key: cord-1000289-lgvjz2j6
authors: Shi, Jidong; Li, Hongbian; Xu, Fang; Tao, Xiaoming
title: Materials in Advanced Design of Personal Protective Equipment (PPE): A Review
date: 2021-09-08
journal: Mater Today Adv
DOI: 10.1016/j.mtadv.2021.100171
sha: 39db959b874562de761c3d16cdd6998d8c02e7ce
doc_id: 1000289
cord_uid: lgvjz2j6

The outbreak of Covid-19 pandemic has aroused tremendous attention towards personal protective equipment (PPE) in both scientific research and industrial manufacture. Despite decades of development in PPE design and fabrication, there’s still much room for further optimization, in terms of both protection performance and wear comfort. Interdisciplinary efforts have been devoted in this research field in recent years. Significantly, the innovation of materials, which brings about improved performance and versatile new functions for PPEs, has been widely adopted in PPE design. In this minireview, recent progress in the development of novel materials and structural designs for PPE application are presented in detail, with the introduction of various material-based strategies for different PPE type, as well as the examples which applies auxiliary components into face masks to enrich the functionalities and improve the personal feelings in pandemic period.

The Covid-19 pandemic has dramatically changed the world since its outbreak at the end of 2019. Up to now, more than 184 million people have been infected, with 3.98 million deaths caused worldwide. [1] Despite the recent development of vaccines which are still skeptical in preventing infections and causing side effects, the most effective approach to slow down the spread of the Covid-19 coronavirus is to wear PPEs, which include not only face masks and shields for ordinary people, but also protective clothes and glasses for medical workers surrounded by high concentration of the coronavirus.

In this context, the research of PPE has been greatly boomed. According to the searching result in Web of Science, the PPE related literatures have been dramatically ramped up from 357 in 2019 to 1734 in 2020, indicating the unprecedented enthusiasm in this field amongst the scientific society. Besides, PPE manufacture is also an industry worth billions of dollars. The productivity of face masks in China has reached more than 110 million per day in 2020, [2] and will be sure to continuously increase before the attenuation of the pandemic.

Except for the PPEs for coronavirus protection, there are multiple types of PPEs applied in various scenarios, such as the armors and helmets for the protection from mechanical impacts, respirators and gloves to resist the chemical toxicants, as well as protective clothes to block the thermal hazards and electromagnetic radiations. Each of them has undergone at least several decades of development, and commercial products have been massively manufactured. However, there is always room for the progress in PPE design and fabrication. For example, surgical masks, which most people wear in this pandemic, have high filtration efficiency only towards particles larger than 300 nm, [3] while the coronavirus is 65-125 nm in diameter, [4] which could still penetrate through the surgical mask and infect the wearer. The N95 masks have better filtration efficiency towards small particles, while it could also block the air flow for breath, and the wearer could develop both physiological [5] and psychological syndromes [6] after long-term use of N95 masks. Therefore, there's great motivation for the optimization of face masks to achieve high filtration efficiency while maintaining the breathability.

Besides, the insufficient protection property, high weight and bulkiness, as well as the low breathability and long-term durability also necessitates the innovation in PPE design. On the other hand, smart elements could be introduced to PPEs to bring in new functions to assist the protection process, such as sensors and energy harvesters. The development of PPEs is an interdisciplinary subject combining materials science, textile engineering, ergonomics and medical science, etc., which aims to protect people from various hazards to the most extent.

In this minireview, recent advances in the novel strategies to improve PPE performance in recent years will be discussed. We will first provide an overview of the materials and structural designs for personal protective application, divided by different PPE types. Then we will give an introduction of the state-of-the-art commercial masks with auxiliary designs, which represents the recent efforts in developing smart PPEs for everyday use. This minireview is ended by the discussion of the prospects and challenges in future PPE design and optimization.

Despite the wide application of PPEs in various occupational and civil scenarios, there are still great efforts to develop new materials and structures for improved protection performance. In this section, we will introduce the materials in advanced PPE design in recent years, which is divided based on the specific PPE type.

PPEs against mechanical impacts, such as helmets and armors, are essential in both military and civil applications. There are two basic requirements in the design of mechanical PPEs. The first is enough strength to prevent the PPE from broken apart; and the second is the capability to absorb the impact energy so that the load transferred to PPE wearers could be minimized.

In antient society, armors are the most common type of mechanical PPE for the J o u r n a l P r e -p r o o f abundance of wars. In order to effectively avoid the penetration of spears and bullets, the armors are usually made of intrinsically hard materials with high mechanical strength, such as metals and ceramics. However, due to their undeformable nature, the impact load could be ballistically transferred to the wearer which still causes injury.

