*2.2. Mycelium in Architecture*

The use of mycelium-based composites as building materials has been explored in recent decades [15]. The most common approach has been to create mycelium building blocks that are assembled into larger structures. However, this application often relies on substructures, and is geometrically quite limiting due to the inherent properties of mycelium that only allows for structures in compression [7,16]. Studies have also been carried out on monolithic mycelium constructions, but these systems require either large scaffolds or extensive reinforcement systems that in most cases take over the structural functions and reduce the mycelium to a surface finishing, rather than a load-bearing material [17].

3D printing mycelium is an emerging research area that uses the mass-customization opportunity as the main research driver. While articulated surfaces created through 3D printing help mycelium growth [18], for large-scale structures, time-efficient additive manufacturing processes have not yet been developed, and the directional dependency of the fabrication method can cause the resulting components to display relatively low structural performance [19].

In order to compensate for the lack of tension and bending resistance of mycelium, research on reinforcing mycelium has been developed recently. Woven textiles, wood fibers, or 3D-printed spatial lattices are among the methods used [20–23]. However, these studies either heavily rely on manual production, or currently present very limited data about the effects of the reinforcement on the mycelium-based composite strength. For construction applications, to date only mycelium-based foam (MBF) and mycelium-based sandwich composites (MBSC) have been developed and investigated for their properties [24]. The latter uses natural fiber textiles on top and bottom of the components in order to increase bending resistance [25].

### *2.3. Additive Manufacturing with Timber*

Current additive manufacturing technologies for producing high complexity objects are mainly based on inorganic materials. New processes that allow 3D printing with organic materials have been recently developed [26,27]. For the fused deposition modelling of timber, wood is ground to particles and mixed with various thermoplastics to create continuous printing filaments or pellets [28]. Both these materials cause timber to lose its natural material structure that provides strength, resulting in relatively weak printed structures [29].

In recent years, researchers proposed a fabrication method for architectural elements, using continuous natural timber fiber filaments through robotic fabrication [30,31]. The aim was to produce structural elements by combining the advantages of continuous fiberbased manufacturing with bio-based materials. This method can achieve highly controlled, sustainable, surface-like [32] and optimized geometries [33], as it can be seen in Figure 1.

**Figure 1.** Robotic fiber laying process with processed willow strips from the research project TETHOK—Textile Tectonics for Wood Construction, University of Kassel. **Figure 1.** Robotic fiber laying process with processed willow strips from the research project TETHOK—Textile Tectonics for Wood Construction, University of Kassel.

### *2.4. Contribution 2.4. Contribution*

Combining the previously introduced mycelium composites and wood-based additive manufacturing processes, we propose a novel wood-veneer–mycelium bio-composite and its construction method for carbon neutral, circular building elements. As mycelium has excellent compression properties, but low tension and bending resistance, the integration of tailored continuous wood fibers in the composite is expected to increase the structural capabilities of mycelium-based components, while still being composed of exclusively natural materials. To demonstrate its potential in the context of architecture, we describe the material concept and its production process, and present results regarding its characterization with reinforcement strategies. Combining the previously introduced mycelium composites and wood-based additive manufacturing processes, we propose a novel wood-veneer–mycelium bio-composite and its construction method for carbon neutral, circular building elements. As mycelium has excellent compression properties, but low tension and bending resistance, the integration of tailored continuous wood fibers in the composite is expected to increase the structural capabilities of mycelium-based components, while still being composed of exclusively natural materials. To demonstrate its potential in the context of architecture, we describe the material concept and its production process, and present results regarding its characterization with reinforcement strategies.

