Antibacterial Surfaces Mechanisms, Design and Development
The spread of disease-causing microorganisms through high-touch surfaces and their increased tolerance against antimicrobials and the host immune system is responsible for several fatal diseases. By the year 2050, Antimicrobial resistance (AMR) is expected to cause 10 million deaths annually and a loss of US$100 trillion. Today, some bacterial species (e.g., Carbapenem-resistant Enterobacteriaceae group of bacteria) are immune to all major classes of available antibiotics. This has encouraged the scientific community to develop alternatives to antibiotics to fight the AMR. Primary sources of spread and resistance acquisition among bacteria include cross-contamination of surfaces in hospitals, catheters, stethoscopes, and surgical tools. Any such abiotic surface is vulnerable to bacterial colonization that begins with a few primary colonizers attaching themselves to the surface to condition it for further attachment of arriving bacteria. After initial attachment, bacteria start to proliferate and develop into surface-bound colonies. It then forms a robust protective layer of biofilm that brings advantages to bacterial survival against environmental odds. Hence, the initial stage of attachment is a weak link in the bacterial journey to forming a protective biofilm. Exploiting this weak link, nanostructured surfaces hinder initial attachment by physically rupturing the cell without the involvement of any chemical or biocides, hence are consistently called “promising” in controlling bacterial proliferation. Although various theories over the past few years have tried to explain the behavior of bacteria on these nanostructures, there is a lack of consensus on the precise mechanism that leads to bacterial death. To efficiently restrain bacterial colonization, it is of profound importance to understand the fundamental cause of bacterial death on these nanopillars. Only such fundamental understanding can guide us to the answer to the question: What precise nanopillars feature participate in bacterial cell-rupture and how? In this doctoral dissertation, we investigated the mechano-response of E. Coli cells as it attaches itself to a regular array of precise dimension-controlled nanopillars. Overcoming the fabrication limitations, two sets of ordered arrays of nanopillars by varying one dimension at a time makes it possible to study the involvement of individual dimensions on the response of single bacterial cell, which is crucial in understanding the rupture mechanism. The bacterial cell extends out via thread-like projections in the direction of neighboring pillars to establish contact with them. At a particular interpillar spacing (pitch) of straight pillars, the attached nanopillars appear to bend towards the cell due to the application of force. This displacement of pillars and hence the force increases with interpillar spacing. Bactericidal efficacy was proportional to the applied force, and hence interpillar spacing. The method of calculating force applied by bacteria on nanopillars adds direct experimental evidence towards the proposed mechanism of bacterial interaction with nanopillars at the single-cell level. We have focussed on one bacterial strain E. Coli; however, this method of studying bacterial-nanopillar interaction can pinpoint the governing parameter for cell rupture for different bacterial strains. After establishing the fundamentals of mechano-bactericidal mechanism, the subsequent work progresses to dual action antibacterial surfaces that aim towards studying alternatives to biocide coatings aiding from mechanical rupture of cells. A common non-selective way to kill bacteria without using antibiotic chemicals, and hence following the risk of developing antibacterial resistance, is to use photocatalytic materials. They produce reactive oxygen species (ROS) in the presence of light and water that cause bacterial death on the surface. The dual action surfaces benefit from nanostructures and photocatalytic antibacterial coatings over it. We establish the design principles of such “dual-action” surfaces, and answer several open questions, for example: which material should the nanostructures be made of? What is the optimum photocatalyst thickness? What geometries are most effective? In this work, TiO2 is used as the photocatalytic coating on nanostructures made of Si and SiO2. It is demonstrated that TiO2-coated “black-silica" (nanostructured SiO2), is more effective in producing the bactericidal effect. The bacterial kill rate is improved by 73% on replacing the underlying Si nanopillars with SiO2 nanopillars. To understand the dynamics of light absorption and subsequent ROS diffusion in such systems, FDTD and FEM simulations were used for modeling. FDTD simulations show that parasitic absorption in the underlying base pillar of high extinction coefficient leads to significant loss of incident optical energy. Hence, the “total absorption” of a system can be a misleading proxy for photocatalytic activity. Only absorption in the photocatalyst (TiO2) matters, which can be enhanced by fabricating nanopillars with a more transparent material like SiO2 or PDMS, having a low extinction coefficient. Further, FDTD coupled with FEM simulations shows that taller nanopillars don’t always lead to higher bulk ROS concentration, despite more absorption. Beyond 5 µm height, ROS are unable to diffuse out of the nanopillar forest. After articulating the design rules, the next step is to come up with a scalable process that can be deployed as practical antibacterial surfaces. In this work, we further extend the effectiveness of the TiO2-coated B-Si. By substituting TiO2 with TiO2 nanoparticles, the effective surface area for the production of ROS increases significantly. The extraction of photocarriers also improves because bulk of TiO2 is always within a few nm of a surface. The films are fabricated with three different techniques, all of which are scalable to large-areas. We establish the impact of the different techniques on the film’s topology and ability to kill bacteria. Antibacterial photocatalytic coatings are a promising alternative; however, the band gaps of most metal oxides are too wide, requiring UV/blue illumination. To deal with this, we discovered a new antibacterial photocatalyst, Mn2V2O7 (MVO), that works in ambient light or low-intensity solar radiation. The β-phase has a bandgap of 1.7 eV, so MVO absorbs visible light up to 600 nm.7 Under visible light, MVO reduces bacterial load by four orders of magnitude. MVO can be coated into films by drop-casting, which kills 76% of bacteria. In conclusion, work done in this thesis address the problem of spread of antimicrobial resistant bacteria via surfaces. We establish the mechanism of interaction of bacteria with nano-pillars also called as mechano-bactericidal mechanism. This formulates the understanding behind contact-kill mechanism of nanostructures. We extended efficiency of nanopillars by coating it with photocatalytic material that non-selectively degrades any organic material including bacterial cells, hence adds as a second line of defense again bacterial colonization. Using FEM and FDTD simulations, we articulated the design rules of such coated nanostructures. We developed technique to coat mesoporous photocatalyst on these nanostructures allowing larrge area deployment. At last, we overcame the UV-activated limitation of photocatalysts by enabling a visible light-activated antibacterial material suitable for large area coatings.