High Adhesion Research Articles

Impact of Surface Pretreatment on the Corrosion Resistance and Adhesion of Thin Film Coating on SS316L Bipolar Plates for Proton-Exchange Membrane Fuel Cell Applications.

Coated SS316L is a potential alternative to the graphite bipolar plates (BPPs) used in proton-exchange membrane fuel cells (PEMFCs) owing to their low manufacturing cost and machinability. Due to their susceptibility to corrosion and passivation, which increases PEMFC ohmic resistance, protective and conductive coatings on SS316L have been developed. However, coating adhesion is one of the challenges in the harsh acidic environment of PEMFCs, affecting the performance and durability of BPPs. This study compares mechanical polishing and the frequently adopted chemical etchants for SS316L: Adler's, V2A, and Carpenter's etchant with different etching durations and their impact on the wettability, adhesion, and corrosion resistance of a Nb-coated SS316L substrate. Contact angle measurements and laser microscopy revealed that all etching treatments increased the hydrophobicity and surface roughness of SS316L substrates. Ex situ potentiodynamic and potentiostatic polarization tests and interfacial contact resistance analysis revealed high corrosion resistance, interfacial conductivity, and adhesion of the Nb-coated SS316L substrate pretreated with V2A (7 min) and Adler's (3 min) etchant. Increased hydrophobicity (contact angle = 101°) and surface roughness (Ra = 74 nm) achieved using V2A etchant led to the lowest corrosion rate (3.3 µA.cm-2) and interfacial resistance (15.4 mΩ.cm2). This study established pretreatment with V2A etchant (a solution of HNO3, HCl, and DI water (1:9:23 mole ratio)) as a promising approach for improving the longevity, electrochemical stability, and efficiency of the coated SS316L BPPs for PEMFC application.

Compression molding of ethylene–octene copolymer‐toughened polypropylene/graphene surfaces with actively controlled shape‐morphing microarchitectures induced by friction and wear

AbstractSuperhydrophobic microarchitectured polyolefin surfaces are currently used intensively in various industrial applications. However, the deployment of products made of these materials into practical application is typically constrained by their inferior dry sliding behaviors, which stem from their limited strength and toughness. To obtain reinforced and toughened superhydrophobic microstructured surfaces that can be easy to demold and overcome friction in the workplace, elastomeric ethylene–octene copolymer (POE) and rigid graphene (GP) were introduced into the polypropylene (PP) matrix to prepare microstructured PP/POE/GP surfaces by compression molding. The elongation at break is significantly improved by 2000% and reached up to 520.33%. The contact angle (CA) of the microstructured PP/POE/GP surface increases to 154.4°. They exhibit superhydrophobic and low adhesion characteristic, that is, lotus effect. The enhanced toughness of PP/POE/GP composites reduces wear debris and damage to microarchitecture during the abrasion process. Even after the microstructured PP/POE/GP surfaces were worn after a distance length of 3000 mm, they still exhibited superhydrophobic, but high adhesion characteristic, that is, petal effect. The controlled shape‐morphing microarchitectures formed on the microstructured PP/POE/GP surface abraded after 1000 mm, possessing wetting stability during droplet impacting.Highlights The elongation at break of composites was improved by 2000% through adding POE. The composite microstructure deforms to consume energy during abrasion and POE reinforces this energy dissipation process. POE improves fracture toughness and wetting stability of composites.

Size-dependent ability of AtaA to immobilize cells in Acinetobacter sp. Tol 5

Microbial cells serve as efficient and environmentally friendly biocatalysts, but their stability and reusability in practical applications must often be improved through immobilization. Acinetobacter sp. Tol 5 shows high adhesiveness to materials due to its large cell surface protein AtaA, which consists of 3630 amino acids (aa). Previously, we developed a method for immobilizing bacteria using AtaA. Herein, we investigated the cell immobilization ability of in-frame deletion (IFD) mutants of AtaA with different sizes in Tol 5. Mini-AtaA, which consists of 775 aa and is functional in Escherichia coli, was produced and present on the cell surface; however, mini-AtaA showed no immobilization ability in Tol 5. A cell immobilization assay was performed with cells expressing 16 IFD mutants of AtaA with different sizes, revealing that a length of at least 1417 aa was required for the sufficient immobilization of Tol 5 cells; thus, the minimum length needed to achieve the adhesive function of AtaA varies among bacterial species. The constructed mutant library of AtaA ranging from 3630 to 775 aa will allow researchers to quickly and easily explore the optimal size of AtaA, even for bacteria newly introduced to AtaA.

