Volume 6 Issue 1
Feb.  2021
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Synthesis and Applications of Fungal Mycelium-based Advanced Functional Materials

  • Over the last couple of decades, the introduction of living systems to material science for the synthesis of functional materials from biological resources is receiving immense consideration. This is also in accordance with the need for green and sustainable development of new materials. For example, the growing concerns of the degradation of synthetic plastics are shifting the direction of materials-related research to the use of polymeric materials acquired from renewable resources. For example, the fungal mycelium-based materials are produced by growing the vegetative part of mushroom-forming fungi on different organic substrates. Such fungi are known for their ability to degrade agricultural wastes such as straws and sawdust. The mycelium-based composites having tailored structural, physical, chemical, mechanical, and biological properties are relying on the strain, feeding substrate, and the manufacturing process. The mycelium cell wall mainly contains the chitin, glucans, proteins, and lipids, whose concentrations depend upon the feeding substrate that ultimately defines the final properties of the synthesized materials. The mycelium-based functional materials with tunable properties are synthesized by selecting the desired components and the synthesis method. The pure and composites of stiff, elastic, porous, less dense, fast-growing, and low-cost mycelium-derived materials with efficient antimicrobial, antioxidant, and skin whitening properties pave their way in various applications such as construction, packaging, medicine, and cosmetics. This review describes the synthesis and structural organization of mycelium-based materials. It further discusses the effect of different factors on the material properties. Finally, it summarizes different applications of mycelium-based materials in medicine, cosmetics, packaging, and construction fields.
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Synthesis and Applications of Fungal Mycelium-based Advanced Functional Materials

    Corresponding author: Muhammad Wajid Ullah, wajid_kundi@hust.edu.cn
    Corresponding author: Guang Yang, yang_sunny@yahoo.com
  • a. Department of Biomedical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
  • b. Department of Chemical Engineering, College of Engineering, Dhofar University, Salalah, Sultanate of Oman

Abstract: Over the last couple of decades, the introduction of living systems to material science for the synthesis of functional materials from biological resources is receiving immense consideration. This is also in accordance with the need for green and sustainable development of new materials. For example, the growing concerns of the degradation of synthetic plastics are shifting the direction of materials-related research to the use of polymeric materials acquired from renewable resources. For example, the fungal mycelium-based materials are produced by growing the vegetative part of mushroom-forming fungi on different organic substrates. Such fungi are known for their ability to degrade agricultural wastes such as straws and sawdust. The mycelium-based composites having tailored structural, physical, chemical, mechanical, and biological properties are relying on the strain, feeding substrate, and the manufacturing process. The mycelium cell wall mainly contains the chitin, glucans, proteins, and lipids, whose concentrations depend upon the feeding substrate that ultimately defines the final properties of the synthesized materials. The mycelium-based functional materials with tunable properties are synthesized by selecting the desired components and the synthesis method. The pure and composites of stiff, elastic, porous, less dense, fast-growing, and low-cost mycelium-derived materials with efficient antimicrobial, antioxidant, and skin whitening properties pave their way in various applications such as construction, packaging, medicine, and cosmetics. This review describes the synthesis and structural organization of mycelium-based materials. It further discusses the effect of different factors on the material properties. Finally, it summarizes different applications of mycelium-based materials in medicine, cosmetics, packaging, and construction fields.

1.   Introduction
  • Fungi are eukaryotic organisms displaying huge diversity in morphology and lifestyle and have the potential to colonize large areas (Ibrar et al., 2020). For example, individuals of the genus Armillaria have been identified to colonize ≥ 1000 hm2 of soil, making them the most abundant organisms on earth (Smith et al., 1992). The fungi colonize their substrate through elongated filamentous cells called hyphae, which grow and form a three-dimensional (3D) interwoven filamentous network, known as mycelium. The mycelium secretes enzyme and degrades different substrates into simpler components that can be used as nutrients. Fungi utilize these nutrients and increase their biomass, both by growing on the surface of the substrate as well as penetrating into it (Vidal-Diez de Ulzurrun et al., 2017), while some grow out of the substrate and form a compact or fluffy layer called as 'fungal skin'. Among the different types of fungi, the mushroom-forming fungi are known for their ability to degrade the agricultural waste such as straws and sawdust containing the lignocellulose (Hawksworth and Lücking, 2017).

