Ultrathin Flexible Piezoelectric Sensors for Monitoring

Journal of Micromechanics and Microengineering

ACCEPTED MANUSCRIPT

Ultrathin Flexible Piezoelectric Sensors for Monitoring Eye Fatigue To cite this article before publication: Chaofeng Lu et al 2017 J. Micromech. Microeng. in press https://doi.org/10.1088/1361-6439/aaa219

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MS. NO. JMM-103325.R

Revised manuscript

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Ultrathin Flexible Piezoelectric Sensors for Monitoring Eye Fatigue Chaofeng Lü1,2,3†*, Shuang Wu1†, Bingwei Lu4, Yangyang Zhang1, Yangkun Du5, Xue Feng4 1. College of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, P. R. China

2. Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou 310027, P. R. China

3. Soft Matter Research Center, Zhejiang University, Hangzhou 310027, P. R. China

4. Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P. R. China

5. Department of Engineering Mechanics, Zhejiang University, Hangzhou 310027, P. R. China

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† These authors contribute equally to this work.

*Corresponding author, E-mail: [email protected]

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Abstract: Eye fatigue is a symptom induced by long-term work of both eyes and brains. Without proper treatment, eye fatigue may incur serious problems. Current studies on detecting eye fatigue mainly focus on computer vision detect technology which can be very unreliable due to occasional

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bad visual conditions. As a solution, we proposed a wearable conformal in vivo eye fatigue monitoring sensor that contains an array of piezoelectric nanoribbons integrated on an ultrathin flexible substrate. By detecting strains on the skin of eyelid, the sensors may collect information about eye blinking, and, therefore, reveal human’s fatigue state. We first report the design and fabrication of the piezoelectric sensor and experimental characterization of voltage responses of the piezoelectric sensors. Under bending stress, the output voltage curves yield key information about

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the motion of human eyelid. We also develop a theoretical model to reveal the underlying mechanism of detecting eyelid motion. Both mechanical load test and in vivo test are conducted to convince the working performance of the sensors. With satisfied durability and high sensitivity, this sensor may efficiently detect abnormal eyelid motions, such as overlong closure, high blinking

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frequency, low closing speed and weak gazing strength, and may hopefully provide feedback for assessing eye fatigue in time so that unexpected situations can be prevented.

Keywords: ultrathin flexible piezoelectric sensor; eye fatigue; eyelid motion; real-time monitoring;

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AUTHOR SUBMITTED MANUSCRIPT - JMM-103325.R1

electromechanical analyses 1


1. Introduction Eye fatigue has been recognized as an important public health issue that is most commonly

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caused by reading, writing, or driving. Eye fatigue has serious adverse impacts on maintaining daily behavior and physical fitness. For example, eye fatigue may induce ophthalmic diseases, among

which computer vision syndrome is a common condition that results from focusing the eyes on computers or other digital devices for protracted, uninterrupted periods [1]. In contrast, fatigue

driving that is always accompanied with eye fatigue could be most dangerous and may even lead to mass casualties [2, 3]. A survey suggests that among all injuries or deaths, traffic accidents from

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work to home is one of the major causes while fatigue is responsible for up to 20~30% of all road

fatalities [4]. For professionals who engage in special occupations, e.g. surgeons, eye fatigue can result in work faults and cause potential safety hazard [5]. Therefore, developing a fatigue

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monitoring system and assessment method is essential for preventing accidents.

Among various techniques, most researchers have been focusing on those developed for collecting physiological signals such as electroencephalogram (EEG), electrocardiogram (ECG), or

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pulse rate [6, 7]. However, these methods are not convenient for practical use under work or driving circumstances because contacting hard electrodes and cables are indispensable. Head-mounted devices developed for monitoring eyelid motion and eye gaze [8], although remove the inconveniences of using electrodes and cables, are subject to limitations for their distractions to human’s daily activities. To avoid these distractions, computer vision detect technologies have been

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proposed to monitor eye fatigue through real-time eye movement capturing [9]. However, the success of applying these vision detect techniques relies significantly on image resolution, light condition, and other environmental conditions, and is not yet satisfying in veracity and instantaneity [10].

