**3. Design of the measurement device**

vessels or destroys them, sensitivity to tactile sensations is restricted. Most diabetes patients, even at an early stage of the disease, have reduced sensitivity to tactile sensations in the fingers

Diabetic neuropathy (DPN) is caused by the degradation of axons in peripheral nerves, which decrease nerve function in slow progression. The rate of degeneration depends on the ability of the patients to control their glycemic index; thus, it varies with each individual. Nerves are distributed throughout the body and vary in function. For that reason, neurological diagnostic methods vary depending on the parts and functions of the nerve distributed, thus making uniform standards difficult to formulate. For example, in the event of dysuria arising from DPN, the patient may consult a urologist. However, a patient who feels discomfort or numbness in the sole of the foot owing to DPN may consult an orthopedist, and both patients may not consult a diabetes specialist until their condition has degraded significantly. DPN presents a variety of symptoms that patients are likely to consult a range of specialists for the same underlying condition. Diagnosis of DPN is so complicated and time-consuming that even

Neuropathy can also be caused by other diseases, but DPN is distinguished by a few symptoms. DPN presents diffuse neuropathy, with bilateral symmetry. The nerve failure is focused on

and feet. The extent of the decline is a measure of the progress of the condition.

**Figure 2.** Innervation density of tactile receptors [18].

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many diabetes specialists are not equipped for quantitative studies.

sensory functions, and DPN tends to progress from peripheral nerves inward.

#### **3.1. A compact SMA actuator to generate micro-vibrations**

To generate the physical stimuli, an SMA wire was employed. Within the typical operating temperature range, SMA has two phases, each with a different crystal structure and therefore different properties. The first is a high-temperature phase, called the Austenite phase, and the second a low-temperature phase, called the Martensite phase. When the temperature exceeds a critical threshold (70°C), the SMA alternates between the two phases, causing the crystal structure, and therefore the shape of the SMA, to change. SMA has been widely used in actuation and sensing applications and in the aerospace, automotive, and biomedical sectors.

When SMA is formed into a thin wire, its length originally 3 mm at a low-temperature phase will change at a known temperature. In the current study, the SMA wire (Toki Corp., BioMetal, BMF75) was used to create a compact actuator, the characteristics of which are shown in **Figure 3**. When the temperature of an SMA wire passes T1 (68°C), the wire begins to shrink up to 5% lengthwise at the temperature T2, reaching a minimum at T2 (73°C). As the temperature is reduced, the wire gradually returns to its initial length.

As the alloy has an electrical resistance of 0.6 ohms per 1 mm, its length can be controlled by supplying a pulse current. This instantaneously increases the temperature, shrinking the wire.

**Figure 3.** Characteristics of the SMA wire [3].

**Figure 4.** Pulse signal for driving SMA [3].

When the pulse current is halted, the body instantly cools, returning to its initial length. The shrinkage and return are fully synchronized with the ON/OFF pulse current, as shown in **Figure 4**. The magnitude of the vibration created can be precisely controlled by the amplitude of the pulse signal H and the duty ratio W/L. For an efficient operation, the SMA temperature must be maintained within the range T1–T2. In our design, a pulse-width modulated (PWM) rectangular wave signal with an arbitrary frequency, amplitude, and duty ratio is generated by a PC and is then amplified to drive the SMA actuator. The amplifier drives the SMA actuator at frequencies up to 300 Hz. The voltage amplitude is variable and controlled by the current. According to the measurement results of our research so far, the SMA wire shrinks by ~2 μm at the maximum according to the duty ratio of the pulse current. Therefore, according to the duty ratio, the overall length of the SMA wire was observed to be shrinking from 0.1 to 2 μm. The detailed driving pulse signal for each amplitude level of vibration is shown in **Table 1** [3].

While most SMAs have a slow response time, the BMF75 wire with a diameter of 75 μm can respond within less than 1 ms and was used to create the compact vibration actuator.

The subject only touches the actuator lightly to eliminate the disturbance of the actuator due

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The vibration stimulus generated by the SMA wire is transmitted to the subject through the round-head pin (**Figure 5**) described below. The pin actually touched by the subject is shown

to the skin reaction force.

**Table 1.** Driving signal for each amplitude level.

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**Table 1.** Driving signal for each amplitude level.

