As clearly reported in the literature, the chemistry of wood is mainly based on the combination of a few elements (namely carbon, hydrogen, and oxygen), which are organized in three main structures (i.e., cellulose, hemicellulose, and lignin), hence giving rise to the formation of a bio-composite material, possessing high mechanical strength and toughness, low density, and ease of processing [2<\/a>,3<\/a>]. Undoubtedly, structure and composition mainly affect the overall properties of wood, notwithstanding that the specific mechanical features are determined by the crystallization propensity of cellulose, and in particular the so-called crystalline region of wood, in which the linear macromolecular chains of cellulose are highly arranged in parallel with each other, in contrast to the amorphous region, characterized by high cellulose disorder [4<\/a>]. Further, wood is a multifaceted material, characterized by complex hierarchical structures, high anisotropy due to the presence of open channels aligned in the growth direction, peculiar micro-, meso-, and macro-porosity, and opacity [5<\/a>,6<\/a>].<\/p>\n\n\n\n Despite its exploitation over a wide application range, wood suffers from several issues that somewhat limit its potential uses. In particular, the high anisotropy and the presence of OH groups account for high water sorption and subsequent expansion, hence favoring the formation of cracks and the occurrence of shrinkage phenomena, leading to the overall worsening of its quality and performance [7<\/a>]. To limit these drawbacks, several approaches have been exploited so far, involving both chemical and physical treatments. As an example, Rousset and co-workers demonstrated the effectiveness of short-term pyrolysis treatments carried out under mild conditions (i.e., inert gas environment and temperatures between 160 and 250 \u00b0C) for enhancing the dimensional stability and durability of wood, thanks to the occurrence of hemicellulose degradation phenomena and the partial removal of both free and bound water [8<\/a>]. Another proposed approach refers to high-intensity microwave treatments performed on woods possessing high moisture content: the high energy transferred by the microwave source gives rise to very rapid moisture evaporation that in turn increases the pressure inside the wood cell cavities, hence making the \u201cswollen\u201d biocomposite suitable for further physico-chemical modifications [9<\/a>].<\/p>\n\n\n\n Further, several chemical treatments have been proposed so far for wood: they usually involve selected chemical products able to either interact (i.e., react) with lignin, hemicellulose, and cellulose or to fill the semi-closed wood cells, thus providing new thermal, mechanical, and optical features. As a result, the chemical-modified wood usually shows enhanced properties, specifically referring to its flame retardance, dimensional stability, hardness, electrical behavior, mechanical strength, and abrasion resistance, among others [10<\/a>,11<\/a>,12<\/a>,13<\/a>]. In this context, both as a new material (to investigate as it is, also after densification [14<\/a>]) and a starting intermediate product for further modifications\/functionalizations, transparent wood, from its discovery and morphological characterization in 1992 by S. Fink [15<\/a>], started to acquire great importance as a new material with high potentialities. However, this new concept was somewhat abandoned until 2016, when two research groups, one from the KTH Royal Institute of Technology (Sweden) [16<\/a>] and the other from the University of Maryland (USA) [17<\/a>], concurrently rediscovered the potential of transparent wood. Since then, the delignification methods have been considerably improved, as well as the physico-chemical approaches for its further functionalization. As a result, nowadays, transparent wood can be considered a new, sustainable, smart \u201cbuilding block\u201d for the design of novel functional and structural systems, showing interesting and impactful uses in different advanced sectors, ranging from transparent solar cell windows and optoelectronic components to light-transmitting buildings and structural materials for the automotive industry. This review is aimed at providing the reader with an up-to-date overview of the recent progress in transparent wood-based materials, highlighting their current potential in different advanced sectors, the latest achievements, and, finally, giving some perspectives about further developments for the forthcoming years.Go to:<\/a><\/p>\n\n\n\n Transparency is the physical property of allowing light to pass through a material without appreciable light scattering. On a macroscopic scale, in which object dimensions are much larger than the wavelength of light, photons can be said to follow Snell\u2019s law (Figure 1<\/a>).<\/a>Figure 1<\/a><\/p>\n\n\n\n Refraction of light at the interface between two media of different refractive indices, with n2<\/sub> > n1<\/sub>. Since the velocity is lower in the second medium (v2<\/sub> < v1<\/sub>), the angle of refraction \u03b82<\/sub> is less than the angle of incidence \u03b81<\/sub>.<\/p>\n\n\n\n On the other hand, translucent objects also allow light to pass through, but not necessarily by following Snell\u2019s law. If a translucent material is composed of components with different refractive indices (RIs), photons can be scattered over one of the two interfaces or internally.<\/p>\n\n\n\n A transparent material consists of components with a uniform refractive index [18<\/a>], with the general appearance of one color or any combination leading to a brilliant spectrum of each color. The opposite property of translucency is opacity.