Morphological Analysis of Several Bamboo Species with Potential Structural Applications

polymers Article Morphological Analysis of Several Bamboo Species with Potential Structural Applications Asier Elejoste 1,*, Alfonso Arevalillo 2, Nagore Gabilondo 1, Amaia Butron 2,* and Cristina Peña-Rodriguez 1,*   Citation: Elejoste, A.; Arevalillo, A.; Gabilondo, N.; Butron, A.; Peña-Rodriguez, C. Morphological Analysis of Several Bamboo Species with Potential Structural Applications. Polymers 2021,13, 2126. https://doi.org/10.3390/polym13132126 Academic Editors: Raul Fangueiro, Jörg Müssig and Diana Ferreira Received: 29 May 2021 Accepted: 23 June 2021 Published: 28 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 ‘Materials + Tecnologies’ Research Group, Chemistry and Environmental Engineering Department, Faculty of Engineering, Gipuzkoa, University of the Basque Country (UPV/EHU), Plaza Europa 1, 20018 Donostia, Spain; [email protected] 2TECNALIA, Basque Research and Technology Alliance (BRTA), Sede Azpeitia, Área Anardi 5, E-20730 Azpeitia-Gipuzkoa, Spain; [email protected] *Correspondence: asier[email protected] (A.E.); [email protected] (A.B.); [email protected] (C.P.-R.) Abstract: Bamboo constitutes a family of plants that are very promising and interesting as renewable materials for both large and small structure construction. To be used as an alternative to traditional materials; the understanding of its morphology and mechanical behavior is of crucial importance. As the distribution of fibers and vascular bundles differs for each type of bamboo; several bamboo types have been characterized: Phyllostachys aurea (PA), Arundinaria amabilis (AA) and Dendrocalamus strictus (DS). Morphological analysis has been performed by optical (OM) and scanning electron microscopy (SEM). Differences in density; surface morphology and wall thickness have been found. In fact; PA and AA have shown a great morphological regularity; while DS presents the greatest thickness; to the point that it can be considered full culm. The plant’s own ducts constitute a very important factor for future impregnations and the optimization of mechanical properties for structure construction. Keywords: bamboo; Phyllostachys aurea; Arundinaria amabilis; Dendrocalamus strictus; morphology 1. Introduction Due to increasing environmental concerns, the scientific community in the field of materials science is striving to find new renewable structural materials. In this context, bamboo is gaining greater recognition in the marketplace amongst consumers of sustainable and environmentally friendly materials [ 1 – 5 ], representing a greener alternative than the normally employed less sustainable materials. Indeed, in comparison to wood, which is characterized by a long growth cycle [ 6 , 7 ], the compressive strength of bamboo is two times higher compared to concrete and its tensile strength is almost equal to that of steel. Different tests have shown that the ultimate tensile strength of bamboo ranges from 140 to 280 N/mm 2 , equal to mild steel [ 8 ]. By using bamboo as a construction material, economic and ecological benefits are obtained, as can be seen in the study by Nayak et al. [ 9 ], which compared the cost of steel reinforcement with bamboo reinforcement. This study showed that some bamboo species have a tensile strength equivalent to that of mild steel and work very well in bending. It is also an added benefit in earthquake-prone areas due to the energy absorption capacity of bamboo. This study concludes that the use of bamboo as a substitute for steel reinforcement is a good option in low-cost structures, being three times cheaper. Bagchi et al. [ 10 ] conducted several similar studies on the use of bamboo as a construction material to replace standard concrete material. They also came to the conclusion that the relationship between bamboo’s mechanical properties and cost makes it a very good alternative to concrete as an environmentally friendly building material, due to its high strength and low cost [ 11 ]. Bamboo regenerates faster and produces less wastage of Polymers 2021,13, 2126. https://doi.org/10.3390/polym13132126 https://www.mdpi.com/journal/polymers Polymers 2021,13, 2126 2 of 13 raw material in pruning. In addition, several authors indicate that bamboo retains larger amounts of carbon than trees, which is crucial for the reduction of the greenhouse effect and the fight against climate change [ 12 – 14 ]. Bamboo is included in the Bambusoideae subfamily, with around 90 genera and more than 1200 species. Bamboo grows naturally in all the continents, being especially abundant in Asia, with 65% of the total world production, mainly in China, followed by Central and South America with 28% and Africa with 7%. However, Brazil possesses the widest variety of bamboo species [ 15 – 17 ]. Bamboo