In situ multilevel covalent crosslinking binder with high ionic conductivity for high-performance Si anodes

Silicon (Si) anode has emerged as a promising material for high-energy-density lithium-ion batteries (LIBs), thanks to its high theoretical capacity. However, the commercial application of Si anodes is hindered by inadequate cycling performance and poor interfacial stability, stemming from unfavorable stress conditions. In this study, a multiple crosslinking binder with high ionic conductivity is proposed combining high mechanical strength and high adhesion to current collector and active Si particles by coupling poly(acrylic acid), polyvinyl alcohol, and polyethylene glycol. The designed polymer binder not only efficiently dissipates the inner stress with its multiple crosslinked networks, but also establishes a fast lithium-ion conduction pathway to facilitate the electrode reaction kinetic. The resultant Si anode using the designed binder exhibits significantly improved cycling stability and superior rate performance. This work presents a straightforward yet highly effective method to alleviate the detrimental stress incurred by high-capacity anodes in high-energy LIBs.

Frictional Properties of Biomimetic Micro-Hexagonal-Textured Surfaces Interacting with Soft Counterfaces under Dry and Wet Conditions.

Biomimetic micro-hexagonal-textured surfaces have sparked interest for their application in fields that demand high friction and adhesion, such as micro-robotics and biomedicine. Despite extensive research conducted on this specific microstructure, its friction behavior against soft counterfaces remains a topic that has not been fully investigated yet. This study examines how micro-hexagon textures behave when they come into contact with engineered and biological materials like gelatin and chicken skin in dry and wet conditions. The results show clearly that under dry contact conditions, flat surfaces generate higher friction compared to hexagon micropattern surfaces. Under wet conditions, hexagon micropattern surfaces generate higher friction compared to flat surfaces. In wet conditions specifically, the static coefficient of friction is up to 13 times greater than that of a flat specimen against glass, up to 11 times greater against gelatin, and up to 6 times greater against chicken skin. For the dynamic coefficient of friction, the patterned surface demonstrates a maximum increase by a factor of 28 against glass, 11 against gelatin, and 5 against chicken skin. These results further develop our knowledge of these hexagon micropattern surfaces and pave the way for their utilization in future technological advancements in which soft and wet counterfaces are to be considered, such as in biomedical applications that can benefit from increased friction in wet conditions for better control and stability.

Effect of the mechanical densification process in wood material on the surface adhesion strength of varnishes

This research aimed to determine the impact of the mechanical densifying process of wood material on the varnish surface adhesion strength. Specimens from black pine (Pinus nigra) and Uludag fir (Abies bornmuelleriana Mattf.) were subjected to densification in a hydraulic press at 140 °C to the extent of 25% and 50% in the radial direction. While densification increased the surface adhesion strength of the varnish layer in black pine, the value decreased in fir. Regarding the interaction between densification ratio, surface treatment, and wood type, the highest surface adhesion strength of the varnish layer was found in black pine + unsanded surface + 25% densification, and the lowest was in Uludag fir + unsanded surface + 50% densification. It can be stated that the densification process creates high adhesion values for the polyurethane varnish in the black pine wood type. The sanding process has an intensifying effect on these values, and the products that were obtained from the polyurethane varnished samples do not require sanding. Considering these situations can provide significant advantages in projects with wood materials subjected to the densification process.

Improved Liver Intravital Microscopic Imaging Using a Film-Assisted Stabilization Method.

Intravital microscopy (IVM) is a valuable method for biomedical characterization of dynamic processes, which has been applied to many fields such as neuroscience, oncology, and immunology. During IVM, vibration suppression is a major challenge due to the inevitable respiration and heartbeat from live animals. In this study, taking liver IVM as an example, we have unraveled the vibration inhibition effect of liquid bridges by studying the friction characteristics of a moist surface on the mouse liver. We confirmed the presence of liquid bridges on the liver through fluorescence imaging, which can provide microscale and nondestructive liquid connections between adjacent surfaces. Liquid bridges were constructed to sufficiently stabilize the liver after abdominal dissection by covering it with a polymer film, taking advantage of the high adhesion properties of liquid bridges. We further prototyped a microscope-integrated vibration-damping device with adjustable film tension to simplify the sample preparation procedure, which remarkably decreased the liver vibration. In practical application scenarios, we observed the process of liposome phagocytosis by liver Kupffer cells with significantly improved image and video quality. Collectively, our method not only provided a feasible solution to vibration suppression in the field of IVM, but also has the potential to be applied to vibration damping of precision instruments or other fields that require nondestructive ″soft″ vibration damping.

