The presence of pathogenic microorganisms on the material surfaces can lead to significant healthcare and environmental problems. In recent times, different strategies have been defined to prevent the proliferation and adhesion of microorganisms on medical devices; or materials for food storage, and packaging [1
]. Moreover, the biofilm formation on the materials surfaces can limit their functionality, leading to critical health related complications [3
]. Furthermore, antibiotic-resistant microorganisms have emerged due to the extensive use of antibiotics or biocidal to impair their growth. Thus, it is necessary the development of new drug free materials that could avoid the increase of antibiotic-resistant microorganisms.
Biofilm is an aggregate of microorganisms that attaches to wet surfaces and multiplies, forming a slimy matrix of extracellular polymeric substances (EPS), thus creating an optimum environment to develop biofilms [4
]. The EPS is composed of polysaccharides, proteins, lipids, and nucleic acids, forming a highly hydrated polar mixture that contributes to the three-dimensional structure of the biofilm. The biofilm formation is established in five stages: attachment, colonization, development, maturation, and active dispersal. Figure 1
presents a scheme of the stages of development of biofilm. In the attachment stage, the microorganisms are reversibly absorbed to the biotic or abiotic surface by weak van der Waals forces bonds. In contrast, in the colonization stage, stronger hydrophilic/hydrophobic bonds are established with the surfaces allowing them to proliferated and secret EPS [4
]. In the maturation stage, a three-dimensional structure contains channels that distribute nutrients and signal molecules in the biofilm. In the last stage, called active dispersion, the cells are detached, either singly or in clumps, and colonize other locations [5
]. The formation and development of biofilm depend on many factors, such as the specific bacteria strain, the properties of the material’s surface, the environmental condition (pH, temperature, and nutrients), among others [6
]. Biofilms are responsible for biocorrosion, biofouling (accumulating microorganisms in surfaces), and reservoir souring, causing many constraints in different industries [4
The main mechanisms of antimicrobial action by which antimicrobial compounds affect microorganism are protein synthesis inhibition, cell wall disruption, and nucleic acid inhibition. Antimicrobial compounds can act as suppressing protein synthesis targeting the ribosomal subunits or protein folding, thus inhibiting their active role. They can also disrupt cell walls, causing an increase of permeability of the membrane, leading to the leakage of intracellular constituents. In addition, they are able to inhibit nucleic acid mechanism by suppressing the replication of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) [8
]. Currently, there is no deep knowledge on the mechanism of action against pathogenic microorganisms and the way they will act on surfaces.
The conventional methods to disinfect surfaces uses antimicrobial reagents, such as antibiotics, fungicides, antiviral drugs, and nonpharmaceutical chemicals [2
]. Antifouling agents are also employed since they prevent the adsorption on the surface and/or kill/inhibit the growth of microbes, preventing the biofilm formation. Antimicrobial or antibacterial agents are classified as a subclass of antifouling agents, and these materials present biocidal activity [9
The extensive use of these compounds can cause concern due to their environmental pollution potential, and the development of microbial resistance. The use of antimicrobial agents can be limited, and they cannot achieve high and durable local concentrations on the surface and providing lessen disinfection of the materials surfaces [2
]. Therefore, it is important to develop new antimicrobial agents able to prevent microbe’s adhesion and proliferation on materials surfaces, and reduce their negative effects.
The antimicrobial agents are classified into two categories, organic and inorganic. The organic antimicrobial agents include natural biopolymers, for instance, the chitosan, cellulose and lignin, phenols, halogenated compounds, and quaternary ammonium salts [10
]. The inorganic antimicrobial agents comprise, for example, metals, or metals bonded with phosphates, and metal oxides. The most common metallic nanoparticles or metal oxides used are silver, copper, titanium oxide, zinc oxide, magnesium, and calcium oxide [10
]. In the literature, several studies explore the use of different antimicrobial agents by incorporation or applied as coatings on materials surfaces. Among them, the antimicrobial potential of natural derived lignocellulosic compounds remains still unexplored.
Since lignocellulosic materials are mainly composed by the biopolymer with antimicrobial activity, cellulose and lignin, these materials have revealed antimicrobial potential. The present review explores the use of the lignocellulosic compounds, cellulose, hemicellulose and lignin, and lignocellulosic fibers as antimicrobial agents, highlighting their antimicrobial potential to be applied for different technological applications from the environment to the health.
