A materials-science perspective on tackling COVID-19

The ongoing SARS-CoV-2 pandemic highlights the importance of materials science in providing tools and technologies for antiviral research and treatment development. In this Review, we discuss previous efforts in materials science in developing imaging systems and microfluidic devices for the in-depth and real-time investigation of viral structures and transmission, as well as material platforms for the detection of viruses and the delivery of antiviral drugs and vaccines. We highlight the contribution of materials science to the manufacturing of personal protective equipment and to the design of simple, accurate and low-cost virus-detection devices. We then investigate future possibilities of materials science in antiviral research and treatment development, examining the role of materials in antiviral-drug design, including the importance of synthetic material platforms for organoids and organs-on-a-chip, in drug delivery and vaccination, and for the production of medical equipment. Materials-science-based technologies not only contribute to the ongoing SARS-CoV-2 research efforts but can also provide platforms and tools for the understanding, protection, detection and treatment of future viral diseases.

The novel coronavirus SARS-CoV-2 causes coronavirus disease 2019 (COVID- 19) and has spread around the world, leading to a global pandemic 1 . The number of confirmed cases has reached 27,486,960 and the number of deaths has risen to 894,983 across 200 countries (as of September 9, 2020) 2 . The Center for Systems Science and Engineering at Johns Hopkins University 3 has reported an even higher infection rate, with 27,699,112 confirmed cases and 900,203 deaths, indicating highly efficient transmission at an exponentially increasing rate 1 . The data from the World Health Organization (WHO) suggest that SARS-CoV-2 has a case fatality rate of about 3.3%, which is higher than that of the 1918 influenza pandemic (2%) and the 1957 influenza pandemic (0.6%) 4 . The rate of the spread of SARS-CoV-2 is 40-fold higher than that of SARS-CoV 1 , which makes it more difficult to control. Therefore, addressing the SARS-CoV-2 pandemic requires scientific collaborations across borders and disciplines 5 .
Viruses are able to adapt to changing environmental conditions and rapidly evolve 6 . Therefore, accurate, rapid and long-term detection strategies, efficient protection and frequent updating of treatments are needed to control the spread of viruses, which requires multidisciplinary solutions. In addition, only a global network can achieve coordinated detection, protection, containment and therapy. Materials science has long played a key role in the rapid response to emerging viral diseases by developing instruments that reveal the structures of viral proteins, by designing point-of-care devices for accurate and rapid detection and diagnosis 7,8 , and by formulating therapeutic treatment and vaccination strategies 9 .
In this Review, we discuss the role of materials science in antiviral research and highlight the importance of collaborations with virologists, epidemiologists and clinicians in addressing the global pandemic caused by SARS-CoV-2 (ref. 1 ). We examine viral transmission, infection and disease progression ( fig. 1a), and discuss viral protection, detection, treatment and vaccination from a materials-science viewpoint ( fig. 1b). Finally, we investigate the future role of materials science in studying infectious viruses and in the treatment of viral diseases. Indeed, the emergence of a new pandemic that is potentially more severe than the SARS-CoV-2 pandemic is not a matter of if but when 1,10 . Therefore, universal platform technologies offered by materials science will also play a crucial role in the understanding, protection, diagnosis, treatment and prevention of future viral outbreaks.

Materials science against viruses
Materials science is important in all areas of antiviral research, including investigation of viral structure and biology, protection, detection, treatment and vaccination.
A materials-science perspective on tackling  to the host cell through binding of the S protein to the angiotensin-converting enzyme 2 (ACE2) receptor, allowing it to enter the cell and release its viral RNA ( fig. 1a). In the cell, the virus hijacks the machinery of the host cell to synthesize RNA and produce structural proteins, which assemble into new viruses that are released from the host cell. The replication cycle is then repeated 26,27 . Not only humans but also animals (cats, golden hamsters and ferrets) 28,29 can be infected by SARS-CoV-2.
The most straightforward method for protection is physically isolating viruses by masks, gloves, face shields and protective suits. Surgical masks are made of three different fibre layers to prevent the entry of viruses; the outside layer is designed to stop liquid splash; the interlayer acts as a barrier against viruses and bacteria; and the inner coating absorbs moisture exhaled by the wearer 30 . Different materials can be synthesized to act as protective coatings, such as polyimide thin films with nanoporous membranes 31 and carbon-based materials 32 . A polymer nanofibre filter, with the surface chemistry optimized to match that of particulate matter, allows both transparency to sunlight and sufficient airflow 33 . A self-powered air filter composed of an ionic liquid-polymer composite and a sponge network also enables efficient removal of particulate matter, including nanoparticles 34 (fig. 2d). Although these materials have not yet been used for viral protection owing to safety and cost concerns, they could be modified and introduced for virus protection in the foreseeable future, which will require research efforts to simplify synthesis procedures and guarantee safety.