covers a growth area of almost 38,000 million hectares, approximately 3.2% of the total forest area around the world. Regarding the cultivation of bamboo, it is estimated that there are 22 million hectares around the world with nearly 4000 different uses [ 18 ]. The cultivation of bamboo is desirable as it protects against soil erosion, can be grown in acidic land and it is often used to recover degraded areas as it spreads to inhospitable regions where other plants will never survive. However, bamboo is still not well known across Western civilization [12]. As a structural material, bamboo provides both high specific strength and rigidity, and low water absorption compared to wood [ 19 , 20 ]. The specific strength of bamboo is also higher than that of other common structural materials such as wood-based composites, concrete, and steel [ 21 , 22 ]. However, as with other species, bamboo usually presents a hollow culm and, subsequently, mechanical properties vary along the plant, limiting its use in construction as a structural material [ 23 ]. Due to its morphological characteristics, it is mainly used in the form of bamboo derivatives, for furniture production, flooring and in construction and civil engineering [ 24 ]. Indeed, bamboo has been used as a homogeneous construction material only in the form of laminated boards or complex composite materials, once the initial bamboo culms are destroyed and reprocessed; i.e., the fibers are separated and later put back together artificially [ 19 , 25 – 32 ]. However, the entire bamboo culm could be satisfactorily used for construction purposes, such as railings, balconies, or small structures to support electrical wiring, taking advantage of the native effective orientation of cellulose fibers. In order to use bamboo in its original form as a structural material, the first step must be to understand its morphology, thus enabling its treatment in a more efficient way. The schematic structure of bamboo plants is shown in Figure 1. Bamboo is composed of culms with solid transverse diaphragms or nodes separating different hollow inter-nodal regions (Figure 1a). Unidirectional cellulosic fibers are oriented parallel to the longitudinal axis of the culm (Figure 1b) and embedded in the parenchyma tissue matrix composing the circular cross section (Figure 1c) [ 12 ]. This parenchyma tissue matrix lignifies and hardens as the culm matures, leading to increased density and improved mechanical properties. Only a few studies have been carried out on the morphology of bamboo, most of them focused on Guadua or other species not analyzed in this work [ 1 , 23 ]. However, even if the reported studies are mostly related to other properties or practices of bamboo, they usually refer to its morphology [ 20 , 24 , 27 , 33 , 34 ] revealing the necessity of a deeper comparative study in order to better understand the internal structure of the plant and the way to use it. The knowledge related to the morphology of the plant is required in order to solve the two main limitations of bamboo: the durability of the structure and the heterogeneity of the plant in terms of mechanical properties. One of the proposed strategies is the impregnation of the culm of bamboo with suitable resins. For that, in this work, the fiber and duct distribution has been analyzed for three bamboo species to identify the most suitable impregnation conditions in order to generate materials with efficient transmission of stresses from the matrix to the fibers. Polymers 2021,13, 2126 3 of 13 Figure 1. Bamboo scheme: ( a ) view of the plant from the ground to the beginning of the branches, (b) longitudinal and transverse sectioned view and (c) circular cross section. 2. Materials and Methods 2.1. Materials The most common bamboos used for the manufacture of small structures have been selected for this work. Culms of PA bamboo (Phyllostachys aurea), AA bamboo (Arundinaria amabilis), and DS bamboo (Dendrocalamus strictus) were gently provided by Bambusa. They were impregnated by immersion in borax salts for preservation. The employed materials were young bamboo culms grown after at least six cutting/growing cycles of the plant. The culms obtained in the seventh grow of the bamboo plant can be considered homogeneous in terms of morphological and mechanical properties. PA comes from Anji County, Huzhou, Zhejiang, at an altitude of 200 to 500 m, with an ambient temperature of 18 ◦ C with highs of 28 and lows of − 3, humidity of 5.47 ± 0.61%, presenting a density of 863 kg/m 3 (362 kg/m 3 if the hollow center is taken into account). AA comes from Anji County, Huzhou, Zhejiang, at an altitude of 200 to 500 m, with an ambient temperature around of 18 ◦ C with highs of 28 and lows of − 3, humidity of 5.72 ± 0.15%, presenting a density of 940 kg/m 3 (497 kg/m 3 if the hollow center is taken into account). DS comes from Kanchanaburi, Thailand at an altitude of 400 to 600 m with an ambient temperature between 10 and 30 ◦ C and a humidity of 6.6 ± 0.7%, presenting a density of 624 kg/m 3 . For all the study and subsequent tests, the entire culm has been used, as the final objective is the use of the complete culm in structures. 