Besides, the rigid and heavy armor could cause dramatic discomfort to the soldier by limiting his mobility. In modern society, safety helmets have been increasingly used to protect the wearer against the impact by high-velocity or high-weight objects. Typically, the safety helmets are comprised of rigid outer shell, which is made of stiff plastics such as polycarbonate (PC) and acrylonitrile butadiene styrene (ABS), and buffer layer, which is made of deformable paddings such as polyurethane (PU), and expanded polystyrene (EPS). [7] In recent years, polymers reinforced by high modulus fibers are also applied in assembling the mechanical PPEs, for the improved strength and reduced brittleness compared with plastics. Composites with high mechanical performance have been prepared by embedding aramid fibers, polybenzoxazole (PBO) fibers, carbon fibers, and glass fibers, etc. into polymer matrix [8, 9] . And the mechanical properties of the final composites are dramatically affected by the continuity as well as the weaving architectures of the fibers. [10, 11] Compared with single component polymers, the preparation of fiber reinforced composite is relatively laborious, especially for the synthesis of fibers. Recently, Lin et al. tackled this issue by extracting Kevlar fibers and low-melting polyester (PET) fibers from the selvage of discarded fabrics, and assembled these recycled fibers together with nylon fiber into a non-woven fibrous network. A sandwiched structure was constructed by enclosing the high strength PET interlayer with two as-prepared fibrous networks ( Fig. 1a and 1b) . [12] Together with the dramatic advantage in the reduction of feedstock consumption and textile waste, the resulted composite also demonstrates optimal burst strength and tensile strength of 1957 N and 425 N. The lightweight, environmentally friendly and mechanically robust composite serves as a promising candidate in mechanical PPE application.

For materials as mechanical supports, there's usually a trade-off between mechanical strength and flexibility. While the tradeoff could be effectively overcome J o u r n a l P r e -p r o o f by the introduction of shear thickening fluids (STFs). STFs are usually concentrated colloidal suspensions. Upon the infliction of shear force, the suspended particulates could aggregate which dramatically increase the viscosity of the suspension. [13] Therefore, STFs-based mechanical PPEs provides simultaneously more flexibility and comfort in daily wear and the enough protection upon mechanical impact. Fowler et al.

encapsulated an STF composed of silica nanoparticles and poly(ethylene glycol) into a spacer fabric, which decreases the peak force by 66% upon impact, compared to the pure fabric. [14] Since STFs sometimes brings about problems in sealing due to its liquid nature, a solid-state shear thickening gel was developed and impregnated into the Kevlar fabric for ballistic protection (Fig. 1c) . [15] With the addition of carbon black into the gel, the composite could not only adsorb 21.6% of the impact energy (Fig. 1d ), but also monitor the impact intensity through the change of electrical resistance when integrated into a protective helmet.

Except for the selection of material, novel structural designs have also been adopted for reinforcing the mechanical strength, absorption of more impact energy, and prevention of crack propagation in PPEs. Since a porous structure usually has lower modulus than its bulk counterparts, it could deform both elastically and plastically in controlled manner under impact, which efficiently absorbs and dissipates the input mechanical energy. [16] Foam structures with uniform and periodic porosity are relatively easy to be prepared, while they could still collapse under high load due to the global buckling behavior. In comparison, a hierarchical graded porous structure demonstrates higher strength and energy absorption ability due to the structural response to compressive stress by stages. [17] That is, when the compressive stress is small, the softer parts of the graded structure undertake the main deformation, while with the increase of stress, the stiffer parts begin to deform. Such behavior renders a better-organized structural adaption towards the applied pressure and more sufficient absorption of the input mechanical energy. [18, 19] With the development of additive manufacturing, various complex hierarchical structures with microscale feature size could be facilely prepared. [20] [21] [22] [23] The graded honeycomb structure could also be applied as a liner of a J o u r n a l P r e -p r o o f helmet, demonstrating nearly twice energy absorption rate and 37% lower transferred load compared to homogeneous honeycomb structure. [24] On the other hand, reinforcement of the foam structure by continuous cushioning fillers could efficiently improve the structural stability without much sacrifice in flexibility. [25] For example, the filling of flexible polyurethane (PU) into the pores of 3D printed polylactide (PLA) lattice could decrease the jerk by 9%, displacement by 17% and increase the energy adsorption by 23% under impact, at an expense of 21% mass increase. [26] Despite the high mechanical strength for anti-impact PPEs, small defects inevitably exist within them, which could develop into cracks upon mechanical impacts.