When the pulse current is halted, the body instantly cools, returning to its initial length. The shrinkage and return are fully synchronized with the ON/OFF pulse current, as shown in **Figure 4**. The magnitude of the vibration created can be precisely controlled by the amplitude of the pulse signal H and the duty ratio W/L. For an efficient operation, the SMA temperature must be maintained within the range T1–T2. In our design, a pulse-width modulated (PWM) rectangular wave signal with an arbitrary frequency, amplitude, and duty ratio is generated by a PC and is then amplified to drive the SMA actuator. The amplifier drives the SMA actuator at frequencies up to 300 Hz. The voltage amplitude is variable and controlled by the current. According to the measurement results of our research so far, the SMA wire shrinks by ~2 μm at the maximum according to the duty ratio of the pulse current. Therefore, according to the duty ratio, the overall length of the SMA wire was observed to be shrinking from 0.1 to 2 μm. The detailed driving pulse signal for each amplitude level of vibration is shown in **Table 1** [3]. While most SMAs have a slow response time, the BMF75 wire with a diameter of 75 μm can

**Figure 3.** Characteristics of the SMA wire [3].

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**Figure 4.** Pulse signal for driving SMA [3].

respond within less than 1 ms and was used to create the compact vibration actuator.

The subject only touches the actuator lightly to eliminate the disturbance of the actuator due to the skin reaction force.

The vibration stimulus generated by the SMA wire is transmitted to the subject through the round-head pin (**Figure 5**) described below. The pin actually touched by the subject is shown

**3.3. Tactile display for the detection of diabetes mellitus**

way that the tips of two fingers were in contact with the array.

diabetic subject with severely compromised tactile sensitivity.

**Figure 7.** Presentation of tactile vibratory stimuli [3].

As shown in **Figure 6**, eight actuators were arranged as arrays. Two of these made up the tactile presentation area. The patient placed the index and middle fingers on these in such a

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The presentation of vibratory stimuli makes use of higher-level tactile perceptual processes [21]. The pins in each array were driven by the pulse current signals with a time delay, as shown in **Figure 7**. This was expected to create an apparent perception of movement and the subject to experience a vibrating object moving from Ch. 1 (fingertip) to Ch. 8 (the second finger joint). The apparent movement of the stimuli could be controlled by varying the time delay of the pins. To confirm that perception of apparent movement could be generated, a pilot study was run, using three healthy subjects, in which the frequencies and the amplitudes were varied using different time delays. Based on the results, the amplitude of the vibrations was divided into 30 levels. The lowest amplitude represented a stimulus that was difficult for healthy people with normal tactile sensitivity to perceive, while the strongest could be perceived even by a

As shown in **Figure 4**, the amplitude of vibration was controlled by selecting the parameters W [ms]: pulse width, L [ms]: period, and H [V]: amplitude. These parameter values were carefully selected to allow the vibration to be increased linearly from level 1 to 30 (**Table 1**). To examine the lowest threshold of tactile sensitivity of the index and middle fingers, a tactile sensation threshold (TST) score or peripheral neuropathy vibration (PNV) score were used. The subject was asked to place the index and middle fingers on the pin arrays. Tactile stimuli were then presented at different frequencies and amplitudes and in randomized directions. Using "yes" or "no" responses, the system measured the threshold of tactile perception and its relationship to the severity of attenuation. We named our proposed method the finger method.

**Figure 5.** Structure of vibration actuator [3].

in **Figure 6**. As shown in **Figure 6**, the test equipment is shaped in a manner such that the subject can lightly touch the middle and index fingers on the pin array.

Tests were conducted at room temperature, 20–30°C, controlled by air conditioning. We did not use any electromagnetic shielding as the actuator will need to function in unshielded clinical settings.

#### **3.2. Vibration actuator with a round-head pin**

To make the actuator usable for tactile screening of diabetes, the micro-vibration generated by the SMA wire required amplification. A round-headed pin was therefore fixed at the center of the SMA wire, transforming the movement of the SMA wire into vibration. As shown in **Figure 5**, the actuator comprised an SMA wire, 75 μm in diameter and 3 mm in length, and a round-headed pin, 1.4 mm in diameter and 3 mm in length.

