress; its measurement or estimation using biological information can serve as the foundation for methods of objectively assessing bodily activity and physical and physiological stress in humans [2]. A method is available for measuring the skin potential activity, which serves an index for evaluating MWL. When humans experience mental burden such as tension or excitement, emotional perspiration appears at skin sites such as the palm and sole [3]. Skin potential varies according to this emotional state. The extant methods measure skin potential activity considering the entire arm, as illustrated in Fig. 1. The probe electrode, reference electrode, and earth electrode are attached to the palm, forearm refraction portion, and forearm portion, respectively. As this method measures the parameters by attaching electrodes, it is suitable for continued evaluation of MWL. However, as it necessitates pasting the electrodes over the whole arm, it is not suitable for providing instantaneous feedback if the subject requires it. If skin potential activity (and consequently emotional sweating) can be measured with an electrode placed only on the palm and by the subject gripping the device, development of a portable load-evaluation interface for measuring MWL is feasible. Therefore, in this study, we developed the prototype of a portable MWL evaluation interface and investigated whether skin potential activity can be evaluated with an electrode of this device kept in contact with the palm.

Conventional method of attachment

Full size image

Emotional sweating is a reaction to mental excitement. This reaction is rapid and involves the cortical premotor cortex, limbic system, hypothalamus, etc. It is evaluated by physiological indexes such as perspiration amount and skin potential activity (SPA). SPA is categorized into: SPR (Skin Potential Reflex), which is an alternating current (AC) component, and SPL (Skin Potential Level), which is a direct current (DC) component. SPL is a reaction of arousal degree. SPR responds with mental arithmetic and thinking. The amplitude reflects the strength of the stimulus [1].

It has been confirmed in previous studies that the amount of perspiration differs at each part of the hand such as fingers and mother-child spheres. Instead of pasting the electrode on the body, we conceived a device that enabled measurement by it being grasped. Figure 2 illustrates the proposed device. Considering the nature of the actual application, a mobile terminal case was utilized. In addition, as illustrated in Fig. 2, by attaching the electrode to the side of the case rather than vertically, measurement was possible by touching any point on the side surface of the case. Electrodes were prepared using tin foil tape. The probe, reference, and ground electrodes were attached to the side of the measurement case. When gripping the case, the finger and palm were required to touch the case at the electrodes. Consequently, we created two devices: (1) one in which the electrodes were distributed and pasted over both sides faces of the case; and (2) one in which all the electrodes were pasted over one side face of the case.

The proposed device

Full size image

In the case of (1), wherein electrodes are attached to both sides, there is a possibility of measurement of the skin potential activity from the difference in the amount of perspiration at the contacting parts. However, in the case of (2), wherein the electrode is attached to one side, as the positions of all the electrodes are close to each other, the amounts of perspiration recorded by them are highly similar. Consequently, there is a possibility that large skin potential activity may not be measurable. Therefore, we examined whether skin potential activity can be measured by varying the widths and positions of the electrodes.

4.1 Protocol

Subjects took rest for approximately 3 min before the experiment started. Then, the subjects closed their eyes, and as a stressor, pain was stimulated on the forearm by striking three times with a rubber hammer. To ensure uniform intensity of stimulation, we unified the quality of rubber, length to extend (15 cm), width to hook (5 cm). The subjects were five healthy male university students in their twenties. For comparison of the proposed devices, measurement was carried out simultaneously using a skin potential meter (Nishizawa Electric Co., Ltd. SPN-01). The skin electrometer was attached to the subjects’ left hand by the conventional attachment method. On the subjects’ right hand, the proposed devices were placed and grasped by the subject and measurements were taken. The width and position of the electrodes are presented in Fig. 3 and Table 1.

Position of electrodes

Full size image

4.2 Evaluation Method

Emotional sweating by pain sensory stimulation is evaluated by skin potential. “Spr" signify the skin potential value measured by the extant research method, and Nspr is that by the proposed device. The proposed method is evaluated using the ratio of the amplitudes of Spr and Nspr.

4.3 Experimental Results and Discussion

(1) Electrodes Attached to both Sides: Figure 4 (a) illustrates the waveform when the width of the search electrode on Subject 1 is 3 mm, and Fig. 4 (b) illustrates the waveform when the width is doubled to 6 mm. In the figure, the time at which the stimulus is provided is set as 0 s. The reaction to the stimulus in the case of both Nspr and Spr occurred approximately 2 s after stimulation. The fact that the reactions appeared simultaneously in the two methods indicates that the proposed device has the capacity to measure mental sweating. However, the proposed method recorded lower amplitude than the conventional method. The amplitude ratio is presented in Table 2. Although there are individual differences, for three of the subjects, amplification was increased when the area of the probe electrode was doubled. Moreover, the amplitude reversed for all the subjects. The probable cause for this is that the amount of perspiration was larger on the palm than on the finger.

Results of case (1)

Full size image

(2) Electrodes Attached to One Side: The results are illustrated in Fig. 5.

Results of case (2)

Full size image

For this device, in the case of both Nspr and Spr, the reaction occurred approximately 2 s after stimulation. This demonstrated that the proposed device is capable of measuring emotional sweating. The amplitude ratio is presented in Table 3. From Table 3, it can be observed that the amplitude measured using this method is marginal when compared to that by the conventional method. This marginal difference in impedance is attributed to the close proximity of the measurement site. However, it was demonstrated that the amplitude increased as the width of the search electrode was widened. An increase in the width of the search electrode led to a decrease in the impedance. As a result, the potential fluctuation on the search electrode side becomes marginal. Thus, potential fluctuation on the reference electrode side is considered to be emphasized.

In this study, we developed two prototype portable MWL evaluation interfaces and investigated the effectiveness of measuring skin potential activity using hand contact with the electrode. Fluctuations in the waveform were observed in both the devices, one of which had electrodes attached on both sides of the device and the other had electrodes attached to one side of the device. Moreover, the feasibility of burden measurement in the palm area was demonstrated.