Liquid-Infused Surfaces: A Review of Theory, Design, and Applications

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Liquid-Infused Surfaces: A Review of Theory, Design, and Applications Martin Villegas,† Yuxi Zhang,‡ Noor Abu Jarad,† Leyla Soleymani,† and Tohid F. Didar*,†,‡,§ †

School of Biomedical Engineering, ‡Department of Mechanical Engineering, and §Institute for Infectious Disease Research, McMaster University, 1280 Main Street West, L8S 4L8 Hamilton, Ontario, Canada

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S Supporting Information *

ABSTRACT: Due to inspiration from the Nepenthes pitcher plant, a frontier of devices has emerged with unmatched capabilities. Liquidinfused surfaces (LISs), particularly known for their liquid-repelling behavior under low tilting angles (150° and 0 or lays on top if Sow(a) < 0, as shown in Figure 2a, left and right, respectively. The spreading factor Sow(a), where “o”, “w”, and “a” are represented by oil, water, and air, correspondingly, is defined by eq 4 below.51 Sow(a) ≡ γwa − γow − γoa

(4)

where γwa is the water−air surface tension, γow is the oil−water surface tension, and γoa is the oil−air surface tension. Similarly, the spreading factor of oil on the solid substrate can be defined when in contact with air or water in eq 5 and eq 6, accordingly. Sos(a) ≡ γsa − γos − γoa

(5)

Sos(w) ≡ γsw − γos − γow

(6)

Depending on the four-phase system at hand, the oil will completely wet (encapsulate) the surface, partially wet, or will not be present under thermodynamic conditions. Additionally, the penetration condition is related to the substrate roughness by the contact line cos θ = (ϕ − r)/(r − 1), where r is the same surface roughness factor defined by Wenzel’s equation and ϕ is the fraction of projected area occupied by the liquid, analogous to the fraction factor in Cassie’s equation.51 Returning to the three main criteria for LIS devices, a solid will have a higher affinity for the “oil” over water if −γow(r − 1)/(r − ϕ) < Sos(w) < 0. Moreover, the lubricant will wet and encapsulate the surface if Sos(a) ≥ 0.51 The other conditions are presented in the inset of Figure 2a, and their derivations can be reviewed in the original publication.51 However, it is possible to make the following observations: (i) if the lubricant is not present underneath the droplet, the droplet can fall into a Wenzel state and pin on the substrate; (ii) if the oil is partially present, then the droplet will slide, but interactions with the solid will cause frictional forces; (iii) if the lubricant does not encapsulate or impregnate the surface, eventually the droplet will reach a dry surface and get pinned. Semprebon et al. formulated an analytical model to explain contact angle and contact angle hysteresis on a LIS.52 They concluded that the contact angle on a LIS is not a constant material property but an energy balance where the three fluids meet (air, lubricating liquid, and probing liquid), as displayed in Figure 2b. In 2015, Schellenberger et al. demonstrated that the

F = mg sin α = ρΩg sin α

(7)

Here, F is the gravitational force, Ω is the droplet’s volume, ρ represents the density of the droplet, α is the angle of inclination, 8521

DOI: 10.1021/acsnano.9b04129 ACS Nano 2019, 13, 8517−8536

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ACS Nano and m and g correspond to the mass of the droplet and the acceleration due to gravity, respectively. The friction experienced by a droplet on a flat surface can then be defined by the surface tension between water and air γwa, the contact area Rc, the maximum (θmax), and minimum angles (θmin), as well as a constant κ, to compensate for the surface roughness and other experimental parameters.1 Ff = γwaR cκ(cos θmin − cos θmax )

over the lubricant and does not slide during this regime due to a relatively stationary droplet−lubricant interface (Vi).

FABRICATION OF LISs Physical and Chemical Methods to Fabricate LISs. Two main factors that govern surface wettability are surface chemistry and surface roughness.55 For this reason, several approaches have been extensively studied to modify both the surface topography and chemical features of LISs during the manufacture of these surfaces. There are two primary methodologies to prepare LISs: one is to modify pre-existing roughened surfaces with the proper chemical coating to match the chemistry of the lubricant, and the second is to roughen the surfaces of low-surface-energy substrates.32 In some cases, a combination of both methods is applied.32 Whereas obtaining a compatible surface chemistry between the solid and the lubricant is crucial to the creation of LIS, having a textured surface is not.8 However, it has been revealed that having a textured material greatly improves the LIS interface and promotes lubricant retention compared to planar devices.44,56 Recently, liquid-infused coatings have been integrated into many kinds of materials, including metals such as gold,57,58 aluminum,37,59,60 copper,61,62 stainless steel,47,63,64 and titanium,56 in addition to nonmetallic substances such as wood,65 glass,53,66 and silicon.7,54,67,68 Moreover, a great variety of polymers have also been studied as substrates, for example, polyvinyl chloride,8,69 polycarbonate,8 polytetrafluoroethylene (PTFE),7,8,70,71 poly(methyl methacrylate),8 PDMS,72−74 and polystyrene (PS),8,13 among others. Several methods have been used to create micro/nanostructures onto the base of LISs, which include chemical and physical etching, emulsion and phase separation, mold transcribing methods, spin-coating, spraying, electrospinning, electrochemical deposition, nanoparticle/nanopillar assembly, polymer-induced wrinkling,75,76 and layer-by-layer and bottom-up manufacturing approaches.6,32,33,35 For cases where the inherent properties of the material do not match those of the lubricant, a chemical coating can be applied to provide higher affinity to the lubricant. There are two main approaches to induce these low-energy chemical coatings. The first one involves a chemical modification using liquid-phase deposition (LPD), which can be realized by dip-coating or spray-coating technologies. The second one uses chemical vapor deposition (CVD) to functionalize the surface. Among these two techniques, CVD is a more advantageous approach as it creates more uniform coats, specifically for nonplanar morphologies or when the surface is not directly exposed; in addition, it requires fewer reagents than LPD.70,77−79 Furthermore, it has also been reported that the LPD process has the potential to erode the material being coated as there is a possibility that corrosive byproducts are left in the solution after the chemical reaction has taken place. The effects of such byproducts are mitigated through the CVD process.70 On the other hand, the LPD process is easily scalable, a nontrivial task for chemical vapor deposition, as the CVD process requires specialized equipment or the use of vacuum chambers where parallelization is difficult. Table S1 (in the Supporting Information) provides a comprehensive summary of reported designs for liquid-infused surfaces. This table includes the substrate materials, physical or chemical modifications, the lubricants used, as well as a plethora of applications. As discussed in previous sections, the behavior and stability of liquid-infused surfaces are not just substrate-dependent. For this