Detection of infected patients. Rapid identification of infected patients helps limit the spread of viruses. Immunoassays and nucleic-acid-amplification tests using PCR are widely used and reliable methods for detecting viral proteins and nucleic acids, respectively. The accuracy of the results of immunoassays, such as radioimmunoassay 35 , enzyme-linked immunosorbent assay 36 and the immune colloidal gold technique 37 , depends on the sensitivity of the instruments and materials. The immune colloidal gold technique is an immunoassay based on labelling the antibody with plasmonic gold nanostructures, enabling a fluorescent readout once the antigen is captured by the antibody. This technique can be used for the detection of viral antigens, for example, of the Zika and dengue viruses 7 . Compared with conventional singleplex approaches, this multiplexed immunoassay can amplify near-infrared fluorescence by up to 100 times, substantially improving the imaging of antibody binding. In addition, the immune colloidal gold technique requires only 1 μl of human serum, owing to its high sensitivity ( fig. 3a). Nanowire field-effect transistors can be combined with immune colloidal gold assays 38 for real-time and direct electrical detection of single virus particles based on conductance changes after antigen-antibody binding ( fig. 3b). Similarly, an external electrical pulse can be applied to incorporate graphene quantum dots and gold-embedded polyaniline nanowires during the virus-accumulation step to increase the sensitivity of virus detection, for example, for the detection of the hepatitis E virus 39 (fig. 3c). 1 Real-time PCR with reverse transcription (rRT-PCR) is commonly used for the detection of viral DNA or RNA 40 , and is the recommended technique for the detection of the single-stranded RNA virus SARS-CoV-2 (ref. 41 ). However, PCR-based techniques for nucleic-acid detection come with trade-offs in terms of efficacy, cost, simplicity and speed. Alternatively, the CRISPR effectors Cas12 (refs 42,43 ) and Cas13a 44 can be applied for nucleic-acid detection. For example, specific high-sensitivity enzymatic reporter unlocking (SHERLOCK) is a fluorescence-based or lateral-flow-based assay using CRISPR nucleases for nucleic-acid detection 45 . Interestingly, a prophylactic antiviral CRISPR strategy using Cas13 has also been proposed to inhibit SARS-CoV-2 in human cells 46 . Single-molecule nucleic-acid sensitivity and selectivity can also be achieved using a biosensing platform based on microcavities. In this platform, the hybridization kinetics of matching and non-matching strands are detected by monitoring microcavity resonance wavelength shifts caused by interaction of the molecules with nanorods 47 (fig. 3d). This label-free technology has the advantage of transiently monitoring molecular interactions, allowing statistical analysis of interaction kinetics and increasing sensor lifetime. Nanopore-based platforms can also be used for targeted sequencing, and have already been applied for SARS-CoV-2 virus detection 48 .

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The immune system responds to viral infections and induces detectable symptoms, such as high body temperature. Non-contact detection of body temperature is mainly performed to preliminarily detect infected patients by using infrared thermal-imaging systems, for example, hand-held infrared temperature guns. However, the temperature gun is inaccurate at cold ambient temperatures, because the temperature is calculated based on a Seebeck coefficient 49 close to the body temperature, which is not available at low temperatures.  b | Materials science contributes to the development and optimization of protective equipment and provides technologies and tools for the analysis of SARS-CoV-2, for example, high-resolution imaging, for sequencing (PCR) and protein analysis (immunoassays), for viral detection, for vaccine and treatment development and delivery, as well as by contributing advanced materials for clinical instruments, for example, filters for extracorporeal membrane oxygenation (ECMO) machines. SNP, single nucleotide polymorphism.
Although laboratory assays have been developed for identifying infected patients, the detection and diagnosis of severe viral infections, such as COVID-19, may also require clinical diagnostic methods, such as computed tomography (CT), ultrasonic imaging, magnetic resonance imaging, single-photon emission computerized tomography and positron emission tomography. For example, CT can be used to detect interstitial changes and small patchy shadows in the lung, as well as extrapulmonary bands, in patients infected with SARS-CoV-2 (ref. 50 ). However, instruments and professional medical staff are limited, and, therefore, portable diagnosis instruments 51,52 and artificial intelligence (AI) 53 should be further developed and combined with clinical diagnosis to improve the detection process and guarantee the accuracy of results.

Treatment and vaccination.
