Metal-phenolic Networks Research Articles

Particle Engineering via Supramolecular Assembly of Macroscopic Hydrophobic Building Blocks.

Tailoring the hydrophobicity of supramolecular assembly building blocks enables the fabrication of well-defined functional materials. However, the selection of building blocks used in the assembly of metal-phenolic networks (MPNs), an emerging supramolecular assembly platform for particle engineering, has been essentially limited to hydrophilic molecules. Herein, we synthesized and applied biscatechol-functionalized hydrophobic polymers (poly(methyl acrylate) (PMA) and poly(butyl acrylate) (PBA)) as building blocks to engineer MPN particle systems (particles and capsules). Our method allowed control over the shell thickness (e.g., between 10 and 21 nm), stiffness (e.g., from 10 to 126 mN m-1 ), and permeability (e.g., 28-72 % capsules were permeable to 500 kDa fluorescein isothiocyanate-dextran) of the MPN capsules by selection of the hydrophobic polymer building blocks (PMA or PBA) and by controlling the polymer concentration in the MPN assembly solution (0.25-2.0 mM) without additional/engineered assembly processes. Molecular dynamics simulations provided insights into the structural states of the hydrophobic building blocks during assembly and mechanism of film formation. Furthermore, the hydrophobic MPNs facilitated the preparation of fluorescent-labeled and bioactive capsules through postfunctionalization and also particle-cell association engineering by controlling the hydrophobicity of the building blocks. Engineering MPN particle systems via building block hydrophobicity is expected to expand their use.

Metal-polyphenol “prison” attenuated bacterial outer membrane vesicle for chemodynamics promoted in situ tumor vaccines

As natural adjuvants, the bacterial outer membrane vesicles (OMV) hold great potential in cancer vaccines. However, the inherent immunotoxicity of OMV and the rarity of tumor-specific antigens seriously hamper the clinical translation of OMV-based cancer vaccines. Herein, metal-phenolic networks (MPNs) are used to attenuate the toxicity of OMV, meanwhile, provide tumor antigens via the chemodynamic effect induced immunogenic cell death (ICD). Specifically, MPNs are assembled on the OMV surface through the coordination reaction between ferric ions and tannic acid. The iron-based “prison” is locally collapsed in the tumor microenvironment (TME) with both low pH and high ATP features, and thus the systemic toxicity of OMV is significantly attenuated. The released ferric ions in TME promote the ICD of cancer cells through Fenton reaction and then the generation of abundant tumor antigens, which can be used to fabricate in-situ vaccines by converging with OMV. Together with the immunomodulatory effect of OMV, potent tumor repression on a bilateral tumor model is achieved with good biosafety.

Modification of cellulosic adsorbent via iron-based metal phenolic networks coating for efficient removal of chromium ion

Surface modification of durian rind cellulose (DCell) was done by utilizing the strong coordination effect of polyphenol-based metal phenolic networks (MPNs). MPNs from Fe(III)-tannic acid (FTN) and Fe(III)-gallic acid (FGN) were coated on DCell via a self-assembly reaction at pH 8, resulting in adsorbent composites of FTN@DCell and FGN@DCell for removal of Cr(VI). Batch adsorption experiments revealed that FTN coating resulted in an adsorbent composite with higher adsorption capacity than FGN coating, owing to the greater number of additional adsorption sites from phenolic hydroxyl groups of tannic acid. FTN@DCell exhibits an equilibrium adsorption capacity at 30°C of 110.9 mg/g for Cr(VI), significantly higher than FGN@DCell (73.63 mg/g); the adsorption capacity was increased at higher temperature (i.e., 155.8 and 116.8 mg/g at 50°C for FTN@DCell and FGN@DCell, respectively). Effects of pH, adsorbent dose, initial concentration, and coexisting ions on Cr(VI) removal were investigated. The kinetics fractal-based model Brouers-Sotolongo indicates the 1st and 2nd order reaction for Cr(VI) adsorption on FTN@DCell and FGN@DCell, respectively. The isotherm data can be described with a fractal-based model, which implies the heterogeneous nature of the adsorbent surface sites. The Cr(VI) adsorption via surface complexation with phenolic hydroxyl groups was confirmed by evaluating the functional groups shifting. FGN@DCell and FTN@DCell were found to have good reusability, maintaining over 50 % of their adsorption efficiency after four adsorption-desorption cycles. Environmental assessment with Arabidopsis thaliana demonstrated their potential in eliminating the Cr(VI) phytotoxic effect. Thus, this study has shown the efficient and economical conversion of durian waste into environmentally benign adsorbent for heavy metal treatment.

