Slippery Surfaces

Howell C, Vu TL, Lin JJ, Kolle S, Juthani N, Watson E, Weaver JC, Alvarenga J, Aizenberg J. Self-Replenishing Vascularized Fouling-Release Surfaces. ACS Appl. Mater. Interfaces. 2014;6 (15) :13299-13307. Full TextAbstract

Inspired by the long-term effectiveness of living
antifouling materials, we have developed a method for the self-
replenishment of synthetic biofouling-release surfaces. These
surfaces are created by either molding or directly embedding
3D vascular systems into polydimethylsiloxane (PDMS) and
filling them with a silicone oil to generate a nontoxic oil-
infused material. When replenished with silicone oil from an
outside source, these materials are capable of self-lubrication
and continuous renewal of the interfacial fouling-release layer.
Under accelerated lubricant loss conditions, fully infused vascularized samples retained significantly more lubricant than equivalent nonvascularized controls. Tests of lubricant-infused PDMS in static cultures of the infectious bacteria Staphylococcus aureus and Escherichia coli as well as the green microalgae Botryococcus braunii, Chlamydomonas reinhardtii, Dunaliella salina, and Nannochloropsis oculata showed a significant reduction in biofilm adhesion compared to PDMS and glass controls containing no lubricant. Further experiments on vascularized versus nonvascularized samples that had been subjected to accelerated lubricant evaporation conditions for up to 48 h showed significantly less biofilm adherence on the vascularized surfaces. These results demonstrate the ability of an embedded lubricant-filled vascular network to improve the longevity of fouling-release surfaces.

Yao X, Hu Y, Grinthal A, Wong T-S, Mahadevan L, Aizenberg J. Adaptive fluid-infused porous films with tunable transparency and wettability. Nature Materials. 2013;12 :529-534. Full TextAbstract
Materials that adapt dynamically to environmental changes are currently limited to two-state switching of single properties, and only a small number of strategies that may lead to materials with continuously adjustable characteristics have been reported1-3. Here we introduce adaptive surfaces made of a liquid film supported by a nanoporous elastic substrate. As the substrate deforms, the liquid flows within the pores causing the smooth and defect-free surface to roughen through a continuous range of topographies. We show that a graded mechanical stimulus can be directly translated into finely tuned, dynamic adjustments of optical transparency and wettability. In particular, we demonstrate simultaneous control of the film's transparency and its ability to continuously manipulate various low-surface-tension droplets from free-sliding to pinned. This strategy should make possible the rational design of tunable, multifunctional adaptive materials for a broad range of applications.
Cui J, Daniel D, Grinthal A, Lin K, Aizenberg J. Dynamic polymer systems with self-regulated secretion for the control of surface properties and material healing. Nat. Mater. 2015;14 (8) :790-795. Publisher's VersionAbstract
Approaches for regulated fluid secretion, which typically rely on fluid encapsulation and release from a shelled compartment, do not usually allow a fine continuous modulation of secretion, and can be difficult to adapt for monitoring or function-integration purposes. Here, we report self-regulated, self-reporting secretion systems consisting of liquid-storage compartments in a supramolecular polymer-gel matrix with a thin liquid layer on top, and demonstrate that dynamic liquid exchange between the compartments, matrix and surface layer allows repeated, responsive self-lubrication of the surface and cooperative healing of the matrix. Depletion of the surface liquid or local material damage induces secretion of the stored liquid via a dynamic feedback between polymer crosslinking, droplet shrinkage and liquid transport that can be read out through changes in the system’s optical transparency. We foresee diverse applications in fluid delivery, wetting and adhesion control, and material self-repair.
Leslie DC, Waterhouse A, Berthet JB, Valentin TM, Watters AL, Jain A, Kim P, Hatton BD, Nedder A, Donovan K, et al. A bioinspired omniphobic surface coating on medical devices prevents thrombosis and biofouling. Nature Biotechnology. 2014;32 (11) :1134-1140. Full TextAbstract

Thrombosis and biofouling of extracorporeal circuits and indwelling medical devices cause significant morbidity and mortality worldwide. We apply a bioinspired, omniphobic coating to tubing and catheters and show that it completely repels blood and suppresses biofilm formation. The coating is a covalently tethered, flexible molecular layer of perfluorocarbon, which holds a thin liquid film of medical-grade perfluorocarbon on the surface. This coating prevents fibrin attachment, reduces platelet adhesion and activation, suppresses biofilm formation and is stable under blood flow in vitro. Surface-coated medical-grade tubing and catheters, assembled into arteriovenous shunts and implanted in pigs, remain patent for at least 8 h without anticoagulation. This surface-coating technology could reduce the use of anticoagulants in patients and help to prevent thrombotic occlusion and biofouling of medical devices.