Currently available antiviral drugs can be broadly divided based on their mechanism of action into penetration and husking inhibitors, DNA polymerase inhibitors, reverse transcriptase inhibitors, protein inhibitors, neuraminidase inhibitors and broad-spectrum antiviral drugs [54][55][56] . Indeed, the majority of antiviral drugs currently used in the clinic are polymerase inhibitors, which are specific for a given viral enzyme. However, owing to their specificity, polymerase inhibitors cannot be applied for the treatment of infections with new viruses, such as SARS-CoV-2.
In contrast to bacteria, which can often be treated with antibiotics owing to their similar structure and replication mechanisms, the structure and replication of viruses greatly differ between types and species. Therefore, only few broad-spectrum antiviral drugs exist 57,58 . For example, cyclodextrins modified with mercaptoundecane sulfonic acids mimic the key viricidal effects of heparan sulfates without causing systemic toxicity 59 . Alternatively, antiviral nanoparticles have been developed with flexible and long linkers that mimic heparan sulfate proteoglycans. The nanoparticles generate high forces (~190 pN), which irreversibly deform the virus 60 (fig. 4a). Based on the spatially defined arrangement of ligands for the viral spike protein, bacteriophage capsids have also been designed that carry a multivalent binder. The bacteriophages can cover the entire virus www.nature.com/natrevmats envelope and, thus, prevent its binding to the host cell 61 .
In addition, nanosponges derived from the plasma membranes of human lung epithelial type II cells or of human macrophages can neutralize SARS-CoV-2 by displaying the protein receptors necessary for SARS-CoV-2 entry into host cells 62 . Similarly, biomimetic cell membrane vesicles can act as delivery platforms. Cell membrane vesicles have excellent biocompatibility and possess the inherited functions of the source cells, such as long circulation time (red blood cells) 63 , specific targeting moieties and receptors 62,64 . For example, virus-mimetic nanovesicles can deliver antiviral antigens and mimic the viral property to trigger specific immunogenicity 65 . Nanodecoys composed of cell membranes of the Zika virus and gelatin nanoparticles can be administered to capture the Zika virus and inhibit Zika virus entry and infection 66 . Nanomaterials may also be applied to control the cytokine storm associated with pneumonia, which is a common symptom in patients with severe

Binding between IgG and
IgA with antigens is evaluated by measuring the fluorescence intensities of the two dyes. b | Nanowire-based detection of single viruses. Binding of the virus to a specific antibody (Ab) leads to a change in conductance. c | An external electrical pulse and biosensors based on graphene quantum dots (GQDs) and gold-embedded polyaniline nanowires (AuNP-PAni) can be used for the detection of hepatitis E virus (HEV). The biosensor electrode, which is based on anti-HEV Ab-conjugated to nitrogen and sulfur codoped graphene quantum dots and gold-embedded polyaniline nanowires (Ab-N,S-GQDs@AuNP-PAni), can capture HEV. The HEV concentration is determined from the pulse-induced impedimetric response. d | A singlemolecule whispering gallery mode biosensor platform using plasmonic gold nanorods can be used to detect single nucleic acid molecules. EDC, 1-ethyl-3- COVID-19. For example, neutrophil membrane-coated poly(lactic-co-glycolic acid) nanoparticles can neutralize pro-inflammatory cytokines and suppress inflammation in inflammatory arthritis, which might also be applicable for pneumonia 67 .
The development of new antiviral drugs is challenging and requires thorough knowledge of the infection and replication mechanisms of the specific virus. For example, based on the genome sequence similarity between SARS-CoV and SARS-CoV-2, the serine protease TMPRSS2 was identified to be crucial for S protein priming and, thus, for SARS-CoV-2 cell entry via ACE2. Therefore, a TMPRSS2 inhibitor may provide a treatment strategy to prevent SARS-CoV-2 infection 68 . Remdesivir, which is a nucleotide analogue prodrug that inhibits RNA-dependent RNA synthetase, has also been explored for COVID-19 treatment 69 . Preliminary data of the Randomised Evaluation of COVID-19 Therapy (RECOVERY) trial, testing dexamethasone as a treatment strategy for hospitalized COVID-19 patients, shows a lower 28-day mortality rate of treated patients than patients in the standard-care group, supporting a  www.nature.com/natrevmats potential treatment effect of dexamethasone in patients who need respiratory support 70 . Injection or inhalation of dexamethasone nanoformulations may promote the treatment efficacy on COVID-19 through targeting of the drug to hyperactivated immune cells, potentiating its antioedema activity and antifibrotic effects 71 . Moreover, neutralizing antibodies [72][73][74] and nanobodies (antibody fragments) 75,76 have been proposed for COVID-19 treatment. A recent clinical trial (NCT04313127), testing neutralizing antibodies to prevent COVID-19, had shown satisfactory results and is in further evaluation 77,78 . However, achieving a high concentration of neutralizing antibodies is crucial for efficacy and, thus, aluminium-based adjuvants, such as those used for PiCoVacc 79,80 , may be helpful.
