Effectiveness Evaluation of Silicone Oil Emulsion In Situ Polymerization for Dehydration of Waterlogged Wooden Artifacts

Organosilicon materials have shown potential as dehydration agents for waterlogged wooden artifacts. These materials can polymerize under normal conditions to form polymers with favorable mechanical strength, antibacterial properties, and aging resistance. However, the insolubility of most organosilicon hindered their penetration into waterlogged wood, which may lead to an unwanted cracking. This study aimed to evaluate the effectiveness of polydimethylsiloxane (PDMS) and hydroxy-terminated polydimethylsiloxane (PDMS-OH) with low viscosity and moderate reactivity for dehydrating waterlogged wooden artifacts from the Nanhai No.1 shipwreck. Four surfactants ((3-aminopropyl) triethoxysilane (APTES), alkyl polyoxyethylene ether (APEO), tri-methylstearylammonium chloride (STAC), and fatty alcohol polyoxyethylene ether (AEO)) and cosurfactant were employed to transform the two kinds of water-repellent silicone oils into eight groups of highly permeable oil-in-water (O/W) emulsions. Under the catalysis of a neutral catalyst, in situ polymerization occurred within the wood cells. Group P2-2 formulated with PDMS-OH and APEO showed the best efficiency in maintaining the dimensions of the wood during dehydration. The dehydrated wood exhibited a natural color and texture with a minimal volume shrinkage rate of 1.89%. The resulting polymer adhered uniformly to the cell walls, effectively reinforcing the wood cell structure. The weight percent gain of the wood was only 218%, and the pores of the cell lumen were well maintained for future retreatment. This method effectively controlled the sol-gel reaction process of the organosilicon and prevented damage to the wooden artifact during the dehydration process. Moreover, the dehydrated wood samples only experienced a low weight gain of 17% at 95% relative humidity (RH), indicating their great environmental stability.

Brushite mineralised Scots pine (Pinus sylvestris L.) sapwood - revealing mineral crystallization within a wood matrix by in situ XRD

Dicalcium phosphate dihydrate (CaHPO4·2H2O, DCPD, brushite) crystals were synthesised within Scots pine sapwood via a wet-chemistry route from aqueous solutions of Ca(CH3COO)2 and NH4H2PO4 salts. SEM/EDS analysis was used to assess the saturation of the wood cell lumina and cell wall as well as morphological features and elemental composition of the co-precipitated mineral. Brushite mineral crystallization and crystallite growth within the wood matrix was studied by in situ XRD. The chemical composition of the mineral before and after the dissolution was evaluated using FTIR spectroscopy. The overall impact of brushite on the thermal behaviour of wood was studied by TGA/DSC and TGA/DTA/MS analysis under oxidative and pyrolytic conditions. Bending and compression strength perpendicular and parallel to the fibre directions as well as bending strengths in longitudinal and transverse directions of the mineralised wood were also evaluated. Results indicate the viability of the wet-chemistry processing route for wood reinforcement with crystalline calcium phosphate (CaP)-based minerals, and imply a potential in producing hybrid bio-based materials that could be attractive in the construction sector as an environmentally friendly building material.

Multifunctional Flame-Retardant, Thermal Insulation, and Antimicrobial Wood-Based Composites

Wood has been used in a variety of applications in our daily lives and military industry. Nevertheless, its flammability causes potential fire risks and hazards. Improving the flame retardancy of wood is a challenging task. Herein, a phytic acid-based flame retardant (referred to as AMPA) was synthesized based on supramolecular reactions between melamine and p-amino-benzene sulfonic acid followed by a reaction with phytic acid using deionized water as the solvent. A composite wood was prepared by removing lignin to tailor the unique mesoporous structure of the material, followed by coating AMPA on the surfaces of wood microchannels. The limiting oxygen index of wood has been improved to 52.5% with the addition of 5.6 wt % AMPA. The peak heat release rate for the prepared composite wood was reduced by 81% compared to that for delignified wood, which demonstrates the excellent flame-retardant performance of the prepared composite wood. Furthermore, AMPA and mesoporous structures endow antimicrobial and thermal insulation functions. Hence, this work provides a feasible method for preparing flame-retardant wood-based materials for diversified applications.

