Most fungi are multicellular organisms. They display two distinct morphological stages: the vegetative and reproductive. The vegetative stage consists of a tangle of slender thread-like structures called hyphae singular, hypha , whereas the reproductive stage can be more conspicuous.
The mass of hyphae is a mycelium. It can grow on a surface, in soil or decaying material, in a liquid, or even on living tissue.
Example of a mycelium of a fungus : The mycelium of the fungus Neotestudina rosati can be pathogenic to humans. The fungus enters through a cut or scrape and develops a mycetoma, a chronic subcutaneous infection. Most fungal hyphae are divided into separate cells by endwalls called septa singular, septum a, c. In most phyla of fungi, tiny holes in the septa allow for the rapid flow of nutrients and small molecules from cell to cell along the hypha.
They are described as perforated septa. The hyphae in bread molds which belong to the Phylum Zygomycota are not separated by septa. Instead, they are formed by large cells containing many nuclei, an arrangement described as coenocytic hyphae b. Fungi thrive in environments that are moist and slightly acidic; they can grow with or without light. A bright field light micrograph of c Phialophora richardsiae shows septa that divide the hyphae.
Like animals, fungi are heterotrophs: they use complex organic compounds as a source of carbon, rather than fix carbon dioxide from the atmosphere as do some bacteria and most plants.
In addition, fungi do not fix nitrogen from the atmosphere. Like animals, they must obtain it from their diet. However, unlike most animals, which ingest food and then digest it internally in specialized organs, fungi perform these steps in the reverse order: digestion precedes ingestion. First, exoenzymes are transported out of the hyphae, where they process nutrients in the environment. Then, the smaller molecules produced by this external digestion are absorbed through the large surface area of the mycelium.
As with animal cells, the polysaccharide of storage is glycogen rather than the starch found in plants. Fungi are mostly saprobes saprophyte is an equivalent term : organisms that derive nutrients from decaying organic matter. They obtain their nutrients from dead or decomposing organic matter, mainly plant material.
Fungal exoenzymes are able to break down insoluble polysaccharides, such as the cellulose and lignin of dead wood, into readily-absorbable glucose molecules. The carbon, nitrogen, and other elements are thus released into the environment. Because of their varied metabolic pathways, fungi fulfill an important ecological role and are being investigated as potential tools in bioremediation.
Some fungi are parasitic, infecting either plants or animals. Fungi can reproduce asexually by fragmentation, budding, or producing spores, or sexually with homothallic or heterothallic mycelia. Perfect fungi reproduce both sexually and asexually, while imperfect fungi reproduce only asexually by mitosis.
In both sexual and asexual reproduction, fungi produce spores that disperse from the parent organism by either floating on the wind or hitching a ride on an animal.
Fungal spores are smaller and lighter than plant seeds. The giant puffball mushroom bursts open and releases trillions of spores.
The huge number of spores released increases the likelihood of landing in an environment that will support growth. The release of fungal spores : The a giant puff ball mushroom releases b a cloud of spores when it reaches maturity. Fungi reproduce asexually by fragmentation, budding, or producing spores.
Fragments of hyphae can grow new colonies. Mycelial fragmentation occurs when a fungal mycelium separates into pieces with each component growing into a separate mycelium.
Somatic cells in yeast form buds. During budding a type of cytokinesis , a bulge forms on the side of the cell, the nucleus divides mitotically, and the bud ultimately detaches itself from the mother cell.
The most common mode of asexual reproduction is through the formation of asexual spores, which are produced by one parent only through mitosis and are genetically identical to that parent. Spores allow fungi to expand their distribution and colonize new environments. They may be released from the parent thallus, either outside or within a special reproductive sac called a sporangium.
Types of fungal reproduction : Fungi may utilize both asexual and sexual stages of reproduction; sexual reproduction often occurs in response to adverse environmental conditions. There are many types of asexual spores. Conidiospores are unicellular or multicellular spores that are released directly from the tip or side of the hypha. Other asexual spores originate in the fragmentation of a hypha to form single cells that are released as spores; some of these have a thick wall surrounding the fragment.
Yet others bud off the vegetative parent cell. The single cell then sends out hyphae to help establish the fungus and gather food. After the spore has sent out its hyphae, they will eventually meet up with the hyphae of another mushroom. After the sexual process of reproduction has begun, the mushroom forms the structures of a "fruiting body" that will eventually produce and disperse spores.