Vaccines remain the most efficient strategy to protect against viral infections, because they generate specific antibodies 81 . Following infection, the virus causes inflammation and stimulates the release of chemokines, which attract leucocytes to the infected area to destroy the virus 82 . At the same time, phagocytes engulf viruses, disassemble them, deliver them to nearby lymph nodes and present the antigens to other immune cells, such as B and T cells, to initiate the production of antibodies that specifically recognize viral antigens 82 . However, upon first contact with a virus, the immune response is slow, and can only help patients to recover, but not to prevent the disease. Once antigen-specific T and B cells are activated, they proliferate and generate memory cells. Therefore, upon second contact with the same type of virus, the immune system can respond quickly and efficiently. Pulmonary surfactant biomimetic liposomes can be applied to encapsulate and deliver an agonist of the interferon gene inducer, that is, stimulator of interferon genes, to increase the immune response against influenza viruses 83,84 (fig. 4b).
The purpose of a vaccine is to protect the body against a specific virus, capitalizing on the memory of the immune system, so that the immune system can eliminate the virus upon first contact. Vaccine design includes the identification of an antigen and adjuvant, which boosts the immune response caused by the anti gen 85 , as well as an appropriate delivery platform. A variety of vaccines have been developed, including liveattenuated vaccines, inactivated vaccines, toxoid vac cines, subunit vaccines, conjugate vaccines, DNA vaccines, mRNA vac cines and recombinant vector vacci nes. mRNA vaccines 86 , which directly induce antigen expression, provide an interesting alternative for RNA viruses, such as the Zika virus and SARS-CoV-2, which are susceptible to mutation and, therefore, to drug resistance 6 . mRNA vaccines require nanocarriers for encapsulation and stabilization of the mRNA 87,88 ; for example, mRNA can be encapsulated in lipid nanoparticles to protect mice against Zika 89 virus infection ( fig. 4c).
Although there is no mRNA vaccine yet approved for any disease, several clinical trials are under way. For example, Pfizer Inc., in a collaboration with BioNTech AG, is working on the phase II/III clinical trial of BNT162 mRNA vaccines for COVID-19 (NCT04368728) 90 and has just announced data from preclinical studies 91 . Moderna, Inc. is currently conducting phase II clinical trials (NCT04232280) with an mRNA vaccine encoding a monoclonal antibody against cytomegalovirus 92 . In addition, the mRNA-1273 vaccine, developed by Moderna, Inc., against SARS-CoV-2 is currently in phase III clinical trials (NCT04470427) 93 . These mRNA vaccines are all delivered by lipid nanoparticles.
Finally, medical devices, such as ventilators, are important for the treatment of infected patients, to provide oxygen and ensure airflow in the lungs. Alternatively, extracorporeal membrane oxygenation (ECMO) can be applied in patients with severe respiratory failure to promote their recovery process 94 . In this procedure, venous blood is drained from the body, oxygenated using a gas-exchange device and then returned to the body 95 (fig. 4d). ECMO has also been applied for the treatment of COVID-19 patients 94 .
Possibilities for materials science Materials science has long played an important role in the study of viruses and in the treatment of viral infections 18,37,44,96-106 ( fig. 5). In the future, materials science and engineering will certainly continue to contri bute to confronting viral outbreaks by providing platform technologies and tools for virus research, ensuring efficient and comfortable protection, performing rapid, accurate and low-cost detection, and enabling timely and effective treatment and vaccination.

Physical protection.
