Development of Mushroom Mycelium Composites for Footwear Products

Abstract

This research aims to reduce solid waste, resource depletion, and material toxicity in the footwear industry. Mycelium, the root structure of mushrooms, binds together substrate materials as it grows, offering opportunities for composite development. Mycelium composites were developed using edible mushroom species alongside other natural materials. The 4 × 2 experiment tested four mushroom species (reishi, oyster, king oyster, and yellow oyster) and two fabric levels (with or without a natural fabric mat). Scanning electron microscopy images confirmed mycelium growth within the composite and around the substrates. Two-way analysis of variance tests found that both species and fabric significantly affected the density, and the species significantly affected the compressive strength. A positive and significant linear relationship was found between density and compressive strength, with higher density leading to higher compressive strength. The compressive strength of the mycelium composites, especially those made from king oyster mycelium, provides opportunities for renewable and biodegradable footwear inputs.

Keywords

mycelium composite material footwear sustainability

Global footwear production amounted to 23 billion pairs in 2015 (World Footwear, 2016). After purchase, footwear is worn until damaged, out of style, or otherwise unwanted by the consumer. This unwanted footwear, as well as clothing, must go somewhere, and current methods of disposal, reuse, and recycling are failing to address this volume of waste. The U.S. Environmental Protection Agency (n.d.) found that 10.5 million tons of textile waste were sent to landfills in the United States in 2015, comprising 7.6% of all municipal solid waste landfilled.

Current apparel and footwear production methods follow a cradle-to-grave model in which resources are consumed to produce a good, which is then used and disposed of, rendering all materials useless (McDonough & Braungart, 2002). McDonough and Braungart (2002) proposed a cradle-to-cradle model, where products are designed to become part of a metabolism in which inputs are not used up and waste does not exist. The biological metabolism takes natural resources and creates outputs that can be safely decomposed, while the technical metabolism recycles or remanufactures synthetic nutrients (Jacques, Agogino, & Guimarães, 2010; McDonough & Braungart, 2002). Product inputs comprise whatever is taken from the earth to make and sustain the product, while outputs include everything that comes from the product and returns to the environment in the form of liquid, gas, or solid waste (Ashton, 2018). Current apparel and footwear inputs and outputs, as well as the cradle-to-grave model, make the soft goods industry as a whole unsustainable.

Synthetic composite materials sometimes used in footwear are generally created using nonrenewable inputs or resources that are renewable but limited in availability, which makes these materials unsustainable to produce (Jiang, Walczyk, McIntyre, & Chan, 2016). It also may be difficult to separate different synthetic components, often requiring landfilling as opposed to recycling and leading to poor end-of-life options. Additionally, the cost to create and dispose of these composite materials is typically high (Jiang, Walczyk, McIntyre, & Chan, 2016). Instead, bio-based materials, defined as “material of which one or more of its components are sustainably grown and are fully renewable,” could help limit pollution and solid waste generation (Lelivelt, Lindner, Teuffel, & Lamers, 2015).

In this research, we investigate bio-based materials in the form of composites made from mushroom mycelium. Most mushrooms are composed of filaments called hyphae that join together to create a mat-like structure, the mycelium (Hanson, 2008). The goal of this research is to develop a wearable mushroom mycelium composite to be used as a shoe sole that could completely biodegrade at the end of its life. Mycelium composites have been successfully used as a compostable Styrofoam substitute to protect computers and other fragile shipments (Holt et al., 2012). These material attributes potentially enable mycelium composites to provide support for a wearer’s foot and cushioning against the hard ground.

Literature Review

Footwear

Most shoes are comprised of three parts: the upper, which contains everything above the sole; the lower, which includes the insole, sole, and outsole that make up the bottom of the shoe; and the grindery, which includes any additional parts that may be attached to the upper or lower (Muthu, 2013). Shoe soles are more specialized than all other shoe components and are meant to fit a wide variety of functions based on the desired shoe attributes (Cohn, 1969). Insoles support the bottom of a shoe (along with the outsole, the piece that lies between the shoe and the ground) and give the upper something to attach to (Choklat, 2012). Components are prepared through cutting, machining, and prestitching and are attached by stretching the upper over the top of a last, or shoe mold, and connecting it to the lower (Staikos & Rahimifard, 2007).

