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|Slippery Liquid-Infused Porous Surfaces|
Slippery liquid-infused porous surfaces (SLIPS) seek to emulate the properties of the amazing Nepenthes pitcher plant. The inner walls of the plant are naturally coated with a lubricating fluid that produces a robust, slick, durable, self-cleaning, anti-icing and self-healing lubricious surface capable of repelling just about any type of foreign matter. The secret to these surfaces is a nanostructured top layer that is infused with a water-based lubricating fluid making it slippery and lubricous.
The fabrication of a man-made SLIPS surface first involves adding texture or roughness to a material. Electrospinning, sandblasting, etching, electrodisposition, and a variety of other methods are used to make a surface hydrophobic (or to make a porous hydrophobic surface). A lubricant is then selected that will completely wet the surface and occupy all the nooks and crannies created by the nanotexturing. The lubricant must also be immiscible to the drop phase liquid. The hydrophobicity of the surface (due to the inherent non-wetting properties of the solid phase material or as a result of the hydrophobicity induced by a surface modification to the solid) ensures that the lubricant is not displaced by the drop phase liquid. In short, the drop phase rests entirely on the lubricant and does not actually touch the solid surface. The result is a total lack of pinning. The drop will move with very little force or with a very low tilt angle. In other words, drop mobility is greatly enhanced.1 This is what allows the ketchup to slide out of the SLIP bottle without clinging. (More on ketchup in item 4 below.)
Man-made SLIP surfaces are being developed for a variety of real-world applications and often are competing against traditional superhydrophobic designs. This month we will examine seven applications for SLIPS:
1. As biomedical applications turn to polymeric solutions, problems arise when bacteria attach to the polymeric surfaces which leads to infection and rejection. Researchers have proposed using SLIPS for biofilm generation.2 Coaxial electrospinning3 is used to produce a porous surface which is then infused with black seed oil and antimicrobial materials. The result is a slippery surface which rejects the attachment of contaminants that can retard tissue repair and recovery.
2. Researchers at Harvard have proposed a nanotextured nanoporous tungsten oxide coating which can be applied to steel surgical tools.4 The result is a rugged anti-corrosive surface which can repel virtually any type of liquid while still maintaining structural integrity. In addition to non-fouling surgical tools, this SLIP-enhanced steel could also be used on spinnerets for bioprinting and even marine applications.
3. Medical devices can also benefit from SLIPS to reduce bacterial infection, rejection and blood clotting. The same technology discussed in the paragraph above is used by license at FreeFlow Medical to reduce catheter-associated clotting and infections.5
4. We first reported in 2013 how the next generation of ketchup bottles may feature SLIPS technology developed by a company called LiquiGlide with roots at MIT.6 Despite a lot of news coverage,7 I have yet to see a ketchup bottle that uses SLIPS. Perhaps the day will come soon.
5. SLIPS can also be made icephobic. Researchers at Harvard have shown how SLIPS surfaces can be optimized to prevent drops of condensation from freezing by enabling them to slide off a frozen surface.8 The surfaces can also reduce ice adhesion and prevent surfaces from ice build up. Wind turbines, aircraft, marine vessels, and refrigeration applications could all benefit from icephobic SLIP surfaces.
6. Industrial vessels for sticky fluids like adhesives and paint can be made so no residue is left behind by converting the inner walls to SLIP surfaces. A company called Adaptive Surface Technologies has developed a product called SLIPS Repel which does exactly that.9 The company uses a nanotextured coating and then selects a lubricant that will work optimally with the sticky fluid in use.
7. A ship moves inefficiently when biofouled surfaces (e.g., covered in barnacles or other micro-organisms) slow it down. A barnacle cannot latch onto a SLIPS surface due to its slippery nature. Commercial products such as SLIPS paint are making it easier to reduce biofouling for a variety of marine applications.10 Excellently, biocides can also be added to the lubricant to further retard the accumulation of biogrowth.
Needless to say, there are countless additional applications for SLIPS - many of which have yet to be discovered and fully exploited. Keep reading our monthly newsletter as we strive to keep you abreast of the latest development in engineered surface technologies.
As 2022 winds down and we approach a
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