### **3. Materials and Methods 3. Materials and Methods**

### *3.1. Selection of the Base Materials 3.1. Selection of the Base Materials*

### 3.1.1. Wood Veneer Species 3.1.1. Wood Veneer Species

We made a pre-selection of wood species indigenous to Germany, based on their availability at the time of the research, and data from the literature that proved their compatibility with mycelium growth: beech (*Fagus sylvatica*), maple (*Acer pseudoplatanus*), oak (*Quercus robur*), and spruce (*Picea abies*) veneers and willow branches of the genus *Salix americana.* Initial binding tests with these selected species were carried out at the University of Kassel. Maple demonstrated the best wood–wood bond with the selected binding method and was chosen as the reinforcement material. We made a pre-selection of wood species indigenous to Germany, based on their availability at the time of the research, and data from the literature that proved their compatibility with mycelium growth: beech (*Fagus sylvatica*), maple (*Acer pseudoplatanus*), oak (*Quercus robur*), and spruce (*Picea abies*) veneers and willow branches of the genus *Salix americana.* Initial binding tests with these selected species were carried out at the University of Kassel. Maple demonstrated the best wood–wood bond with the selected binding method and was chosen as the reinforcement material.

H. Heitz Furnierkantenwerk from Melle, Germany supplied FSC (Forest Stewardship Council) certified maple veneer edge-bands that were 12 mm wide and 0.5 mm thick in spools. They are produced by lining up veneer sheets and joining them with fully glued finger joints. Non-woven cellulose-based fleece on one side of the roll is then added with PVAc dispersion glue to ensure that the material does not easily break during application. Due to the commercially available veneer rolls using a small amount of glue for their joining during production, the presented composites are not yet fully bio-based. However, custom veneer rolls made with bio-adhesives could be produced and utilized in future studies. H. Heitz Furnierkantenwerk from Melle, Germany supplied FSC (Forest Stewardship Council) certified maple veneer edge-bands that were 12 mm wide and 0.5 mm thick in spools. They are produced by lining up veneer sheets and joining them with fully glued finger joints. Non-woven cellulose-based fleece on one side of the roll is then added with PVAc dispersion glue to ensure that the material does not easily break during application. Due to the commercially available veneer rolls using a small amount of glue for their joining during production, the presented composites are not yet fully bio-based. However, custom veneer rolls made with bio-adhesives could be produced and utilized in future studies.

### 3.1.2. Substrates 3.1.2. Substrates

We made a pre-selection of the substrates based on the availability of the raw materials mainly as waste stream in Europe: hemp fibers, hemp hurds, pine wood sawdust and shavings, and Silvergrass (Miscanthus) shavings. For the purpose of this study, only hemp hurds were used, which were collected from Bafa GmbH (Malsch, Germany), a local wood mill. We made a pre-selection of the substrates based on the availability of the raw materials mainly as waste stream in Europe: hemp fibers, hemp hurds, pine wood sawdust and shavings, and Silvergrass (Miscanthus) shavings. For the purpose of this study, only hemp hurds were used, which were collected from Bafa GmbH (Malsch, Germany), a local wood mill.

### 3.1.3. Mycelium Species

The mycelium mother culture of *Ganoderma lucidum* (*G. lucidum*) was purchased from Tyroler Glückspilze (Innsbruck, Austria) in the form of grain spawn and stored at 4 ◦C for up to four weeks. This selection was mainly made due to the already known faster growth rate on hemp hurds, and its availability in Europe. *Ganoderma lucidum* was grown on hemp hurds and subsequently reinforced with maple veneers to carry out a series of physical and mechanical tests on lightweight and dense veneer-reinforced mycelium-based composites. rate on hemp hurds, and its availability in Europe. *Ganoderma lucidum* was grown on hemp hurds and subsequently reinforced with maple veneers to carry out a series of physical and mechanical tests on lightweight and dense veneer-reinforced mycelium-based composites.