3. Lignocellulosic Fibers
Natural fibers are mainly classified into three different classes in accordance with the origin of the fiber: plant, animal, and mineral [79
], as shown in Figure 5
, and the main chemical composition of each fiber in Table 2
. The lignocellulosic fibers are mainly classified in bast, grass, seed/fruit, leaf or hard fibers, stalk, and wood (hardwood and softwood) [81
]. The plant fibers are also classified as primary and secondary plants. The primary plants are cultivated for their fiber, such as jute or hemp, and the secondary plants are grown for the fruit, but the by-products of the plants are used to produce fibers, for example, banana and pineapple [80
The lignocellulosic fibers present unique properties, such as low specific weight, high specific strength, good mechanical properties, good thermal, and acoustic insulation properties. These materials from renewable sources are environmentally friendly, high availability, low cost, biodegradability, low amount of energy during fiber processing, contributing to a lower emission of carbon dioxide, and they do not produce harmful gases [79
]. As the main drawbacks, the natural fibers present high hydrophilicity, which causes high moisture absorption, poor matrix-fiber interfacial adhesion, and low fiber dispersion when combined with polymer matrices [80
]. However, this can be overcome with the fiber surface modification or the use of coupling agents [79
Traditionally, natural fibers are used to produce ropes, fabrics, cords, and threads. Engineering applications can be broader since they can be used in the automotive, packaging, paper, marine, and aerospace industries [79
3.1. Wood Fibers
As previously described, the origin of the lignocellulosic material influences the antibacterial and antifungal activity. Munir et al. [91
], described the evaluation of the antibacterial activity of wood disks from European fir (Abies alba
), American red oak (Quercus rubra
), European oak (Quercus spp
.), and European beech (Fagus sylvatica
). From them, the European oak species showed positive activities against a Gram-negative bacteria, P. aeruginosa
, and a Gram-positive bacteria E. faecalis
In polypropylene composites with wood flour, the origin of the wood flour was also observed. The antifungal activity of wood flour from Chinese white poplar (Populus tomentosa
), moso-bamboo (Phyllostachys heterocycla
), Chinese fir (Cunninghamia lanceolata
), Ramin (Gonystylus bancanus
), Chinese white pine (Pinus armandii
), river red gum (Eucalyptus camaldulensis
), western red cedar (Thuja plicata
), and rubberwood (Hevea brasiliensis
) was tested against A. niger
, Trichoderma viride
, Penicillium funiculosum
, Aureobasidium pullulans
, and Chaetomium globosum
]. The authors concluded that the mold growth resistance depended on the wood origin, with Chinese fir, red gum, and red cedar showing better activity against the fungi tested. They also established a relationship between the lower sugar content of wood fiber rendered WPC with higher fungal resistance, suggesting that composite from wood with less sugar content presented higher antifungal activity.
The use of wood flour in polymer composites with antimicrobial activity is reported in the literature for different polymer matrices, such as polyvinyl chloride (PVC) [93
], polyhydroxyalkanoate (PHA) [94
], polylactic acid (PLA) [96
], poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHVB) [97
] and polyvinyl alcohol (PVA) [98
]. The composites with PHA and PLA presented good antibacterial activity against E. coli
. The PHA composites with a higher quantity of wood (20% or more) in the composition exhibit more antibacterial activity [94
]. In the case of PLA composites [96
], the author used triclosan, an antibacterial material in different concentrations. The authors observe that PLA/triclosan composites showed lower antibacterial activity than the composites with PLA/wood flour/triclosan against E. coli
, proving that the wood flour increases the antibacterial activity.
Not only the origin of the material influence the antimicrobial activity; so too does the type of fiber. Treinyte et al. [98
] prepared composites with PVA with materials from the same tree, pine, but they used pine needles and pine bark. The composite with pine needles did not present antifungal activity, but the composites with pine bark partially suppressed Trichoderma viridescens
and did not affect the other fungus. Overall, the pine materials did not present good antimicrobial activity.
The combination of wood material with non-wood materials also revealed promising results. Jamili et al. [93
] produced PVC composites with wood and wood dyed with walnut shell wood by extrusion process. The composites showed a reasonable reduction of the growth of S. aureus
and E. coli
; however, the composites dyed with walnut shells proved to be the most antibacterial due to the presence of phenolic and naphthoquinone compounds in walnut.