Superhydrophobic Modification of Sansevieria trifasciata Natural Fibres: A Promising Reinforcement for Wood Plastic Composites

Sansevieria trifasciata fibre (STF) is a lignocellulosic material which could be utilised for reinforcement composites. Surface modification is often needed to improve the compatibility of hydrophilic STF and hydrophobic resin. In this study, treatments for natural fibres to attain superhydrophobic properties were carried out using silica nanoparticles and fluorosilane. Sansevieria trifasciata fibres (STF) were subjected to treatment by deposition of silica (SiO₂) nanoparticles which were prepared by the sol-gel method, then followed by modification with fluorosilane, namely 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (PFOTS). The presence of SiO₂ nanoparticles and PFOTS were evaluated using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). The crystallisation properties and thermal behaviour of STF were studied through X-ray diffraction (XRD) and thermogravimetric (TGA) analysis, respectively. The hydrophobicity of STF was determined by water contact angle (WCA) measurement. The results show that nanoscale SiO₂ particles were deposited on the STF surface, and PFOTS were covalently linked to them. The SiO₂ nanoparticles provide surface roughness to the fibres, whereas the long-chain fluorine on PFOTS lowered the surface free energy, and their combination in these treatments has successfully modified the STF surface from hydrophilic into superhydrophobic with a WCA of 150° and sliding angle of less than 10°. Altogether, a non-toxic, simple, and promising method of imparting hydrophobicity on natural fibres was developed, opening new opportunities for these fibres as reinforcement for composite parts.

Physical Properties of Fast-Growing Wood-Polymer Nano Composite Synthesized through TiO2 Nanoparticle Impregnation

Mangium (Acacia mangium Willd.) is a fast-growing wood that is widely grown in Indonesia. The impregnation method is needed to improve the qualities of the wood. In this study, TiO₂ nanoparticle (79.17 nm) was produced using the hydrothermal method. The purpose of this study was to analyze the effect of TiO₂ nanoparticle impregnation on the density and dimensional stability of mangium and the effectiveness of the presence of TiO₂ nanoparticle in wood in degrading pollutants. The mangium samples (2 cm × 2 cm × 2 cm) were placed inside impregnation tube. The impregnation solutions included water (untreated), 1% TiO₂ nanoparticle, and 5% TiO₂ nanoparticles. The samples were analyzed for density, weight percent gain (WPG) dan bulking effect (BE). Samples were also analyzed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). TiO₂ nanoparticle resulted in an increase in density, WPG, and BE-treated mangium. Based on XRD and FTIR results, TiO₂ nanoparticle was successfully impregnated into mangium wood. Scanning electron microscopy-energy-dispersive X-ray spectroscopy analysis indicated that TiO₂ nanoparticle covered the surface of the wood cells. The TiO₂-impregnated mangium wood has a higher photocatalyst activity than untreated, indicating better protection from UV radiation and pollutants.

Fire-Retardant Property of Hexasubstituted Cyclotriphosphazene Derivatives with Schiff Base Linking Unit Applied as an Additives in Polyurethane Coating for Wood Fabrication

A series of new hexasubstituted cyclotriphosphaze derivatives containing Schiff base linkages were successfully synthesized and characterized. The series contains different terminal substituents of pentyl and tetradecyl. Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and carbon, hydrogen, and nitrogen (CHN) elemental analysis were used to characterize the intermediates and final compounds, while the thermal stability of the final compounds is evaluated with a thermogravimetric analysis (TGA) test. The final compounds are physically added to the polyurethane coating formulation and then applied to the wood panel using a brush and the compound’s fire-retardant properties are evaluated using the limiting oxygen index (LOI) test. In this research, compound 3b showed good thermal stability compared to compound 3a. In terms of LOI results, polyurethane with an LOI value of 21.90% was employed as a matrix for wood coating and the value increased to 24.90% when this polyurethane is incorporated with 1 wt.% of the compound 3b. The increase in the LOI value indicates that the wood coating containing hexasubstituted cyclotriphosphazene compounds exhibits excellent fire-retardant properties as additives.

