Recent Developments in Slippery Liquid-Infused Porous Surface Coatings for Biomedical Applications.

Slippery liquid-infused porous surface (SLIPS), inspired by the Nepenthes pitcher plant, exhibits excellent performances as it has a smooth surface and extremely low contact angle hysteresis. Biomimetic SLIPS attracts considerable attention from the researchers for different applications in self-cleaning, anti-icing, anticorrosion, antibacteria, antithrombotic, and other fields. Hence, SLIPS has shown promise for applications across both the biomedical and industrial fields. However, the manufacturing of SLIPS with strong bonding ability to different substrates and powerful liquid locking performance remains highly challenging. In this review, a comprehensive overview of research on SLIPS for medical applications is conducted, and the design parameters and common fabrication methods of such surfaces are summarized. The discussion extends to the mechanisms of interaction between microbes, cells, proteins, and the liquid layer, highlighting the typical antifouling applications of SLIPS. Furthermore, it identifies the potential of utilizing the controllable factors provided by SLIPS to develop innovative materials and devices aimed at enhancing human health.

Infusing Silicone and Camellia Seed Oils into Micro-/Nanostructures for Developing Novel Anti-Icing/Frosting Surfaces for Food Freezing Applications.

Undesired ice/frost formation and accretion often occur on food freezing facility surfaces, lowering freezing efficiency. In the current study, two slippery liquid-infused porous surfaces (SLIPS) were fabricated by spraying hexadecyltrimethoxysilane (HDTMS) and stearic acid (SA)-modified SiO2 nanoparticles (NPs) suspensions, separately onto aluminum (Al) substrates coated with epoxy resin to obtain two superhydrophobic surfaces (SHS), and then infusing food-safe silicone and camellia seed oils into the SHS, respectively, achieving anti-frosting/icing performance. In comparison with bare Al, SLIPS not only exhibited excellent frost resistance and defrost properties but also showed ice adhesion strength much lower than that of SHS. In addition, pork and potato were frozen on SLIPS, showing an extremely low adhesion strength of <10 kPa, and after 10 icing/deicing cycles, the final ice adhesion strength of 29.07 kPa was still much lower than that of SHS (112.13 kPa). Therefore, the SLIPS showed great potential for developing into robust anti-icing/frosting materials for the freezing industry.

Inhibiting host-protein deposition on urinary catheters reduces associated urinary tract infections.

Microbial adhesion to medical devices is common for hospital-acquired infections, particularly for urinary catheters. If not properly treated these infections cause complications and exacerbate antimicrobial resistance. Catheter use elicits bladder inflammation, releasing host serum proteins, including fibrinogen (Fg), into the bladder, which deposit on the urinary catheter. Enterococcus faecalis uses Fg as a scaffold to bind and persist in the bladder despite antibiotic treatments. Inhibition of Fg-pathogen interaction significantly reduces infection. Here, we show deposited Fg is advantageous for uropathogens E. faecalis, Escherichia coli, Pseudomonas aeruginosa, K. pneumoniae, A. baumannii, and C. albicans, suggesting that targeting catheter protein deposition may reduce colonization creating an effective intervention for catheter-associated urinary tract infections (CAUTIs). In a mouse model of CAUTI, host-protein deposition was reduced, using liquid-infused silicone catheters, resulting in decreased colonization on catheters, in bladders, and dissemination in vivo. Furthermore, proteomics revealed a significant decrease in deposition of host-secreted proteins on liquid-infused catheter surfaces. Our findings suggest targeting microbial-binding scaffolds may be an effective antibiotic-sparing intervention for use against CAUTIs and other medical device infections.

The mechanisms of anti-icing properties degradation for slippery liquid-infused porous surfaces under shear stresses.

HYPOTHESIS: Loss of anti-icing properties of slippery liquid-infused porous surfaces (SLIPS) in conditions of repetitive shear stresses is the intrinsic process related to peculiarities of SLIPS structure. EXPERIMENTS: The study of the evolution of the ice adhesion strength to superhydrophobic surfaces (SHS) and SLIPS during repetitive icing/de-icing cycles measured by a centrifugal method was supplemented with the estimation of change in capillary pressure inside the pores, and SEM analysis of the effect of multiple ice detachments on surface morphology. FINDINGS: Obtained data indicated that although for freshly prepared SLIPS, the ice shear adhesion strength at -25 °C was several times lower than for SHS, repetitive icing-deicing cycles resulted in dramatic SLIPS degradation. In contrast, SHS showed weak degradation at least during 50 cycles. Additional to the depletion of an impregnating oil layer, other mechanisms of SLIPS degradation were hypothesized and tested. It was shown that lower capillary pressure required to displace air by water from the surface texture for SLIPSs compared to SHSs resulted in deeper water/ice penetration inside the grooves. The accelerated destruction of the mechanical texture caused by the Rehbinder effect constitutes another mechanism of SLIPSs degradation.