3.2. Non-Wood Fibers
Different studies propose non-wood fibers as antimicrobials agents. These fibers can be presented as found in nature, or with different treatments, or combined with polymers. Kalinoski et al. [99
] studied different hydrogels prepared with poplar wood and sorghum dissolved with ionic liquids. They compared the antimicrobial activity against E. coli
of the lignocellulosic hydrogels with hydrogels prepared with commercial lignin, xylan, and cellulose. They verified that the hydrogels compositions based on cellulose/lignin, cellulose/lignin/xylan, and poplar presented a significant reduction of E. coli
The work by Gonçalves et al. [100
] shows that cork has antibacterial activity against S. aureus
, demonstrating that after 90 min of incubation, the reduction of the bacteria was about 100%. This behaviour is similar to the value obtained for a commercial product known to inhibit bacteria growth. The cork behaviour in the presence of E. coli
, showed a bacterial reduction of only 36%. These results are explained by the differences in the bacteria’s cell wall, and the Gram-negative bacteria present an outer membrane that acts as a barrier. Francesko et al. [101
], functionalized the cork particles with silver nanoparticles produced in the presence of chitosan or 6-deoxy-6-(ω-aminoethyl) aminocellulose. The work reveals that only the functionalization of cork particles with silver nanoparticles increased the antibacterial activity against E. coli
and S. aureus
In order to increase the antimicrobial activity of lignocellulosic fibers, some authors treated fibers with other compounds. Ketema et al. [102
] proved that the cotton fibers treated with nettle leaf extract present antibacterial properties against E. coli
and S. aureus
. Li et al. [103
], removed the lignin and hemicellulose from the hemp fibers and impregnated them with a Cinnamon derivative. The authors concluded that the fibers treated with cinnamon derivatives also have higher antimicrobial activity against brown-rot and white-rot fungi.
In the work by Thakur et al. [104
], coconut fiber also showed antimicrobial activity. The authors modified the coconut fibers by biografting the lignin structure with ferric acid and proved that they have antibacterial activity against E. coli
than S. aureus
. Lazić et al. [105
] prepared flax fibers with different hemicellulose content and lignin and combined them with Ag nanoparticles. The samples with a lower lignin concentration present higher antimicrobial activity against E. coli
, S. aureus
, and fungi C. albicans
The incorporation of different lignocellulosic materials in PBAT/starch composites was studied by Spiridon et al. [106
]. The composites were prepared with celery fibers, poplar seed hair fibers, pomace, and Asclepias syriaca
fibers. The pomace composite was the only material that presented inhibition against both bacteria E. coli
and S. aureus
. Torres-Giner et al. [107
] prepared composites with poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and coconut fibers and coconut fibers impregnated with essential oregano oil by extrusion process. The films with the functionalized coconut fiber, presented antibacterial activity even for a low concentration of the fibers. Guna et al. [90
] prepared composites with sabai fiber and polypropylene (PP). The sabai fibers presented higher antibacterial activity against S. aureus
and B. cereus
compared to E. coli
and S. marcescens
and better inhibition against the fungus Cryptococcus
than A. niger
4. Intellectual Property
Patent rights are designed to confer only a market opportunity. Furthermore, they create the opportunity for patent owners to obtain higher returns for products or services of the claimed technical solution [108
]. There is a particular increasing interest in materials demonstrating efficient use of renewable resources, that is reflected by the increasing number of publications during the recent years. A search carried out in Espacenet Patent Search Database (Jan 2021) revealed a total of 84 applications related to strategies containing lignocellulosic materials and claiming antibacterial properties. The survey revealed that the vast majority of the patented technologies are based on different application areas, whereas nine patent cases on the wood-polymer composite (WPC) materials. Table 3
reports the recent patents and technologies that have shown us a wider application in lignocellulosic antibacterial materials. As expected in this area, the majority of antibacterial properties of several lignocellulosic materials are achieved by adding the inorganic or organic agents or incorporating the antibacterial agent on a coating that is further applied in the lignocellulosic material.