The mycelium mother culture of *Ganoderma lucidum* (*G. lucidum*) was purchased from Tyroler Glückspilze (Innsbruck, Austria) in the form of grain spawn and stored at 4 °C for up to four weeks. This selection was mainly made due to the already known faster growth

### *3.2. Fabrication 3.2. Fabrication*

3.1.3. Mycelium Species

### 3.2.1. Robotic Wood Fiber Laying 3.2.1. Robotic Wood Fiber Laying

*Biomimetics* **2022**, *7*, x FOR PEER REVIEW 5 of 21

We developed a custom fabrication process to lay the continuous wood fibers robotically. The process consisted of the following sub steps: a single wood strip was extruded at a time, and the material was cut when a change in extrusion direction was needed. This allows for complex tool paths, the creation of multi-directional reinforcement patterns and controlled anisotropy. Two approaches can be used for the layering: placing the veneers with the same direction or similar directions at once, then moving on to the next layer (Figure 2); or printing one line from a different direction at a time, which results in a structure with interwoven fibers [31]. We developed a custom fabrication process to lay the continuous wood fibers robotically. The process consisted of the following sub steps: a single wood strip was extruded at a time, and the material was cut when a change in extrusion direction was needed. This allows for complex tool paths, the creation of multi-directional reinforcement patterns and controlled anisotropy. Two approaches can be used for the layering: placing the veneers with the same direction or similar directions at once, then moving on to the next layer (Figure 2); or printing one line from a different direction at a time, which results in a structure with interwoven fibers [31].

**Figure 2.** Robotic fiber laying process: (**a**) Fiber laying in direction one; (**b**) Fiber laying in direction two; (**c**) Completed 2D lattice. **Figure 2.** Robotic fiber laying process: (**a**) Fiber laying in direction one; (**b**) Fiber laying in direction two; (**c**) Completed 2D lattice.

We designed two types of 2D veneer lattices to reinforce the mycelium blocks/boards for this study: high- and low-density lattices (Figure 3). While the low-density lattice had two veneer strips in the longitudinal direction and four in the transversal direction, the high-density lattice had three and seven veneer strips, respectively. In each lattice, we placed the veneers to form a frame that was 19 cm x 8 cm in a total of two layers that were perpendicular to each other. The veneer strips were fixed at their ends using double-sided tape during printing. After all strips with the same direction were laid down, the second layer of strips perpendicular to the first layer was added, and the lattices were ready for the ultrasonic welding of the intersection points. We designed two types of 2D veneer lattices to reinforce the mycelium blocks/boards for this study: high- and low-density lattices (Figure 3). While the low-density lattice had two veneer strips in the longitudinal direction and four in the transversal direction, the high-density lattice had three and seven veneer strips, respectively. In each lattice, we placed the veneers to form a frame that was 19 cm × 8 cm in a total of two layers that were perpendicular to each other. The veneer strips were fixed at their ends using double-sided tape during printing. After all strips with the same direction were laid down, the second layer of strips perpendicular to the first layer was added, and the lattices were ready for the ultrasonic welding of the intersection points. *Biomimetics* **2022**, *7*, x FOR PEER REVIEW 6 of 21

**Figure 3.** Veneer lattices produced: (**a**) Low-density lattice; (**b**) High-density lattice (dimensions in mm). **Figure 3.** Veneer lattices produced: (**a**) Low-density lattice; (**b**) High-density lattice (dimensions in mm).

### 3.2.2. Ultrasonic Wood Welding for Wood–Wood Binding 3.2.2. Ultrasonic Wood Welding for Wood–Wood Binding

Precedents of continuous wood fiber laying research have explored synthetic binders, such as UV-curing glue, contact glue and hot melt glue [31]. Since mycelium growth is incompatible with synthetic materials, and the goal of producing a 100% bio-based composite cannot be achieved with the binders investigated to date, it was necessary to re-Precedents of continuous wood fiber laying research have explored synthetic binders, such as UV-curing glue, contact glue and hot melt glue [31]. Since mycelium growth is incompatible with synthetic materials, and the goal of producing a 100% bio-based

Ultrasonic welding is a common adhesive-free joining method used in many indus-

The wood welding was performed with an ultrasonic welding horn, a generator that uses 20 kHz frequency, and a flat-ended sonotrode provided by Weber Ultrasonics (Karls-

