The type of strain used for the synthesis of mycelium-based materials greatly influences the properties of the resultant composite. Mostly, the white-rot filamentous species are used to grow the mycelium-based materials. This group of fungi has the ability to adapt to diverse habitats. Interestingly, this group of fungi produces non-toxic materials by degrading the toxic compounds, like terpenes, in the feeding substrate. For example, a study reported that Trametes multicolor produced a velvety soft skin at the substrate surface with flexible and foam-like structure, while P. ostreatus produced a material with a firm and rough surface when grown on rapeseed straw (Appels et al., 2019). In another study, Ganoderma material grown on cotton plant biomass materials demonstrated a bending strength of 7-26 kPa (Ziegler et al., 2016). Similarly, the material produced by P. ostreatus was more firm as compared with the one produced by G. lucidum, when cultivated on cellulose, where the addition of dextrose into the growth medium substantially increased the elasticity of both fungal-based materials (Haneef et al., 2017). The color of materials synthesized by different fungal strains varies accordingly. For example, the T. multicolor produced dark-brown, whereas P. ostreatus produced light-brown fiberboard-like material upon heat pressing (Appels et al., 2019).
The type of substrate used for the growth of mycelium is another important factor contributing to the material properties. Substrates having intact natural fibers give strain-hardening characteristics to mycelium-based materials by providing strength and preventing shear failure. Moreover, the natural fibers reduce the cracking events during the shearing and improve Young's modulus of mycelium-based materials (Yang et al., 2017). A study reported a 300% increase in the strength of pure mycelium-based materials when sand was mixed with wood chips. The addition of silica, methyl-cellulose, and agarose further increased the water holding capacity of materials (Jones et al., 2017). The nature of the substrate also influences the inflammation of fungal composites. For example, lignin decreases the combustion as its cyclic rings are decomposed into aromatic fragments which constitute the major components of char, while the cellulose favors combustion (Gibson, 2006). The composites grown on oak sawdust demonstrated high tensile strength as compared with those prepared from the beech sawdust (Faruk et al., 2012). In a study, P. ostreatus grown on potato dextrose demonstrated frequent hyphal collapse, low hyphal width, and less content of chitin as compared with those grown on cellulose. The synthesized material with low polysaccharides/chitin ratio showed high water absorption, a high rate of elongation, and low Young's modulus (Haneef et al., 2017). The water holding capacity of the fungal-based mycelium is greatly dependent on the substrate. For example, the water content of T. multicolor-based material ranged between 5.8%-7.2% and 7.6%-9.6% when growth on cotton and rapeseed straw, respectively (Appels et al., 2019).
The structural features of mycelium-based composites can be effectively improved through cold or heat pressing. In general, pressing increases material density and reduces porosity. It also assists the reorientation of fibers horizontally in a plane and reduces their thickness, thus increase the contact between the fibers at overlapping points (Thoemen and Humphrey, 2006). The mechanical properties of mycelial-based materials are also greatly influenced by the pressing temperature. A study reported that the composites of P. ostreatus-rapeseed straw showed less stiffness and tensile stress upon cold pressing, as compared to high tensile strength, stiffness, bending properties, and low rupture strain when pressed under heat. Overall, the cold and hot pressings of P. ostreatus-rapeseed straw composites resulted in a two- and three-fold increase in density, respectively. Heat pressing also increased the elastic and flexural moduli of T. multicolor-rapeseed straw composites and ensured a uniform thickness of the synthesized P. ostreatus-rapeseed straw composite (Appels et al., 2019). According to Liu et al. (2019), heat pressing may negatively affect the thermal decomposition temperature of the composites; however, it increases the thermal stability of siliceous layer, thus indicating its suitability for producing fire-resistant materials. Heat pressing of G. lucidum-cotton stalk composite at 200 ℃ resulted in 4.6 MPa modulus of rupture, 680 MPa modulus of elasticity, and 0.18 MPa internal bonding strength. The study proposed that the esterification, repolymerization, and formation of hydrogen bonding at high pressing temperatures are key for the better features of the composite (Liu et al., 2019). A possible reason for the formation of hydrogen bonding at a higher temperature could be the interaction of hydroxyl groups of substrate cellulose nanofibrils with the crosslinkers or radicals during the fungi-induced degradation of the substrate (Widsten et al., 2004). At a pressing temperature of 200 ℃, a repolymerization of lignin takes place by the free radicals and acidolysis. Meanwhile, the esterification between the amino acids in the substrate and mycelium improves the interfacial binding (Liu et al., 2019). In another study, heat treatment of chitosan films obtained from mycelium biomass of Aspergillus niger led to structure reorganization and reduced solubility, suggesting the amidation of materials as a result of heat treatment (Solodovnik et al., 2017). The summary of various factors that can influence the mycelium based material properties are summarized in Table 1.
