Given the impact of COVID-19, and how the illness spreads via contact with infected respiratory droplets, engineers at IIT Bombay wanted to answer a basic question:

Is it possible to design surfaces where the survival of coronavirus is inherently limited?

The Bombay team discovered that two properties determine a surface’s antiviral capabilities: wettability, or whether a given surface will be wetted by water, and physical texture.

By adjusting wettability and texture, the IIT Bombay researchers studied how to best create an antiviral surface and limit the spread of coronavirus.

Two basic geometries of surface physical texture were considered, namely rectangular parallel grooves (shown above in figure 1a) and rectangular pillars (shown above in figure 1b)

"To achieve the strongest antiviral effects, the surface with taller and closely packed pillars, with an intrinsic contact angle, or wettability, of 60°, is required," Dr. Sanghamitro Chatterjee, Department of Mechanical Engineering at IIT Bombay told Tech Briefs.

Surfaces with taller and closely packed pillars — high ℎ and low ?, in the above image — with a contact angle of around 60 degrees show the strongest antiviral effect or shortest drying time.

Dr. Chatterjee, lead author and a postdoctoral fellow in the mechanical engineering department, worked alongside IIT Bombay mechanical engineering professors Janani Srree Murallidharan, Amit Agrawal, and Rajneesh Bhardwaj.

Their research, "Designing Antiviral Surfaces to Suppress the Spread of COVID-19 ," appeared this month in the journal Physics of Fluids.

The survival duration of coronavirus on these surfaces is much shorter than typically available surfaces. Knowledge of the general rules, governing mechanism, and optimized conditions, however, for achieving the desired virucidal effects are still lacking, said Dr. Chatterjee.

In a short Q&A with Tech Briefs below, Dr. Chatterjee, in coordination with the IIT Bombay research team, explains how to incorporate the study's conclusions into new designs.

Tech Briefs: If a surface's wettability and its physical texture determine its antiviral properties, what does that mean for design? What surfaces should be re-designed, do you think?

Dr. Sanghamitro Chatterjee: The surfaces could be decorated with rectangular parallel grooves or with rectangular pillars. These are simplest geometries which are easy to fabricate, and therefore have been considered in our design.

Hydrophobic, or non-wetting, surfaces are the most susceptible ones for survival of coronavirus. Our study indicates that for designing antiviral surfaces, first a chemical treatment should be involved to transfer a hydrophobic surface to a hydrophilic one. Thereafter, physical texture needs to be done in order to achieve the optimum antiviral effects. Surfaces with taller and closely packed pillars and having an intrinsic contact angle around 60° exhibit the strongest antiviral effects.

When a droplet is deposited on a surface, the angle ? between the droplet surface and the substrate surface, measured within the liquid, is called the contact angle. The surface is said to be wettable or hydrophilic if ?<90°. Conversely, if ?>90°, the surface is said to be non-wetting or hydrophobic. (Image Credit: S. Chatterjee, J. S. Murallidharan, A. Agrawal, and R. Bhardwaj)

Tech Briefs: What design recommendations do you have, based on these conclusions?

Dr. Chatterjee: Our work proposes the design methodology of the process and helps to disseminate knowledge of parameters needed to engineer the surface for the shortest possible virus survival time. For instance, using a chemical treatment, a hydrophobic surface can be rendered hydrophilic, and fabricating nano or microscale features on it helps to reduce the virus survival time by 50%. In particular, taller, closely packed structures and pillars are the most conducive to eliminate the virus. Although it may not be possible to change the material of the surfaces around us to the materials on which the survival time of coronavirus is short (such as porous and metallic materials), our work presents a way to shorten the life-times of coronavirus by employing surface engineering.

Our recommendations can be adopted on a surface that may come in frequent contact with infected aerosols, such as dentistry instruments, face shields, frequently-touched surfaces (e.g., a door handle), etc., for quicker obliteration of the coronavirus.

Tech Briefs: Is it possible to modify existing surfaces, or is the idea to create all-new surfaces? Are these surfaces easy to create and maintain?

Dr. Chatterjee: It is possible to modify the existing surfaces. A chemical treatment which form hydrophilic compounds/coating (e.g., silicon dioxide, aluminum hydroxide) on the surface can readily impart hydrophilicity on an existing hydrophobic surface. Furthermore, physical texture is possible by well-known lithographic techniques, such as electron beam lithography, focused ion beam (FIB), and photo lithography. The designs shown [in the above image] can easily be fabricated using these techniques.

The physical textures formed by electron beam lithography, focused ion beam (FIB), and photo lithography are permanent. However, the optimum wettability obtained by chemical methods can be may be lost or degraded upon exposure to erosion which can be caused by rough water flow or to dirt which may lead to adsorption of carbon on the surface. Therefore, care should be taken to avoid such circumstances.

Tech Briefs: Did any conclusions surprise you?

Dr. Chatterjee: We were surprised that the surface’s wettability and physical texture together determine the antiviral properties. One would not achieve the best results by continuously tailoring any one of these parameters. The most conducive antiviral effect lies within an optimized range of both wettability and texture.

Secondly, in regards to the physical texture, it is surface area factor ?, i.e., the ratio between the actual and the projected surface area that matters and needs to be tailored, irrespective of the specific geometry of the texture. One would not need to delve into the details of the specific geometry of the textures. Just obtaining optimized ? and wettability would render the best antiviral effect. Another surprising fact was that while a couple of previous studies reported antibacterial effects by designing superhydrophobic surfaces, our research indicate that antiviral surface design can be achieved by surface hydrophilicity.

Tech Briefs: What’s next? Can this surface treatment be used on illnesses other than coronavirus?

Dr. Chatterjee: In the future, the model presented here readily extends to other respiratory diseases such as Influenza A or other diseases that spread through fomite transmission. Therefore, the surface engineering process can be universalized to mitigate the spread of such disease. Since we have analyzed antiviral effects by a generic model independent of the specific geometry of the texture, one may fabricate any geometric structures as per convenience and availability of the fabrication technique (such as focused ion beams, chemical etching, etc.) to achieve the same outcome.

Furthermore, our model disclosed that a range of wettability and texture may render the optimized antiviral effects. Based upon the availability of the fabrication technique, one may obtain any value within the reported range, and thereby optimizing the operating time and cost. This way, design and fabrication of antiviral surfaces and application to different disease may be a future course of work.

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