Metal-phenolic networks derived CN-FeC hollow nanozyme with robust peroxidase-like activity for total antioxidant capacity detection

A tannate-iron network-derived peroxidase-like catalyst loaded with Fe ions on carbon nitride (C3N4) was reported for detection of total antioxidant capacity (TAC) in food in this study. Metal-phenolic networks (MPNs) was employed to form a low coordination compound on C3N4, and calcined catalyst formed hollow structure with abundant and uniform Fe sites and surface folds. CN-FeC exhibited significant peroxidase-like activity and high substrate affinity. The homogeneous distribution of amorphous Fe elements on the C3N4 substrate provides more active sites, resulting in provided excellent catalytic activity to activate H2O2 to ·OH, 1O2 and O2·–. The established CN-FeC/TMB/H2O2 colorimetric system can detect AA in the concentration range of 5‐40 μM, with the detection limits of 1.40 μM, respectively. It has good accuracy for the detection of vitamin C tablets, beverages. Taken together, this work shows that metal-phenolic networks can be an effective way to achieve efficient utilization of metal atoms and provides a promising idea for metal-phenolic networks in nanoparticle enzyme performance enhancement.

Metal-Phenolic Networks for Chronic Wounds Therapy.

Chronic wounds are recalcitrant complications of a variety of diseases, with pathologic features including bacterial infection, persistent inflammation, and proliferation of reactive oxygen species (ROS) levels in the wound microenvironment. Currently, the use of antimicrobial drugs, debridement, hyperbaric oxygen therapy, and other methods in clinical for chronic wound treatment is prone to problems such as bacterial resistance, wound expansion, and even exacerbation. In recent years, researchers have proposed many novel materials for the treatment of chronic wounds targeting the disease characteristics, among which metal-phenolic networks (MPNs) are supramolecular network structures that utilize multivalent metal ions and natural polyphenols complexed through ligand bonds. They have a flexible and versatile combination of structural forms and a variety of formations (nanoparticles, coatings, hydrogels, etc.) that can be constructed. Functionally, MPNs combine the chemocatalytic and bactericidal properties of metal ions as well as the anti-inflammatory and antioxidant properties of polyphenol compounds. Together with the excellent properties of rapid synthesis and negligible cytotoxicity, MPNs have attracted researchers' great attention in biomedical fields such as anti-tumor, anti-bacterial, and anti-inflammatory. This paper will focus on the composition of MPNs, the mechanisms of MPNs for the treatment of chronic wounds, and the application of MPNs in novel chronic wound therapies.

Direct Assembly of Metal‐Phenolic Network Nanoparticles for Biomedical Applications

AbstractCoordination assembly offers a versatile means to developing advanced materials for various applications. However, current strategies for assembling metal‐organic networks into nanoparticles (NPs) often face challenges such as the use of toxic organic solvents, cytotoxicity because of synthetic organic ligands, and complex synthesis procedures. Herein, we directly assemble metal‐organic networks into NPs using metal ions and polyphenols (i.e., metal‐phenolic networks (MPNs)) in aqueous solutions without templating or seeding agents. We demonstrate the role of buffers (e.g., phosphate buffer) in governing NP formation and the engineering of the NP physicochemical properties (e.g., tunable sizes from 50 to 270 nm) by altering the assembly conditions. A library of MPN NPs is prepared using natural polyphenols and various metal ions. Diverse functional cargos, including anticancer drugs and proteins with different molecular weights and isoelectric points, are readily loaded within the NPs for various applications (e.g., biocatalysis, therapeutic delivery) by direct mixing, without surface modification, owing to the strong affinity of polyphenols to various guest molecules. This study provides insights into the assembly mechanism of metal‐organic complexes into NPs and offers a simple strategy to engineer nanosized materials with desired properties for diverse biotechnological applications.

Direct Assembly of Metal-Phenolic Network Nanoparticles for Biomedical Applications.

