4.1.3. Material Shrinkage

Cylindrical specimens grown for compression testing shrank, on average, by 5.4% of their original height. These specimens measured 17 or 34 cm in height and were grown and dried inside PVC tubes. The specimens grown were taller than those produced in previous research projects and were dried inside their mold (i.e., PVC pipes). This set-up may have resulted in friction between the specimen and the internal walls of their mold, thus limiting shrinkage. In terms of horizontal shrinkage (i.e., in diameter), the shrinkage of uncompacted specimens from inoculation to the beginning of the mechanical testing was approximatively 3.52%.

For the rectangular specimens grown for bending testing, compacted materials shrank by an average of 12.68% in addition to the 50% compaction. Uncompacted specimens shrank by 9.29%. The specimens' width was reduced by around 4.93% for the uncompacted specimens. The lower shrinkage in width compared to height likely resulted from the friction caused by the bottom of the molds.

### *4.2. Compression Testing*

In our analysis, the fungal species did not have a distinct effect on the compressive Young's modulus of the mycelium-based materials. In the literature, fungal species have been shown to affect the mechanical properties of mycelium-based materials. For instance, *Ganoderma* spp. generally have a higher strength than *Pleurotus* spp. [12], but Haneef et al. found contradictory results [7]. The effect of fungal species seems to depend on the hyphal types (i.e., monomitic, dimitic, and trimitic), which can be generative, binding, and/or skeletal [30,31].

Compressive Young's modulus was positively correlated with sample density. Density is known to have a significant effect on the stress–strain curve response of mycelium materials [48]. The correlation between porosity/density and compressive modulus was previously discussed in [13]. The density of the final mycelium-based materials can be controlled in two main steps of the process: during substrate processing and application of post-growth treatments. For example, sawdust is known to be denser than straw. However, as mycelium growth is based on oxygen access, it is usually reduced in the center of dense substrates [5,13]. By comparison, compaction after mycelial growth will partly break the mycelium network. Therefore, a trade-off between both options could be further studied. When comparing materials grown from *Pleurotus ostreatus*, materials made from mixture S (substrate with fine particles) showed an increase in density by 30% and resulting increase in compressive Young's modulus by 156% compared to those made from mixture L (without fine particles). In comparison, Rigobello and Ayres found that materials made from substrate particles having sizes ranging from 0.5 to 12.0 mm had a slightly higher density but lower Young's modulus than those made from particles having sizes ranging from 4.0 to 12.0 mm [42]. In their case, the presence of smaller particles reduced Young's modulus. In terms of post-growth treatments, material compaction increased density by 48% when considering all specimens but increased compressive Young's modulus by 511%. For structural applications, the material should be compacted after the growth of mycelium to increase its compressive modulus while avoiding mycelium growth inhibition emerging from a lack of oxygen access. When averaging all samples,

baking increased material density by 4% but increased compressive Young's modulus by 43%. Baking (at 100 ◦C) is supposed to kill the fungus that may damage the mycelial network throughout the specimen. By comparison, drying (at 40 ◦C) is only supposed to put the fungus in a dormant stage. In this regard, the choice of drying or baking the materials depends on the desired application. For instance, keeping the fungus alive can potentially induce self-healing properties, but may not be desired in environments where humans may potentially be exposed to large quantities of fungal spores. The slight humidity difference between samples may be another factor influencing the variation in Young's modulus, since the strength of a material is roughly inversely proportional to its moisture content [45]. However, no correlation was observed between those values.

The substrate preparation, growth conditions, and post-growth processes used in this research resulted in similar or slightly higher compressive Young's moduli than those recorded in previous research projects, as described in Section 1. Introduction [8,9,11,31,45,48]. For instance, materials grown from *Pleurotus ostreatus* on hemp mat reached a compressive strength of 0.19 MPa [31]. In comparison, our mycelium-based samples made of *Pleurotus ostreatus* grown on chipped and sieved straw resulted in compressive Young's moduli ranging from 0.15 to 4.55 MPa. To the authors' knowledge, no studies have tested the mechanical properties of materials grown with *Coprinus comatus*. Current mycelium research explores the use of mycelium-based materials to replace traditional foam materials. In this case, the goal is to obtain high mechanical strength with low density. However, difficulties in manufacturing lead to inconsistencies in mechanical behavior of composite materials. For instance, materials made from the same substrate still show large standard deviations due to varying particle distribution and orientation [42].

Studying the hysteresis and dilatation of mycelium-based materials is important to predict their final dimensions and behavior. Since the height of all specimens increased after the compressive tests, materials experienced some elastic deformation. In addition, since no specimen dilated back to their initial height, plastic deformation also occurred in all materials tested. Drying the grown materials seemed to increase dilatation after compression of materials made from the large mixture. For specimens grown with the small mixture (i.e., containing fine particles), baking resulted in increased dilatation after compression testing. As expected, specimens that experienced higher strain (i.e., uncompacted specimens) were less likely to dilate back to a height closer to their initial height. Depending on the application, various substrate mixtures and post-growth treatments can be employed to target desired material behavior under compression, including maximum load, elasticity, hysteresis, and dilatation.

### *4.3. Bending Testing*

Similar to compression testing, the elastic modulus of the mycelium-based materials was not thoroughly affected by the difference in fungal species, but the modulus was strongly correlated with material density. The results from the *Pleurotus ostreatus* materials were less consistent than those grown from *Coprinus comatus*. The main difference in behavior between the two fungal species was observed in the failure angle. Even if the specimens of each sample were produced with the same process, the variation in elastic moduli remained high, especially for materials grown from *Pleurotus ostreatus*. The variations in elastic moduli and failure angle among samples may partially be explained by the heterogeneous properties of the material due to irregular substrate particle distribution and orientation, along with heterogeneous mycelium growth. Furthermore, results obtained from the bending materials grown with *Pleurotus ostreatus* should be interpreted with care due to the small sample size and testing of materials initially grown as extra.

On average, the presence of fine particles in the substrate mixture resulted in a higher density, yielding higher elastic moduli. The difference between drying and baking did not have a significant effect on the material's elastic modulus, whereas compaction substantially increased the elastic modulus. For instance, the compaction of materials grown from *Coprinus comatus* resulted in a density increase of 100% and an increase in the elastic modulus of 2611%. In comparison to Appels et al., 2019, our compacted then baked specimens achieved flexural moduli in the same range as those of heat-pressed specimens having higher densities [32]. Therefore, the substrate preparation and post-growth treatments can be used to tune the behavior of the materials under bending for the application of interest.
