Fungal mycelium-based biofoam composite: A review in growth,properties and application

Abstract

Recent years have shown a surge in interest in incorporating living systems into materials research to synthesize functional materials using biological resources. Among these, mycelium-based materials, notably biofoam, have emerged as innovative solutions for repurposing organic wastes that were previously considered unusable. The growth of mycelium, vital for the synthesis of biofoam, is influenced by a multuple of factors including substrate composition, moisture content, temperature, nutrient availability, pH levels, oxygen concentration, and measures for contamination control. Additionally, physical stimulation techniques have been explored to enhance mycelium growth, ranging from cold stress-induced adaptations to electrical shock-induced modifications and optimization of sound treatments and light exposure. This review highlights the growing interest in biofoam composite materials, a novel class of environmentally friendly and cost-effective materials that are gaining popularity, for advancing sustainable construction practices. Biofoam composites use organic fungal growth as a low-energy bio-fabrication process to transform abundant agricultural byproducts and waste into viable alternatives to energy-intensive manufactured building materials. Their versatility in composition and manufacturing methods allows them to be used in a wide range of applications, including insulation and door cores, panelling, flooring, and furniture components. Notably, biofoams outperform synthetic foams and engineered wood in terms of thermal insulation, sound absorption, and fire resistance, making them highly promising for construction industry. Besides, due to its customizable composition and production method, biofoam can be used in the replacement of foams, leather, wood, and plastics in a variety of applications such as water treatment and filtration, medical supplies and healthcare applications. However, despite their remarkable properties, biofoam typically serve as non- or semi-structural supplements to traditional construction materials due to inherent limitations. Nevertheless, the useful material properties of these materials, combined with their low cost, ease of manufacture, and environmental sustainability, imply that they will have an important part to play in the development of environmentally friendly materials in the future.

Keywords

Biofoam mycelium growth mechanical physical stimulant

Introduction

Recent years have seen a significant increase in interest from businesses and academics regarding mycelium, a type of thready fungi (mushroom), as a novel low-energy bio-fabrication and waste upcycling method.^1–4 In a very natural organic process, mycelium combines organic matter with a web of hyphal microfilaments to produce high-value composite materials ^1–3,5,6 as well as low-value materials, like packaging, from troublesome agricultural and industrial waste products at little to no cost to the company.^7,8 This mycelium binder component is a dispersed agricultural residue interface (substrate filler) that resembles the matrix phase of chemical compound composites and is intended to serve as a load transfer medium between fibrous agricultural residue and the composite.^3,9 Mycelium-derived materials have a number of benefits over conventional synthetic materials, including biodegradability, low environmental impact, and low carbon footprint. They are also less expensive, denser, and use less energy.^5,10,11 By combining a wide range of viable substrates with managed processing procedures like growing environment and hot pressing, mycelium-derived materials can be customized to meet specific structural and functional requirements, such as fire resistance, thermal and acoustic insulation.^1–3,5 This not only makes it possible for them to be used as environmentally friendly alternatives for synthetic planar materials (like plastic films and sheets) and larger low-density objects (like synthetic foams and plastics),^2,3,12,13 but it also means that they can be used as semi-structural materials (like paneling, flooring, furniture, and decking).^6,14–16 The main factors limiting mycelium materials' current application and use are their typical foam-like mechanical properties, rapid water absorption, and numerous data gaps regarding material properties. These limitations necessitate further research and development of these materials in addition to their targeted use in specific applications such as insulation, door cores, paneling, flooring, cabinetry, and other furniture.

Fungal structure and properties

The structural and vegetative diversity of fungi ranges from unicells like yeasts to filaments that can be solitary or grouped into strands, cords, or more complex rhizomorphs. Few members of the Chytridiomycetes and yeasts are non-mycelial; the vegetative body of the majority of saprophytic fungi is the mycelium. The hypha, which is the fundamental component of the mycelium, is typically filamentous unless physiological conditions are abnormal.¹⁷ 64,000 fungi are known to exist. It is likely that the number of species that have been discovered so far has been probably a fraction of the total number that could exist. Hawksworth (1991) estimates a total of 1.5 million possible fungal species, implying that there are at least three million fungal species in existence, which is more than any other biological group with the exception of insects. Many parasitic, mutualistic, or commensal fungi rely on insects as hosts, and the range of these fungi is typically quite restricted. Many more fungal species are likely to exist in insects than in any other animal or plant group due to the high number of insect species.¹⁸ Fungi can be single-celled or multi-celled, depending on the species. Unicellular fungi are referred to as yeasts, whereas filamentous or mycelial fungi are referred to be multicellular fungi with complicated, multicellular propagation mechanisms. Some fungi can switch from unicellular to multicellular growth depending on the conditions in which they grow, and vice versa.¹⁹

The portion of a fungus that grows is called mycelium. It is composed of the fungus’s developing stem cells. The world’s largest living thing, mycelium, is located in Oregon’s Blue Mountains and spans almost 10 km². Because of its symbiotic relationship with the materials that feed it, Figure 1(A) illustrates how it develops. As seen, it creates tangled networks of branching fibres. The hyphae, which are made up of long cells, are the threads that make up the fibrous mycelium. Figure 1(B) shows the tubular cell wall that encloses all of these septa-separated cells that are connected by internal porous cross walls called septa. The cell wall, which also gives the entire mycelium mechanical strength, shields the hyphae (Figure 1(C)). Chitin, glucans, and proteins like mannoproteins and hydrophobins make up the outer layer of this substance. The hyphal tip’s expansion of the cell membrane and wall is what causes it to grow.²⁰

Figure 1.

Cross-section of mycelium macrostructure and the hyphal microstructure. (Hotz et al., 2023).²¹

Fungal mycelium growth

Extracellular enzymes are produced to solubilize the substrate on which the hyphae grow as the mycelium grows by nutrient absorption. The fungus can utilize a large surface area of the substrate thanks to the way the mycelium grows, collecting and storing nutrients for later use at a low biomass cost. Mycelium grows in culture from an inoculum site by radial hyphae extending in three dimensions. In liquid or water environments, mycelium has a spherical shape; however, on solid substrates, it is more convex, with aerial hyphae ascending above and immersed hyphae penetrating the medium below.¹⁷

As decomposers, animal and plant symbionts, and parasites, fungi are important to all ecosystems. As a result, their ecology is of significant interest. Fungi are thought to have as many as 1.5 million species, many of which have yet to be discovered. Their interactions with their hosts, substrates, competitors such as other fungus, and abiotic variables in their environment are varied, as are their interactions with these. They exhibit a wide range of morphological, reproductive, life-cycle, and dispersion characteristics. Fungal development is the outcome of a combination of biological processes in nature.^17,22

Sexually generated spores are found in the fruit bodies of two fungi: the Ascomycetes and the Basidiomycetes. As a result, the fruit bodies of these two families, together with a third group, the Deuteromycetes, have been referred to as the higher fungi. Although they are linked to the Ascomycetes and Basidiomycetes, these fungi have lost the ability to reproduce and hence lack fruit bodies. Mycelial development is extremely restricted in some lesser fungi. Extensive mycelia are produced by the Oomycetes and Zygomycetes. In short, mycelium grows from an inoculum site through hyphal radial expansion. Mycelium is convex in liquid cultivation but more so on solid substrates. Growing fungus uses a large surface area and fungi are crucial to ecosystems as decomposers, symbionts, and parasites. They vary in morphology, reproduction, life cycle, and dispersal.