Coordination assembly offers a versatile means to developing advanced materials for various applications. However, current strategies for assembling metal-organic networks into nanoparticles (NPs) often face challenges such as the use of toxic organic solvents, cytotoxicity because of synthetic organic ligands, and complex synthesis procedures. Herein, we directly assemble metal-organic networks into NPs using metal ions and polyphenols (i.e., metal-phenolic networks (MPNs)) in aqueous solutions without templating or seeding agents. We demonstrate the role of buffers (e.g., phosphate buffer) in governing NP formation and the engineering of the NP physicochemical properties (e.g., tunable sizes from 50 to 270 nm) by altering the assembly conditions. A library of MPN NPs is prepared using natural polyphenols and various metal ions. Diverse functional cargos, including anticancer drugs and proteins with different molecular weights and isoelectric points, are readily loaded within the NPs for various applications (e.g., biocatalysis, therapeutic delivery) by direct mixing, without surface modification, owing to the strong affinity of polyphenols to various guest molecules. This study provides insights into the assembly mechanism of metal-organic complexes into NPs and offers a simple strategy to engineer nanosized materials with desired properties for diverse biotechnological applications.

Assembly of Metal-Phenolic Networks onto Microbubbles for One-Step Generation of Functional Microcapsules.

The one-step assembly of metal-phenolic networks (MPNs) onto particle templates can enable the facile, rapid, and robust construction of hollow microcapsules. However, the required template removal step may affect the refilling of functional species in the hollow interior space or the in situ encapsulation of guest molecules during the formation of the shells. Herein, a simple strategy for the one-step generation of functional MPNs microcapsules is proposed. This method uses bovine serum albumin microbubbles (BSA MBs) as soft templates and carriers, enabling the efficient pre-encapsulation of guest species by leveraging the coordination assembly of tannic acid (TA) and FeIII ions. The addition of TA and FeIII induces a change in the protein conformation of BSA MBs and produces semipermeable capsule shells, which allow gas to escape from the MBs without template removal. The MBs-templated strategy can produce highly biocompatible capsules with controllable structure and size, and it is applicable to produce other MPNs systems like BSA-TA-CuII and BSA-TA-NiII . Finally, those MBs-templated MPNs capsules can be further functionalized or modified for the loading of magnetic nanoparticles and the pre-encapsulation of model molecules through covalence or physical adsorption, exhibiting great promise in biomedical applications.

GSH-Activatable Metal-Phenolic Networks for Photothermal-Enhanced Chemotherapy and Chemodynamic Therapy.

Chemotherapy (CT) plays an important role in the antitumor process, but the unsatisfactory therapeutic efficacy and the obvious toxic side effects of CT seriously restrict its application. To overcome the limitations of CT, the strategy of chemotherapy enhanced by chemodynamic therapy (CDT) and photothermal therapy (PTT) has been considered a promising approach to improve the anticancer effect. Herein, a novel GSH-activatable Cu2+-Quercetin network (QC) was synthesized via a convenient strategy to load Au nanoparticles (NPs) and DOX, named QCDA, for the synergistic therapy of CT/CDT/PTT. The results showed that QCDA exhibited GSH-sensitive degradation and "cargos" release in cancer cells, and then PTT and CDT caused by Au NPs and Cu+ significantly enhanced the CT effect of DOX and Quercetin on anticancer. More importantly, the PTT and depleted GSH accelerated the Fenton-like ionization process resulting in facilitating the CDT efficiency. Collectively, the multi-mode synergistic strategy of CT/CDT/PTT, which showed an excellent therapeutic effect, maybe a potential therapeutic pathway for anticancer.

Facile Fabrication of Carbon Hybrid Membranes by Fast Self-Assembled Coating with Metal–Phenolic Networks for the Selective Separation of Lead from Water

Facile Fabrication of Carbon Hybrid Membranes by Fast Self-Assembled Coating with Metal–Phenolic Networks for the Selective Separation of Lead from Water

A sustainable and smart fungicide release platform through cocrystal nanocapsules for improved utilization rate and environmental safety

