Ultralong Hydroxyapatite Nanowire-Based Filter Paper for High-Performance Water Purification

Guidelines derived from biomineralized tissues for design and construction of high-performance biomimetic materials: from weak to strong

Living organisms in nature have undergone continuous evolution over billions of years, resulting in the formation of high-performance fracture-resistant biomineralized tissues such as bones and teeth to fulfill mechanical and biological functions, despite the fact that most inorganic biominerals that constitute biomineralized tissues are weak and brittle. During the long-period evolution process, nature has evolved a number of highly effective and smart strategies to design chemical compositions and structures of biomineralized tissues to enable superior properties and to adapt to surrounding environments. Most biomineralized tissues have hierarchically ordered structures consisting of very small building blocks on the nanometer scale (nanoparticles, nanofibers or nanoflakes) to reduce the inherent weaknesses and brittleness of corresponding inorganic biominerals, to prevent crack initiation and propagation, and to allow high defect tolerance. The bioinspired principles derived from biomineralized tissues are indispensable for designing and constructing high-performance biomimetic materials. In recent years, a large number of high-performance biomimetic materials have been prepared based on these bioinspired principles with a large volume of literature covering this topic. Therefore, a timely and comprehensive review on this hot topic is highly important and contributes to the future development of this rapidly evolving research field. This review article aims to be comprehensive, authoritative, and critical with wide general interest to the science community, summarizing recent advances in revealing the formation processes, composition, and structures of biomineralized tissues, providing in-depth insights into guidelines derived from biomineralized tissues for the design and construction of high-performance biomimetic materials, and discussing recent progress, current research trends, key problems, future main research directions and challenges, and future perspectives in this exciting and rapidly evolving research field.

Laser-assisted synthesis of nano-hydroxyapatite and functionalization with bone active molecules for bone regeneration

The main goal of bone tissue engineering research is to replace the allogenic and autologous bone graft substitutes that can promote bone repair. Owing to excellent biocompatibility and osteoconductivity, hydroxyapatite is in extensive research and high demand for both medical and non-medical applications. Although various methods have been developed for the synthesis of hydroxyapatite, in the present study we have shown the use of nanosecond laser energy in the wet precipitation method of nano-hydroxyapatite (nHAP) synthesis without using ammonium solution or any other chemicals for pH maintenance. Here, the present study aimed to fabricate the nanohydroxyapatite using a nanosecond laser. The X-ray diffraction and Fourier transform infrared spectroscopy have confirmed the hydroxyapatite formation under laser irradiation in less time without aging. A transmission electron microscopy confirmed the nano size of synthesized nHAP, which is comparable to conventional nHAP. The length and width of the laser-assisted nHAP were found to be in the range of 50-200 nm and 15-20 nm, respectively, at various laser parameters. The crystallite size obtained by Debye Scherrer formulae was found to be in the range of ∼ 16-36 nm. In addition, laser-assisted nHAP based composite cryogel (nanohydroxyapatite/gelatin/collagen I) was synthesized and impregnated with bioactive molecules (bone morphogenic protein and zoledronic acid) that demonstrated significant osteogenic potential both in vitro in cell experiment and in vivo rat muscle pouch model (abdomen and tibia muscles). Dual-energy X-ray analysis, micro-CT, and histological analysis confirmed ectopic bone regeneration. Micro-CT based histomorphometry showed a higher amount (more than 10-fold) of mineralization for animal groups implanted with composite cryogels loaded with bioactive molecules compared to only composite cryogels groups. Our findings thus demonstrate a controlled and rapid synthetic method for the synthesis of nHAP with various physical, chemical, and biological properties exhibited as comparable to conventionally synthesized nHAP.

Hydroxyapatite and its composite in heavy metal decontamination: Adsorption mechanisms, challenges, and future perspective

Nanohydroxyapatite (n-HAP), recognized by its peculiar crystal architecture and distinctive attributes showcased the underlying potential in adsorbing heavy metal ions (HMI). In this paper, the intrinsic mechanism of HMI adsorption by n-HAP was first revealed. Subsequently, the selectivity and competitiveness of n-HAP for HMI in a variety of environments containing various interferences from cations, anions, and organic molecules are elucidated. Next, n-HAP was further categorized according to its morphological dimensions, and its adsorption properties and intrinsic mechanisms were investigated based on these different morphologies. It was shown that although n-HAP has excellent adsorption capacity and cost-effectiveness, its application is often challenging to realize due to its inherent fragility and agglomeration, the technical problems required for its handling, and the difficulty of recycling. Finally, to address these issues, this paper discusses the tendency of n-HAP and its hybridized/modified materials to adsorb HMI as well as the limitations of their applications. By summarizing the limitations and future directions of hybridization/modification HAP in the field of HMI contamination abatement, this paper provides insightful perspectives for its gradual improvement and rational application.