Effects of Wollastonite on Fire Properties of Particleboard Made from Wood and Chicken Feather Fibers

The present study was carried out primarily to investigate the fire properties of particleboards with 5% and 10% feather content. With regard to the flammability of chicken feathers, separate sets of panels were produced with 10% wollastonite content to determine to what extent it could help mitigate the negative effects of the addition of flammable feathers on the fire properties. It was concluded that the inclusion of 5% of chicken feathers can be considered the optimum level, enough to procure part of the ever-growing needs for new sources of raw material in particleboard manufacturing factories, without sacrificing the important fire properties. Moreover, the addition of 10% wollastonite is recommended to significantly improve the fire properties, making the panels more secure in applications with higher risks of fire. It is further stated thata chicken feather content of 10% is not recommended as it significantly deteriorates all properties (including physical, mechanical, and fire properties).

Review of Functional Treatments for Modified Wood

Wood modification is now widely recognized as offering enhanced properties of wood and overcoming issues such as dimensional instability and biodegradability which affect natural wood. Typical wood modification systems use chemical modification, impregnation modification or thermal modification, and these vary in the properties achieved. As control and understanding of the wood modification systems has progressed, further opportunities have arisen to add extra functionalities to the modified wood. These include UV stabilisation, fire retardancy, or enhanced suitability for paints and coatings. Thus, wood may become a multi-functional material through a series of modifications, treatments or reactions, to create a high-performance material with previously impossible properties. In this paper we review systems that combine the well-established wood modification procedures with secondary techniques or modifications to deliver emerging technologies with multi-functionality. The new applications targeted using this additional functionality are diverse and range from increased electrical conductivity, creation of sensors or responsive materials, improvement of wellbeing in the built environment, and enhanced fire and flame protection. We identified two parallel and connected themes: (1) the functionalisation of modified timber and (2) the modification of timber to provide (multi)-functionality. A wide range of nanotechnology concepts have been harnessed by this new generation of wood modifications and wood treatments. As this field is rapidly expanding, we also include within the review trends from current research in order to gauge the state of the art, and likely direction of travel of the industry.

Improved wet shear strength in eco-friendly starch-cellulosic adhesives for woody composites

Development of eco-friendly adhesives from renewable biomass has attracted considerable attention in recent years. Here, we present a novel approach via combination of waste newspaper (WNP) powder, oxidized glutinous rice starch, and polyamidoamine-epichlorohydrin (PAE) to prepare a formaldehyde-free starch-cellulosic adhesive (SCA) for woody composites. The oxidation treatment made the carboxyl/carbonyl groups more available in starch. Plywood bonded by the optimum SCA with 50 wt% of the WNP powder showed a wet shear strength of 0.83 MPa exceeding that of the oxidized starch adhesive by 130.5 %. During the curing process of SCA, the oxidized starch and WNP fiber participated into the crosslinking reaction with PAE via ester and ether bonds, as evidenced by FTIR analysis. The resulting cured adhesive had enhanced crystalline structures, thermal properties, hydrophobicity, wet-cohesion, rheological properties, and adhesiveness to wood. The SCA showed great potential in wood composites as an alternative to formaldehyde-derived adhesives.

Improvement of Mechanical, Hydrophobicity and Thermal Properties of Chinese Fir Wood by Impregnation of Nano Silica Sol

In this paper, a wood-SiO₂ composite material was prepared via in-situ polymerization using vacuum/pressure impregnation technology using commercial scale nano silica sol and Chinese Fir (Cunninghamia lanceolate (Lamb.) Hook.). Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TG), and water contact angle were used to study the changes in the microstructure and physical and mechanical properties of this composite. The results showed that silica sol can penetrate and distribute into the wood cell cavities and surface of cell walls and hence combine with the substances of wood materials. FTIR results indicated that the –OH groups of wood can polycondense in-situ with silica sol to form Si–O–C covalent bonds, and amorphous SiO₂ formed from Si–O–Si bonds between the –OH groups of silica sol did not change the crystalline structure of wood cell walls. This in-situ formulating composite significantly improved the compact microstructure, thermal and mechanical properties, and hydrophobicity of the composites.

