Nano-patterned surfaces with precisely engineered topographies have emerged as a promising approach to mechanically inactivate enveloped viruses such as SARS-CoV-2. These surfaces rely on physical interactions between nanostructures and viral membranes, leading to structural damage without chemical agents. Black silicon and nanopillar arrays are among the most studied nano-patterned surfaces for antiviral applications. Their fabrication, structure-activity relationships, and potential integration into high-touch medical devices like ventilator components or personal protective equipment (PPE) are areas of active research.

Fabrication methods for these nano-patterned surfaces often involve lithographic techniques to achieve high precision and scalability. Deep ultraviolet (DUV) lithography and reactive ion etching (RIE) are commonly used to create black silicon surfaces, characterized by dense, needle-like nanostructures with high aspect ratios. The process begins with a silicon wafer coated with a photoresist, exposed to DUV light through a mask to define the pattern. Subsequent RIE etches the exposed silicon, forming sharp nanoscale protrusions. The resulting structures typically exhibit heights ranging from 500 to 1000 nanometers and tip diameters below 50 nanometers, dimensions critical for piercing viral envelopes.

Nanopillar arrays are fabricated using nanoimprint lithography (NIL) or electron beam lithography (EBL). NIL offers high throughput by pressing a pre-patterned mold into a polymer or silicon substrate, followed by etching to transfer the pattern. EBL, while slower, achieves higher resolution, enabling precise control over pillar diameter, spacing, and height. Optimal antiviral nanopillars often have diameters between 50 and 200 nanometers, heights of 300 to 600 nanometers, and center-to-center spacing of 100 to 300 nanometers. These parameters ensure sufficient mechanical force is applied to viral particles upon contact.

The antiviral mechanism of nano-patterned surfaces is primarily physical. Enveloped viruses like SARS-CoV-2 possess lipid membranes that are vulnerable to mechanical stress. When a virion lands on a surface of black silicon or nanopillars, the nanostructures penetrate the envelope, disrupting its integrity. Studies using transmission electron microscopy (TEM) have confirmed that viral particles on these surfaces exhibit visible membrane rupture and leakage of genetic material. The effectiveness of inactivation correlates with nanostructure density and sharpness. Surfaces with taller, sharper features achieve higher inactivation rates, often exceeding 90% within six hours of contact.

Structure-activity relationships reveal that spacing between nanostructures is equally critical. If pillars or needles are too widely spaced, viruses may settle between them without encountering sufficient mechanical stress. Conversely, excessively dense patterns may reduce the effective contact area. Experimental data suggest that a spacing of approximately 200 nanometers optimizes viral capture and inactivation. Additionally, the mechanical robustness of the nanostructures is vital for long-term use. Repeated contact with viral particles or cleaning procedures must not blunt or fracture the features, as this would diminish antiviral performance.

Applications in medical settings focus on high-touch surfaces where viral transmission risk is elevated. Ventilator components, such as tubing connectors or touchscreens, could benefit from nano-patterned coatings. These components are frequently handled and may harbor viral particles. Integrating black silicon or nanopillar arrays onto such surfaces could reduce contamination without altering workflow. Similarly, PPE like face shields or respirator casings could incorporate these nanostructures to enhance passive protection. The key advantage is the durability of the effect, as mechanical inactivation does not deplete over time like chemical coatings.

Challenges remain in scaling up production and ensuring compatibility with existing medical materials. Silicon-based nanostructures are brittle and may not adhere well to flexible polymers used in PPE. Researchers are exploring transfer techniques to imprint nanopatterns onto stainless steel or polycarbonate substrates, which are more common in medical devices. Another consideration is the potential for nanostructures to trap other debris, which could shield viruses from mechanical damage. Regular cleaning protocols must be designed to maintain surface functionality without damaging the nanostructures.

Testing the antiviral efficacy of these surfaces involves standardized protocols. Surfaces are inoculated with a known concentration of virus, incubated under controlled conditions, and then assayed for remaining infectivity. Comparisons between nano-patterned and flat surfaces consistently show significant reductions in viral load on the former. For example, one study reported a 96% reduction in SARS-CoV-2 infectivity on nanopillar surfaces after 24 hours, compared to control surfaces. These results are promising but must be validated under real-world conditions, including varying humidity and temperature.

Future directions include optimizing nanostructure designs for broader viral coverage. While enveloped viruses are particularly susceptible, non-enveloped viruses may require different mechanical approaches. Hybrid surfaces combining multiple nanostructure geometries could offer broader-spectrum activity. Additionally, advances in roll-to-roll nanoimprinting could lower production costs, making these surfaces viable for widespread use in hospitals and public spaces.

In summary, nano-patterned surfaces like black silicon and nanopillar arrays represent a novel strategy to mechanically inactivate enveloped viruses. Their fabrication relies on advanced lithography techniques to achieve precise, scalable patterns. The antiviral effect stems from physical disruption of viral membranes, with performance dictated by nanostructure geometry. Applications in ventilator components and PPE could reduce transmission risks in healthcare settings, provided challenges in material compatibility and scalability are addressed. Continued research will refine these surfaces for practical, real-world deployment.