WPC are a group of innovative materials consisting of mainly renewable resources. Typically, the concept is based on the selection of waste materials and by-products from wood and agricultural industry as raw materials that are combined with thermoplastic or thermosetting matrices and a small amount of additives that are further processed by melt-based technologies to obtain the desired product. Furthermore, with the increase in the variety and content of filled lignocellulose, the fast development of the industry, and the continuous expansion of the application fields, the resistance of WPC to biotic and abiotic factors has decreased significantly. Thus, there are some effective innovations claiming antibacterial resistance, such as the invention CN104893331A [109
], that pretreats the lignocellulosic powder by spraying a chitosan-nanosilver composite as antibacterial agent on the wood powder surface. After that, the material is mixed and combined during the processing with a thermoplastic such as polyolefins. The resulted WPC material has favorable resistance to S. aureus
, E. coli
, P. aeruginosa
, being a safe and environmentally friendly product solution. In this area, it is evident that the metal oxide particles have an effective antimicrobial activity against common harmful microorganisms and have been used to pre-treat the lignocellulosic fraction before compounding. For instance, the CN106752049A [110
], reports the use of titanium dioxide (TiO2
), the invention CN101659751A [111
], includes the addition of zinc oxide (ZnO) or the invention CN108841188A [112
], reports a WPC material that uses carbon nanofibers to enhance thermal conductivity, and the antibacterial agent is a silver-zinc composite.
A different concept can be found in the embodiment CN105350741A [113
], where the WPC flooring system comprises on the wear surface a decorative layer based on polyvinyl chloride (PVC) that contains the antibacterial agent, and it is applied through a rolling method. According to the claims, the antibacterial coating is a photocatalyst layer that is also applied in the CN106183293A [114
]. Similarly, the innovation CN109731747A [115
], discloses the use of a nano-oxide suspension by indicating a layer of polydimethylsiloxane that is sprayed on the surface of the WPC and further dried to prepare an anti-corrosion antibacterial composite material. The nano-oxide suspension is nano-aluminum oxide, a nano titanium dioxide, and/or a nano zinc oxide suspension, resulting in a claimed antiseptic, antibacterial wood fiber composite material.
Besides, the innovation reports the technology to prepare an antibacterial WPC by using sawdust, peanut shell, rice husk, several crop straws, linen rods, and cotton rods. The lignocellulosic raw material is crushed and pulverized into powder and dried and further combined with thermosetting or a thermoplastic by using an extrusion process. In the composition, it is also used other additives that may confer the claimed property.
also presents other strategies and uses claiming the antibacterial property. Among the patents, the EP2199046A1 [117
], describes the use of organic compounds, in this case, oligomeric or polymeric tannins that are covalently bonded onto the surface of wood or other lignocellulosic materials by enzymatically catalyzed oxidation. In this regard, the modified lignocellulosic surface shows improved antibacterial (bactericidal or bacteriostatic) properties compared to the untreated surfaces. In the innovation CN108724381A [118
], a technology for wood floor based on an antibacterial impregnation process is proposed that comprises a vapor treatment chamber to use reduced amounts of the antibacterial agent.
Additional patents focus on the potential of the chemical constituents of the lignocellulosic materials. The invention CN105506765A [119
], reports a method for produce a functional regenerated cellulose fiber. In particular, the invention comprises the steps of dissolving a cellulose pulp, introducing a material containing a graphene structure and non-carbon non-oxygen elements (e.g., Fe, Si and Al), and obtaining a spinning dope. The functional regenerated cellulose fiber has far-infrared antibacterial properties, bacteria resistance, and bacteriostasis with relevance for clothing, home textiles, or special protective clothing for industrial use.
Lignin has also been considered to develop polyurethane products, as reported in the invention CN105637036A [120
]. Polyurethanes foams are typically formed by the reaction of a resin comprising at least one polyol, a surfactant, a catalyst and a blowing agent, and the isocyanate comprising two or more isocyanate groups. The polyols used are usually derived from petroleum products. Nevertheless, due to environmental concerns, the industry is now attempting to replace petroleum products with bio-based solutions, for instance, derived from biomass, such as agricultural waste or biomass in forests. In the invention, an antibacterial agent is added to the composition, the lignin is less expensive and less harmful to the environment than the traditional petro-based polyols.
Lignocellulosic materials are considered non-toxic, odorless, non-pollution, and non-radioactive and can be used as a food packaging film. However, cellulose molecules contain a large number of hydroxyl groups, which tend to form intermolecular and intramolecular hydrogen bonds that increases crystallinity. The invention CN107934198A [121
], presents the use of ellagic acid for modifying lignocellulose to reduce the effect of hydrogen bonding and crystallinity changes on the crystal structure of cellulose. The remarkable result is that the thermoplastic processing performance of cellulose is greatly improved, and at the same time, the thermal stability performance is ensured as well as the barrier function of the biopolymer film targeting packaging application.