Fungus type/strain Factor Improved material property Reference T. multicolor Rapeseed straw Flexible and soft skin (Appels et al., 2019) P. ostreatus Rapeseed straw Firm and rough skin (Appels et al., 2019) T. multicolor Heat pressing Uniform thickness, dark brown material (Appels et al., 2019) P. ostreatus Heat pressing Three-fold increase in density, light brown material (Appels et al., 2019) P. ostreatus Cold pressing A two-fold increase in density (Appels et al., 2019) T. multicolor Cotton straw Enhanced water retention ability (Appels et al., 2019) G. lucidum Cotton biomass High bending strength (Ziegler et al., 2016) P. ostreatus Cellulose Firm (Haneef et al., 2017) P. ostreatus Cellulose and dextrose Enhanced elasticity, high water holding capacity (Haneef et al., 2017) G. lucidum Cellulose and dextrose Enhanced elasticity (Haneef et al., 2017) G. lucidum Cellulose Loose structure (Haneef et al., 2017) G. lucidum Cotton straw Enhanced elasticity (Liu et al., 2019) G. lucidum Heat pressing Enhanced elasticity (Liu et al., 2019) A. niger Heat pressing Reduced solubility (Solodovnik et al., 2017)
Table 1. Summary of roles of different factors influencing material properties.
As processing parameters such as growth time and conditions and material-drying methods vary for different strains and substrates, thus substantially affect the properties of the materials. For example, the incubation time generally depends on the size of the material, and it ranges from 5 to 42 days for different fungal strains and depends upon the nature of the substrate (Jiang et al., 2013b; Haneef et al., 2017). Materials grown for a longer period are thermally more stable and less porous, thus extended incubation time increases the material's strength. As mycelium grows, the spaces between the fibers are occupied, and fibers are linked strongly, thus enhancing the overall density (Yang et al., 2017). In contrast, the extended incubation time may result in complete degradation of the substrate, which acts as the reinforcement material; thus, it contributes to improving the elastic stiffness and reduces the shearing behavior (Yang et al., 2017). Similarly, favorable growth conditions vary from specie to specie on different substrates. For example, the incubation temperature varies between 21℃ and 30 ℃ for different fungal species. Similarly, the average pH level for optimal growth of various fungi ranges from 5 to 8, while the humidity level ranges between 70% and 100% (Haneef et al., 2017; Appels et al., 2019). Mycelium requires oxygen for growth and to produce carbon dioxide. Typically, a low carbon dioxide content initiates the formation of a fruiting body; hence a high carbon dioxide level should be maintained to prevent the formation of the fruiting body and ensure efficient mycelium growth (Lelivelt et al., 2015). Similarly, high-density materials are synthesized in the dark with low carbon dioxide concentration and in light with high carbon dioxide concentration, indicating an interrelationship between light and carbon dioxide on their combined effect on fiber density (Appels et al., 2019). A recent study proposed a biochemical solution to regulate the formation of the fruiting body. The study reported that the use of glycogen synthase kinase-3 (GSK-3) inhibitors in the cultivation medium inhibited the formation of the fruiting body in Pleurotus djmour strain and supported the mycelium growth (Chang et al., 2019). This approach is easy to handle, cost-effective, and reliable.
There are several methods that are operated at a broad temperature range and for different time intervals to dehydrate and denature the materials. For instance, infrared oven heating, infrared lamp heating, microwave heating, and oven baking are usually carried out for 2 h at 60-125 ℃ (Haneef et al., 2017). Similarly, the conventional heating with solar dryers is carried out for 8 h at 60 ℃ or for 2 h at 220 ℃ (Jiang et al., 2013b), while drying with a machine is carried out at 60 ℃ for 24 h (Lelivelt et al., 2015). Typically, the dried samples cannot be regarded as absolutely dried due to the humidity factor from the environment. The moisture content of the dried sample varies from 0.6% to 20.0%. Oven with air circulation could be the best choice for the complete drying of the synthesized materials. The thoroughly dried materials have less thermal conductivity as compared to the materials with locked moisture. Moreover, heat drying stops the mycelium growth, and its filaments lack internal bonding; thus, these appeared flat. Consequently, the elastic moduli of dried samples are generally higher than that of the living materials (Yang et al., 2017).