Design of a self-powered triboelectric face mask.

Usage of a face mask has become mandatory in many countries after the outbreak of SARS-CoV-2, and its usefulness in combating the pandemic is a proven fact. There have been many advancements in the design of a face mask and the present treatise describes a face mask in which a simple triboelectric nanogenerator (TENG) with an electrocution layer may serve the purpose of filtration and deactivation of SARS-CoV-2. The proposed mask is designed with multilayer filters, in which the inner three layers act as a triboelectric (TE) filter and the outer one as an electrocution layer (EL). The viral particles experience a mildshock in EL due to the electric field produced between the electrocution layers by contact electrification. Four pairs of triboelectric series fabrics, i.e. polyvinylchloride (PVC)-nylon, polypropylene (PP)-polyurethane (PU), latex rubber-PU, polyimide (PI)-nylon are studied to establish the efficacy of the mask. The motional force exerted on triboelectric filter materials can produce sufficient electric power to activate EL. The proposed mask can be used by a wide range of people because of its triboelectric self-powering (harvesting mechanical energy from daily activities, e.g. breathing, talking or other facial movements) functionalities to ensure effective filtration efficiency. More importantly, it is expected to be potentially beneficial to slow down the devastating impact of COVID-19.

Introduction

While the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, also commonly known as a novel coronavirus and 2019-nCoV) gains significant prevalence worldwide, scavenging a curative remedy has become the main thrust to the scientists across the globe. So far widely accepted thumb rule of preventing the spread in society are maintaining social distancing, keeping hand hygiene, and wearing masks. Quarantine is another adopted way if anybody come to contact to any infected person directly or indirectly. A few countries have made it compulsory to wear a mask whenever a person going outside. Alike personal protective equipment (PPE), homemade do-it-yourself (DIY) cloth mask has attracted great attention to combat the novel coronavirus. Though the performance of various fabrics used in such a homemade mask is still under observation, their response is likely to be anticipated for combatting the novel pandemic. In regards to the present scenario, the recent report by Konda et al. illustrated the filtration efficiency of various combination of fabrics for different sizes of the aerosol and they are found to be effective for the wide bracketed (10 nm - 6 μm) particles [1]. Their study unveils aerosol filter made by the combination of chiffon and silk to be significantly efficient and a hybrid combination of non-woven fabrics with different thread count per unit shows effective filtration efficiency for nano size particles. Improper and loose-fitting might prevent the working of that mask as the filtration is based on the principle of static electricity. It has been studied that among respiratory droplets, such as aerosols (<5 μm) and droplets (>5 μm), water droplets and liquefied gas droplets act as the medium on which SARS-CoV-2 can traverse over a distance when the size is relatively small unlike larger droplets that can settle down easily [2]. Even though direct contact with an infected patient is the primary cause of the spread of SARS-CoV-2, transmission is also possible over a certain distance by the highly virus-containing droplets which are found to remain stable for more than 24 h [3,4]. COVID positive patients can also passively spread viruses to the people surrounding them. As per the underlying precaution made by WHO 2020a, wearing a face mask is mandatory to combat COVID-19 [5]. In case of transmission of virus by droplets, the larger liquefied droplets evaporate or break down quickly with the formation of smaller sized droplets. These droplets are transported by air rather than gravitation and get enough liberation to traverse in the air for several meters [6]. The five basic types of aerosol filtration include the electrostatic attraction that takes predominant role alike gravity sedimentation, inertial impaction, interception and diffusion [1,7,8]. All these mechanisms are solely size-dependent, i.e. gravitational forces come into play for larger sized droplets (>1 μm), whereas sedimentation and impaction are only applicable for medium-sized particles (>1 μm–10 μm). Lesser the size of the particles (100 nm–11 μm), more is the tendency to get diffused by mechanical interception and Brownian motion [1]. Interestingly, nanosized particles can easily slide between the opening network of non-woven fibers [9]. In such cases, electrostatic attraction (EA) significantly attract the viral particles to the fibers. The principle of integrating EA with nanogenerators capable of generating triboelectric charges from the wasted mechanical energy from our routine activities (e.g. respiration, talking and other facial expressions can be utilized favorably in mask design which has been conceptualized in the present study as represented in Fig. 1 [10].

Fig. 1

Schematic representation of the proposed triboelectric multilayers comprising self-powered mask. Inner three layers (from face side) are acting as TE filter and outer layer is the EL made with conducting mesh.

The concept of triboelectric nanogenerator (TENG) is first proposed in 2012 by Zhong Lin Wang's group [11]. Attempts have been also made on TENG based power bank using different mechanical agitation/vibration coming out from natural sources [12]. Various advantages, e.g. simplest design possibilities, compactness, noise free output signal and cost-effectiveness makes TENG a promising and attractive research topic mainly in the sensing and energy harvesting domains. The effectivity of nanogenerators as auto energy harvestor has been proved in several works [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]].

In recent days, wearable energy harvesting technology obviate the disadvantages of portable batteries. So, the prime features of TENG includes flexibility, stretchability, weaveability, water-resistant, and comprising of high surface charge density [[32], [33], [34], [35], [36], [37]]. In particular, paper based TENG concept has been reported as a green, eco-friendly, and efficient mechanical energy harvester [34]. This energy harvesting system works in correspondence of human motion [35]. It was found very interesting where a water balloon based TENG was employed to harvest water wave energy. This type of self-supported system exhibiting high throughput of current ~147 μA and voltage of 1221 V that indicates an effective approach of blue energy harvesting [36]. The other amazing application of TENG is sleep monitoring using smart bedsheets which are made up of conductive fibers and elastomeric materials with a wave structure. This smart textile device is integrated to self-powered remote warning system in the case of an aged non hospitalized patients falling down from the bed [37,38]. A different type of energy harvesting for powering up wearable electronics has been demonstrated with a photo-rechargeable concept which is capable of constantly deliver sufficient amount of electric power [39,40]. Contextually, daily-used beddings like pillows were integrated to a fluffy pile of self-powered triboelectric body-motion sensors. In this work, different body gestures (turning-over, breathing, mild snoring, serve snoring or teeth-grinding) has been demonstrated to convert into electrical signals [35].