<\/p>\n\n\n\n When light encounters a material, it can interact with it in a variety of ways. These interactions depend on the wavelength of the light and the nature of the material. Photons interact with an object through a combination of reflection, absorption, and transmission. Materials transmitting much of the light that falls on them and reflecting little are called optically transparent, a behavior typically exhibited by many liquids, which are characterized by the absence of structural defects (voids, cracks, etc.). Materials that do not transmit light are called opaque. Many of them have a chemical composition that includes centers that may selectively interact with only some of the photons constituting white light. They absorb some portions of the visible spectrum while reflecting others, thus giving rise to color. The attenuation of light of all frequencies and wavelengths is due to the combined mechanisms of absorption and diffusion [19<\/a>]. <\/p>\n\n\n\n Many animals that float near the water surface are highly transparent, which results in almost perfect mimetization. Some marine animals, such as jellyfish, have gelatinous bodies, which are composed mainly of water and are largely transparent [20<\/a>]. Planktonic gelatinous animals are also very transparent. However, transparency is difficult for bodies composed of materials that have refractive indices that differ from water.<\/p>\n\n\n\n Transparency in the air is more difficult to achieve, though a partial example is found in the glass frogs of the South American rainforest, which have translucent skin and pale greenish limbs. Several species of Central American butterflies and many dragonflies and related insects also have wings that are mostly transparent, thus protecting them from predators.<\/p>\n\n\n\n However, with the exception of some tender parts, most plant samples are not sufficiently transparent to allow a view of the internal structures. Conversely, the tissues should be sectioned into slices thin enough to allow for observation by optical means. This is especially true for hard and heavily lignified materials such as wood. <\/p>\n\n\n\n To obtain a visual indication of the internal part of an object, the reconstruction of numerous thin sections in series has been used. This can be virtually achieved by some tomography techniques or by the direct visualization of thin slices.<\/p>\n\n\n\n An alternative approach was proposed by Spalteholz in 1911, who rendered gross organs transparent by immersion in organic liquids with appropriate refractive indices.<\/p>\n\n\n\n The result was a clear three-dimensional image of the macroscopic structures.<\/p>\n\n\n\n The technique consists of combining the refractive indices of the sample and the medium in which it is incorporated: this leads to the whitening of the organ, which allows the observation of its internal structure. This is achieved by replacing the interstitial fluid sample with a fluid having a similar refractive index. However, the liquids used (formalin, acetone, etc.) are toxic, irritating, and volatile, thus being poorly suitable for anatomic studies.<\/p>\n\n\n\n In 1977, while examining a collection of samples, Hagens considered why plastics were used to embed the samples instead of the opposite, hence stabilizing them from the inside and allowing them to be picked up.<\/p>\n\n\n\n With this in mind, Hagens soaked kidney slices in acetone, followed by its replacement with polymethylmethacrylate under a vacuum. The final sample was transformed into a black, non-transparent mass within an hour. He repeated the experiment using liquid silicone rubber, which had more favorable light-refractive properties. This successful procedure was first reported in a patent [21<\/a>] and led to the technique that is known now as plastination<\/em> or forced polymer impregnation [22<\/a>].<\/p>\n\n\n\n Analogously, biologists have shown a similar interest in the anatomy of wood. In this respect, the common technique was that proposed in 1959 by Braun, who, in a long and meticulous study, reconstructed the vessel development of a Populus<\/em> tree from a series of transverse sections, finding that not all run parallel, thus forming a network structure [23<\/a>].<\/p>\n\n\n\n On this basis, in 1992, S. Fink [15<\/a>], inspired by Hagen\u2019s work on plastination, proposed this approach to better study the structure of wood at a greater depth.<\/p>\n\n\n\n Observing the structures within the wood in situ at a greater depth requires one to render the wood samples transparent to light through immersion in liquids with a similar refractive index.<\/p>\n\n\n\n To understand this approach, it is first necessary to outline some principles of the behavior of light in wood and other materials: when light passes through a partially transparent material, a part is reflected on the outer and inner surface, a part is absorbed, and a part is refracted. The amount of absorption is due to the coloring of the object: in particular, darker objects absorb more light than brighter ones. In plants, coloration is mainly due to chlorophyll, tannins, and other polyphenols. Therefore, colored samples must undergo a chemical modification by the action of a bleaching solution to become more transparent.