*4.2. The Effects of Total Growth Time on Compressive Strength*

Substrate mixtures have been grown for five (X23, X32), six (X24, X33, X42), and seven (X34, X43) weeks. According to the literature [25], the longer growth time causes more organic substrate degradation, which means less substrate and more hyphal structures. Since most of the compressive strength of MBC is from the substrates, longer growth times result in less compressive strength. The results from our mechanical tests are also in line with the literature [7,25]. Figure 5 shows the average compressive strength for each substrate mixture (A, B, C, D, and E) grown for five, six, and seven weeks. For each substrate mixture, the compressive strength decreases by increasing the total cultivation time. *Biomimetics* **2022**, *7*, x FOR PEER REVIEW 8 of 15

**Figure 5.** Compressive strength (kPa) of treatments with different growth times. **Figure 5.** Compressive strength (kPa) of treatments with different growth times.

### *4.3. The Effects of Varied Bag/Formwork Growth Times on Compressive Strength 4.3. The Effects of Varied Bag/Formwork Growth Times on Compressive Strength*

Treatments with the exact total growth times have been grown for different time frames within bags and formworks. We tested this feature to find the optimal duration of growth in each phase of MBC cultivation. For each substrate mixture (A, B, C, D, and E), we have two sets of treatments cultivated for five weeks (X23 and X32), three sets for six Treatments with the exact total growth times have been grown for different timeframes within bags and formworks. We tested this feature to find the optimal durationof growth in each phase of MBC cultivation. For each substrate mixture (A, B, C, D, and E), we have two sets of treatments cultivated for five weeks (X<sup>23</sup> and X32), three sets for

weeks (X24, X33, and X42), and two for seven weeks (X34 and X43). Our tests show that more extended cultivation in formworks yields better mechanical performance than longer cul-

(X24, X33, and X42) regarding the cultivation time. The samples grown in molds for a more

**Figure 6.** Compressive strength (kPa) of treatments with the six-week cultivation time and different

works compared to the growth in bags yielded stronger compressive strengths.

The duration of cultivation has an inverse relation to the compressive strength of the material. We cultivated mixtures for 5, 6, and 7 weeks. The material cultivated for five weeks showed higher compressive strength in all the cases. This result is in accordance with the published literature [7]. For all the samples, the extended time of growth in form-

Role of partial growth time on compressive strength

X24 X33 X42 Treatments

extended time show better mechanical performance.

partial growth times.

0

Compressive Strength (kPa)

0

100

200

300

Compressive Strength (kPa)

400

500

six weeks (X24, X33, and X42), and two for seven weeks (X<sup>34</sup> and X43). Our tests show that more extended cultivation in formworks yields better mechanical performance than longer cultivation times within bags for all substrate mixtures and cultivation sets. Figure 6 shows the average compressive strength of all the substrate mixtures cultivated for six weeks (X24, X33, and X42) regarding the cultivation time. The samples grown in molds for a more extended time show better mechanical performance. we have two sets of treatments cultivated for five weeks (X23 and X32), three sets for six weeks (X24, X33, and X42), and two for seven weeks (X34 and X43). Our tests show that more extended cultivation in formworks yields better mechanical performance than longer cultivation times within bags for all substrate mixtures and cultivation sets. Figure 6 shows the average compressive strength of all the substrate mixtures cultivated for six weeks (X24, X33, and X42) regarding the cultivation time. The samples grown in molds for a more extended time show better mechanical performance.

Treatments with the exact total growth times have been grown for different time frames within bags and formworks. We tested this feature to find the optimal duration of growth in each phase of MBC cultivation. For each substrate mixture (A, B, C, D, and E),

A B C D E

**Figure 5.** Compressive strength (kPa) of treatments with different growth times.

Five Weeks Six Weeks Seven Weeks (X23+ X32)/2 (X24+ X33+ X42)/3 (X34+ X43)/2

*4.3. The Effects of Varied Bag/Formwork Growth Times on Compressive Strength* 

*Biomimetics* **2022**, *7*, x FOR PEER REVIEW 8 of 15

Compressive Strength of Different Cultivation Times

**Figure 6.** Compressive strength (kPa) of treatments with the six-week cultivation time and different partial growth times. **Figure 6.** Compressive strength (kPa) of treatments with the six-week cultivation time and different partial growth times.

The duration of cultivation has an inverse relation to the compressive strength of the material. We cultivated mixtures for 5, 6, and 7 weeks. The material cultivated for five weeks showed higher compressive strength in all the cases. This result is in accordance with the published literature [7]. For all the samples, the extended time of growth in formworks compared to the growth in bags yielded stronger compressive strengths. The duration of cultivation has an inverse relation to the compressive strength of the material. We cultivated mixtures for 5, 6, and 7 weeks. The material cultivated for five weeks showed higher compressive strength in all the cases. This result is in accordance with the published literature [7]. For all the samples, the extended time of growth in formworks compared to the growth in bags yielded stronger compressive strengths.