Fungal mycelium growth environment

Diverse microorganisms live in a variety of habitats and have a variety of growth requirements, including nutrition availability, pH balance, osmotic pressure, and temperature.²³ For fungus, there are several factors that influence their structure and reproduction, as well as their host range and their substrate choice. These factors include environmental variables that affect the fungi directly, as well as their hosts and their vectors.¹⁷ Fungi eat organic material. These substances can be wastes like animal excrement or plant exudates, as well as living or dead plants, animals, especially insects. Along with mineral particles, liquids, and gases, soil contains a significant amount of organic material. For this reason, hyphae need to have the ability to spread across surfaces, pierce through loose and dense substrata, and cross obstructions to reach other substrata. Depending on their environment, the hyphae of the majority of fungi can perform the aforementioned tasks to varying degrees.¹⁸ Organic matter is the main component of fungi’s diet. The environment contains things like insects, plants, and other species as well as wastes like animal excrement and plant exudates. Organic matter makes up a sizeable portion of soil, which also contains mineral particles, liquid, and gas phases. As a result, in order to reach nearby substrata and access nutrients, hyphae need to be able to spread across surfaces, pierce loose and dense substrates, and cross gaps. Although the hyphae of most mushrooms can execute the following functions to variable degrees, the hyphae of some fungus are more capable than others.²² Solid state fermentation systems, which offer microorganisms with an environment comparable to their natural habitat, are known for their improved growth, morphological diversity, and differentiation.²⁴

Besides, the most important environmental variables are the specimens' moisture content and the temperature in which fungal are placed. Mycelium grows at the ambient temperature range and requires a relatively humid atmosphere in which to develop. Because of this, mycelium is typically grown with the help of humidifiers or sprinkler systems.²⁵ Figure 2 depicts the relationship between time, moisture content, and the growth rate coefficient. Negative growth rates at severe temperatures and a non-zero growth rate at low moisture content have been observed as a result of a parabolic regression model and a short experimental period.²⁶

Figure 2.

Calculated growth rate coefficient for given moisture content (MC) and temperature (T) conditions ²⁶.

On another research, on potato dextrose agar (PDA) and sabouraud dextrose agar (SDA), Pleurotus ostreatus generated cottony-textured, regular density, and regular mycelium development, but Ganoderma lucidum produced floccose-textured, low density, and sparse mycelium growth on PDA. As shown in Table 1, temperature and growth media had a direct effect on the mean mycelium growth of P. ostreatus and G. lucidum. After 12 days of inoculation, P. ostreatus keeps growing higher than G. lucidum on PDA (23.28 cm) and SDA (9.85 cm) at 22°C, and on PDA (5.43 cm) and SDA (4.43 cm) at 30°C. P. ostreatus developed mycelium throughout a greater temperature range of 22 to 30°C on both PDA and SDA. P. ostreatus and G. lucidum mycelium developed maximally at a temperature of 22°C.²⁷ P. ostreatus was described by Yang et al. as a frequently cultivated fungal strain due to its ability to grow in a wide temperature range and utilize a variety of nutritional conditions. As a result, the fungal strain P. ostreatus was chosen to evaluate the influence of different growing conditions on mycelium growth.

Table 1.

Effect of various temperatures and growth media on fungal strains P. Ostreatus and G. Lucidum²⁷

Temperature (°C)	Growth media	Mean radial mycelium growth (cm)
Temperature (°C)	Growth media	G. lucidum	P. ostreatus
22	PDA	9.03	23.28
	SDA	1.00	9.85
30	PDA	1.00	5.43
	SDA	1.00	9.85
37	PDA	1.00	1.00
	SDA	1.00	1.00

To conclude, the amount of moisture present in the samples and the temperature at which the fungi are kept and grown are the two environmental factors that are considered to be of the greatest concern. Besides, other research shows that both temperature and growing medium had an immediate impact on the mycelium growth.

Fungal mycelium growth phase and kinetic

For the majority of their life cycle, hyphae, which are cylindrical cells that lengthen by expanding at one end, make up the majority of fungi.¹⁸ Microorganisms can grow on solid substrates without the presence of free-flowing water, which is known as solid-state culture (SSC). The growth rate of a mycelial colony is statistically dependent on the growing medium’s chemical composition.²⁸ Monitoring and managing culture factors such as biomass content, pH, temperature and moisture can be challenging when the substrate is heterogeneous, lacks free water and has low conductivity. Agriculture management and logistics modelling are difficult without an understanding of growth kinetics. The engineering aspects of this process are extensively studied in the literature using the logistic model. Although it does not perfectly fit the entire growth curve, the logistic equation for growth rate is adequate. It is much more acceptable to use an exponential and logistic growth model with temperature-dependent specific growth rates. This model can estimate the entire growth rate profile, particularly at temperatures lower than the ideal condition where growth rate does not exhibit a clear peak.²⁹ Solid-state fermentation (SSF) process control requires reliable microbial growth assessment. Quantitative fractal geometry can measure fungus growth. Digital image analysis could determine fungal growth from morphological changes in SSF mycelia matrix. Dynamic imaging combined with kinetic models based on quantitative fractal geometry helped determine fungal biomass in solid-state fermentation. Mycelia-matrix fractal dimension variation rates indicated fungal growth rate selectivity. This strategy is applicable in many circumstances.³⁰ The growth rate of mycelium under natural conditions varies widely since it is dependent on the mycelium species, the soil composition, the temperature, the humidity, and the pH of the water.³¹

The three typical growth phases that follow the inoculation of a suitable media are shown in Figure 3 (lag phases, exponential phases, and stationary phase). Less population growth happens during the lag phase as the inoculated cells get used to their new chemical and physical environments. Although the lag phase’s duration varies depending on the species, it is inversely related to each species' pace of growth. Species that grow more quickly have shorter lag phases. As for exponential phase, growth occurs when optimal conditions exist, and the amount of biomass generated by the population increases according to cell number, dry weight, nucleic acid content, and protein content. Last but not least is the stationary phase, which fungal cells enter when essential nutrients are depleted or toxic substances build up. During this phase, the specific growth rate stops and the biomass remains largely stable. It was long believed that fungi could survive without additional nutrients, but if this phase persisted indefinitely, cells might start to die.^32,33

Figure 3.

Growth phase in culture medium.³³

Empirical growth equations can be used to mathematically model growth throughout the exponential phase of the growth cycle (linear, exponential, logistic, two phase).³⁴ In the presence of adequate nutrition and a conducive habitat, a population of unicellular microorganisms will develop. When environmental conditions are kept constant, cells expand proportionally and maintain their average size and composition throughout time. Quantity of cells, dry weight, and the proportion of nucleic acids and proteins in the population are all on the rise in parallel. This means that these and other appropriate population variables, such as optical density, can be examined to monitor growth. That the population is growing at an accelerating rate can be shown from such statistics.³³

Fungal mycelium growth factors

On rock surfaces, in soil, in the sea, in fresh water, at extremes of heat and cold, on dry substrates, and in concentrated solutions, mycelium can grow practically anywhere that contains organic carbon. In culture, the established mycelium grows by a slow-moving mycelial front that separates the individual mycelium and is made up of airborne and submerged hyphae.¹⁷ Fungal contamination occurs quickly, however when water is condensed on shifting humidity and temperature conditions, it takes longer for fungal development to occur. Interpretation of moisture measurements in building materials, particularly in gypsum board is problematic because moisture’s complicated behaviour makes it hard to determine its microbiological state based on a single measurement. It is also possible that the material’s concentration of culturable fungi is exaggerated due to the influence of ambient moisture and temperature. However, some molds, like Penicillium species, are able to endure environmental stress and thrive in high humidity environments.³⁵ Besides, fungal growth on construction materials was not limited by temperature, since fungus survived at temperatures as low as 10°C. Fungi growth was unaffected by the air’s relative humidity. It is possible for fungi to thrive at low levels of air humidity if there is sufficient water on the surface.³⁵ To compare initial growth rates at different temperatures with growth at the optimum temperature, Table 2 shows initial growth rates at various temperatures. Increasing temperatures accelerated the colony’s growth. The fungi at first expanded linearly, but then their growth slowed drastically.³⁶

Table 2.

Initial growth rates of Aspergillus fumigatus and Penicillium sp at various temperature relative to the optimum temperature.³⁶

T °C	Growth rate at T relative to growth rate at the optimum T
T °C	A.Fumigatus (%)	Penicillium sp. (%)
4	0	0
9-10	18	52
9-22	40	100
30	100	85

Hence, as long as the medium’s moisture content was above a certain critical value, growth of fungus was independent of the moisture levels. It was crucial that the atmosphere be as dry as possible in order for this condition to be achieved as soon as possible. So long as the surface is damp, fungal development is unaffected by the relative humidity of the air.