Other materials are found to be highly efficient mechanical energy harvester in terms of its extraordinary ferroelectric and dielectric properties [13]. Contextually, the usage of ferroelectric PVDF nanofibers based matisproved to be prevalent in several applications of human personalized healthcare monitoring system as reported by Ghosh and Mandal [21]. Unlike TENG, piezoelectric nanogenerator (PENG) is also found to be interesting as an efficient mechanical energy harvester as elucidated by Mutsuda et al. The report describes the application of a piezoelectric painting coated flexible device as excited by wave forces [22]. A macrotriangle-prism-shaped of PU foam and polytetrafluoroethylene (PTFE) film based TENG has been successfully integrated the functions of spring, spacer and electrode [23]. The main essence of the work is to avoid the usage of full-wave rectifiers for charging the storage capacitor. Instead of that, TENG is integrated with Bennet doubler conditioning circuit in order to enhance the energy per cycle, totally stored energy and charging efficiency. Recent findings on the use of cloth mask during the outbreak of Influenza in 2009 appeared to be inadequate to conclude the selection of fabrics since the filtration efficiency is up to mark. Though the recent study envisaging that the thread counts of non-woven fibers are playing a key role in electrostatic filtration. Furthermore the performances of the face mask is possible to improve significantly by judicially selecting two extreme series of triboelectric series based cloths [1]. When the triboelectric charges are considered in the concept of cloth based face mask, however so far the concept of self-powered TENG has not been attempted. Thus in the present study, body motion activated three layers (one triboelectric positive layer and two triboelectric negative layers) comprising TENG based face mask is conceptualize (as show in in Fig. 1), proposed and experimentally clarified where the idea of electrocution is also introduced.

Proposed design of face mask

The proposed multilayered face mask comprises of the triboelectric series materials (TSM) (three inner layers, out of them if one layer is tribo positive then other two layers are tribo negative) and the outer one is metallic mesh comprising electrocution layers (ELs). The prototype design of the self-powered mask is shown in Fig. 2 a. The working mechanism of the proposed multilayered mask is depicted in Fig. 2b. It is speculated that the proposed mask can filter and inactivate virus containing aerosols both during breath in and breath out condition. The EL produce sufficient electric field by taking the stored charges from the capacitor which is charged by the TENG. Due to the mesh like structure of the ELs, when any droplet gets in between, the current tends to flow due to the low resistance path created by the droplet itself (it has been considered as 1KΩ of resistance). Hence, the circuit gets closed and an instantaneous current suppose start to flow through the circuit. In consequence, the droplet would possible to evaporates if it is a water droplet. For other case, it is highly expected that virus would get deactivated. The triboelectric layers provide extra protection to the mask wearer in addition to generating the electric potential. In Fig. 2b, Cs is the charge storing capacitor, and Rs is the series resistance connected with one of the ELs to increase time constant of the discharge circuit. Otherwise, a single discharge would destroy all the energy stored in the capacitor. In order to transfer the induced charge from TLs to the storing capacitor (Cs), necessary electrode connection, have been made from each TLs.

Fig. 2

(a) Embodiment of proposed triboelectric self-powered mask, (b) Block diagram of TENG based self-powered mask, where VTU stands for voltage tripler circuit unit.

Since the electric field energy density (where E is electric field and is permittivity of free space) has a quadratic relation with charge density σ, so increasing the triboelectric charges is one of the major factor to improve the resulting energy density between ELs [[26], [27], [28], [29]]. That can be primarily done using structural optimization of different triboelectric series materials, surface modification and surface charge engineering [25,30,31]. Interestingly, nucleocapsid protein crowned SARS-CoV-2 possesses surface electrostatic potential characteristics that reinforced the present design concept [32]. Keeping all these in mind, four combinations of TSMs have been chosen, each with a pair of positive and negative TSM connected to capacitor via voltage tripler circuit unit (VTU) [41]. As a mechanical energy harvester, expiration and inspiration during talking, any other lip activities, facial gestures are also possible to utilize as the prime source of power enhancement to induce static electricity in between the triboelectric layers (TLs) of the proposed mask, besides regular inhaling or exhaling mechanism. The detailed characteristics, e.g. effective TECD, Qa and saturation voltage among the four pairs of TSM are illustrated in Table 1 . The self-powered induced potential allows the conducting mesh framing of these layers to transfer charges between them. The layer is capable of acquiring the static charges so that any viral particle having surface charge can easily be inactivated in the outer layer. The activated EL can adhere electrically the charged viral particles coming from the vicinity of the wearer. The novelty of this design lies in the self-activation of the EL through the breathing cycle of the wearer and has provision to deal with aerosols as well as droplets (gaseous, liquid). The essence of the proposed multilayered self-powered mask is fully protective from virus infection, comfortable texture and wide accessibility that also assures no internal respiratory problem (breathing issue) to the wearer since sufficient porosities are there in TLs. The novelty of the proposed mask is self-powered, i.e. it does not require any external power to activate the EL.

Table 1

TECD, Qa and saturation voltage of pair of four selected fabrics can be used in the proposed mask design.