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Recent progresses in flexible/stretchable electronics [11-14], that may be integrated in conformal contact with human skin and deform into arbitrary shapes without detachment from the epidermal surface, have offered new strategies to retrieve vita signals. Representative devices based on flexible/stretchable electronics include sensors for measuring body temperature[15], pulsing [16],

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breathing [17], electro cardio[18], and blood flow[19] etc. as well as for supercapacitors [20-22]. Since the eye fatigue may be recognized through the eyelid motion [23], we herein introduce an 2

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ultrathin piezoelectric sensor to monitor the real-time eyelid motion, thus making assessment on the fatigue state of eyes. Here, we chose lead zirconium titanate (PZT) as the functioning material to

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monitor the eyelid motion. This is due to fact that the processing technics for PZT are well-developed due to its high Curie temperature, and the piezoelectric coefficient of PZT is high which makes it able to output as high voltage signal as possible when subject to a relative small

mechanical deformation. The core functional part of the sensor is made of PZT nanoribbons and the sensor has an entire thickness of the order 10 μm so that it can maintain conformal contact with the eyelid skin during eye blink. Mechanical load test and in vivo test are carried out to validate the

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functionality and efficiency of the sensor for monitoring the real-time eyelid motion. A simple

theoretical electromechanical model is also developed to correlate the output voltage of the sensor with the physical motion of the eyelid, thus further demonstrate the functionality of the sensor for

2. Sensor design and fabrication

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monitoring and assessing eye fatigue.

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Fig. 1(a) illustrates the exploded-view of the ultrathin flexible ribbon-structured sensor. The main functional element is designed into a sandwich-like structure with a layer of PZT ribbon (1.5 μm) laminated between top (Au/Cr, 200 nm/10 nm) and bottom (Pt, 300 nm) electrodes (Fig. 1b). The PZT sensing module consists of 6 groups of 2 such ribbon structures that are electrically connected in parallel by a layer of Au/Cu (100 nm/600 nm) interconnects. Each of the 6 groups is

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connected in series to its neighboring group so as to increase the output voltage. The whole functional module is encapsulated by a thin spin-casted layer of polyimide (PI) at above and Kapton substrate beneath.

Fig. 2 provides the schematic diagrams of the fabrication process of the functional module of

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the sensor. Here, we adopt the well-developed MEMS techniques [24-26], e.g. metal deposition and mask lithography, to create the sandwich-like PZT structures. First, for a PZT film with a bottom electrode Pt on a silicon wafer, a layer of Au/Cu is sputtered onto the PZT film as an upper electrode, and, photolithography and wet etch process is used subsequently to define the previously designed

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patterns. Every layer is designed smaller in size than the layer below to make place for further mask lithography. This functional element is then integrated with a receiving substrate of Kapton using 3


the well-established technique of transfer printing [27]. As preparation, we use a thick mask of hard-baked (250oC for 1 hour) PI as protection layer for the PZT elements and undercut the

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sacrificial SiO2 layer with hydrofluoric acid (H2O : 49%HF=1:3 by volume). A PDMS stamp serves as vehicle to retrieve the PZT element from the donor substrate at room temperature. Following the

law of fracture competition [28], a fast peeling speed guarantees the capture of element. We then print the PZT element onto a Kapton substrate with a layer of pre-cured PI (from poly (pyromellitic

dianhydride-co-4, 40-oxydianiline) amic acid solution; 4,000 rpm for 30 s, pre-cured on a hot plate

at 110oC for 2 mins) by compressing the PDMS stamp. Heating the system up to around 150oC for a

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few minutes may fully cure the tacky interface and finish the process of transfer printing. After removing the stamp, another layer of PI is spin-casted on the top as the isolation layer. To enhance the electric responses, another round of sputtering and patterning of Cu/Au are conducted to

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interconnect all functional elements electrically in series and parallel following the manner as mentioned before. Since the top and bottom electrodes are of different heights, a negative photoresist enpi202 is studied and introduced to perforate horizontal paths for interconnection.