    The mycelium-based materials, both alone and in the form of composites, are produced by growing the vegetative part of the mushroom-forming fungi on different organic substrates. The pure and composite mycelium-based materials vary in terms of their synthesis process: for example, pure mycelium-based materials are obtained through complete degradation of the substrate whereas composite mycelium-based materials are produced by heating/drying the substrate during the colonization (Pelletier et al., 2013; Islam et al., 2018). The structural properties of pure mycelium-based materials depend on the fungal strain, nature of the substrate, growth conditions, and post-synthesis processing (Appels et al., 2018; 2019). In contrast, the structural properties of mycelium-based composites depend on the nature of the substrate and the type of additives used to reinforce the material, in addition to the type of fungus, environment during colonization, and post-synthesis processing (Jones et al., 2017; Appels et al., 2019). In general, the filamentous species used for the production of mycelium-based composites belong to the group of white-rot fungi, which show good colonization rate and are capable of degrading a large variety of organic biomass as well as purify substrates comprising of deleterious aromatic compounds such as terpenes (Eastwood et al., 2011; Floudas et al., 2012). Mycelium belonging to genera basidiomycetes, such as Pleurotus ostreatus and Trametes versicolor, gives more stiffness and strength to the synthesized material (Lelivelt et al., 2015). Moreover, the materials synthesized by P. ostreatus upon utilization of cellulose are more firm than those synthesized by the Ganoderma. lucidum, where the addition of dextrose further enhances the elasticity of materials manufactured by both fungal species (Haneef et al., 2017). The mechanical, physical, and thermal properties of the synthesized materials are greatly affected by the post-synthesis pressing temperature. For example, a high pressing temperature of 200 ℃ develops hydrogen bonding between the G. lucidum-produced mycelium and cotton stalk particles, thus greatly contributed to the enhanced physico-mechanical properties of the composite (Liu et al., 2019).

    The production process of mycelium-based composites is considered eco-friendly due to the valorization of waste material, thereby protecting the ecosystem through the mining of natural resources. Moreover, the fungi are capable of adaptive growth according to the environmental conditions, and a great variety of substrates are utilized by them as feedstock. Additionally, the developed mycelium-based composites are solely comprised of biocompatible components; thus, these are biodegradable in nature (Chambergo and Valencia, 2016). The diverse bio-adoptability and habitat make fungi the potential candidate for tunable material synthesis. A schematic illustration describing the different steps involved in the synthesis of mycelium-based materials is given as Fig. 1.

    Figure 1.  Schematic illustration of synthesis process of mycelium-based composites detailing key steps and possible variations in processes during each step.

    Fungal-based materials find potential application in different fields, including biomedical (Khamrai et al., 2018; Wang et al., 2020), paper (Jones et al., 2019), packaging (Abhijith et al., 2018), cosmetic (Mohorčič et al., 2007), textile (Jiang et al., 2013a), and construction industry (Jones et al., 2020). For example, a study reported the development of curcumin-loaded mycelium-based wound healing patch with great mechanical strength and stiffness. The developed patch demonstrated sustained release of curcumin from mycelium matrix at the wound site, which is appropriate for cell proliferation during wound healing (Khamrai et al., 2018). The mycelium materials are also replacing the traditional packaging materials, like Styrofoam, and emerging as a new source for sustainable packaging material (Abhijith et al., 2018). Due to their low-cost production, high acoustic absorption, low thermal conductivity, and fire resistance nature, the mycelium-based composites could be promising materials in the development of thermal and acoustic insulation panels, thus can find useful applications in the construction industry (Jones et al., 2020).

    Considering the tailored structural, physico-mechanical, thermal, and biological properties of mycelium-based materials, this review is aimed to provide a comprehensive overview of its synthesis mechanisms as well as various factors influencing its production and structural and functional properties. The detailed understating of each factor affecting the characteristics of the final product support to optimize the production process of the desired material. It further discusses the functionalization of fungal mycelium for its application in different fields, including biomedical, cosmetics, packaging, and construction industry.

