Antibacterial high-genus polymer vesicle as an "armed" drug carrier

Physically and Chemically Compartmentalized Polymersomes for Programmed Delivery and Biological Applications

Multicompartment polymersomes (MCPs) refer to polymersomes that not only contain one single compartment, either in the membrane or in the internal cavity, but also mimic the compartmentalized structure of living cells, attracting much attention in programmed delivery and biological applications. The investigation of MCPs may promote the application of soft nanomaterials in biomedicine. This Review seeks to highlight the recent advances of the design principles, synthetic strategies, and biomedical applications of MCPs. The compartmentalization types including chemical, physical, and hybrid compartmentalization are discussed. Subsequently, the design and controlled synthesis of MCPs by the self-assembly of amphiphilic polymers, double emulsification, coprecipitation, microfluidics and particle assembly, etc. are summarized. Furthermore, the diverse applications of MCPs in programmed delivery of various cargoes and biological applications including cancer therapy, antimicrobials, and regulation of blood glucose levels are highlighted. Finally, future perspectives of MCPs from the aspects of controlled synthesis and applications are proposed.

In-depth study of anticancer drug diffusion through a cross-linked -pH-responsive polymeric vesicle membrane

Post-encapsulation and release of the anticancer drug doxorubicin hydrochloride (DOX·HCl) through cell-like transmission functions of polymeric vesicles were studied using cross-linked pH-responsive polymeric vesicles. The vesicles were fabricated for the first time via the redox-initiated reversible addition-fragmentation chain transfer dispersion polymerization in ethanol-water mixture, using 2-(diisopropylamino)ethyl methacrylate and glycidyl methacrylate, and the vesicle membrane was modified post-cross-linking by using ethylenediamine. A phase diagram was constructed for reproducible fabrication of the polymeric vesicles, and well-shaped vesicles were formed when the target degree of polymerization of the hydrophobic polymer chains was equal to or higher than 50 with solid content in the range of 10-30 wt%. The cross-linked vesicle membrane served as a gate enabling "open" and "closed" states in response to pH stimulation. Up to 50% drug loading efficiency and 39% drug loading content could be achieved, and in vitro release of the DOX-loaded vesicles in aqueous buffer solutions showed a much faster DOX release rate at pH 5.0 than at pH 6.5. The polymeric vesicles were of very low cytotoxicity to A549 cells up to the concentration of 2 mg/mL, and the IC₅₀ of DOX-loaded vesicles were higher than that of the free DOX. The intracellular DOX release study indicated higher cellular uptake capability for DOX-loaded vesicles than that of free DOX.

Intelligent design of polymersomes for antibacterial and anticancer applications

Polymersomes (or polymer vesicles) have attracted much attention for biomedical applications in recent years because their lumen can be used for drug delivery and their coronas and membrane can be modified with a variety of functional groups. Thus, polymersomes are very suitable for improved antibacterial and anticancer therapy. This review mainly highlighted recent advances in the synthetic protocols and design principles of intelligent antibacterial and anticancer polymersomes. Antibacterial polymersomes are divided into three categories: polymersomes as antibiotic nanocarriers, intrinsically antibacterial polymersomes, and antibacterial polymersomes with supplementary means including photothermal and photodynamic therapy. Similarly, the anticancer polymersomes are divided into two categories: polymersomes-based delivery systems and anticancer polymersomes with supplementary means. In addition, the bilateral relationship between bacteria and cancer is addressed, since more and more evidences show that bacteria may cause cancer or promote cancer progression. Finally, prospective on next-generation antibacterial and anticancer polymersomes are discussed. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Biology-Inspired Nanomaterials > Lipid-Based Structures.