Effect of Surface Treatments Applied to 3D Printed Permanent Resins on Shear Bond Strength.

3D-printed permanent resins have recently been introduced to produce permanent restorations. The aim of this study is to evaluate the effect of surface treatment methods on the shear-bond strength (SBS) between 3D-printed permanent resins and adhesive cement. In this study, samples were produced using digital light projection (DLP) and stereolithography (SLA) 3D printers with two permanent resins (Crowntec, Saremco and Permanent Crown, Formlabs) in accordance with manufacturer guidelines. The samples were separated into three groups: sandblasting, hydrofluoric acid and no surface treatment. The surface profile (Ra, Sa) of the samples was examined with a confocal microscope (Smartproof 5, Zeiss). Then, a self-adhesive resin cement was applied to the samples in a transparent mold (2.38 mm diameter) in accordance with ISO 29022:2013. A universal testing machine was used to perform SBS test. A stereomicroscope was used to analyze the different types of fractures. A two-way analysis of variance (ANOVA) test was used to assess the data (p<0.05). The samples with sandblasting applied to the surface showed higher surface roughness values than the samples with hydrofluoric acid (p<0.001). 3D printer technology and surface treatment methods affected SBS values (p<0.001). Sandblasting groups higher SBS values were than in the hydrofluoric acid group (p<0.001). Sandblasting revealed cohesion fractures, which indicated a stronger bond, while hydrofluoric acid displayed adhesive and mix fractures. When sandblasting was applied to the surface of the samples prepared using permanent resins, higher adhesion was achieved with adhesive cement. Key words:3D printing, Shear bond strength, Sandblasting, Stereolithography.

High Adhesion of Hot-Dip Galvanized Steel and Silicone Adhesive through Si Amorphous Film

High Adhesion of Hot-Dip Galvanized Steel and Silicone Adhesive through Si Amorphous Film

A new DEM calibration method for wet and stick materials based on the BP neural network

The assessment of flow characteristics and the Discrete Element Method (DEM) calibrations for Wet and Sticky Materials (WSMs) are significantly demanded by the mining and transportation industries. However, the angle of repose (AoR), as a common DEM calibration method, has been facing challenges in adopting for WSMs. Due to the relatively high adhesion and cohesion of WSMs, it is difficult to form repeatable material heaps, resulting in inaccurate measurements of the AoRs. Therefore, this study aims to calibrate DEM parameters for WSMs under various conditions, utilizing a modified AoR test method and the Back Propagation-Neural Network (BP-NN). In the modified AoR test method, WSMs are discharged from a funnel using a screw feeder and fall closely against a vertical baffle to form a heap in a constrained direction. The new method combines the features of the traditional AoR test and the slump plane test to accurately measure the AoRs for different WSMs, including wet white sand, bulk coal, and corn powders. Test results are analysed to show the advantage of the modified AoR test. Furthermore, an innovative DEM calibration method based on the BP-NN is developed. By training the BP-NN with the database established according to the relationship between the measured and simulated AoRs, the DEM calibration parameters can be predicted rapidly for WSMs with different particle sizes and moisture contents. Validation is conducted for the wet white sand, from which the deviations and relative mean deviations between the experiment AoRs (Ae) and the calibration AoRs (Ac) are less than 2°and 4 % respectively, indicating the capability of the developed method for WSMs DEM calibrations.

Breathable, Adhesive, and Biomimetic Skin-Like Super Tattoo.