(8)

Based on the definitions for advancing and receding angles, it follows that maximizing both of these parameters is indicative of a quasi-spherical droplet with minimal contact area to the surface. These droplets are capable of rolling on the surface, and for this reason, the sliding angle can sometimes be referred to as the “roll-off” angle (Figure 3b).53 LISs increase the complexity of the system by adding a lubricating layer. Droplets on these fourphase systems, composed of two immiscible liquids along a solid, are notorious for their low sliding angles. Although most LIS systems have been characterized as sliding, in reality, it has been shown that this property is entirely dependent on the viscosities of the two liquids.51 Smith et al. demonstrated that, in situations where high viscosity lubricants are used, a droplet will roll rather than slide over the lubricant (Figure 3d).51 This behavior was modeled by eq 9: ij η R yz Vi ∼ jjjj1 + o zzzz V ηw h { k

−1

(9)

where V is the velocity of the droplet, Vi is the velocity at the water−oil interface, h is the lubricant’s thickness, R is the droplet’s radius, and ηo and ηw are the dynamic viscosities of oil and water, respectively. Keiser et al. built on these observations and showed two distinct regimens for droplet mobility on liquidinfused materials. In the first scenario, when the droplet has a much higher viscosity than the lubricant, such that ηw ≫ ηo, the droplet velocity scales with sliding angle. In other words, the friction appears linear to the droplet. V∼

ρgR2 sin α ηw

(10)

On the other hand, if ηw ≪ ηo, the droplet velocity scales by sin3/2 α, as shown by eq 11, where β represents the dissipation at the tip of the meniscus54 and the other factors are the same as explained above. V∼

(ρg )3/2 R3 γoaϕ3/2βηw

sin 3/2 α (11)

This equation indicates that the droplet experiences nonlinear frictional forces. This friction is hypothesized to come from the leading and lagging oil menisci and their viscous effects (Figure 3e,f).54 Furthermore, by comparing the equations above, it is visible that the droplet velocity is independent of surface microtexture when ηw ≫ ηo but scales as an inverse of ϕ3/2 when ηw ≪ ηo. Therefore, as the fraction of microstructures increases, so does the frictional forces acting on the droplet. For the flat and textured surfaces, the frictional forces are dependent on the adhesion and wetting characteristics of the liquid and the surface. On the other hand, the four-phase systems, which involve a lubricant layer, greatly depend on the characteristics of both fluids, proving that at least two mobility regimes exist. Moreover, it has been shown that the droplet rolls 8522


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ACS Nano Table 1. Physicochemical Properties of 10 Different Lubricantsa

lubricant perfluorodecalin perfluoroperhydrophenanthrene perfluorotripentylamine Krytox 100 Krytox 103 heptane octane decane dodecane 1-butyl-3-methylimidazolium hexafluorophosphate (BMIm)

average molecular weight (g/mol)

boiling point (°C)

liquid density (g/mL)

vapor pressure (Torr)

kinematic viscosity (cSt)

surface tension (dyn/cm)

refractive index (-)

oxygen solubility (mL O2/100 mL)

ref

462 630 821 N/A N/A 100.21 114.23 142.29 170.34 284.19

140−143 215 215 N/A N/A 90.549 125.1 173.8 214 N/A

1.93 2.02 1.910 1.835 1.90 0.6795 0.703 0.730 0.7495 1.38

6.25 0.122 0.113 N/A N/A 39.99 11.03 1.46 0.135 N/A

2.94 8.0 11.0−17.0 6.76 42.7 0.568 0.771 1.26 1.788 N/A

19.3 21.6 18 19 N/A 20.14 21.62 23.83 25.35 N/A

1.31 1.31 1.30 N/A N/A 1.3855 1.398 1.411 1.421 N/A

49 37 N/A N/A N/A N/A N/A N/A N/A N/A

13,58,69 79 8 48 48 82 83 84 85 51

a

Table adapted with permission from ref 81. Copyright 2018 Martin Villegas.

Figure 4. Anticorrosion applications. (a) Schematic illustration of carbon fiber−lubricant LIS formation process. (b) Scanning electron microscopy (SEM) images of dendritic structures and dendrite growth through an electrodeposition process to create initiation sites. (c) SEM images of hydrophobic carbon fibers deposited using a chemical vapor deposition process. (d) Schematic of corrosion experimental setup for superhydrophobic and LIS devices. (e) SEM image displaying the level of corrosion on aluminum (left), superhydrophobic aluminum (middle), and LIS aluminum (right). (f) Schematic illustration of hydrophobic tetradecanoate on steel (TAH/LS) film formation process. (g) Schematic illustration of corrosion protection mechanisms between superhydrophobic (top) and LIS-treated steel (bottom). (h) Corrosion propagation images for steel, superhydrophobic steel, and LIS steel with lubricants with increasing viscosities, in order from left to right. Panels a−c adapted with permission from ref 61. Copyright 2014 Elsevier. Panels d and e adapted with permission from ref 90. Copyright 2017 Elsevier. Panels f−h adapted with permission from ref 63. Copyright 2015 Elsevier. 8523


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be done in each specific area, these results prove invaluable for the future applications of LISs.