Formation of octacalcium phosphate with incorporated dicarboxylate ions containing disulfide bonds

Octacalcium phosphate (OCP) is a layered compound capable of incorporating carboxylate ions within its interlayer structure. In this study, we successfully synthesised OCP with incorporated 3,3'-dithiodipropionate ions. Our finding is beneficial for the development of novel OCP-based materials with dynamic properties derived from disulfide bonds.

Hydroxyapatite: A journey from biomaterials to advanced functional materials

Hydroxyapatite (HAp), a well-known biomaterial, has witnessed a remarkable evolution over the years, transforming from a simple biocompatible substance to an advanced functional material with a wide range of applications. This abstract provides an overview of the significant advancements in the field of HAp and its journey towards becoming a multifunctional material. Initially recognized for its exceptional biocompatibility and bioactivity, HAp gained prominence in the field of bone tissue engineering and dental applications. Its ability to integrate with surrounding tissues, promote cellular adhesion, and facilitate osseointegration made it an ideal candidate for various biomedical implants and coatings. As the understanding of HAp grew, researchers explored its potential beyond traditional biomaterial applications. With advances in material synthesis and engineering, HAp began to exhibit unique properties that extended its utility to other disciplines. Researchers successfully tailored the composition, morphology, and surface characteristics of HAp, leading to enhanced mechanical strength, controlled drug release capabilities, and improved biodegradability. These modifications enabled the utilization of HAp in drug delivery systems, biosensors, tissue engineering scaffolds, and regenerative medicine applications. Moreover, the exceptional biomineralization properties of HAp allowed for the incorporation of functional ions and molecules during synthesis, leading to the development of bioactive coatings and composites with specific therapeutic functionalities. These functionalized HAp materials have demonstrated promising results in antimicrobial coatings, controlled release systems for growth factors and therapeutic agents, and even as catalysts in chemical reactions. In recent years, HAp nanoparticles and nanostructured materials have emerged as a focal point of research due to their unique physicochemical properties and potential for targeted drug delivery, imaging, and theranostic applications. The ability to manipulate the size, shape, and surface chemistry of HAp at the nanoscale has paved the way for innovative approaches in personalized medicine and regenerative therapies. This abstract highlights the exceptional evolution of HAp, from a traditional biomaterial to an advanced functional material. The exploration of novel synthesis methods, surface modifications, and nanoengineering techniques has expanded the horizon of HAp applications, enabling its integration into diverse fields ranging from biomedicine to catalysis. Additionally, this manuscript discusses the emerging prospects of HAp-based materials in photocatalysis, sensing, and energy storage, showcasing its potential as an advanced functional material beyond the realm of biomedical applications. As research in this field progresses, the future holds tremendous potential for HAp-based materials to revolutionize medical treatments and contribute to the advancement of science and technology.

Wet End Chemical Properties of a New Kind of Fire-Resistant Paper Pulp Based on Ultralong Hydroxyapatite Nanowires