To this end, there are also novel designs to improve the fracture toughness of the antiimpact PPEs. The introduction of discontinuous staggered structure could be a solution, for the crack propagation could cease at the interface between staggered layers. Inspired by the structure of conch shells, a 3-layer prototype was prepared by 3D printing in 2017. (Fig. 1e and 1f ). Within each layer, stiff VeroMagenta plastic serves as the base, with soft Tangoblackplus rubber lamellae periodically sandwiched within the VeroMagenta plastic. For the restraint of crack propagation, the orientations of the Tangoblackplus rubber lamellae in each layer are staggered by 45°, so that the propagated cracks could be arrested at the interface between layers. [20] The 3-layer hierarchical structure demonstrates 85% and 70% higher energy absorption than the bulk VeroMagenta plastic and 1-layer sandwich structure, respectively. One drawback of this prototype is that cracks could still propagate freely within each layer. Recently, Wu et al. developed a fibrous Bouligand structure with fibrils lamellae twisted continuously over depth, like a spiral stair. [27] The propagation of cracks follows the twisted lamellae which absorb greatly increased energy than propagating in a straight line, so that the crack toughness is dramatically improved.

J o u r n a l P r e -p r o o f shear thickening gel infiltrated Kevlar fabric. Reprinted with permission from ref. [15] (e) The schematic of the three-tier cross lamellar structure for conch shell. (f) The man-made 3D printed three-tier cross lamellar structure. Reprinted with permission from ref. [20] .

Chemical hazards are ubiquitous in many occupations. For example, doctors should be protected from the viruses and bacteria in the aerosol around the hospital;

Soldiers should stay away from chemical warfare agents (CWAs) in the battlefield;

Environmental workers should reduce the inhalation of volatile organics. Even ordinary human beings also suffer from the growing air and water pollution. Therefore, great efforts have been devoted in the design of novel PPEs for chemical protection.

Reducing the pore size of face respirators and chemical protection clothes down below the particulate size could isolate the protege from certain chemical hazards, such as microorganisms, while it also brings difficulty in breathing and releasing the thermal stress. [28] Materials with high surface area, such as activated carbon, [29] have been J o u r n a l P r e -p r o o f commonly applied for assembling face masks and chemical protective clothes due to the strong physical adsorption ability and low fabrication cost, while the low filtration efficiency, the easy saturation by non-toxic adsorbents, as well as the risk for secondary toxicant release, still restricts its application in high-risk occupational circumstances.

In this section, we focus on PPEs which could chemically interact with the chemical hazards by permanently convert the toxicants into harmless agents or generate alert signals to the proteges.

PPEs with different chemical modifications are usually applied for different target toxicants for protection. Metal-Organic Framework (MOF), which contains metal nodes bridged by organic ligands, have been widely applied for assembling the chemical protective clothes for their high porosity, chemical stability and abundant functionalization sites. [30] Among various MOF types, Zr-based MOFs, such as UiO-66-NH2, are particularly suitable for the degradation of organophosphate-based CWAs. [31] [32] [33] [34] One issue concerned in the application of MOF in PPEs is the poor adherence between the modified MOFs and underlying fabric. To this end, an oxidebased interlayer could be introduced to anchor the MOF molecules. For example, a highly efficient CWA protective fabric could be obtained by conformally coating a polyamide-6 fiber mat with atomic layer deposited TiO2 film, followed by in situ growth of UiO-66-NH2 (Fig. 2a) . [32] The as-prepared CWA protective fabric could decompose the nerve agent soman by 50% in merely 2.3 min (Fig. 2b ). Since the introduction of rigid oxide layer decreases the flexibility of the PPEs, alternative solutions includes directly mixing the MOF and fiber precursors followed by electrospinning, [35, 36] as well as using hot pressing to ensure a strong bonding between MOF and fibers. [36] Besides, the warning function could also be introduced in MOF modified chemical protective clothes. Making use of the decolorization of the toxicants during the degradation process, the functionality of the chemical protective clothes could be real-time monitored. [37] The introduction of colorimetric warning dramatically alleviates the potential danger from the dysfunction of the PPEs.

Antimicrobial PPEs are essential to protect medical workers from possible infections. Deposition of Ag nanoparticles (AgNPs) on polymeric fiber mats is a simple way to endow antimicrobial properties on PPEs, [38, 39] while the antibacterial tests for AgNPs coated PPEs are usually conducted in solution, the performance in gaseous environment is doubtful. Organic macromolecules with nitrogen-halogen moieties usually demonstrate strong oxidative property by releasing of oxidative halonium ions in aqueous environment. [40] , and could be decorated onto fiber mats for antimicrobial applications. [41, 42] Among various macromolecules of this type, N-halamine is most commonly applied despite its weak intrinsic toxicity. [40, 43] In contrast with AgNPs, ion release by N-halamine could take place utilizing the environmental humidity, facilitating its application for inhalation protection (Fig. 2c ). [43] In order to further enhance the biosafety and environmental friendliness of the antimicrobial PPE, textiles functionalized by botanic extracts have been designed. [44, 45] With natural antimicrobial ingredient such as phenolic compounds, some bionic extracts are lethal to a variety of microorganisms. Usually, the bionic extracts are volatile and easily oxidable, which necessitates proper encapsulation for a controlled release and lasting functionality.