Obligate aerobes, like other fungi, need oxygen for a wide range of energy-related metabolic processes. The process of aerobic respiration involves a series of processes that use oxygen from the surrounding air as a reactant. Fungal digestion, assimilation, respiration, translocation, and synthesis, all of which are mediated by enzymes, are sensitive to temperature changes. As heterotrophs, fungi and most bacteria have three specific needs in their food source or substrate. Fungi, like all other organisms, require significant amounts of nitrogen for the synthesis of proteins and other cell contents or products such as nucleoproteins, lipoproteins, enzymes, and the chitin present in hyphal cell walls.³⁷

Biofoam composite fabrication

Contrary to cellulose, which is the main structural polymer in plant cell walls, useful structural polymers like chitin and chitosan, as shown in Figure 4, are naturally occurring and renewable sources in fungi. The primary structural element of the exoskeletons of the majority of insects and other arthropods is a linear macromolecule called chitin, which is made up of N-acetylglucosamine units.³⁸ A significant dipole moment and hydrogen bonding between macromolecule chains give it a nanofibril tensile strength of 1.6-3.0 GPa.³⁹

Figure 4.

Chemical structure of chitin, chitosan and cellulose.⁴⁰

Chitin, other polysaccharides like glucans, mannoproteins, chitosan, polyglucuronic acid, or cellulose, and smaller amounts of proteins and glycoproteins in hyphae that form mycelium of hyphae filaments make up the dense and complex fibrous network of fungal cell walls.^41,42 Due to their composition, mycelium possesses the mechanical properties of lignocellulosic substances like wood and cork.⁴³ Contrarily, mycelium composites, which combine agricultural residues with a mycelium binder to form a dispersed substrate filling phase, have lower densities and elastic moduli than pure mycelium and are thus classified as foams.^44,45 Due to the significant amount of air stored between and within the porous and loosely packed substrate filler, this is a frequent occurrence.²

Due to the use of waste material in the production of mycelium-based composites, the exploitation of natural resources is reduced to a minimum, preserving the ecosystem. In addition, fungus may adapt their growth to the conditions of the environment and use a wide range of substrates as feedstock.⁴⁶ One of the main benefits of biofoam composites is that they are produced using a low-energy, natural process that sequesters carbon. In Figure 5, any substance that can aid in the development of fungi, such as carbohydrates, is necessary as a precursor.^1,47 Low-cost lignocellulose agricultural or forestry waste is frequently used as fibre substrates, like straw, or particulate matter, like sawdust, to keep the cost of biofoam composites low and to facilitate waste recycling and the circular economy.^3,48,49 The unfortunate side effect of using cheap, low-quality materials as substrates to keep costs low and environmental sustainability high is that it prevents fungal development, which degrades the properties of the composite. Higher quality and more expensive substrates, such as nutrient-rich wheat grains and saw dusts, are occasionally used in place of less expensive ones when mechanical characteristics are of the utmost importance.^12,50,51

Figure 5.

Development process of biofoam composite.⁴⁷

Regardless of the quality, substrates are initially soaked in water to hydrate them. Moisture is crucial for fungal growth, and the duration of this stage varies depending on the substrate.⁴⁷ The soaking time for rice hulls is less important than it is for inoculation media like wheat grains, which expand significantly and need at least 48 h of soaking.⁴⁸ The hydrated raw material is first homogenized using low-energy mechanical operations, such as a kitchen blender, or it can be ground or milled depending on the functional and production scale, in order to increase the growing surface area.⁵¹ To eliminate microbiological competition from already present bacteria and fungi, the macerated raw material is then sterilized. Either a high-temperature oven, which has the drawback of drying out the substrate, or a pressure cooker or autoclave, which maintains the substrate’s moisture content and is therefore advised, can be used to accomplish this. Chemicals can also be used to sterilize the substrate, such as hydrogen peroxide, but they are less efficient than other sterilization techniques, leading to higher contamination rates.⁵²

The lignocellulosic material is bonded into three-dimensional geometries that reflect the mold form into which the substrate is packed during the construction of the composite using a natural fungal growth process.^1,2 The lignocellulosic substrate is inoculated by adding and evenly distributing 10-32 wt percent of any element of fungal biomass into the mold’s lignocellulosic material.^1,53 Examples of this element of fungal biomass include spores in liquid solution, hyphae, or fruiting body tissue produced on a nutrient-rich substrate, such as wheat grains. Spores can start to grow in a variety of environments, but they need a substrate that is rich in nutrients because they initially struggle to grow on inferior materials. Using grain- or sawdust-based inocula, which provide nutrient-rich substrates, early growth can be supported on low-grade substrates. These inocula, however, are more challenging to disperse evenly and have fewer initial growth spots.⁴⁸

Following inoculation, the molds can be stored at room temperature or in a temperature-controlled environment at 25°C–27°C for a period of days to months, depending on the fungal species and substrate utilized, as well as the degree of bonding required.^1,54 Environmental conditions are definitely more cost-effective and energy-efficient to maintain than high-temperature environments, but they might result in a slower rate of growth. When composite materials are grown, they can be taken from molds and dried in an oven or hot press to reduce moisture and kill the fungus. At the same time, this ensures that it cannot develop or spread and stiffens the composite material.⁵³ Industry prefers hot-pressing and oven drying because they are the fastest methods of drying, while hot-pressing also consolidates and densifies the material, resulting in better mechanical qualities. When produced on nutrient-rich substrates, fully processed mycelium composite materials are totally biodegradable and contain around 95 wt % lignocellulosic material bound with about five wt % fungal mycelium.⁵⁵

Biofoam composites are suitable for non-structural building applications such as insulation materials and door cores because of their foam-like physical and mechanical qualities. This type of composite material is currently commercially available in the United States and Indonesia, however there is no published information on its physical or mechanical qualities.^56,57 A popular commercial product in the EU and the US is biofoam composite acoustic insulation foams, which are promoted as renewable materials having acoustic absorption capabilities that compete with other typical and traditional commercial construction materials.^55,57 With the development of highly flexible polymer-like mycelium-based materials, textile applications are increasingly drawing interest. These materials are currently sold as completed products rather than raw ingredients .⁵⁷ However, the physical and mechanical qualities of mycelium composites impregnated with soy-based resin and then cured may allow them to be used in semi-structural applications including wall paneling, floor covering, cabinetry, and other furnishings.^58–60

Although these materials have been offered commercially for more than 10°years, adoption has been slow. IKEA is interested in using mycelium-based packaging, and Dell packs business servers with mycelium foams.^61,62 Mycelium materials are still a niche good, however, valued by a very small subset of artists and creators. Who are now employed to produce a variety of items, such as chairs and lampshades, as well as artistic buildings like Philip Ross' “Mycotectural Alpha” tea house and the 12 m tall “Hy-Fi” organic compostable tower that was on display at the New York Museum of Modern Art in 2014.^1,63–66 A mycelium materials patent monopoly, a lack of commercial industrial feasibility, a lack of confidence in this new material platform for uses other than packaging, or a lack of awareness among industry and the general public could be the causes of this underutilization. But there is growing interest in mycelium materials, with businesses operating in the Netherlands, Indonesia, Italy, Belgium, and the United States.^{56–58,67–73} Additionally, research is being conducted in these countries as well.^{56–58,67–73}

Physical stimulant for enhancing fungal growth

Impact of cold stress on fungal enzyme activity

The study regarding the effects of cold stress on fungal enzyme activity and its relevance in biotechnology applications takes on added significance when we delve into specific research findings from the cited articles. These investigations, conducted on various fungal species, yield valuable insights into the multifaceted relationship between cold stress and fungal responses, thereby providing a deeper understanding of their biotechnological potential.

