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Energy harvesting from the human body-movement during walking by integrating piezoelectric PVDF film in the garment

  1. Abstract

Commercialization of smart textiles depends on the ability to replace the bulky batteries because of their short lifespan. In this project, we developed sportswear that harvests electric energy from human motion during walking. The strategic location to attach the film was optimized by measuring the bending angle, bending frequency and imparted pressure on to the garment during the movement of our body.

Total four flexible piezoelectric polyvinylidene difluoride (PVDF) polymeric film added to the sportswear, harvesting on average 112.2mW power which can be stored in textile-based wearable supercapacitor.

1.1    Polyvinylidene difluoride (PVDF)

Piezoelectric materials are not an asymmetric shape, it’s formed of many crystal units. Whenever they undergo mechanical strain (bending) the misbalance crystal cells, resulting in a surface charge.

If all the charges are externally removed, they must be bent again to produce those charges which finally results of AC current production. Reverse piezoelectricity follows the same pattern by misbalancing their crystalline cell by the application of electricity.

As far as our knowledge, PVDF shows the strongest piezoelectric property among all polymer. It’s a thermoplastic fluoropolymer with linear polymer chain –[CF2–CH2]– basically produced by melt spinning or solution spinning can be processed into fibers and films. PVDF is highly chemically resistant and biocompatible.

Presence of two electronegative fluorine in the monomer of PVDF results in its ferroelectric, pyroelectric and piezoelectric properties. Among four phases of PVDF, only β phase shows piezoelectricity generally transformed from α phase by cold drawing or electric field.

A planar zigzag form of β phase allows F atoms to come closer and overlap in Van Dar Waals radii resulting in the piezoelectric property.

  1. Experimental procedure

2.1    Materials and instruments

Table 1: Specification of PVDF
Symbol Parameter PVDF Units
t Thickness 28 μm
d31 23 Piezo Strain

Constant

(10-12)

C/N

d33 -33
g31 216 Piezo Strain

Constant

(10-3)

Vm/N

g33 -330
K31 12% Electromechanical

Coupling Factor

Kt 14%
C Capacitance 380 pF/

cm2

Y Young’s

Modulus

2-4 1×109

N/m2

PVDF (molecular weight 60-70kg/mol) film with silver ink coated [9]Table 1 was purchased from MEAS through TechshopBD. An Arduino, capacitor, battery, resistance of 1MΩ, LED light, jumper wire, were also purchased from the same supplier.

A sealant was purchased from the local market. Sportswear, knee support, figure support, elbow support was purchased from the local market and used without any modification.

2.2    Circuit design

A circuit was built with the following connection (fig:1) to harvest energy for different frequency and bending angle. As microcontroller can’t be operated at a voltage above 5V, a resistor of 1MΩ connected in parallel to reduce voltage. A capacitor was connected in parallel to collect the charge as electric power. In addition, a LED light was used for real-time evaluation of energy produced from the film. Arduino was programmed using fade value command and analogWrite library so that the intensity of LED changes with respect to energy produced.

Circuit with Arduino
Figure 1: Circuit with Arduino, External capacitor is used to store energy to prove a continuous output of current.

The flexible PVDF film was bent at a various angle (10, 40, 55, 70) and different frequency (1Hz, 1.5Hz, 2Hz, 2.5Hz) in order to, measure voltage built on capacitor and current produced. Data was collected at a controlled environment without any interruption from sound, air circulation, moisture droplet, skin contact as all these effects the actual vibration of PVDF.

2.3    Human locomotion

Interpretation of left knee bending during walking
Figure 2. a) Interpretation of left knee bending during walking. Knee movement during single gait cycle of individual leg from right heel contact to left heel contact. b) sagittal plane1 angle during a single gait cycle of knee, hip and ankle flexion.
Figure 2. b) from Whittle’s Gait Analysis interprets integral movement of knee with respect to bending angle. At the beginning of the gait cycle heels are in contact to the ground, left leg at back warded, right leg is forwarded and legs are straightened perfectly. As the body moves forward the right foot touches the ground flexing the left knee. Left knee flex around 60-70⁰ bending angle before heel touching the ground. There are other several movements and bending at lower limbs such as ankle and hip. Ankle bends around 10⁰ and hips bend 25⁰ on average through the whole gait cycle with constantly changing direction and amplitude Figure 2 b).

2.3.1    Upper limb and trunk2 motion

Bending angle of upper limbs and trunks are measured with respect to 3 planes namely sagittal, frontal3 and horizontal4 plane.

A standard and physiologist prescribed walking pattern suggest to walk with holding the elbow at 90⁰ and swinging the arm from shoulder not from the elbow as it produces waste of energy.

Synchronization of opposite arms and legs are should be maintained for a proper balance of body while walking. Although during regular walking elbow is bent at around 45-50⁰ on average.

In order to keep the arm swinging from the shoulder, the arm moves forward and backward from the plain of the shoulder, create a bending at the shoulder joint. There are many other movements in trunk such as shoulder pelvis bending at 7.2⁰, the proximal curvature at 7⁰, shoulder rotation 6.9⁰ and shoulder pelvis rotation 13⁰.