2.2. Methods A Nikon OM E80i microscope was used for optical microscopy (OM) (Nikon Corporation, Japan) characterization both in transmission and reflection modes. The samples were firstly immersed in distilled water, under vacuum for water penetration, and then maintained in boiling water for 1 min. The softened samples were cooled down and cut using a Leica Ultracut R microtome equipped with a diamond knife in rectangular pieces of 0.2 mm in thickness for transmission and 10 mm for reflection. Scanning electron microscopy (SEM) (Hitachi, Ltd., Chiyoda, Tokyo, Japan) characterization was performed with a Hitachi S-4800 microscope at different magnifications. SEM micrographs of 15 nm gold-coated samples were obtained at 10 kV. 3. Results and Discussion 3.1. General Morphology of Bamboo The culm is the part between the roots and branches, which makes up approximately two thirds of the plant (Figure 1a). The general morphology of the bamboo culm was Polymers 2021,13, 2126 4 of 13 analyzed by OM in both reflection and transmission modes. Images obtained from the cross-section (transversal cut) of AA bamboo culm are shown in Figure 2. Figure 2. OM micrographs from the cross-section of AA bamboo as obtained by ( a ) reflection and ( b ) transmission modes. As could be observed, the fibers were more visible in the images obtained in reflection mode (Figure 2a). The fiber packages appear darker due to the loss of light at the bottom of the fibers, whereas the parenchyma appeared bright due to its higher reflection capacity. This image is virtually the negative of the image obtained in transmission (Figure 2b). The different elements of the culm can be identified with more detail from the OM images shown in Figure 3. Figure 3. OM micrographs from the cross-section of AA bamboo: ( a ) transversal cut and ( b ) vascular bundle in detail. As can be observed, vascular bundles make up the repeating pattern all along the cross-section of the culm (Figure 3b). Those fiber packages or sheaths are responsible for maintaining the shape and structural integrity of the plant. The largest fiber sheath is composed of xylem or protoxylem, whereas the smallest one, always closer to the outer part of the bamboo culm, contains phloem fibers. Vascular bundles are the brightest groupings seen in the image due to the light transmission through the fibers. On the other hand, the darkest areas observed in the image, those between the fiber packages, are small parenchyma cells (Figure 3b). Parenchyma, as one of the so-called fundamental tissues, is involved in a wide variety of functions such as photosynthesis, storage, processing of organic substances and tissue regeneration. Parenchyma appeared darker in this case due to the higher density of the tissue of small-sized cells. In addition, other vascular elements that allow light transmission appear in Figure 3. These refer to the metaxylem vessels, which are the pipes responsible for water and food transport throughout the plant. These ducts are clearly observed in Figure 3a. As is well known, lignin is the aromatic biopolymer responsible for reinforcing the structure of major plants, keeping all plant cells stuck together, which is also the case for bamboo. Moreover, the role of lignin in metaxylem Polymers 2021,13, 2126 5 of 13 vessels is especially important because as it is not water-soluble; it prevents the metaxylem vessels from collapsing due to the water flow. Lignin also contributes to thermal stability by providing structural rigidity, the lignin content ranges from 18.35% to 25.80% [35–37]. In summary, three elements can be differentiated in the inner part of the culm: fibers, ducts or metaxylem vessels, and parenchyma (Figure 3b). The fibers are the structural part of the plant, providing the required strength to stand upright and cope with the external mechanical stress. The ducts ensure the circulation of water and nutrients, while the parenchyma cells participate in food storage. Finally, lignin keeps this network of cells, conduits, and fibers stuck together. The distribution of these elements in terms of percentages and densities depends not only on the species analyzed, but also on the part of the culm section that is being analyzed. From the OM images of the transition zone in AA bamboo shown in Figure 4, the appearance of different features can be observed. Fiber sheaths are almost geometrically perfect in shape (Figure 4a), while if parenchyma is analyzed (Figure 4b), cells show different shapes