For instance, in the case of Flammulina velutipes (enoki mushroom), shed light on the molecular intricacies of its response to cold stress through quantitative proteomic analysis.⁷⁴ By utilizing iTRAQ-based quantitative proteomics coupled with 2D LC-MS/MS, these researchers identified 63 differentially expressed proteins primarily involved in metabolic processes. This discovery underscores the complexity of the fungal response to cold stress, involving a cascade of molecular changes. These proteins could potentially serve as markers for cold stress resistance in mycelium. Further research could focus on isolating and characterizing these markers to develop strategies for enhancing enzyme production in response to cold stress, ultimately bolstering biotechnological applications relying on fungal enzymes.

Similarly, Fu et al.'s (2016) research on Pleurotus eryngii subsp. Tuoliensis (Bailinggu) demonstrates the power of molecular-level investigations.⁷⁵ They identified differentially expressed unigenes associated with various pathways in response to cold stimulation using RNA-Seq technology. This knowledge can be harnessed to explore the functional significance of these pathways, potentially leading to the development of targeted biotechnological applications. For example, if specific pathways are found to enhance the production of particular enzymes or bioactive compounds under cold stress conditions, biotechnologists could optimize these pathways for industrial purposes.

Furthermore, Rawn’s study (1991) on Sclerotium rolfsii highlights the potential for understanding the factors triggering sclerotia formation in response to cold treatment.⁷⁶ This understanding can be applied to further research aimed at optimizing the conditions and factors influencing sclerotia formation in S. rolfsii. Enhanced control over sclerotia formation could have significant implications in biotechnology, particularly in the production of valuable secondary metabolites or bioactive compounds produced by this fungus.

Moreover, Mattoo and Nonzom’s investigation (2022) into methods for inducing sporulation in endophytic fungi through cold treatment and alternate light and dark cycles emphasizes the importance of exploring innovative techniques for biotechnological applications.⁷⁷ Developing improved sporulation methods can be pivotal for taxonomic and ecological studies and may also have applications in the production of bioactive compounds or specialized enzymes produced during sporulation.

Consequently, the research findings from these cited articles exemplify the intricate responses of fungi to cold stress and their potential applications in biotechnology. The methodologies employed, ranging from quantitative proteomics to RNA-Seq, offer a toolkit for researchers to further explore the molecular mechanisms and pathways involved. These insights pave the way for innovative biotechnological strategies, including optimizing enzyme production, enhancing mushroom cultivation, and improving the control of critical fungal processes, ultimately contributing to sustainable biotechnology practices.

Electrical shock-induced adaptations in mycelium for enhanced fungal production

The investigation of employing electrical shock as a means to stimulate mycelium growth for improved mushroom production has gained significant importance due to the growing demand for sustainable and cost-effective cultivation methods.^78,79 Traditional approaches have faced challenges in efficiently meeting this demand, prompting the need for innovative methods to enhance yield while maintaining cultivation simplicity.

The central focus of this matter revolves around optimizing mycelium growth, particularly in Pleurotus species. While conventional cultivation techniques are functional, they stand to benefit from unconventional growth stimulants such as electrical shock. This study delves into the potential of electrical stimulation as a strategy to enhance growth.^80,81

The research methodology involves an in-depth analysis of existing studies, with a specific emphasis on research involving white oyster mushrooms (Pleurotus florida) and grey oyster mushrooms (Pleurotus sajor-caju). Various levels of electrical voltage, in combination with physical treatments, have been applied to these fungi. Significantly, the use of a 9V electrical shock during the cultivation of white oyster mushrooms yielded the highest production yield.⁷⁸ For grey oyster mushrooms, electrical shock treatment has emerged as a promising technique, resulting in significant improvements in both yield and quality.^79,80

The findings from this investigation highlight the potential of electrical shock in inducing adaptive responses in mycelium, ultimately leading to substantial enhancements in mushroom production. Electrical stimulation offers a cost-effective and environmentally sustainable approach to optimize cultivation practices.^78–80 Additionally, it underscores the necessity for further exploration of electrical stimulation parameters to fully unlock its potential.

The implications of this research are wide-ranging, introducing innovative cultivation techniques for mushrooms. Future studies should prioritize refining electrical stimulation parameters, exploring synergistic effects between electrical shock and other physical treatments, and examining how different mushroom species respond to this stimulating approach.^78–81

Sound treatment optimization for fungal cultivation

Numerous studies underscore the importance of refining sound-based treatment methods to enhance mushroom cultivation. These research efforts aim to tackle the fundamental challenge of improving mushroom growth and yield through the application of diverse acoustic and physical treatments. A range of experiments has been conducted to assess the effects of different sound treatment approaches, intervals between treatments, and treatment durations on the cultivation of grey oyster mushrooms (P. sajor-caju).

There is a consistent focus on the impact of acoustic sound treatments, involving various stimuli such as thunderstorm sounds, energetic music, soothing instrumentals, Quranic recitations, and a control group.^78,82 The key findings from these studies reveal a significant increase in mycelium growth rate and overall yield when exposed to acoustic sound treatments at 75 dB. These results highlight the potential for further exploration and refinement of acoustic sound treatment methods to enhance mushroom cultivation.

Moreover, the investigations delve into sound treatment intervals, ranging from no treatment to intervals of 5, 10, 15, and 20 days.⁷⁸ The results consistently indicate that sound treatment at 5-days intervals produces the most favorable outcomes in terms of promoting mushroom growth and increasing yield. Future research goals aim to delve deeper into the specific genes influenced by acoustic sound treatment, with the ultimate objective of applying these findings to ecological agriculture practices.⁷⁸

In parallel with acoustic treatments, the studies explore the effects of various physical treatments, including high sound intensity, bright light, low temperature, and electrical shock.^79,83 Notably, electrical shock treatment emerges as particularly promising, displaying the highest yield and shortest spawning time. This discovery prompts further investigation into combining different physical treatments to optimize mushroom production.⁷⁹

Furthermore, the research utilizes advanced genetic analysis techniques like DDRT-PCR to assess the genetic expression of mushrooms exposed to acoustic sound treatment.⁸⁴ These studies reveal that acoustic sound treatment has varying effects on the expression of specific genes. This underscores the importance of identifying and studying these specific genes in the context of acoustic sound treatment for ecological agriculture applications.⁸⁴

Lastly, the studies explore the potential of ultrasonic sound treatment by investigating different durations of exposure.⁸⁰ The results indicate that a duration of 1.5 min of ultrasonic treatment leads to the shortest time for various growth stages and the highest yield, suggesting promising advantages for mushroom cultivation.⁸⁰

Impact of light spectrum on fungal growth

Numerous studies have delved into the effects of various environmental factors on fungal development. These factors encompass temperature, humidity, and substrate composition, and their interaction with light conditions significantly influences fungal growth.^85–87 Maintaining an optimal temperature range is crucial for mushroom cultivation because temperature variations can profoundly affect growth rates and the formation of fruiting bodies. Furthermore, humidity levels must be carefully regulated, as insufficient moisture can impede mycelium growth, while excessive moisture can result in contamination.⁸⁸

Moreover, the importance of substrate composition should not be underestimated. Multiple studies have demonstrated that different types of organic materials used as substrates can impact not only the growth rate but also the nutritional content and flavor of mushrooms.^89,90 For instance, research has investigated the use of agricultural waste substrates, like cottonseed hulls or sawdust, to optimize mushroom cultivation in arid regions. These studies aim to enhance both the economic viability and sustainability of mushroom production in diverse environments.^89,90

In addition to environmental factors, the role of genetics in fungal growth is an emerging area of interest. Recent research has uncovered that genetic factors play a substantial role in how fungi respond to different light spectra.⁹¹ Understanding the genetic mechanisms governing light sensitivity in various mushroom varieties is essential for tailoring cultivation strategies. This research holds the potential to optimize growth conditions for specific mushroom species, ultimately enhancing yield and quality.⁹¹

Furthermore, the application of technology, including microcontrollers and sensors, has been explored to automate and optimize environmental control in mushroom cultivation.⁹² These technological innovations aim to address challenges associated with temperature and humidity fluctuations, ensuring consistent growth conditions. This technology-driven approach shows promise for future advancements in mushroom farming, potentially increasing production efficiency and yield.⁹²