reason, the next section will expand on the properties of some of the most commonly used liquid lubricants to facilitate the design of LIS systems. Choice of Lubricants and Probing Liquids. Choosing a lubricant is one of the most essential steps when designing a LIS. Table 1 highlights some of the physical properties of the lubricants commonly used on LISs. It is evident that the choice of lubricants is versatile, and specific properties can be chosen depending on the required applications. For example, for devices where optical transparency is required, matching the refractive index between lubricant and substrate is required.48 For applications requiring long-term stability or increased temperatures, more viscous oils can be chosen. These oils tend to have low vapor pressures and high boiling points.47 Furthermore, nonfluorinated hydrophobic lubricants, such as silicone oils or hydrocarbon oils, can be used against aqueous solvents. Similarly, polar lubricants can be used to repel low surface tension hydrocarbons, allowing for a greater variety of lubricants. As mentioned earlier, to obtain omniphobic repellency, a lubricant which is immiscible with organic and aqueous solvents can be used. Typically, fluorous lubricants can be used such as perfluorodecalin, Fluorinert (FC70), or Krytox oils. Choosing the correct lubricant and probing liquid is of great importance when designing a liquid-infused surface. Traditionally, scientists used contact angles, spread coefficients, and interfacial tensions to describe the thermodynamic states discussed above. However, Preston et al. argued that these methods were measured empirically to justify previous experiments and could not predict new LIS combinations.80 In 2017, their group combined previous models with the one proposed by van Oss, Chaudhury, and Good (vOCG). This model takes into account various intermolecular forces independently, in which the “Lewis acid−base contribution to interfacial energy are considered”.80 The model formulated by Preston et al. had a 90% accuracy at predicting new and existing LIS combinations and gave insight into alternate liquid−substrate combination capable of repelling very low surface tension probing liquids such as butane (γ = 13 mN m−1). This was achieved by conjugating high surface energy substrates with a polar perfluorinated lubricant.80 Similar results were also achieved by the work of Sett et al., which studied the miscibility and cloaking properties of eight testing liquids versus nine different lubricants, ranging from fluorinated and hydrocarbons to polar lubricants.66 These results can be used as an initial step when choosing a lubricant for a specific application. On a different note, Howell et al. demonstrated that the lubricant layer of a LIS on a closed channel is highly stable and independent of flow rate, lubricant type, or surface type under physiologically relevant ranges (100−1600 μL min−1).69 Moreover, their results showed that the most disruptive force introduced to the system was caused by an air−water interface that was generated through the insertion of air bubbles.69 Although some of the lubricant was stripped off in the presence of an air−water interface, their results proved that structured surfaces are better equipped for lubricant retention compared to flat channels.69 These findings open the opportunities for LISs to be used in closed-channel platforms, ideal for chemical and oil transportation, microfluidics, or even point-of-care diagnostic tools with continuous monitoring, as long as no air interfaces are introduced into the system. Even though further testing needs to

APPLICATIONS OF LIQUID-INFUSED SURFACES Anticorrosion. Metals and metal alloys are widely used in the fabrication of marine structures including wind turbines, marinas, oil rigs, and vessels. Metals such as copper, zinc, aluminum, steel, and their alloys are of great importance for marine applications. Thus, the anticorrosion properties of these metals are extensively studied.86 Corrosion of metallic materials can lead to a huge economic loss in the marine industry. Inspired by lotus leaves, superhydrophobic surfaces have been proposed as an anticorrosive strategy. This mechanism works by entrapping a layer of air in the microcavities of rough surfaces to prevent water penetration via capillary action.87,88 However, these superhydrophobic coatings, made of carbon fibers grown on a zinc substrate, were proven to be unstable over long-term immersion tests in 3.5% saline solution environments, ultimately resulting in corrosion.86 These surfaces fail because they achieve a metastable state at best, turning researchers toward creating LISs as an alternative corrosion-resistant option. In 2014, a liquid-infused carbon fiber matrix, infused with a perfluoroalkyl ether, displayed exceptional corrosion inhibition performance, outperforming its superhydrophobic counterpart (Figure 4a).61 Superhydrophobic and liquid-infused surfaces were fabricated on copper alloy substrates via the electrodeposition of copper to create hierarchical structures (Figure 4b). These micro/ nanostructures acted as catalytic sites for the deposition of carbon fibers through a CVD process, inducing sponge-like structures (Figure 4c) capable of entrapping a lubricant or air layer.61 The corrosion properties were characterized through electrochemical impedance spectroscopy and the potentiodynamic polarization measurements, confirming the superiority of LISs compared to superhydrophobic surfaces.61 Han et al. fabricated a hierarchical porous structure by spin-coating a silicon wafer with PS−PEO/FeCl3 solution. The resulting iron oxide structure can be liquid-infused with either hydrocarbon oils or water to repel bubbles or immiscible fluids.89 This versatile coating has the potential to be applied for pipeline transportation or marine structure to reduce fouling and increase the material’s longevity. Aluminum and steel alloys are other commonly used materials in the maritime industry. Upon recognizing the importance of this application, Tuo et al. fabricated a layered double hydroxide (LDH) coating on aluminum foil (Figure 4d,e). When these surfaces were impregnated with an oil-based lubricant, a metastable LIS was formed. The resultant surface was capable of corrosion resistance in seawater conditions.90 Similarly, Yanga et al. applied a perfluorinated lubricant-infused hydrophobic iron tetradecanoate (TAH) film onto high-strength low-alloy steel (HSLA) and examined the resultant anticorrosion properties (see Figure 4f−h). It was found that the rough architecture of the steel surface dramatically enhanced the retainment of lubricant.63 Moreover, a LIS coating with lubricant thickness over 80 μL displayed corrosion protection better than that of other control groups. Interestingly, lubricant viscosity significantly reduced corrosion on these surfaces, as seen in Figure 4h (right column). Mechanical damage is another major concern faced in the marine industry. Recently, a liquid-infused surface with the ability to self-heal was fabricated on a copper alloy. These surfaces maintained their anticorrosion properties as illustrated by Shi et al.62 8524


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Figure 5. Anti-icing. (a) Images of ice formation on different aluminum vs SLIP surfaces. (b) Schematics of the fabrication procedure for electrochemical coating to increase surface roughness. (c) Schematic of self-lubricating anti-icing device. (d) Average ice adhesion strengths on four different test surfaces. (e) Preparation process of the micropore-arrayed silicon wafer surface. (f) Schematic representation of joule-heated LIS device. (g) Schematic of setup used for the frosting experiments. (h) Dispersion comprising carbon nanofibers and a fluoroacrylic copolymer solvents sprayed onto a plain glass slide. Panels a and b adapted from ref 59. Copyright 2012 American Chemical Society. Panels c−e adapted from ref 99. Copyright 2013 American Chemical Society. Panels f−h adapted from ref 101. Copyright 2016 American Chemical Society. Further permissions related to this material should be directed to the American Chemical Society.

