Powering of IOTs through single jersey wearable tribo-electric nano generator

A simple, lightweight, and easy to develop Single Jersey Wearable Tribo-Electric Nano Generators (SJ-WTENG) were constructed using Cotton and Acrylic fabrics (Triboelectric series materials). Fabrics were also coated with Maghemite (γ-Fe2O3) nanoparticles (13 nm) to increase the electrical conductance of the samples. Compression and Vertical contact-separation modes were adopted for studying the performance of the developed samples. Along with a Single WTENG sample, the outputs of two samples connected in series were also measured. To study the effect of developed Maghemite (γ-Fe2O3) nanoparticle coating, non-coated fabrics WTENGs were also constructed and tested. The maximum voltage reached with the Maghemite (γ-Fe2O3) nanoparticle-coated SJ-WTENG samples was a time-varying signal of 7.68 volts peak to peak volts with an approximate frequency of 50.5 Hertz. A shotky diode-based full bridge rectifier was used to get the DC voltage. The rectified DC signal was observed to be 5 volts which was enough to light up an LED with a threshold voltage of 1.7 volts DC as well as charge 3.7 volts, 3.6Ah Li-ion battery pack. Results confirmed that the application of Maghemite (γ-Fe2O3) nanoparticles was useful in augmenting the output of the proposed SJ-WTENG design. The proposed system can be used to power the battery powered IOT (Internet of Things) devices, widely used in medical and body sensor network applications.