Footwear materials may be selected based on price, aesthetics, and/or performance characteristics (Cohn, 1969). Around 40 different materials are employed when manufacturing a single pair of shoes (Muthu, 2013). Shoes commonly include leather, canvas, polyurethane, polyvinyl chloride (PVC), rubbers, plastics, and polymeric materials (Muthu, 2013; Staikos & Rahimifard, 2007). Most insoles are made from expanded sheet materials comprised of resin-treated cellulose materials or thermoplastics, sometimes alongside reinforcement layers (Choklat, 2012; Cohn, 1969). Many of the common footwear inputs are a mixture of different synthetic materials, which limits end-of-life opportunities for shoes.

Furthermore, many of the footwear inputs contain toxic materials. PVC gives off dioxins that bioaccumulate in the environment and human bodies, disrupting hormone levels (Cao et al., 2014); zinc oxide, often used in the vulcanization of rubber, is soluble in water and poisonous to aquatic life (Ingre-Khans, Ruden, & Breitholtz, 2010). Leather is tanned with the toxic substance chromium, and conventional rubber soles include lead and plastic, which enter the soil and air with use over time and end their lives in landfills where their nutrients are lost forever (McDonough & Braungart, 2002). Landfill space is dwindling, and as landfilled products break down, by-products leach out and contaminate soil, air, and groundwater (Staikos & Rahimifard, 2007). The number of municipal waste landfills in the United States decreased from 6,326 in 1990 to 1,956 in 2014 (Statista, n.d.).

Footwear manufacturing is intensely impactful and produces goods that are designed for only one life, alongside by-products that cannot be reused or safely disposed of (Jacques et al., 2010). Material selection and production typically contribute considerably to the environmental impact of footwear (Jacques et al., 2010). Rather than cleaning up this chemical and solid waste after it is produced, Staikos, Heath, Haworth, and Rahimifard (2006) recommend a “proactive approach” to preventing waste generation by making material improvements, including replacing conventional materials with biodegradable footwear inputs.

Natural Fiber Composite Materials

Recently, researchers have begun using natural fibers as reinforcements in polymer composites due to the cost, biodegradability, and performance characteristics of many of these fibers (Zhao, Mao, Yang, & Hamada, 2017). Cellulose fibers specifically are cheap, plentiful, lightweight, sturdy, and generally soft, making them promising for use as reinforcements in place of traditional synthetic materials (Hadjadj et al., 2016). Hadjadj et al. (2016) found that cellulose fibers enhanced the tensile strength and modulus of their polyurethane-based composite, providing support for the addition of cellulose reinforcement materials to other composites.

Mycelium Composites

Mycelium is a dense network of thin hyphae strands that grow and fuse together into a continuous material; it can act as a matrix that binds other natural substrates into a lightweight composite material (Lelivelt et al., 2015). As it grows, the mycelium secretes enzymes to degrade the substrates while simultaneously binding them together (Joint Nature Conservation Committee, n.d.). Since all of the raw materials and growing process are natural, the mycelium composite is fully biodegradable (Holt et al., 2012).

Previous mycelium composite researchers (Pelletier et al., 2017; Yang, Zhang, Still, White, & Amstislavski, 2017) have used Basidiomycetes-based fungi, such as the Ganoderma sp. (Holt et al., 2012). Haneef et al. (2017) used Ganoderma lucidum and Pleurotus ostreatus species, while Lelivelt, Lindner, Teuffel, and Lamers (2015) used P. ostreatus and Coriolus versicolor species. Attias, Danai, Tarazi, and Grobman (2017) tried four species, including P. ostreatus, Pleurotus salmoneo-stramineus, Pleurotus pulmonarius, and Aaegerita agrocibe mushrooms, of which P. ostreatus was found most promising for creating design and architectural materials.

