6. Targets for LbL Coating and Potential Applications against Coronaviruses as Well as General Antiviral Applications and Challenges
The applications that are discussed in this section are in a sense a crude method of applying highly sophisticated materials and techniques that were developed for in vivo or diagnostic purposes. A so-called nanotrap set of applications has been developed that can be used for the detection of pathogens [114]. These traps rely on the recognitions between antigens and antibodies that were already discussed above. An example of carbohydrate trapping was illustrated for chitosan fibers that were functionalized with sialyllactose, facilitating the capture of influenza viruses via the virus hemagglutinin glycoprotein on its surface [115].
Carbohydrate microarrays have been reviewed comprehensively [116,117] and these were developed to recognize certain pathogens based on glycan-binding properties of lectins. Either glycan or lectin is indicative of a particular pathogen. Numerous analytical methods, for example, fluorescence spectrometry, mass spectrometry, and surface plasmon resonance spectrometry methods are employed for qualitative and quantitative techniques.

6.1. Water Sterilization
All humans need to drink water and have access to water to ensure personal hygiene. In the context of the recommendation to wash hands frequently to remove coronavirus and other pathogens, humans need clean water. However, water can easily be contaminated with viruses.
It was proven that representative coronaviruses, mouse hepatitis virus, and transmissible gastrointestinal virus survived for weeks, even in pasteurized, settled sewage. This suggested that sewage and contaminated water sources posed a risk for coronavirus transmission [118]. Currently, numerous research groups have started to establish if SARS-CoV-2 is present in sewage water and traces of the virus have been found in numerous sewage sources [119]. SARS-CoV-2 was also recently detected in the stool of an asymptomatic child [120] and remains in the gastrointestinal tract of pediatric patients for a prolonged period even after clearance from the lungs [121]. It is suggested that the fecal-oral route of transmission should not be excluded as a route of virus transmission [122], however, it is still being investigated if SARS-CoV-2 will spread through contaminated water [123].
An indirect form of LbL coating was effected by the manufacturing of lignin core particles. These lignin cores were coated with prepared cationic lignin particles. Negatively charged cowpea chlorotic mottle viruses and the cationic lignin particles were highly effective in removing the viruses from the water. It is suggested to develop this material into water filtration devices [124].
A microfiltration membrane was developed to filter water and reduce viral counts of a model bacteriophage. The LbL coating of the filters was performed with the synthetic polycation, poly(ethylene imine) which was covalently immobilized onto the poly(ether sulfone) water membrane material. Silver and copper nanoparticles were also coated in the LbL layers, and although an effective antiviral performance was seen, leached markedly into the filtered water [125].
In a proof-of-concept study, silicon wafers were LbL-coated with synthetic polyelectrolytes in bilayers. Antibacterial effects were apparent and also antiviral activity against the H1N1 influenza virus. Although the coated material proved to be 100% bactericidal against waterborne bacteria, it demonstrated 60% virucidal activity against an H1N1 droplet after coating three layers of the polyelectrolyte. However, as the coating was increased to 7.5 bilayers, 100% virucidal activity was observed [126].
LbL coating of nanofiltration membranes with gallic acid and PEI could effectively adsorb selected antibiotics from water. Although not directly related, an antiviral effect can be envisioned by this kind of LbL coating [127].
The successful disinfection of aqueous poliovirus and rotavirus solution was achieved by coating glass slides with synthetic poly(ethylene imine), PEI, cations [128]. Numerous other examples of LbL coatings on surfaces were also shown to be antimicrobial against many waterborne organisms, yet antiviral effects were not reviewed or were simply mentioned briefly. It is, however, apparent that the same intermolecular forces that lead to interaction between surfaces and viruses occur between surfaces and other microorganisms [129,130,131,132].