Electronic tattoo, capable of imperceivably acquiring bio-electrical signals from the body, is broadly applied in healthcare and human-machine interface. Tattoo substrate, the foundation of electronic tattoo, is expected to be mechanically mimetic to skin, adhesive, and breathable, and yet remains highly challenging to achieve. Herein, the study mimics human skin and design a breathable, adhesive, and mechanically skin-like super tattoo substrate based on an ultra-thin film (≈2µm). Similar to skin, super tattoo demonstrates strain-adaptive stiffening properties with high tear energy (5.4 kJ·m-2) and toughness (1.3 MJ·m-3). Superior to skin, it exhibits high adhesion, ionic conductivity, and permeability. A variety of conductive electrodes can be processed on it, showing the universality toward an ideal platform for electronic tattoo with stable and low contact impedance. Super tattoo-based electrodes can imperceivably and accurately monitor weak electromyography (EMG) of swallowing on the junction, providing effective guidance for rehabilitation training of dysphagia.

Solvation Chemistry Enables Eutectogels with Ultrahigh Autofluorescence, Toughness, and Adhesion under Extreme Environments

AbstractCompared to conventional gels, eutectogels encompassing deep eutectic solvents (DESs) circumvent the shortboards including poor temperature resistance, high cost, and undesired toxicity. However, existing eutectogels suffer weak intra‐ and interchain interactions, which culminates in vanishing autofluorescence, weak mechanic and feeble adhesion. Although myriad strategies have been proposed to boost the polymer/polymer interactions within gels, the processes are time‐consuming, which are not suitable for mass production or emergent deployment. Herein, to deal with it, a facile solvation chemistry is leveraged to build dynamic rigid network via incorporating active polymer backbones and DESs. Within these eutectogels, DESs play a magnet‐like role, and physically cluster polymer chains together via multiple noncovalent forces, thus intensifying polymer/polymer and DESs/polymer interactions. This solvation network bestows rare autofluorescence and unprecedentedly high strength, toughness, and adhesion among reported analogues, even in harsh conditions. Moreover, the mechanism of solvation chemistry is fully scrutinized for the first time. It is believed that this facile strategy can be easily expanded to the orchestrations of other soft materials.

Transparent, robust, and anti-fingerprint silicone coating with a three-dimensional cross-linked network enabled by hydrosilylation reaction

Silicone coatings with anti-smudge, anti-fingerprint, abrasion resistance, and ultraviolet (UV) resistance properties are increasingly demanded for touch screens as the development of artificial intelligence and human-computer interaction interfaces. Herein, a transparent, robust, and self-cleaning silicone coating was developed by incorporating cage-like structures and long carbon chains into a 3D cross-linked network via the rational combination of polymethylhydrosiloxane (PMHS), PSS-Octavinyl substituted (VPOSS), lauryl acrylate (LA), and 1,2-epoxy-4-vinylcyclohexane (EVCH) through hydrosilylation reaction with the presence of Karstedt catalysts. After introducing sealed diphenyliodonium phosphate (I-200), the silicone coating (PVLE-I-200) demonstrated excellent pencil hardness, anti-smudge, and abrasion resistance, and its average transmittance of visible light reached about 92 %. Owing to the low surface energy carbon chains, the coating showed amphiphobic properties and excellent anti-fingerprint property. The binding between cyclohexyl epoxy and glass substrate contributed to the high adhesion of the coating, which remained nearly intact and slightly reduced surface wettability after 500 friction cycles of sandpaper. In addition, the PVLE-I-200 coating demonstrated UV resistance which could withstand 200 h of UV irradiation attributed to the cage-like structure of VPOSS. The functional silicone coating provides insights into the development of environmentally friendly coatings for touch screens with high transparency, abrasion resistance, anti-smudge, and anti-fingerprint properties.

Tailored extracellular matrix-mimetic coating facilitates reendothelialization and tissue healing of cardiac occluders

Minimally invasive transcatheter interventional therapy utilizing cardiac occluders represents the primary approach for addressing congenital heart defects and left atrial appendage (LAA) thrombosis. However, incomplete endothelialization and delayed tissue healing after occluder implantation collectively compromise clinical efficacy. In this study, we have customized a recombinant humanized collagen type I (rhCol I) and developed an rhCol I-based extracellular matrix (ECM)-mimetic coating. The innovative coating integrates metal-phenolic networks with anticoagulation and anti-inflammatory functions as a weak cross-linker, combining them with specifically engineered rhCol I that exhibits high cell adhesion activity and elicits a low inflammatory response. The amalgamation, driven by multiple forces, effectively serves to functionalize implantable materials, thereby responding positively to the microenvironment following occluder implantation. Experimental findings substantiate the coating's ability to sustain a prolonged anticoagulant effect, enhance the functionality of endothelial cells and cardiomyocyte, and modulate inflammatory responses by polarizing inflammatory cells into an anti-inflammatory phenotype. Notably, occluder implantation in a canine model confirms that the coating expedites reendothelialization process and promotes tissue healing. Collectively, this tailored ECM-mimetic coating presents a promising surface modification strategy for improving the clinical efficacy of cardiac occluders.