The growth of mushroom mycelium requires nutrients, appropriate temperature, moisture, and air. Mycelium generally seeks high concentrations of carbon, nitrogen, oxygen, sulfur, phosphorus, and potassium; carbohydrates (glucose, cellulose, starch, and lignin) are the source of carbon and energy (Jones, Huynh, Dekiwadia, Daver, & John, 2017). For ideal mycelium growth, substrates high in cellulose are preferable; not only does this discourage other species from flourishing, since many cannot easily break down cellulose, but also natural woody materials can contribute to high tensile strength in a mycelium-based composite (Jones, Huynh, et al., 2017). Haneef et al. (2017) used pure cellulose, as well as a cellulose–potato dextrose broth mixture, as carbohydrate nutrients to develop mycelium composites. Many researchers used agricultural by-products, including rice husks and wheat grain (Arifin & Yusuf, 2013; Jones, Bhat, Wang, Moinuddin, & John, 2017); switch-grass, rice straw, sorghum stalks, flax shive, kenaf fiber, cotton bur fiber, and hemp pith (Pelletier, Holt, Wanjura, Bayer, & McIntyre, 2013; Pelletier et al., 2017); corn stover and millet (Tudryn, Smith, Freitag, Bucinell, & Schadler, 2018); sawdust pulp, millet grain, and wheat bran (Yang et al., 2017); and wood chips (Attias, Danai, Tarazi, & Grobman, 2017) as nutrients and to offer support for the mycelium composite structure. Holt et al. (2012) used blends of cotton by-products, starch, and gypsum to develop mycelium composites.

Several researchers have included fibers or fabric mats as part of the composite. The inclusion of natural fibers increased the Young’s and shear moduli, compressive strength, and elastic stiffness across samples in one study and also helped prevent cracking on the surface of the material (Yang et al., 2017). Lelivelt et al. (2015) tested wood chips, hemp hurd, loose hemp fiber, and nonwoven hemp fiber mats and found the hemp mat enabled the densest growth of mycelium in their experiments and showed the highest strength and stiffness. A research group developed sandwich or laminate mycelium composite structures that included textile materials such as woven fabrics made from jute, flax, and Biomid cellulose fibers (Jiang, Walczyk, & McIntyre, 2014, 2017; Jiang, Walczyk, McIntyre, & Bucinell, 2016).

During the growing process of mycelium composites, inoculated substrates were added to various molds for 4 to 30 days at temperatures ranging from 21 °C to 30 °C and a relative humidity of 30–80% (Haneef et al., 2017; Holt et al., 2012; Jones, Bhat, et al., 2017; Lelivelt et al., 2015; López Nava, Méndez González, Ruelas Chacón, & Nájera Luna, 2016; Pelletier et al., 2013, 2017). Jiang, Walczyk, McIntyre, and Chan (2016) placed mycelium materials in semipermeable growth bags on growth racks for 5 days at room temperature and high humidity (> 95%). Tudryn, Smith, Freitag, Bucinell, and Schadler (2018) grew mycelium in a filter patch bag for 4 days, then loosely packed it into tile molds to grow 4 additional days before being flipped and grown for 2 more days.

In order to halt the growth and maintain the desired structure, the material must be heated and dried. This step also prevents fruit bodies from forming on the surface of the material (Jiang, Walczyk, McIntyre, & Chan, 2016). Drying time in past studies varied from 2 hr (Haneef et al., 2017; Lelivelt et al., 2015) to 46–48 hr (Arifin & Yusuf, 2013; Jones, Bhat, et al., 2017; López Nava et al., 2016). Temperatures ranged from 25 °C (López Nava et al., 2016) to 125 °C (Lelivelt et al., 2015). López Nava, Méndez González, Ruelas Chacón, and Nájera Luna (2016) let the samples dry in direct sunlight at 25 °C for 48 hr, a method that is more sustainable due to its limited energy use.