Preparation, Test, and Analysis of a Novel Aluminosilicate-Based Antimildew Agent Applied on the Microporous Structure of Wood

Fungi play a considerable role in the deterioration of lignocellulose materials, as their activities either affect the esthetic properties or lead to decay of the host materials. The new generation of organic-inorganic preservatives, which are copper-based but chrome- and arsenic-free, is a subject of many research works. Mildew fungus prevention, treatment of affected materials, and their successive conservation are essential to the woodworkers. To prevent degradation and prolong the service life of wood, a sol-gel organic-inorganic procedure was employed in this study. Aluminum sulfate (Al₂(SO₄)₃), copper sulfate (CuSO₄·5H₂O), and boric acid (H₃BO₃) were introduced into phosphoric acid (H₃PO₄) and water glass as an antimildew agent, with different treatment concentrations (0.7, 1.4, and 2%). Wood was inoculated with Aspergillus niger and Trichoderma viride after new treatment based on the inorganic preservative. The changes in wood surface, structural chemistry, and the crystalline structure of the treated wood were examined by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD), respectively. The growth of the two mildew fungi showed distribution, and evidence of mildew covering only the untreated wood surfaces and an increase in the crystallinity of wood was observed after the process. The study suggests that the two mildew fungi investigated herein could be prevented by sol-gel coating with a Si-Al-Cu-P antimildew agent.

Cellulose and Lignin Nano-Scale Consolidants for Waterlogged Archaeological Wood

Waterlogged archaeological wood comes from submerged archaeological sites (in lake, sea, river, or wetland) or from land waterlogged sites. Even if the wooden object seems to have maintained the original size and shape, the wood is more or less severely decayed because of chemical and biological factors which modify the normal ratio of cellulose and lignin in the cell wall. Drying procedures are necessary for the musealization but potentially cause severe shrinkages and collapses. The conservation practices focus not only on removing water from wood but also on substituting it with materials able to consolidate the degraded wood cell walls like polymers (e.g., PEG), sugars (e.g., lactitol), or resins (e.g., Kauramin). In the present work three different nano-scale consolidants were tested: lignin nanoparticles (LNPs) obtained form beech wood via a non-solvent method involving dialysis; bacterial nanocellulose (BC) obtained from cultures fed with agro-alimentary waste; cellulose nanocrystals (CNC) chemically extracted from native cellulose. Waterlogged archaeological wood samples of different species (oak, elm, stone pine, and silver fir) characterized by different levels of degradation were impregnated with the consolidants. The treatments efficiency was evaluated in terms of macroscopic observation of treated samples, anti-shrink efficiency (ASE) and equilibrium moisture content (EMC). The results obtained for the three consolidants showed substantial differences: LNPs and CNCs penetrated only about a millimeter inside the treated wood, while BC formed a compact layer on the surface of the cell walls throughout the thickness of the samples. In spite of successful BC penetration, physical evaluation of treatment efficiency showed that BC nanoparticles did not obtain a satisfying consolidation of the material. Based on the reported results more focused test protocols are optimized for future consolidation experiments.

Innovative Wood Surface Treatments Based on Nanotechnology

This work reviewed innovative wood surface treatments based on nanotechnology. It is well documented in the literature that the cell walls of wood present significant porosity; this porosity is on a molecular scale. The main reason for the use of nanotechnology in wood science and technology is the unique characteristic of nano-based materials to effectively penetrate deeply into wood substrates, which, in turns, results in the alteration of their surface chemistry. This subsequently causes an improvement in wood properties. Any potential change in the wood properties due to treatment with nanomaterials is based on the higher interfacial area which is developed due to the treatment. This occurs because the number of particles is significantly reduced to the nanoscale. The nanomaterials improve the properties of wood as a raw material and alter its original features to a limited extent. However, their potential impact on both health and the environment should be addressed by applying tools such as life-cycle assessments. This will avoid mistakes being made in which new technologies are released on the market prior to an impact assessment having been carried out.