Looking at intellectual property (IP) in the same database and combining the words cellulose or lignin with antibacterial property, we obtain thousands of IP results. One area of particular interest due to the social impact and high added-value of the products is the biomedical field sector. In this regard, hydrogels are a swelling body that has a three-dimensional polymer network structure, formed by physical crosslinking or chemical crosslinking, containing a large amount of water but insoluble in water [122
]. The range of benefits includes softness, rich moisture content and good biocompatibility to be used in biomedical, tissue engineering, sensors, among others. The invention CN110240774A [123
], discloses a polyvinyl alcohol (PVA) that contains a large amount of hydroxyl groups formed by physical or chemical crosslinking to form a hydrogel. The use of lignin in the mixture with a solvent reinforces the PVA, resulting in hydrogels by freeze–thaw method or solvent exchange method. The hydrogels show a high-strength lignin/polyvinyl alcohol composite with good electrical conductivity and antibacterial activity.
A different use in the invention AU2007236166A1 [124
], relates to biomedical foam articles to treat chronic wounds, which are formed by spraying a polymer onto a wound surface to form a three-dimensional spatial shape, covering the wound surface and is also highly absorbent. The most frequent forms of chronic wounds by far are decubitus ulcers (caused by chronic pressure), chronic venous ulcers of the legs (caused by chronic venous insufficiency) and diabetic ulcers (caused by angiopathy and neuropathy). The standard treatment of chronic wounds follows the principle of “moist wound healing” with different wound contact materials. In this particular case biomedical foam composition uses naturally ionic biopolymers based on carbohydrates such as cellulose derivatives, for example cellulose acetate phthalate, cellulose acetate succinate, cellulose acetate trimellitate, hydroxypropylmethylcellulose phthalate, carboxymethylcellulose, and also natural biopolymers such as lignin contributing to the wound healing process. Furthermore, in the field of biomedical materials, there are inventions taking advantage of the strength and antibacterial properties of bamboo fiber. The patent CN106075601A [125
], discloses a bamboo fiber that is prepared as a porous material as a reinforcing phase of hydroxyapatite/polylactic acid composite material, or in the patent CN108607116A [126
], that presents a method to combine bamboo fiber with nano-apatite, where both inventions claim application in bone tissue engineering scaffold materials.
The IP in this field showed that depending on the natural lignocellulose material innovation, it can be applied in outdoors, or indoor houses and finishing, hospitals, antibacterial package, biomedical and other places that have high requirement on the anti-aging, mildewproof and antibacterial performances of the material.
5. Conclusions and Future Perspectives
Lignocellulosic materials are widely used in several production sectors such as construction, furniture, packaging, or the automotive industry. Several studies have highlighted the potential of these natural fibers or their chemical constituents on different polymer-matrix systems. In addition, their antimicrobial effects have been recognized. In the last few years, the use of lignocellulosic materials has grown, mainly to their characteristics; high availability, environmentally friendly, from renewable sources, low cost, and biodegradability. This review presents an overview of the most recent advances that demonstrates the potential of the lignocellulosic-based materials, cellulose, hemicellulose, lignin, and lignocellulosic fibers to be used as antimicrobial agents. In this area, the antimicrobial activity of the materials has emerged from the combination of the lignocellulosic source with antimicrobial agents from inorganic or organic origin. The intellectual and industrial property similarly shows products following the same routes; however, there are several innovations in the field that claims antibacterial activity only because at least one of the constituents present in the material is known to have an antibacterial effect. Thus, further research efforts in respect of these findings are needed, preferably in the presence of certain bacteria or fungi showing the inhibition of bacterial growth.
The potential of lignocellulosic as new drug-free polymers is extensive, but this area is still virtually unexplored, especially as antimicrobial or anti-biofouling materials for industries, such as healthcare, environmental, textile, space engineering, among others. By unlocking the full potential of the antimicrobial properties of lignocellulosic materials, it will be possible to fully disclose their potential, bringing new links of knowledge between the areas involving the synthesis of natural fibers, polymer matrices, and microbiology.