Fungal mycelium-based biofoam composite material properties

Influence of the mycelium binder on biofoam composite mechanical performance

It is common to blame the components of biofoam composites made from fungal mycelium for their poor mechanical performance.^12,60 The mycelium binder is quite strong, with tensile strengths of up to 25 MPa⁹³ and up to 200 MPa for fruiting body extract.⁹⁴ This suggests that low fungal growth density, which restricts the amount of mycelium binder and mycelium binder to substrate filler interface, is more likely to be the cause of poor mechanical performance. The type of fungus used to bind agricultural filler dispersed in mycelium composites has a significant impact on the growth density and level of interfacial bonding at the mycelium-substrate interface, which varies greatly between species and substrates.⁹⁵ The material’s mechanical properties appear to be impacted by this. How well a particular substrate supports a particular fungus species depends on natural evolutionary factors. At moderate temperatures, mesophilic microflora thrives in the natural world. Thermophilic microflora then prefers hot environments for growth, and so forth. On the other hand, mesophiles begin to flourish as the temperature rises. The thermophiles are left with only cellulose and hemicelluloses after they have consumed sugars, amino acids, and organic acids. There are numerous instances of this in nature, such as when the first colonizers that grow more quickly consume the simple sugars that are available while leaving the more complex sugars for the later colonizers. As a result, these groups have developed a natural affinity for the various carbon sources, which has a significant impact on a fungus' ability to flourish on a given substrate.^96,97 White rot fungi like those in the Trametes, Ganoderma, and Pleurotus genera as well as the Phylum Basidiomycota are typically used because a significant portion of mycelium is grown on lignocellulosic agricultural by-products and wastes, which typically lack optimal fungal nutrients like easily utilizable simple sugars (e.g., fructose, glucose, and sucrose).^2,5,44,48,51

The structure of the mycelium binder network that surrounds biofoam composites also affects how they behave mechanically. Mono-, di-, and tri-mitic hyphal networks, for instance, are good example of this in basidiomycetes.⁹⁸ Basidiomycetes have been found to have up to three different types of hyphae, including generative, binding, and skeletal hyphae, each of which has a different cell wall thickness, internal structure, and branching characteristics.^1,99 The various hyphal types found in a species are described using the mitic system. Trimitic species contain all three types of hyphae, while monomitic species contain only generative hyphae.⁴⁰ Dimitic species typically contain two different hyphal types (often generative and skeletal). The thin-walled, hollow, branching “generative” hyphae and the densely branched, solid, “skeletal” hyphae are two different types of filaments found in fungi. In binding (ligative) fungi, thick-walled, solid, and heavily branching hyphae are also present. It is generally accepted that more complex hyphal systems, like trimitic systems, are superior.^98,100,101 Due to the thickness of the hyphal system’s walls and the amount of water contained within its cells, the biomass has particular qualities.¹⁰² The mechanical characteristics of wood-rot fungus hyphae are still poorly understood, despite research into the tensile characteristics of fungal hyphae used in fermentation, which have estimated ultimate tensile strengths of up to 24 MPa and elastic moduli of up to 140 MPa.^103–105 Instead of generative hyphae (monomitic hyphal systems), which are hollow and contain cytoplasm, binding hyphae (dimitic and trimitic hyphal systems) are thought to be in charge of material strength.^106,107 The tensile strength (0.04 MPa) and flexural strength (0.22 MPa) of biofoam composites containing trimitic species, such as T. Versicolor or multicolor, are higher than those containing monomitic species, such as P. ostreatus (0.01 MPa tensile strength, 0.06 MPa flexural strength), when grown on rapeseed straw, although there is no evidence to support this.⁴⁴ When grown on hemp, Trametes versicolor has a better compressive strength than P. ostreatus which is 0.26 MPa compared with 0.19 MPa.⁵² The presence of structural polymers such as chitin and chitosan, on the other hand, is restricted to the thin hyphal cell wall, which also contains polysaccharides (e.g., galactose, mannose, and fucose), phosphate, proteins, lipids, and mineral salts^41,93 that raises the question of the significance of hyphal structure, with mycelial biomass (binder) amount likely to have a greater impact on mechanical performance than hyphal structure.

Influence of the substrate filter on the mycelium-based biofoam composite mechanical performance

The physical and mechanical characteristics of as-grown mycelium are frequently influenced by the substrate, which acts as the dispersed substrate filler phase in the composite material. Natural fibres and straw, which are agricultural byproduct filler phases, have lower densities (60–130 kg/m3) than substrates made from forestry byproducts, like sawdust (87–300 kg/m3) (Table 3). Data on the mechanical characteristics of biofoam composites for different substrate groups are scarce.

Table 3.

Density, tensile, compressive, and flexural material properties of as-grown biofoam composites comprising fibrous and particulate dispersed agricultural filler substrate.^12,45,51

Loading	Substrate type	Substrate	Density (kg/m³)	EMPa	Ultimate MPa
Tension	Fibrous	Rapeseed straw	115	3.0	0.025
	Particulate	Beech sawdust	170	13.0	0.05
		Red oak sawdust	300	1.30	0.18
Compression	Fibrous	Flax hurd	99	0.73	-
		Wheat straw	192	-	0.17
		Hemp hurd	94	0.64	-
	Particulate	Pine shavings	87	0.14	-
		Red oak sawdust	300	1.0	0.49
		White oak sawdust	552	-	1.1
Flexure	Fibrous	Cotton fibre	130	1.0	0.05
		Rapeseed straw	115	1.5	0.14
	Particulate	Beech sawdust	170	9.0	028

The tensile properties of biofoam composites have been studied to a great extent. Sawdust substrates have a wide range of reported tensile values (0.05-0.18 MPa), but as shown in Table 3, sawdust does seem to have higher tensile strengths than straw substrates (0.01-0.04 MPa). However, the mechanical properties of the substrates have no relationship with the tensile properties of the as-grown sawdust-based biofoam composites. Beech wood sections have a tensile strength that is similar to or greater than red oak (5.5 MPa) perpendicular to the grain (5-7 MPa) ^108,109 but biofoam composites made from beech sawdust have a significantly lower tensile strength (0.05 MPa) than biofoam composites made from red oak sawdust (0.18 MPa). This shows that the dispersed substrate filler has less of an impact on the tensile properties of biofoam composites than the failure of the mycelium binder, and that nutrient-rich substrate are required for the establishment of a dense mycelium network and the enhancement of the tensile properties of biofoam composites as opposed to strong substrates. Due to their low cost, some lower-grade substrate materials, like agricultural waste and byproducts, are appealing. However, these materials frequently lack ideal fungal nutrients, like easily utilizable simple sugars (such as fructose, glucose, and sucrose), and instead contain more complex carbon sources (such as cellulose and lignin).¹¹⁰ These lignocellulosic substrates support the growth of white rot fungi, but other agricultural byproducts, like rice hulls, contain significant amounts of minerals that prevent fungal growth.54 Less interaction between hyphae and organic matter results from the reduced fungal growth on these harder to come by substrate types, which negatively affects the tensile strength of the mycelium binder (which holds the mycelium together).^12,55,111

Unfortunately, the information that is available about the compressive properties of biofoam composites is contradictory and scant. The compressive moduli of biofoam composites made from fibre hemp and flax hurd substrates were found to be higher than those of particulate pine shavings by Elsacker et al.⁵¹ It should be noted, however, that their study only tested 70% to 80% strain and did not look at compressive strength (0.64 and 0.73 MPa compared to 0.14 MPa, respectively). Contrarily, according to Ghazvinian et al.¹¹² when biofoam composites were grown on white oak sawdust and wheat straw substrates, the compressive strength of the sawdust particulate substrate was found to be significantly higher than the fibrous straw (1.1 MPa vs 0.17 MPa, respectively). The compressive modulus (1 MPa) and strength (0.49 MPa) of biofoam composites with a red oak sawdust substrate were only evaluated by Travaglini et al.¹² It seems likely that particle substrates like sawdust provide the composite with superior compressive qualities over fibrous substrates like straw, despite significant gaps in the characterization of biofoam composites under compressive loading conditions. Higher porosity results in lower mechanical performance and the compression properties of porous materials are highly correlated with their porosity and pore size.^45,113 This suggests that the porosity and compressive properties of the substrate filler, the composite itself, and the degree to which the fungus digests the filler, thereby increasing its porosity, are all important factors in determining how effectively biofoam composites compress air.⁴⁷ Additionally, it was found that the compressive strength of biofoam composites is typically unrelated to the particle size of the substrate filler.¹¹⁴