2.   Synthesis and Structural Organization of Mycelium Matrix
  • Fungal mycelia are capable of producing biofilm adhering to the surfaces of solid substrates and spherical pellets in liquid cultures (Villena et al., 2010). The growth, branching, and networking of mycelium primarily take place through a highly polarized process called 'tip extension'. Two discrete mathematical models are commonly used to study the behavior of individual hyphae and characteristics of mycelium. In the first phase of growth, the mycelia produce a large number of hyphae to form a network on the surface of the substrate (Nopharatana et al., 2003), which continuously grow at the tip via tip extension and form a tubular structure (Levina and Lew, 2006). The tip extension occurs in many organisms along the cell wall, generally in specialized tissues such as root hairs and pollen tubes. Among all tip-growing cells, high calcium content is found at the tips (Steinhorst and Kudla, 2013). The calcium required for hyphal growth is released by the calcium-containing vesicles through IP3 induction (Levina and Lew, 2006). The septation initiation network (SIN) proteins are involved in septa formation in the filamentous fungi. The actin cytoskeleton regulates the formation of septa, which has a central role in maintaining the mono- and dikaryotic nuclear number in the growing hyphae (Raudaskoski, 2019). Usually, the hyphae diameter varies between 1 μm and 30 μm while their length ranges from few microns to several meters. A new hypha grows to an angle of 42°-47° to the long axis of the existing hyphae and forms an interlinked network called mycelium (Money, 2016). The branch thickness and network nature mainly depend on the nutrition and growth conditions. During the substrate colonization, the mycelium grows in and around the individual particle and glue the discrete particles to form a solid composite. As the colony stretches out, the vegetative hyphal fusion connects the discrete hyphae and form a leaf-like lattice structure (Du and Perré, 2020). In addition, the mycelium responds to the internal damage by re-growing, strengthening, and rejoining the adjacent branches, which is an important feature for the production of mycelium-based materials. The regrowth and rejoining of branches provide alternate pathways for developing channels responsible for intercommunication and transportation. Any accentuated damage to the hyphal network may result in a denser and robust network.

    The fungal cell wall is mainly composed of chitin, β-glucan, and glycoproteins, such as mannose and hydrophobins (Fig. 2). The outer surface is rich in glucans acting as mucilage, while the inner layer contains chitin microfibrils, which are covalently cross-linked with other polysaccharides such as glucans (Ruiz-Herrera and Ortiz-Castellanos, 2019). The most abundant glucans found in fungi are β-glucans, which accounts for 42%-50% of total glucans present in the cell wall, in contrast to α-glucans accounting for 28% of total glucans present in a typical fungal cell wall (Synytsya and Novák, 2013). Seven different classes of β-glucans are found in the members of kingdom fungi, β-1, 3-unbranched glucans, β-1, 3-glucans with rare β-1, 6-branches of a single glucose unit, β-1, 3-phosphorylated glucans, glucans comprising of β-1, 3- and β-1, 4-joined glucose moieties, β-1, 3-glucans with extensive β-1, 6-branching, glucans with abundant β-1, 6-bound glucose units, and glucans containing β-1, 3, β-1, 4, and β-1, 6-joined moieties. Most species contain Laminarin β-glucans made of glucosyl units containing β-1, 3-linkages (Free, 2013). Cellulose, a polysaccharide containing β-1, 4-linkages, is part of the cell wall in very few fungal species (Kang et al., 2018). Chitin, a polymer of N-acetylglucosamine, is essential for cell wall integrity. The amount and localization of chitin are specie-specific. Chitin deacetylase (CDA) enzyme involved in chitin synthesis is found in both the cell membrane and cell wall. Besides maintaining the integrity of the cell wall, chitin is also responsible for the linkage between the cell wall and capsule for epithelial adhesion (Goldman and Vicencio, 2012; Free, 2013). The composition and localization of cell wall polysaccharides continuously change to adapt to the environmental condition, thus protect the fungi against the harsh environment.

    Figure 2.  Schematic presentation of mycelial materials on diverse scales.