Shrinking and expansion of the SMA wire was continuously synchronized by the ON/OFF pulse current. This induced vibration in the round-headed pin, allowing even diabetic patients with reduced tactile sensitivity to recognize the tactile stimuli when the vibration pins were brought into light contact with the fingertips.

**Figure 6.** Tactile input for diabetes screening [3].

#### **3.3. Tactile display for the detection of diabetes mellitus**

**Figure 5.** Structure of vibration actuator [3].

**3.2. Vibration actuator with a round-head pin**

brought into light contact with the fingertips.

round-headed pin, 1.4 mm in diameter and 3 mm in length.

clinical settings.

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in **Figure 6**. As shown in **Figure 6**, the test equipment is shaped in a manner such that the

Tests were conducted at room temperature, 20–30°C, controlled by air conditioning. We did not use any electromagnetic shielding as the actuator will need to function in unshielded

To make the actuator usable for tactile screening of diabetes, the micro-vibration generated by the SMA wire required amplification. A round-headed pin was therefore fixed at the center of the SMA wire, transforming the movement of the SMA wire into vibration. As shown in **Figure 5**, the actuator comprised an SMA wire, 75 μm in diameter and 3 mm in length, and a

Shrinking and expansion of the SMA wire was continuously synchronized by the ON/OFF pulse current. This induced vibration in the round-headed pin, allowing even diabetic patients with reduced tactile sensitivity to recognize the tactile stimuli when the vibration pins were

subject can lightly touch the middle and index fingers on the pin array.

**Figure 6.** Tactile input for diabetes screening [3].

As shown in **Figure 6**, eight actuators were arranged as arrays. Two of these made up the tactile presentation area. The patient placed the index and middle fingers on these in such a way that the tips of two fingers were in contact with the array.

The presentation of vibratory stimuli makes use of higher-level tactile perceptual processes [21]. The pins in each array were driven by the pulse current signals with a time delay, as shown in **Figure 7**. This was expected to create an apparent perception of movement and the subject to experience a vibrating object moving from Ch. 1 (fingertip) to Ch. 8 (the second finger joint). The apparent movement of the stimuli could be controlled by varying the time delay of the pins.

To confirm that perception of apparent movement could be generated, a pilot study was run, using three healthy subjects, in which the frequencies and the amplitudes were varied using different time delays. Based on the results, the amplitude of the vibrations was divided into 30 levels. The lowest amplitude represented a stimulus that was difficult for healthy people with normal tactile sensitivity to perceive, while the strongest could be perceived even by a diabetic subject with severely compromised tactile sensitivity.

As shown in **Figure 4**, the amplitude of vibration was controlled by selecting the parameters W [ms]: pulse width, L [ms]: period, and H [V]: amplitude. These parameter values were carefully selected to allow the vibration to be increased linearly from level 1 to 30 (**Table 1**).

To examine the lowest threshold of tactile sensitivity of the index and middle fingers, a tactile sensation threshold (TST) score or peripheral neuropathy vibration (PNV) score were used. The subject was asked to place the index and middle fingers on the pin arrays. Tactile stimuli were then presented at different frequencies and amplitudes and in randomized directions. Using "yes" or "no" responses, the system measured the threshold of tactile perception and its relationship to the severity of attenuation. We named our proposed method the finger method.

**Figure 7.** Presentation of tactile vibratory stimuli [3].

#### **3.4. Experimental procedures for detection by diabetes mellitus subjects based on tactile sensation threshold scores**

disorders in this fashion leads to patient fatigue and boredom, and we cannot expect to see any response for the weakest stimuli. Therefore, we developed a protocol to shorten the inspection time. We began examining all subjects at the middle stimulus intensity of 15. The test stimulus was presented to the subject two or three times. Only subjects who correctly answered 66.7% or more with the test stimulus, that is, examinees who correctly answered at least twice, are next presented with stimulus intensity of 7, which is halfway between the minimum and middle intensities. Subjects who do not detect a stimulus at intensity 15 are presented next with a stimulus intensity of 22. This process significantly reduces the time needed to determine a subject's reaction threshold. Ultimately, the lowest stimulus intensity that was detected more than two-thirds of the presented intensity was defined as the tactile

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threshold for that subject.

firmed [1].

in diagnosing DPN.

used to evaluate DPN.