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Then, we hot-press a row of ACF cable onto the two ends of the circuit as for external connection (Fig. 1c). Finally, a layer of encapsulation is spin-casted onto the functional module as the overall protection. Upon two hours of polarization with a 90V DC voltage source and simultaneous 130oC heating, the PZT sensor is eventually activated.

3. Results and discussion

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Mechanical load test is performed first to validate the functionality of the sensor (length 2.0 cm) by forcing it to out-of-plane deformation with the help of a vibration exciter. The sensor is perfectly bonded onto a silicone ribbon (thickness 1.0 mm, length 5.0 cm) at the center part, for which one end is fixed on a glass chunk and the other is rigidly anchored with the hand tip of the vibration

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exciter (Fig. 3a). The vibration exciter outputs a triangular wave of 4 Hz to deform the structure back and forth with a maximal end-to-end displacement of  L  2.5 mm (Fig. 3b and red line in Fig. 3d). The output voltage of the sensor exhibits a very stable feature with a peak-to-valley value

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of about 200 mV (Fig. 3c) that is sufficient large to record using an oscilloscope. This demonstrates the reliability of the sensor for detecting the periodic mechanical deformation of the 4

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silicone ribbon substrate. The correlation between output voltage of the sensor and the input mechanical stimulation

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can be captured by an analytical electromechanical model. Under the action of an end-to-end compression  L(t ) , the sensor (length L0 ) undergoes buckling with an out-of-plane displacement in a sinusoidal configuration, and the amplitude may be approximated as 2



 L  L0 [24]. The PZT ribbons, together with the electrodes, bend with the buckled sensor,

 PZT  4

zp

L

L0

L0

, where  is the ratio of bending

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with an axial strain given analytically as

rigidity per unit width of the two-layered substrate (PI plus silicone ribbon) to that of the substrate with PZT ribbons and electrodes, while zp is the distance of the PZT ribbon from the bottom of

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the substrate. The axial strain in the PZT ribbon induces an electrical charge of

Q  e31 Ap PZT  33 Ap VPZT hp [29], where e31 , 33 are the effective piezoelectric constant

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and permittivity, Ap and hp are the effective area and thickness of a single PZT ribbon, while

VPZT is the voltage drop across the PZT ribbon. Since the current PZT sensing module consists of 12 PZT ribbons with every 2 connected in parallel and then in series, the total output voltage V may be determined according to the definition of current and Ohm’s law by the following equation

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3hp e zh d dV  V  24 31 p p dt 33 Ap R 33 L0 dt

L ,

(1)

L0

with R denoting the electrical resistance of the oscilloscope. The input end-to-end displacement

 L may be derived by integrating the above equation with respect to time t . The material, geometrical, and circuit properties of the current sensor are e31  40.5 10

12

C/N ,

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33  8.854  108 F/m [30, 31], Ap  1.216 10 7 m 2 , hp  1.5 μm , zp  51.986 μm ,

  0.808 , and R  10 MΩ . By inputting the measured voltage in Fig. 3(c), the calculated

end-to-end displacement (blue line) agrees very well with the actual input signal (red line in Fig.