For mycelium composites, initial indicators of success include growth rating and density rating systems (Attias et al., 2017). Holt et al. (2012) found a density range of 66.5–224 kg/m³ for their mycelium samples, while Yang, Zhang, Still, White, and Amstislavski (2017) reported a density range of 160–280 kg/m³ for their samples. Scanning electron microscopy (SEM) images have been used to fully examine the material’s structure (Jiang et al., 2014; Tudryn et al., 2018). While properties such as acoustic impedance (Pelletier et al., 2013) and thermal conductivity (Yang et al., 2017) have been measured for special applications of building insulation materials, mechanical (flexural, compressive, and tensile) properties have often been measured to determine performance. Researchers typically use different standard methods, and comparison of results across testing standards does not give an accurate picture of sample performance. For example, Lelivelt et al. (2015) reported compressive strengths of 24–93 kPa for their best performing mycelium composite samples, while Yang et al. (2017) reported 350–570 kPa compressive strengths for their best performing samples. However, Lelivelt et al. (2015) measured compressive strength at 10% deformation and Yang et al. (2017) used American Society for Testing and Materials (ASTM) Standard D2166, which measures compressive strength at failure.

Many researchers in this field have sought out natural replacements for foams like polystyrene (Arifin & Yusuf, 2013; Holt et al., 2012; Yang et al., 2017). Arifin and Yusuf (2013) found that mycelium was a promising replacement for polystyrene foam because of its biodegradability, lack of toxic components, and renewable inputs. It also produces 10 times less carbon dioxide and uses around 8 times less energy than its foam counterpart, and its comparative strength makes it appealing for a variety of materials (Arifin & Yusuf, 2013). Others have strived to find a viable and natural replacement for plastics due to their prevalence in the world (Jones, Bhat, et al., 2017; López Nava et al., 2016; Tudryn et al., 2018). Ecovative Design, LLC (Green Island, NY), has developed the MycoComposite™ platform, which can be used for packaging; home accessories; and large panels and blocks for construction, theater sets, acoustic paneling, and wetland rafts (Ecovative Design, n.d.).

Several researchers have evaluated mycelium’s viability as a material input for shoe production (Jiang et al., 2014, 2017; Jiang, Walczyk, McIntyre, & Chan, 2016). In their studies, they used jute or flax fabric plies bound together by mycelium and filled with a thick but lightweight core of mycelium and plant waste. Sandwich structure composites gain increased strength through these additional layers (Jiang et al., 2014). Researchers created a shoe sole–shaped mold to form fabric plies into a structured fabric mold of the same shape in which to grow the mycelium (Jiang et al., 2014, 2017; Jiang, Walczyk, McIntyre, & Chan, 2016).

Method

Materials and Experimental Design and Process

The mushroom species tested included P. ostreatus (oyster), Pleurotus citrinopileatus (yellow oyster), Pleurotus eryngii (king oyster), and G. lucidum (reishi). The mushroom materials were in the form of sawdust spawn blocks, with all three oyster species acquired from Phillips Mushroom Farms (Kennett Square, PA), and the reishi blocks purchased from Everything Mushrooms (Knoxville, TN). An undyed nonwoven fabric mat with a fiber content of 45% recycled jute, 40% recycled cotton, and 15% cornstarch was obtained from the packaging of a Green Chef (Boulder, CO) food subscription box. Other nutrients and materials used in the experiments were unbleached all-purpose flour (General Mills, Minneapolis, MN), whole husk psyllium (a dietary supplement; Organic India Pvt. Ltd., India), and whole chicken feathers (Allen Laboratory, College of Agriculture and Natural Resources, University of Delaware).

A 4 × 2 experiment was designed. The independent variables under investigation were the mushroom species (four types) and the fabric mat (two levels: with and without a fabric mat). The dependent variables were the resulting performance characteristics, as determined by the testing methods outlined below. The controlled variables included the substrates and ratios (excluding the fabric mats), and temperature and time used to grow and deactivate the mycelium.