**4. Verification of early detection of DPN**

**4.2. Validation of DPN evaluation for diabetic patients**

**4.1. Pilot study to confirm tactile reduction in long-term diabetic patients**

The device was first used in a pilot study of 15 diabetic patients with a long history of treatment, and a significant decrease in tactile sensation compared with healthy subjects was con-

The device was next used to validate the evaluation of DPN in diabetic patients [2]. Based on the criteria [19] for diagnosis of DPN provided by the American Diabetes Association (ADA), tactile sensation was quantified, and a comparison was made of patients with and without

The goal of this part of the study was to investigate the effectiveness of the proposed method

A cross-sectional study was conducted of 52 type 2 diabetic outpatients. Patients were evaluated for DPN using the ADA criteria, the Michigan Neuropathy Screening Instrument (MNSI), and our proposed finger method. Patients were assigned to probable DPN or non-DPN groups, based on the ADA criteria. The finger method was used to produce a PNV score from the index and middle fingers, using the three procedures introduced above: PNV 1, PNV 4, and PNV 8. The scores ranged from 1 to 30, and comparisons were made between the two groups.

The PNV scores of the DPN group were significantly higher (P < 0.01). The PNV scores for the right fingers of the DPN and non-DPN groups were 10.2 ± 7.4 and 3.4 ± 3.3 in PNV 1, 20 ± 4.9

Overall, the tactile threshold of the DPN group was higher than that of the non-DPN group. The results suggested that the finger method, performed using the proposed device, can be

and 10.7 ± 5.3 in PNV 4, and 23.2 ± 4.9 and 14.6 ± 7.8 in PNV 8, respectively (**Table 2**).

DPN. A significant reduction in tactile sensitivity was confirmed in the DPN group.

Three different procedures were performed. In the first, tactile stimuli were presented to both fingers simultaneously, in a single direction starting at the fingertips (Pattern 6 in **Figure 8**). Subjects were asked if they had perceived the stimuli. This procedure is known as the tactile sensation threshold 1 direction test (TST-1) or PNV 1 direction test (PNV 1) and was used to investigate the perception of tactile stimuli in two fingers. In the second procedure, a moving stimulus was presented to one of the two fingers in a random direction, and the subject was asked to identify both the finger and the direction of movement. In this procedure, known as the tactile sensation threshold 4 direction test (TST-4) or PNV 4 direction test (PNV 4), the subject was asked to identify the tactile perception as matching one of the four patterns shown in **Figure 8**. In the third procedure, known as the tactile sensation threshold 8 direction test (TST-8) or PNV 8 direction test (PNV 8), stimuli moving in random directions were applied to one or both fingers, and the subject was asked to match the finger(s) and direction of movement with one of the same eight patterns.

In all procedures, the examination began at a stimulus intensity of 15. Based on the accuracy of the answer given, the next round started at an intensity of 22 or 7.

Again, based on the accuracy of the answer given, in the next round, a stimulus intensity of 26, 19, 11, or 4 was presented to the subject. The stimulus intensity was then changed until the subject gave a correct answer 66.7% or more of the time. This TST score or PNV score was defined as the tactile threshold. The value for the tactile threshold was defined as the lowest value among the 30-stage stimulus intensity in which subjects were able to correctly answer more than 66.7%.

To reduce the examination time as much as possible, we applied a protocol to stimuli levels in 30 successive stages.

In our preliminary research on healthy subjects, we gradually increased the stimuli intensity from the weakest stimulus to the strongest. Inspecting patients with obvious neurological

**Figure 8.** Eight patterns of moving directions of tactile stimuli [3]. Mid = middle finger; Indx = index finger.

disorders in this fashion leads to patient fatigue and boredom, and we cannot expect to see any response for the weakest stimuli. Therefore, we developed a protocol to shorten the inspection time. We began examining all subjects at the middle stimulus intensity of 15. The test stimulus was presented to the subject two or three times. Only subjects who correctly answered 66.7% or more with the test stimulus, that is, examinees who correctly answered at least twice, are next presented with stimulus intensity of 7, which is halfway between the minimum and middle intensities. Subjects who do not detect a stimulus at intensity 15 are presented next with a stimulus intensity of 22. This process significantly reduces the time needed to determine a subject's reaction threshold. Ultimately, the lowest stimulus intensity that was detected more than two-thirds of the presented intensity was defined as the tactile threshold for that subject.