Longevous ionogels with high strength, conductivity, adhesion and thermoplasticity

The longevity of flexible electronics substantially influences their viability for practical use. At present, the service life of pliable devices utilizing conductive gels is frequently compromised due to factors inclusive but not limited to solvent loss. Taking cues from the steadfast structure of human tissues, we hereby delineate a solvent-assisted strategy to fabricate ionogels, predicated on a supramolecular network of hydrogen bonds. Upon subjecting these ionogels to realistic operational environments, it has been empirically ascertained that they maintain a steadfast mechanical and electrical consistency for a minimum duration of 22 months. In terms of dynamic stretchability, they effortlessly endure one million stretch cycles, even when faced with a notch. The induction of a supramolecular structure within the ionogel not only confers exceptional mechanical attributes but also simultaneously enhances conductivity, a feat paralleled by few recent explorations. Furthermore, these ionogels boast of superior anti-freezing and adhesive characteristics, including a toughness extending up to 17.96 MJ/m3 at an ultra-low temperature of −50 °C, and an adhesive capacity permitting movement and bearing of items weighing over 10,000 times its own weight, the lap-shear strength can reach 30.2 kPa. Intriguingly, these gels demonstrate thermoplastic behavior, which facilitates their easy reshaping into diverse forms. Such compelling attributes amplify the potential of supramolecular ionogels, positioning them as formidable contenders for flexible electronics in stringent and challenging scenarios.

Soybean protein “mechanically interlocked” bonding of polysaccharide and MXene to improve the strength and toughness of biomass adhesive

Simultaneously improving the strength and toughness of soybean protein adhesives is a huge challenge. The multi-scale enhancement strategy proposed in this study introduced MXene nanosheets and oxidized sodium alginate (OSA) to improve mechanical properties. The polymer network framework with high crosslink density dissipates energy by restricting the slip of the SPI molecular chains, and MXene act as stress dispersers in the polymer network in order to solve the challenge of the strength-toughness balance. The prepared biomass-based adhesives with high adhesion (dry/wet shear strength 1.93 MPa/1.33 MPa), high toughness (adhesion work 810.9 mJ), high thermal stability, high residual rate (92.53 %), low moisture absorption rate (11.18 %), and long storage time (12 d). Additionally, the application of SOM-5 adhesive reduces the burden of petrochemical resource extraction for adhesive production and contributes to the sustainable development of the wood industry. Most importantly, it presents an adoptable strategy for resolving the tension between strength and toughness of composite materials.

Synthesis of hemostatic aerogel of TEMPO-oxidized cellulose nanofibers/collagen/chitosan and in vivo/vitro evaluation

The treatment of internal hemorrhage remains challenging due to the current limited antibacterial capability, hemostatic efficacy, and biocompatibility of hemostatic materials. The TEMPO-oxidized cellulose nanofibers/collagen/chitosan (TCNF/COL/CS) hemostatic aerogel was developed in this work by physically encasing COL in a sandwich structure and electrostatically self-assembling polyanionic TCNF with polycationic CS. In vitro coagulation experiments revealed the favorable procoagulant properties of TCNF/COL/CS along with high adhesion to erythrocytes and platelets. TCNF/COL/CS significantly increased the hemostatic efficacy by 59.8 % and decreased blood loss by 62.2 % in the liver injury model when compared to Surgicel®, the most frequently used hemostatic material. Furthermore, it demonstrated outstanding biodegradability both in vitro and in vivo, and a substantial increase in resistance (96.8 % against E. coli and 95.4 % against S. aureus) compared to TCNF. The significant hemostatic and biodegradable characteristics of TCNF/COL/CS can be ascribed to its interconnected porous structure, increased porosity, and efficient water absorption, along with the synergistic effect of the three constituents. The TCNF/COL/CS aerogel shows significant potential to control internal bleeding. A novel plant-derived nanocellulose composite aerogel has been described here for the first time; it has outstanding antibacterial characteristics, higher biocompatibility, and outstanding hemostatic characteristics in vivo.