3.   Factors Influencing Material Properties
  • 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).

4.   Applications of Mycelium-based Materials
  • Recently, different living organisms are introduced in the field of material science and nanotechnology and extensively evaluated for their ability to produce biopolymers. A concern of researchers is to produce materials that are ecofriendly and economical with valuable properties (Park et al., 2016). For example, bacterial cellulose (BC)-based materials are used for various biomedical applications such as wound healing, bio-sensing, and drug delivery (Jasim et al., 2017; Ullah et al., 2017; Li et al., 2018; Ul-Islam et al., 2019b; Farooq et al., 2020). The natural biopolymers such as chitin and cellulose are the main components of mycelium. Moreover, the purification of mycelium is easier as compared with cellulose from both plant and microbial origins. The mycelium films are easily purified by heating at 60 ℃ for 2 h (Haneef et al., 2017), whereas the BC films need repeated washing with boiled sodium hydroxide solution followed by washing with sterilized water (Kim et al., 2019), while plant-based cellulose requires extensive physico-chemical and mechanical treatment for the removal of hemicellulose, lignin, and other impurities (Ul-Islam et al., 2019a; Ullah et al., 2019). Mushrooms have been used in traditional medicines since ancient times. Many filamentous fungal species are known for their therapeutic properties. The fruiting body of mushrooms and mycelia are rich in substances possessing antiviral, antimicrobial, antiaging, anti-inflammatory, hypocholesterolemic, and antioxidant properties (Gunawardena et al., 2014).

    Several fungi also serve as the substrate for the synthesis of nanomaterials for different applications (Fig. 3). The fungi-mediated nanoparticles are synthesized by adding metal precursors in the fermentation culture, which is taken up by the mycelium biomass and reduced into respective nanoparticles (Guilger-Casagrande and de Lima, 2019). The post-synthesis biomass is disrupted through a chemical treatment to release nanoparticles (Xu et al., 2015; Molnár et al., 2018). For example, the mycelium biomass produced by Phanerochaeete chrysosporium is used to synthesize Cds quantum dots (Chen et al., 2014) and metallic nanoparticles (Vigneshwaran et al., 2006). The ZnO nanoparticles prepared from fungal polysaccharides have improved biocompatibility for potential use in broad-spectrum biomedical applications (Xu et al., 2015). Porous and stiff 3D structure of mycelium film makes it a potential candidate for wound healing. A study reported the development of curcumin-loaded mycelium film as a wound healing patch, which offered a sustained release of curcumin (Khamrai et al., 2018). The developed patch can be an effective drug delivery wound healing system due to the known anti-inflammatory, antitumor, antioxidant in nature (Aggarwal and Sung, 2009). It is well-established that the therapeutic efficiency of the drug-loaded patch system is associated with the concentration of the drug and its release kinetics from the patch system. A constant release of the drug supports the continuous availability of the drug at the infection site (Mohanty et al., 2012). Triterpenoid compounds (TCs) isolated from Morchella mycelium are effective for the treatment of dementia and cancer (Wang et al., 2020). The TCs possess good antioxidant and antitumor activities. The TCs obtained from Morchella mycelium-significantly suppressed the proliferation of PC-3, HT-29, HepG2, HeLa, and HepG2 cell lines (Wang et al., 2020). Another study revealed good antioxidant and antiaging activity of acetylated mycelia polysaccharides (AMPS) of P. djmour (Li et al., 2019). In addition, the fungal mycelia from different species show shielding effects on the kidney, brain, and liver (Xu et al., 2017; Li et al., 2019).

    Figure 3.  Applications of mycelial-based materials in different fields.

  • The cosmetic industry mainly deals with cosmeceuticals used for topical applications and nutricosmetics, which are administered orally. In addition to their positive effect on the skin, the cosmetics products must also be safe without causing any irritation or causing other side effects. For developing effective and safe cosmetic products, the cosmetic industry is constantly in search of ingredients from natural sources owing to their competitive effectiveness and lower toxicity. To this end, the potential of various fungal species has been evaluated appreciated as a traditional source of natural bioactive compounds for centuries (Hyde et al., 2010). Specifically, mushrooms contain volatile organic compounds such as polyphenolics, polysaccharides terpenoids, vitamins, and several others, which are useful for hair and skin (Poucheret et al., 2006; Ahmad et al., 2013; Wu et al., 2016). These compounds show excellent antiaging, anti-wrinkle, and skin whitening effects (El Enshasy and Hatti-Kaul, 2013; Kalač, 2013). For example, the Shiitake species are used as a skin exfoliator, which encourages rapid skin renewal and improves skin elasticity and brightness (Liu, 2002).

