Energy Harvesting from Human Biomechanical

IETE Journal of Research

ISSN: 0377-2063 (Print) 0974-780X (Online) Journal homepage: http://www.tandfonline.com/loi/tijr20

Energy Harvesting from Human Biomechanical Energy for Health-monitoring Devices Parul Chaudhary & Puneet Azad To cite this article: Parul Chaudhary & Puneet Azad (2018): Energy Harvesting from Human Biomechanical Energy for Health-monitoring Devices, IETE Journal of Research To link to this article: https://doi.org/10.1080/03772063.2018.1530074

Published online: 22 Oct 2018.

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IETE JOURNAL OF RESEARCH https://doi.org/10.1080/03772063.2018.1530074

Energy Harvesting from Human Biomechanical Energy for Health-monitoring Devices Parul Chaudhary1,2 and Puneet Azad1 1 Department of Electronics & Communication Engineering, Maharaja Surajmal Institute of Technology, New Delhi 110058, India; 2 University School of Information, Communication & Technology, GGSIP University, Delhi 110078, India

ABSTRACT

KEYWORDS

Conversion of human biomechanical energy into electrical form leads to the development of triboelectric nano-generator (TENG) for the operation of low-power health-monitoring devices. Here, we report a self-powered energy conversion system for sustainable operation of health-monitoring devices consisting of a TENG, an ac–dc converter, and a buck converter. The portable TENG comprising films of polytetrafluoroethylene, Mylar, and aluminium converts rotational motion into useful electricity. We have demonstrated the use of TENG as a power source for health-monitoring devices such as pulse rate monitor and thermometer. It is also capable of measuring the speed of cycling using an odometer and moisture content in the atmosphere using a hygrometer. The maximum energy is found to be 1269 µJ/cm3 across a 1000 µf capacitor. Also, the maximum power across a 2 M resistor is 240 µW. Such portable systems can act as a power source by harvesting human biomechanical energy while cycling for low-power electronics useful for medical diagnostics and fitness monitoring.

Energy harvesting; Buck converter; TENG; Health monitoring; Skin temperature; Pulse rate

1. INTRODUCTION The rapid growth and inventions in technology have led to the innovations of fitness and health-monitoring devices beneficial for our society. Regular monitoring of vital signs such as skin temperature, heart (pulse) rate, blood pressure, and humidity of atmosphere plays a key role in preventing any health hazard or injury while running, cycling, or exercise. Also, it plays an important role in alarming an emergency in elderly and chronically ill people. Temperature monitoring is essential as the skin temperature begins to fall by 3–4°C immediately after starting the exercise. Hyperthermia and hypothermia are extreme situations when the temperature deviates from its normal level and causes complicated health risks in humans leading to heat stroke, heat exhaustion, and heat cramps [1,2]. Higher skin temperature results in dehydration as it is directly related to the sweat level. High level of skin temperature gradient between feet may predict neuropathic ulceration [3], which demands the need for self-monitoring. Pulse rate is another important factor, which is associated with cardiovascular diseases in human beings. It gives vital information regarding various physiological factors relating to health and nervous systems. A normal heart rate of 60–100 beats/minute is considered healthy for a resting human, but it varies with the intensity physical activity [4], and may result in sudden cardiac arrest. Thus, its recovery is essential to © 2018 IETE

prevent any health hazard by regular monitoring especially during cycling or physical exercise [5]. Pulse rate is the physical sensation of a heartbeat felt through the vascular system of arteries. Pulse and heart rates differ only when the conditions affect only the heart and the blood vessels. Blood pressure is another important indicator for checking the status of the cardiovascular system [6] and is strongly related to instantaneous heart rate, which increases during exercise. Another important health diagnostic tool is a hygrometer for measuring the relative humidity of the atmosphere. The evaporation of water from the skin depends on the concentration of water relative to the temperature in the atmosphere. The sweat from our body does not get absorbed when the humidity in the atmosphere is very high. Sweating makes us uncomfortable and sticky. The value of humidity and air temperature dictates human comfort working conditions. It controls various factors like skin moisture, thermal tactile sensation in fabrics, and air quality. Midrange humidity level also controls the many airborne diseases and improves the working conditions. It should be maintained within the range of 40–60% [7,8]. Thus, health monitoring has become progressively important keeping in mind the safety and health concerns of people of all ages. Numerous health benefits are associated with physical exercises, including cycling, which has a positive relation with cardiorespiratory and inverse relation with