replacement of facilities and can even introduce potential safety hazards. Over the past years, great efforts have been made to explore deicing methods. Generally speaking, these methods can be divided into either active or passive strategies. The common active deicing methods include electrothermal strategies93 and pneumatic impulse techniques,94 which are rarely used as they become inefficient and costly. On the other hand, passive methods use natural forces, which significantly lessen the implementation costs. Superhydrophobic surfaces have been widely investigated as potential passive anti-icing strategies, but researchers have shown that these techniques fail in high humidity environments, where a strong interlock forms between ice and the superhydrophobic surface.95,96 Recently, using LISs has emerged as an effective moisture-resistant and ice-phobic strategy. The physical and chemical conditions of LISs create an ultrasmooth interface which reduces ice nucleation sites and lowers the water crystallization rate on these surfaces.97

In addition to possessing anticorrosion properties, LISs are also able to prevent microbiological adherence. Microbial fouling can corrode a marine structure as well as reduce a marine vessel’s hydrodynamics, therefore increasing fuel consumption for marine transportation.5 In order to circumvent this problem, Wang et al. generated a porous aluminum surface functionalized with perfluorinated silane and infused with perfluoropolyether, which efficiently prevented SRB−biofilm formation.60 One of the biggest drawbacks of LIS devices causing failure is the loss of the lubricating layer. For this reason, more attention should be drawn toward developing LISs with long-lasting lubricants to eliminate marine corrosion. Anti-icing. Anti-icing coatings play an important role in transportation equipment manufacturing and infrastructure such as with aircraft wings,91 power transmission lines,92 and air conditioning systems.93 Ice accretion on these devices has the potential to induce extra costs for the maintenance and 8525


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Figure 6. Medical. (a) Device performance while in contact with whole blood. (b) Endoscope modification with a disposable glass coverslip. (c) Images taken with endoscope in contact with lung secretions in an ex vivo lung. (d) Schematic of LIS material and biofilm testing. (e) Photographs of biofilm formation on PTFE and LIS. (f) Fluorescent image of biofilm attachment reduction by LIS for S. aureus vs control. (g) Schematic of blood repellency on LIS. (h) Whole blood sliding angle on LIS and control surfaces. (i) Schematic of the porcine arteriovenous shunt model and photographs of polyurethane cannulae. (j) Schematic illustration of catheter omniphobic coating. (k) Images of blood clot formation on different catheter surfaces and their respective SEM image; scale bars are 50 μm. Panels a−c adapted with permission from ref 103. Copyright 2016 National Academy of Sciences of the United States of America. Panels d−f adapted with permission from ref 7. Copyright 2012 National Academy of Sciences of the United States of America. Panels g−i adapted with permission from ref 8. Copyright 2014 Springer Nature. Panels j and k adapted with permission from ref 70 under a Creative Commons license.

tests, demonstrating high durability of the coating.98 As for ecofriendly lubricant exploration, Chen et al. reported a hygroscopic polymer-grafted porous silicon substrate (Figure 5c), which had natural deicing properties that inflicted no harm to the environment.99 The self-lubricating liquid water layer showed relatively low ice adhesion strength (Figure 5d). The illustrated process (Figure 5e) resulted in the formation of an anti-icing surface with self-healing and abrasion resistance properties. The cross-linked hygroscopic polymers were inserted into silicon via a free radical polymerization method. To improve ice removal efficiency, researchers combined LIS technology with knowledge of thermogenesis to induce functional nanoparticles on the coating, which enabled the surface to absorb particles capable of locally melting the ice. To demonstrate this concept, Yin et al. reported a liquid-infused coating embedded with functional particles. More specifically, the coating contained uniformly distributed Fe3O4 magnetic nanoparticles with photothermogenic properties that were capable of increasing the temperature on the surface.100 A different approach was applied by Elsharkawy et al., where a

In an experimental study, Kim et al. electrochemically modified aluminum fins and sheets through the deposition of polypyrrole to enhance the roughness of the surface, followed by chemically fluorinating the surface with a CVD process. Finally, adding Krytox 100 lubricant created an icephobic surface (Figure 5a).59 Ice and melted water on these surfaces could be easily removed by weak shear force, such as via gravity, wind, and vibrations.59 Figure 5b illustrate that treated aluminum (Al) samples maintained smaller contact angle hysteresis with changing temperatures, showing the efficiency in removing condensed water. However, lubricated-infused hydrophobic aluminum had drastically smaller ice surface coverage relative to that of other control groups. Ice nucleation at the edge was still the main contributor of ice coverage on the LISs.59 This is a significant issue which remains unsolved.59 Furthermore, Juuti et al. demonstrated a slippery liquidinfused coating with TiO2 nanoparticles. This nanocoating dramatically decreased ice adhesion strength to a value of 12 kPa, while still maintaining the anti-icing properties of the surface. Importantly, the surface underwent cyclic ice adhesion 8526


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Figure 7. Self-healing. (a) LIS fabrication through femtosecond laser direct writing. (b) Water droplet sliding on LIS device after self-healing. (c) Schematic illustration of the critical condition for self-healing. (d) SEM images of PCDTPT film (top) and sliding angles (bottom) displaying a reduction of sliding angles with the reduction of oil viscosity on a self-healing surface. (e) Schematic of surface wettability recovery after self-healing. (f) Scheme demonstrating the synthesis of LIS with multiple self-healing mechanisms, i.e., strong covalent bonds from polymer−magnetic nanoparticle interactions and weak polymeric interactions. Panels a and b adapted with permission from ref 106. Copyright 2017 John Wiley and Sons. Panels c and d adapted with permission from ref 105. Copyright 2018 Royal Society of Chemistry. Panels e and f adapted from ref 107. Copyright 2017 American Chemical Society.