Compared with N-halamine, the efficiency for bacteria killing is far inferior for natural bionic extracts (hours vs days), indicating that PPEs with bionic extracts could only be used in ambient environment, and not suitable for pathogen abundant environment, such as hospitals.

Except for utilizing the intrinsic chemical reactivity for degrading the toxicants, there are also chemical PPE designs relying on external stimulus, mostly light, for the generation of reactive oxygen species (ROS), which could attack pathogens and organic toxicants and protect the PPE wearers. A prominent advantage of photocatalytic detoxification is the rechargeability of the reactivity, which is different from the abovementioned chemical reagents which are irreversibly consumed by toxicants. A face mask integrated with TiO2 nanowire networks was prepared by Horváth et al. (Fig. 2d) , which could deactivate various pathogenic viruses under UV illumination (Fig. 2e) . [46] Since the percentage of UV in solar radiation is very low, the applicability of the TiO2 J o u r n a l P r e -p r o o f nanowire integrated mask is limited. A rechargeable daylight driven bioprotective nanofibrous membrane could be achieved through modifying electrospun PVA-co-PE film with benzophenone derivatives, which could efficiently generate ROS under visible light. [47] The bactericidal and virucidal efficiency reaches 99.9999% and 99.999%, respectively. Moreover, the ROS generated by light could be stored, which still kills pathogens under dark condition.

Although versatile chemical PPEs have been designed and prepared by introducing functional coatings to conventional textiles, the long-term functionality should also be further evaluated for the possible degradation and detachment of the functional coating. Besides, the cost issue also needs to be considered before the substitution of passive PPEs by chemically functionalized PPEs in practical scenarios.

For the future research of chemical PPEs, the combination of detoxification and sensing of the chemical hazards could be further explored. permission from ref. [32] . (c) The schematic for the deactivation of airborne pathogens J o u r n a l P r e -p r o o f by H-halamine. Reprinted with permission from ref. [43] (d) A face mask integrated by TiO2 nanowire network. (e) The mechanism for the photocatalytic degradation of microbial targets. Reprinted with permission from ref. [46] 

The introduction of electricity to PPEs, whether by building a functional electrical circuit or generating electrical charges on them, could dramatically enrich the functionality of PPEs. In this section, three scenarios of electrically functional PPEs are introduced. Firstly, a smart PPE could be endowed with sensing function, which could either detect the external stimulus or record the physiological signals of the wearer.

Secondly, PPEs could be integrated with energy harvesters to provide the power supply of essential electrical devices. Thirdly, tribo/piezoelectric charges could be induced in air filters which benefits the adhesion of particulate matters (PMs).

As illustrated in last section, toxic gases have been a serious threat in many occupations. Although chemical PPEs could isolate or detoxify them to protect the wearer, there's still danger from the unconscious failure of the PPEs. A quick and reliable detection of toxic gases could effectively assist the protection process by warning people of the chemical hazards in the surroundings, for which gas sensor integrated PPEs have been developed. Carbon nanomaterials, such as carbon nanotube (CNT) and graphene, have been widely adopted in building chemical sensors, for the adsorption of analytes dramatically affects the carrier transport within the nanostructures. [48, 49] To prevent the exfoliation of carbon nanomaterials from the textile substrate, Wang et al. directly transformed Kevlar fabric to graphene at pre-defined positions through laser irradiation (Fig. 3a) . [50] The in situ carbonization process guarantees a seamless bonding between graphene and underlying substrate. A selfpowered NO2 sensor based on the graphene/Kevlar fabric was integrated into a face respirator which has a low detection limit of 10 ppm (Fig. 3b) . On the other hand, gas sensor integrated PPEs should be capable to distinguish different toxic gases and avoid crosstalk. To this end, a smart face mask which could separately detect ethanol vapor, J o u r n a l P r e -p r o o f methanal and ammonia gas was developed. [48] The three gas sensors are constructed by coating a nylon fiber by single-walled carbon nanotubes (CNTs), multi-walled CNTs and Zn nanoparticles decorated single-walled CNTs, respectively, followed by weaving the fiber sensors into a face mask. Each sensor is sensitive to one type of toxic gas and insensitive to the others, so that the three types of gas could be separately distinguished.