and sizes, grouped in a much more erratic way. Figure 4. Optical micrographs from the transition zone of AA bamboo: ( a ) transversal cut of fiber sheaths and (b) transversal cut of parenchyma. As can be appreciated from SEM images of PA bamboo shown in Figure 5, the vascular bundles constitute the pattern that is repeated throughout the transversal section of the bamboo culm. Taking into account that the growth of the plant occurs only apically, the form and distribution of the fibers, metaxylem vessels and ducts of the culm is maintained from the base to the end. However, this pattern is slightly affected near the nodes, where more radial contact appears between the fibers and the ducts, thus giving the plant a greater resistance to torsional stresses [ 38 ]. In the internodes, both the ducts and the fibers are axially oriented, while the nodes provide the transverse interconnection. As can be seen in OM micrographs shown in Figure 6, in some nodes, small packages of fibers can be found even in the transverse direction (indicated with arrows). This distribution, in addition to improving the resistance to torsion or combined forces [ 39 – 42 ], also allows the plant to retain its integrity and makes the culm fracture much more difficult [ 39 , 40 , 43 , 44 ]. Thus, the node is the most resistant part of the bamboo, as can be read in the study by Chen [ 45 ], which, among the data provided, we can find a tensile strength of 130.93 MPa for internodes and 162.12 MPa for nodes. In order to analyze the morphological features close to the limits of the circular section at bamboo culm, longitudinal cuts were also performed, as shown in Figure 7. These areas are difficult to observe in the transverse cuts as they are usually damaged during sample preparation. When cutting the specimens, the softer inner part is deformed by crushing, while the most brittle outer one tends to splinter. In Figure 7a the outer part is clearly visible, composed by the cortex and the epidermis and the wax that protects the culm [ 38 ]. In the inner part (Figure 7b), the highest concentration of parenchyma cells is observed along with sclerenchyma cells, which are usually dead cells with thickened and lignified secondary walls [ 23 ]. The sclerenchyma, one of the three types of fundamental tissues in Polymers 2021,13, 2126 6 of 13 plants, is composed of rigid and non-stretchable cells and is generally found in disused parts of plant bodies, such as mature stems [ 23 , 46 , 47 ]. As can be seen, the straightness of the lines that appear diminishes closer to the inner part, aggravated especially by the disordered accumulation of cells. Figure 5. SEM images of transversal cut of PA bamboo with different magnifications. Figure 6. Optical micrograph from the transversal cut in a node of AA bamboo. Figure 7. OM micrographs from longitudinal cuts of AA bamboo: (a) outer part and (b) inner part. In the innermost part of the culm, the size of fiber packages gradually decreases to disappear, as can be seen in Figure 7b. The parenchyma cells take on more relevance, Polymers 2021,13, 2126 7 of 13 with large areas composed exclusively of parenchyma observed. The amount of metaxylem vessel is also increased in this area. This specific way of grouping together the different cells, with the greater importance of fibers outside and food storage inside, turns the bamboo culm into a bar with an extraordinary geometry. For any bar, regardless of the material, mechanical stresses (tensile, compression, torsion, etc.) will concentrate at the outermost part. Therefore, the evolution of species has made most of the fibers concentrated at the area in which they are most needed. This concentration decreases when approaching the inner part, and finally completely disappears. In Figure 5, the complete section of the bamboo wall can be seen. In that image, the ducts distribution can be easily identified. It can be observed that in the outer part of culm, ducts become smaller until they disappear, due to the fact that the vascular bundles are massive and compressed. In the inner part of the culm, they become larger until they disappear, and with them, also the fibers. In this way, with respect to the structural elements of the bamboo culm, the longitudinal fibers of the culm and the lignin keep them compact and provide tensile, compression and flexural strength. Instead, the transversal fibers located in the nodes and the lignin prevent relative longitudinal displacement between fibers, providing torsion strength. In Figure 8, the elements of vascular bundle and parenchyma are shown by SEM images with higher magnification. Along the whole length of the metaxylem vessel small holes are appreciated, for supplying nutrients to cells (Figure 8a). In the zone of parenchyma cells (Figure 8b), the aforementioned small ducts are also