Mycelium Composite Growing Process

The fabric, feathers, and growing containers were sterilized at 80–90°C in a Stabil-Therm^® constant temperature oven (Blue M Electric Co, Blue Island, IL), while lab materials were wiped with an alcohol solution before coming into contact with the mycelium mixture. Per 100 g of sawdust spawn block mixture, the composite ingredients included 1.2 g flour, 0.3 g chicken feathers, 2.8 g husk psyllium, 50 ml water, and 1 g of textile for the samples that included a textile mat. Husk psyllium, flour, chicken feathers, and water were added to the sawdust spawn block and thoroughly mixed. This mixture was then added on top of a layer of dampened textile mat placed in the mold.

To create cylindrical samples for testing, the material was grown in 250-ml glass beakers and pressed down during the filling process to create a dense sample. Based on advice from Everything Mushrooms suggesting that oyster mushrooms need more airflow than other species, all three Pleurotus species’ samples were left uncovered in the growing chamber, while Ganoderma samples were covered loosely with plastic wrap with holes punched in the top to allow for airflow while still trapping in moisture.

The mycelium composites were grown in an environmental chamber (Thermal Product Solutions, New Columbia, PA, Lunaire, Model No. CEO910-4) set at 25°C for all trials. Water was added to the open samples as needed, approximately once or twice a day until the top layer was visibly damp to maintain high moisture content. Samples were grown for 7 days but were taken out of the molds and flipped on the 6th day, allowing air to more fully reach the previously contained portions of the samples and ideally yielding a more fully developed sample. Afterward, samples were heated at around 90°C for 2 hr in an oven.

To develop mycelium composite shoe soles, the ingredients were placed in a custom silicone mold with shoe sole shape. The growing ingredients and process were the same.

Testing

The growth of the mycelium and the structure of the cylindrical composite were examined using a SEM; Hitachi, S-4700). Prior to measurement and testing, the samples were conditioned in an environmental chamber at 23°C and 50% relative humidity for more than 40 hr. The thickness and diameter of the samples were then measured using a caliper. Samples were also weighed using a digital scale. Compressive strength at 10% deformation was measured using a Tinius Olsen (Horsham, PA) H5KT benchtop tester in accordance with ASTM D1621-16 standard method (ASTM, 2016). Two-way analysis of variance (ANOVA) tests were used to evaluate the effects of the independent variables on sample density and compressive strength.

Results and Discussion

Mycelium Composite Samples

For each species of mushroom, 10 cylindrical samples were developed, including 5 with fabric and 5 without. Figure 1 shows some mycelium composite samples. The samples had an average thickness of 45.0 mm (SD = 1.5) and diameter of 60.5 mm (SD = 0.8). The average weight of the samples was 41.3 g.

Figure 1.

Oyster mycelium composite samples (left: without fabric; right: with fabric on top).

The growing process and material inputs yielded abundant mycelium growth and substrates bound together to form a cohesive composite. The inclusion of renewable and inherently nontoxic flour, husk psyllium, chicken feathers, and natural textile fibers provided sufficient starch, cellulose, and protein nutrients for mycelium growth. By soaking up water, husk psyllium helped keep the mycelium damp during the growing stage and also appeared to act as a gel that helped hold the materials together. The inclusion of chicken feathers and textile waste provided structural support for the mycelium composites and a useful application of agricultural and textile waste that otherwise requires costly and unsustainable disposal.