<\/p>\n\n\n\n On the other hand, light scattering (i.e., refraction + reflection) occurs in all parts where objects having a different nature (i.e., different refractive index) meet\u2014that is, at the boundaries between the air, vacuole, cell wall, intercellular spaces, cellulose, hemicellulose, lignin, starch, cytoplasm, and chloroplasts.Go to:<\/a><\/p>\n\n\n\n Wood has a complex hierarchical microstructure, characterized by high anisotropy, in which cellulose, hemicellulose, and lignin, its three main components, are interconnected to form a network. <\/p>\n\n\n\n Cellulose is a polysaccharide consisting of \u03b2(1\u21924) linked D-glucose repeating units with a molecular mass in the range of 104<\/sup>\u2013105<\/sup> g\/mol. Cellulose macromolecular chains mutually interact through H-bonds and van der Waals forces, giving rise to microfibrils, which include both crystalline and amorphous cellulose [24<\/a>]. The combination of microfibrils forms larger fibrils and lamellae.<\/p>\n\n\n\n Hemicellulose is also a polysaccharide but with a more complex composition that includes xyloglucans, xylans, mannans, and glucomannans. The most important biological role of hemicellulose is its contribution to cell wall strengthening through its interaction with cellulose and lignin [25<\/a>].<\/p>\n\n\n\n Lignins are formed by the polymerization of cinnamyl alcohols (monolignols), which differ in structure depending on the plant type (Figure 2<\/a>). With a molar mass up to 3 \u00d7 105<\/sup>, lignin constitutes an integral part of the cell wall, with chemical bonds to both the above polysaccharides, conferring mechanical strength to the plant. However, it has been shown that the major portion of lignin is more often covalently linked to hemicellulose (i.e., xylan and glucomannan).<\/p>\n\n\n\n While polysaccharides are hydrophilic, lignin is hydrophobic, thus acting as an obstacle to water absorption into the cell wall. As a result, lignin is responsible for the effective conduction of water along the plant vascular tissue.<\/p>\n\n\n\n Wood is composed of numerous cells that are mainly aligned along the longitudinal axis. The lignocellulosic wall of cells contains a primary layer (P) and a secondary layer (S). The cell wall consists of microfibril bundles, containing microfibrils, nanofibrils, elemental fibrils, and macromolecular chains.<\/p>\n\n\n\n The secondary layer is in turn composed of three sub-layers, S1, S2, and S3, having a thickness of 0.1\u20130.3, 1\u20135, and 0.1 \u03bcm, respectively (Figure 3<\/a> and Figure 4<\/a>) [26<\/a>].<\/p>\n\n\n\n <\/p>\n\n\n\n <\/p>\n\n\n\n The cell walls are bonded by the compound middle lamella (CML), constituted of the primary wall and the middle lamella. Each layer within the secondary cell wall can be considered as a natural fiber-reinforced composite, where the stiff hydrophobic crystalline cellulose microfibrils are closely packed in a hydrophilic matrix of amorphous cellulose, hemicellulose, and lignin. The central and thickest layer, S2, is the most important structural component of the cell wall and provides mechanical support for the tissue.Go to:<\/a><\/p>\n\n\n\n The complexity of the wood structure, its anisotropy, the presence of interfaces among different materials, and light-absorbing chemical groups are responsible for the opacity of wood. However, the application of proper techniques has resulted in modified wood with a high degree of transparency.<\/p>\n\n\n\n For this purpose, two general approaches have been proposed: lignin removal and lignin modification.<\/p>\n\n\n\n <\/p>\n\n\n\n …<\/p>\n\n\n\n <\/p>\n\n\n\n To read the full article, please go to: https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC9788626\/<\/strong><\/p>\n","protected":false},"excerpt":{"rendered":" Abstract – Human history is largely characterized by the massive use of wood, the most well-known natural composite material, possessing unique thermal, mechanical, and environmental features that make it suitable for several applications, ranging from civil engineering, art, and household uses, to business uses (including furniture, stationery, shipbuilding, and fuel). Further, as a renewable and […]<\/p>\n","protected":false},"author":59,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_seopress_robots_primary_cat":"none","nova_meta_subtitle":"The present manuscript aims at providing the reader with some perspectives about its novel functionalizations and applications","footnotes":""},"categories":[5572],"tags":[5838,12430,11785,11828,5820],"supplier":[21575],"class_list":["post-120593","post","type-post","status-publish","format-standard","hentry","category-bio-based","tag-bioeconomy","tag-buildingmaterial","tag-composites","tag-lignin","tag-wood","supplier-nih-national-library-for-medicine-national-center-for-biotechnology-information"],"_links":{"self":[{"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/posts\/120593","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/users\/59"}],"replies":[{"embeddable":true,"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/comments?post=120593"}],"version-history":[{"count":0,"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/posts\/120593\/revisions"}],"wp:attachment":[{"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/media?parent=120593"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/categories?post=120593"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/tags?post=120593"},{"taxonomy":"supplier","embeddable":true,"href":"https:\/\/renewable-carbon.eu\/news\/wp-json\/wp\/v2\/supplier?post=120593"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}2. Making Objects Transparent<\/h3>\n\n\n\n
3. Wood Structure<\/h3>\n\n\n\n
![\"Figure](\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC9788626\/bin\/materials-15-09069-g002.jpg\" "\\"Click")
![\"Schematic](\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC9788626\/bin\/materials-15-09069-g003.jpg\" "\\"Click")
![\"Hierarchical](\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC9788626\/bin\/materials-15-09069-g004.jpg\" "\\"Click")
4. Current Methods for Obtaining Transparent Wood<\/h3>\n\n\n\n