fibers Review Plant Fibers and Phenolics: A Review on Their Synthesis, Analysis and Combined Use for Biomaterials with New Properties Roberto Berni 1,2 , Giampiero Cai 1 , Jean-Francois Hausman 3 and Gea Guerriero 3,* 1 Department of Life Sciences, University of Siena, Via P.A. Mattioli 4, I-53100 Siena, Italy 2 Trees and Timber Institute-National Research Council of Italy (CNR-IVALSA), Via Aurelia 49, I-58022 Follonica (GR), Italy 3 Environmental Research and Innovation Department, Luxembourg Institute of Science and Technology, 5, Avenue des Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg * Correspondence: [email protected]; Tel.: +352-275-888-5023 Received: 13 June 2019; Accepted: 26 August 2019; Published: 31 August 2019 Abstract: Devising environmental-friendly processes in biotechnology is a priority in the current economic scenario. Weare witnessing a constant and steady push towards finding sustainable solutions to societal challenges by promoting innovation-driven activities minimizing the environmental impact and valorizing natural resources. In bioeconomy, plants are among the most important renewable sources of both fibers (woody and cellulosic) and phytochemicals, which find applications in many industrial sectors, spanning from the textile, to the biocomposite, medical, nutraceutical, and pharma sectors. Given the key role of plants as natural sources of (macro)molecules, we here provide a compendium on the use of plant fibers functionalized/impregnated with phytochemicals (in particular phenolic extracts). The goal is to review the various applications of natural fibers functionalized with plant phenolics and to valorize those plants that are source of both fibers and phytochemicals. Keywords: plant fibers; cellulose; secondary metabolism; phenolics 1. Introduction Plants are bioresources providing valuable raw materials, such as fibers and wood, a major commodity [1], as well as phytochemicals displaying bioactivity and used in the healthcare, pharmaceutical, cosmetic, and nutraceutical industries [2]. Plant fibers have been used since the dawn of time [3]; the increasing environmental concerns draw attention on their exploitation, instead of synthetic ones, in the industrial sector. Plant fibers can be woody (as those found in the xylem), or rich in crystalline cellulose (as, for example, the bast fibers produced by flax or hemp). Depending on their physicochemical characteristics, plant fibers can, therefore, be used for different purposes. For example, lignocellulosic fibers extracted from the stalk of fiber crops such as hemp are used for the manufacture of bricks (known as “hempcrete”) in buildings: Such fibers (which contain silica [4]) are mixed with lime, resulting in the formation of a lightweight cementitious material with insulating properties. Cellulosic fibers (such as those extracted from the cortex of hemp) find application in the biocomposite sector: They have several advantageous features, namely length (>100 mm in primary bast fibers [5]) and high tensile strength, because of the parallel orientation of the cellulose microfibrils to the fiber axis. Bast fibers are embedded in the stem cortex (Figure1) and are glued together principally by pectin; therefore, to be used, they need to be separated from the surrounding tissues. This process is known as “retting” and is performed via the intervention of microorganisms (fungi, such as Ascomycota, Basidiomycota, and Zygomycota and bacteria belonging to the phyla Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes [6]) Fibers 2019, 7, 80; doi:10.3390/fib7090080 www.mdpi.com/journal/fibers Fibers 2019, 7, x FOR PEER REVIEW 2 of 17 Fibers(fungi,2019 such, 7, 80 as Ascomycota, Basidiomycota, and Zygomycota and bacteria belonging to the phyla2 of 17 Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes [6]) secreting enzymes acting on cell secretingwall polysaccharides enzymes acting [7] (e.g. on, cellpectinases wall polysaccharides degrading the [middle7] (e.g., lamellas pectinases which degrading “glue” together the middle the lamellasbundles whichof bast “glue”fibers). together the bundles of bast fibers). FigureFigure 1. CrossCross section section of of the the stem stem of ofa representative a representative fiber fiber crop, crop, stinging stinging nettle nettle (Urtica (Urtica dioica dioica L.), andL.), anddetails details of enzyme of enzyme-treated-treated cortical cortical peels peelswith some with someseparated separated bast fibers. bast fibers.