Slippery Surfaces

Harvard Professor Joanna Aizenberg shows David Pogue the Nepenthes Pitcher Plant, a carnivorous plant whose slippery surface inspired a non-stick material invented by her lab.

Unwanted surface interactions are a limiting factor nearly everywhere liquid is handled or encountered: they create drag in transport systems, trigger blood clotting, nucleate aircraft icing, and promote biofouling. Despite intensive study, state-of-the-art repellent surfaces have remained poorly suited for many substances, environments, and budgets. Inspired by the carnivorous pitcher plant, we have invented a fundamentally different conceptual approach to surface design that avoids the inherent limits of current strategies. Based on this approach, we have created surfaces that show almost perfect slipperiness toward polar and organic liquids, complex substances such as blood, oil, and ketchup, a genetically diverse range of bacteria and algae, and solid materials such as ice, dust, and insects. The surfaces can be optimized to function under extreme conditions, adapt to stimuli, and self-heal, and can be constructed from a variety of low-cost materials and applied to surfaces as diverse as medical tubing, planes, and refrigerators. 

Our approach comes from the same concept the pitcher plants use to make insects slide down their leaves into their digestive juices. We infuse a porous substrate with a lubricating fluid such that the overlying film, rather than air or solid, serves as the slippery interface. Based on extensive theoretical and experimental characterization, we have defined materials requirements for the lubricant to form a locked-in, stable, inert surface. These surfaces outperform state-of-the-art surfaces in scope (water, hydrocarbons, crude oil, blood, ice, bacteria), liquid mobility (contact angle hysteresis <2.5o), resilience (recovery within 0.1-1 s); and pressure tolerance (up to ~676 atm). Since low-surface-energy porous solids are abundant from small to large sacles, highly omniphobic surfaces can be easily generated without specialized fabrication facilities. 

Currently our group is developing this technology to meet emerging needs in biomedical fluid handling, fuel transport, antifouling, anti-icing, optical imaging, harsh environments, and other areas. 


See SLIPS in action by following the links below: 

Interviews: 

"The Makers" of The Mind of the Universe features Professor Aizenberg, International science documentary series, VPRO, May 21, 2017.

Nova episode on Making Stuff Wilder, October 23, 2013. 

Deadly pitcher-plant inspires super slippery nano-surface, Reuters, February 6, 2012. 

“Extreme Biomimetics” talk by Professor Aizenberg, Disruptive Ideas, TEDxBigApple, New York, February 4, 2012. 

“Slippery when wet (or anytime)”, CBC interview with Professor Aizenberg, October 8, 2011. 

Material World, BBC interview with Professor Aizenberg, September 21, 2011. 
 


In the news: 

Slippery liquid surfaces confuse mussels to stop them from sticking to underwater structuresHarvard press release, August 17, 2017. 

ARPA-E Making Progress Toward Achieving Mission, Report by the National Academies of Sciences, Engineering, and Medicine, June 13, 2017.

Creating a slippery slope on the surface of medical implantsHarvard press release, November 1, 2016. 

An unobstructed view into the human body, Harvard press release, September 26, 2016

Harnessing engineered slippery surfaces for tissue repair, Harvard press release, May 18, 2016.

Mid-Atlantic Seabed Drilling Expedition, BBC Science in Action podcast (starting at 14:08 minutes), October 23, 2015. 

Super-slick material makes steel better, stronger, cleaner, Harvard press release, October 20, 2015. 

The secrets of secretion, Harvard press release, June 22, 2015. 

What Is ARPA-E Up to Now?, Innovation, May 2015. 

Artificial membrane system uses fluid-filled pores for smooth moving, Materials 360, April 9, 2015. 

Literally Nothing Will Stick To This New Slippery Surface, Fast Company, March 27, 2015. 

Fabrics of Life, Nature, Outlook, March 25, 2015. 

Pores for thought over separation issues, Materials Today, March 10, 2015. 

Gating mechanism under pressure, Nature, News and VIews, March 5, 2015. 

Fluid–filled pores separate materials with fine precision, Harvard press release, March 5, 2015. 

New silicone infused with silicone oil = super-slippery, and proven to keep bacteria from growing on medical tubing, Reddit, February 20, 2015. 

SLIPS team heads to Capitol Hill, Harvard press release, February 12, 2015. 

Novel non-stick material joins portfolio of slippery surface technologies, Harvard press release, February 10, 2015. 