Joule-heated liquid-infused surface was produced on carbon nanofibers (Figure 5f,g).101 Using a spray deposition method to coat the surface with a material having repellent properties can reduce manufacturing costs and creates the opportunity for a simplified procedure to coat a wide range of surfaces and to

recoat the surface after going through damage, wear, or longterm usage (Figure 5h).101 Such development is highly useful for future developments in the field. Biomedical Applications. Recently, medical devices have been coated with liquid-infused coatings to improve their 8527


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ACS Nano functionality and lifespan.8 The reduction of biofilm adhesion is critical for the proper function of medical devices such as catheters, heart valves, and prosthetic implants.8 Biofilm adhesion, also known as biofouling, occurs when bacteria bind to a surface and increase the rate at which they deposit macromolecules and proliferate.7 Biofouling on medical devices results in 65% of bacterial-related infections for humans, which can lead to an immune response causing failure of the device.102 Biofouling causes persistent infections, resulting in nearly 100,000 deaths in the United States annually.7 Sunny et al. demonstrated that biofouling inhibits the operation of medical devices such as endoscopes by causing the obstruction of the field of view.103 In response to this, they utilized a LIS-covered lens for endoscopy.103 This was achieved by binding silica nanoparticles to a transparent glass substrate and infusing the substrate with silicone oil or a perfluorinated lubricant. Implementation of this material significantly reduced the adhesion of biospecies to the lens, thus improving the images taken throughout the procedure even after repeated submersions into blood and mucus.103 As witnessed in Figure 6a,b, the uncoated endoscope failed after one dip in whole porcine blood, whereas the LIS coating repelled blood and biofilm for over 100 repeated cycles.103 Furthermore, when testing the LIS-coated endoscope in an ex vivo lung, the LIScoated scope had a distinct advantage over the uncoated endoscope (Figure 6c). Moreover, the LIS-coated endoscope eliminates the loss of vision caused by blood occlusion. In another study, Epstein et al. showed that LIS prevented 96−99% of biofilm adhesion over a 7 day period, which was significantly better (35× more effective) compared to a precursor polyethylene glycol-coated surface.7 Figure 6d shows a schematic representation of the fabrication process for a liquid-infused surface in preparation for a biofilm adhesion test, where a porous PTFE substrate was infused with a perfluorinated lubricant.7 The biofilm formation was observed using fluorescence micrography after a 48 h incubation period of P. aeruginosa on either a LIS or superhydrophobic PTFE surfaces.7 As observed in Figure 6f, the LIS is effective at reducing biofilm formation compared to its superhydrophobic counterpart. In a different study conducted by Chen et al., the biocompatibility of a LIS was tested by examining the effect of LISs on macrophage viability. It was reported that an expanded polytetrafluoroethylene substrate infused with perfluorocarbon liquid displayed no inhibition of macrophage adhesion, viability, phagocytosis, and bactericidal activity; the macrophages were successfully able to engulf and kill S. aureus bacteria.71 Thus, in the rare case that a LIS surface is subject to bacterial adhesion, the immune system will be capable of targeting the infection.71 The biomedical applications of LISs do not end here. A recent study conducted by Leslie et al. has proven that LISs also reduce platelet adhesion.8 In the experiment, a stable liquid-infused surface was created without the need for surface roughness (Figure 6g−i). These devices were capable of repelling full blood (as shown in Figure 6i), while at the same time reducing the number of platelets adsorbed onto the surface.8 These findings indicated that LISs can prevent thrombosis in LIS-treated medical tubing in an ex vivo arteriovenous model, which is a significant step forward in the medical field.8,70 Using LIScoated devices could eliminate the need for anticoagulant uptake by patients, reducing the risk of postoperative bleeding, thrombocytopenia, and hypertriglyceridemia, among other complications.8

Furthermore, Badv et al. fabricated an antithrombogenesis catheter via a straightforward CVD method to tether a biocompatible lubricant onto the surface (Figure 6j,k).70 The SEM images in Figure 6k display the lack of platelets or protein adsorbed on the treated catheters, thus proving a feasible and straightforward approach can be used to treat medical devices. In short, this research indicates that LISs can be used to improve the safety and function of medical devices and implants. As with any medical device, approval by the regulatory authorities is still one of the biggest hurdles required to bring new technologies to the market. Luckily, companies such as NuSil are already commercializing silicone medical lubricants to reduce insertion forces of medical devices, such as needles, cannulas, and cutting edges.104 The future looks bright for LISs in the medical field, and it is expected that more companies will join the growing market. Self-Healing. One of the significant limitations of slippery surfaces is that they lose their superwettability upon mechanical abrasion.105 To overcome this issue, scientists have investigated self-healing surfaces, which spontaneously repair damage without direct manipulation.105 When damage occurs on the surface of a LIS, the lubricant flows toward the damaged area due to the fluidic nature of the lubricant layer, allowing these surfaces to retain their self-cleaning properties.105 These self-healing properties were reported by Yong et al., where a polyamide-6 (PA6) substrate was modified by femtosecond laser abrasion and the addition of a lubricant layer.106 After mechanical damage, the surfaces recovered their slippery nature (Figure 7a,b). In another study, Wang et al. demonstrated the selfhealing capabilities of a conductive poly[4-(4,4-dihexadecyl-4Hcyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-alt-[1,2,5]thiadiazol[3,4-c]pyridine] (PCDTPT) substrate affixed with silicone oils, via the creation of a directional gradient and the production of an anisotropic surface.105 The researchers used a spin-coating method to investigate the lubricant’s thickness variability and found that a critical lubricant thickness was required to maintain the self-healing properties of the surface (Figure 7c,d).105 Another approach to creating a self-healing surface has been explored in a study conducted by Jin et al.107 LISs were created by dual cross-linking magnetic nanoparticles (MNPs) and dopamine molecules with PDMS and glycidyl methacrylate (Figure 7f).107 Silicone oil was then infused into the surface as a lubricant due to its high affinity for PDMS.107 Using this strategy, strong covalent bonds were formed between the MNPs and dopamine molecules, thus reinforcing the mechanical properties of the material (Figure 7e).107 The PDMS and glycidyl methacrylate formed weak “sacrificial” polymer bonds which could rapidly re-form after damage occurred.107 In addition, Jin et al. have provided a technique for fabricating LIS surfaces that retain slippery properties despite exposure to threatening external stimuli.107 For example, results indicated that after being exposed to a sunlamp for 2 h, the material recovered with 78.25% healing efficiency.107 Unfortunately, LISs exhibit reduced long-term stability, scalability, durability, or applicability to a wide range of substrates. Thermal stability, mechanical robustness, and chemical durability are important aspects for the long-term usage of LISs. Using liquids that have different surface tensions enhance the long-term stability of the surfaces and make them more resilient. Testing the mobility of liquid droplets after being cooled from a high temperature is one of the ways to test how LISs could retain their omniphobic properties. Many reported surfaces have shown that they could withstand high temper8528