(a) Transversal (a) Transversal cross section cross sectionstained stainedwith Safranin with Safranin and Alcian and Alcianblue (FASGA blue (FASGA staining staining),), showing showing in blue in the blue cellulosic the cellulosic bast fibers bast fibersand in and red inlignin; red lignin; (b) cortical (b) cortical peels separated peels separated from the from stem the bottom stem bottom internodes internodes and treated and treated for 24 h for at 2450 h°C at with 50 ◦C TEXAZYM with TEXAZYM BFE (INOTEX BFE (INOTEX Ltd, Czech Ltd, Czech Republic Republic)) showing showing some some separated separated bast bastfibers; fibers; the theinset inset shows shows one one separated separated bast bast fiber. fiber. 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Some effects. plant Some species, plant suchspecies, as the such fiber as cropsthe fiber hemp crops and hemp nettle, and are multi-purpose,nettle, are multi as-purpose, they produce as they both produce high yields both ofhigh cellulosic yields of fibers cellulosic and phytochemicals fibers and phytochemicals [2,9]. [2,9].In this review we will illustrate recent examples of natural fibers functionalized (i.e., with a chemicalIn this bond) review or impregnated we will illustrate with phytochemicals recent examples (more of natural specifically fibers phenolics functionalized which show(i.e., strongwith a antimicrobialchemical bond) activities or impregnated thanks to with their phytochemicals chemical heterogeneity (more specifically [10]). The goal phenolics is to highlight which show the value strong of plantsantimicrobial as renewable activities resources thanks of to biomass their chemical (fibers) heterogeneity on one hand and [10] of). The molecules goal is withto highlight biological the e valueffects (e.g.,of plants antimicrobial) as renewable on the resources other. More of biomass specifically, (fibers) the on review one hand is divided and of into mol fiveecules parts: with One biological part will describeeffects (e.g. the, synthesisantimicrobial) and properties on the other. of plant More fibers, specifically, one section the will review be devoted is divided to the into phenylpropanoid five parts: One pathway,part will onedescribe paragraph the synthesis will summarize and properties the current of plant methods fibers, used one tosection extract will and be quantify devoted/identify to the phenolicsphenylpropanoid from plant pathway, matrices, on ae sectionparagraph will will describe summarize the antimicrobial the current propertiesmethods used of plant to extract phenolics and andquantify/identify the last part willphenolics report from recent plant data matrices, on the combined a section usewill of describe plant fibers the antimicrobial and phenolics. properties We will concludeof plant phenolics the survey and with the anlast outlook part will on report future recent prospects data concerningon the combined the manufacture use of plant and fibers use and of functionalizedphenolics. We plantwill fibers.conclude the survey with an outlook on future prospects concerning the manufacture and use of functionalized plant fibers. 2. Properties of Plant Fibers and Characterization of Cellulose 2. PropertiesThe chief of load-bearing Plant Fibers component and Characterization of plant fibers of (both Cellulose gelatinous and lignocellulosic) is cellulose. This polymerThe chief of glucoseload-bearing units linkedcomponent by β-1,4-linkages of plant fibers is linear, (both which gelatinous makes itand possible lignocellulosic) for the glucan is chainscellulose. to pack This tightlypolymer via of hydrogen glucose units bonds linked and van by β der-1,4 Waals-linkages interactions is linear, [ 11which]. It ismakes synthesized it possible by thefor rosettethe glucan terminal chains complex to pack [12 ],tightly consisting via hydrogen of six particles, bonds each and with van up der to Waals six cellulose interactions synthase [11] (CESA). It is catalyticsynthesized subunits. by the rosette terminal complex [12], consisting of six particles, each with up to six celluloseCellulose synthase exists (CESA) in di ffcatalyticerent polymorphs, subunits. with cellulose-I being the native form and type Iβ the mostCellulose abundant exists inin different higher plants. polymorphs, It has with a crystalline cellulose- andI being an the