Introduction
Along with the advancements in green energy production through renewable resources, there are other ways through which waste is utilized to generate power without harming the environment.Investigations have been carried out to harvest energy from ambient human body movement which previously no one thought would be another source of power production.Energy harvesting through human body movement is cheaper as it does not utilize resources like fossil fuel or carry out nuclear reactions.Two such approaches have been adopted to fulfill the green energy research; Energy harvesting from the Piezoelectric effect and energy harvesting from the Triboelectric effect.The piezoelectric effect termed as Piezo Electric Nano Generator (PENG) [1] is based on electric current generation in crystalline structure when a force/pressure is applied resulting in an increase of free electrons in the conduction band due to the breaking of crystalline bonds.On the other hand, the Triboelectric Nanogenerator (TENG) concept was discovered by John Carl in 1757 that the materials are either negatively or positively charged [2].Since materials acquire charges to exhibit the Tribo-electric effect.A triboelectric effect/charging is observed when there is a separation between the surfaces of the two tribo-electric materials, after bringing those in close contact with each other [3].The higher the magnitude of polarity between two tribo-electric materials, the more the electrical charge generation [4,5].
The triboelectric effect can be achieved by three methods: Vertical contact separation, In-plane sliding, and Single-electrode mode [2,4].Vertical-contact separation involves cyclic separation and re-contact of the oppositely charged triboelectric materials for continuous power generation.By pressurizing materials together, charge transfer commences because of the closed contact of the surfaces, and it also makes one of the surfaces acquire a positive charge while the other becomes negative.Upon recovering from deformation, an electric field sets up in between, and a potential difference is induced.AC flow in an external circuit under repeated contact and separation of the layers of triboelectric materials [2,4].It was found that the addition of a conductive material layer in the form of coatings or nano-materials on the Tribo-electric materials improved their performance [3,5,6].There are several benefits of applying Nanomaterials on triboelectric nanogenerators; especially for smart textiles.A variety of methods have been developed and there are various constructing materials to serve the purpose.Harvesting of energy through tribo-electric effect, which utilize nanomaterials for desired output, are termed as Tribo-electric nano generators (TENGs).TENGs have diverse applications especially in wearable devices for health monitoring, motion sensing, wearable interface devices and sports applications [7][8][9] and, thus many researchers have proposed different designs by coalescence of numerous types of nano structures and Tribo-electric materials.Wearable TENGs can be manufactured by using Textiles and non-textile materials [3,4,6,[9][10][11][12][13].Since textiles are easily available in different forms i.e. fibers, yarns, fabrics and garments and several materials (conductive and non-conductive), which can easily be transformed into wearable TENGs [12,14,15].Garments deform as per body movement while performing different daily chores; and especially during workout or sporting activities.It does not only provide health benefits but at the same time the forces applied by different body regions in motion, require conversion into energy and it requires wearable TENGs.These products are capable of energy harvesting, energy storing and self-powered to charge IOTs [9,12,16,17].Now-a-days, wearable technology is in massive demand [12].Textile based TENGs can be produced through several methods i.e. by utilizing materials from tribo-electric series but electric charges generation would not be enough for the charging of small batteries/IOTAs.However coating of metals on the tribo-electric materials or by deposition of Nano rods/tubes, Nano-fibers or Nano particles (NPs) enhances charges production and hence electrical current/ output voltage/ power [3,10,11,13].Conductive coatings or NPs deposition on the tribo-electric materials provides an additional conductive path to the accumulated charges on the surface of materials therefore they can light up LEDs or charging IOTs.Output of TENGs produced from Textiles is greatly affected by the constructing materials properties, structure properties and manufacturing techniques [4].A composite based TENG film was constructed by using thermoplastic polyurethane (TPU) matrix with polyethylene glycol (PEG) additives and polytetrafluoroethylene (PTFE) nanoparticle [6,18].Optimal output power measured was 16.8 mW at an external resistance of 200 kΩ.However, for composites flexibility, softness washing are concerns plus the cost due to manufacturing process is higher.In 2017, a highly stretchable, self-charging and storing TENGs was manufactured using knitted textiles.It was able to produce peak power density of approx.85 mW•m -2 and lit at least 124 light-emitting diodes [17].Because of the knitted structure, which is inherently elastic, TENG was flexible and since constructing material was textiles, it was light in weight, stable and washable either.It was a good attempt as most of the garments are constructed from knitting technology.In another investigation, interlock-knitted fabrics were constructed from Polyamide (PA) composite yarn coated with silver and then by silicon rubber.Woven fabric was also integrated with this knitted structure which eventually transformed into a 3D tactile sensor.This 3D sensor was stretchable and can be self-powered by human and other machines motions [19].
Recent studies have also employed Electro-spinning technology to manufacture electrospun polyvinylidene fluoride (PVDF) fibers and yarns which harvested energy of high value i.e. 40.8 V, 0.705 μA cm -2 , and 9.513 nC cm -2 .These yarns can easily be converted into fabrics.It has an additional property of softness along with other properties inherited due to using textile materials and structures [10,19].A simple and low-cost embroidery technique has also been utilized to manufacture textile based TENGs and it was outputting high voltage of 113 V [20].It was because of large contact area was created for high charge density through pile structure embroidery.Carbon cloth was also converted into wearable electronics by combining it with PTFE and applying nickel-copper bimetallic hydroxide Nano wrinkles to modify surface topography for more power generation [3].It produced an output of 1.323 mW cm −2 after using nickel-copper bimetallic hydroxide nano wrinkles.Conductive textiles are available in yarns and fabric forms so in one of the studies silver coated Nylon yarns along with Cotton were employed for scalable core-spun coating yarn-based triboelectric nanogenerators (CSCY-TENGs) manufacturing [14].Output voltage of 174 V, peak power density of 275 mW m −2 and average power density 57 mW m −2 were achieved.This system can be integrated with carpets and shoes, for energy harvesting by applying pressure or compression through bio-motion.An advanced triboelectric nano generator was designed and converted into a smart Glove.This system was operated on multimode mode energy harvesting and impact energy was utilized for power generation.This Smart glove worked well in energyscavenging and safeguarding areas.Kevlar fabric PDMS, multi-walled carbon nanotube (MWCNT) and carbonyl irons layers were assembled for this type of nano-generator construction [21].The output voltage measured was 10.4 volts at 10 MΩ under compression.With increasing swing amplitudes voltage increased from 0.47 V, 1.01 V and 2.52 V while sustaining maximum force applied while grasping different objects.Different materials like Polyester, Silver coated, Nylon, Acrylic yarns and fabrics, along with different types of coatings and nano-materials have been employed to construct TENGs [5,19,22,23].They are able to harvest sufficient energy to charge batteries.Most of the TENGs operated either by Vertical contact-separation mode or through compression by human body movements.These textiles based TENGs can easily be converted into different products according to desired applications and modes of operation.Stretchability is also required for wearable TENGs and for this some studies utilized silicone rubber.It may cause rashes to the human body and become uncomfortable for the user.Breathability and moisture management are also important requirements which are mostly improved by using natural fibers like cotton, wool, and silk.
In this investigation, lightweight, flexible, breathable, low-cost, and easy to manufacture wearable TENGs were developed.Single jersey (SJ) weft knitted structure was selected because it the highest production rate for garments manufacturing and easily constructed in all knitting machines.We termed our TENGs as SJ-WTENGs.Single jersey Cotton (+polarity, density 1.52 g cm −3 ) and Acrylic fabrics (−polarity, density 1.16 g cm −3 ) are inherently Tribo-electric in nature with opposite polarities.These materials are easily and abundantly available all over the world.These materials do not harm the human skin either and show improved output for TENGs manufacturing.In Single Jersey Tribo-electric Nano generators (SJ-TENGs), energyharvested by paired effects of contact-electrification and electrostatic induction to transform mechanical stress/ compression into electrical energy and the phenomena has been discussed above.For more production of the electrons; smooth and continuous flow of required amount of current and for the desired voltage generation, Maghemite (γ-Fe 2 O 3 ) NPs are synthesized and then applied on Cotton and Acrylic fabrics.The results of Maghemite (γ-Fe 2 O 3 ) NPs coated and non-coated SJ-TENGs are also compared to understand the phenomena and importance of improving conductivity of the conventional Tribo-electric materials.SJ-TENGs are also analyzed as a Single unit and in another attempt; two single TENGs are connected in series to light a LED and charging of a battery too.The SJ-TENGs designed in this study can easily be converted into wearable garments that can harvest energy for charging batteries of the portable IOT and embedded system devices.