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Figure 8. Droplet manipulation. (a) Device function schematic and (b) still frame of droplet movement by acoustic waves. (c) Schematic of low voltage reversible electrowetting on LIS. (d) Repeated switching of droplet with electrowetting on LIS. (e) Device schematic of conductive lubricant-infused nanostructured surfaces. (f) Still frame of paramagnetic droplet movement by magnetic actuation. Panels a and b adapted with permission from ref 67. Copyright 2017 The American Physical Society. Panels c and d adapted with permission from ref 122. Copyright 2017 Royal Society of Chemistry. Panels e and f adapted with permission from ref 58. Copyright 2018 John Wiley and Sons.

detection,116 tissue engineering,116 chemical and biological assays,110 drug screening,109 biosensing,117 lab-on-a-chip systems,116 and point-of-care diagnostics.117 In the past years, great efforts were made to increase droplet mobility on these systems by introducing superhydrophobic surfaces. Superhydrophobic surfaces reduce droplet contact with the surface while in a Cassie−Baxter state, thus reducing the capillary forces pinning the droplet. Despite these efforts, droplets on superhydrophobic surfaces can fall into a Wenzel state and remain pinned.118 Recent approaches have applied a liquid-infused conformal layer that reduces the contact between the droplet and the surfaces and increases droplet mobility, while mitigating droplet pinning. Moreover, observations on LIS systems have demonstrated that droplets on nanotextured surfaces that are lubricated with oils display high contact angles (with the solid surface) and low contact angle hysteresis.53,119 In the past 2 years, different droplet actuation methods on LIS have emerged, thus allowing for multiplexed operations. One scheme to create droplet propulsion involves the generation of a Laplace pressure gradient. This can be produced either through the creation of a topographical or chemical gradient.120 Guan et al. used this technique to promote transportation and localization of droplets using V-shaped open channels.120 Luo et al. demonstrated a different approach for droplet manipulation

atures and mechanical damage, in addition to coming in contact with acids and bases.108 Droplet Manipulation. Traditionally, microfluidic chips have been fabricated with a series of channels to promote a plethora of functionality, including droplet generation, splitting, fusion, fission, sorting, and mixing.109,110 These devices are capable of doing this by manipulating the microscale physics of fluidic flow. Furthermore, emerging devices have deviated from using micromechanics alone and have integrated other actuation principles, such as electrostatics,111 acoustic waves,112 capillary action,113 and magnetism.114 Even though these devices show excellent functionality, they tend to require precise flow control and intricate designs, which makes them time-consuming, technology-intensive, and costly to fabricate.115 Channel-less microfluidic devices offer a promising alternative, allowing for discretized solvents and digital microfluidic templates to be used, which do not require an external pump to control the fluid dynamics. Instead, these microfluidic chips require higher ingenuity and more complex designs, which promotes easier electronic integration and higher volume control within the chips. One of the biggest hurdles scientists face with droplet microfluidics is droplet mobility. Droplet manipulation is crucial for the operation of the system and opens the door to a multitude of functionalities including cell 8529


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Figure 9. Other applications. (a) Water droplet coalescence. (b) Air-stable water noncoalescent droplets. (c) Experimental setup for electrical measurements of oil thickness between noncoalescing droplets. (d) Experimental setup for water sliding, pinning, and collection using a LIS film. (e) Schematic of flexed and rigid LIS films for water collection. (f) Images of flexed versus rigid water collection film. Unidirectional droplet movement in (g) superhydrophobic surfaces and (h) liquid-infused surfaces. (i) Controllable flow of differently colored water, based on the directionality of the microcilia. Panels a−c adapted with permission from ref 128. Copyright 2014 National Academy of Sciences of the United States of America. Panels d−f adapted from ref 131. Copyright 2017 American Chemical Society. Panels g−i adapted with permission from ref 132. Copyright 2017 John Wiley and Sons.

using acoustic waves (see Figure 8a,b). By utilizing a ZnO film on a Si substrate and a surface acoustic wave actuator, they demonstrated a highly efficient device with proper energy and momentum transfer, which reduced the threshold power required for droplet mobility (up to 85%) and achieved high droplet velocity.67 An alternative method for droplet actuation involves the use of electrowetting principles. In electrowetting, an electrical potential is introduced which changes the wetting behavior of a surface, usually from hydrophobic to hydrophilic. This phenomenon can be used to move droplets between electrodes, as well as combine and separate droplets to create chemical assays. In 2014, Hao et al. manipulated droplets via electrowetting on liquid-infused films and termed the technique EWOLF. This method achieved a fast droplet motion response, while retaining complete reversibility (changing the wetting behavior from hydrophobic to hydrophilic and back) over multiple cycles.57 Furthermore, they demonstrated that EWOLF could be implemented in a dynamic liquid lens for fast optical focusing.57 In 2017, Brabcova et al. achieved reversible low hysteresis control between a droplet and a liquid-infused film.121 In the same year, He et al. demonstrated a robust and reversible electrowetting property on LIS that

enabled manipulation of droplets under low-voltage settings, while maintaining the dielectric constant and layer thickness.122 These results can be seen in Figure 8c,d, which depict the change in contact angle with changes in the voltage applied to the system. In another study, Wang, Heng, and Jiang reported that an anisotropic conductive polymer coating on an indium tin oxide (ITO) substrate demonstrated a multitude of properties including self-healing, slipperiness, and electrowetting with different lubricant viscosities.105 Moreover, their experiments illustrated that increased silicone oil viscosity increases the critical self-healing thickness and increases the voltage required to move the droplet.105 Using this strategy, the electrowetting LIS systems can be optimized via the application of lubricants with the proper viscosities.105 Guo et al. have used conductive poly(3-hexylthiophene) (P3HT) fibers and silicone oil to control the direction of droplets, as well as the reversible control of conductive droplets under the application of voltage. P3HT are porous and directional fibers which exploit favorable anisotropic sliding angles, obtaining the reversible electrical control of a drop’s slide.123 In addition, Wang et al. reversibly controlled the movement of conductive droplets by also applying voltage. They fabricated different anisotropic surfaces with various scales of roughness via an interfacial freezing 8530