Physical protection provides a key strategy to prevent viral infection. Materials science can be applied to optimize physical-protection platforms in terms of preparation, manufacturing, material performance and structure, and to balance comfort and practicality. Commercial face masks can reduce virus emission from infected patients by providing a physical barrier with a certain cut-off size 30 . However, masks could also be designed to incorporate drugs or physical obstacles for killing the virus and stopping the transmission; for example, the virus could be damaged by heat generated by photothermal materials, such as graphene 32 , or by producing reactive oxygen species through a photodynamic process [107][108][109] . The design of reusable masks could further address personal protective equipment shortages, as experienced during the COVID-19 pandemic. Reusable masks would need to retain filtration ability and remain safe after decontamination, such as ultraviolet irradiation, moist heat and vaporous hydrogen peroxide 110 . To address this challenge, fibres, such as washable polypropylene fibres coated with assembled metal-organic frameworks 111 or thermally stable electrospun polyimide/zeolitic imidazolate framework-8 nanofibres 112 , could be applied that tolerate heat, ultraviolet irradiation 113 and hydrogen-peroxide treatment. In addition, a self-cleaning capability could be implemented by changing the surface chemistry of the material or by coating materials with photothermal materials 32,109 or hydrophobic membranes 31 to capture, kill and release the virus; for example, likewise, using filters composed of metal-organic frameworks with photodynamic ability 109 or graphene with photothermal ability 32 . Functional nanofibres, for example, fibres modified with amino-functionalized silica particles 114 or nanodecoys 66 capable of capturing viruses through charge interactions or by active recognition, could also be used for masks and manufactured by nanospinning 115 or 3D printing 116,117 . To address the problem of fogging when wearing safety glasses and masks, a layer of nonionic surfactant could be added to the surface of protective equipment. Environmental contamination may also play a role in SARS-CoV-2 transmission 118 . Therefore, long-lasting and sprayable antiviral materials (for example, functional nanogels can multivalently interact with viruses and have adhesive effects on the surface) 119 could be developed that can be applied on surfaces to keep the environment and protective equipment clean.
Virus detection. Materials science offers tools and techniques to improve the speed, sensitivity, accuracy and portability of virus-detection methods, in particular, for the isolation of nucleic acids and proteins from viruses. For example, magnetic particles can be used to capture nucleic acids or proteins in a sample, which can then be separated by application of a magnetic field 120 . Integrating microfluidics with isolation techniques can reduce the amount of reagents, increase isolation speed and allow automation 121 . Moreover, microfluidics can be introduced to rapidly mass-synthesize uniform magnetic particles 122 and to efficiently isolate viruses using magnetic-particle-based microfluidics 123 . In addition, DNA 124 or RNA nanostructures 125 that contain complementary oligonucleotides to virus DNA or RNA could be combined with microfluidics for virus isolation. Although current nasal swabs are effective in collecting virus samples, nasal swabs with a controlled porous architecture could be functionalized with materials that can capture viruses through multivalent interactions 119 , which would enable the collection of viruses even at very low concentrations.
Similarly, aggregation-induced-emission (AIE) technology or quantum dots could be integrated with immune colloidal gold techniques to increase viral-detection sensitivity 126 . In addition, materials-science approaches, such as ionic-liquid-based materials 127,128 , could be applied to improve the stability of extracted nucleic acids or proteins for further investigation. Improving the sensitivity, specificity and simplicity of virus-detection methods is important to ensure accurate and rapid testing of samples. For example, an Fe-Au nanoparticle-coupling approach, combining PCR amplification with the quantitative measurement of Au nanoparticles using inductively coupled plasma mass spectrometry 129 , which was originally developed for the detection of circulating tumour DNA, could also be applied for the sensitive and specific detection of SARS-CoV-2 RNA. Microarrays, which have been developed for the precise control and manipulation of fluids, combine sample preparation, reaction, separation and detection on one chip of a few square centimetres 130 , and could be used in combination with surface-enhanced Raman spectroscopy for rapid virus detection using nanoparticle aggregates or arrays that can generate surface-enhanced Raman spectroscopy signals 131,132 . Low-cost point-of-care devices, designed with microfluidics, could also be adapted for viral detection, requiring only small sample volumes. Microfluidics can further be used for the production of designed nanoparticles 133,134 , which are used for viral detection, treatment and vaccination.
Plasma protein profiles have been investigated as disease-related biomarkers using mass-spectroscopybased proteomics; similarly, viral protein biomarkers could be detected in blood 135 . However, the accuracy of protein identification by mass-spectrometry-based proteomics is limited, owing to the concentration range (from pg ml −1 to mg ml −1 ) of the >10,000 proteins in plasma. The protein corona, that is, the shell of biomolecules formed on a nanoparticle upon contact with a biofluid 136 , can be exploited to increase the concentration of proteins for mass spectrometry to overcome the bottleneck of blood-sample-based proteomics and to enable the detection of low-abundance pro teins 137 . Therefore, investigating the protein corona 138,139 may also provide a strategy for virus detection and/or to study the host response. For example, nanoparticles could be designed to adsorb viral protein biomarkers or host response proteins (that is, cytokines) in plasma, serum or other biological samples. The nanoparticles could then be isolated for downstream mass-spectrometry-based proteomics analysis, enabling fully automated, high-throughput viral detection, including nanoparticle-plasma incubation, protein-corona purification using magnetism, protein digestion and liquid chromatography-tandem mass spectrometry 139 . Bioelectronic systems 140 enable the real-time and remotely controlled monitoring of biological information in the body; for example, wearable sensors for the wireless monitoring of uric acid and tyrosine in sweat 141 , glucose levels during diabetes therapy 142 and skin temperature 143 . Bioelectronic devices could also be applied for the long-term monitoring of symptoms of viral infections, such as an increase in temperature or respiratory syndrome. Protective equipment could also be equipped with a sensor that can sense the entry of viral particles and translate the signal to a smartphone. For example, cellular-binding-like events between the virus and host-cell-mimicking sensors (that is, sensors coated with host-cell membranes or multivalent binders) can be detected by thermal-shift measurements based on protein stabilization induced by a ligand 144,145 , which can be translated into a digital signal recorded by a smartphone. Similarly, interactions between viral particles and specific molecules could be detected in protective masks.