Methods and material
2.1.Electrical potential generation in SJ-WTENG using vertical contact separation mode Electrostatic potential between two charged fabric samples in SJ-WTENG is governed by the Gauss law.Let's consider two fabric samples with Electric Fields + E and - E and having charge density s + and s -as shown in figure 1. Electric fields on a Gaussian surfaces, assuming the sheets to be infinite in size, can be expressed by equation (1) and equation (2) [24], Thus the total electric field of SJ-WTENG E W can be calculated by equation (3), Two charged fabric samples in a SJ-WTENG.
Since the fabric samples have finite area, and if, the overlap area between the two samples is assumed to be the same as their physical areas ɣ, then the total Electric field of SJ-WTENG, E WT can be expressed using equation (4), WT Electric potential between fabric samples of SJ-WTENG can be calculated by equation (5) [24], where ø is the physical separation between the fabrics having charges on surfaces.
If sheets made of metal are attached to the charged fabric samples and then conductive path is connected to a load R between both fabric samples, an instantaneous current I W starts to flow (see figure 2).It can be calculated by equation (6), It is evident that when the pressure is applied on the fabric samples assembly it diminishes gap (ø) between fabric layers and hence causes reduction in the potential difference (V) (equation ( 5)).It reduces current flow as represented by equation (6).However, when the fabric layers are separated the amount of voltage and current increase (see equations (5) and (6)).Thus, the compression to create contact followed by the separation leads to variable voltage and current generation if fabric layers are connected through a conductive path as shown in figure 2.

Synthesis of maghemite nanomaterials
Iron nitrate [Fe(NO 3 ) 3 9H 2 O)], ammonium hydroxide, and ethanol were obtained from Sigma-Aldrich for the synthesis of Maghemite nanomaterials and of analytical grade.
Initially, the surface of the iron precursor [Fe(NO 3 ) 3 9H 2 O)] was activated by stirring overnight under a nitrogen atmosphere. 1 molar solution of the precursor was prepared in 25 ml of nitrogen-bubbled ultrapure water.The solution was heated to 60 °C and Ammonium hydroxide (NH 4 OH) was then added drop by drop until the pH 9-9.5 was achieved while continuous stirring.Gel was obtained by aging the above sol for 30 min followed by washing and then centrifuged at 8000 rpm for 5 min.The sediment was dried at 100 °C for 30 min using a water bath to remove excess water from the sample, which was further dried using ethanol, overnight at room temperature.The dried sample was calcinated in air at 350 °C for 15 min to obtain the desired nanomaterials [25].