Pair	TENG materials		TECD (nC/m2)		Effective TECD (nC/m2)	Charge affinity (Qa) (nC/J)			Saturation voltage (V)
	-TSM	+TSM	(−)	(+)		(−)	(+)	Effective charge
1	PVC	Nylon	0.117	4220	4220.12	100	30	130	11.92
2	PP	PU	0.027	3850	3850.03	90	60	150	10.875
3	Latex rubber	PU	0.0106	3850	3850.01	105	60	165	10.875
4	PI	Nylon	0.093	4220	4220.09	70	30	100	11.92

Results and discussion

Mathematical modelling of multilayered self-powered mask

In order to establish the working of the proposed multilayered self-powered mask, tuning of its important parameters is very important as reported by Niu et al. [42,43]. They have studied the design parameters and explored possible techniques for optimization of TENG fundamental modes. The real-time output characteristics, relationship between the TENG parameters and optimum resistance were derived to design theoretical model of contact-mode TENG [43]. The working mechanism of the proposed mask is based on the triboelectricity and proper utilization of the electric field to activate the EL, different factors have been considered (separation distance between TLs, rate of breath-in/breath-out, etc.) and optimized to get effective filtration effect.

To bring the light on these factors, the nature of the droplet is considered to be conducting, the current flowing through the droplet would be 1 mA, considering low resistance (R = 1kΩ) and voltage offered by this droplet under study. Study reveals that the amount of induced current is sufficient enough to inactivate the mutation of any charged virus. For the case of any water droplets, some amount of heat will be generated by means of short-circuiting due to the contact of water particles in between EL. Therefore, heat generated in case of conducting water droplet would be 1 mW. The important factors have been discussed in the following subsections for safety and filtration efficiency of the self-powered mask.

Optimization of separation distance between TLs

It is very important to maintain an electric field of a value lower than that of air discharge field to maintain contact between two TLs. Considering the minimum separation distance (= 0.1 mm), the voltage (1 V) would be sufficient to create high electric field (E), i.e. 104 V/m for the droplet under study.

Safety issues of the mask

In order to verify whether the heat generated causes any problematic to the wearer, the relative movement of the TLs as shown in Fig. 3 during breathing has been considered.

Fig. 3

Schematic representation of TLs with a separation distance of ‘d’, where ‘dx'and ‘A’ indicates the relative movement of TLs and the effective area of each TL, respectively.

Once the droplet comes in contact to the mask, the droplet will try to pass through the plates with high field-which burns the droplet due to current and generated heat.Air specific heat (H) = 10where, Cp = Specific heat at constant pressure (1 kJ/kg), Cv = Specific heat at constant volume (0.718 kJ/kg), density = 1.23 kg/m3

The generated heat is very less that indicate that it would not affect the respiratory system of the wearer.

The equivalent electric circuit and forward and reverse motion of TLs in TENG have been shown in Fig. 4 a–b respectively. In context, the forward and reverse motion of TLs is equivalent to a DC source. In the equivalent circuit, is the equivalent DC voltage, is the equivalent resistance andis the equivalent capacitor of TENG. Due to friction, one part of breathing energy will be lost as heat and the remaining part will be used to electrify the layers by overcoming the work functions of the materials used. The friction due to breathing resembles a triangular shape with a constant DC part and an oscillating part. So, the equivalent circuit is a constant DC battery () connected with a resistance () for accounting heat and all other possible losses due to mechanical arrangements and a vibrating capacitor (). Therefore due to the associated heat loss and mechanical loss, the filtration efficiency cannot be attained to its maximum value, i.e. 100%. Considering the practical case, the effective breathing pressure has been taken in to account. Active area of TLs has been considered for collecting the static charge for charging the capacitor (C ) and this charge is used for the electrocution of the droplets at ELs.

Fig. 15

Characteristics of (a) current and (b) power during electrocution of droplets as a function of orifice diameter.

Equivalent electrical circuit diagram of TENG where (a) the combination of internal resistance (R) and capacitance (C) and (b) forward and reverse motion of TLs is shown.

Effect of breathing cycle to power up the mask

The present study has immense significance in order to study the effect of the relative movement of the TLs on the induced charges. The relative movement of any of TLs depends on the pressure exerted by the inhalation and exhalation. The following section will highlight the key parameters which have mutually contributed to charge generation. Any face mask must be free from breathing problems. The present study is highlighted this factor. The following parameters have been considered during calculation of the effect of breathing cycle of a normal human being.

volume of air per inhaling ~0.5 L

breathing pressure ~ ±5 cm of H2O (water)

maximum pressure change during one breathing cycle is 10 cm of water ~100 mm of water

normal air pressure ~100 kPa

pressure variation during breathing ~1.0 kPa

Several finding reveals that the normal breathing rate (NBR) of a human is 12–20/min [44]. Therefore, the frequency of breathing would be 0.2 Hz (considering 12/min NBR). Additionally, one complete breathing cycle requires 5 s (inhaling and exhaling take 2.5 s each).

If (mm) is the normal separation between TLs and (mm2) is the area of each plate, then the volume of the space between this plates is (mm3). The relative displacement dx of the TLs is proportional to the excess pressure dp. So, . Consequently, work done (dW) by exhaling or inhaling is given by(where, P = atmospheric pressure in Pascal = 105 Pa, dp~103 Pa, area of each TLs is (A) = 2500 mm2,

= pconstant depends on mass of the layer under study and other material properties of the plate; = force exerted on the TLs due to extra pressure)

The average available energy from breathing per breathing cycle

The charging of the mask mainly depends on the type of triboelectric materials used in the study. Based on the highest triboelectric charge density (TECD), four pairs of TSMs have been selected as mentioned in Table 1. Based on this, the simulation has been carried out. The four pairs of TSM impart different TECD. It has been considered as effective TECD, i.e., being a variable. In order to obtain the work done per breathing (inhaling and exhaling) cycle, the charge affinity comes into account.

Considering the charge affinity (Qa), the required charge possible per breathing,

Effect of shifting of TSMs on charge density

The governing relation (equation 5) between the electric field () and relative displacement () between triboelectric layers has been illustrated in Fig. 3 and the other prime characteristics of the proposed mask have been calculated using equations, 5 to 9.