The mechanical impact to the head could be lethal. The measurement of the impact force is significant for both medical treatment of the injured person and investigation of the accident. Typically, the operational range of most piezoresistive and capacitive pressure sensor is in kPa level, while the impact force during an accident could reach several MPa. Therefore, pressure sensor with extended operational range should be designed. Wang et al. encircled the side face of a cylinder PDMS by a fabric strain sensor, which could record the normal pressure within the range of 0-8 MPa with a sensitivity of 1 MPa -1 . [51] Such pressure sensor could be integrated in helmets or armors for measuring impact force. Furthermore, for a better protection from blast triggered impact, a feedback system could also be introduced. [52] For people working in harsh environment, real time monitoring of their physiological parameters is essential to guarantee their well-being. Besides, the high weight and low breathability of PPEs could inflict much mechanical and thermal stress to the wearer, which also necessitates physiological monitoring to avoid the heatstroke and asphyxia of the wearer. Breath flow monitoring could be achieved through the design of a smart face mask integrated by piezoresistive [53] or triboelectric [54, 55] pressure sensors, which could reflect both the respiration condition of the wearer and functioning status of the face mask. PPEs capable for comprehensive physiological monitoring are also developed by integrating various sensors, such as temperature sensor, heart rate sensor, respiration sensor and motion sensor into the PPEs. [56, 57] With advanced computing technologies and artificial intelligence (AI), a comprehensive assessment could be made for the PPE wearer so that a timely feedback is provided before the occurrence of severe health problems.

Further efforts in sensor integrated PPEs lies in the display, storage and transmission of the sensing data, so that the wearer himself or the supervisor could readily get the information and take measures. The integration of LEDs [48] or electrochromic devices [58] could be a solution for visible demonstration of the data, while they are still too rough in displaying the result. Reliable data transmission necessitates advanced connectivity solutions, such as Bluetooth, WIFI and mobile network (5G). Intelligent sensing systems with decision making and feedback ability is also being pursued with improved functionality and reduced size.

For people working in remote areas, there could be a serious deficiency of electric power, which brings great inconvenience and even danger to them. Since batteries are usually bulky and heavy which are suitable to carry along, the utilization of biomechanical energy could be a novel solution to provide essential power supply. PPEs could be applied as the platform for the integration of generators. For example, a triboelectric nanogenerator (TENG) and an electromagnetic generator (EMG) made by the double deck structure of Al-Kapton-Al could be integrated into a safety helmet, which could harvest the biomechanical energy from head motion (Fig. 3c) . [59] After AC-DC transformation by rectifiers, the TENG could power an LED and a wireless pedometer (Fig. 3d) . Generators could also be mounted on gloves, which collect the biomechanical energy from hand motion; [60, 61] and masks, which harvest the mechanical and thermal energy of the respiration air flow. [55] In all, generators within PPE could bring about life-saving significance to the wearer working in harsh environment, while the necessity of rectifying devices and low power output could be the limitation for biomechanical energy harvesters.

The most common and efficient approach for the filtration of PMs is to wear face masks. However, there's always a tradeoff between filtration efficiency and air permeability in the design of pore size for face masks. The introduction of electrical potential within face masks could dramatically improve the filtration efficiency of PMs by electrostatic adsorption (Fig. 3e and 3f) , [62] without losing much breathability. The J o u r n a l P r e -p r o o f typical approaches to introduce electrical potential in face mask include mounting TENGs [63, 64] and integrating electret materials (PVDF, BaTiO3 et al.), [62, 65, 66] by which voltage of several hundred volts could be generated under the respiration air flow. Since in both ways the generated electrical potential increases with the speed of air flow, which further raises the adsorption efficiency of PMs, the deterioration of filtration efficiency under high speed air flow could be largely prevented. [62] Despite the advantages above, the adsorption of PMs could clog the porous fiber mats, leading to degraded performance after long-term operation. LEDs and a wireless pedometer. Reprinted with permission from ref. [59] . (e) The SEM image showing the PMs adhered on PVDF fiber mat. (f) The long-term filtration efficiency for PM2.5 and PM10 by PVDF fiber mat. Reprinted with permission from ref. [62] .