Wyss Institute launches SLIPS company, Harvard press release, October 29, 2014. Read more at SLIPS Technologies

Carnivorous Plant Inspires Anticlotting Medical Devices, Scientific American podcast, October 15, 2014. 

Slippery When Coated: Helping Medical Devices Prevent Blood Clots, NPR, October 12, 2014. 

Bioinspired coating for medical devices repels blood and bacteria, Harvard press release, October 12, 2014. 

Super-Slick Material Keeps Ice From Forming, Technology Review, July 2, 2014. 

Scientists Modify Cotton and Polyester to Display Repellent Properties, The Crimson, February 26, 2014. 

Stain-free, self-cleaning clothing on the horizon, Harvard press release, January 13, 2014. 

Pulling On The Shade, American Scientist, September 1, 2013. 

New coating turns ordinary glass into super-slippery glass, Harvard press release, August 2, 2013. 

Fluid-Infused Porous FIlms Dynamically Adjust Transparency and Wettability, Materials 360, April 22, 2013. 

Scientists Design New Adaptive Material Inspired by Tears, Popular Mechanics, April 17, 2013. 

Morphing 'fabric' shifts shape to repel or grip water, New Scientist, April 9, 2013. 

New material can halt runny liquids on demand, BBC, April 9, 2013. 

Cry me a river of possibility: Scientists design new adpative material inspired by tears, Harvard press release, April 8, 2013. 

Ice-Phobic Surfaces that are Wet, ACS Nano, August 9, 2012. 

SLIPS Blitz Biofilms, Nature, August 9, 2012. 

Harvard scientists' breakthrough could stop biofilm formation, Food Production Daily, August 9, 2012. 

SLIPS liquid repeller is inspired by carnivorous plants, enemy to insects and graffiti artists alike, Engadget, August 3, 2012. 

Super slippery surface prevents biofilms, PNAS, July 31, 2012. 

New coating evicts biofilms for good, Harvard press release, July 30, 2012. 

Ice Curbs, National Science Foundation's Discovery Files (podcast), June 27, 2012. 

'Ice-Phobic' Airplane Wings, Wall Street Journal, June 22, 2012. 

SLIPS receives a 2012 R&D 100 Award from R&D Magazine, honoring it as one of the 100 most technologically significant products of the previous year. June 20, 2012. 

No Scraper Required: Ice Rolls Off Metal, Discovery, June 16, 2012. 

Slippery Coating Keeps Metals Frost-Free, Chemical and Engineering News, June 15, 2012. 

Keeping Metal Surfaces Ice, Frost Free, The Hindu, June 14, 2012. 

Ultra-Antifreeze Keeps Ice From Even Forming, Smithsonian, June 12, 2012. 

A new spin on anti-freeze, Harvard press release, June 11, 2012. 

Super-slippery material could mean end to having to wait for ketchup, The Telegraph, November 13, 2011. 

Slippery Slope: researchers take advice from carnivorous plant, Harvard press release, September 21, 2011. 

Slippery when wetted, Nature News and Views, September 21, 2011. 

Carnivorous plant inspires super slippery material, New Scientist, September 21, 2011. 

Pitcher plant inspires super slippery surface, Chemical and Engineering News, September 21, 2011. 

Ketchup bottle problems solved, The Telegraph, November 13, 2011. 

Carnivorous plant shows its slippery side, Financial Times, September 30, 2011. 

A great non-friction story: Researchers create ‘world’s most slippery surface’, Daily Mail, September 22, 2011. 

Scientists create wonder material, Metro Herald, p. 10, September 22, 2011. 

Copied from pitcher plants; destined for ketchup bottles, Popular Science, November 14, 2011. 

Pitcher plant perfect, Nature Chemistry, vol. 3, pp. 834, October 24, 2011. 

Pitcher plant inspires ultimate non-stick surface, Chemistry World, September 22, 2011. 

Plant technology used to create super-repellent material, The Engineer, September 26, 2011. 

Flesh-eating plant inspires super-slippery material that repels everything, Discover Online, September 21, 2011. 

Invention Solves Ketchup Dilemma, The Harvard Crimson, November 20, 2011. 

Source: Nature 
Michael Nosonovsky of the University of Wisconsin-Milwaukee: "The development of SLIPS typifies two themes that are likely to dominate the field of biomimetic and functional surfaces in coming years." 