Due to inductive effect, the three triboelectric layers with conducting painting on one side of the triboelectric materials is equivalent to a parallel plate capacitor. Between two conducting paints, there is the triboelectric materials an air gap in between two triboelectric layers. Fig. 5 indicates the schematic representation of the inductive effect of TENG, where C is the equivalent capacitance of the three triboelectric layers where C1 and C2 are two capacitances formed inside the mask structures. The space between the conducting paints (acting as two parallel conducting plates) is filled with triboelectric layers (dielectric) and the gap is filled with air. This is a real capacitor and has resistive and inductive effects along with capacitive effect. There is also loss effect due to imperfection of the triboelectric layers (not a dielectric of infinite resistance as considered ideally), the polarization of the triboelectric layers, and vibrational loss due to alternating field. The electrodes drawn by conducting paints used here in mask design are not possible to be perfectly (ideal) conducting, rather the layers have resistive effect. Another important effect is the inductive effect. When the triboelectric layers are charged, the electrical charges are transferred through the conducting paints drawn electrodes to the EL. One metallic mesh of EL is positively charged and other is negatively charged that creating an electric field in between the plates. That is an electric field exists inside the triboelectric layers and the air-gap. This field creates a displacement vector and same amount of ) in air, where and is the electric field within the tribolayers and air. But is same throughout, as is perpendicular to the separation between triboelectric layers and air. As the charge density charges with time, the displacement vector ( changes with time.

Fig. 5

Schematic representation of the inductive effect occurring in TSMs of TENG in the context of working principle of face mask.

So, there exists which is equivalent to the displacement current. If constant, there is only a transient phenomenon, which produces a constant magnetic field surrounding the vector. As is not constant vector, rather changes with time which produces an opposite e.m.f., proportional to the rate of charge of , where I is displacement current.

So,

This is equivalent to an inductance. Also, the distance between two layers charges with time, which causes an inductive effect.

So,  =

That is + =  = , this L is called the equivalent series inductance (ESL) of a real capacitance. Therefore, equation (6) can be modified as equation (7).

As the inductance (few ) is quite small and the breathing frequency is 0.02 Hz, the inductive effects involved in equation (7) is negligible. where, = atmospheric pressure in Pa;

= effective charge-affinity of triboelectric series material in C/J;

change of pressure in Pa;

relative displacement in m;

= permittivity of free space in F/m;

electric field in V/m;

voltage between two triboelectric layers in V;

current between two triboelectric layers through droplet (mA);

power generated between two triboelectric layers in mW.

The generation of voltage () under 1 kPa of change of pressure amplitude in terms of the relative separation between two TLs pairs (displacement), such as PVC-nylon, PP-PU, latex rubber-PU and PI-nylon have been depicted in Fig. 6 a–d, respectively. It is important to note that four different triboelectric fabric pairs are considered here, where charge affinity (Qa) is one of the major governing factor of output voltage generation. On the other hand, breathing pressure (dp) also plays a critical role in static charge generation in TLs.

Fig. 6

The variation of voltage (V) due to displacement of TLs, such as combination of (a) PVC-nylon, (b) PP-PU, (c) latex rubber-PU and (d) PI-nylon under~1.0 kPa.

Effect of the shape of TLs on induced charges

Alignment of TLs and movement of aerosols inside the layers are factors upon which the induced triboelectric charge depends. A deeper insight into the movement of the viral particles describes that initially the droplet is not in contact with the layers, but stay inside the space between the two layers. Hence, the effective field inside the droplet is expected to be reduced. However, current flow through the droplet is possible as water being the good conductor of electricity. Therefore, it affects the virus or any living organism irrespective of the size of the droplets. The droplet will be polarized and a current will start flowing through the droplet when droplets come in contact to the triboelectric layers. Different alignments of the triboelectric layers are shown in Fig. 7 a–b. The droplet act as a dipole inside an electric field () within the layers and it will be accelerated towards the negative plate, ‘’. As aerosol is considered, it will have a speed of ‘’ parallel to the layers. In order to make contact between the particles and triboelectric layers, the acceleration ‘’ is likely to be insufficient to reach the droplet from plate ‘’ to ‘’. Therefore, the alignments of these layers are optimized as shown in Fig. 7b. The motion of the particles inside the layer‘’ and ‘’ is shown in Fig. 7c.

Fig. 7

Alignment of triboelectric layers and movement of aerosols inside the plates in two different configurations, such as when layers are arranged in (a) parallel fashion, (b) curvey fashion and (c) movement of the droplet inside the electric field (E) distribution.

Power management in electrocution layer

The mask is designed in a way that the inbuilt charges are drive the EL, the important part of the mask itself. This phenomena affirms the designed mask can be driven smartly without drawing power from external source. The proposed mask is so designed as attached electrode contact-mode TENG, which is based on vertical charge separation mechanism [13]. Fig. 8 a represents two different dielectric layers (here termed as plates) (selected in such a way so that they have two distinct type of TECDs, such as one is σ+ and another is σ−) of thicknesses of d1 and d2, respectively. These plates are stacked face to face as two TLs. Here one positive TL is sandwiched between two negative TLs for experiencing similar output voltage during exhaling or inhaling. The plates are stacked in such a way so that the two metal layers at the outer surface of these two dielectrics are acted as electrodes. For simplicity, it is assumed that the relative distance (dx) between two TLs is likely to be varied by the mechanical friction during inhaling and exhaling condition of the wearer by the same amount as shown in Fig. 8 b. This phenomena makes them to come in contact with each other. Hence, the inner surfaces of the two TLs get opposite triboelectric charges (σ+ and σ−) with equal charge density, in turn contact electrification resulted. In brief the design assures the equal distribution of charge during inhalation and exhalation period.

Fig. 8

The schematic represents (a) dielectric-to-dielectric attached-electrode in vertical contact-mode TENG and (b) induced charges during exhaling and inhaling cycle.