Mobile technologies offer the possibility of track-andtrace systems that provide information on COVID-19infected patients, including age, gender, symptoms and geographical areas. These systems can alert users when they are in high-risk areas via an app to slow down the spread of SARS-CoV-2. Mobile technology can also be used to collect data for analysis and to inform guidelines for government policies. For example, a COVID-19 symptoms tracker app has been developed. The app has already collected data from over 2.8 million participants, providing information about risk factors, symptoms, clinical results and geographic hotspots. The data are publicly available and important in the response to a pandemic; however, privacy concerns and ethical considerations should also be considered 146 . In addition, AI, such as machine learning, can be applied to large datasets of clinical results to help clinicians in their diagnosis. For example, an AI system has been developed based on the CT database of COVID-19 patients, enabling rapid diagnosis 147 .
Finally, point-of-care devices are particularly important in areas with low resources; for example, low-cost and portable gene-sequencing devices that can be operated without the need for trained medical staff. Accurate, real-time monitoring of viral transmission can further help to understand the infection process; for example, bioluminescent microscopy can be used to image the transmission of a virus with an inserted luciferase gene in mice 148,149 or ferrets 150 . Similarly, fluorescent protein genes such as green fluorescent proteins and red fluorescent proteins genes 151-153 could be integrated into the SARS-CoV-2 genome for tracking transmission in vitro and in animal models.
Drug discovery and delivery. Materials science can also contribute to the development of new technologies, such as thermal proximity coaggregation 144 and time-resolved wide-angle X-ray scattering 154 , which allow the dynamic observation of structural protein changes in host cells in response to a virus, and virus-drug interactions, to accelerate and improve the process of drug discovery. In addition, drug development relies heavily on studies in animal models, which cannot entirely recapitulate the human response and often inaccurately predict the safety and efficacy in humans. Alternatively, microphysiological systems, such as organ-on-a-chip platforms, in which engineered human tissues are connected by physiologically relevant flows, can be applied for drug development, as well as for toxicity and efficacy evaluation. Similarly, organoids derived from stem cells can provide macroscale models of human tissues featuring the complexity and spatial heterogeneity of human organs. However, compared with animal models, organs-on-a-chip and organoids would fail in discovering systemic immunity-modulation drugs, because they cannot recapitulate the organ microenvironment 155,156 . Moreover, the lifespan of these platforms would need to be prolonged to allow the generation of mature tissues undergoing homeostasis 156 . Importantly, organoids can be used for disease modelling and for the evaluation of antiviral drug toxicity and treatment effects [157][158][159] . For example, bat and human intestinal organoids were used to confirm that bat intestinal cells can be infected by SARS-CoV-2 and that the human intestinal tract is a possible transmission route of SARS-CoV-2 (ref. 160 ). Furthermore, it could be demonstrated in SARS-CoV-2infected kidney and human blood vessel organoids that soluble human ACE2 can inhibit SARS-CoV-2 infection 161 . Materials science can improve organoid cultures by developing chemically defined, tunable, synthetic scaffolds that guide the assembly of human cells into tissues and that support organoid maturation 162,163 .
Nanoparticles can enable the effective delivery of drugs, improving circulation time, enhancing targeting efficiency and reducing potential adverse side effects. Nanocarriers can also be used to deliver antiviral therapeutics into cells to intervene with the viral-replication cycle at the molecular level or to deliver plasmid DNA 164 and mRNA 165 for vaccination. Vaccine efficiency and safety are of the utmost importance, and, therefore, US Food and Drug Administration (FDA)-approved materials should be used as delivery platforms; for example, a lipid-nanoparticle-encapsulated mRNA, encoding the receptor binding domain of SARS-CoV-2, has been reported as a vaccine candidate 166 . The FDA-approved lipid nanoparticle improves the stability of the mRNA and the transfection efficiency in vivo.