Pesticides have always been restricted by environmental factors and utilization rate. Smart delivery systems are important to modify pesticides and improve utilization rate, but the economic and environmental risks of the carriers limit their practical applications. Here, responsive release nanocapsules were constructed with metal-phenolic networks and pyraclostrobin - thiophanate-methyl (PYR-TM) cocrystal core to modify the poor physicochemical properties and safety of pesticides. PYR-TM was prepared by sustainable mechanochemical method and deposited by metal-phenolic networks in water, producing the PYR-TM@TA-Cu nanocapsules. The crystal structure and DFT calculations proved that TM improved the photostability of PYR by 75.7-fold through competitive energy absorption of photon and molecular conformation modulation. With a loading capacity up to 91.2%, PYR-TM@TA-Cu exhibited pH-responsive release behavior triggered by acidic conditions, which showed that 43.6%, 51.4%, 62.4% and 81.6% of encapsulated PYR were released at pH 7.9, 6.2, 5.3, and 4.0, respectively. Additionally, PYR-TM@TA-Cu exhibited favorable fungicidal activity (EC50 = 0.123 mg/L) in controlling Botrytis cinerea. The greenhouse control efficiency of fungal infection was enhanced from12.0% to 70.7% by PYR-TM@TA-Cu after 7 days spraying. The coating of metal-phenolic network improved the washing persistence of the pesticides from 51.26% to 79.88%, the security for zebrafish by 42-fold, and decreased the leaching potential of the pesticides. Meanwhile,PYR-TM@TA-Cu showed no phytotoxicity on the growth of corn and cucumber plants. Overall, this study provides insight into efficient and sustainable agricultural production with the construction of an eco-friendly nanocapsules platform by cocrystal engineering strategy.

Metal-phenolic networks modified two-dimensional magnetic MXene-based composites for highly efficient enrichment of phosphopeptides

Efficient and specific enrichment of phosphopeptides is a precondition for the research of phosphoproteomics-related processes, while the development of more superior affinity materials remains a long-term challenge. In this work, Zr-tannic acid metal-phenolic networks (MPNs) modified magnetic composites based on two-dimensional MXene substrates, denoted as Ti3C2Tx@Fe3O4@MPNs-TA/ZrIV, were facilely prepared and innovatively applied for phosphopeptides enrichment. By virtue of large specific surface, excellent magnetic responsiveness, and massive affinity sites provided by TA/ZrIV-based MPNs, the Ti3C2Tx@Fe3O4@MPNs-TA/ZrIV composites exhibited outstanding enrichment performance with high sensitivity (1 fmol μL−1), excellent selectivity (β-casein: bovine serum albumin = 1:5000, molar ratio), and satisfactory repeatability (5 times). Encouraged by the remarkable enrichment results, we further employed the Ti3C2Tx@Fe3O4@MPNs-TA/ZrIV composites to detect phosphopeptides from non-fat milk and human saliva, which confirmed the great potential of Ti3C2Tx@Fe3O4@MPNs-TA/ZrIV composites in phosphopeptide enrichment from intricate biological specimens. The enrichment results demonstrate that Ti3C2Tx@Fe3O4@MPNs-TA/ZrIV composites are ideal candidates for the research of phosphoproteomics and also promote the application of surface-functionalized MXene-based materials in the biomedical field.

Development of novel EGCG/Fe loaded sodium alginate-based packaging films with antibacterial and slow-release properties

Antibacterial food packaging with slow and sustained release of antibacterial agents exerts a crucial role in ensuring food quality and safety. Herein, metal-phenolic networks based on epigallocatechin gallate (EGCG) and iron (Fe), as well as thyme essential oil nano-emulsion (TEON) were successfully incorporated into sodium alginate (SA) matrix to prepare SA-based films through solution casting method. The effects of EGCG/Fe and different concentrations of TEON on the morphology, structure, physical properties, and functional characteristics of SA-based films were systematically analyzed. Results showed that the addition of EGCG/Fe and TEON significantly enhanced the mechanical intensity, flexibility, thermal stability, UV shielding, water vapor barrier performance, and oxygen barrier ability of the SA-based films. In addition, the introduction of TEON made the SA-based films slow-release of thyme essential oil, which obviously enhanced the antioxidant and dynamic antimicrobial properties of the films. Ultimately, the SA-based film revealed an outstanding preservation effect on pork by significantly reducing the increase rate of pH value and total plate count of pork. This developed SA-based films loaded with EGCG/Fe and TEON were expected to become a potential antibacterial material with slow-release properties for food packaging applications.

Biomimetic Epigallocatechin Gallate-Cerium Assemblies for the Treatment of Rheumatoid Arthritis.