Slippery Liquid-Infused Porous Surfaces on Aluminum for Corrosion Protection with Improved Self-Healing Ability.

Slippery liquid-infused porous surfaces (SLIPSs) can be formed by impregnating lubricants in porous surfaces with low surface energy. In this study, SLIPSs have been obtained on practically important aluminum with a porous anodic alumina layer by impregnating lubricants containing organic additives. The additive-containing lubricants change the surface slippery even without prior organic coating of the porous alumina surface. The additive-containing SLIPSs reveal a low water sliding angle of <5° and markedly improved corrosion resistance in an acetic acid solution containing chloride. The SLIPSs are formed by the in situ adsorption of the organic additives on the porous alumina surface. The scratched defects induce corrosion of the organic coating-type SLIPSs, whereas the additive-containing SLIPSs sustain high corrosion resistance even after introducing scratch defects. The adsorption of the organic additive in lubricants and refilling of the lubricant are responsible for the self-healing of the corrosion resistance. Thus, the additive-containing SLIPSs are promising self-healing corrosion-resistant surfaces.

Design novel three-dimensional network nanostructure for lubricant infused on titanium alloys towards long-term anti-fouling.

Titanium alloys, recognized as a marine material with great potential, are currently facing serious biofouling problems, which greatly limits its application range. To improve the antifouling performance of titanium alloys, three unique surface of three-dimensional network, grass-like and linear nanostructures were obtained on titanium alloys via hydrothermal treatment in this work. Further, slippery liquid-infused porous surfaces (SLIPSs) were fabricated on titanium alloys via infusing PFPE lubricant into these nanostructures. Water contact angles and sliding angles of SLIPSs were measured to evaluate the effect of nanostructures on the stability of PFPE lubricant layer. Anti-fouling capability of SLIPSs were investigated by quantifying the cells of chlorella and phaeodactylum tricornutum (P. tricornutum)adhered to titanium alloys. The results shows that all the SLIPSs exhibited remarkable inhibition capacity for the settlement of chlorella and P. tricornutum. Among them, the SLIPS with three-dimensional network nanostructure displayed the longest-term anti-fouling performance, and its reduction rate of P. tricornutum and chlorella reaching 77.2 % and 84.5 % after being cultivated for 21 days, respectively, indicating that there existed a positive correlation between the stability of lubricant layer in the artificial seawater and the antifouling effect.

Designing Splicing Digital Microfluidics Chips Based on Polytetrafluoroethylene Membrane.

As a laboratory-on-a-chip application tool, digital microfluidics (DMF) technology is widely used in DNA-based applications, clinical diagnosis, chemical synthesis, and other fields. Additional components (such as heaters, centrifuges, mixers, etc.) are required in practical applications on DMF devices. In this paper, a DMF chip interconnection method based on electrowetting-on-dielectric (EWOD) was proposed. An open modified slippery liquid-infused porous surface (SLIPS) membrane was used as the dielectric-hydrophobic layer material, which consisted of polytetrafluoroethylene (PTFE) membrane and silicone oil. Indium tin oxide (ITO) glass was used to manufacture the DMF chip. In order to test the relationship between the splicing gap and droplet moving, the effect of the different electrodes on/off time on the minimum driving voltage when the droplet crossed a splicing gap was investigated. Then, the effects of splicing gaps of different widths, splicing heights, and electrode misalignments were investigated, respectively. The experimental results showed that a driving voltage of 119 V was required for a droplet to cross a splicing gap width of 300 µm when the droplet volume was 10 µL and the electrode on/off time was 600 ms. At the same time, the droplet could climb a height difference of 150 µm with 145 V, and 141 V was required when the electrode misalignment was 1000 µm. Finally, the minimum voltage was not obviously changed, when the same volume droplet with different aqueous solutions crossed the splicing gap, and the droplet could cross different chip types. These splicing solutions show high potential for simultaneous detection of multiple components in human body fluids.

Liquid-Infused Structured Titanium Surfaces: Antiadhesive Mechanism to Repel Streptococcus oralis Biofilms.