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wrinkled electrode with a lubricant layer with and without a fluorosilane intermediate coating.58 This liquid-infused surface reduced blood coagulation and demonstrated conductivity and electrochemical charge transfer for use in biosensing.58 In 2014, Boreyko et al. investigated the noncoalescing property of oilinfused surfaces. This was induced through the introduction of a lipid bilayer between the droplets. Importantly, these droplets did not require liquid immersion and provide an easy platform to study ion channels (Figure 9a−c).128 Moreover, this air-stable interface bilayer has the potential to be used in the sensing of airborne molecules.128 In previous sections of this report, the requirements for creating a stable liquid-infused surface via capillary forces brought by surface tension was described. In recent years, Irajizad et al. demonstrated a LIS composed of a ferrofluid.129 This device did not require any particular modification but spontaneously produced a texturized surface upon the introduction of a magnetic field, thus displaying remarkable icephobic properties.129 Recently, Wang et al., built on this idea to fabricate ferrofluid-containing liquid-infused porous surfaces.130 These devices displayed a multitude of functionality, from destroying biofilm formation to the manipulation of nonmagnetic colloidal particles and the modulation of friction.130 In another study, a wind-resistant water collection membrane was created with high elastic deformability.131 This membrane was capable of manipulating the degree of water coalescence and water sliding by tuning the membrane’s morphology in real time (see Figure 9d−f). In the area of microfluidics, Villegas et al. demonstrated a fabrication process using a liquid-infused 3D-printed mold to fabricate PDMS microchannels with smooth topology. This fabrication process provides a practical solution to overcome the surface roughness of 3D-printed molds to eliminate the need for timeconsuming and costly fabrication methods of photolithography while achieving smooth surface quality and smooth fluid profiles.72 Another application of LIS in the area of microfluidics is the fabrication of unidirectional wetting properties revealed by Cao et al.132 These surfaces not only enable anisotropic wettability but are capable of separating aqueous solutions into different channels in situ (Figure 9g−i).132 Furthermore, this technique provides a feasible template-free method for smart liquid manipulation systems.132 Badv et al. created biofunctional LIS interfaces that demonstrated the ability to capture specific analytes and repel background biological materials. Co-deposition of fluorosilane and aminosilane on a glass surface created an anchor for embedding functional biomolecules (i.e., anti-CD-34 antibody) within the lubricant layer to specifically capture endothelial cells from whole blood while repelling undesired cells and biomolecules.27 Using similar methods will be invaluable for the development of biosensors where limit of detection is often deteriorated by the background signals or signal loss caused through nonspecific adsorption or biofouling.133−136 In the following section, we will discuss how the development in LIS over the past decade is expected to impact the materials, devices, and processes we will use in the future.

technique, whereby lubricants where infused into the porous films. Using a p−n heterogeneous poly(3-hexylthiophene-2,5diyl)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/ PCBM) binary system and controlled freezing speed, mass ratio, and total solution concentration, P3HT and PCBM are fabricated into anisotropic films.124 A similar approach was researched by Han et al., whereby photoelectric principles were applied to change the wettability of zinc-oxide-coated ITO. These surfaces changed from hydrophobic to hydrophilic upon the introduction of light stimuli. Furthermore, these devices allow for the manipulation of droplet motion, capable of creating precise patterned shapes.125 Another instance of a reversible sliding-pinning LIS was introduced by Wang et al., where the phase-change properties of parafilm were exploited by manipulating the temperature, providing anisotropic sliding.126 These surfaces have shown different approaches to fabricate reversible droplet movement or pinning, providing a valuable platform for microfluidics, for medical or chemical applications, such as bioprinting, patterning, and biochip assay manipulation. Electric manipulation is generally dependent on different factors, including surface charge, pH, ion concentration, and temperature.127 When it comes to life sciences, many of these factors can also affect biological samples, limiting the efficacy of this approach when utilizing certain biomolecules. Magnetic manipulation provides an alternative approach for handling biomolecules. One of the proposed strategies involves capturing biomolecules onto paramagnetic micro/nanoparticles and manipulating them via an external magnetic field. Using these principles, scientists have investigated different technologies coupled with magnetic actuation on LISs. For example, Beyzavi et al. digitally manipulated droplets on a planar circuit board printed with microcoils to drive ferrofluid droplets on a reservoir lubricated with silicone oil.115 Similarly, Hosseini et al. illustrated a droplet manipulation method with a friction-less surface. This surface provided adequate charge transfer for use in electrochemical assays and electrochemical sensors.58 They achieved magnetic actuation by patterning a layer of permalloy onto a gold-coated polystyrene substrate. The application of an external magnetic field created a magnetic field gradient capable of long-range droplet actuation as shown in Figure 8e,f.58 Droplet microfluidic control is a promising area of research, although the fabrication and applications of these techniques requires further exploration and improvement. However, LISs have demonstrated superior droplet motion while operating at lower energies. Perhaps the biggest contribution of LISs for droplet mobility would be in overcoming lubricant loss problems. To overcome this issue, perhaps LISs with semiliquid lubricants or greases could be explored. Emerging and Nonconventional Applications of Liquid-Infused Surfaces. The applications mentioned above represent a small fraction of the grand plethora of applications explored by researchers. When it comes to analytical science, the detection of analytes in fluids with high sensitivity and accuracy is of great importance. A recent study conducted by Yang et al. has proven that a perfluorinated lubricant-infused glass substrate with a nanoporous Teflon membrane can provide a multitude of distinctive properties. For example, Yang et al. demonstrated that the fabricated surface was slippery and omniphobic and enhanced the signal measured (into the sub-femtomolar level) using Raman scattering by concentrating and delivering analyte to specific surface-enhanced Raman scattering detection areas.128 Recently, Hosseini et al. fabricated a conductive liquid-infused nanostructured electrode by combining a

FUTURE OUTLOOK Liquid-infused surfaces have made a tremendous impact on many research fields, and the real-life applications of this technology are seemingly endless. This trend of innovation will undoubtedly continue; however, there are several challenges that must be addressed in future studies to further refine this technology. It is critical to enhance lubricant stability in 8531


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ACKNOWLEDGMENTS This work was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, Ontario Early Researcher Award Grant and McMaster start-up funds to Tohid F Didar.