In 2014, a new type of the fire-resistant paper based on ultralong hydroxyapatite (HAP) nanowires was reported by the author’s research group, which had superior properties and promising applications in various fields, such as high-temperature resistance, fire retardance, heat insulation, electrical insulation, energy, environmental protection, and biomedicine. The wet end chemical properties of the fire-resistant paper pulp are very important for papermaking and mechanical performance of the paper, which play a guiding role in the practical production of the fire-resistant paper. In this paper, the wet end chemical properties of a new kind of fire-resistant paper pulp based on ultralong HAP nanowires are studied for the first time by focusing on the wet end chemical parameters, the effects of these parameters on the properties such as flocculation, retention, draining, and white water circulation of the fire-resistant paper pulp, and their effects on the properties of the as-prepared fire-resistant paper. The experimental results indicated that the wet end chemical properties of the new kind of fire-resistant paper pulp based on ultralong HAP nanowires were unique and entirely different from those of the traditional paper pulp based on plant fibers. The wet end chemical properties of the fire-resistant paper pulp were significantly influenced by the inorganic adhesive and its content, which affected the runnability of the paper machine and the properties of the as-prepared fire-resistant paper. The flocculation properties of the fire-resistant paper pulp based on ultralong HAP nanowires were affected by the conductivity and Zeta potential. The addition of the inorganic adhesive in the fire-resistant paper pulp based on ultralong HAP nanowires could significantly increase the conductivity of the fire-resistant paper pulp, reduce the particle size of paper pulp floccules, and increase the tensile strength of the fire-resistant paper. In addition, the fire-resistant paper pulp based on ultralong HAP nanowires in the presence of inorganic adhesive exhibited excellent antibacterial performance. This work will contribute to and accelerate the commercialization process and applications of the new type of the fire-resistant paper based on ultralong HAP nanowires.

Microwave-Assisted Hydrothermal Rapid Synthesis of Ultralong Hydroxyapatite Nanowires Using Adenosine 5'-Triphosphate

Ultralong hydroxyapatite (HAP) nanowires are promising for various biomedical applications owing to their chemical similarity to the inorganic constituent of bone, high biocompatibility, good flexibility, excellent mechanical properties, etc. However, it is still challenging to control the formation of ultralong HAP nanowires because of the presence of free PO₄³⁻ ions in the reaction system containing the inorganic phosphate source. In addition, it takes a long period of time (usually tens of hours) for the synthetic process of ultralong HAP nanowires. Herein, for the first time, we have developed an eco-friendly calcium oleate precursor microwave hydrothermal method using biocompatible adenosine 5′-triphosphate (ATP) as a bio-phosphorus source and water as the only solvent for the rapid synthesis of ultralong HAP nanowires. The controllable hydrolysis of ATP can avoid the premature formation of calcium phosphate nuclei and uncontrollable crystal growth. Microwave heating can significantly shorten the synthetic time from tens of hours required by the traditional heating to 1 h, thus achieving high efficiency, energy saving and low cost. The as-prepared ultralong HAP nanowires with high flexibility have lengths of several hundred micrometers and diameters of 10~20 nm, and they usually self-assemble into nanowire bundles along their longitudinal direction. The as-prepared ultralong HAP nanowire/chitosan porous scaffold has excellent bioactivity, good biodegradation and cytocompatibility owing to the bioactive adenosine adsorbed on the surface of ultralong HAP nanowires. It is expected that ultralong HAP nanowires will be promising for various applications in the biomedical fields, such as bone defect repair, skin wound healing, and as a drug nanocarrier.

Robust and Multifunctional 3D Graphene-Based Aerogels Reinforced by Hydroxyapatite Nanowires for Highly Efficient Organic Solvent Adsorption and Fluoride Removal

In view of the serious perniciousness and complex diversity of actual wastewater systems, exploiting a robust and multifunctional adsorbent material featuring high sorption efficiency, broad-spectrum applicability, and excellent recyclability in treating multifarious pollutants in water (such as oils and fluoride ions) is highly required; however, it is still a daunting goal to pursue to date. In this work, novel mechanically robust and exceptional graphene oxide/hydroxyapatite nanowire (GO/HAPNW) aerogels (RGHAs/polydopamine (PDA)@RGHAs) with adjustable surface wettability were developed through a facile sol-gel self-assembly technology and subsequently optional bioinspired hydrophilic modification. Thanks to the reinforcing effect of HAPNWs with higher aspect ratio, a remarkably improved mechanical robustness (including superior compressibility and superelasticity) was acquired for the resulting aerogels. Based on the cooperative effect of the favorable selective wetting properties (i.e., hydrophobic/oleophilic for RGHAs) and the excellent mechanic stability, the aerogels displayed an outstanding sorption performance for diverse oils/organic solvents accompanied with a prominent recyclability. Specifically, a fairly high adsorption capacity of as high as 191 times of its own mass (for pump oil) was achieved based on a fast adsorption kinetic process. More importantly, superamphiphilic three-dimensional (3D) PDA@RGHAs revealed an extraordinary removal capability for water-soluble fluoride ions, exhibiting a superior equilibrium adsorption capacity (q_e, 9.93 mg/g), which is distinctly superior to those of low-dimensional fluorine adsorbent materials recently reported. Accordingly, the as-prepared 3D aerogels combining both superior oil/organic solvent adsorption and excellent defluorination capability reveal a competitive application prospect toward effective intricate oily wastewater purification.

