Topic2- Liquid Infused Structured Surfaces with Exceptional Anti-Biofouling Performance
Wetting, biofilms, bacteria, SLIPS, superhydrophobic, antifouling
Bacteria are capable of forming layers of films on the surface of materials. They can be found on medical equipment such as catheters and implants, food processing plants, plumbing etc. It is difficult to eliminate these biofilms from the surface once attachment has been achieved because they are highly resistant to environmental stresses, antimicrobial agents, and immune response. These resilient cells are capable of surviving chlorine treatment, biocide treatment over a period of 7 days, and iodine solution treatment for up to 15 months. It is obvious that chemical treatment does not resolve the problem. As a result, researchers have looked to change the surface properties of the materials they are using for these applications in an effort to prevent biofilm formation. Some of the methods that have been proposed include creating a surface that releases anti-bacterial agents and modifying the surface chemistry of materials to prevent protein adsorption. These methods have proven to work for a shorter period of time but over long time scales or in extreme environments (i.e., extreme pH, salinity, and UV environments) bacterial formation is still prominent. Epstein et. al have fabricated a Slippery Liquid Infused Porous Surface (SLIPS) that is capable of preventing 99.6% bacterial attachment.
Slippery Liquid Infused Porous Surface
Despite efforts to establish surfaces that repel the adhesion of bacteria, the underlying problem seems to be that solid surfaces have fixed atoms that allow for surface interactions with the adhesive proteins of the bacteria to take place eventually which leads to permanent attachment. The hypothesis is that if the surface molecules have some mobility, then these permanent interactions will be more difficult to establish. To test this hypothesis, a stable liquid interface was created taking inspiration from a pitcher plant. In order to create this stable liquid interface, a porous solid material was used to trap liquid within the pores. The incredible property of this material is that the liquid is still locked in place by the pores even when submerged in other fluids. In order to achieve this, there are three requirements that need to be fulfilled: a.) the liquid locked within the porous solid needs to have more of an affinity to the solid to which it is attached than the fluid it will be submerged in, b.) the solid porous surface must maximize surface area for the liquid to be immobilized in, and c.) the liquid within the pores must be immiscible with the fluid it will be interacting with. Epstein et al is not only able to create this material but also demonstrate poor biofilm attachment.
Formation of SLIPS
A substrate, polytetrafluoroethylene (PTFE), is nano patterned or roughened creating pores. The SLIPS liquid, perfluoropolyether (Krytox-103), is poured into the porous PTFE. The excess liquid is removed at which point it is ready to be tested for bacterial biofilm formation. The following Figure 1 demonstrates the process.
Bacteria was deposited on three different surfaces in order to compare the performance of SLIPS to other surfaces. The first surface tested was a PTFE membrane which is known to be superhydrophobic. The second surface tested was SLIPS and the third surface was a fluoro-silanized patterned silicon wafer which is capable of repelling water. With the exception of a few colonies of bacteria on the SLIPS, bacterial biofilms were observed on the PTFE and silicon wafer. The comparison between the PTFE and SLIPS substrate is made in Figure 2. When the bacteria was grown on the controls, a film formed on the surface whereas the SLIPS substrate allowed the biofilm to slide right off without leaving behind stains on the surface.
To date, PEGylated titanium surfaces have been able to reduce biofilm formation by 86% (leaving behind 14% biofilm) after allowing bacterial incubation of 5 hours. However, after this time, biofilm growth is extremely high. To test the efficiency of the PEGylated titanium surface compared to the SLIPS surface, the SLIPS surface was incubated with bacteria for 7 days. After 7 days, the PEGylated titanium surface had 35 times the amount of bacteria on its surface compared to SLIPS as shown in Figure 3.
Relevance to Topic 2: Wetting
The purpose of this paper was to create a surface that prevented wetting. While superhydrophobic surfaces have been created in the past to combat biofilm formation, they do not perform well enough to be implemented. The Aizenberg group found that the adhesive proteins of bacteria are able to attach to most surfaces creating biofilms that completely wet the surface which is an indication of their adhesive nature. In order to avoid this, they created their own surface with a liquid interface that prevented wetting. They were able to find that when the liquid containing the bacteria evaporated from their fabricated SLIPS, the contact angle remained the same as when they first deposited the droplet containing bacteria on the surface. The remaining bacteria could then be easily removed using adhesive tape. On the other hand, when the porous TEFLON control was used, the liquid spread and wetted the surface forming a coffee ring of bacteria that could not be removed. The Aizenberg group was able to successfully create a surface that prevented wetting by bacterial biofilms.
 Epstein, A. K., Wong, T.W., Belisle, R.A., Boggs, E.M., and Aizenberg, J. (2012) Liquid-infused structured surfaces with exceptional anti-biofouling performance. Proceedings of the National Academy of Sciences. 109(33):13182-13187.