6.2. Air Filtration
As with water, coronaviruses survive on inanimate surfaces [133] that include metal, glass, wood, paper, silicon, surgical gloves, plastic, and non-stick PTFE surfaces for a period of up to nine days on plastic and two days on steel [134]. It was also observed that MERS-CoV could survive in the air for much longer durations than influenza virus strains [135].
However, living tissue such as mucosal membranes harbors these viruses [136]. As with the need for water, humans need to breathe and are, therefore, naturally exposed to all these surfaces. The need for broad-spectrum protection against airborne microbes has been expressed. Three protective measures against airborne spread of viruses have been suggested to ensure the antiviral efficiency of filter materials and lastly, that these materials are not limited to only one type of device or surface [137].
Wearing facial masks is a commonly recommended prophylactic measure against the spread of airborne pathogens [138,139] such as the influenza A virus [140]. It is even recommended that patients suspected of being infected with SARS-CoV-2 should be fitted with masks during surgery or procedure requiring anesthesia [141] and also during transport to the hospital [142]. FFP2 and FFP3/N95 masks offer significant protection against airborne particles and even then the FFP3 masks filter at least 99% of airborne particles, leaving a gap for some particle penetration. These masks also still pose the risk of improper sealing [143].
It has been shown that airborne viruses, for example, norovirus, adenovirus, and torque teno virus can be detected in air samples [144] on for instance PTFE filters that were used to detect SARS-CoV [145], and MERS by the employment of a specialized air sampling instrument [146].
Several viruses, especially respiratory viruses are effectively distributed by airborne droplets [147,148], and very obviously the human coronaviruses such as SARS-CoV [145,149,150], MERS [146,151,152], and SARS-CoV-2 [153,154,155,156] even though some controversy exists regarding respiratory transmission as the only means of distribution [157]. Some examples of viral air filtration are discussed next.
HEPA filters were coated with charged carbon nanotubes. During the filtration study, the nanotubes were shown to be 92% effective in the filtration of a viral bacteriophage, MS2, as a model of viral particle [158]. There are virtually no reports on coated antiviral air filters, let alone employing an LbL coating method to create these masks. These are rapid inactivation action, broad-spectrum inactivation not only against specific strains, and lastly universal application on various media, not just facial masks [159].
The coating of the filter fabric of these masks with PEI achieved more than 99.999% antiviral activity against the T4D bacteriophage virus. These coatings were achieved by dip coating, however, LbL nanocoating was not reported [159].
A disposable polyimide layer has been coated onto porous silicon-based masks, including an N95 mask. The layer proved to be hydrophobic and prevented the accumulation of droplets on the mask almost completely. The film can be peeled off after use and replaced to reuse the mask [160].
A comprehensive review of cellulose-based air filters has shown that cellulose is an effective material to produce antiviral masks. Cellulose ester materials that are anionic are still seen as the most effective antiviral material in filters and again affirms the potential interaction of positively charged spike proteins and negative GAGs. However, LbL nanocoating of cellulose has both been described as a method to produce antiviral filters [161]. In an unrelated application, our research has shown that cellulose can be coated easily employing LbL coating with, for example, chitosan as coating materials [162]. It is proposed that it would be possible to modify a facial mask that is based on a cellulose fabric with antiviral GAGs.
There are indications that LbL nanocoating of air filtration devices can make a significant contribution to the prevention of virus distribution and be very relevant to the prevention of the respiratory variants of human coronaviruses.

6.3. Textiles
In addition to wearing facial masks, protective clothing is another important measure against the spread of pathogens. Cotton is one of the most commonly employed textiles used to manufacture clothing.
Cotton fabric, which was ready for use by the textile industry, was LbL nanocoated. The alternating layers comprised of PSS and poly(allylamine) bilayers followed by bilayers of PSS and chitosan. The coated fabric was washed in water to remove the excess polyelectrolyte with or without ultrasonication. The fabric that was washed with ultrasonication produced a more uniformly coated fabric than without. The coated material was exposed to antimicrobial tests and significant antibacterial activity was shown toward S. aureus and some activity was still maintained after washing the fabric with a non-ionic detergent. Up to 80% of bactericidal activity was maintained for fabric samples that were washed with ultrasonication compared to samples washed without. The latter samples retained approximately 30% bactericidal effect. Antiviral effects were not investigated [163].