SEM Images

The SEM images of mycelium growth and interactions between the mycelium and the substrates are shown in Figure 2. SEM images of the mycelium matrix from reishi and king oyster samples (Figure 2A and 2B) indicate that there is successful mycelial growth within the composites, confirming mycelium’s ability to grow under these experimental conditions. Substrates include woody and plant materials such as the sawdust on which the spawn was inoculated, as well as the husk psyllium added to the mixture. Figure 2C and 2D show the embedding of woody materials into the composite. Both reishi and king oyster mycelia interlaced with the larger mulch-like pieces and the smaller round particles, meaning these substrates provided adequate nutrition to allow the mycelium to grow. Figure 2E indicates that the mycelium consumed a small portion of the chicken feathers as nutrients during the growing process and was able to interweave with the feather fibers, successfully embedding the feathers into the composite; the image in the lower left corner of Figure 2E depicts a closer view of the same interweaving. The unconsumed portion of the chicken feathers, as in Figure 2E, could provide structural support for the composite. The SEM image of Figure 2F provides information about how well integrated the fabric mats at the bottom of the samples are with the rest of the composite. It is unclear whether the mycelium penetrates into the fibers or how deep the strands go, but it is clear that mycelium grew on the surface of and bonded to the fibers. The mycelium fibril matrix fused multiple fibers in the mat with the rest of the composite, firmly attaching it to the underside. Mycelium’s ability to bond to the fibers in the fabric mat also suggests that the mycelium consumed some of the fibers to provide energy for growth. A vast majority of fibers were not consumed by the mycelium, as in Figure 2F, and could provide structural support for the composite.

Figure 2.

Scanning electron microscopy images of mycelial growth and interactions with substrates (A) Reishi mycelium matrix. (B) King oyster mycelium matrix. (C) Reishi mycelium bonding woods. (D) King oyster mycelium bonding woods. (E) Reishi mycelium bonding to chicken feather. (F) King oyster mycelium bonding to textile.

The Effect of Mushroom Species and Fabric on Composite Density and Compressive Strength

Mycelium composites’ density and compressive strength data are shown in Figure 3. The density of all composites ranged from 285.57 to 353.92 kg/m³, with the highest density for oyster samples without fabric. The standard deviations (error bars in Figure 3) varied from 2.17 to 18.9 kg/m³. The density of the mycelium composites is higher than cork’s density, which is in the range of 130–250 kg/m³ (Anjos, Rodrigues, Morais, & Pereira, 2014). The compressive strength of all composites ranged from 124.80 to 340.08 kPa, with the highest compressive strength from king oyster samples without fabric. The standard deviations for compressive strength were between 15.2 kPa and 100 kPa.

Figure 3.

Density and compressive strength of mushroom mycelium composites.

The R ² value of a two-way ANOVA test evaluating the effects of both independent variables on density was .872. The general linear model fits the data well. The interaction between the species and fabric was not significant (p > .05), but the main effects of both species and fabric on density were found to be significant, as shown in Table 1. The fabric contributed to a lower density overall, presumably due to its loose structure and low weight. A post hoc least square difference (LSD) test (significance level = .05) separated the species into three groups based on their density: (oyster = king oyster) > yellow oyster > reishi.

Table 1.

Two-Way Analysis of Variance Table: Effects of Species and Fabric on Density.

Source	Type III Sum of Squares	df	Mean Square	F	Significance
Species	23,269.25	3	7,756.42	67.19	.000
Fabric	1,247.58	1	1,247.58	10.81	.002
Species × Fabric	735.36	3	245.12	2.12	.117
Error	3,693.90	32	115.43
Corrected total	28,946.09	39

Note. df = degrees of freedom.

The two-way ANOVA test that evaluated the effects of both independent variables on compressive strength yielded an R ² value of .757, indicating a good fit of the general linear model. The interaction between fabric and species was not significant (p > .05) nor was the effect of fabric on compressive strength (p > .05). Species was found to have a significant effect (p = .000) on compressive strength. Results are shown in Table 2. A post hoc LSD test (significance level = .05) determined that the species can be grouped from highest to lowest compressive strength as: king oyster > oyster > (reishi = yellow oyster). Overall, king oyster showed excellent compressive strength, oyster showed good compressive strength, and reishi and yellow oyster did not perform as well as the other two species.

Table 2.

Two-Way Analysis of Variance Table: Effects of Species and Fabric on Compressive Strength.

Source	Type III Sum of Squares	df	Mean Square	F	Significance
Species	240,451.52	3	80,150.51	32.42	.000
Fabric	5,071.51	1	5,071.50	2.05	.162
Species × Fabric	1,003.33	3	334.44	.14	.938
Error	79,108.51	32	2,472.14
Corrected total	325,634.86	39

Note. df = degrees of freedom.