Secondary Amine Pendant β-Peptide Polymers Displaying Potent Antibacterial Activity and Promising Therapeutic Potential in Treating MRSA-Induced Wound Infections and Keratitis

Interest in developing antibacterial polymers as synthetic mimics of host defense peptides (HPDs) has accelerated in recent years to combat antibiotic-resistant bacterial infections. Positively charged moieties are critical in defining the antibacterial activity and eukaryotic toxicity of HDP mimics. Most examples have utilized primary amines or guanidines as the source of positively charged moieties, inspired by the lysine and arginine residues in HDPs. Here, we explore the impact of amine group variation (primary, secondary, or tertiary amine) on the antibacterial performance of HDP-mimicking β-peptide polymers. Our studies show that a secondary ammonium is superior to either a primary ammonium or a tertiary ammonium as the cationic moiety in antibacterial β-peptide polymers. The optimal polymer, a homopolymer bearing secondary amino groups, displays potent antibacterial activity and the highest selectivity (low hemolysis and cytotoxicity). The optimal polymer displays potent activity against antibiotic-resistant bacteria and high therapeutic efficacy in treating MRSA-induced wound infections and keratitis as well as low acute dermal toxicity and low corneal epithelial cytotoxicity. This work suggests that secondary amines may be broadly useful in the design of antibacterial polymers.

Polymer Vesicles for Antimicrobial Applications

Polymer vesicles, hollow nanostructures with hydrophilic cavity and hydrophobic membrane, have shown significant potentials in biomedical applications including drug delivery, gene therapy, cancer theranostics, and so forth, due to their unique cell membrane-like structure. Incorporation with antibacterial active components like antimicrobial peptides, etc., polymer vesicles exhibited enhanced antimicrobial activity, extended circulation time, and reduced cell toxicity. Furthermore, antibacterial, and anticancer can be achieved simultaneously, opening a new avenue of the antimicrobial applications of polymer vesicles. This review seeks to highlight the state-of-the-art of antimicrobial polymer vesicles, including the design strategies and potential applications in the field of antibacterial. The structural features of polymer vesicles, preparation methods, and the combination principles with antimicrobial active components, as well as the advantages of antimicrobial polymer vesicles, will be discussed. Then, the diverse applications of antimicrobial polymer vesicles such as wide spectrum antibacterial, anti-biofilm, wound healing, and tissue engineering associated with their structure features are presented. Finally, future perspectives of polymer vesicles in the field of antibacterial is also proposed.

Macromolecular Nanotherapeutics and Antibiotic Adjuvants to Tackle Bacterial and Fungal Infections

The escalating rise in the population of multidrug-resistant (MDR) pathogens coupled with their biofilm forming ability has struck the global health as nightmare. Alongwith the threat of aforementioned menace, the sluggish development of new antibiotics and the continuous deterioration of the antibiotic pipeline has stimulated the scientific community toward the search of smart and innovative alternatives. In near future, membrane targeting antimicrobial polymers, inspired from antimicrobial peptides, can stand out significantly to combat against the MDR superbugs. Many of these amphiphilic polymers can form nanoaggregates through self-assembly with superior and selective antimicrobial efficacy. Additionally, these macromolecular nanoaggregrates can be utilized to engineer smart antibiotic-delivery system for on-demand drug-release, exploiting the infection site's micoenvironment. This strategy substantially increases the local concentration of antibiotics and reduces the associated off-target toxicity. Furthermore, amphiphilc macromolecules can be utilized to rejuvinate obsolete antibiotics to tackle the drug-resistant infections. This review article highlights the recent developments in macromolecular architecture to design numerous nanostructures with broad-spectrum antimicrobial activity, their application in fabricating smart drug delivery systems and their efficacy as antibiotic adjuvants to circumvent antimicrobial resistance. Finally, the current challenges and future prospects are briefly discussed for further exploration and their practical application in clinical settings.

Engineering of Janus-Like Dendrimers with Peptides Derived from Glycoproteins of Herpes Simplex Virus Type 1: Toward a Versatile and Novel Antiviral Platform

Novel antiviral nanotherapeutics, which may inactivate the virus and block it from entering host cells, represent an important challenge to face viral global health emergencies around the world. Using a combination of bioorthogonal copper-catalyzed 1,3-dipolar alkyne/azide cycloaddition (CuAAC) and photoinitiated thiol-ene coupling, monofunctional and bifunctional peptidodendrimer conjugates were obtained. The conjugates are biocompatible and demonstrate no toxicity to cells at biologically relevant concentrations. Furthermore, the orthogonal addition of multiple copies of two different antiviral peptides on the surface of a single dendrimer allowed the resulting bioconjugates to inhibit Herpes simplex virus type 1 at both the early and the late stages of the infection process. The presented work builds on further improving this attractive design to obtain a new class of therapeutics.