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P. CHAUDHARY AND P. AZAD: ENERGY HARVESTING FROM HUMAN BIOMECHANICAL ENERGY

Figure 1: SEM images of (a) aluminium, (b) PTFE, and (c) Mylar

all-cause mortality [9]. It also helps in reducing diabetes and improves the overall health [10]. Recent advances in wireless technologies, internet of things (IOTs), and low-power electronics have led to the inventions and use of wearable medical devices for effective health monitoring. Such battery-operated devices may rely on ambient energy such as solar energy [11–14], piezoelectric energy [15–18], thermal energy [19–21], triboelectric energy [22–27], and pyroelectric energy [28,29].These self-sustaining sources are cost-effective with no operational and maintenance cost. Here, we have considered the triboelectric effect, which involves charge transfer due to the frictional contact between two materials. These charges are collected via an external circuit to generate electricity. An interesting prototype of wearable fibrebased TENG composed of fibre super capacitor for harvesting human biomechanical energy was reported in 2015 [30]. The design presents a self-charging power system to convert human motion into useful electricity. Wearable TENG consisting of the nanofibrous membrane converts human biomechanical energy such as simple hand tapping into electricity [31]. It involves the electrospinning method for enhancing triboelectric polarity and different design approaches for enhancing device robustness, performance, stability, and resistance. Another article proposed humidity-resisting (HR)-TENG for harvesting energy from human biomechanical motion for wearable electronics. Special electrospun nanofibrous membranes are used for eliminating the adverse effects of humidity on the output [32]. In the present study, we have attempted a triboelectric-based simplified design by converting human biomechanical energy during cycling for running low-powered health-monitoring devices.

2. MATERIALS AND METHODS The present experimental investigations have been conducted using the films of MYLAR and polytetrafluoroethylene (PTFE) as triboelectric materials and aluminium as an electrode. MYLAR is obtained from UKI Insulations, Lancashire (UK), with a thickness of 50 μm and PTFE from Dalau Fluoropolymer Products (UK)

with a thickness of 125 μm. Mylar has an affinity to attract positive charges, while PTFE lies far away in triboelectric series and becomes negatively charged when they come in frictional contact with each other. Scanning electron microscopy (SEM) images are shown in Figure 1 for MYLAR and PTFE, which shows the surface quality of films. The development of triboelectric charge during sliding motion is strongly affected by the surface quality of the films. Figure 2 illustrates a typical schematic of triboelectric nano-generator (TENG) from human biomechanical energy attached to the rear gear of the bicycle. It is designed by rapping Mylar and PTFE on the outer and inner cylinders on the top of films of aluminium layers. The radii of inner and outer cylinders are 3.75 and 3.95 cm and the length of both the cylinders is 22 cm. However, in real applications, the gear of the cycle will be responsible for the vertical motion of the inner cylinder. The circular motion of the gear is converted to vertical motion of the inner cylinder, which moves in and out of the outer cylinder. Opposite charges are generated on the films of PTFE and Mylar due to rubbing of cylinders, which are induced on the aluminium electrodes on both the sides. The charges are extracted from the aluminium surfaces by disengaging the two cylinders during vertical motion. A current will flow from one electrode to another generating an electric potential to cancel the induced potential due to triboelectricity. The output voltage from TENG is converted into the dc value using a bridge rectifier and is further stepped down to 1.5 and 3 V using a dc–dc buck converter for running different low-powered devices such as the pulse rate monitor (watch), thermometer, hygrometer, odometer for speed of the cycle, etc. The circuit diagram of the experiment as shown in Figure 3 comprises a TENG, a bridge rectifier consisting of four BAT 54 schottky diodes, and a buck converter for stepping down the voltage for running low-powered devices. Firstly, the output of the rectifier is connected to different capacitors and the voltages are recorded on the Fluke 287 Electronic logging multimeter. Energy density is computed across all the load capacitors


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Figure 2: Schematic for TENG from human biomechanical energy

Figure 3: Circuit diagram of the experiment

using the relation E = 12 Cin V 2

(1)

The output of the capacitor is fed to the buck converter in which the Gate control of the MOSFET is powered from the dc voltage obtained from the TENG itself. The buck converter steps down the capacitor voltage to a suitable level for low-power electronics related to health monitoring. When the switch is closed, the diode is reverse biased, and the voltage across the inductor [33] is Vl = Vs − Vo =

Ldi , dt

(2)

To calculate the change in current while the switch is closed, Equation (2) is modified for a duty cycle D, as follows: di iL iL Vs − Vo = = = dt t DT L