### *4.4. Digital Image Correlation (DIC)*

For the cubic samples of treatments X24, X33, and X42, we used the DIC setup with two 5 MP (2448 × 2048 pixels by 50 fps) and two Schneider Xenoplan 1.9/35 mm compact series lenses. This setup lets us capture the behavior of cubic samples under compression. The cubic samples were speckled prior to the test, and the speckles' movement during the test was monitored. This monitoring allowed us to have a more detailed quantitative study of the material's mechanical behavior and qualitatively study its behavior. The detailed movement data of speckles enables access to different displacement and strain amounts happening throughout the loading process. Mapping these on the loading timeline enables us to have more precise results. First, the samples' mechanical strength and elastic moduli were calculated using the DIC system data (reported in Table A1). These results verified the results from the extensometer attached to the MTS machine with less than a 5% difference. The system also allowed us to calculate other engineering characteristics of the material, such as the principal strains, shear moduli, and the Poison ratio. Besides that, the images taken from the system and the correlation between images reveal how the material behaves under compression.

One of the results from the tests and the study of the images show that treatments with substrate mixtures with more sawdust content (Sample A) behave with toughness and show a peak in their stress/strain diagrams. In comparison, treatments with more straw content in their substrate mixtures (Sample E) behave with hardness and reach the fracture point without showing plastic behaviors. Figures 7 and 8 show the stress/strain diagrams of cubic samples of treatments A<sup>33</sup> and E33 and some images of their behavior under compression.

For the cubic samples of treatments X24, X33, and X42, we used the DIC setup with two 5 MP (2448 × 2048 pixels by 50 fps) and two Schneider Xenoplan 1.9/35 mm compact series lenses. This setup lets us capture the behavior of cubic samples under compression. The cubic samples were speckled prior to the test, and the speckles' movement during the test was monitored. This monitoring allowed us to have a more detailed quantitative study of the material's mechanical behavior and qualitatively study its behavior. The detailed movement data of speckles enables access to different displacement and strain amounts happening throughout the loading process. Mapping these on the loading timeline enables us to have more precise results. First, the samples' mechanical strength and elastic moduli were calculated using the DIC system data (reported in Table A1). These results verified the results from the extensometer attached to the MTS machine with less than a 5% difference. The system also allowed us to calculate other engineering characteristics of the material, such as the principal strains, shear moduli, and the Poison ratio. Besides that, the images taken from the system and the correlation between images reveal how the ma-

One of the results from the tests and the study of the images show that treatments with substrate mixtures with more sawdust content (Sample A) behave with toughness and show a peak in their stress/strain diagrams. In comparison, treatments with more straw content in their substrate mixtures (Sample E) behave with hardness and reach the fracture point without showing plastic behaviors. Figures 7 and 8 show the stress/strain diagrams of cubic samples of treatments A33 and E33 and some images of their behavior un-

The images show that the cubic sample of the A33 treatment develops a crack in the center and deforms before reaching the peak point (near its 10% strain). In comparison, the sample of the E33 treatment does not show large cracks and reaches the maximum strain without fracture. Most treatments with substrate mixtures with more straw content

**Figure 7.** Stress-strain diagram and actual images of cubic sample A33. **Figure 7.** Stress-strain diagram and actual images of cubic sample A33.

*4.4. Digital Image Correlation (DIC)* 

terial behaves under compression.

der compression.

have shown this behavior.

**Figure 8.** Stress-strain diagram and actual images of cubic sample E33. **Figure 8.** Stress-strain diagram and actual images of cubic sample E33.

Figures 9 and 10 show the correlation of images from cubic samples E24 and E42 and their stress/strain diagrams. The other samples of substrate mixture E show the same behavior. Studying the correlated images and the internal strains of the cubic samples also show that the material works in compression with more tendency to use its toughness. The images show that the cubic sample of the A<sup>33</sup> treatment develops a crack in the center and deforms before reaching the peak point (near its 10% strain). In comparison, the sample of the E<sup>33</sup> treatment does not show large cracks and reaches the maximum strain without fracture. Most treatments with substrate mixtures with more straw content have shown this behavior.

1 2 3 4 6 5 Figures 9 and 10 show the correlation of images from cubic samples E24 and E42 and their stress/strain diagrams. The other samples of substrate mixture E show the same behavior. Studying the correlated images and the internal strains of the cubic samples also show that the material works in compression with more tendency to use its toughness.

Sample E24

Stress (kN)

**Figure 9.** Stress-strain diagram and correlated images of cubic sample E24.

0 5 10 15 20 Strain (%)

Figures 9 and 10 show the correlation of images from cubic samples E24 and E42 and their stress/strain diagrams. The other samples of substrate mixture E show the same behavior. Studying the correlated images and the internal strains of the cubic samples also show that the material works in compression with more tendency to use its toughness.

0 5 10 15 20 Strain (%)

Sample E33

1 2 3 4 5

**Figure 9.** Stress-strain diagram and correlated images of cubic sample E24. **Figure 9.** Stress-strain diagram and correlated images of cubic sample E24.