Particle geometry has had little impact on the flexural strength of biofoam composites, which experience maximum compressive stress on one surface, zero tension in the midplane, and maximum tensile stress on one side when bent.¹¹⁵ While cellulose fibres are likely to be damaged and deteriorated by extensive fungus growth on air-exposed surfaces, reducing the beneficial properties of the fibres present, fibrous geometries can improve the tensile capabilities of the surfaces and subsequently the flexural characteristics of the composite as a whole if they are oriented in the direction of loading.¹¹⁶ Mycelial density is highest at air-exposed surfaces and lowest in the core, where growth may be constrained or even impossible depending on the porosity of the substrate filler.^40,117 Air transmission is essential for fungal growth. The low flexural stiffness (1-3 MPa) and strength (0.06-0.22 MPa) of cotton fiber-based composites (1 MPa and 0.05 MPa, respectively) were used to support the lack of improvement in the flexural properties of biofoam composites with fibrous surfaces, though fibrous straw-based composites did show improved flexural stiffness (1-3 MPa) and strength (0.06–0.22 MPa). However, a particle Beech sawdust substrate resulted in a composite with noticeably higher flexural modulus (9 MPa) and strength (0.29 MPa), which suggests that the nutritional makeup of the substrate encouraged the growth of a dense, continuous mycelium binder on the composite’s air-exposed surface. Results from Tudryn et al. ,¹¹⁸ who discovered that increased nutrition at homogenization increased specific flexural stress and specific flexural modulus due to the presence of a larger, more continuous mycelium binder, support the idea that the substrate nutrient profile affects composite flexural properties. Overall, the value of any particular substrate in strengthening the composite is more strongly influenced by its nutrient profile, with more nutrient-rich substrates facilitating more fungal growth and bonding. This is because failure frequently occurs in the mycelium binder rather than the substrate filler, regardless of the loading situation. Unfortunately, unless additional processing techniques like hot or cold pressing, resin infusion, or hybridization are used to increase mechanical performance, this restricts the use of low-grade agricultural and forestry residues to foam-like biofoam composites.⁵⁵

Additionally, as shown in Table 4, the bio-composite material has a higher compression modulus and strength when compared to Expended polystyrene (EPS), indicating that it is stronger in compression than EPS.

Table 4.

Result of compression test on mycelium composite.¹¹⁹

Sample	Mycelium Bio-composite		Expended polystyrene (EPS)
Sample	Compression modulus (E_c) GPa	Compressive strength (rM) MPa	Compression modulus (E_c) GPa	Compressive strength (rM) MPa
1	0.00041	0.432	0.0000352	0.00929
2	0.00036	0.288	0.0000957	0.00898
3	0.00016	0.352	0.0000608	0.00858
4	0.00013	0.0326	0.000000737	0.00805

Thermal conductivity properties of mycelium-based biofoam composites for insulation applications

Biofoam composites with high-performance natural insulators like hemp and straw fibres have low thermal conductivities (0.04-0.08 W/mK) and low densities (57-99 kg/m3) (Figure 6). Their ability to compete with common commercial thermal insulation materials like glass wool (57 kg/m3, 0.04 W/mK) and extruded polystyrene (XPS, 34 kg/m3, 0.03 W/mK) is made possible by this.¹²⁰ In addition to naturally occurring insulators like kenaf (105 kg/m3, 0.04 W/mK), and sheep wool (18 kg/m3, 0.05 W/mK). Lower thermal conductivities are associated with better insulating materials, which are primarily influenced by density and only slightly by moisture content.^121–123 Thermal conductivity of hemp concretes, a bio-composite material made of hemp shives and lime, will increase by 54% when the density is increased by 67%, but only by 15%–20% when the relative humidity is increased by 90% (from completely dry to% RH).¹²²

Figure 6.

Density (kg/m3) and thermal conductivity (W/m∙K) of biofoam composites produced using various substrates.^2,121,131

There is a strong correlation between material density and thermal conductivity because low density materials with extremely low thermal conductivity (26.2 ×10-3 W/mK at 0.1 MPa, 300 K) contain a significant amount of dry air.¹²⁴ As a result of the abundance of air, low density materials are excellent thermal insulators,¹¹⁸ which trap a lot of air between the fibres in the insulation, giving it its useful insulation properties due to their porous structure and low bulk density of the bundled fibres.^125,126 Their thermal insulation properties are significantly influenced by their density, moisture content, and fibre type.¹²⁷ Using wheat straw as a filler, biofoam composites have reported thermal conductivities of 0.04 W/mK⁵¹ and 0.08 W/mK⁵⁰ respectively. However, the accuracy of the former value, which is associated with a higher composite density (94 kg/m3 as opposed to 57 kg/m3) and is significantly lower than the conductivity of straw bales themselves (0.07-0.08 W/mK), seems to be in question.¹²⁸ Additionally, it was noted that the thermal conductivities of hemp fiber-based biofoam composites (0.04 W/mK)⁵¹ were significantly lower than those of hemp concretes (0.1 W/mK).¹²⁹ Even biofoam composites made with substrates that have less effective insulation, like cotton carpel substrates (0.10-0.18 W/m K),² have thermal conductivity values that are comparable to gypsum (0.17 W/m K), high density hardboard (0.15 W/m K), plywood (0.12 W/m K), and both hardwoods and softwoods (0.16 W/m K).¹³⁰ Therefore, biofoam composites are a useful, economical, and environmentally responsible alternative to conventional commercial building insulation materials.

In addition, this bio-composite was also designed to replace thermoplastics in non-structural and semi-structural applications such as insulation foams, furniture, decking, etc. Mycelium composites can replace synthetic foams and comparable materials in non-structural and semi-structural applications. Despite burning, these cheap and eco-friendly organic materials produced less heat, mass loss, and CO₂. Not only did the mycelium produce non-toxic and non-combustible gases, but they also lowered oxygen levels within the pyrolysis zone, providing thermal insulation. Because of these properties, mycelium composites are ideal substitutes for standard synthetic insulation foams, furniture, and decking utilized in the building and construction industry.¹³²