Source: C&EN (ACS) 
Philippe Brunet, an expert on omniphobic materials at France’s University of Paris Diderot, says he is impressed by the innovative strategy. “In my opinion, this study does not simply represent an improvement in liquid-repellent surfaces, but rather a revolution in the field,” he says. “I am sure many researchers reading this paper would think as I do: ‘I wish I would have thought about these surfaces before.’” 

Source: Chemistry World (RCS) 
"This really is a new direction," says Steven Bell, director of innovative molecular materials at Queen's University Belfast, UK. "Many of us have been working on improving the durability of 'lotus effect' materials but this now offers us an alternative way to try to reach the same objectives." 

Source: ABC (Australia) 
Professor Robert Short, director of the Mawson Institute at the University of South Australia, says the results are highly impressive and the simplicity of the approach is particularly striking. 
"The combination of solids and liquids uses capillary action to bring lubricant to the surface of nano/microengineered structures," Short says. 
"Furthermore, the ability of the surface to 'heal' sets it apart from other cutting-edge approaches." 

Source: Discover 
Walter Federle from the University of Cambridge, who discovered the structure of the pitcher plant’s peristome, says,“It’s really exciting to see that this principle has inspired the authors and allowed them to develop something that could prove extremely useful.” 

Source: New Scientist 
"It's interesting that it combines self-lubrication, self-healing and self-cleaning, which are different 
processes,"
 says Michael Nosonovsky of the University of Wisconsin-Milwaukee. "It's a new type of 
smart material."

Juthani N, Howell C, Ledoux H, Sotiri I, Kelso S, Kovalenko Y, Tajik A, Vu TL, Lin JJ, Sutton A, et al. Infused polymers for cell sheet release. Scientific Reports. 2016;6 (1) :26109.Abstract

Tissue engineering using whole, intact cell sheets has shown promise in many cell-based therapies. However, current systems for the growth and release of these sheets can be expensive to purchase or difficult to fabricate, hindering their widespread use. Here, we describe a new approach to cell sheet release surfaces based on silicone oil-infused polydimethylsiloxane. By coating the surfaces with a layer of fibronectin (FN), we were able to grow mesenchymal stem cells to densities comparable to those of tissue culture polystyrene controls (TCPS). Simple introduction of oil underneath an edge of the sheet caused it to separate from the substrate. Characterization of sheets post-transfer showed that they retain their FN layer and morphology, remain highly viable, and are able to grow and proliferate normally after transfer. We expect that this method of cell sheet growth and detachment may be useful for low-cost, flexible, and customizable production of cellular layers for tissue engineering.

Chen J, Howell C, Haller CA, Patel MS, Ayala P, Moravec KA, Dai E, Liu L, Sotiri I, Aizenberg M, et al. An immobilized liquid interface prevents device associated bacterial infection in vivo. Biomaterials. 2017;113 :80-92.Abstract

Virtually all biomaterials are susceptible to biofilm formation and, as a consequence, device-associated infection. The concept of an immobilized liquid surface, termed slippery liquid-infused porous surfaces (SLIPS), represents a new framework for creating a stable, dynamic, omniphobic surface that displays ultralow adhesion and limits bacterial biofilm formation. A widely used biomaterial in clinical care, expanded polytetrafluoroethylene (ePTFE), infused with various perfluorocarbon liquids generated SLIPS surfaces that exhibited a 99% reduction in S. aureus adhesion with preservation of macrophage viability, phagocytosis, and bactericidal function. Notably, SLIPS modification of ePTFE prevents device infection after S. aureus challenge in vivo, while eliciting a significantly attenuated innate immune response. SLIPS-modified implants also decrease macrophage inflammatory cytokine expression in vitro, which likely contributed to the presence of a thinner fibrous capsule in the absence of bacterial challenge. SLIPS is an easily implementable technology that provides a promising approach to substantially reduce the risk of device infection and associated patient morbidity, as well as health care costs.

Sunny S, Cheng G, Daniel D, Lo P, Ochoa S, Howell C, Vogel N, Majid A, Aizenberg J. Transparent antifouling material for improved operative field visibility in endoscopy. Proc. Nat. Acad. Sci. 2016;113 (42) :11676-11681.Abstract

Inspection devices are frequently occluded by highly contaminating fluids that disrupt the visual field and their effective operation. These issues are particularly striking in endoscopes, where the diagnosis and treatment of diseases are compromised by the obscuring of the operative field by body fluids. Here we demonstrate that the application of a liquid-infused surface coating strongly repels sticky biological secretions and enables an uninterrupted field of view. Extensive bronchoscopy procedures performed in vivo on a porcine model shows significantly reduced fouling, resulting in either unnecessary or ∼10–15 times shorter and less intensive lens clearing procedures compared with an untreated endoscope.