J o u r n a l P r e -p r o o f Thermal hazards are common for many occupations, such as firefighting and metallurgical industry. Besides, as mentioned in previous sections, long-time wearing

of PPEs could bring about much thermal stress and discomfort to the wearer, causing the rise in body temperature and heart rate, which may further leads to heatstroke and cardiovascular diseases. [67, 68] PPEs with thermal insulation interlayers could isolate the wearer from the environmental heat, while the thermal stress is even harder to be released from the wearer. [69] [70] [71] To this end, thermally functional materials as well as structural designs have been introduced to PPEs to overcome this tradeoff, so that both external and internal heat could be kept away from the human body. In this section, two types of thermally functional PPEs will be introduced: One is PPEs integrated with heat absorbing materials, including phase change materials (PCMs) or liquid coolant; the other is PPEs with engineered pore structure which could dissipate heat through thermal radiation.

Since phase change process of PCMs could absorb or release much heat without temperature change, the PCMs could be regarded as a thermal reservoir which help to stabilize the local temperature upon massive heat input. PCMs could be integrated into protective clothes and helmets to block the thermal hazards in specific workplace. The thermal protection performance of PCM-based thermal liner depends dramatically on the phase change temperature and latent heat, which necessitates the careful selection of PCM type. With phase change temperature close to skin burn temperature and high latent heat, paraffin is the commonest PCM type in PPE application. [72] In addition, the phase change temperature of paraffin could be tuned by changing the number of carbon atoms, for fitting the application in different scenarios. Pure paraffin usually has low thermal conductivity, causing insufficient absorption of heat. To this end, fillers with high thermal conductivity could be added, such as graphene, CNTs and metal nanoparticles. [73] Besides, since paraffin is not fire retardant, it should be encapsulated when applied in firefighting clothes. [72, 74] Liquid cooling is a common strategy for thermal protection of industrial facilities, J o u r n a l P r e -p r o o f which is also applied in the design of thermal PPEs. Water-containing channels could be knitted into cotton fabrics, which resulted in a liquid cooling garment. [75] The liquid cooling garment could resist temperature rise by not only the high heat capacity, but also the circulation of water to carry local heat away. High electrical voltage up to several kV could be applied to accelerate the circulation of dielectric coolant in a stretchable pump, while the safety issue should be considered (Fig. 4a) . [76] Skin simulant with penetrating channels connecting a water reservoir was also prepared, which serves as an artificial sweating system under heat exposure. [77] The evaporation of sweat could dramatically enhance energy absorption, as well as weight reduction of the whole system.

Radiative heat transfer contributes greatly to the thermal exchange between human body and outside world. Since human body could dissipate heat by emitting infrared radiation with wavelength ranging between 7-14 μm, there is also designs in PPE to boost this radiative cooling process for personal thermal management. In 2016, Hsu et al. first developed this idea by preparing a nanoporous polyethylene (PE) membrane with pore size ranging from 50 to 1000 nm. [78] Due to the comparability in feature size, visible light is scattered by this membrane which renders the membrane opaque in naked eyes. On the other hand, PE contains mostly C-C and C-H bonds with narrow absorption peaks around certain wavelengths (3.5 μm, 6.8 μm, 13.9 μm). Therefore, it is transparent to most IR radiation which facilitates the dissipation of heat ( Fig. 4b and   4c ). As a result, the skin temperature covered by the PE membrane is 2.7 ℃ lower than that covered by cotton (Fig. 4d) . Compared with IR transparent materials, IR emissive materials could actively absorb heat and convert it to IR radiation, which enable the dissipation of heat from both external environment and the human body. [79] For the blockage of radiant heat flux in specific working scenario, such as firefighting and metallurgical industry, increasing the reflectance of thermal PPEs by deposition of metal nanoparticles could be adopted. [80, 81] Despite the rapid development in the design for thermal PPE, there are still Besides, the encapsulation of PCMs and cooling liquids should be meticulously conducted for their leaking could bring danger to the wearer. For radiative cooling system, there might be privacy concern for the IR transmissive membrane could be transparent under IR camera. permission from ref. [78] .

UV radiation, taking up ~5% of total solar radiation, could partially penetrate through the ozone layer and harm the skin of ordinary people. Besides, hazardous radiations, such as UV, X-rays, and high-power lasers, are common in many working scenarios (eg: medical test, materials processing and characterizations, optical communications). PPEs for radiation shielding have been developed for both specialized and civil use. For textiles without specific anti-radiation design, a high J o u r n a l P r e -p r o o f textile thickness and compactness is needed for complete radiation blockage, which dramatically increases the discomfort for the wearer. [82] Therefore, novel materials for absorbing the electromagnetic radiation, especially UV, could be introduced for building the anti-radiation PPEs.

The radiation shielding materials could be divided into inorganic and organic ones.

The mechanism of radiation shielding in inorganic systems is the absorption of the electromagnetic wave for the transition of electrons from valence to conduction band.