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3d). This indicates that the mechanical deformation of the silicone ribbon substrate can be well predicted by the theoretical electromechanical model together with the voltage outputs of the PZT 5


sensor. The in vivo test of the sensor yields a good relevancy between the output voltage and the

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actual motion of the eyelid (Fig. 4). The volunteer wears the sensor on the upper eyelid by a thin layer of biocompatible adhesive for medical use, and the sensor keeps as flat as possible when the

eye is closed (Fig. 4a). The adhesive used here was proved to be strong enough to bond the sensor

on the eyelid for several hours, but will not pose any uncomfortable feeling to the volunteer since the overall bending rigidity of the device/adhesive to that of the eyelid is only 0.69×10-4. A slight

out-of-plane buckling deformation almost in a sinusoidal form is observed in the sensor together

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with the eyelid when the eye opens (Fig. 4b). The output voltage in Fig. 4(d) exhibits a steady variation (constant frequency and peak-valley value) that reveals very well the four stages of a normal eyelid motion, i.e. closing, closure, opening and gazing, which correspond to the voltage

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rising until the peak, dropping back to zero, inverse rising to the valley, and decreasing until and remaining zero (Fig. 4c). For the normal eyelid motion in Fig. 4(c) and 4(d), the first three stages occurs within an extremely short period (~0.2 s) that is identical to published statistics [32]. In

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contrast to the mechanical load test, the substrate here is replaced by the eyelid with a typical thickness and modulus of 1.0 mm and 3.7 MPa [33, 34] , while zp  9.642 μm and

  0.793 . Using the measured voltage for normal eyelid motion in Fig. 4(d), the calculated end-to-end displacement of the sensor also exhibits a periodic feature (Fig. 4e), where an average gazing platform of 0.6~1.0 mm stands for a lucid and energetic mental state. The sharp decreasing

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of the displacement right after the blink attributes to a slight relaxation of the eyelid after opened consciously to a maximal extent during the in vivo test. An overlong closure during a series of normal eyelid motions is detected by the sensor which corresponds to the part of zero voltage before its inverse rising (Fig. 5a). This overlong closure

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may indicate high possibility of eye fatigue and is of great concern in the assessment of driving safety because it represents a completely blind driving distance [1-4]. Fig. 5(b) captures the moment of sudden rapid eye blinking that is normally recognized as a stress behavior of self-sobering when people feel tired or sleepy [35]. This can also be a supporting evidence of eye

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fatigue. Fig. 5(c) demonstrates a gradually decreasing in voltage amplitude together with the lag of 6

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inverse rising, which provides typical combined signals for bluntness of eyelid behavior and thus suggests a state of sever eye fatigue. This eye fatigue phenomenon is further predicted by the

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electromechanical model for the end-to-end displacement of the sensor (Fig. 5d). After the gazing stage (nonzero platform), the closure keeps for a longer period (zero displacement) than that of the normal eye blink (Fig. 4e). In addition, the weakening phenomenon of the gazing state is also observed from the theoretical predictions where the nonzero platform decreases (Fig. 5d).

4. Conclusions

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In summary, an ultrathin flexible piezoelectric sensor for monitoring eye fatigue was designed,

fabricated and tested. The sensor was fabricated using the conventional MEMS procedures together with the technique of transfer printing. The key functional module of the sensor is designed in an

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ultrathin configuration of microscale so that it may contact conformally with the eyelid surface any distraction to the eyelid due to its extremely small bending rigidity. In addition, the use of piezoelectric materials makes the sensor free from external batteries. Both mechanical and in vivo

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tests validated the functionality and efficiency of the sensor to retrieve input mechanical deformations or eyelid motions, thus demonstrating its feasibility of monitoring the real-time eyelid motion. A simple electromechanical model was also developed to correlate the output voltage of the sensor with the eyelid motion. The predicted results may provide key information of the eyelid motion, e.g. blinking frequency, closing speed, closure period, and gazing period, that can also be

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reflected by the output voltage in time as evidences for assessment of eye fatigue. The results here suggest an innovative and efficient way of detecting eye fatigue and may provide guidance for

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future design of sensing devices.

Acknowledgements

C.L. acknowledges the financial supports by the National Natural Science of China under

grant no. 11322216 and 11621062. X.F. acknowledges the financial supports by the National

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Natural Science Foundation of China under grant no. 11320101001.