Chitosan-alginate LbL was also reported to impart antibacterial effects on cotton fabric, however, antiviral activity was not determined [164]. Chitosan also afforded antibacterial properties when alternately layered with sulfonated lignin or boric acid onto cotton fabric [165].
Lignin, an abundant product of unexplored commercial value, that is produced from wood pulp during paper manufacturing also impart antibacterial properties to coated textiles [165,166].
Another popular application of the LbL coating of cotton fabrics was to create flame-retardant cloth [167]. Examples include the coating of the material with flame-retardant laponite nanoclay particles [168], cationic starch, and montmorillonite nanoclay particles in alternate layers [169].
Nylon-66, a popular synthetic textile, was also rendered flame-retardant by LbL-nanocoating with cationic chitosan and anionic alginate [170] or anionic phytic acid [171]. Another synthetic textile, polyester, were coated with synthetic PSS and PDDA to impart significant antibacterial properties to the fabric [172,173].
Dual-purpose, environmentally friendly, antibacterial, and flame-retardant layers of poly(hexamethylene guanidine phosphate) and the seaweed-derived alginate could also be coated on cotton fabric [174].
Another example of synthetic polyelectrolyte LbL nanocoating that showed significant antibacterial properties comprises of N-halamine-derived copolymer polyelectrolytes, however, the coated cotton fiber had to be immersed in household bleach to elicit antiviral activity. Despite this, the amount of chlorine that was absorbed could be controlled depending on the bilayer thickness [175].
Another approach to producing a coated textile is to coat fibers that are used to weave the textile. Some drug delivery approaches were employed to create chitosan-alginate fibers loaded with a protein drug [176]. Wood pulp fibers were also successfully LbL-coated with chitosan and carboxymethylcellulose [177] and this fiber coating could, for example, lead to paper strengthening [178]. Antibacterial vascular grafts were produced by LbL nanocoating of chitosan and heparin onto the graft materials [179]. It is suggested that pre-coated fibers can be produced and subsequently weaved into an antiviral textile. Again, a scarcity of literature was found that focused on antiviral activity. It provides an “inside-out” method of textile coating instead of coating the finished textile after it was weaved from uncoated fibers. Polysaccharides may again prevail as an effective pre-treatment option prior to weaving fabrics.
Antiviral activities of polysaccharides, especially of unmodified GAGs or uncomplicated naturally derived polysaccharides such as chitosan and pullulan have been reviewed [180], yet at the time of writing, almost no literature could be found where these polysaccharides were LbL-coated onto textiles specifically for an antiviral effect. Antibacterial and improvement of the physical properties of coated textiles seem to be the focus of comprehensive reviews with brief mentioning of antiviral effects [173,181,182].
Despite the lack of explicit LbL nanocoating of textiles, it is illustrated that coating of fabrics could be an effective antiviral countermeasure. Surgical gowns were, for example, laminated with a 3-fold layer of poly(propylene) as the outer layer, PTFE as the middle layer, and a non-woven polyester layer as the innermost layer. PTFE proved essential in curbing viral adhesion and penetration through the fabric [183,184].
A survey of the patent literature also revealed almost no applications of LbL-nanocoating in antiviral settings regarding textiles.

6.4. Medical Devices
Seriously ill patients with respiratory distress, and also other patients with other conditions such as cardiovascular disease often require intimate contact with medical devices. Examples of devices include prosthetic heart valves, orthopedic implants, intravascular catheters, artificial hearts, cardiac pacemakers, etc. [185]. Wettability of the surfaces has been shown to play a crucial role in the adhesion of viruses to whichever surface is present. Glass slides were successfully coated with silanes of various hydrophobicity and showed that more hydrophobic surfaces were most efficient in capturing influenza A viruses. This points again to a form of nanotrapping and inactivation of the virus by surface interactions [186]. Glass slides were also LbL coated with polyanionic and polycationic chitosan to eliminate S. aureus. It was implied that this LbL process will benefit medical devices with antibacterial properties [187].