Rheumatoid arthritis (RA) is an autoimmune and inflammatory disease that is so far incurable with long-term health risks. The high doses and frequent administration for the available RA drug always lead to adverse side effects. Aiming at the obstacles to achieving effective RA treatment, we prepared macrophage cell membrane-camouflaged nanoparticles (M-EC), which were assembled from epigallocatechin gallate (EGCG) and cerium(IV) ions. Due to its geometrical similarity to the active metal sites of a natural antioxidant enzyme, the EC possessed a high scavenge efficiency to various types of reactive oxygen species (ROS) and reactive nitrogen species (RNS). The macrophage cell membrane assisted M-EC in escaping from the immune system, being uptaken by inflammatory cells, and specifically binding IL-1β. After tail vein injection to the collagen-induced arthritis (CIA) mouse model, the M-EC accumulated at inflamed joints and effectively repaired the bone erosion and cartilage damage of rheumatoid arthritis by relieving synovial inflammation and cartilage erosion. It is expected that the M-EC can not only pave a new way for designing metal-phenolic networks with better biological activity but also provide a more biocompatible therapeutic strategy for effective treatment of RA.

Impact of metal-phenolic networks on the structural and functional properties of soy protein

The modification of soy protein refers to the process of altering the physical, chemical, or functional properties of soy protein through various methods to improve its applicability in various food and non-food industries. Metal-phenolic networks have been proven effective in modifying proteins. In this study, the metal-phenolic networks (MPNs) consisting of (−)-epigallocatechin-3-gallate (EGCG) and Fe3+ were used to modify SPI for the first time. The successful assembly of the MPNs and structural properties of SPI were investigated with UV, FTIR, energy-dispersive X-ray spectroscopy and fluorescence spectroscopy. Results indicated that the modified SPI by MPNs showed reduced surface hydrophobicity and improved foaming properties. Additionally, the emulsions stabilized by MPNs showed smaller droplet size and higher modulus with increasing complex concentration. This study demonstrates the feasibility of MPNs as a novel modification method to improve the functional properties of soy protein.

Metal phenolic networks (MPNs)-based pH-sensitive stimulus responsive nanosystems for drug delivery in tumor microenvironment

The tumor microenvironment (TME) is a dynamic and complex system characterized by different cellular, molecular, and physiological heterogeneity. Therefore, conventional drug delivery strategies often failed to deliver an effective dosage to kill cancerous cells. In recent years, the development of nano-based drug delivery has expanded into a wide range of clinical applications to address the limitations of free therapeutics and voyage the biological barriers. Recently, stimuli-responsive metal phenolic networks (MPNs) have recently become a focus of intense scientific investigation in biomedical applications, and offer a versatile platform for controlled release of therapeutics in TME. This review provides a timely and thorough description of the use of a MPNs in the rapidly evolving field of advanced drug delivery with insights unique to pH-stimulus responsive systems. This review aims to offer an in-depth understanding of the potential use of MPNs as pH-sensitive therapeutics carriers in TME. We briefly explained the endogenous pH-associated factors affecting the therapeutics delivery in TME. A cross comparison of the advantages and disadvantages of MPNs with other counterparts are highlighted to elucidate their key design principles for pH-stimulus responsive drug delivery in TME. In later sections, pros and cons of MPNs as well as current challenges and future perspectives are also discussed, which will deepen our understanding of designing, fabricating, and applying MPNs for advanced therapeutics.

Employing single valency polyphenol to prepare metal-phenolic network antitumor reagents through FeOOH assistance.

Metal-phenolic networks (MPNs) have been conventionally formed by the coordination of metal ions and polyphenols, which can responsively release metal ions and polyphenols under the trigger of tumor microenvironment (TME), showing high potential in the field of antitumor. However, MPNs are mainly limited to multi-valency polyphenols, and the unavailability of single valency polyphenols greatly hinders their applications even though they have excellent antitumor activity. Herein, we demonstrate a FeOOH assisted preparation method for MPNs antitumor reagent by introducing the complexes of Fe3+, H2O and polyphenol (Fe(H2O)x-polyphenoly) in the preparation process, which overcomes the shortage of single valency polyphenols. Taking apigenin (Ap) as an example, Fe(H2O)x-Apy complexes are foremost formed, in which the Fe(H2O)x is capable of hydrolyzing to generate FeOOH, thus producing Fe3+-Ap networks-coated FeOOH nanoparticles (FeOOH@Fe-Ap NPs). Under the stimulation of the TME, FeOOH@Fe-Ap NPs could release Fe2+ and Ap, realizing ferroptosis and apoptosis for tumor combination therapy. In addition, FeOOH can shorten transverse relaxation time, acting as a T2-weighted magnetic resonance imaging contrast agent. The current efforts provide an alternative strategy for constructing MPNs by exploiting single valency polyphenols, which strengthens the potential of MPNs in antitumor applications.