Acoustic properties of mycelium and its composited as noise barriers

Due to its inherent low-frequency absorption (b1500 Hz), mycelium makes a superior acoustic absorber, while cork and commercial ceiling tiles perform better at reducing road noise.¹³³ Due to this peculiar quality, biofoam/mycelium foam can be combined with other substances to improve its low frequency absorption capabilities. A biofoam composite made of agricultural waste bound by mycelium, on the other hand, can offer a wider range of acoustic absorption with 70–75 % or more absorption for perceived road noise.³ A-weighted decibels are used to express the relative loudness of air sounds in decibels, with the magnitude of low frequency sounds (b1000 Hz) being reduced to account for the reduction in human hearing sensitivity at low frequencies and the magnitude of higher frequency sounds remaining uncorrected.¹³⁴ Humans can interpret street noise (frequency range 700-1300 Hz), human speech (frequency range 85-255 Hz), and dog barking (frequency range 500-1500 Hz) in this way.^135–138 Examples of acoustic absorbers that are fibrous, porous, or reactive resonators include nonwovens, fibrous glass, mineral wools, felt, and foams.^139,140 Absorbers prevent sound from building up in enclosed spaces and reduce the volume of reflected noise by converting the mechanical motion of air molecules travelling in sound waves to low-grade heat.¹³⁹ When compared to typical reference absorbers such as commercial ceiling tiles (61 dBa), urethane foam board (64 dBa), and playwood (65 dBa), all biofoam composites evaluated reduced perceived road noise (45.5–60 dBa) (Figure 7(A), (B)). The best individual substrate fillers for acoustic absorption were rice straw (52 dBa), hemp pith (53 dBa), flax shive (53.5 dBa), sorghum fibre (54 dBa), and switchgrass (55 dBa) (Figure 7(A)). The best filler combinations were rice straw-sorghum fibre (45.5 dBa), rice straw-cotton bur fibre (47 dBa), and sorghum fiber-switchgrass (47 dBa), which together produced even better acoustic absorption (50-50 wt%) (Figure 7(B)). Biofoam composites' high acoustic absorption qualities are due to their porous, fibrous nature. The air flow resistance of a material is significantly influenced by the impedance and propagation constants used to measure its acoustic properties, with higher airflow resistance being associated with greater acoustic absorption.¹⁴¹ In biofoam composites, the fibres act as frictional elements, preventing acoustic wave motion and lowering its amplitude as the waves attempt to pass through the material’s tangled channels and turn into heat.¹⁴² Thin fibres have better acoustic absorption qualities because they have more fibres per unit length and move more quickly.^143,144 Additionally, it is crucial to take into account the surface pore concentration and geometry because both tortuosity and porosity are necessary for effective damping.¹³⁹ Additionally, the porosity and airflow resistance of porous materials have an impact on the height and width of sound wave peaks, whereas tortuosity has an effect on the high frequency acoustic properties of porous materials.¹³⁹ Less dense, more open structures in nonwoven fibrous materials are better at absorbing low-frequency sound (500 Hz), while denser structures are better at absorbing high-frequency sound (2000 Hz).¹³⁸ The amount of acoustic absorption decreases as a material is compressed because its thickness is reduced.¹⁴⁵ As a result, neither hot nor cold pressing should ever be used on biofoam composites used as acoustic absorbers.

Figure 7.

A) weighted perceptual road noise for biofoam composites comprising an individual substrate compared to traditional acoustic absorbers and B) 50–50 wt% mixtures of selected substrate fillers.

Colors: green cross: 45.5–50.0 dBa, orange line: 50.5–55.0 dBa, red dot: 55.5–60.0 dBa, grey: traditional reference absorbers.

Table 5 reveals that EPS has a lower sound absorption coefficient than the mycelium bio-composite, which means that the mycelium bio-composite prevents some sound waves from traveling through it. It has a sound absorption coefficient of 0.2275, compared to 0.13 for EPS. Composites can be utilized for sound insulation because their average sound absorption coefficient is above 0.2.¹²⁰

Table 5.

Sound absorption coefficient obtained from impedance test.¹²⁰

SI no	Frequency (Hz)	EPS	Mycelium Bio-composite
1	250	0.04	0.17
2	500	0.05	0.19
3	1000	0.10	0.33
4	16,000.04	0.33	0.22

Water absorption properties of biofoam composites

At present, one of the biggest obstacles standing in the way of biofoam composites' broad application in material science applications is their slow rate of large-scale water absorption. The weight of biofoam composites typically increases by 40–580 wt% when they are exposed to water for 48–192 h at a time.^{2,13,43,50,146} The strong affinity of biofoam composites for water absorption is due to their typical constituents of cellulosic fillers, which contain a variety of accessible hydroxyl groups,¹⁴⁷ as well as the hydrophilic porous mycelium binder and biologically derived filler phases, which facilitate wicking and wicking-related.^148–150 Within 48 h of contact with water, air-dried biofoam composites comprising a fibrous substrate of rapeseed straw or cotton bur fiber absorb around 530–550 wt percent moisture (Figure 8(A)). Biofoam composites, such as acoustic and thermal insulation, can absorb a lot of water, although these applications can be found in indoor or dry areas that are not even affected by the weather, minimizing this issue. Within the first 3 hours, the weight of both rapeseed straw- and cotton bur fiber-based composites increases by approximately 220 wt percent (Figure 8(B)). Until the material reaches saturation (around 580 %), water uptake continues at a decreased rate for up to 48 h before slowing and eventually ceasing (Figure 8(A)). Rapeseed straw is rich in cellulose (48.5 % by weight) and pentosans (17 % by weight),¹⁵¹ while cotton bur fibers are mostly made of cellulose (98 wt percent with b0.5 wt % pentosan).¹⁵² Polymers derived from pentoses, which are water-soluble, have been known to boost the biocomposite’s ability to absorb water, whereas cellulose’s hydroxyl groups attract water molecules.^147,153 As an alternative, beech sawdust-based biofoam composites have a weight gain of only 23 wt % during 3 hours of water contact, which gradually rises to 43 wt % over 192 h (Figure 8(A)). Beech sawdust contains 48% cellulose in addition to 26 wt percent of hydrophobic lignin (HPL)¹⁵⁴ which, along with its higher material density and lower void content of the fine particle substrate filler, is probably what causes it to absorb less water. Additionally, the water uptake of hot-pressed or cold-pressed biofioam composites is less than half that of air-dried composites (250 wt% vs 580 wt %). (Figure 8(A), 8(B)). This is most likely owing to the smaller volumes of pressed materials, which restrict capillary action and consequently water absorption.¹⁵⁵ A cold-pressed biofoam composite is less absorbent (214 wt % after 48 h, 238 % after 192 h) than hot-pressed composites, but they reach saturation faster because they are initially more hydrated, making them more efficient than the dryer hot-pressed composites at absorbing water. When lignocellulosic polysaccharide components, such as hemicelluloses, are heated to temperatures more than 160°C, the quantity of free hydroxyl groups decreases, which in turn reduces water absorption.^156,157 Nevertheless, because hot pressing primarily heats treatments mycelium-rich surfaces, any improvement in water absorption capabilities due to hemicellulose depolymerization is likely to be observed only with more uniform temperature application to the lignocellulosic core, such as oven drying. Along with the use of particulate substrate fillers and pressing, a variety of bio-based coatings, such as polyfurfuryl alcohol resin (PFA), have demonstrated promise in reducing water absorption in natural fiber composites¹⁵⁸ which could be used to improve the water-resistance of bio-foam composites.

Figure 8.

Moisture uptake (wt.%) of air dried (solid lines) and hot and cold pressed (dotted lines) fibrous (P. ostreatus on cotton bur, orange, T. versicolor on rapeseed straw, red) and particulate (T. versicolor on beech sawdust, green) biofoam composite materials resulting from continuous contact with a water surface over (a) 192 h with (b) the most rapid absorption period (0–6 h) magnified.⁴³

Termites resistance of biofoam composites

In many nations around the world, termites pose a threat to residential and commercial buildings, resulting in billions of US dollars in yearly structural damage.¹⁵⁸ The annual damage they cause to New Orleans exceeds US$100 million, despite the fact that they are more prevalent in Africa, Asia, South America, and Australia.¹⁵⁹ The biological and lignocellulose components that make up biofoam composites don’t naturally resist termites. By choosing the right substrate and using termiticides, it can be enhanced.¹⁶⁰ Low mass losses from termite infestation over 4 weeks (16-53 wt%) and high termite mortality rates (directly correlated to termite treatment success) are two characteristics of hemp composites. Kenaf-based composites have the highest mass losses of any untreated mycelium composite (43%–62%), but have a moderate to 100% termite mortality rate. Corn-based composites have mass losses of 42%–43% and mild to moderate termite mortality. The most effective natural termiticides are guayule resin (flavonoid, cinnamic, terpenoids, and p-anisic acid bioactive chemicals) and vetiver oil (bioactive compounds - and -vetivone).^161,162 For treated biofoam composites, a single coating of these oils results in total termite mortality and mass losses of 18–28 wt percent, and 16–27 wt percent for untreated biofoam composites. In comparison to untreated composites (42–62 wt%) and an untreated southern yellow pine (Pinus taeda) reference sample (80 wt%), this mass loss is significantly lower. When commercial borax termiticides are used, termite infestation results in a mass loss of 28–40 %. For biofoam composites, the effects of the fungi Daedaleopsis confragosa, Ganoderma resinaceum, and T. versicolor are comparable. Two additional biofoam composite degradation factors that need further study are mould and weathering resistance.