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Figure captions

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Figure 1 The flexible piezoelectric sensor for monitoring eyelid motion and eye fatigue. (a) Schematic diagram of multilayered structure of the sensor from tilted view and top view; (b) Optical microscope images during fabricating processes; (c) Photograph of a PZT sensor with ACF cables for external connection; (d) Photograph of a PZT sensor on Eco-flex substrate revealing its flexibility. Figure 2 Fabrication flow chart of the flexible piezoelectric sensor.

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Figure 3 Mechanical load test on the PZT sensor and theoretical predictions on the input mechanical load. (a) Experimental setup of the mechanical load test; (b) Bending of the

piezoelectric sensor together with the silicon ribbon substrate under an end-to-end

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displacement load ΔL; (c) The output voltage of the sensor under a triangular wave load input; (d) The theoretical prediction of the input load compares well with actual triangular load.

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Figure 4 In vivo monitoring of normal eyelid motion and the theoretical predictions. (a) Photograph of the PZT sensor with eyelid closed; (b) Photograph of the PZT sensor with eyelid open; (c) Correspondence of the output voltage to the four stages of a period of eyelid motion in normal mental state; (d) Output voltage of a series of normal eyelid motion; (e) Theoretical predictions of the end-to-end displacement of the sensor based

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on the measured voltage in (d) and its correspondence to the real-time eyelid motion. Figure 5 In vivo monitoring of three main types of abnormal eyelid motion and theoretical predictions. (a) Output voltage of eyelid motions with a significant overlong closure before the next eye opening; (b) Output voltage of normal eyelid motions with a

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subsequent transition to hastily eye blinking; (c) Output voltage of eyelid motions with gradual bluntness that represents a sever eye fatigue; (d) Theoretical predictions of the end-to-end displacement of the sensor based on the measured voltage in (c).

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Au PZT Pt

(a)

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Figure 1

Perforated(PI)

Tilted view

1cm

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ACF cable

an

(c)

(b)

us

Interconnections Top view

Ac

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Eco-flex

Kapton


Au/Cr PZT Pt SiO2 Si

cri pt

Figure 2 Photolithography and etching

Undercut of SiO2

After deposition of Au/Cr Printing

Peeling up

Kapton Isolation with PI

Interconnection with Au/Cu

ce

pte

dM

an

PI

us

PDMS stamp

Ac

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Page 12 of 15

Au/Cu

Page 13 of 15

cri pt

Figure 3 (b)

(a)

L0 - D L

0 -100 1

2

3 Time (s)

4

5

2 1 0 -1

ce

pte

dM

-200

Actual Theoretical

3

0

1

2 3 Time (s)

an

100

4

2cm

us

(d)

200

Displacement (mm)

Voltage (mV)

(c)

Ac

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4

5


(c)

(b)

Voltage (mV)

(a)

20

10 Closing 0 -10

10 0

0

1

2 3 Time (s)

4

5

ce

pte

dM

-20

0.8

1.0

1.2 1.4 Time (s)

1.6

1.8

Normal eye blink

Normal gazing zone

1.2 0.6 0.0

an

-10

Opening

us

20

(e) 1.8

Eyelid opening

Displacement (mm)

Voltage (mV)

Eyelid closing

Closure

Gazing

-20 0.6

(d)

cri pt

Figure 4

Ac

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Page 14 of 15

0

1

2 3 Time (s)

4

5

Page 15 of 15

Figure 5

0

1

2 3 Time (s)

4

5

0 -10 2 3 Time (s)

ce

pte

1

0 -10 -20 0

4

1

5

2 3 Time (s)

4

5

Overlong closure

Normal gazing zone

1.2

an

10

0

10

(d) 1.8

20

-20

20

hastily eye blink

cri pt

-10

30 Normal eye blink

us

0

Displacement (mm)

Voltage (mV)

Voltage (mV)

10

-20

(c)

(b)

Overlong closure

dM

Voltage (mV)

(a) 20

Ac

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0.6 0.0

0

1

2 3 Time (s)

4

5