    Various fungal-derived moisturizers improve the physical and chemical structure of the skin and make it soft, moist, and wrinkle-free by maintaining the water content of stratum corneum and surface lipids (Sator et al., 2003). For example, carboxymethylated polysaccharides obtained from Tremella mushroom retain 65.7% of moisture even after 96 h, thus can be used as an effective moisturizer (Wang et al., 2015). In a study, the use of 0.05% Tremella polysaccharides as skincare products demonstrated better moisture retention ability as compared with the 0.02% hyaluronic acid (Liu and He, 2012). As the aging process disrupts and collapses the elastin and collagen, the use of antiaging products, for example, antioxidants can play an effective role in maintaining the body repair system (Lupo and Cole, 2007; Ocampo et al., 2016). Different fungi such as Lentinula edodes and Volvariella volvacea are a rich source of such antioxidants as well as enzymes, which can be found applications in many cosmetic products (Dubost et al., 2006; Keles et al., 2011). A study reported that the extracts from Pleurotus cornucopiae applied to mice showed positive effects against atopic dermatitis (Tomiyama et al., 2008). In another study, the extract from Pleurotus nebrodensis served as an effective skin whitener, thus could be used in the bioactive materials against skin discoloration (Dangre et al., 2012). Similarly, bioactive compounds isolated from mycelium of Ganoderma fromosanum decreased the surface pigmentation by inhibiting the tyrosinase enzyme, responsible for cutaneous pigmentation (Hsu et al., 2016). Besides, several marine fungal species also show inhibitory effects against tyrosine, thus can be potentially used in the preparation of skin whitening products (Lee et al., 2003). A high concentration of extracellular melanin produced by Gloiocephalus trichum simplex showed efficient UV absorbance, indicating its potential usage in the development of sunscreens (Jalmi et al., 2012). These studies suggest that fungi can pave the way into the cosmetic industry with multiple applications.

  • Over the past few decades, rapid urbanization has resulted in the development of significant pressure on the construction industry for the continuous supply of traditional construction materials such as bricks, cement, insulation panels, and others. The production of conventional construction materials demands high energy as well as cause air, water, and land pollution (Madurwar et al., 2013). According to a study, a typical dwelling uses up to 36% of the lifetime energy (Sartori and Hestnes, 2007). The mycelium growth on agriculture byproducts and waste has attracted researchers for the production of low-energy construction materials as well as an approach for waste recycling (Madurwar et al., 2013). Moreover, the mycelium-based materials offer several advantages over traditional materials, including low-cost, biodegradability, and less environmental impact and density. The utilization of broad range substrates in combination with controlled processing techniques allows the production of mycelium-derived materials with the desired structure and function for specific applications. The mycelium-based composites grown on straw and hemp fibers serve as a natural insulator due to their low density and low thermal conductivity (Collet and Pretot, 2014), which usually have a strong connection (Kadoya et al., 1985; Uysal et al., 2004). In less dense materials, a large amount of dry air is present in free air spaces, which lessens the thermal conductivity. This feature makes less dense materials as superb thermal insulators. The mycelium-based biofoams offer great potential for application as alternative insulation materials for building and infrastructure development (Yang et al., 2017).