Camera-guided instruments, such as endoscopes, have become an essential component of contemporary medicine. The 15–20 million endoscopies performed every year in the United States alone demonstrate the tremendous impact of this technology. However, doctors heavily rely on the visual feedback provided by the endoscope camera, which is routinely compromised when body fluids and fogging occlude the lens, requiring lengthy cleaning procedures that include irrigation, tissue rubbing, suction, and even temporary removal of the endoscope for external cleaning. Bronchoscopies are especially affected because they are performed on delicate tissue, in high-humidity environments with exposure to extremely adhesive biological fluids such as mucus and blood. Here, we present a repellent, liquid-infused coating on an endoscope lens capable of preventing vision loss after repeated submersions in blood and mucus. The material properties of the coating, including conformability, mechanical adhesion, transparency, oil type, and biocompatibility, were optimized in comprehensive in vitro and ex vivo studies. Extensive bronchoscopy procedures performed in vivo on porcine lungs showed significantly reduced fouling, resulting in either unnecessary or ∼10–15 times shorter and less intensive lens clearing procedures compared with an untreated endoscope. We believe that the material developed in this study opens up opportunities in the design of next-generation endoscopes that will improve visual field, display unprecedented antibacterial and antifouling properties, reduce the duration of the procedure, and enable visualization of currently unreachable parts of the body, thus offering enormous potential for disease diagnosis and treatment.

Aizenberg J. Slippery Liquid-Infused Porous Surfaces. The Journal of Ocean Technology. 2014;9 (4) :113-114.Abstract

Marine biofouling, the process of accumulation of microorganisms, plants, algae and animals on submerged surfaces, is an age-old problem associated with any maritime activity affecting commercial and recreational shipping activities, naval operations, aquaculture facilities and marine renewable energy structures alike. The adverse effects of marine biofouling include the increase of drag on ship hulls, damage to ships and maritime equipment such as corrosion, the spread of diseases in aquaculture and the distribution of invasive species causing extensive damage to coastal ecosystems and the benefits derived from them. An estimated global annual total of $60 billion in fuel cost alone can be saved by the application of marine antifouling coatings, making the treatment of marine biofouling a necessity not an option.

Park K-C, Kim P, Grinthal A, He N, Fox D, Weaver JC, Aizenberg J. Condensation on slippery asymmetric bumps. Nature. 2016;531 (7592) :78-82.Abstract
Controlling dropwise condensation is fundamental to water-harvesting systems, desalination, thermal power generation, air conditioning, distillation towers, and numerous other applications. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and to be shed as quickly as possible. However, approaches based on microscale, nanoscale or molecular-scale textures suffer from intrinsic trade-offs that make it difficult to optimize both growth and transport at once. Here we present a conceptually different design approach—based on principles derived from Namib desert beetles, cacti, and pitcher plants—that synergistically combines these aspects of condensation and substantially outperforms other synthetic surfaces. Inspired by an unconventional interpretation of the role of the beetle’s bumpy surface geometry in promoting condensation, and using theoretical modelling, we show how to maximize vapour diffusion flux at the apex of convex millimetric bumps by optimizing the radius of curvature and cross-sectional shape. Integrating this apex geometry with a widening slope, analogous to cactus spines, directly couples facilitated droplet growth with fast directional transport, by creating a free-energy profile that drives the droplet down the slope before its growth rate can decrease. This coupling is further enhanced by a slippery, pitcher-plant-inspired nanocoating that facilitates feedback between coalescence-driven growth and capillary-driven motion on the way down. Bumps that are rationally designed to integrate these mechanisms are able to grow and transport large droplets even against gravity and overcome the effect of an unfavourable temperature gradient. We further observe an unprecedented sixfold-higher exponent of growth rate, faster onset, higher steady-state turnover rate, and a greater volume of water collected compared to other surfaces. We envision that this fundamental understanding and rational design strategy can be applied to a wide range of water-harvesting and phase-change heat-transfer applications.