Briefly, a part of the heat energy takes out the electron from the plates by overcoming the work function of the plate material and the plates get charged, consequently rest of the energy is lost as heat energy (. Due to the breathing the plates move in both direction, i.e. +ve and –ve (forward and backward) with respect to its normal positions as accordance with the breathing pressure. The relative shifting of TLs with respect to time follow a triangular nature. So for forward and backward movement will act as same charging process as shown in Fig. 9 .

Fig. 9

Forward and backward exhaling pressure as a function of time which is applied to TLs during inhaling and exhaling.

During the pressure from 0 to T/2, the charge is going to increase and reach a value, whereas the same decreases during T/2 to T as shown in Fig. 10 . This nature of changing pressure affirms the rate of charging decreases (neglecting the minor leakage that occurs during the period).

Fig. 10

Modulus exhaling pressure as a function of time experienced by TLs during inhaling and exhaling.

Main thing is that at each cycle of rubbing (friction) it seems that charge will increase linearly without any limit. But, what happens is that when some charge density is created at the surfaces, then these charges repels the other charges and rate of increase of force decreases. Hence forth, one electron needs an energy to come out of from the material. On the other hand, as the charge density increases there is another energy required to overcome that repulsion. In consequence, when charge density becomes zero, the energy required to extract one electronic charge becomes equal to the work function.

As time passes on, the charge density increases and an extra amount of energy is required to emit an extra electron. This extra energy is definitely proportional to the existing charge density, that is Therefore, energy required to extract one electron will be ; is proportionality constant.where, is electronic charge, is a constant . So, the energy required for amount of charge is and energy required per unit time is . If the plate area is A, then charge is distributed over plate. So, and it can be written as

Let, so, the energy required to extract amount of energy is .

As is an energy. It can equivalently take as a current and as resistance.

So, is like a voltage drop across a resistance . [Since].

Now voltage between two plates is

We know that the source of our energy is breathing energy converted into motional energy and then electrostatic energy. The nature of the motion has already been discussed. The Fourier analysis of the motion is

Since the motion of the TLs are periodic, we can represented by Fourier analysis. Where is the constant part and motion of the plate consisting of both D.C. and A.C. components. But for simplicity D.C part is taken as consideration. Then is like a battery (energy source) of e.m.f. =  (say) and the voltage drop is equal to V = . Then,

So, is the charge density at particular time t and eqn (15) can be written as.

Therefore, the induced charge density found to be dependent on time (t) inherently, so eqn (17) becomes

An interesting fact to be observed is that the developed voltage, current and corresponding power get saturated after certain period of time. Therefore, developed charge in the proposed mask can be increased for a period of time () until and unless the charge density reach its saturation limit (). The time required to reach is much larger than the time of one forward and backward motion of the triboelectric layers.

Contextually, it has been studied thoroughly to observe the effect of charge density on the output voltage, current and power characteristics. Since, the four combinations of triboelectric materials are so chosen based on their distinctive nature including their wide bracketed size of charge density. Since, this is not practical to transfer 100% of energy during mechanical agitation of one TL with other TL is likely to be impractical. Henceforth, different charge density of these selected TSM pairs has illustrated in Table 1.

The output current and power characteristics of EL at varying percentage, particularly there different values, such as 75, 50 and 25% of effective TECD of different pair of TSM combinations, such as PVC-Nylon, PP-PU, Latex rubber-PU and PI-Nylon are depicted in Fig. S1 (a ~ c), Fig. 11 (a ~ c), Fig. S1 (d ~ f) and Fig. S1 (g ~ i) respectively. Conclusively, the proposed multilayer mask can be capable of electrocuting the target charged virus effectively even if 25% of the saturation charge density can be achieved in the EL. The study signified that the time required to reach the saturation charge density is of the order of few milliseconds for all the four selected triboelectric pairs. From Fig. 12 , obtained from equation no. 18, it has been clearly observed that the time required to reach saturation charge density is 25 ms, which is likely to be very less. Though the total charge generated (per second) by the TENG is less, the storage capacitor (Cs) can store much amount of charge, in turn this can be used to electrocute several droplets. In this way, the storage capacitor plays a significant role in this self-powered mask, On the other hand, the values of the time constants of charging are calculated on theoretical basis but the factor of heat loss and other losses due to different mechanical aspects are assumed herein empirically. The shorter range (ms) assures that the wearer no needs to wait for the instant effect of motion of triboelectric layers, which is the prime benefits of using this mask.

Fig. 11

Output current and power in the electrocution layer considering the combinations of PP-PU triboelectric pairs when (a) 75%, (b) 50% and (c) 25% effective charge density are present.

Fig. 12

Response profile of saturation charge density of capacitor (Cs of Fig. 2b) at different time constant (ζ = 2, 3, 4, 5).

Power harvesting in electrocution unit

The variation of capacitance, dissipation and equivalent series resistance (ESR) are affected to some extent depending on the environmental conditions (particularly if one considered the fluctuation of relative humidity) as observed by the experiments. Though this affects the functionality of the system not to an appreciable amount, the current through the electrocution circuit (when droplets enter in between the ELs) produce some amount of heat that enables dehumidification to some extent and helps to nullify the effect of moisture to some extent [42]. If the frequency of the mechanical energy source increases then the number of contact per second is also increases (up to a certain frequency) and hence there is tunning possibilities of input mechanical energy depending on physical motions, such as here mainly breathing frequencies are considered. For example, if breathing frequency is reach to 2 Hz (which is 10 times more than normal frequency rate), the harvested energy will increase to a large extent. The power exhaled by a healthy male and female person is 1.12 W and 0.5W which is sufficient enough to utilize in TENG to use as an self-powered mask [45]. Relevant experimental results of TENG made with selected triboelectric pairs are presented in Table 2 along with the theoretical calculations. It indicates the charge affinity of selected triboelectric materials (referred to Table 1) are playing a significant role that governs the efficiency of the EL.