Characterization
The structure of the nanohybrids was analyzed by x-ray diffraction (XRD).The XRD patterns were recorded using an x-ray diffractometer (Rigaku Mini Flex II, Japan) employing a graphite monochromator and Cu Ka radiation (l 0.15406 nm).Infrared absorption spectra were measured at room temperature on an FTIR spectrometer (Nicolet 5DX FT-IR, USA).were indexed either for maghemite [26].De-by Scherer equation was exercised for the estimation of nanoparticle size 13 nm.

Characterization of the maghemite nanomaterials
FTIR measurements, as shown in figure 4, confirmed the presence of Maghemite.Absorption bands corresponding to the stretching of the Fe-O bond were observed at 636 and,694 cm −1 [27].The peak at 3450 cm −1 can be assigned to the-H stretch from carboxy groups (COOH and C-OH) on the surface of the nanohybrids [28].The band centered at,1636 cm −1 represents the stretch mode of a carbonyl group(CO) [29].

SJ-WTENG sample preparation
Samples of Weft knitted single jersey fabrics were knitted by using yarns of Cotton and Acrylic.Cotton (+polarity, density 1.52 g cm −3 ) and Acrylic (−polarity, density 1.16 g cm −3 ) belong to triboelectric series materials [16,30].The fabrics constructed were lightweight, stretchable, and had good breathability.AT250F circular weft knitting technique was made fabric samples of size 4 × 6 inches.To improve the performance of the nano generator, Maghemite nanoparticles (NPs = 13 nm) having conductivity 2.135 micro-Siemens per centimeter, were applied on the fabric samples using Spray coating technique.A sonication method was selected for NPs dispersion in an ethanol medium so that it can be sprayed over the prepared fabric samples.Fabrics were then dried at 100 degrees centigrade for 5 min, depicted in figure 5.
In order to develop Single Jersey Wearable Textile nano generators (SJ-WTENG), using non-coated and NPs coated Cotton and Acrylic fabrics; initially a piece of Aluminum foil (conductivity 38 million Siemens per meter [31,32] was pasted to a card sheet (to avoid distortion), and then connected to wires to allow flow of the static  charges which would be stored by the conductive material as a result of Triboelectric effect.Selected Triboelectric materials i.e.Cotton and Acrylic (with and without NPs) were then separately joined the Aluminum foils using double sided tape.Single Jersey Wearable Textile nano generators (SJ-WTENG) were now ready for testing.

Experimental set up
Developed knitted SJ-WTENG samples, both NPS coated and non-coated, were tested based on the mathematical model presented in section 2.1.A full bridge rectified circuit was developed, and the power signals were studied on oscilloscope and multi-meter.In bread board of 400 points bridge rectifier circuit was connected in which the point at which two common positive ends of diodes were met, was connected to the positive probe while the point at which two common negative ends of diodes were joint to the negative clamp.A 16 volts 47 micro farad capacitor was employed to smoothen the DC output voltage of the SJ-WTENG.An LED (threshold voltage is 1.7 DC volts) was placed as a load to observe the output of SJ-WTENG.The points where the alternating negative and positive ends of diodes were joined; those were selected as the input points for the connection of wires of the SJ-WTENG samples developed for this investigation.
Complete experimental set up for the demonstration of Triboelectric effect using single SJ-WTENG (noncoated and coated); and in another arrangement, two SJ-WTENG samples were connected in series to find if we able to get more output, see figure 6.
Application work was also carried out while measuring generated by SJ-WTENG.For the purpose, an experiment was conducted where lithium-ion battery pack was charged from output of the SJ-WTENG.Experimental setup of battery charging is shown in figure 7. A 3.7volts 3.6 Ah lithium-ion battery pack was selected.To access the performance, battery voltage was measured before and after the charging.SJ-WTENG samples were connected to a mechanism which was developed based on Contact-separation mode of operation.A wooden wheel connected to the wooden levers and an attachment for connecting SJ-WTENG samples to generate contact followed by separation for charges development.

