Anti-Microbial Property of SiO2 or SnO2 Thin Film Coated Resin Nanostructures
Authors: Matsushita Yuito , Shimizu Tomohiro , Shingubara Shoso , Ito Takeshi 
Published: December 2024

Nano and micro patterns in nature show many beneficial properties. For examples, the wings of cicadas and dragonflies have sprawled with nano-sized bumps which express bactericidal and antibacterial property physically. The physical antimicrobial property is a promising new tool to combat antimicrobial resistance (AMR) bacteria. The number of deaths related to AMR infection was estimated over 70,000 which was firstly reported by O’Neil. The value increased to 1,270,000 only 4 years after that report. Therefore, new antimicrobial technologies that are not chemically driven are needed. The bactericidal mechanism of nanostructured surface is complex, but the main mechanism is attributed to the stretching effect, which means that cell membrane between the nanostructures is pulled downward and laterally. This physical stress induces reactive oxygen species (ROS) and autolytic protein activity. Sometime, sharp nanostructure could pierce the cell membrane directly. Many researchers have tried to develop materials with anti-microorganism including inorganic and organic materials. In the meanwhile, resin is fit to mass production using nano-imprint lithography (NIL), which has potential for inexpensive supply.
There are some reports of antibacterial and bactericidal properties of resin nanostructures. The wettability of the substrate surface has been shown to affect bacterial adhesion and antibacterial performance. Resin surface usually shows relatively hydrophobic. Nevertheless, the hydrophilic surface would increase the number of attached cells since bacterial cell membrane had hydrophilicity, which means that the hydrophilic surface would kill bacteria because many cells attach to the nanostructures. In addition, mechanical property is also important to achieve high antibacterial performance. The higher elastic modulus resin shows the higher antibacterial performance. Therefore, it needs to tune the surface wettability and mechanical property of resin nanostructures. Even though, there is a limit to how much the resin itself can change those properties. We change the surface characteristics of resin nanostructures depositing SiO2 or SnO2 ultra-thin layer by atomic layer deposition and evaluated the anti-bacterial properties among them.
Anodic Aluminum Oxide (AAO) with regular nanoholes was used as a mold to fabricate nanostructures on the surface of a resin film. The surface of an Al plate was flattened by electropolishing, and then anodized and wet-etched to fabricate regular nanoholes of AAOs with various parameters (pitch: 200 nm or 300 nm, depth: 100 nm, 150 nm and 200 nm, diameter: 100 nm and 150 nm). The nanoholes were imprinted to resin (Cyclo orefin polymer: COP) film using thermal nano-imprint lithography. Atomic layer deposition (ALD) was introduced to coat the SiO2 or SnO2 ultra-thin layer (10 nm) on the resin film to change the wettability of the surface, increase the elastic modulus. This is expected to be one of the new processes for mass production of resin nanostructures with antimicrobial properties. We evaluated the antibacterial properties with and without the thin films. In general, ALD can form atomic- or molecular-level layers in a single deposition cycle. Many ALDs need relatively high temperatures up to 100 °C, which would damage the resin. Therefore, we introduced the room-temperature atomic layer deposition method (supported by Cool ALD®) . Surface wettability was evaluated by contact angle, and elastic modulus was measured by Atomic Force Microscopy (AFM).
Fig.1 shows SEM and AFM images of fabricated sample, which indicated that fine naonopillars were arranged regularly. The water contact angle (WCA) of the resin nanostructures coated with the thin film changed drastically from 130.2° to 7.2°, showing strong hydrophilicity. The local elastic modulus was larger than that without the thin film. In antibacterial tests, nanopillars coated with SiO2 layers showed antibacterial activity against E. coli; nanopillars coated with SnO2 layers were effective in antiviral tests. These results indicate that thin film coatings have great potential to increase the antibacterial and antiviral effects of polymeric nanopillar arrays.

Figure 1