Table 2

Output of TENGs made with different pairs of triboelectric materials under exhalation.

TENG materials	Gender	Generated charge (Q_a×E_a) (nC)	Generated voltage (Calculated) (V_T) (V)	Estimated energy (12Q_aV_T) (μW)	Generated voltage (Expt.)(V_P) (V)	Obtained energy (12Q_aV_P) (μW)	Efficiency (μW)
PVC-Nylon	Male	291.2	11.92	1.736	7.0	1.019	0.764
	Female	130	11.92	0.775	7.0	0.455	0.341
PP-PU	Male	336	10.875	1.827	7.0	1.176	0.882
	Female	150	10.875	0.816	7.0	0.525	0.394
Latex rubber-PU	Male	369.6	10.875	2.010	6.0	1.109	0.832
	Female	165	10.875	0.897	6.0	0.495	0.371
PI-Nylon	Male	224	11.92	1.335	5.0	0.560	0.420
	Female	100	11.92	0.596	5.0	0.250	0.188

 = Harvesting energy from human exhalation [45]; Efficiency is estimated under 75% of charge collection.

Comparision with others mask

The study illustrated in sec. 3.1. concludes a mathematical formulation considering all the parameters in ideal condition. Here, the recycling of breath energy occurs by converting the motional energy to electrical energy. In short, the energy during breathing per minute and its total conversion to electrical energy have been assumed in this study. It is mention worthy that the all the parameters are taken ideally to shape the modelling of the proposed mask design. Though the filtration efficiency seems to reach 100% theoretically, but achieving 100% efficiency in practice is not possible due to the effect of different factors interfering. But the proposed methodology affirms that the mask can effectively filter out the charged viral particles even with 10–30% conversion efficiency (considering the competent effect of other environmental factors) which is sufficient enough to activate the ELwhich is the heart of the proposed work.

The observation concludes that the continuous breathing does not lead to linear increase in voltage. In addition, the value of capacitance is fixed and charge gets increased due to friction. Consequently, the charge will reach saturation and no further increase in charge occurs in the proposed model. Hence, the model follows relationship throughout the mechanism.

The key contribution of the TLs is to induce triboelectric charges through contact electrification between triboelectricpairs is likely to be utilized using the EL to disinfect the incoming SARS-CoV-2 contained aerosols. The storage capacitor ( holds the induced charges in a way to be utilized in two different ways. In addition, the incorporation of the outer EL provides extra protection to the wearer considering the case when the incoming aerosol contains SARS-CoV-2. In such cases, the charged SARS-CoV-2 can be disinfected by electrocution in the EL giving double protection through middle and inner TLs. In summary, the proposed technology can block the novel coronavirus through double action of charge adsorption, and electrocution by triboelectrification, thus providing effective protective role to combat the deadly impact of SARS-CoV-2. In light of this context, the reported masks based on different technologies have been discussed below and illustrated in Table 3 . It is interesting to note that common type of N95 mask is integrated with nanoporous membrane to get better filtration efficiency. The silicon based nanoporous polymer was used as template where its hydrophobic property is utilized. Aerosols can be filtrated using the face mask with 85% of efficiency [46]. Lustig et al. have explained a seven layered fabrics based face mask for getting better efficiency [47]. In this case, hydrophilic, absorbent type common fabrics were used. Mainly, pulsed aerosols were targeted to filter out here. Nanoscale aerosol filtration technique has been used with commonly available fabrics such as cotton, synthetic, synthetic blends, synthetic cotton blends woven fabrics [48]. Zhong et al. have proposed a technique where kitchen paper towel, laboratory paper towel were used as fabric base material in which salt-based soaking strategy has been applied for testing [49]. An interesting approach of graphene mask has been proposed by Zhao et al. in which a third generation CW-lift graphene is used as a superhydrphobic layer to combat with any infectious viral entities [50]. It has been also reported that cotton, polyester, nylon, silk spun bonded hydrophobic fabrics were used to make efficient face mask [51]. Even house hold available material such as elastic T-shirt, quilting cotton, assorted fibers were used to fabricate face mask that indicates besides custom made nanofiber based mask, textile based masks are also gains prevalence due to its cost effectiveness and reusable possibilities along with adequate filtration abilities [52]. Recent study also been taken where customized made nanofibers made mask were also investigated where the reusabilities has also given one of the prime focus [53].

Table 3

Comparative study of the reported literatures with proposed mask.