Antimicrobial Polymeric Structures Assembled on Surfaces

Pathogenic microbes are the main cause of various undesired infections in living organisms, including humans. Most of these infections are favored in hospital environments where humans are being treated with antibiotics and where some microbes succeed in developing resistance to such drugs. As a consequence, our society is currently researching for alternative, yet more efficient antimicrobial solutions. Certain natural and synthetic polymers are versatile materials that have already proved themselves to be highly suitable for the development of the next-generation of antimicrobial systems that can efficiently prevent and kill microbes in various environments. Here, we discuss the latest developments of polymeric structures, exhibiting (reinforced) antimicrobial attributes that can be assembled on surfaces and coatings either from synthetic polymers displaying antiadhesive and/or antimicrobial properties or from blends and nanocomposites based on such polymers.

High-genus multicompartment vesicles evolved from large compound micelles

High-genus multicompartment vesicles (HGMVs) are self-assembled from block copolymers containing fluorescent and photo-responsive azobenzene groups.

Biodegradable Antibacterial Polymeric Nanosystems: A New Hope to Cope with Multidrug-Resistant Bacteria

The human society is faced with daunting threats from bacterial infections. Over decades, a variety of antibacterial polymeric nanosystems have exhibited great promise for the eradication of multidrug-resistant bacteria and persistent biofilms by enhancing bacterial recognition and binding capabilities. In this Review, the "state-of-the-art" biodegradable antibacterial polymeric nanosystems, which could respond to bacteria environments (e.g., acidity or bacterial enzymes) for controlled antibiotic release or multimodal antibacterial treatment, are summarized. The current antibacterial polymeric nanosystems can be categorized into antibiotic-containing and intrinsic antibacterial nanosystems. The antibiotic-containing polymeric nanosystems include antibiotic-encapsulated nanocarriers (e.g., polymeric micelles, vesicles, nanogels) and antibiotic-conjugated polymer nanosystems for the delivery of antibiotic drugs. On the other hand, the intrinsic antibacterial polymer nanosystems containing bactericidal moieties such as quaternary ammonium groups, phosphonium groups, polycations, antimicrobial peptides (AMPs), and their synthetic mimics, are also described. The biodegradability of the nanosystems can be rendered by the incorporation of labile chemical linkages, such as carbonate, ester, amide, and phosphoester bonds. The design and synthesis of the degradable polymeric building blocks and their fabrications into nanosystems are also explicated, together with their plausible action mechanisms and potential biomedical applications. The perspectives of the current research in this field are also described.

Strategies from nature: polycaprolactone-based mimetic antimicrobial peptide block copolymers with low cytotoxicity and excellent antibacterial efficiency

PCL₁₆-b-K_n diblock copolymers exhibit excellent antibacterial activities and low cytotoxicity. Meanwhile, they act by the pore-forming bactericidal mechanism without inducement of drug resistance.

Evolution of diverse higher-order membrane structures of block copolymer vesicles

An evolutionary route to polymer vesicles with diverse higher-order membrane structures has been discovered.

Synthesis, Self-Assembly, and Biomedical Applications of Antimicrobial Peptide-Polymer Conjugates

Antimicrobial peptides (AMPs) have been attracting much attention due to their excellent antimicrobial efficiency and low rate in driving antimicrobial resistance (AMR), which has been increasing globally to alarming levels. Conjugation of AMPs into functional polymers not only preserves excellent antimicrobial activities but reduces the toxicity and offers more functionalities, which brings new insight toward developing multifunctional biomedical materials such as hydrogels, polymer vesicles, polymer micelles, and so forth. These nanomaterials have been exhibiting excellent antimicrobial activity against a broad spectrum of bacteria including multidrug-resistant (MDR) ones, high selectivity, and low cytotoxicity, suggesting promising potentials in wound dressing, implant coating, antibiofilm, tissue engineering, and so forth. This Perspective seeks to highlight the state-of-the-art strategy for the synthesis, self-assembly, and biomedical applications of AMP-polymer conjugates and explore the promising directions for future research ranging from synthetic strategies, multistage and stimuli-responsive antibacterial activities, antifungi applications, and potentials in elimination of inflammation during medical treatment. It also will provide perspectives on how to stem the remaining challenges and unresolved problems in combating bacteria, including MDR ones.