observed, allowing nutrient transmission. The transversal connection between vessels and parenchyma cells is shown in detail in Figure 8c. The liquid flow in the transversal direction is possible due to the interconnection of cells by these small pits (Figure 8b). This is because the horizontal movement of conductive vessels towards the neighboring parenchyma tissue and the fibers occurs solely by diffusion, thus being very slow [ 38 ]. Moreover, the circulation through the vessels is reduced when the culm is harvested and matured. Therefore, tyloses and limos produced on the neighboring parenchyma cells move towards the vessels, blocking the conductive cavity [ 38 ]. This phenomenon is clearly observable in the bundle of Figure 8d. Figure 8. SEM images of ( a ) longitudinal cut of AA, detail of vessel, ( b ) parenchyma cells of AA, (c) detail of vessels and cells of AA and (d) obstructed vessels of DS. Polymers 2021,13, 2126 8 of 13 3.2. Morphological Comparison of Different Species Figure 10 shows photographs of AA, DS, and PA bamboo culms both before and after water immersion. DS bamboo presents a very rough surface with high thickness, whereas PA bamboo shows a perfectly finished thinner surface. There are also differences regarding their wetting behavior after water immersion. Unlike the AA and DS samples, in the case of PA bamboo, the water impregnation is clearly visible in Figure 10. Figure 9shows the OM images for the inner and outer part of the three different species, together with those of the transition zones. Regarding the inner parts, it can be observed that the central part of DS bamboo (Figure 9b) presents exactly the same morphology as the inner part of the rest of the species. It is not always hollow, even though in the central part almost everything is parenchyma. Small clusters of fibers with their vessels are found, which allow the conduction of nutrients to the cells of the central part. However, on closer observation, it is noticeable that fiber packages in DS bamboo present an erratic orientation. The premise that the smallest bundles will always be closer to the outside of the bamboo culm is not fulfilled in this case. It can be stated that each grouping is facing a different side. On the contrary, as can be seen in Figure 9a,c, both AA and PA maintain the aforementioned structure, following the path discussed throughout the manuscript. Figure 9. OM images of the inner part of ( a ) AA, ( b ) DS and ( c ) PA species, together with those corresponding to the transition zones of ( d ) AA, ( e ) DS and ( f ) PA species and the corresponding images of the outer parts of (g) AA, (h) DS and (i) PA. Analyzing the surface area ratio occupied by fibers and parenchyma, differences were found between the three species. The area to be analyzed was the outer part, since it is the most interesting one in terms of mechanical properties. As could be observed in Figure 11, the proportion changes as it goes deeper into the culm. The main difference was found in the epidermis, which is defined in the first 50 µ m for PA and AA species, while for DS, it extends beyond 200 µm. Polymers 2021,13, 2126 9 of 13 Figure 10. Pictures of AA, DS and PA bamboo culms before (( a ), ( b ), ( c ), respectively) and after (( d ), (e), (f), respectively) water immersion. Figure 11. Fiber percentage in the transversal section of the culm of AA, PA, and DS bamboo species. The external zone. Regarding the inner parts, two other characteristics can be found for DS bamboo that could directly affect the mechanical properties. The first one is the distance between the fiber sheaths. As AA presents lower parenchyma cells density, the fibers are present. On the other hand, in the case of both the AA and DS species, the fiber sheaths appeared to be almost black, while for PA, they were clearer due to higher light reflection, indicating lower fiber density. Furthermore, in the case of PA, the fiber sheaths are much smaller than in the DS and AA species, undoubtedly being the weakest inner part among the three species. The so-called transition zone between inner and outer zones corresponds to a region of 5 mm close to the skin in the case of DS (Figure 9e), and to the intermediate zone between the outer and inner regions for the rest of the species. This part of DS, compared to that of AA (Figure 9d), shows that the distance between fiber groups is somewhat higher. In addition, more metaxylem ducts are observed but not as much as in PA (Figure 9f). Differences in the metaxylem ducts of AA and DS species can be observed in Figure 9g,h, respectively. In the case of DS, these ducts are present not only in all the external parts but also bordering the epidermis. This fact could allow for a better resin impregnation in the external part, which could lead to a significant improvement of the mechanical