Applications of mycelium-based biofoam composite

Environmental sustainability in packaging and construction

Mycelium-based materials offer an appealing answer to the environmental issues linked with traditional packaging and construction materials. Utilizing agricultural by-products and fungal mycelium, these materials provide a renewable and biodegradable substitute for petroleum-based plastics and synthetic foams commonly used in packaging.¹⁶³ As for the construction, mycelium-based materials serve a dual purpose. Firstly, they can be utilized as insulation, effectively reducing energy consumption and carbon emissions in buildings.¹⁶⁴ This is particularly significant as buildings account for a significant portion of global energy usage and greenhouse gas emissions. Secondly, mycelium-based materials have the unique ability to absorb sound, thus improving the acoustics of indoor spaces. Enhanced acoustic properties contribute to a more comfortable and pleasant environment, benefiting both residential and commercial settings.¹⁶⁵ By adopting mycelium-based materials in packaging and construction, this can mitigate environmental impact, promote sustainability, and create healthier living spaces for future generations.

Water treatment and filtration

As the global water scarcity issue continues to escalate, there is a growing urgency to develop innovative solutions for water treatment and filtration. Mycelium-based materials offer a sustainable alternative for addressing this challenge. Mycelium, the root-like network of fungi, possesses remarkable properties that make it well-suited for water filtration applications. Mycelium has a high-water retention capacity, meaning it can hold onto significant amounts of water. This characteristic is advantageous for water filtration systems, as it enables the material to efficiently capture and retain waterborne contaminants during the filtration process. Additionally, mycelium exhibits hydrophobic properties, meaning it repels water.¹⁶⁸ This property is crucial for creating effective membranes for water filtration, as it allows the material to selectively allow water molecules to pass through while blocking contaminants. Mycelium-based membranes offer a sustainable solution for water filtration. Unlike traditional filtration materials, which often rely on non-renewable resources and may contribute to environmental pollution, mycelium membranes utilize renewable resources and are biodegradable. This means that after their useful life, they can decompose naturally, reducing the environmental impact associated with water treatment technologies.

Biocomposites for various industries

Industries spanning packaging, furniture, and construction stand to gain substantially from integrating mycelium bio-composites into their processes. These innovative materials boast a host of advantages, making them versatile and well-suited for a wide array of applications.^166,167 Their inherent malleability allows for shaping into various forms, offering designers and manufacturers flexibility in product design. Additionally, mycelium bio-composites are lightweight, which not only facilitates ease of handling and transportation but also contributes to overall resource efficiency. Furthermore, mycelium bio-composites exhibit remarkable fire resistance properties, making them inherently safer for use in applications where fire risk is a concern. This feature enhances their suitability for construction materials and other contexts where fire safety standards must be met. Moreover, the biodegradability of mycelium bio-composites ensures that they can break down naturally at the end of their lifecycle, without contributing to the accumulation of non-recyclable waste.¹²⁰ This aligns closely with sustainability objectives, as it reduces the burden on landfills and minimizes environmental pollution. The use of bio-composites in various industries can lessen their dependence on finite resources such as petroleum-based plastics and synthetic materials. This approach not only aids in alleviating the depletion of natural resources but also diminishes the environmental damage linked with extracting, processing, and disposing of such materials.^169,170

Medical and healthcare applications

Mycelium-derived materials offer exciting prospects in the fields of biomedicine and healthcare due to their unique properties and potential benefits. Cosmeceuticals and nutricosmetics derived from mycelium provide natural alternatives to traditional cosmetics. Unlike many conventional products that contain synthetic ingredients, mycelium-based skincare products are inherently gentle on the skin, reducing the likelihood of irritation and inflammation.^166,171 This is particularly advantageous for individuals with sensitive skin or those prone to allergies, as mycelium materials are less likely to trigger adverse reactions. Mycelium films possess a porous structure that makes them well-suited for wound healing applications. These films can serve as scaffolds for tissue regeneration, providing a supportive environment for cells to proliferate and heal damaged tissue. Additionally, the porous nature of mycelium films facilitates the exchange of nutrients and waste products, further promoting the healing process. Some studies suggest that certain types of mycelium may exhibit antimicrobial activity, which can help prevent infections in wounds. By inhibiting the growth of harmful bacteria, mycelium films can reduce the risk of complications associated with wound healing, such as infection and delayed healing.^166,172,173

Leather substitutes

The development of mycelium-based materials as alternatives to traditional leather represents an important move towards sustainability and ethical consumerism. Mycelium-based leather substitutes are a compelling alternative to both animal-derived leather and synthetic alternatives due to their comparable mechanical properties and numerous environmental benefits. One of the key advantages of mycelium-based leather alternatives lies in their capacity to mimic the texture, flexibility, and durability of real leather.¹⁷⁴ This makes them extremely versatile and suitable for a variety of applications, including the manufacture of shoes, bags, garments, and other leather goods. Consumers looking for cruelty-free and sustainable alternatives can now enjoy products that closely resemble traditional leather without contributing to animal exploitation or environmental damage. In addition, mycelium-based leather substitutes have numerous environmental benefits over traditional leather and synthetic materials. Mycelium-based leather substitutes are biodegradable, which means they decompose naturally at the end of their lifecycle, as opposed to synthetic materials, which can remain in the environment for centuries. This is consistent with circular economy principles, in which products are designed to be regenerative and contribute to a closed-loop system of resource use.

Conclusion and future directions

Exploring future directions is critical in the context of advancing mycelium biocomposites to identify potential avenues for further research and development. This section delves into the changing landscape of mycelium-based materials, emphasizing emerging trends and novel approaches. From advancements in production techniques to novel applications, the following discussion provides insights into the promising opportunities and key challenges that are ahead. Mycelium-based technology continues to develop rapidly, with numerous potential future applications and advancements. Mycelium has emerged as a sustainable alternative to traditional packaging materials such as expanded polystyrene, with companies such as Ecovative demonstrating its suitability for packaging applications. Furthermore, mycelium has been studied as a support material for electronic circuit boards, with properties such as heat and thermal resistance, lightweight construction, modulable shape, and hydrophobicity.¹⁷⁵ Standardizing production processes for mycelium-based composites can help future research and development efforts in a variety of ways, including ensuring consistency and reproducibility, which are critical for progressing research in this field. Furthermore, by developing standardized manufacturing methods, researchers can implement quality control measures to ensure the dependability and quality of mycelium-based materials. In terms of the features and functionality of the material, this may result in more dependable and predictable results. The efficiency and optimization areas are the next frontier for research. By comprehending the critical factors influencing material properties, researchers can better optimize the production process and attain the desired qualities. Standardized production procedures can also aid in fulfilling industry standards and legal requirements, both of which are necessary for the commercialization and broad use of mycelium-based materials in a variety of applications, including construction. Standardized procedures can also contribute to the development of stakeholder and consumer confidence in the content.¹⁷⁶ In addition to the promising applications and advancements in mycelium-based technology, it is critical to understand and address the numerous challenges that come with its implementation. These challenges span a wide range, from concerns about its mechanical properties, which affect its structural integrity and durability, to its tendency to absorb water, which can affect its stability and performance in a variety of environments. Furthermore, the longer production time required for cultivating mycelium-based materials raises logistical and economic concerns. Furthermore, ensuring termite resistance is critical for applications involving mycelium in structural or outdoor settings, necessitating research into effective deterrents. Furthermore, comprehensive documentation of mycelium’s material properties is necessary for understanding its behavior under different conditions and informing design and engineering decisions.^177–179 Addressing these multifaceted challenges through comprehensive study is important for maximizing mycelium’s potential as a sustainable and versatile material solution.

Footnotes

Acknowledgements

The authors acknowledge and greatly appreciate for the financial support from the Fundamental Research Grant (FRGS/1/2020/TK0/UNIMAP/03/4) from Ministry of Higher Education, Malaysia.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Noorulnajwa Diyana Yaacob

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