Vascular prostheses were coated successfully with alternating layers of human serum albumin and the GAG, heparin, and other carbohydrates such as dextran sulfate. These multilayered contacts prevented the clotting factor, fibrinogen from causing thrombosis which is a serious complication of contact with the medical device [188].
Although not always disclosing full information, self-eliminating coatings on medical devices that comprise mainly of polypeptides were successfully applied to medical devices that could deliver biological substances after implantation into the human body [189], applied to urethral catheters and stents to prevent mucosal tissue of the urethra to block openings of the catheter or stent that will lead to prevention of fluid drainage. These coatings also released antimicrobials although antivirals were not mentioned specifically [190,191]. In a comprehensive review, the future of LbL nanocoating in the medical profession was pointed out, however, antiviral LbL applications were not mentioned [192].
It was interesting to find that LbL-nanocoating of medical devices also revealed very few literature entries and even fewer regarding coating with polysaccharides or antiviral coating.
It was reported that catheter silicon tubes that contained chlorohexidine-containing PEG-micelles were coated with poly(acrylic acid) as polyanion. Again the purpose of this study was to afford antibacterial action to the silicon tubes [193].
Dental implants are another medical device that benefited from antibacterial LbL coating with PSS and PAH. Metronidazole was coated in the final bilayer and this provided the antibacterial effect to prevent periodontal infections [194].
It is again apparent, at the time of writing, that antiviral coatings of medical devices have not been the main focus of research or innovation efforts. Figure 5 depicts a summary of our suggested methods of contributing in an almost crude, passive way to curb viral infections.

6.5. Challenges to LbL Nanocoating as an Antiviral Measure
It is seen from the literature that LbL nanocoating has been successful against many microorganisms, however, it has not seen significant testing against viruses. It is uncertain how long a surface will remain efficient in trapping viruses, and regular replacement of the materials is probable. Recently [160] it was shown that self-cleaning reusable face masks could be manufactured by coating the textile with hydrophobic silicon dioxide over etched pores of the mask.
On the other hand, it was shown that superhydrophilic chitosan/carboxymethylcellulose LbL coating of silicone ophthalmic devices encouraged the flow of tears and prevented bacterial biofilm formation. The devices also contained antibacterial agents that were released and contributed to the overall effect [195]. Perhaps polysaccharides can be employed in the same way, however, they might require chemical modification. It is also not established if hydro- or hydrophilic coatings will be the most effective.
It has been illustrated that orthopedic implants could be coated with drug-containing niosomes that prevented bacterial adhesion to the implant [196]. Other antibacterial coatings almost always contained antibacterial drugs. Antiviral drugs are not so abundant and other agents might have to be included in the LbL polysaccharide films. These are yet to be identified.
There are no studies that have reported the saturation load that these coatings might accommodate. The disposal of materials that have come in contact with viruses is of concern and protocols for handling this waste should be developed. Not only do they contribute to waste production [197], but also possibly to accumulation of a virus at the place of disposal. Antibacterial textiles have been utilized to manufacture sportswear, underwear, bedding, mattresses, wound dressings, and hospital gowns [198]. These items need laundering in order to reuse and pollute water sources, potentially leading to the accumulation of viruses in the water [199]. Perhaps a disposable product is a better alternative than a reusable product such as illustrated for disposable towels and baths that prevent the accumulation of skin bacteria in patients under nursing care [200].
Clearly many challenges need to be addressed to ensure the effectiveness of these coatings against viruses. The way has, however, been paved by, for example, successful antibacterial and antifungal applications in many settings where humans are exposed to many different surfaces.