Phosphonium-Functionalized Polymer Micelles with Intrinsic Antibacterial Activity

New approaches to treat bacterial infections are badly needed to address the increasing problem of antibiotic resistance. This study explores phosphonium-functionalized block copolymer micelles as intrinsically antibacterial polymer assemblies. Phosphonium cations with varying alkyl lengths were conjugated to the terminus of a poly(ethylene oxide)-polycaprolactone block copolymer, and the phosphonium-functionalized block copolymers were self-assembled to form micelles in aqueous solution. The size, morphology, and ζ-potential of the assemblies were studied, and their abilities to kill Escherichia coli and Staphylococcus aureus were evaluated. It was found that the minimum bactericidal concentration depended on the phosphonium alkyl chain length, and different trends were observed for Gram-negative and Gram-positive bacteria. The most active assemblies exhibited no hemolysis of red blood cells above the bactericidal concentrations, indicating that they can selectively disrupt the membranes of bacteria. Furthermore, it was possible to encapsulate and release the antibiotic tetracycline using the assemblies, providing a potential multimechanistic approach to bacterial killing.

Preparation and Antibacterial Mechanism Insight of Polypeptide-Based Micelles with Excellent Antibacterial Activities

Traditional antibiotics usually sterilize in chemical ways, which may lead to serious drug resistance. By contrast, peptide-based antibacterial materials are less susceptible to drug resistance. Herein we report the preparation of an antibacterial peptide-based copolymer micelle and the investigation of its membrane-penetration antibacterial mechanism by transmission electron microscopy (TEM). The copolymer is poly(l-lactide)-block-poly(phenylalanine-stat-lysine) [PLLA₃₁-b-poly(Phe₂₄-stat-Lys₃₆)], which is synthesized by ring-opening polymerization. The PLLA chains form the core, whereas the polypeptide chains form the coronas of the micelle in aqueous solution. This micelle boasts excellent antibacterial efficacy against both Gram-positive and Gram-negative bacteria. Furthermore, TEM studies clearly reveal that the micelles pierce and then destroy the cell membrane of the bacteria. We also compared the advantages and disadvantages of two general methods for measuring the Minimal Inhibitory Concentration (MIC) values of antibacterial micelles. Overall, this study provides us with direct evidence for the antibacterial mechanism of polypeptide-based micelles and a strategy for synthesizing biodegradable antibacterial nanomaterials without antibiotic resistance.

Synthesis and Mechanism Insight of a Peptide-Grafted Hyperbranched Polymer Nanosheet with Weak Positive Charges but Excellent Intrinsically Antibacterial Efficacy

Antimicrobial resistance is an increasingly problematic issue in the world and there is a present and urgent need to develop new antimicrobial therapies without drug resistance. Antibacterial polymers are less susceptible to drug resistance but they are prone to inducing serious side effects due to high positive charge. Herein we report a peptide-grafted hyperbranched polymer which can self-assemble into unusual nanosheets with highly effective intrinsically antibacterial activity but weak positive charges (+ 6.1 mV). The hyperbranched polymer was synthesized by sequential Michael addition-based thiol-ene and free radical mediated thiol-ene reactions, and followed by ring-opening polymerization of N-carboxyanhydrides (NCAs). The nanosheet structure was confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM) studies. Furthermore, a novel "wrapping and penetrating" antibacterial mechanism of the nanosheets was revealed by TEM and it is the key to significantly decrease the positive charges but have a very low minimum inhibitory concentration (MIC) of 16 μg mL(-1) against typical Gram-positive and Gram-negative bacteria. Overall, our synthetic strategy demonstrates a new insight for synthesizing antibacterial nanomaterials with weak positive charges. Moreover, the unique antibacterial mechanism of our nanosheets may be extended for designing next-generation antibacterial agents without drug resistance.

Reversible morphology transitions of supramolecular polymer self-assemblies for switch-controlled drug release

A novel method for switch-controlled drug release was developed through the reversible morphology transitions of supramolecular branched copolymer self-assemblies. The reversible transitions from vesicles to nanoparticles were successfully achieved by alternating UV and visible light irradiation to obtain morphology-controlled drug release in a switch mode.