Filter paper-based colorimetric analysis: An instrument-free strategy for semiquantitative naked-eye detection of food colorants

A filter paper-based colorimetric strategy for instrument-independent visual detection of artificial food colorants (FCs) was developed in this study. Indicator papers were prepared via the one-step polycondensation of silane coupling agents onto glass microfiber filter papers, and colorimetric cards with a fine one-to-one correspondence between their colors and FCs concentrations were straightforward obtained by the extraction of FCs with indicator papers by virtue of electrostatic interaction and hydrophobic effect. Filter papers post-modified via such a simple way were proved to be of improved binding class selectivity and colorimetric sensitivity, allowing for in-situ colorimetric assay of FCs in an unprecedently wide range of applicable pH (1.0-12.0) with high reliability and fine versatility. Finally, the semiquantitative naked-eye determination of FCs (Allura red, brilliant blue and sunset yellow) in real-world drink samples was experimentally confirmed to be feasible by comparison with the findings of UV-vis absorption spectra, HPLC and mass spectra.

Fabrication of novel bio-adsorbent and its application for the removal of Cu(II) from aqueous solution

As eco-friendly adsorption material, hydroxyapatite (Ca₅(PO₄)₃OH, HA) has been extensively applied to the removal of heavy metal ions. However, separating and recovering of HA powder after the adsorption process limits their application. Alginate-based composite beads (HCA) encapsulated with HA and cellulose were designed to remove Cu(II) from aqueous solution. Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used for characteristic analysis. An extensive discussion in terms of HCA adsorption capacity, effect of various Cu(II) concentration, and analysis of the involved mechanisms of Cu(II) removal on the biosorption. HCA beads showed that the maximum adsorption capacity for Cu(II) of 64.14 mg/g at pH = 5 with 8 h contact time. The Langmuir adsorption isotherm and second-order kinetic model gave the closest fit. HCA beads display good regeneration ability after four cycles and offer potentiality for practical application.

Multifunctional Photocatalytic Filter Paper Based on Ultralong Nanowires of the Calcium-Alendronate Complex for High-Performance Water Purification

Semiconductor photocatalysts and membrane separation technology have been widely used in the field of water treatment. Usually, the particles of traditional semiconductor photocatalysts are easy to aggregate, difficult to separate from the liquid phase after photocatalysis, and may even cause secondary pollution. On the other hand, the membrane separation technology is also facing the problem of sharp decreases in removal efficiency and water flux caused by the membrane fouling. However, it is an attractive and promising solution to combine two technologies of photocatalysis and membrane separation for high-performance water treatment. In this work, we have developed the calcium oleate precursor solvothermal method to synthesize ultralong nanowires (UNWs) of Ca-alendronate (Ca-ALN) complex for the first time. Experimental results and data analysis indicate that the as-prepared Ca-ALN ultralong nanowires are an n-type semiconductor with an energy band gap of 3.41 eV. A new type of multifunctional photocatalytic filter paper has been developed based on ultralong nanowires of Ca-ALN complex (Ca-ALN-UNWs) and cellulose fibers (CFs). The as-prepared Ca-ALN-UNW/CF photocatalytic filter paper exhibits multifunctions of photocatalysis, adsorption, and filtration, which can be used for high-performance treatment of the wastewater containing various pollutants such as heavy-metal ions, dyes, antibiotics, and bacteria. The active oxygen species produced by the Ca-ALN-UNW/CF photocatalytic filter paper under light illumination are determined by electron spin resonance, and the energy band gap and photoelectric properties of the material are tested by ultraviolet-visible diffuse reflectance spectroscopy and electrochemical workstation. The pure water flux of the Ca-ALN-UNW/CF photocatalytic filter paper is very high, which can reach 2230.5 L m^-2 h^-1 under a working pressure of 0.1 MPa. The Ca-ALN-UNW/CF photocatalytic filter paper is promising for various applications such as highly efficient water purification and in the biomedical field.