Vs − Vo DT (5) L When the switch is open, the diode is forward biased and the voltage across the inductor is

(il )closed =

Vl = −Vo =

where Vl is the voltage across the inductor, Vo is the output voltage,Vs is the input dc value (from rectifier), L is the inductor value and i is the current flowing through the inductor.

or

Rearranging Equation (1), we get

(il )open = −

di Vs − Vo = dt L

(3)

(4)

di Vo =− dt L

Ldi dt

(6)

(7)

Vo (1 − D)T (8) L For steady state operation, the initial inductor current should be the same at the end, so net inductor current

4


over the period is zero. Thus, (il )closed + (il )open = 0

(9)

Using Equations (5) and (9), we have Vs − Vo Vo DT + (1 − D)T = 0 L L

(10)

Solving we get Vo = Vs D

(11)

Thus, the output is regulated by the value of the duty cycle and is either less or equal to the input value. Furthermore, the output power across the load resistor is given as V2 P = out RL

(12)

3. EXPERIMENTAL RESULTS The open circuit voltage and short circuit current generated through TENG are shown in Figure 4 reaching maximum values of 8.45 V and 24.76 μA. It is demonstrated that the open circuit voltage and short circuit current maintain levels of above 8 V and 20 μA, respectively, for the entire duration. These ac signals are converted into uni-directional signals using a bridge rectifier and the output voltage is stored in different capacitors (4.7, 10, 100, 220, 470, 1000, 2200, and 4700 μF). The variation of the output voltage with respect to time across different capacitors without buck converter is demonstrated in Figure 5. It is observed that there is a rapid increase in voltage across small capacitors as compared to bigger capacitors. The voltage across 4.7 μF capacitor rises immediately to 6 V in 38 s achieving a maximum

Figure 4: (a) Open circuit voltage and (b) short circuit current

Figure 5: Output voltage vs. time plots across load capacitor without load resistance

of 7.8 V in 240 s. Similarly, the 10 μF capacitor achieves 6 V in 60 s. Bigger capacitors take more time to charge but retain charge for a longer duration, which is helpful in running low-powered electronic devices. The maximum voltage attained by 4700 μF capacitors is 0.6935 V in 360 s. Due to a smaller time constant, small capacitors charge faster and attain saturation in less time as compared to big capacitors. The stored energy in various capacitors is illustrated in Figure 6, with a maximum energy of 1269 μJ/cm3 across the 1000 μf capacitor as mentioned in Equation (1). The energy stored gradually increases in bigger capacitors as compared to small capacitors. The voltage across all the capacitors is varying and will not be useful for running any device. A buck converter of Figure 3 steps down the voltage of the capacitor (Cin ) to achieve a constant output for running health-monitoring devices such as the thermometer and pulse rate monitor.


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Figure 6: Energy stored in the capacitor under no load conditions Figure 7: Output voltage across buck converter

Table 1 shows the inductances and duty cycles for driving the gate of MOSFET of the buck converter. The MOSFET (ZVNL110) utilizes the energy from a similar TENG with smaller cylinders of 3 cm height, which produces a voltage of 1 V for the operation. It is shown in Table 1 that an output voltage across a 100 μF capacitor of 1.5 V is obtained using a 81 μH inductor with 25% duty cycle and 3 V using a 270 μH inductor with 45% duty cycle for running devices. The output voltage across the buck converter is shown in Figure 7 using 81 and 270 μH inductors. The current requirement of devices running at 1.5 and 3 V is mentioned in Table 1; hence, a maximum power of 48 μA is required to run all the devices. Furthermore, different load resistors are connected in parallel across the output capacitors (Cout ) of Figure 3. It is shown in Figure 8 that a maximum of 240 μW of power is obtained across a resistance of 2 M, which is sufficient to run the devices stated in Table 1. Figure 9(a) illustrates the demonstration of TENG with the engagement of the inner and outer cylinders due to the circular motion of the flywheel (gear). The actual circuit of Figure 3 for ac–dc conversion and stepping down the voltage using the buck converter is as shown in Figure 9(b). Figure 9(c–f) illustrates various vital devices related to health monitoring such as skin temperature measurement using the thermometer, atmosphere humidity using

Figure 8: Power vs. load resistances

a hygrometer, pulse rate using a pulse watch, and speed of cycling using an odometer. The assembly of the odometer comprises a sensor-magnet unit integrated on the wheel tire of the bicycle. The sensor comprises a wire coil, which generates a small current every time the wheel faces the magnet. Thus, the generated signal is sent to the receiver cum display device installed on the handle of the cycle and speed is demonstrated. The odometer can perform multiple functions, including speed and distance measurement for fitness application. The observations of the

Table 1: Diﬀerent devices powered using buck converter Voltage across Cin 7.8 V 7.8 V

Inductor value

Duty cycle

Output voltage

Devices powered

Current requirement

81 µH 270 µH

25% 45%

1.5 V 3V

Thermometer Pulse rate watch, Hygrometer, Odometer

9 µA 3 µA, 5 µA, 16 µA

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be used to calculate the physiological strain index value [38,39].