Lessons learned from cancer vaccines can benefit the development of antiviral vaccines. Biocompatible materials have been explored for drug delivery in cancer, to improve the stability and pharmacokinetics of payloads and to prevent premature release or degradation. In addition, nanomaterials can be modified to target the delivery to antigen-presenting cells and to enable co-delivery of viral antigens and adjuvants to the lymph nodes 167 , for example, by using an albumin-hitchhiking approach 168 or by targeting phagocytic cells to promote lymph-node transportation 169 . Nanomaterials can also be designed to respond to a specific environment or to external stimuli, allowing the control of payload release and preventing undesired immune reactions, such as immune toxicities in normal tissues caused by payload leakage or an immune cytokine storm caused by rapid payload release 87 , for example, pH-responsive nanoparticles 170 or light-responsive nanoparticles 171 . Cancer vaccines can be designed to integrate diagnosis and treatment using nanomaterials with imaging abilities 172 , for example, to monitor the delivery of antigens and adjuvants in vivo, which may also be relevant for antiviral vaccines. Real-time data on the location and treatment responses could be generated by introducing NIR-II bioluminescence 173 , NIR-II chemiluminescence 174 or NIR-II fluorescence 175 to enable the rational design of nanoscale materials and bench-to-bedside translation of vaccines. Materials science also enables the design of self-administration platforms; for example, vaccines formulated as microneedle patches 176 , which slowly release payloads upon self-administration. Reproducibility and scalability also play major roles for the translation of nanomaterials for drug delivery. Microfluidic systems can be used for the controlled, reproducible and high-throughput synthesis of libraries of nanoparticles. Nanomaterials also play a role in virus treatment, for example, to provide cofactors, such as Zn 2+ , which inhibits RNA polymerase and, thus, transcription 177 . Zn-based nanomaterials that bind RNA polymerase could also be designed and administered to inhibit virus replication. In contrast to dendrimers or polymers, the valency and spatial structures of DNA nanostructures can be easily designed. In addition, DNA nanomaterials are non-toxic and stable. Therefore, DNA nanostructures with particular shapes, modified with virus-targeting aptamers, can be applied for virus detection and inhibition 124 . Owing to their customized spatial structure, such DNA nanostructures precisely match the viral surface, enabling efficient virus capture.
Inhalation as a route of drug and vaccine administration is particularly relevant for viruses causing respiratory symptoms, such as SARS-CoV-2, because it allows small-molecule drugs 178 and vaccines 179 to directly target the lungs, in which the virus resides 180 . Inhalation is particularly advantageous in resource-poor regions, because no medical staff is needed and self-administration of drugs or vaccines improves patient compliance. Inspired by the approved intranasal spray vaccine FluMist 105 , similar inhaled drugs or vaccines could be encapsulated using smart nanoparticles to promote their stability and to realize long-term and controlled release. In addition, inhaled nanoparticles encapsulating drugs or vaccines can be modified with specific antibodies targeting the cells in which the viruses reside to kill the viruses, or immune cells to trigger an immune response. This strategy could also decrease potential side effects of drugs and vaccines by avoiding drug delivery to healthy tissues. For example, nanomaterials could be coated with a specific ligand that binds to receptors on host cells of the virus in the lungs. Therefore, following inhalation, the nanoparticles would inhibit virus-host cell interactions.
In the case of SARS-CoV-2, the ACE2 receptor or the serine protease TMPRSS2 could be targeted 68,181 . Nanoparticles can also be modified with elements of host-cell membranes to inhibit viral entry 64,66 ; for example, nanomaterials can be decorated with host-cell membranes derived from human lung epithelial type II cells or human macrophages 62 to capture SARS-CoV-2 and inhibit infection. Similarly, using nanomaterials that can generate free radicals 56 or heat 32 could improve the antiviral efficacy of such approaches.
Nanoscale materials have a similar size to viruses, which allows them to interact with the virus at the same size level. Cyclodextrins modified with mercaptoundecane sulfonic acids 59 , nanoparticles modified with long linkers that mimic heparan sulfate proteoglycans 60 , nanodecoys 64 , cellular nanosponges 62 or DNA nanostructures 124 could bind the virus to inhibit infection by inhibiting virus entry 56 .