The preparation of hydroxyapatite nanowires and nanorods via aliphatic micelles as soft templates

Hydroxyapatite nanoparticles were tunably synthesized via the use of an aliphatic–ethanol–water three-phase mixture system using micelles as soft templates via an emulsion–hydrothermal synergistic method.

A scalable, low-cost and green strategy for the synthesis of ultralong hydroxyapatite nanowires using peanut oil

A scalable green and low-cost synthesis of ultralong hydroxyapatite nanowires using peanut oil is reported, which can be scaled up for large-scale low-cost production of ultralong hydroxyapatite nanowires and the fire-resistant inorganic paper.

Liquid Transport and Real-Time Dye Purification via Lotus Petiole-Inspired Long-Range-Ordered Anisotropic Cellulose Nanofibril Aerogels

Nowadays, large-scale oriented functional porous materials have been sought after by researchers. However, regulation of the long-range uniform and oriented structures of the material remains a challenge. Herein, ultralong anisotropic cellulose nanofibril (CNF) aerogels with uniformly ordered structures of pore walls inspired by lotus petioles were constructed by applying external speeds to counterbalance the growth driving forces of ice crystals. Based on the growth law of ice crystals, the ice crystals grew at a stable rate when the applied external speed was 0.04 mm/s, ensuring the consistent orientation of the large-scale CNF aerogel. The aerogel exhibited a rapid long-range directional transport ability to different liquid solvents, delivering ethanol up to 40 mm from bottom to top within 50 s. Moreover, by introducing rectorites with good cation-exchange properties, the resulting long-range composite possessed an enhanced adsorption capacity for methylene blue. Furthermore, aerogel successfully achieved real-time dye purification at a long distance, such as fast dye adsorption or selective adsorption. This flexible and straightforward strategy of fabricating ultralong oriented CNF aerogel materials is expected to promote the development of functional aerogels in directional liquid transport and sewage treatment.

Advanced Materials Based on Nanosized Hydroxyapatite

The development of new materials based on hydroxyapatite has undergone a great evolution in recent decades due to technological advances and development of computational techniques. The focus of this review is the various attempts to improve new hydroxyapatite-based materials. First, we comment on the most used processing routes, highlighting their advantages and disadvantages. We will now focus on other routes, less common due to their specificity and/or recent development. We also include a block dedicated to the impact of computational techniques in the development of these new systems, including: QSAR, DFT, Finite Elements of Machine Learning. In the following part we focus on the most innovative applications of these materials, ranging from medicine to new disciplines such as catalysis, environment, filtration, or energy. The review concludes with an outlook for possible new research directions.

Customized Cellulose Fiber Paper Enabled by an In Situ Growth of Ultralong Hydroxyapatite Nanowires

Cellulose fiber (CF) paper is a low-cost, sustainable, and flexible substrate, which has gained increasing interest recently. Before practical usage, the functionalization of the pristine CF paper is indispensable to meet requirements of specific applications. Different from conventional surface modification or physical mixing methods, we report in situ growth of ultralong hydroxyapatite nanowires (HAPNWs) with lengths larger than 10 μm on the CF paper. HAPNWs are radially aligned on the surface of CFs, creating a micro/nanoscale hierarchical structure. By means of the excellent ion exchange ability of HAP and the hierarchical structure, the functions of the CF paper can be easily customized. As a proof-of-concept, we demonstrate two kinds of functional CF paper: (1) the photoluminescent CF paper by doping Eu³⁺ and Tb³⁺ ions into the crystal lattice of HAPNWs and (2) the superhydrophobic CF paper by coating poly(dimethylsiloxane) on the HAPNW hierarchical structure, which can be applied for self-cleaning and oil/water separation. It is expected that an in situ growth of ultralong HAPNWs will provide an instructive guideline for designing a CF paper with specific functions.