Immature fruting body. The mature fruiting body can have various structures. The picture at left is that of an Amanita , one type of mushroom. The fruiting body may contain a cap, stalk, ring, volva, and gills. The cap normally houses the spore producing surface of the fruiting body. In the case of the Amanita , the spore-producing cells are in the gills, but in other types of mushrooms, spores are produced in tubes or inside the cap.
By comparing this illustration to the spore above, it is evident which parts of the spore develop into specific structures of the fruiting body. Mature fruiting body. Types of Mushrooms. In cup fungi, the spore-producing asci are located on the inner surface of the mature fruiting body. Spores are released in a cloud when the asci break open.
To exclude the effect of material damage on the response, the tests were performed within small strains. The rate-dependence of the material behavior is not significant in this range of strain rates. Stress relaxation behavior and rate sensitivity of mycelium: a stress vs. In this section we present a two-scale model developed to represent the mechanical behavior of mycelium.
The model can be further used to solve boundary value problems or to represent composites with mycelium matrix. Given the network microstructure of the material, the most comprehensive model would be a random fiber network with fiber properties, structure and spatial distribution of fiber density matching those of the actual material. Such a model would include a very large number of fibers and would be intractable with the current modeling and simulation capabilities.
To address this problem we develop a two-scale model. On the larger scale, comparable with the macroscopic scale of the samples tested, we use a stochastic continuum representation. The density and hence the mechanical behavior are allowed to change from sub-domain to sub-domain on this scale, with a characteristic length scale. This model takes into account density fluctuations with a length scale much larger than the network scale, i.
The constitutive behavior of each such sub-domain is provided by a sub-scale random fiber network model in which individual fibers are explicitly represented.
This coupling is in the spirit of sequential multiscale models We present next each of these models, the coupling, the calibration, and the validation procedures. The microscale model is a representative volume element RVE of a random fiber network of specified density.
The network is generated using a Voronoi tessellation algorithm. A set of randomly distributed seed points are generated inside a cubic domain and are used to generate a Voronoi tessellation. Fibers are defined along all edges of the resulting tessellation which results in an interconnected fiber network. The coordination number, i. A representative network configuration generated using this procedure is illustrated in Fig.
Random fiber network model: a representative network configuration, b projected view of a thin slice of the entire network and c comparison of the model network red and mycelium blue projected mesh size distributions.
These relations allow calibration of the network density directly based on the mass density of the physical network. As an additional check of the resulting stochastic structure, we compare the topology of the model obtained using this procedure with the mycelium microstructure. To this end, we consider images of mycelium obtained by SEM Fig.
Given the finite depth of focus of the instrument, these images are actually 2D projections of the network structure within slices of the mycelium structure. These are compared with corresponding projections of slices of similar thickness of the model network. Figure 9 b shows such a slice projection of the model network. The two projected structures are compared in terms of the mesh size distribution. Mesh size is calculated as the diameter of the greatest circle that fits within the void spaces in the 2D projected images.
Each of these distributions is obtained by averaging over multiple replicas of the respective structure. The two distributions match closely, which indicates that the geometry of the model network reproduces the measurable features of the actual mycelium network and no calibration is needed. In the actual mycelium, the diameter of individual filaments varies in the range from 0.
The mechanical behavior of the chitin wall is considered elastic-plastic with the elastic modulus, Poisson ratio and yield strength reported in the literature for chitin, 2.
In the finite element model constructed based on the structure of the RVE Fig. The number of elements per fiber is selected such to keep the element aspect ratio close to 5.
The model is subjected to boundary conditions that mimic the experimental set-ups and solved using the general purpose finite element solver Abaqus version 6. In uniaxial tension, top boundary nodes are subjected to prescribed displacement in the loading direction while bottom boundary nodes are constrained in the loading direction.
To study the network behavior in compression, we introduce two rigid surfaces at the top and bottom boundaries of the network and network is compressed by displacing the top surface while keeping the bottom surface stationary.
Additionally, in compression, we incorporate surface based contact between the rigid surfaces and beam element surfaces similar to At small strains the network exhibits a linear elastic response and the stiffness is identical in tension and compression.
This is in agreement with experimental observations Fig. This is due to the pronounced bending of filaments and their reorientation in the direction perpendicular to the compressive axis. Representative deformed network configurations with displacement contours are illustrated in Fig.
Figure 10 d,e illustrates the stress-strain response of networks with seven different densities averaged over three realizations under tension and compression respectively each curve is an average over 3 realizations.