Figure 9: Demonstration of biomedical applications: (a) TENG, (b) actual circuit, (c) measurement of skin temperature using thermometer, (d) measurement of humidity and temperature using hygrometer, (e) pulse rate monitor watch, and (f) odometer for speed measurement

above-mentioned vitals are useful in preventing dehydration, heatstroke, and fatigue. A range of ideal values, risk levels, and observed variations for these parameters are given in Table 2. Figure 9(c–f) illustrates the measurement of skin temperature, relative humidity of atmosphere, pulse rate, and speed of the cycling. All the measured parameters are below the risk level for a 32-year-old male person as mentioned in Table 2. The combination of heartbeat rate and skin temperature can

This parameter is an essential evaluator for people at risk of heat stress and heat stroke during any physical activity. It rates the physiological strain on a scale of 0–10, which gives the risk amount of suffering from it. Furthermore, to check the feasibility of the experiment, the variation of skin temperature and pulse rate is recorded while cycling for a healthy person as shown in Figure 10. Skin temperature is an important parameter affecting the aerobic performance through the impact of hypohydration [35]. It is a state which results in loss of body water. As observed from Figure 10(a), there is a 1.2°C fall in skin temperature over a period of 12 min of cycling by a 32-year-old male. The fall in skin temperature during the muscular effort is due to the vasoconstriction during exercise. It is a state in which constriction of blood vessels takes place resulting in an increase in blood pressure. It is observed that with a decrease in cycling speed to 10 km/h during 10th minute, the skin temperature increases to 33.2°C but decreases further with increase in speed. A normal pulse rate of 60–100 beats/minute is considered healthy at rest, but high pulse rate may result in sudden cardiac arrest. As shown in Figure 10(b), the pulse rate increases with the speed of cycling and goes to a maximum of 148 beats/min. At the 10th minute, when the speed of cycling falls to 10 km/h, the pulse rate decreases and attains a value of 135 beats/min.

Table 2: Ideal level and risk level of various factors for human Level

Pulse rate (bpm)

Pulse rate during exercise

Skin temperature after exercise

Environment humidity

Ideal level Risk level Observed variations

60–100 100 above [34,35] 70

220-A > 220-A [36] 70–148

31–37°C Below 30°C and above 38°C [34,35] 33–34.2°C

45–55% > 60% [37] 53%

Note: A is age of the person.

Figure 10: Variation of (a) speed and skin temperature with time and (b) speed and pulse rate with time


4. CONCLUSION In the present study, PTFE and Mylar are used as triboelectric materials with aluminium as an electrode for energy harvesting by converting rotary motion into sliding motion. An attempt has been made to give a demonstration of converting human biomechanical energy into electricity for running health-monitoring devices such as skin temperature and pulse rate measurement. The voltage produced during the motion is stepped down using the buck converter to 1.5 and 3 V, which replaces the traditional batteries in the devices. Furthermore, the assembly is capable of generating 1269 μJ/cm3 across a 1000 μf capacitor and 240 μW of power across 2 M resistor. It is also capable of observing the speed of cycling and distance travelled.

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Authors Puneet Azad received the PhD degree from Gautam Buddha University, Greater Noida (UP), India, and ME degree from Delhi Technological University (formerly Delhi College of Engineering), Delhi, India. He is presently the Head of Department and Associate Professor in Department of Electronics and Communication Engineering in Maharaja Surajmal Institute of Technology (GGSIP University), New Delhi, India. His research interests are energy harvesting using different techniques, power electronics and their applications. Corresponding author. [email protected]

Email:

[email protected];

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Parul Chaudhary received the BE and MTech degree (with honors) in Electronics and Communication engineering from the Maharashi Dayanand University, Rohtak, in 2009 and 2011, respectively. She is currently working as Assistant Professor in Maharaja Surajmal Institute of Technology (GGSIP University) Delhi, India. Her research interests include design and analysis of various energy harvesting techniques and renewable energy systems. Email: [email protected]