2.3.2    Frequency of walking

Children and teenagers take a higher number of steps/min while walking than senior people comparatively. Infants younger than 2 years take above 200 steps/min whereas, teenagers take 144 steps/min at most and people of age ranging from 19-70 years old take 96-136 steps/ mins.

Although people of different age have different stride length 5 which depends on the length of the leg, that is the age of people

3     Result and discussion

3.1    Characterization of PVDF

PVDF film is best suitable for energy harvesting from human body movements as it is flexible and might be easily integrated into textiles with simple sealant adhesive (highly viscous). PVDF is highly resistant to almost all chemicals such as acids, and bases with a response time of less than 100µsec.

PVDF has almost effect on aging which can withstand a million pressure cycle from 0-200kPa. PVDF also shows pyroelectricity; it also creates electricity whenever the surrounding temperature change, makes it highly suitable for the ambient application.

Again, PVDF doesn’t require long strips of length, rather smaller the stripe length higher the current as the bending applied on smaller surface area comparatively. This property makes PVDF suitable for application in the body without sacrificing the comfort of a human.

A film (2×1 cm) was characterized in relation to the bending angle. The circuit was designed in a manner its voltage doesn’t cross 5V. As a result, the maximum current produced was 20mA.

The whole voltage was divided into battery and capacitor. Capacitor continuously saves charges produced from the film surface and the charges the battery attached in the circuit. The power generated from PVDF was stored in capacitor and then supplied to the battery. Power generation can be calculated from formula P=VI

Amount of current produced at 10⁰ bending angle of PVDF
Figure 3 Amount of current produced at 10⁰ bending angle of PVDF LDT 01 with respect frequency a)1HZ b)1.5Hz c)2Hz d)2.5 Hz

Amount of current produced basically depends on the frequency of bending, strip length and bending angle. As charges produced by misbalancing the structure of unit cells, the charge is produced every time it’s bent. Figure 3 shows currently produced due to bending at 10⁰ at a different frequency.

It is the enlarged graph of the original graph to properly understand the points of current production. Current always fall to its initial value when it returns to starting point as the film seems to be straight for microseconds.

Current generation was almost constant for the frequency of 1.5Hz, 2Hz, 2.5Hz, current generation raises to 20mA at each bending and falls to 14mA resulting in an average of 17mA continuous production.

While in 1Hz frequency fall of current is higher comparatively resulting in a lower average current. We monitored similar results while bending at 70⁰ and different bending angle (figure 4).

Amount of current produced at the 70⁰ bending of PVDF
Figure 4: Amount of current produced at the 70⁰ bending of PVDF LDT01 with respect to the frequency a)1Hz b)1.5Hz c)2Hz

3.2    Integration of PVDF in textiles

Optical image of cross-sectional view demonstrating the pasting of film with fabric
Figure 5 a) Optical image of cross-sectional view demonstrating the pasting of film with fabric. Optical image of different region of sportswear b) shoulder joint c) elbow d) knee

PVDF might be attached to textiles in different forms such as shoulder straps, spacer fabric, added to textiles with high viscous sealant.

For sportswear integration, comparatively stable and comfortable process is adding with sealant as stitching might damage final garments quality. Moreover, thermal bonding alters PVDF property.

We pasted the film on the inner side of fabric by a highly viscous silicon glue sealent (figure 5- a). The sealant creates firm and flexible film at top of PVDF film with smooth finish that is comfortable in wearing.

Later the coating was cooled down to room temperature. Solid adhesive after cooling is rubbery flexible comfortable to skin. Total 4 PVDF was added to upper limb 2 at shoulder and 2 at elbow in regular sportswear made of spandex fibre (figure 5- b, c, d).

3.3       Real-time current generation from upper and lower limb

Real-time simulation current production from PVDF
Figure 6: Real-time simulation current production from PVDF at a) Knee b) Shoulder joints c) elbow while walking at a constant frequency

Figure 6 shows real-time value of current producing from single a) knee, b) shoulder joint c) elbow. An average of 15.5 mA current, 3.4V voltage in capacitor was produced from one shoulder joint created by swinging the arm during walking.

We find total power produced from single shoulder to be 57.2 mW using eqn (1). Each elbow produces 15mW and each knee produced 40mW electric power from body movements while walking.

Elbow is held at a constant angle, so bending was less frequent although walking frequency was same. As a result, elbow produce less energy than shoulder joints and knee joints. Knee and shoulder are most promising area of human body to produce energy as they undergo high frequency and high bending motion.

  1. Conclusion

In this paper, we confirm that the PDVF film harvest electrical power from shoulder joints, knee and elbow bending while walking and the film generates on average 112.2 mW electrical power that is much higher than traditional application such as backpack 40mW laminated cantilever with 16µW, 10µW from PZT cantilever.

This smart sportswear generates such high energy without altering the comfort of garments. Moreover, the film and the coating are washable, durable, bendable and stretchable.

  1. A plain passing through midline such as naval and dividing human body in two halves considering bilateral asymmetry gives motion such as thumb, knee, elbow flexion
  2. Portion of the body after removing arms and legs.
  3. A plain passing through the body dividing it into interior and posttrial halves, any kind of lateral movements such as elevation and depression while jumping
  4. A plain dividing the whole body into top and bottom halves. Any kind of rotation occurs in this plane
  5. Difference between two heels measured from their center while both heels are at ground.
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