The curves show qualitatively similar behavior as discussed above. A 3D stochastic continuum model is used at the macroscale to obtain a homogenized representation of the macroscopic mycelium mechanical behavior, Fig. The domain is divided in subdomains, each being assigned a network density sampled from a distribution. In principle, the constitutive behavior of each such sub-domain would be provided by a microscale RVE loaded with the strain of the respective macroscale sub-volume.
This procedure is computationally expensive and requires tracing the deformation history of each element. RVEs of seven different densities are considered Fig. These numerically defined constitutive laws are then assigned randomly to sub-domains of the continuum model.
The volume fractions corresponding to each density correspond to the distribution of densities of the actual mycelium. Procedure used to map the microscale network behavior to subdomains of the continuum stochastic macroscale model. The insets show realizations of the network microscale RVEs corresponding to each of the 7 density bins; c Comparison of normal strain distribution measured on the surface of the physical sample red and macroscopic model blue corresponding to the mass density distribution in b for three values of the far field applied strain in the linear response regime of the mycelium; d Comparison of global stress-strain response predicted by the model symbols and experiments solid lines ; Error bars indicate range of five realizations.
In order to identify the fluctuations of density on the mesoscale, i. We iterate through this procedure until the distribution of computed strains matches the distribution of measured strains.
Figure 11 summarizes the procedure and the results. We consider that the distribution of mass density in the mycelium follows a beta distribution, Fig. The mass density values are assigned to the sub-domains of the continuum model in an uncorrelated way.
This is identical to requiring that the correlation length of the mass density distribution in the physical sample is equal to the size of the subdomains considered. The mean of the distribution is equal to the actual macroscopic sample density. This leaves only the variance of the distribution to be identified based on the procedure stated above. Figure 11 c shows the probability distribution functions for the normal strains measured on the surface of the mycelium sample in the loading direction with DIC, and computed with the continuum model at the end of this optimization procedure.
Distributions are shown for three levels of macroscopic strain in the linear elastic range. This corresponds to the density distribution function shown in Fig. Once the density distribution is defined, the subdomains are being assigned one of the constitutive behaviors shown in Fig. Specifically, we used an elastic-plastic material model with parameters such as elastic modulus, yield strength calculated from the network stress-strain responses as shown in Fig. Figure 11 d shows the comparison of the measured and predicted macroscopic true stress- true strain behavior in both tension and compression.
The curves represent the average of five realizations of the continuum macroscale model and the bars represent the range of the respective replicas. The model prediction is identical to that of mycelium in the respective strain range. It is possible that in the physical sample damage of hyphae takes place during deformation. In absence of reliable damage properties available for hyphae filaments, we choose not to incorporate damage into the network model. In compression, the model predicts accurately the onset of localization and the strain hardening beyond the yield point.
It is emphasized that this is a prediction of the model and was not fitted. The only calibration needed was that of the distribution of densities in the continuum model. We presented morphological and mechanical characterization of a novel biomaterial derived from fungal mycelium. The experimental results revealed the most significant characteristics of mycelium under tension and compression. In tension, the material response is linear elastic at low strain, and then the material yields and undergoes strain hardening before rupture.
On the other hand, the bio polymer behaves similar to open cell foam under uniaxial compression, where the stress-strain curve shows an initial linear-elastic regime followed by a plateau regime with softened response. Furthermore, when subjected to successive loading and unloading cycles, mycelium exhibits strain dependent hysteresis and stress softening effect Mullins effect from cycle to cycle. The mechanical properties show significant variation with material density.
The elastic modulus results in the range to kPa for the given range of densities for both in tension and compression. The measured yield strength is in the order of 40—80 kPa, whereas the ultimate strength in tension varies from — kPa depending on material density.
A multiscale continuum model incorporating microstructural details of the network and which accounts for spatial density variation in the actual material was developed. It is demonstrated that the model represents the topology of mycelium at the network scale. By comparing the strain distribution in the model with the strain distribution measured through DIC, we calibrate the distribution of density in the material on the mesoscale. The stress-strain curves on the mesoscale are uniquely defined by the local density and fiber properties obtained from the literature.
The stochastic continuum model taking into account density fluctuations predicts global stress-strain curves in close agreement with the experimental results in both tension and compression. This model can be used to investigate the mechanical behavior of mycelium based composites, which represents the next stage of this investigation. The error has been fixed in the paper. Klemm, D. Cellulose: fascinating biopolymer and sustainable raw material.
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