As the veneer rolls have fleece on one side, initial welding tests were made comparing the welds of wood to wood, wood to fleece and fleece to fleece sides. The material was always placed with the wood side facing the welding horn to avoid the fleece from sticking onto the welding horn. Once the robot reached the intersection point to be welded, the pressure was applied by moving the robot arm down in the vertical direction. Then, the welding was performed by a signal of the digital control unit connected to the generator.

using ultrasonic energy at high frequencies that produce mechanical vibrations, which results in heat due to the friction between the two elements to be joined. Heat melts thermoplastic materials and binds the parts together after cooling [34]. In recent decades, this method has been used to weld thin woo, through heat softening and melting lignin in wood and binding the materials with entangled fibers [35]. Considering that no adhesives are needed for joining, this method was chosen as the wood–wood binding strategy for

search alternative binding methods.

our custom manufacturing process.

bad, Germany) mounted on a robotic arm (Figure 4).

**Figure 4.** (**a**) Robotic welding process; (**b**) Welded intersection point close-up.

composite cannot be achieved with the binders investigated to date, it was necessary to research alternative binding methods. is incompatible with synthetic materials, and the goal of producing a 100% bio-based composite cannot be achieved with the binders investigated to date, it was necessary to re-

3.2.2. Ultrasonic Wood Welding for Wood–Wood Binding

**Figure 3.** Veneer lattices produced: (**a**) Low-density lattice; (**b**) High-density lattice (dimensions in

Precedents of continuous wood fiber laying research have explored synthetic binders, such as UV-curing glue, contact glue and hot melt glue [31]. Since mycelium growth

*Biomimetics* **2022**, *7*, x FOR PEER REVIEW 6 of 21

Ultrasonic welding is a common adhesive-free joining method used in many industries, including automotive, electronic, and medical, due to its speed. It is performed by using ultrasonic energy at high frequencies that produce mechanical vibrations, which results in heat due to the friction between the two elements to be joined. Heat melts thermoplastic materials and binds the parts together after cooling [34]. In recent decades, this method has been used to weld thin woo, through heat softening and melting lignin in wood and binding the materials with entangled fibers [35]. Considering that no adhesives are needed for joining, this method was chosen as the wood–wood binding strategy for our custom manufacturing process. search alternative binding methods. Ultrasonic welding is a common adhesive-free joining method used in many industries, including automotive, electronic, and medical, due to its speed. It is performed by using ultrasonic energy at high frequencies that produce mechanical vibrations, which results in heat due to the friction between the two elements to be joined. Heat melts thermoplastic materials and binds the parts together after cooling [34]. In recent decades, this method has been used to weld thin woo, through heat softening and melting lignin in wood and binding the materials with entangled fibers [35]. Considering that no adhesives are needed for joining, this method was chosen as the wood–wood binding strategy for our custom manufacturing process.

The wood welding was performed with an ultrasonic welding horn, a generator that uses 20 kHz frequency, and a flat-ended sonotrode provided by Weber Ultrasonics (Karlsbad, Germany) mounted on a robotic arm (Figure 4). The wood welding was performed with an ultrasonic welding horn, a generator that uses 20 kHz frequency, and a flat-ended sonotrode provided by Weber Ultrasonics (Karlsbad, Germany) mounted on a robotic arm (Figure 4).

mm).

As the veneer rolls have fleece on one side, initial welding tests were made comparing the welds of wood to wood, wood to fleece and fleece to fleece sides. The material was always placed with the wood side facing the welding horn to avoid the fleece from sticking onto the welding horn. Once the robot reached the intersection point to be welded, the pressure was applied by moving the robot arm down in the vertical direction. Then, the welding was performed by a signal of the digital control unit connected to the generator. As the veneer rolls have fleece on one side, initial welding tests were made comparing the welds of wood to wood, wood to fleece and fleece to fleece sides. The material was always placed with the wood side facing the welding horn to avoid the fleece from sticking onto the welding horn. Once the robot reached the intersection point to be welded, the pressure was applied by moving the robot arm down in the vertical direction. Then, the welding was performed by a signal of the digital control unit connected to the generator.