Viral infections, such as SARS-CoV 182 and dengue 183 virus, can cause cytokine storms as a result of an excessive immune response 184 . SARS-CoV-2 infection can also induce a cytokine storm in some patients, which contributes to inflammation and pneumonia 185 . Lessons from nanotechnology-based cancer immunotherapy can inform treatment designs to modulate the cytokine storm by delivering anti-inflammatory drugs or inhibitors to inflammation sites. In particular, bioresponsive nanomaterials 186-188 that target inflammation sites can deliver anti-inflammatory drugs and inhibitors. However, the targeting efficacy and payload leakage still need to be optimized to decrease side effects. In addition, the fabrication of clinical-grade materials with the ability of modulating cytokine storms remains a challenge.

Medical equipment and instruments.
Materials science can be applied to optimize filters in ventilators with self-cleaning and virus-inhibition ability. 3D printing of ventilators could be introduced and optimized to fabricate ventilators with more functions. Similarly, ECMO machines, which are applied as life-saving treatment for severe respiratory diseases, such as COVID-19, suffer from high complexity, the need for highly trained staff, low portability and concerns about potential harm 189 , such as changes in thrombotic 190 and immune-related factors 189 . A small device could be developed that combines the blood pump and the oxygenator to improve portability and reduce thrombotic issues, by decreasing the time that blood spends outside the body. Indeed, portable ECMO devices, such as the Maquet Cardiohelp device, which combines the pump and oxygenator in one unit 191 , have been developed. To decrease the size of such devices, wireless technology and energy-converting materials could be applied. In addition, bioelectronic systems and wireless mobile technology could be integrated in ECMO devices to monitor blood-status changes in real time. The gas-exchange membranes, which are a key component of ECMO machines, are usually made of hollow fibres, such as silicone rubber fibres 192 or polymethylpentene materials 193 . Their resistance to blood flow and their gas-exchange capacity could be further improved by using 3D-printable biocompatible materials, such as respiratory-membrane-derived materials.

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Outlook Viral-disease outbreaks require a rapid response to obtain basic knowledge, but also continuous antiviral research. Materials science plays an important role in many aspects of antiviral research, including the study of viral biology and transmission, protection, identification of infected patients, treatment and vaccination. The investigation of new viral strains requires virus isolation and purification, size, structure and component determination, and analysis of infection, replication and spread. To prepare for new viral outbreaks and to accelerate and coordinate our response, portable and inexpensive devices for viral isolation and purification from samples such as blood or saliva are needed; for example, a hand-powered ultra-low-cost centrifuge 194,195 . To investigate virus structure and composition, electron microscopy, cryo-electron microscopy, confocal microscopy and sequencing technologies need to be applied. Additionally, the real-time dynamics of the replication of a single virus in host cells and tissues, including RNA replication, protein formation and cell-to-cell transmission, need to be assessed, for example, using super-resolution imaging or time-resolved wide-angle X-ray scattering. Advanced materials-enabled technologies, such as thermal-proximity coaggregation 144 and time-resolved wide-angle X-ray scattering 154 , could further enable the observation of interactions between drugs and viruses. These technologies will require more powerful instruments with high temporal and spatial reso lution, as well as high sensitivity. However, spatial reso lution should not be achieved at the expense of time resolution, for example, in super-resolution microscopy. The development of portable and inexpensive imaging equipment, such as the Octopi 196 , BiteOscope 197 and scale-free vertical tracking microscopy 198 , are particularly important in low-resource settings. However, the nanoscale settings in viral research provide a challenge for portable optical-imaging systems, owing to the diffraction limit. Thus, the time resolution might be more important than the spatial resolution in this case.
During viral outbreaks, large amounts of masks and personal protective equipment need to be rapidly produced, for example, using 3D printing 117 and nano-electrospinning for the fabrication of nanofibres to compose filters. Virus detection further needs to be quick and done with high accuracy and at a low price, for example, by distributing self-testing kits, which have a sample-collection tool for viral capture (enabled by a specific surface chemistry, for example, a coating that can capture viruses by multivalent interactions 119 at low concentrations), and the results are shown on a strip 43,199 . Sharing of results and data is also an important aspect in antiviral research, in particular, during viral-disease outbreaks.
The delivery of antiviral drugs and vaccines will greatly benefit from the use of vehicles based on biocompatible materials to reduce systemic toxicity, to improve circulation time, for the co-delivery of multiple components, to increase drug or vaccine stability and to target specific cells or tissues. Drugs and vaccines need to be preclinically tested in cells and virus-infected animal models. Alternatively, engineered ex vivo systems can be applied, for example, organoids and organs-on-a-chip, which allow the assessment of drug effects in real time. The production of drugs or vaccines may further require the use of robots and microfluidic technologies to enable scalability and to be able to respond to high demand. To tackle future challenges, the collaboration between different scientific fields, clinicians and industry will be required.
Published online 14 October 2020