Synthesis and microstructural properties of the scaffold based on a 3-(trimethoxysilyl)propyl methacrylate–POSS hybrid towards potential tissue engineering applications

The biomimetic organic–inorganic scaffold with the chemical composition, structural dimensions, topography, and microstructural properties that fulfills the requirements for hard-tissue engineering was developed.

Rationally Engineering Dual Missions in One Statistical Copolymer Nanocapsule: Bacterial Inhibition and Polycyclic Aromatic Hydrocarbon Capturing

Effective inhibition of bacteria and removal of carcinogenic organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) are important technical challenges in water purification because most of the traditional filter membranes are prone to being biologically contaminated by bacteria and difficult to filter off PAHs. Herein we present the synthesis and characterization of a novel multifunctional nanocapsule (vesicle) based on a statistical copolymer, poly[[2-hydroxy-3-(naphthalen-1-ylamino)propyl methacrylate]-stat-[2-(tert-butylamino)ethyl methacrylate]] [P(HNA₂₃-stat-TA₂₀)], which can be easily synthesized in one step. The TA moiety is engineered for effective bacterial inhibition, while the HNA moiety is in charge of the capturing of PAHs by π-π stacking. The nanocapsules can effectively inhibit bacteria and quickly reduce the pyrene content in water to an extremely low residual concentration of 5.6 (in 1 min) or 0.56 (in 60 min) parts per billion (ppb). Moreover, this rational engineering principle could be extended by statistically copolymerizing HNA with other functional monomers for designing a range of multifunctional nanomaterials.

Preparation of polymersomes in pure water for facile antibacterial applications

We report the facile synthesis of antibacterial polymersomes in pure water, which show good antibacterial activities against both Gram-positive and Gram-negative bacteria and can be sprayed in places which are susceptible to bacterial attack.

pH-Controlled multiple-drug delivery by a novel antibacterial nanocomposite for combination therapy

The modified cationic silica based stimuli responsive polymer–clay nanocomposite with pH responsive release manner could improve the targeting performance.

Functionalised nanoparticles complexed with antibiotic efficiently kill MRSA and other bacteria

Antibiotic-resistant bacterial infections are a vexing global health problem and have rendered ineffective many previously-used antibiotics. Here we demonstrate that antibiotic-linkage to surface-functionalized silica nanoparticles (sNP) significantly enhances their effectiveness against Escherichia coli, and Staphylococcus aureus, and even methicillin-resistant S. aureus (MRSA) strains that are resistant to most antibiotics. The commonly-used antibiotic penicillin-G (PenG) was complexed to dye-labeled sNPs (15 nm diameter) containing carboxyl groups located as either surface-functional groups, or on polymer-chains extending from surfaces. Both sNPs configurations efficiently killed bacteria, including MRSA strains. This suggests that activities of currently-ineffective antibiotics can be restored by nanoparticle-complexation and used to avert certain forms of antibiotic-resistance.

Amphiphilic nanocapsules entangled with organometallic coordination polymers for controlled cargo release

A class of new amphiphilic nanocapsules entangled with organometallic coordination polymers has been developed for the first time. Poly(2-(N,N-dimethyl amino)ethyl methacrylate)-b-polystyrene capped with β-cyclodextrin (β-CD) (CD-PDMAEMA-b-PS) is first synthesized using sequent RAFT polymerization of styrene and 2-(N,N-dimethyl amino)ethyl methacrylate with xanthate modified β-CD as chain transfer agent. The end group of β-CD is allowed to include 4,4'-bipyridine through host-guest inclusion to yield PDMAEMA-b-PS terminated with an inclusion complex of β-CD and bipyridine (bpy-PDMAEMA-b-PS), which is then used as surfactant to prepare emulsion droplets in toluene/water mixture. Upon addition of Ni(II), bipyridine coordinates with Ni(II) to form coordination polymers in the periphery of emulsion droplets, affording amphiphilic capsules entangled with organometallic coordination polymers, as confirmed by GPC, (1)H NMR, SEM, TEM, DLS, and so on. The organometallic coordination polymer capsules are capable of encapsulating organic cargoes. Interestingly, encapsulated cargoes can be extracted from the capsules without damaging the capsules. Such capsules are potential candidates for encapsulating and controlled release of organic cargoes.