Potential contamination of the coastal zone by eroding historic landfills

Historically solid waste was commonly landfilled in the coastal zone in sites with limited engineering to isolate waste from adjacent coastal environments. Climate change is increasing the likelihood that these historic coastal landfills will erode releasing solid waste to the coastal zone. Historic coastal landfills are frequently located near designated ecological sites; yet, there is little understanding of the environmental risk posed by released waste. This research investigated inorganic and organic contaminant concentrations in a range of solid waste materials excavated from two historic coastal landfills, and the potential ecological impact should eroded waste be released to the coastal environment. Contaminant concentrations in the analysed waste materials exceeded sediment quality guidelines, indicating erosion of historic coastal landfills may pose a significant environmental threat. Paper and textile wastes were found to make a significant contribution to the total contaminant load, suggesting risk assessments should consider a wide range of solid waste materials.

The Implication of Benzene–Ethanol Extractive on Mechanical Properties of Waterborne Coating and Wood Cell Wall by Nanoindentation

The waterborne coating uses water as its solvent, which will partially dissolve wood extractives when it is applied to wood surfaces. This influences both the coating curing process and the mechanical properties of the cured coating. To investigate these influences, the mechanical properties of waterborne polyacrylic coating on control and extractive-free wood surfaces were investigated by nanoindentation. Reductions to elastic modulus (Er) and hardness (H) of the coating layer was observed in the wood cell walls adjacent to or away from coating layers. Extraction treatment resulted in significant decrease of the Er and H of the coating layer on extractive-free wood surface comparing with control wood, but the values slightly increased for extractive-free wood cell walls compared to a control. Er and H of coating in wood cell lumen were higher than the average value of coating layer on wood surface in both the control and extractive-free wood. The Er of wood cell wall without coating filled in lumen was significantly higher than those of filling with coating. However, there was no distinct difference of H. The Er and H of CCML in extractive-free wood were 15% and 6% lower than those in control ones, respectively.

Improvement of mechanical, humidity resistance and thermal properties of heat-treated rubber wood by impregnation of SiO2 precursor

The SiO₂ precursor solution was impregnated into heat-treated rubber wood to enhance its mechanical and flame-retarding properties. Test specimens were randomly divided into four groups, i.e., untreated (U), heat-treated (HT), impregnated SiO₂ precursor before heat treatment (ISB) and after heat treatment (ISA). Results showed that, compared with HT wood specimens, the modulus of rupture (MOR) and compression strength of ISB and ISA wood specimens were both increased. The hygroscopicity of modified wood was decreased and the dimension stability was consequently improved. Surprisingly, the hardness of ISB specimens increased by 43.65%. The thermogravimetric (TG) examination showed that the incorporation of silicon retarded the thermal decomposition and improved the thermal stability of wood. Furthermore, the scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) revealed that the SiO₂ gel was deposited in the cell wall, The Fourier transform infrared spectroscopy (FTIR) showed the formation of Si–O–Si and Si–O–C covalent bonds. The X-ray diffraction (XRD) tests indicated that the impregnation of SiO₂ precursor had slight effect on the crystalline structure of the wood.

Robust and Low-Cost Flame-Treated Wood for High-Performance Solar Steam Generation

Solar-enabled steam generation has attracted increasing interest in recent years because of its potential applications in power generation, desalination, and wastewater treatment, among others. Recent studies have reported many strategies for promoting the efficiency of steam generation by employing absorbers based on carbon materials or plasmonic metal nanoparticles with well-defined pores. In this work, we report that natural wood can be utilized as an ideal solar absorber after a simple flame treatment. With ultrahigh solar absorbance (∼99%), low thermal conductivity (0.33 W m^-1 K^-1), and good hydrophilicity, the flame-treated wood can localize the solar heating at the evaporation surface and enable a solar-thermal efficiency of ∼72% under a solar intensity of 1 kW m^-2, and it thus represents a renewable, scalable, low-cost, and robust material for solar steam applications.

Micelle, microemulsions, and gels for the conservation of cultural heritage

Past restorations performed with acrylic and vinyl polymers showed detrimental effects to wall paintings that lead to the complete disfiguration of the painted surfaces. The removal of these materials performed with the traditional solvent-based methodology represents a real challenge to conservators and usually achieves very poor results. This review reports on the new palette, nowadays available to restorers, based on microemulsions, micellar systems, physical and chemical gels specifically formulated for the cleaning of cultural heritage artefacts. These systems have been developed in the last twenty years within the cultural framework of colloids and surface science.