Alginate-modulated continuous assembly of iron/tannic acid composites as photothermally responsive wound dressings for hemostasis and drug resistant bacteria eradication

Multifunctional dressing materials are highly required to combat multidrug resistant bacteria in wound infections. Here an alginate-based aerogel dressing is reported that combines photothermal bactericidal activity, hemostatic property, and free radical scavenging for skin wound disinfection and accelerated wound healing. The aerogel dressing is facilely constructed by immersing a clean nail (Fe) in a mixed solution of sodium alginate (Alg) and tannic acid (TA), followed by freezing, solvent replacement, and air drying. The Alg matrix plays an essential role in modulating the continuous assembly process between TA and Fe to allow the homogenous distribution of TA-Fe metal-phenolic networks (MPN) in the resulting composite, without forming aggregates. The photothermally responsive Nail-TA/Alg aerogel dressing is successfully applied in a murine skin wound model infected with Methicillin-resistant Staphylococcus aureus (MRSA). This work provides a facile strategy to integrate MPN with the hydrogel/aerogel matrix through in situ chemistry, which is promising for developing multifunctional biomaterials and biomedicine.

PH-Responsive Nanogel for Photothemal-Enhanced Chemodynamic Antibacterial Therapy

Chemodynamic therapy (CDT) and photothermal therapy (PTT) have been widely studied as treatment alternatives to anbiotics to avoid the occurrence of bacterial resistance because of their negligible bacterial resistance and high antibacterial efficiencies. Herein, we developed an infection microenvironment-responsive nanogel (STF) to successfully achieve a PTT synergized CDT for bacterial infection. Through a one-step/pot method, STF nanotherapeutics consisting of Fe3+/sodium alginate (SA) based nanogels and Fe3+/tannic acid (TA) based metal–phenolic networks (MPNs) were conveniently prepared by the coordination-triggered simultaneous self-assembly of SA, TA, and Fe3+ ions. Afterward, their photothermal and catalytic performances were intensively investigated. Moreover, the sterilization effect of the STF nanogel was evaluated through in vitro and in vivo bactericidal assays. The STF nanogel exhibited a photothermal enhanced peroxidase-like activity because of the good photothermal effect of MPNs. In addition, its Fenton activity was further accelerated by an extra amount of Fe2+ derived from the degradation of Fe3+-chelating agents (SA) and reduction of TA. Together, the STF nanogel possessed a photothermal/Fe2+ dual-augmented catalytic property. In vitro and in vivo antibacterial assays proved its satisfactory bactericidal efficiency owing to PTT/Fe2+ dual-enhanced CDT. Our research findings suggested that the developed intelligent bactericidal nanogel has great potential to be used as an antibiotic alternative to avoid bacterial resistance in future clinical treatments.

Recent Advances of Fe(III)/Fe(II)-MPNs in Biomedical Applications.

Metal-phenolic networks (MPNs) are a new type of nanomaterial self-assembled by metal ions and polyphenols that have been developed rapidly in recent decades. They have been widely investigated, in the biomedical field, for their environmental friendliness, high quality, good bio-adhesiveness, and bio-compatibility, playing a crucial role in tumor treatment. As the most common subclass of the MPNs family, Fe-based MPNs are most frequently used in chemodynamic therapy (CDT) and phototherapy (PTT), where they are often used as nanocoatings to encapsulate drugs, as well as good Fenton reagents and photosensitizers to improve tumor therapeutic efficiency substantially. In this review, strategies for preparing various types of Fe-based MPNs are first summarized. We highlight the advantages of Fe-based MPNs under the different species of polyphenol ligands for their application in tumor treatments. Finally, some current problems and challenges of Fe-based MPNs, along with a future perspective on biomedical applications, are discussed.