Development of an in-situ forming collagen-based hydrogel as a regenerative bioadhesive for corneal perforations

Corneal injuries play a significant role in global visual impairment, underscoring the demand for innovative biomaterials with specific attributes such as adhesion, cohesion, and regenerative potential. In this study, we have developed a biocompatible bioadhesive for corneal reconstruction. Derived from Collagen type I, naturally present in human corneal stromal tissue, the bioadhesive was cross-linked with modified polyethylene glycol diacrylate (PEGDA-DOPA), rendering it curable through visible light exposure and exhibiting superior adhesion to biological tissues even in wet conditions. The physicochemical characteristics of the proposed bioadhesive were customized by manipulating the concentration of its precursor polymers and adjusting the duration of photocrosslinking. To identify the optimal sample with maximum adhesion, mechanical strength, and biocompatibility, characterization tests were conducted. The optimal specimen, consisting of 30 % (w/v) PEGDA-DOPA and cured with visible light for 5 min, exhibited commendable adhesive strength of 783.6 kPa and shear strength of 53.7 kPa, surpassing that of commercialized eye adhesives.Additionally, biocompatibility test results indicated a notably high survival rate (>100 %) of keratocytes seeded on the hydrogel adhesive after 7 days of incubation. Consequently, this designed bioadhesive, characterized by high adhesion strength, robust mechanical strength, and excellent biocompatibility, is anticipated to enhance the spontaneous repair process of damaged corneal stromal tissue.

Research on the Structural–Phase and Physical–Mechanical Characteristics of the Cr3C2-NiCr Composite Coating Deposited by the HVOF Method on E110 Zirconium Alloy

Composite coatings based on chromium carbide (Cr3C2) and nickel–chromium alloys (NiCr) are widely used due to their unique properties, including high heat resistance, wear resistance and corrosion resistance. This article studies the structural–phase and physical–mechanical characteristics of Cr3C2-NiCr composite coatings applied by high-velocity oxygen fuel to E110 zirconium alloy. The HVOF method was chosen to create coatings with high adhesion to the substrate and excellent performance properties. Analysis of the microstructure of the cross-section showed the thickness of the modified surface layer from 75 to 110 μm, depending on the processing modes. Energy dispersive X-ray spectral analysis revealed the presence of elements Cr, Ni, C and O in the coating composition. Structural–phase analysis confirmed the formation of coatings with a high concentration of Cr3C2 carbide particles and NiCr (nickel–chromium) phases. The resulting composite coatings based on Cr3C2-NiCr had a significantly high microhardness, ranging from HV 1190 to HV 1280, and the friction coefficient varied in a significant range from 0.679 to 0.502 depending on the processing conditions. The maximum adhesion strength was 9.19 MPa per square centimeter.

Atomic layer deposition of biocompatible multifunctional ZnO-TiO2 nanocoatings on the surface of additively manufactured nitinol

In order to enhance the corrosion resistance and bioactivity of the additively manufactured nitinol alloy, ZnO-TiO2 (ZTO) nanocoatings have been synthesised by atomic layer deposition. The supercycle approach was employed to deposit 40 nm coatings with varying ZnO/TiO2 ratios: 5/1, 1/1, 1/5 and 1/20. The results demonstrated that this approach permitted the coating composition to be varied over a wide range. The study of anticorrosion properties in physiological Ringer’s solution revealed that the coatings, depending on their composition, are capable of reducing the corrosion rate of nitinol by a value ranging from 4 to 58 times. In vitro studies have demonstrated high viability, good adhesion and spreading of human osteoblast-like MG-63 and mesenchymal stem FetMSC cells on the surface of all samples except those with high zinc content (ZnO and ZTO-5/1). All coatings induced differentiation of both cell lines in the osteogenic direction and demonstrated antibacterial activity against multi-drug resistant A. baumannii (>96 %) and P. aeruginosa (>90 %) strains. The results indicate the considerable potential of the developed methodology for the ALD of ZTO that combine the biocompatibility of titanium oxide, the antibacterial properties of zinc oxide and their overall stability and effectiveness for the protection of nitinol against biocorrosion.