Multifunctional homopolymer vesicles for facile immobilization of gold nanoparticles and effective water remediation

Homopolymers have been considered as a nonideal building block for creating well-defined nanostructures due to their fuzzy boundary between hydrophobic and hydrophilic moieties. However, this unique fuzzy boundary may provide some opportunities for fabricating functional nanomaterials. Presented in this paper is a pH-responsive multifunctional homopolymer vesicle based on poly[2-hydroxy-3-(naphthalen-1-ylamino)propyl methacrylate] (PHNA). This vesicle is confirmed to be an excellent supporter for gold nanoparticles (AuNPs) to facilitate the reduction reaction of 4-nitrophenol (4-NP). The pH-responsive vesicle membrane favors the effective embedding and full immobilization of AuNPs because it is kinetically frozen under neutral and basic environments, preventing AuNPs from aggregation. Meanwhile, there is a synergistic effect between the AuNPs and the supporter (PHNA vesicle). Due to the π-π interaction between the naphthalene pendants in every repeat unit of PHNA and the extra aromatic compounds, a substrate-rich (high concentration of 4-NP) microenvironment can be created around AuNPs, which can dramatically accelerate the AuNPs-catalyzed reactions. In addition, we proposed a method for more accurately determining the membrane thickness of rigid polymer vesicles from TEM images based on "stack-up" vesicles, which may overturn the measuring method commonly used by far. Moreover, proof-of-concept studies showed that those homopolymer vesicles may be used as a powerful adsorbent for effective water remediation to remove trace carcinogenic organic pollutants such as polycyclic aromatic hydrocarbons to below parts per billion (ppb) level at a very fast rate based on the π-π interaction between the naphthalene pendants in PHNA vesicle and polycyclic aromatic hydrocarbons. Overall, this multifunctional homopolymer vesicle provides an alternative insight on preparing effective recyclable AuNPs-decorated nanoreactor and powerful water remediation adsorbent.

Theranostic vesicles based on bovine serum albumin and poly(ethylene glycol)-block-poly(L-lactic-co-glycolic acid) for magnetic resonance imaging and anticancer drug delivery

Presented in this article is the preparation of a new theranostic vesicle which exhibits excellent in vitro and in vivo T1 magnetic resonance (MR) imaging contrast effect and good anticancer drug delivery ability. The theranostic vesicle has been easily prepared based on an amphiphilic biocompatible and biodegradable dibock copolymer, poly(ethylene glycol)-block-poly(l-lactic-co-glycolic acid) (PEG-b-PLGA) and bovine serum albumin-gadolinium (BSA-Gd) complexes. Dynamic light scattering (DLS), transmission electron microscopy (TEM), UV-vis spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements confirmed the formation and physiological stability of BSA-Gd@PEG-b-PLGA vesicles. Furthermore, the in vitro and in vivo MR imaging experiments revealed their excellent T1-weighted MR imaging function. Red blood cell hemolysis and cytotoxicity experiments confirmed their good blood compatibility and low cytotoxicity. Doxorubicin (DOX) loading and release experiments indicated a more retarded release rate of DOX in those theranostic vesicles than sole PEG-b-PLGA nanoparticles without BSA. Overall, this new biocompatible and biodegradable vesicle shows promising potential in theranostic applications.

Antibacterial polymeric nanostructures for biomedical applications

A topical review on recent advances in the research and applications of antimicrobial polymeric nanostructures, such as silver-decorated polymeric nanostructures, and polymeric micelles and vesicles based on antimicrobial polymers and antimicrobial peptides.

Antibacterial Polypeptide-Grafted Chitosan-Based Nanocapsules As an "Armed" Carrier of Anticancer and Antiepileptic Drugs

Antibacterial polypeptides as ancient immune defense systems are effective against bacteria. Here we report a novel kind of "armed" carrier: an antibacterial polypeptide-grafted chitosan-based nanocapsule with an excellent antibacterial efficacy against both Gram-positive and Gram-negative bacteria. This nanocapsule also has excellent blood compatibility and low cytotoxicity. Patients after tumor surgery may benefit from this "armed" carrier because it is highly anti-inflammation and is able to deliver anticancer and antiepileptic drugs simultaneously.