Improvement of wood properties by impregnation with TiO2via ultrasonic-assisted sol–gel process

Wood with improved properties was successfully prepared by impregnation with TiO₂via ultrasonic-assisted sol–gel method which is an innovative, simple, and time-saving technology.

Silicification of wood adopted for barrel production using pure silicon alkoxides in gas phase to avoid microbial colonisation

The paper presents a new approach, covering wood with silica-based material in order to protect it from spoilage due to microbial colonisation and avoiding the loss of the natural features of the wood. Wood specimens derived from wine barrels were treated with methyltriethoxysilane in gas phase, leading to the deposition of a silica nanofilm on the surface. (29)Si and (13)C solid state Nuclear Magnetic Resonance and Scanning Electron Microscope-Energy Dispersive X-ray analysis observations showed the formation of a silica polymeric film on the wood samples, directly bonding with the wood constituents. Inductively Coupled Plasma-Mass Spectroscopy quantification of Si showed a direct correlation between the treatment time and silica deposition on the surface of the wood. The silica-coated wood counteracted colonisation by the main wine spoilage microorganisms, without altering the migration from wood to wine of 21 simple phenols measured using a HPLC-Electrochemical Coulometric Detection.

Colloid and materials science for the conservation of cultural heritage: cleaning, consolidation, and deacidification

Serendipity and experiment have been a frequent approach for the development of materials and methodologies used for a long time for either cleaning or consolidation of works of art. Recently, new perspectives have been opened by the application of materials science, colloid science, and interface science frameworks to conservation, generating a breakthrough in the development of innovative tools for the conservation and preservation of cultural heritage. This Article is an overview of the most recent contributions of colloid and materials science to the art conservation field, mainly focusing on the use of amphiphile-based fluids, gels, and alkaline earth metal hydroxide nanoparticles dispersions for the cleaning of pictorial surfaces, the consolidation of artistic substrates, and the deacidification of paper, canvas, and wood. Future possible directions for solving several conservation issues that still need to be faced are also highlighted.

(I/O) hybrid alkoxysilane/zirconium-oxocluster copolymers as coatings for wood protection

Novel inorganic-organic hybrid copolymers based on vinyl- or (3-mercaptopropyl)-trimethoxysilane and an organically modified zirconium-oxocluster were investigated as a wood preservation treatment. The copolymers were prepared using a modified sol-gel strategy not involving alkoxysilane pre-hydrolysis and were applied on wood through a dip coating method. Even though the copolymers were mainly present on the surface of the wood, EDX analysis showed also a uniform distribution of silicon and zirconium in the cell wall but not in the lumina. The grafting of the copolymers on wood was confirmed through FTIR, (13)C and (29)Si MAS NMR analysis. The copolymer obtained from (3-mercaptopropyl)trimethoxysilane was post-functionalized with the methacrylic ester of thymol; introduced for testing as a biocide. Preliminary accelerated biological tests against the brown rot fungus Coniophora puteana, showed resistance to the fungus for the samples coated with the vinyltrimethoxysilane copolymer, while uneven results were obtained for the samples coated with the (3-mercaptopropyl)trimethoxysilane copolymer, even when functionalized with the ester of thymol.

Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market

Today cross-cutting approaches, where molecular engineering and clever processing are synergistically coupled, allow the chemist to tailor complex hybrid systems of various shapes with perfect mastery at different size scales, composition, functionality, and morphology. Hybrid materials with organic-inorganic or bio-inorganic character represent not only a new field of basic research but also, via their remarkable new properties and multifunctional nature, hybrids offer prospects for many new applications in extremely diverse fields. The description and discussion of the major applications of hybrid inorganic-organic (or biologic) materials are the major topic of this critical review. Indeed, today the very large set of accessible hybrid materials span a wide spectrum of properties which yield the emergence of innovative industrial applications in various domains such as optics, micro-electronics, transportation, health, energy, housing, and the environment among others (526 references).

Gels for the conservation of cultural heritage

Gels are becoming one of the most important tools for the conservation of cultural heritage. They are very versatile systems and can be easily adapted to the cleaning and consolidation of works of art. This perspective reviews the major achievements in the field and suggests possible future developments.