**Figure 8.** Stress-strain diagram and actual images of cubic sample E33.

Stress (kPa)

**Figure 10.** Stress-strain diagram and correlated images of sample E42. **Figure 10.** Stress-strain diagram and correlated images of sample E42.

The results from the DIC system showed that substrate mixtures with more sawdust content tend to use their hardness, while straw-based substrate mixtures tend to behave with toughness. As tough materials are more resistant to fracturing and are not easily breakable, they seem to be better options for the compressive structural systems working through form. While, for functions that need materials that bear the load by their strength, materials with hardness tendencies are preferable. The results from the DIC system showed that substrate mixtures with more sawdust content tend to use their hardness, while straw-based substrate mixtures tend to behave with toughness. As tough materials are more resistant to fracturing and are not easily breakable, they seem to be better options for the compressive structural systems working through form. While, for functions that need materials that bear the load by their strength, materials with hardness tendencies are preferable.

### **5. Conclusions 5. Conclusions**

The sustainable aspects of MBC make them suitable alternatives for their non-sustainable counterparts in several industries. From foam-like materials in the packaging industry to panels in the building industry, MBC cover many functions with different per-The sustainable aspects of MBC make them suitable alternatives for their non-sustainable counterparts in several industries. From foam-like materials in the packaging industry to panels in the building industry, MBC cover many functions with different performance

formance parameters. As studied in this paper, the AEC industry can also benefit more from the mechanical strength of MBC. These composites offer lightweight, graded, and

This research used agricultural waste (sawdust and straw) to cultivate MBC using locally available fungal species. We presented experiments in which we prepared treatments with five different substrate mixtures of varying sawdust to straw ratios. We tested the effects of the total duration of growth on the compressive strength of MBC cultivated with these treatments. We also tested the effects of varying the duration of growth in bags and the duration of growth in formworks on compressive strength. Our mechanical tests showed the possibility of cultivating a gradient of compressive materials. The results are also verified with a Digital Image Correlation (DIC) system, which also enabled the extraction of other material characteristics for future use in structural form-finding and the

MBC material is lightweight, its dead load is negligible, and it bears the load through the form. So, material grades with tougher properties can enable the designing and building of compressive structural forms. Besides, the harder grades can be used for the functions that bear the light loads through the strength of materials. In the following stages of this research, our goal is to develop computational form-finding methods to design and fabricate compressive structures with MBC that employ the results of our mechanical tests

study of the qualitative behavior of the samples under compression.

as the main inputs in optimizing the structural forms.

parameters. As studied in this paper, the AEC industry can also benefit more from the mechanical strength of MBC. These composites offer lightweight, graded, and biodegradable alternatives to conventional building materials and can help address the environmental problems caused by the AEC industry.

This research used agricultural waste (sawdust and straw) to cultivate MBC using locally available fungal species. We presented experiments in which we prepared treatments with five different substrate mixtures of varying sawdust to straw ratios. We tested the effects of the total duration of growth on the compressive strength of MBC cultivated with these treatments. We also tested the effects of varying the duration of growth in bags and the duration of growth in formworks on compressive strength. Our mechanical tests showed the possibility of cultivating a gradient of compressive materials. The results are also verified with a Digital Image Correlation (DIC) system, which also enabled the extraction of other material characteristics for future use in structural form-finding and the study of the qualitative behavior of the samples under compression.

MBC material is lightweight, its dead load is negligible, and it bears the load through the form. So, material grades with tougher properties can enable the designing and building of compressive structural forms. Besides, the harder grades can be used for the functions that bear the light loads through the strength of materials. In the following stages of this research, our goal is to develop computational form-finding methods to design and fabricate compressive structures with MBC that employ the results of our mechanical tests as the main inputs in optimizing the structural forms.

**Author Contributions:** Conceptualization, A.G., and B.G.; methodology, A.G.; software, A.G.; validation, A.G.; formal analysis, A.G.; investigation, A.G.; resources, A.G.; data curation, A.G.; writing original draft preparation, A.G.; writing—review and editing, B.G.; visualization, A.G., and B.G.; supervision, B.G.; project administration, B.G.; funding acquisition, A.G., and B.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research has been partially funded by Penn State Institute of Energy and Environment Seed Grant, and The H. Campbell and Eleanor R. Stuckeman Fund for Collaborative Design Research Fund.

**Institutional Review Board Statement:** Not Applicable.

**Informed Consent Statement:** Not Applicable.

**Data Availability Statement:** Data might be requested by contacting the corresponding author.

**Acknowledgments:** The authors appreciate the contributions of John A. Pecchia, Fabricio Vieira, and the Mushroom Research Center at Penn State University staff for helping with the cultivation process, and Beth Last, and the Material Characterization Lab at Penn State staff for their support with the mechanical tests.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **Appendix A**

The table below shows the characteristics of treatments used for the experiments and their mechanical strength and elastic moduli regarding the DIC results.


**Table A1.** Characteristics of Treatments and their Mechanical Properties.
