sustainability Review Physico-Mechanical and Thermodynamic Properties of Mycelium-Based Biocomposites: A Review Carolina Girometta 1 , Anna Maria Picco 1,*, Rebecca Michela Baiguera 1, Daniele Dondi 2, Stefano Babbini 3, Marco Cartabia 1,3, Mirko Pellegrini 3 and Elena Savino 1 1 Department of Earth and Environmental Sciences, University of Pavia, Via S. Epifanio 14, 27100 Pavia, Italy; [email protected] (C.G.); [email protected] (R.M.B.); [email protected] (M.C.); [email protected] (E.S.) 2 Department of Chemistry, University of Pavia, Viale Taramelli 10, 27100 Pavia, Italy; [email protected] 3 MOGU S.r.l., Via S. Francesco, 61, 21020 Inarzo, VA, Italy; [email protected] (S.B.); [email protected] (M.P.) * Correspondence: [email protected]; Tel.: +39-0382-984874 Received: 16 December 2018; Accepted: 2 January 2019; Published: 8 January 2019 Abstract: Reducing the use of non-renewable resources is a key strategy of a circular economy. Mycelium-based foams and sandwich composites are an emerging category of biocomposites relying on the valorization of lignocellulosic wastes and the natural growth of the living fungal organism. While growing, the fungus cements the substrate, which is partially replaced by the tenacious biomass of the fungus itself. The final product can be shaped to produce insulating panels, packaging materials, bricks or new-design objects. Only a few pioneer companies in the world retain a significant know-how, as well as the ability to provide the material characterization. Moreover, several technical details are not revealed due to industrial secrecy. According to the available literature, mycelium-based biocomposites show low density and good insulation properties, both related to acoustic and thermal aspects. Mechanical properties are apparently inferior in comparison to expanded polystyrene (EPS), which is the major synthetic competitor. Nevertheless, mycelium-based composites can display an enormous variability on the basis of: fungal species and strain; substrate composition and structure; and incubation conditions. The aim of the present review is to summarize technical aspects and properties of mycelium-based biocomposites focusing on both actual applications and future perspectives. Keywords: mycelium; fungi; biocomposite; foam; sandwich; lignocellulose; physical properties; mechanical properties; thermal properties 1. Introduction Due to the increasing demand for “green” materials and productive processes, extensive literature has been developed about the so-called biocomposite and bio-based materials. Such terms basically aim to highlight the derivation of the raw materials from biological sources, although the intervention of a further biological activity by living organisms is not excluded in the transformation process of the materials themselves. Far from being a univocal definition, biocomposites have been referred to by Dicker et al. (2014) [1] as composite materials, where a bio-polymer or bio-derived polymer is reinforced by natural fibers. It should be noted that natural fibers are completely or partially constituted of biopolymers. Moreover, the term “fiber” has a different meaning in chemistry, materials engineering and botany [2–5]. Analogously, bio-based materials are defined as materials containing at least one component that is biologically produced and completely biodegradable [6]; thus, such a definition does not exclude the presence of other synthetic components [7]. Sustainability 2019, 11, 281; doi:10.3390/su11010281 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 281 2 of 22 Sustainability 2019, 11, x FOR PEER REVIEW 2 of 23 AA majormajor attractive attractive factor factor of of biocomposites biocomposites is is the the possibility possibility to to exploit exploit biological biological wastes wastes and/or and/or residue,residue, such such as as husks, husks, waste waste fibers, fibers, and and residual residu stems.al stems. Waste Waste and residuesand residues are thus are valorized thus valorized rather thanrather discarded, than discarded, according according to the principles to the principles of the circular of the circular economy economy [6,8]. [6,8]. EarlyEarly explorations in in the the use use of offungi fungi as biomat as biomaterialserials began began during during the 1990s the by 1990s the Japanese by the Japanesescientist Shigeru scientist Yamanaka, Shigeru Yamanaka, who researched who researchedmycelium for mycelium the production for the of production paper and of building paper andmaterials building [9]. materialsSince then, [ 9mycelium-based]. Since then, mycelium-based composites have compositesbeen investigated have beenfor commercialization investigated for commercialization[10,11] and explored [10 ,for11] theirand exploredpotential in for several their potential conceptual in projects several conceptual(such as the projects recent exhibition (such as the“Fungal recent Futures”: exhibition www.fungal-futures “Fungal Futures”: www.fungal-futures.com.com, 2016, NL) [12]. Research, 2016, in NL) Europe [12]. Researchon mycelium-based in Europe oncomposites mycelium-based has been composites initiated hasby Maurizio been initiated Montalti, by Maurizio in collaboration Montalti, inwith collaboration Utrecht University with Utrecht (the UniversityNetherlands), (the testing Netherlands), mycelium testing technology mycelium wi technologyth the local with waste the localstreams waste of streamsthe European of the Europeanagricultural agricultural industry. industry.This experimentation This experimentation was funded was first funded by the first Creative by the Industry Creative Fund Industry NL Fund(Stimuleringsfonds) NL (Stimuleringsfonds) and later and from later fromnational national and andEuropean European institutions institutions for for research research (NWO,(NWO, EEC-H2020)EEC-H2020) [ 13[13,14].,14]. FungiFungi (apart(apart from from yeasts) yeasts) are are organisms organisms able able to to give give cohesion cohesion to to incoherent incoherent materials materials due due to to the the productionproduction of of a a mass mass of of microscopic microscopic filaments filaments (called (called hyphae) hyphae) that that form form the the mycelium. mycelium. FungiFungi nevernever produceproduce truetrue tissues;tissues; nevertheless, hyphae can can disp displaylay different different types types and and specializations specializations related related to tosubstrate substrate degradation degradation and and the the development development of of repr reproductiveoductive structures [[15,16].15,16]. AccordingAccording toto thethe availableavailable literature,literature, myceliummycelium inin biocompositesbiocomposites isis coupledcoupled almostalmost exclusivelyexclusively withwith otherother (non-fossil)(non-fossil) materialsmaterials derivedderived fromfrom biological processes, such such as as plant plant materials, materials, to to exploit exploit the the natural natural growth growth of ofthe the fungal fungal organism organism on these on these substr substrates.ates. Since Since the variability the variability of the ofdegradative the degradative processes processes in fungi in is fungienormous, is enormous, lignocellulosic lignocellulosic substrates substrates are colonized are colonized by a plethora by a plethora of species. of species.Nevertheless, Nevertheless, the most theefficient most efficientdegradation degradation of the oflignocellulosic the lignocellulosic compound compound is well-known is well-known to tobe be carried carried out byby wood-decaywood-decay fungifungi (WDF)(WDF) mostlymostly belongingbelonging toto Basidiomycota. Basidiomycota. Wood-decayWood-decay speciesspecies cancan degradedegrade cellulose,cellulose, hemicelluloses hemicelluloses and and lignin lignin by enzymatic by enzymatic or non-enzymatic or non-enzymatic mechanisms, mechanisms, of which selectivityof which isselectivity both species-specific is both species-specific and environmentally and environmenta determinedlly determined [17]. The substrate [17]. The matrix substrate thus matrix is forced thus and is penetratedforced and by penetrated the hyphae, by whichthe hyphae, develops which inside develops as an increasingly inside as an tight increasingly net. Over tight time, net. the Over substrate time, isthe replaced substrate partially is replaced by the fungalpartially biomass by the and fungal the resultingbiomass myceliumand the resulting is able to mycelium strongly cement is able the to substratestrongly itself,cement resulting the substrate in a biocomposite itself, resulting material in a biocomposite (Figure1). material (Figure 1). Figure 1. Cultivation bags with mycelial growth in progress (A) and final biocomposite (B,C). Figure 1. Cultivation bags with mycelial growth in progress (A) and final biocomposite (B,C). Pictures by C. Girometta and R.M. Baiguera. Pictures by C. Girometta and R.M. Baiguera. The fungal colonization of the substrate is often inhomogeneous as confirmed by SEM imaging (FigureThe2). fungal colonization of the substrate is often inhomogeneous as confirmed by SEM imaging (Figure 2). SustainabilitySustainability 20192019,, 1111,, x 281 FOR PEER REVIEW 3 3of of 23 22 Figure 2. Stereomicroscopy and cryo-SEM images of different biocomposites. TRN (A–D) = T. multicolor Figure 2. Stereomicroscopy and cryo-SEM images of different biocomposites. TRN (A–D) = T. on straw without heat pressing; TRH (E–H) = T. multicolor on straw with heat pressing; PCH (I–L) = multicolor