    Mycelium itself is an exceptional acoustic absorber which exhibits a strong inherent low-frequency absorption (b1500 Hz) and beating cork, thus can potentially replace the conventional ceiling tiles to reduce the noise pollution. According to a study, the mycelium-based composites containing agricultural residues could show up to 70%-75% acoustic absorbance (Pelletier et al., 2013). The fibers in mycelium composites act as the frictional elements and interfere with the acoustic wave motion; thus, they can potentially reduce its amplitude as the soundwaves move through the tortuous passages of the material and are converted to heat in the process (Peters, 2013). Thin fibers offer improved acoustic absorption since these can move easily, and a large number of fibers per unit volume results in more twisting paths and aids better air-flow resistance (Jailani et al., 2014). The surface porosity and geometry of mycelium-based materials also play important roles in acoustic absorbance. A study reported a significant effect of less porous material on the sound absorption performance as compared to the highly porous material (Samsudin et al., 2016). Generally, the dense and compact materials absorb more sound energy as compared to the thin and relaxed materials. The mycelium-based composites also possess good fire safety features than conventional building materials such as polystyrene insulation and particle-board, as well as offer better termite resistance by utilizing the natural termiticides (Jones et al., 2020). The mycelium-based materials are safe compared with the traditional high inflammable petroleum-based materials as the former produce less CO2 and smoke on burning and take a long time to flashover (Jones et al., 2018). Such unique features make the mycelium-based composites as viable, low-cost, safe, and environmentally sustainable alternatives to conventional construction materials.

  • In recent years, the globalization of the world's industry increased the demand for packaging materials. The petroleum-derived polystyrene is presently the most used packaging material; however, it is not biodegradable nor recyclable, thus posing a serious threat to the environment. Moreover, the production of polystyrene is an energy-consumable process and not eco-friendly due to the emission of greenhouse gases (Abhijith et al., 2018). The mycelium-based materials due to their lightweight and non-toxic nature could be used for a wide range of packaging applications, including electronics, fragile items, and in the food industry (Fig. 3). The green biocomposites from pure bio-based materials can be used as an alternative to conventional petroleum-derived packaging in different fields (Ziegler et al., 2016). Mycelium has the tendency to adopt the shape of the mold, thus making it an ideal candidate for packaging material. Moreover, with the controlled growth conditions and choice of substrate, it is possible to produce mycelium-materials of own choice with tailored strength, density, and other structural features. A Chinese company developed the mycelium-based packaging material by growing the fungi on wheat straw. The developed material is lightweight, biodegradable, and elastic (Cerimi et al., 2019). In another study, Pycnoporus cinnabarinus produced an orange-red packaging material without the addition of external pigments. The developed material showed high buoyancy, thus can be used for the production of sea buoys (Cerimi et al., 2019). In a recent study, Joshi et al. reported the green synthesis of bioblocks by P. ostreatus by using different agricultural wastes, including wheat bran, sugarcane, sawdust, and a mixture of these materials. The synthesized material were hydrophobic in nature and demonstrated excellent thermal and mechanical stability, which could thus find application as packaging materials as well as wall paneling and filtration membrane for removal of toxic materials (Joshi et al., 2020). Various applications of mycelium-based materials in different fields are summarized in Fig. 3.

5.   Conclusions and Prospects
  • Mycelium growth provides a unique and low-cost bio-fabrication method to recycle the agricultural wastes and byproducts into sustainable biomaterials. The discussed studies provide essential information about the impact of every process parameter on the properties of the synthesized material. The main outline of the mycelium-based materials is derived from three fundamental sets, including 1) components such as fungus type and substrate, 2) processing variables, and 3) applications. Although this framework describes the complex interconnection of processing variables and their effect on the material properties, it also helps to produce the mycelium-derived biomaterials with tunable characteristics. Owing to their low density, low thermal conductivity, and highly porous nature, the mycelium-based materials are more suitable for thermal and acoustic insulation as compared with the synthetic foam and wood fibers. Moreover, these materials are stiff and highly fire-resistant, making them suitable alternatives to typical building materials. Additionally, the mycelium-based materials are lightweight and biodegradable, thus offer an eco-friendly alternative to petroleum-based packaging materials. As a traditional source of natural bioactive compounds, fungi can be used to synthesize biomaterials with potential application in the medicine, pharmacology, and cosmetics industries. To date, only a limited number of fungal species are being explored for their contribution in the field of material science. With interdisciplinary studies combined with genomics, metabolomics, and proteomics, the molecular mechanisms of mycelium synthesis and material biology can be revealed, and more fungal species can find their way into multiple applications.

Conflict of Interest
  • There are no conflicts to declare.

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