Relationship Between Density and Compressive Strength

A potential relationship was indicated between the density and compressive strength of the samples. A scatterplot of the two variables confirmed a positive linear relationship, shown in Figure 4. A linear regression test provided an R ² value of .578 and showed a significant linear relationship between density and compressive strength (p = .000), suggesting that a higher density sample yields a higher compressive strength. Researchers or manufacturers who desire a product that has high compressive strength may consider using species that yield a denser structure or compressing the wet material to create a more tightly packed sample early in the growing stage.

Figure 4.

The relationship between density and compressive strength.

Mycelium Composite Shoe Soles

Figure 5 depicts the viability of creating a shoe sole–shaped composite for use in footwear products. These samples denote the success of growing shoe soles from mycelium and provide support for utilizing mycelium composites for footwear development.

Figure 5.

Mycelium composite shoe sole (bottom: yellow oyster; top: king oyster).

Performance of the Mycelium Composites for Footwear Application

Past fracture tests conducted in the literature suggested ideal traits of the reishi (G. lucidum) mushroom (Haneef et al., 2017). However, in this research, we found that reishi mycelium composite does not have high compressive strength compared to king oyster and oyster. A possible reason is that a more flexible material would also likely be compressed more easily than a stiffer material. The addition of fabric did not contribute to a higher compressive strength, likely for the same reason that the fabric’s soft texture, cushion, and ability to bend do not contribute to a higher resistance when being compressed under weight. Of the four species tested, the king oyster species showed the highest compressive strength, indicating the importance of future use of this mushroom species.

Hessert et al. (2005) reported that while walking, the maximum foot pressures for young and old adults were 329 kPa and 222 kPa, respectively, and the mean foot pressures for young and old adults were 89 kPa and 62 kPa, respectively. The compressive strength (at 10% deformation) results of several samples surpass these values, suggesting that mycelium composites could be used for shoe soles, with preference given to species with higher compressive strength. The king oyster samples without fabric (mean compressive strength of 340 kPa) could withstand the maximum walking pressures of both young and old adults, and even the lowest strength samples (yellow oyster with fabric, mean of 124.8 kPa) would fulfill adults’ average walking pressures. The composite material would also be more suited to a fashion shoe like a sandal rather than a sneaker, which would receive a stronger load when a wearer runs or jumps. Mushroom species with denser structures are also more likely to withstand compression compared to less dense species, suggesting the use of king oyster or oyster species based on their higher density values and the positive linear relationship between density and compressive strength. The king oyster mushroom displayed very good compressive strength, providing opportunities for stiff but tough cork-like shoe soles.

Sustainability Characteristics of the Mycelium Composites

In this research, we applied McDonough and Braungart’s (2002) cradle-to-cradle design concept and Anastas and Zimmerman’s (2003) green engineering principles to develop sustainable materials for footwear products. Six of Anastas and Zimmerman’s (2003) 12 principles of green engineering have been implemented in this research, as listed in Table 3.

Table 3.

Green Engineering Principles Applied in This Research.

Principles	Characteristics of the Mycelium Composites
Principle 1: All material and energy inputs and outputs are as inherently nonhazardous as possible	Only inherently nonhazardous materials including edible mushroom species, waste from food production, and natural textile materials are used
Principle 2: Better to prevent waste than to treat or clean up waste after it is formed	A variety of waste materials are used as starting materials; mycelium composites and fashion products are compostable
Principle 7: Targeted durability, not immortality, should be a design goal	Compressive strength data assure durability; mycelium composites and fashion products are compostable
Principle 8: Design for unnecessary capacity or capability solutions should be considered as a design flaw	Shoe soles are produced in mold, and no excess material is produced
Principle 10: Design must include integration and interconnectivity with available energy and material flow.	Locally available starting materials are used
Principle 12: Material and energy inputs should be renewable rather than depleting	All of the starting materials are natural and renewable

Source. Anastas and Zimmerman (2003).

