Formulation-Dependent Physical, Mechanical, and Water-Exposure Behavior of Hemp-Based Mycelium Composites with Polymeric Additives

Hemp-based mycelium composites are promising bio-based materials, but their heterogeneous structure may strongly affect mechanical performance and stability under water exposure. Six hot-pressed composites based on hemp hurds and chopped hemp straw, colonized with Pleurotus ostreatus or Ganoderma applanatum, were investigated. Depending on formulation, spent coffee grounds, thermoplastic or plasticized starch, and polymeric components introduced as granules, film flakes, or poly(ε-caprolactone) meshes were incorporated. Apparent density, water absorption, dimensional changes during immersion, compression, and three-point bending were evaluated. Mean apparent density ranged from 0.69 to 0.96 g × cm-3 and showed a positive specimen-level association with maximum compressive stress within the investigated plates (Spearman's ρ = 0.845, n = 30). Because only one plate was manufactured per formulation, the observed formulation-specific property profiles represent exploratory observations within the investigated manufactured plates rather than population-level estimates of formulation effects or manufacturing reproducibility. The highest mean maximum compressive stress was 5.7 ± 1.2 MPa, whereas the highest mean maximum flexural stress and flexural modulus were 4.6 ± 2.1 MPa and 627 ± 285 MPa, respectively. Water uptake, dimensional swelling, and retention of measurable integrity followed different formulation-dependent trends; the greatest through-thickness swelling did not necessarily correspond to the fastest loss of cohesion. Overall, density, mechanical response, water uptake, dimensional deformation, and wet-state integrity were related but distinct descriptors of material performance, highlighting the need for multi-property optimization.