Due to the proximity between the bandgap and the UV energy, TiO2 (3-3.2 eV) and

ZnO (3.37 eV) are the most commonly used inorganic UV shielding materials so far. [83] Since the adhesion between inorganic oxides and textiles is usually poor, the oxides could be hybridized with organic matrix, such as PET, before coating on textile substrate for UV blockage. [84] Besides, strongly bonded ZnO nanowires on cotton fabrics could be achieved by in situ growth process. The ZnO nanowires deposited on the inner surface of cotton fibers are durable upon more than 50 washing cycles, retaining a high ultraviolet protection factor (UPF) of 100. [85] On the other hand, inorganic coatings are usually rigid and undeformable. To this end, Liang et al. designed a stretchable UV shielding skin by layer-by-layer assembly of electrospun PU fiber mat and spray coated TiO2 layer (Fig. 5a) . [86] The UVF of the UV shielding skin reaches 10810 at 0% strain and still maintains 5685 at 200% strain ( Fig. 5b and 5c ), thanks to the recoverable sliding of fibers under strain. A potential drawback of this system could be the low breathability due to the densely packed structure, leading to poor wearing comfort. As for the protection against electromagnetic waves with higher energy than UV, such as X-ray, heavy metals could be adopted in PPE design. [87] Compared with inorganic radiation-shielding materials, the organic ones could be directly made into fiber mats or combined with existing textile substrates with high affinity, rendering improved durability for the resulted anti-radiation PPEs. Organic UV shields usually contains aromatic structures with carbonyl groups which demonstrates conformational change upon UV absorption. [83] However, chemical reactions are J o u r n a l P r e -p r o o f usually accompanied with the conformational change, which could release detrimental chemicals and ROS, leading to the harm to the human skin. Besides, the UPF for organic UV shields is usually below 100, which is inferior to the inorganic ones. [88, 89] For the future development of anti-radiation PPEs, there should be multiple considerations except for improving the protection efficiency. The safety issue should be taken into prioritized account, considering the possible permeation of inorganic nanoparticles into skin, as well as the detrimental decomposition products for organic radiation shielding materials. Besides, the ergonomic and aesthetic issue also needs to be considered, especially for the civil UV shielding applications. (c) UPF value of the UV shielding skin under different tensile strain. Reprinted with permission from ref. [86] 

Thanks to the advanced novel materials applied in PPE fabrication in laboratory research, many PPE manufacturers have turned the scientific advancement to commercial products. Specially, after the outbreak of Covid-19 pandemic, face masks J o u r n a l P r e -p r o o f have been a must in everyone's daily life. [90] While the negative influence of face masks on people's comfort, as well as the communications between people, has motivated the design of smarter face masks, which provide auxiliary functionality besides passively isolating the coronavirus. In this section, commercial face masks with auxiliary functional components are introduced, which could represent the new trends in industrial PPE development.

Wearing masks hinders the communications between people by both muffled speech and coverage of facial cues. Transparent face masks made of plastics have been prepared. [91] While at the expense of low wearing comfort due to the poor air permeability and large mechanical mismatch with skin, the plastic mask facilitates only facial expressions in the environment with enough brightness. Aiming to further facilitate the communication between people behind the mask, a company named Razor developed a novel face mask last year which could convey both facial and vocal language regardless of the environmental brightness. [92] In addition to the plastic covering which is intrinsically transparent, luminous additives are introduced which could lights the plastic, so that facial language could even be exchanged in the dark.

Furthermore, for the amplification of voices, a microphone is embedded within the mask, which ensures a clearer conversation behind the mask. In another waterproof N95 face respirator called MaskFone designed by Binatone company, an earbud is incorporated for making better quality phone calls (Fig. 6a) . [93] In another conceptual product, an LED array is stitched into a face mask, by which the motion of the mouth is represented by specific light patterns. [94] The C-Face mask designed by Donut Robotics company took a step further by embedding a translator within the mask, by which the spoken words could be real-time translated into 8 other languages. [95] The daily wearing comfort of the electronics embedded mask still wait to be examined, despite the claim by Razor company that ergonomic design has been adopted to prevent the contact between the electronic module and the face.