This research focused on biological nutrients and the biological metabolism of the “cradle-to-cradle” concept (McDonough & Braungart, 2002). All of the starting materials such as chicken feathers, the natural fiber mat, husk psyllium, and mushroom spawn are natural and renewable, and the mycelium composites are biodegradable. Only edible mushroom species were used, and all other ingredients are inherently nonhazardous, which ensures safe manufacturing, use, and degradation of the mycelium composites. After consumers’ use, the mycelium composite shoe soles can be composted to return their nutrients to the earth for agricultural production including mushroom production, resulting in closed-loop cycling within the biological metabolism (McDonough & Braungart, 2002).

The research was conducted in a university in the Mid-Atlantic region of the United States. The Delmarva Peninsula is a major chicken-producing area in the United States and produced 605 million chickens in 2017 (Delmarva Poultry Industry, Inc., n.d.). Chicken production generates a huge amount of chicken waste including feather, which is costly to dispose of. We found chicken feathers a useful component in mycelium composite development, resulting in a valuable application of local chicken feather waste. The university campus is close to Kennett Square, PA, known as the Mushroom Capital of the World, which holds numerous mushroom farms that produce nearly half of U.S. mushrooms (Morris, 2014). In this research, we demonstrate the feasibility of using locally available materials and waste products in composite production to fulfill McDonough and Braungart’s (2002) statement of “all sustainability is local.”

Conclusions

Mushroom mycelium composites were developed in this study to contribute to the body of literature on mycelium composite materials, including utilizing new mushroom species (i.e., king oyster [P. eryngii] and yellow oyster [P. citrinopileatus]) and incorporating chicken feather waste into the composites. Composites made from king oyster mycelium demonstrated the highest compressive strength. It was found that mycelium can grow into feather fibers, interweave with the fibers, and embed the feathers into the composite material. We offer a valuable application for chicken feather waste from poultry production as an alternative to costly and unsustainable disposal. Although the fabric mats were not found to contribute to the material’s compressive strength, incorporating the natural textile provided a use for postconsumer waste and contributed nutrients to the mycelium that it needs to grow. The use of waste products in natural fiber composites also saves money in production (Väisänen, Das, & Tomppo, 2017), which creates a business case for utilizing fabric waste for composite development.

The compressive strength of the mycelium composite, particularly the king oyster species, provided support for utilizing the material for shoe sole applications. The mycelium composite is made from natural, renewable, inherently nonhazardous materials and is biodegradable. A shoe sole made from mycelium would not be destined for landfills and would contribute nutrients back to the soil for future growth, which contributes to the biological metabolism (McDonough & Braungart, 2002). This provides a possible opportunity for sustainable development in the footwear industry.

The shoe soles (Figure 5) tend to break at the narrowest part if bent. This is a limitation of this research and provides opportunities for future study. Future researchers will test additional mushroom species and growing substrates and evaluate other properties that are related to shoe applications such as flexural properties, abrasion resistance, and the impacts of sunlight and water exposure on the mycelium composite. Future investigation will also include product design to identify the appropriate application of the mycelium composite in footwear. For example, the mycelium composite may be used as midsole, which is between the insole and outsole, to avoid rubbing against a hard surface (ground) and excessive water exposure. The mycelium composite shoe sole may be designed with separate pieces rather than one complete piece to avoid bending when the wearer is walking. The future aim is to create additional shoe components and an entirely biodegradable shoe made from mycelium and other renewable inputs.

Footnotes

Authors’ Note

The contents of this article are solely the responsibility of the grantee and do not necessarily represent the official views of the U.S. Environmental Protection Agency (USEPA). Further, USEPA does not endorse the purchase of any commercial products or services mentioned in the publication.

Acknowledgments

The authors thank Tina Ellore of Phillips Mushroom Farms (Kennett Square, PA) for providing mushroom materials and expertise and Deborah Powell of University of Delaware BioImaging Center for generating the SEM images and helping with analysis.

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) disclosed receipt of the following financial support for the research and/or authorship of this article: This study received funding from U.S. Environmental Protection Agency (Grant ID SU839272).

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