Type of mask	Type of fabric	Remarks
Aerosol FiltrationEfficiency of Common Fabrics Used in Respiratory Cloth Masks [1]	cotton−silk,cotton−chiffon, cotton−flannel	(i)Marks layer are particularly effective at excluding particles in the nanoscale regime (<~100 nm), likely due to electrostatic effects that result in charge transfer with nanoscale aerosol particles. (ii)The enhanced performance of the hybrids is likely due to the combined effect of mechanical and electrostatic-based filtration.
Strapless flexible tribo-charged respiratory facial mask [54]	Multilayer flexible flat filter includes an activated carbon layer	(i)The filtration is based on the activation of carbon layer unlike triboelectricity, reusing the mask necessitated the refilling of carbon. (ii)Working of such type of mask depends on the type of the skin of the wearer. The wearer might suffer from medical adhesive related skin injury (MARSI).
Multilayer composition for a breathing mask [55]	Internal and external spunbonded non-woven fabric, felt type tribocharged non-woven fabrics, a ply of melt-blown microfibre	(i)First intermediate layer of felt-type tribo-charged nonwoven fabric based on at least two different types of fibres suitable for giving the fabric opposite electric charges that enhance the filtration. (ii)The mask can filtrate particle sizes in the range of submicron.
Electrically charged filter and mask [56]	Four layered comprises of three layered liquid charged non-woven fibers and one layered tribocharged non-woven fabric.	(i)The induced temperature due to liquid charge intensity likely to b e less than 40 °C, which is insufficient to combat novel corona virus. (ii)Refilling of polar liquid in the liquid charged fabric might be troublesome as it requires immersion apparatus alike spraying in the form of droplets, mist, shower, etc.
Non-woven film and charged non-woven biological protection mask [57]	The inner layer made of rare earth material ‘zein’ and positive chitosan based outer layer.	(i)The mask basically deals with biological protection, and particularly relates to a nonwoven film and a charged non-woven biological protection mask. (ii)A charged ‘zein’ based nanofiber double-layer film prepared through an electrospinning technique can isolate virus through the dual functions of electrical charge absorption and mechanical isolation.
Mask filter [58]	Sheet material composed of a resin fiber with wounding of copper wire.	(i) A copper wounded woven fiber sheet has been used for initiating corona discharge, i.e. the viral particles comes in close vicinity of the mask filter. (ii) It is mentioned the mask is so designed to provide bactericidal effect unlike virucidal effect.
Mask using frictional & static electricity [59]	Polymer, nylon, cotton, silicon based polymer, polypropylene (PE), polypropylene terephthalate (PET)	(i) The design works based on the electrostatic and triboelectric properties (ii) The mask is its location specific, based on the country specific weather conditions (fine and yellow dust) the structure of the mask has been designed and it cannot be reusable.
Medical protectivebreathing mask [60]	The multilayer made ofchitin fiber or silk fiber, hydrophilic and fwovenchemical fiber fabrics.	(i) The outgoing gas is transferred to the environment through the adsorption-diffusion-desorption process of the hydrophilic group of the functional film. (ii) The embodiment of the developed mask is pretty promising, but the working mechanism of such fiber including the contribution of chemicals involved here is quite ambiguous.
Respiratoryprotection mask [61]	Non-woven ofmelt-blown type fibers	(i) Respiratory protection mask with greater breathability and to reduce breathing resistance. (ii) The mask is intended to retain solid or liquid particles suspended in the air and in particular viruses or bacteria capable of causing diseases such as influenza.
Masks that useelectrostatics of materials to protect healthy individuals from COVID-19 [62]	Nylon cloth sandwichedbetween polypropylene layers	(i) The mask useable to adsorb viral particles between layers produced static electricity. (ii) High chance to cross the electrostatic barrier as clinging on the surface of electrostatic layers requires a low pressure drop of incoming breathe.
Self-poweredelectrostatic adsorption face mask based on a tribo-electric nanogenerator [63]	Poly(vinylidene fluoride)electrospunnanofiber film (PVDF-ESNF)	(i) The ultrafine particulates are electrostatically adsorbed by the PVDF-ESNF, and the R-TENG can continually provide electrostatic charges in this adsorption process by respiration. (ii) R-TENG, the SEA-FM shows that the removal efficiency of coarse and fine particulates is higher than 99.20 wt % and the removal efficiency of ultrafine particulates is still as high as 86.90 wt % after continually wearing for 240 min and a 30-day interval.
Washable MultilayerTriboelectric Air Filter for Efficient Particulate Matter (PM)2.5 Removal [64]	Polytetrafluoroethylene (PTFE) and nylon fabrics	(i) It involves triboelectric air filter to filter out particulate matter (PM). (ii) The mask is washable and exhibits removal efficiency of 84.7% for PM0.5 and 96.0% for PM2.5. (iii) The whole filtration process is operated using linear motor to develop charge which increases the complexity of using the mask.
Telephone mouth piece mask [65]	Synthetic polymer (polyolyfin fiber) and electret treated non-woven web (meltbown web)	(i) It has designed to work using the principle of telephone handset. (ii) The non-woven web is coated with a pressure sensitive adhesive so that the sound energy travels through the air into the microphone and makes the layer vibrate and respective layer converts the sound into electricity to make the outer layer electret so that incoming viral particle can be killed. (iii) Repeatable usage of this mask can loosen the knitted threads thus chances of propagating sound wave get reduced.
Self-powered mask(This work)	PVC-nylon, polypropylene (PP)-PU, latex rubber-PU, PI-nylon	(i) Design of the self-powered mask confirms the capability of the mask to function in response to breathing, talking or any gestures of lips of the wearer with no difficulties of fetching external power source. (ii) Tribo-series fabrics generates the static electricity and charged produced due to static electricity powers up the EL. (iii) Any virus-contained droplets/aerosols can get electrified in the EL layer thus de activating the virus in the tribo field. (iv) The proposed self-powered masks can generate thermal power in the range of 387 mW per second (in an ideal condition) which is more than enough to deactivate any virus in the aerosols.