Vogel N, Belisle RA, Hatton B, Wong TS, Aizenberg J. Transparency and damage tolerance of patternable omniphobic lubricated surfaces based on inverse colloidal monolayers. Nature Communications. 2013;4. Publisher's VersionAbstract
A transparent coating that repels a wide variety of liquids, prevents staining, is capable of self-repair and is robust towards mechanical damage can have a broad technological impact, from solar cell coatings to self-cleaning optical devices. Here we employ colloidal templating to design transparent, nanoporous surface structures. A lubricant can be firmly locked into the structures and, owing to its fluidic nature, forms a defect-free, self-healing interface that eliminates the pinning of a second liquid applied to its surface, leading to efficient liquid repellency, prevention of adsorption of liquid-borne contaminants, and reduction of ice adhesion strength. We further show how this method can be applied to locally pattern the repellent character of the substrate, thus opening opportunities to spatially confine any simple or complex fluids. The coating is highly defect-tolerant due to its interconnected, honeycomb wall structure, and repellency prevails after the application of strong shear forces and mechanical damage. The regularity of the coating allows us to understand and predict the stability or failure of repellency as a function of lubricant layer thickness and defect distribution based on a simple geometric model.
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.
MacCallum N, Howell C, Kim P, Sun D, Friedlander R, Ranisau J, Ahanotu O, Lin JJ, Vena A, Hatton B, et al. Liquid-Infused Silicone As a Biofouling-Free Medical Material. ACS Biomater. Sci. Eng. 2015;1 (1) :43-51.Abstract
There is a dire need for infection prevention strategies that do not require the use of antibiotics, which exacerbate the rise of multi- and pan-drug resistant infectious organisms. An important target in this area is the bacterial attachment and subsequent biofilm formation on medical devices (e.g., catheters). Here we describe nonfouling, lubricant-infused slippery polymers as proof-of-concept medical materials that are based on oil-infused polydimethylsiloxane (iPDMS). Planar and tubular geometry silicone substrates can be infused with nontoxic silicone oil to create a stable, extremely slippery interface that exhibits exceptionally low bacterial adhesion and prevents biofilm formation. Analysis of a flow culture of Pseudomonas aeruginosa through untreated PDMS and iPDMS tubing shows at least an order of magnitude reduction of biofilm formation on iPDMS, and almost complete absence of biofilm on iPDMS after a gentle water rinse. The iPDMS materials can be applied as a coating on other polymers or prepared by simply immersing silicone tubing in silicone oil, and are compatible with traditional sterilization methods. As a demonstration, we show the preparation of silicone-coated polyurethane catheters and significant reduction of Escherichia coli and Staphylococcus epidermidis biofilm formation on the catheter surface. This work represents an important first step toward a simple and effective means of preventing bacterial adhesion on a wide range of materials used for medical devices.
Yao X, Dunn S, Kim P, Duffy M, Alvarenga J, Aizenberg J. Fluorogel Elastomers with Tunable Transparency, Elasticity, Shape- Memory, and Antifouling Properties. Angew. Chem. Int. Ed. 2014;53 (17) :4418-4422. Full TextAbstract

Omniphobic fluorogel elastomers were prepared by photocuring perfluorinated acrylates and a perfluoropolyether crosslinker. By tuning either the chemical composition or the temperature that control the crystallinity of the resulting polymer chains, a broad range of optical and mechanical properties of the fluorogel can be achieved. After infusing with fluorinated lubricants, the fluorogels showed excellent resist- ance to wetting by various liquids and anti-biofouling behavior, while maintaining cytocompatiblity.