To combat implant-associated infections, there is a need for novel materials which effectively inhibit bacterial biofilm formation. In the present study, the antiadhesive properties of titanium surface functionalization based on the "slippery liquid-infused porous surfaces" (SLIPS) principle were demonstrated and the underlying mechanism was analyzed. The immobilized liquid layer was stable over 13 days of continuous flow in an oral flow chamber system. With increasing flow rates, the surface exhibited a significant reduction in attached biofilm of both the oral initial colonizer  Streptococcus oralis and an oral multispecies biofilm composed of S. oralis, Actinomyces naeslundii, Veillonella dispar, and Porphyromonas gingivalis. Using single cell force spectroscopy, reduced S. oralis adhesion forces on the lubricant layer could be measured. Gene expression patterns in biofilms on SLIPS, on control surfaces, and expression patterns of planktonic cultures were also compared. For this purpose, the genome of S. oralis strain ATCC 9811 was sequenced using PacBio Sequel technology. Even though biofilm cells showed clear changes in gene expression compared to planktonic cells, no differences could be detected between bacteria on SLIPS and on control surfaces. Therefore, it can be concluded that the ability of liquid-infused titanium to repel S. oralis biofilms is mainly due to weakened bacterial adhesion to the underlying liquid interface.

Bioinspired Surfaces with Superwettability for Anti-Icing and Ice-Phobic Application: Concept, Mechanism, and Design.

Ice accumulation poses a series of severe issues in daily life. Inspired by the nature, superwettability surfaces have attracted great interests from fundamental research to anti-icing and ice-phobic applications. Here, recently published literature about the mechanism of ice prevention is reviewed, with a focus on the anti-icing and ice-phobic mechanisms, encompassing the behavior of condensate microdrops on the surface, wetting, ice nucleation, and freezing. Then, a detailed account of the innovative fabrication and fundamental research of anti-icing materials with special wettability is summarized with a focus on recent progresses including low-surface energy coatings and liquid-infused layered coatings. Finally, special attention is paid to a discussion about advantages and disadvantages of the technologies, as well as factors that affect the anti-icing and ice-phobic efficiency. Outlooks and the challenges for future development of the anti-icing and ice-phobic technology are presented and discussed.

Development of Laser-Structured Liquid-Infused Titanium with Strong Biofilm-Repellent Properties.

Medical implants are commonly used in modern medicine but still harbor the risk of microbial infections caused by bacterial biofilms. As their retrospective treatment is difficult, there is a need for biomedical materials that inhibit bacterial colonization from the start without using antibacterial agents, as these can promote resistance development. The promising concept of slippery liquid-infused porous surfaces (SLIPS) possesses enormous potential for this purpose. In the present study, this principle was applied to titanium, a common material in implantology, and its biofilm-repellent properties were demonstrated. To simplify prospective approval of the medical device and to avoid chemical contamination, surface structuring was performed by ultrashort pulsed laser ablation. Four different structures (hierarchical micro- and nanosized spikes, microsized grooves, nanosized ripples, and unstructured surfaces) and five infusing perfluoropolyethers of different viscosities were screened; the best results were obtained with the biomimetic, hierarchical spike structure combined with lubricants of medium viscosities (20-60 cSt at 37 °C, 143 AZ, and GPL 104). The surfaces exhibited extremely low contact angle hysteresis, as is typical for liquid-infused materials and a reliable 100-fold reduction of human oral pathogen Streptococcus oralis biofilms. This characteristic was maintained after exposure to shear forces and gravity. The titanium SLIPS also inhibited adherence of human fibroblasts and osteoblasts. Toxicity tests supported the explanation that solely the surface's repellent properties are responsible for the vigorous prevention of the adhesion of bacteria and cells. This use of physically structured and liquid-infused titanium to avoid bioadhesion should support the prevention of bacterial implant-associated infections without the use of antibacterial agents.

Biocompatible slippery fluid-infused films composed of chitosan and alginate via layer-by-layer self-assembly and their antithrombogenicity.

Antifouling super-repellent surfaces inspired by Nepenthes, the pitcher plant, were designed and named slippery liquid-infused porous surfaces (SLIPS). These surfaces repel various simple and complex liquids including water and blood by maintaining a low sliding angle. Previous studies have reported the development of fluorinated SLIPS that are not biocompatible. Here, we fabricated fluid-infused films composed of biodegradable materials and a biocompatible lubricant liquid. The film was constructed using a combination of electrostatic interactions between chitosan and alginate and hydrogen-bonding between alginate and polyvinylpyrrolidone (PVPON) via the layer-by-layer self-assembly method. After chitosan and alginate were cross-linked, the PVPON was removed by increasing the pH to generate porosity from the deconstruction of the hydrogen-bonding. The porous underlayer was hydrophobized and covered by biocompatible almond oil. Blood easily flowed over this biodegradable and biocompatible SLIPS without leaving stains on the surface, and the material is environmentally durable, has a high transmittance of about 90%, and is antithrombogenic. The results of this study suggest that this SLIPS may facilitate the creation of nonfouling medical devices through a low-cost, eco-friendly, and simple process.