Moisture interference on the performance of the mask

Since primarily is it expected that moisture is one of the external parameter that can affect the overall performance of the proposed face mask, so in this context an experimental study (as illustrated in Fig. S2) has been carried out where the performance of face mask under different humidity conditions are investigated. Several combinations of triboelectric materials and EL used for making the face mask have been explored, particularly only PU, combination of PP-PU pair and EL are tested here. Since different triboelectric materials are sensitive to moisture that may reduce the overall efficiency of TENG [[66], [67], [68], [69], [70]], taking into consideration, a capacitor (Cs in Fig. 2b) is introduced in self-powered TENG mask that enable to active the EL whenever it is required. Equivalent electrical circuit of a practical capacitor is shown in Figs. S3 (a ~ c). Phasor diagram of impedance of a practical capacitor is illustrated in Figs. S4 (a ~ d). The variation of capacitance, resistance, and dissipation factor as function of frequency of PU, PP–PU and EL is presented in Figs. S5 (a ~ c), Fig. 13 (a ~ c) and Fig. 14 (a ~ c) respectively under different humid conditions, particularly 60, 70, 80 and 90% of relative humidity (RH) values are considered. The resistance, capacitance, and the dissipation factor of the electrocution layer and the tested triboelectric pair are measured using impedance analyzer (E4990A, Keysight). A humid gas is supplied using a humidifier and humidity has been measured using a hygrometer (HTC-1) as shown in Fig. S2. Corresponding temperature and pressure have been monitored using a thermometer (Lutron TM-946) and pressure gauge, respectively. Here, Re is equivalent series resistance (ESR), and Le is equivalent series inductance (ESI). Rp is the insulator resistance in the range of MΩ (plate separating resistance) as shown in Fig. S3 (a ~ c). Total impedance of the capacitor is, Z = + , Here the RESR is responsible for the DC flow through the capacitor. The RESR depends on many factors only one of which is humid weather. Due to humidity, if RESR of the ELs capacitor changes much, this will cause unwanted flow of current through the layers which discharges the capacitor. This effect cannot be fully avoided. To study the RESR of the ELs capacitor, the variation of RESR is studied and the change observed experimentally shows that humidity affects the efficiently which is not to be accounted for. The quality factor (Q - factor) is measured at different frequencies under different humidity conditions which is given byand Loss tangent, tan δ =  = =

Fig. 13

Variation of (a) capacitance, (b) resistance, and (c) dissipation factor of PP–PU as a function of frequency under different humid conditions (60–90% RH as mentioned in inset).

Fig. 14

Variation of (a) resistance, (b) capacitance, and (c) dissipation factor of EL as a function of frequency under different humid conditions (60–90% RH as mentioned in inset).

Therefore,  = , where is measured for EL of four triboelectric pairs. Characteristics of resistance and impedance with respect to frequency has been represented in Fig. S6a and Fig. S6b respectively.

Filtration efficiency of the proposed mask

Filtration efficiency is usually stated in terms of the percentage of particles of a certain size that would be stopped and retained by a filter medium. The proposed mask is designed with multilayer filters, in which the inner three layers act as a TE filter and the outer one as an EL. The electrocution layer has been designed with two conductive meshes and in between, one insulator material (polypropylene) with thickness of the order of 5 μm has been used as a separator.

The viral particles experience a mild shock in EL due to the electric field produced between the EL by contact electrification. The motional force exerted on TE filter materials can produce sufficient electric power to activate EL. The triboelectric self-powering (harvesting mechanical energy from daily activities, e.g. breathing, talking or other facial movements) functionalities ensure the effective filtration efficiency. The induced triboelectric charges have been utilized with a voltage tripler circuit as shown in Fig. 2b in a storage capacitor (Cs) for electrocution layer to disinfect the incoming and outgoing SARS-CoV-2 contained aerosols.

During the experimentation it has been noted that the variation of current in the EL depends on size of the droplets as well. It was found that 25 mA of current is typically sufficient to disinfect virus particles [[33], [34], [35], [36]]. For efficiency of the mask, tiny droplets were spread on EL from different sized orifice on the cap of containers were used to create droplets were spread on EL. Different orifice diameter on the cap of containers were used to create droplets of different sizes. The voltage, current were experimentally noted and the corresponding powers were calculated, as shown in Fig. 15a and b, respectively. The experiment has been carried out with different types of water (distilled water, mineral water, pond water, underground water, salted water) to check the effect of different composition and found not significant changes.

Conclusion

While the whole world has been suffering from the devastating COVID-19, safety precaution becomes a key concern to save life in the adverse situation. The present report illustrates the design of multi-layered TENG based face mask. The different combination of TSMs have been utilized in order to get better filtration efficiency in terms of TECD. The combination of easily available latex rubber-PU can be utilized in designing self-powered mask based on its highest TECD and an induced power in μW range may be obtained. The study has also brought light into the voltage, current and power generated by the contact electrification of the TLs. Amidst four combinations of TEMs, the pair of latex rubber and PU seems to be the best suited combination to be used as a self-powered multilayer mask. The prototype mask can be activated through breathing cycles (and/or talking or other relevant facial gesture) without the need of any external power source. The accumulated charge produces sufficient electric field to deactivate the exterior protein of the charged viral-particles by electrocution. In extent, the storage facility of the self-powered mask allows the wearer to have instant action of deactivating charged virus with comfort. All in all, the present design of face mask has a great potential to be used not only for critical purposes, but also can be used by anyone as it is cost-effective, active-powered and provides sufficient protection.

Author contributions

BT, DM and RB conceived the idea and designed the plan of the work. DM has selected the combination of fabric materials based on their triboelectric properties. ND has carried out the mathematical calculation of the proposed mask. BG, SB, Sk.B Ali were dedicated to focus on all the data analysis and wrote the paper. The experimental work are carried out by BT, RB, SB and Sk.B Ali. ND and DM also participated in critical data analysis of the experimental observations. All authors have read the entire content and finally approved for submission to publish in Nano Energy.

CRediT authorship contribution statement

Barnali Ghatak: Writing - original draft, Formal analysis, were dedicated to focus on all the data analysis and wrote the paper. Sanjoy Banerjee: Writing - original draft, were dedicated to focus on all the data analysis and wrote the paper, participated in critical data analysis of the experimental observations. Sk Babar Ali: Writing - original draft, were dedicated to focus on all the data analysis and wrote the paper, participated in critical data analysis of the experimental observations. Rajib Bandyopadhyay: Formal analysis, conceived the idea and designed the plan of the work, participated in critical data analysis of the experimental observations. Nityananda Das: Formal analysis, has carried out the mathematical calculation of the proposed mask, also participated in critical data analysis of the experimental observations. Dipankar Mandal: Formal analysis, conceived the idea and designed the plan of the work, also participated in critical data analysis of the experimental observations, has selected the combination of fabric materials based on their triboelectric properties. Bipan Tudu: Formal analysis, conceived the idea and designed the plan of the work, participated in critical data analysis of the experimental observations.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.