Kim P, Kreder MJ, Alvarenga J, Aizenberg J. Hierarchical or Not? Effect of the Length Scale and Hierarchy of the Surface Roughness on Omniphobicity of Lubricant-Infused Substrates. Nano Lett. 2013;13 (4) :1793-1799. Publisher's VersionAbstract
Lubricant-infused textured solid substrates are gaining remarkable interest as a new class of omni-repellent nonfouling materials and surface coatings. We investigated the effect of the length scale and hierarchy of the surface topography of the underlying substrates on their ability to retain the lubricant under high shear conditions, which is important for maintaining nonwetting properties under application-relevant conditions. By comparing the lubricant loss, contact angle hysteresis, and sliding angles for water and ethanol droplets on flat, microscale, nanoscale, and hierarchically textured surfaces subjected to various spinning rates (from 100 to 10 000 rpm), we show that lubricant-infused textured surfaces with uniform nanofeatures provide the most shear-tolerant liquid-repellent behavior, unlike lotus leaf-inspired superhydrophobic surfaces, which generally favor hierarchical structures for improved pressure stability and low contact angle hysteresis. On the basis of these findings, we present generalized, low-cost, and scalable methods to manufacture uniform or regionally patterned nanotextured coatings on arbitrary materials and complex shapes. After functionalization and lubrication, these coatings show robust, shear-tolerant omniphobic behavior, transparency, and nonfouling properties against highly contaminating media.
Howell C, Vu TL, Johnson CP, Hou X, Ahanotu O, Alvarenga J, Leslie DC, Uzun O, Waterhouse A, Kim P, et al. Stability of Surface-Immobilized Lubricant Interfaces under Flow. Chem. Mater. 2015;27 (5) :1792-1800. Full TextAbstract
The stability and longevity of surface-stabilized lubricant layers is a critical question in their application as low- and nonfouling slippery surface treatments in both industry and medicine. Here, we investigate lubricant loss from surfaces under flow in water using both quantitative analysis and visualization, testing the effects of underlying surface type (nanostructured versus flat), as well as flow rate in the physiologically relevant range, lubricant type, and time. We find lubricant losses on the order of only ng/cm2 in a closed system, indicating that these interfaces are relatively stable under the flow conditions tested. No notable differences emerged between surface type, flow rate, lubricant type, or time. However, exposure of the lubricant layers to an air/water interface did significantly increase the amount of lubricant removed from the surface, leading to disruption of the layer. These results may help in the development and design of materials using surface-immobilized lubricant interfaces for repellency under flow conditions.
Sunny S, Vogel N, Howell C, Vu TL, Aizenberg J. Lubricant-infused Nanoparticulate Coatings Assembled by Layer-by-layer Deposition. Adv. Funct. Mater. 2014;24 (42) :6658-6667.Abstract

Omniphobic coatings are designed to repel a wide range of liquids without leaving stains on the surface. A practical coating should exhibit stable repel- lency, show no interference with color or transparency of the underlying substrate and, ideally, be deposited in a simple process on arbitrarily shaped surfaces. We use layer-by-layer (LbL) deposition of negatively charged silica nanoparticles and positively charged polyelectrolytes to create nanoscale sur- face structures that are further surface-functionalized with fluorinated silanes and infiltrated with fluorinated oil, forming a smooth, highly repellent coating on surfaces of different materials and shapes. We show that four or more
LbL cycles introduce sufficient surface roughness to effectively immobilize the lubricant into the nanoporous coating and provide a stable liquid inter- face that repels water, low-surface-tension liquids and complex fluids. The absence of hierarchical structures and the small size of the silica nanoparti- cles enables complete transparency of the coating, with light transmittance exceeding that of normal glass. The coating is mechanically robust, maintains its repellency after exposure to continuous flow for several days and prevents adsorption of streptavidin as a model protein. The LbL process is conceptu- ally simple, of low cost, environmentally benign, scalable, automatable and therefore may present an efficient synthetic route to non-fouling materials.

Daniel D, Mankin MN, Belisle RA, Wong T-S, Aizenberg J. Lubricant-infused micro/nano-structured surfaces with tunable dynamic omniphobicity at high temperatures. Appl. Phys. Lett. 2013;102 :231603. Full TextAbstract
Omniphobic surfaces that can repel fluids at temperatures higher than 100 °C are rare. Most state-of-the-art liquid-repellent materials are based on the lotus effect, where a thin air layer is maintained throughout micro/nanotextures leading to high mobility of liquids. However, such behavior eventually fails at elevated temperatures when the surface tension of test liquids decreases significantly. Here, we demonstrate a class of lubricant-infused structuredsurfaces that can maintain a robust omniphobic state even for low-surface-tension liquids at temperatures up to at least 200 °C. We also demonstrate how liquid mobility on such surfaces can be tuned by a factor of 1000.

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