Although multiple layer design could effectively isolate the virus from entering J o u r n a l P r e -p r o o f the respiratory system, the virus could accumulate outside the mask after long-term wearing, which still brings about danger to the wearer. To this end, the LG company developed a face mask named Puricare which utilizes UV light to deactivate the microbes on the mask. The UV-LED is powered by rechargeable batteries which could sustain for up to 8 hours after a single charging. As a consequence, 99.97% of the particulates (<0.3 micron in size) in air could be blocked. [96] For similar aim, scientists from Massachusetts Institute of Technology (MIT) incorporated a copper mesh into the mask. By applying an electrical current through the mesh, large amounts of ohmic heat are generated, which greatly raise the local temperature and the microbes are killed. [97] Another approach to kill the aerosol pathogens adhered on the mask relies on the radical species produced by high electric field (Fig. 6b ). [98] A self-disinfecting mask designed by Swiss researchers has a sandwich structure with two conductive fabrics separated by a dielectric layer. A voltage of several volts is applied by a rechargeable battery which could ionize the air and generate radical species. And the pathogens could be inactivated in only several minutes. After the purification process, the mask could be recycled for use. Despite the reduction of waste, volatile compounds released upon the applied voltage should be further assessed in terms of their safety concern.

Since respiration process contains much information about health, the collection of respiration parameters, such as respiration rate, flow, as well as the concentration of certain constituents, could facilitate the monitoring of health conditions. To this end, commercial smart masks embedded with respiration sensors have been designed. For example, the Airpop company developed a smart mask which incorporated a sensor network for recording the respiration rate of the wearer (Fig. 6c) . [99] Besides, the smart mask could also monitor the air quality in the surroundings. The manufacturer claims that the data could help monitoring the sleep condition of the wearer. The abovementioned Puricare mask also contains a respiration sensor and a ventilator with dual fans. [96] Through tracking the respiration flow of the wearer, the ventilator could automatically adjust the fan speed which facilitates the air exchange across the mask.

Another smart mask designed by Vita Innovations company comprises a 3D printed J o u r n a l P r e -p r o o f resin as the supporting substrate with a series of biometric sensors embedded. [100] The sensors could simultaneously measure heart rate, blood oxygen level, body temperature and respiration rate of the mask wearer. These data could be transmitted to external devices for display, and an alarm could be triggered when abnormal physiological data appears. The as-designed mask is greatly helpful in the hospital for monitoring the health condition of patients, especially for those waiting for emergency treatment.

The above-mentioned functional components of face masks represent the efforts of manufacturers to make the world better under the long-lasting Covid-19 pandemic.

Despite the abundance of novel smart mask products, the price of them is still too high compared with regular surgical masks (tens of cents for each) and even N95 respirators with earbuds. [93] (b) The sandwiched structure of the self-disinfecting mask which could be purified by built-in high voltage. [98] (c) The schematic of sensor embedded smart mask and the display of recorded data on mobile phone. [99] 

In this minireview, advanced materials and structural designs applied for various Novel smart PPEs have been not only designed and fabricated in lab, but also industrially manufactured for daily applications. With the construction of the infrastructures all over the world, as well as the growing air and water pollution, the development of PPEs will continuously be a research hotspot in the future, even after the end of the Covid-19 pandemic. As the basis for the upgrading of PPE functionalities, the innovation of materials will definitely be focused to achieve better protection performance.

Despite the increasing focus and rapid development of this field, there are still limitations in applying advanced materials in smart PPE fabrication. Firstly, the durability of the protection performance of the as-designed PPEs should be assessed, especially for PPEs with additives of organic or biological molecules for specific capture of pollutants, due to the degradation of these additives with time. Besides, side effects should be taken into account when improving certain properties of a PPE by introducing new materials. Sometimes, the improvement of protection performance is at the expense of decreased personal comfort; and some additives used in PPEs are intrinsically detrimental to some extent. Last but not the least, the high price is always the bottleneck for the popularization of the smart PPEs amongst ordinary people. The cost for smart PPEs preparation could be lowered down by mass production through standardized production line, while the risk for technology disruption and the uncertainty of market demand should also be concerned.

In all, the advancement of materials science has greatly motivated the revolution of PPEs, rendering improved protection efficiency, functionality and wearing comfort.

The development of PPEs will not cease with the recession of Covid-19 pandemic, and more efforts will be enrolled in this field to bring more safety and convenience to the people in need of care.

Natural Science Foundation of SZTU

COVID-19 Map -Johns Hopkins Coronavirus Resource Center

Proceedings of the National Academy of Sciences

Nanoscale Horizons

Carbohydrate polymers

AIP Conference Proceedings, American Institute of Physics2014

Smart materials and structures

Fifth European Workshop on Optical Fibre Sensors, International Society for Optics and Photonics2013

Advanced Fiber Materials

The World's Smartest Mask -Project Hazel

Best Athletic Face Mask with Integrated Earbuds | MaskFone

Coronavirus face mask lights up with moving mouth shapes

FACE」 | donut robotics

Face mask for an active lifestyle | AirPop

Dr. F. Xu thank the support from the National Natural Science Foundation of China

The authors declare no competing financial interest.