operando conditions, especially where the lubricant is in direct contact with air to be suitable for real-life applications. It is essential to develop multifunctional LISs that overcome challenging and sometimes contradictory requirements such as highly specific and sensitive capture/interaction paralleled with highly efficient repulsion. These surfaces could be achieved with the synthesis of new molecules capable of providing proper affinity to the lubricant and capture molecules. Novel applications in medicine are now achievable with this technology, examples include: liquid-infused devices for frictionless catherization and endoscopic procedures; contact lenses with enhanced lubricity and oxygen concentration to increase comfort and eye protection; or the coating of orthodontic retainers and dentures to prevent dental plaque formation. Other industries can also adopt this technology to enhance their productivity and device fabrication. For example, industrial settings may see a reduction in energy and higher throughput upon the addition of optimized LISs. In the marine field, underwater crafts and scuba gear can reduce drag and increase visibility of cameras through the incorporation of LIS technology. There are some challenges that need to be overcome, for instance, to apply a wide range of highperformance LISs developed in research laboratories to reallife systems, new fabrication methods need to be developed because either their performance is highly reliant on the materials properties of the underlying surface, or they rely on processes that are difficult and costly to scale. Other possible areas of innovation with LIS devices are surface patterning and interface engineering to integrate various functionalities into these systems. In the foreseeable future, we believe LISs could be paired with antibacterial agents, proteins, bioreceptors, or tools of synthetic biology to give these surfaces additional functions besides repellency. On a final note, it is important to create LISs that do not rely on fluorine chemistry because of their toxicity and potential harm to the environment. For example, long-chain fluorocarbons can be toxic and are hard to break down.137 Thus, the existence of these long-lived lubricants in the environment is a great concern, and further investigation is needed to develop alternative environmentally friendly lubricants with similar omniphobic properties. Overall, LIS technology has proven to be a promising state-of-the-art approach for overcoming complex engineering problems. Further exploration into this field will open new frontiers for research and industrial development of LIS over time.

VOCABULARY liquid-infused surfaces, a physicochemical coating where a liquid is tethered to a surface, providing a physical barrier resulting in low adhesion, adsorption, physical or chemical interactions with the surface; hydrophobicity, a physical property in which molecules lack an attraction to water, disturbing polar and hydrogen bonding, resulting in an entropy change, leading water to minimize in area; hydrophilicity, a physical property in which molecules are attracted to water through polar or hydrogen bonding, resulting in the spreading of water or a homogeneous miscible solution; oleophobicity, a physical property in which molecules lack an attraction to oils or other nonpolar solvents, resulting in the aggregation of oils; oleophilicity, a physical property in which molecules have a high affinity for oils, causing oils to spread or mix homogeneously when in solution; omniphobicity, a physical property where molecules repel “all”; in terms of surface wettability, it is the property to repel both polar and nonpolar solvents; examples include fluorocarbons and silicones; amphiphobicity, a physical property where molecules repel “both”; in terms of surface wettability, it is the property to repel both polar and nonpolar solvents; examples include fluorocarbons and silicones REFERENCES (1) Law, K.-Y.; Zhao, H. Surface Wetting : Characterization, Contact Angle, and Fundamentals; Springer International Publishing: Switzerland, 2016. (2) Jung, C. K.; Bae, I. S.; Lee, S. B.; Cho, J. H.; Shin, E. S.; Choi, S. C.; Boo, J. H. Development of Painting Technology Using Plasma Surface Technology for Automobile Parts. Thin Solid Films 2006, 506−507, 316−322. (3) Kolasinski, K. W. Surface Science : Foundations of Catalysis and Nanoscience, 3rd ed.; John Wiley and Sons: New York, 2012. (4) Xue, C.-H.; Ji, P.-T.; Zhang, P.; Li, Y.-R.; Jia, S.-T. Fabrication of Superhydrophobic and Superoleophilic Textiles for Oil−Water Separation. Appl. Surf. Sci. 2013, 284, 464−471. (5) Almeida, E.; Diamantino, T. C.; De Sousa, O. Marine Paints: The Particular Case of Antifouling Paints. Prog. Org. Coat. 2007, 59, 2−20. (6) Kreder, M. J.; Alvarenga, J.; Kim, P.; Aizenberg, J. Design of AntiIcing Surfaces: Smooth, Textured or Slippery? Nat. Rev. Mater. 2016, 1, 15003. (7) Epstein, A. K.; Wong, T.-S.; Belisle, R. A.; Boggs, E. M.; Aizenberg, J. Liquid-Infused Structured Surfaces with Exceptional Anti-Biofouling Performance. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 13182−13187. (8) Leslie, D. C.; Waterhouse, A.; Berthet, J. B.; Valentin, T. M.; Watters, A. L.; Jain, A.; Kim, P.; Hatton, B. D.; Nedder, A.; Donovan, K.; Super, E. H.; Howell, C.; Johnson, C. P.; Vu, T. L.; Bolgen, D. E.; Rifai, S.; Hansen, A. R.; Aizenberg, M.; Super, M.; Aizenberg, J.; et al. A Bioinspired Omniphobic Surface Coating on Medical Devices Prevents Thrombosis and Biofouling. Nat. Biotechnol. 2014, 32, 1134−1140. (9) MacCallum, N.; Howell, C.; Kim, P.; Sun, D.; Friedlander, R.; Ranisau, J.; Ahanotu, O.; Lin, J. J.; Vena, A.; Hatton, B.; Wong, T.-S.; Aizenberg, J. Liquid-Infused Silicone As a Biofouling-Free Medical Material. ACS Biomater. Sci. Eng. 2015, 1, 43−51. (10) Kratochvil, M. J.; Welsh, M. A.; Manna, U.; Ortiz, B. J.; Blackwell, H. E.; Lynn, D. M. Slippery Liquid-Infused Porous Surfaces That Prevent Bacterial Surface Fouling and Inhibit Virulence Phenotypes in Surrounding Planktonic Cells. ACS Infect. Dis. 2016, 2, 509−517.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b04129. Summary of materials and lubricants used for different liquid-infused surfaces (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Martin Villegas: 0000-0003-0013-0975 Leyla Soleymani: 0000-0003-4915-2999 Tohid F. Didar: 0000-0002-8757-8002 Notes

The authors declare no competing financial interest. 8532


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