Results and discussion
DC voltage readings were recorded from oscilloscope for the developed SJ-WTENG samples; as a single unit and when two samples were connected in series (for both non-coated and NPS coated fabrics).Voltage generated from the single and multi-SJ-WTENGs are shown in figure 8.
It can be clearly observed from figure 8 that significant coefficient of determination (R 2 ) found in single (R 2 > 0.98) as well as the multi-SJ-WTENGs (R 2 > 0.98).A non-coated SJ-WTENG sample has lower DC voltage magnitude due to lower conduction of generated static charges on the fabric samples.However, significant coefficient of determination (R 2 ) is also found in single (R 2 > 0.98) and multi-series (R 2 > 0.98) SJ-WTENG in    The connection of SJ-WTENG samples having nanoparticles, in series resulted in a time varying periodic signal as shown in figure 9(c).Maximum peak to peak voltage recorded was 7.68 volts with approximate frequency of 50.5 Hertz.Time varying signals were converted to voltage using the shotky diode-based bridge rectifier.A capacitor further stabilized the output voltage, and we were able to get a quite stable DC voltage of 5 volts (see figure 9(a).A LED was also turned-on using output of the developed SJ-WTENG, as shown in figure 9(b).For the current measurement, a 220-ohm resistor was used as load and the current was measured using a digital multimeter.Application and removal of the pressure/compression over the samples made current be stabilized at 2.66 mA which generated a maximum power of 5 v * 2.66 mA = 13.3 milli Watt using samples which were developed by a very simple mechanism.The charging of Lithium-ion battery pack (3.7 Ah) was also achieved by the developed SJ-WTENG and contact-separation mode.The battery was not charged and then power was supplied for five minutes through the setup presented in figure 7 using SJ-WTENG developed in this study.Battery voltage reading was recorded after every one minute.Results obtained are shown in figure 10.When the battery was charged it had a voltage of 3.0 volts which increased up to 3.6 volts after 10 min in continuous charging mode.The significant coefficient of determination (R 2 > 0.847) shows that the battery voltage increased significantly in 10 min which indicates that the proposed technique has the potential to charge the small battery packs for powering IOT devices and portable medical devices.
SJ-WTENGs have flexible and breathable constructing materials which are lighter in weight if compared with opposite polarity polymers.This is an ideal solution to harvest energy from biomechanical activity of routine life.SJ-WTENG samples through this investigation can be employed in a variety of wearable articles such as in sportswear, active wears, shoe making, and socks.

Conclusion
In this research, WTENGs were developed using Single Jersey knitted fabrics made of Cotton and Acrylic which can be employed to manufacture different types of daily wear and specialized garments.Vertical contactseparation and compression electrification modes were selected for the generation of electrical charges.The samples of SJ-WTENG were developed by materials which are easily available.The technology used was very simple and easy to implement.A good step towards harvesting energy by using body movement which apparently seems been wasted.The charges accumulated generated a high potential difference due to cotton and acrylic which was able to charge a Li-ion battery.SJ-WTENG samples generated low and high DC voltage magnitude at significant coefficient of determination (R 2 ) in single (R 2 > 0.9) and multi-series (R 2 > 0.9).Maghemite nanoparticles coating and connection of developed 02 samples in series were found to be a useful solution to generate high voltage which was able to glow a LED and had threshold voltage of 1.7 Volts DC, charge 3.7 volts and 3.6 ampere hour for Li-ion Battery pack.In our future work we will modify the designed SJ-WTENG samples for sportswear and other active wears considering other requirements of these garments in daily life.Three or more SJ-WTENGs can also be combined for higher voltage and current.The possibility of using other materials can also be explored to impart sustainability to save the environment further.

Figure 3
depicts the XRD patterns of the maghemite polymorph of iron oxides.A distorted cubic (P 4 132, ICDD: 391346) maghemite with an average crystallite size of 13 nm was observed.Diffraction peaks in the XRD pattern

Figure 2 .
Figure 2. Current flow during the contact separation method.

Figure 9 .
Figure 9. (a) Voltage of NPs coated multi SJ-WTENG series in oscilloscope, (b) Output LED light up (c) Voltage waveform generated from SJ-WTENG (d) Current measurement with load resistor.

Figure 10 .
Figure 10.Battery voltage measured while been charged with SJ-WTENG.