Combined strategy of blending and surface modification as an effective route to prepare antifouling ultrafiltration membranes

Ultrafiltration (UF) membranes blended with hydrophilic nanomaterials usually exhibit preferable overall performance including the membrane permeability and antifouling capability. However, the improvement in antifouling performance may be not outstanding due to the small amount of nanomaterial distributed near the membrane surface and the limited improvement in membrane hydrophilicity. Notably, excess addition of nanomaterials may lead to the decline in membrane permeability. In order to solve the above problem, we integrated the strategy of blending and surface modification to construct novel hybrid UF membranes. Novel nanohybrid was prepared via tannic acid (TA) coating on hydroxyapatite nanotubes (HANTs) and the subsequent grafting of zwitterionic polyethylenimine (ZPEI). The prepared nanohybrid (HANTs@TA-ZPEI) was incorporated with the polysulfone containing tertiary amine groups to fabricate hybrid membranes via the solution blending and the subsequent immersion-precipitation phase inversion process. Then the matrix was modified with zwitterions via the reaction of tertiary amine group with 1, 3-propane sultone. UF tests were conducted using the bovine serum albumin (BSA) and humic acid (HA) as the representative foulants. Results showed that both the permeability and the antifouling performance of the membranes achieved favorable promotion. Thereinto, the water flux of M-B0.4-Z membrane (pre blended with 0.4 wt% HANTs@TA-ZPEI in the casting solution and post-surface modified) exhibited 2.6 times that of the pristine membrane and the flux recovery ratio (FRR) for BSA and HA attained 93.4% and 96.1%, respectively. By the combination of blending and surface modification, both the membrane permeability and fouling resistant properties could attain remarkable promotion, which exerted the advantages of two methods and made up the deficiency of single blending method.

Adsorptive Applications of Montmorillonite Clay for the Removal of Ag(I) and Cu(II) from Aqueous Medium

The present work aims to investigate the ability of Saudi clay containing montmorillonite to remove Ag(I) and Cu(II) ions from aqueous solutions for waste water purification. The effect of pH, adsorbent mass, metal concentration, and contact time on the removal process has been investigated. The batch method was applied, using solution metal concentrations ranging from 40 to 2000 mg/L. Adsorption percentage and distribution coefficients (Kd) were determined as a function of metal concentration. pH 6 was found to be optimal for the adsorption. Adsorption reached equilibrium in 5 min for both Ag(I) and Cu(II) ions. The study on adsorption’s kinetic characteristics showed the adsorption process of these metal ions was of pseudo-second-order. Furthermore, the adsorption rate of Ag(I) was higher than that of Cu(II), and their adsorption appeared to follow the Langmuir isotherm. From the equilibrium studies, it was observed that the selectivity of Ag(I) was higher than that of Cu(II). The results showed that Saudi clay has the potential to be a suitable adsorbent for Ag(I) and Cu(II) removal from aqueous solutions compared with other adsorbents.

Biomolecule-assisted green synthesis of nanostructured calcium phosphates and their biomedical applications

Calcium phosphates (CaPs) are ubiquitous in nature and vertebrate bones and teeth, and have high biocompatibility and promising applications in various biomedical fields. Nanostructured calcium phosphates (NCaPs) are recognized as promising nanocarriers for drug/gene/protein delivery owing to their high specific surface area, pH-responsive degradability, high drug/gene/protein loading capacity and sustained release performance. In order to control the structure and surface properties of NCaPs, various biomolecules with high biocompatibility such as nucleic acids, proteins, peptides, liposomes and phosphorus-containing biomolecules are used in the synthesis of NCaPs. Moreover, biomolecules play important roles in the synthesis processes, resulting in the formation of various NCaPs with different sizes and morphologies. At room temperature, biomolecules can play the following roles: (1) acting as a biocompatible organic phase to form biomolecule/CaP hybrid nanostructured materials; (2) serving as a biotemplate for the biomimetic mineralization of NCaPs; (3) acting as a biocompatible modifier to coat the surface of NCaPs, preventing their aggregation and increasing their colloidal stability. Under heating conditions, biomolecules can (1) control the crystallization process of NCaPs by forming biomolecule/CaP nanocomposites before heating; (2) prevent the rapid and disordered growth of NCaPs by chelating with Ca2+ ions to form precursors; (3) provide the phosphorus source for the controlled synthesis of NCaPs by using phosphorus-containing biomolecules. This review focuses on the important roles of biomolecules in the synthesis of NCaPs, which are expected to guide the design and controlled synthesis of NCaPs. Moreover, we will also summarize the biomedical applications of NCaPs in nanomedicine and tissue engineering, and discuss their current research trends and future prospects.