Medicinal plants as a cellulose source for the fabrication of poly(lactic acid) composites: A mini-review

Abstract

More recently, natural polymers produced by living organisms have received considerable attention due to their unique properties such as eco-friendliness, biodegradability, and biocompatibility. These polymers possess similar properties to conventional plastics, making them suitable potential plastic substitutes for sustainable development and conservation of finite oil resources. Poly(lactic acid) (PLA) is comparable to commonly used plastics but is inferior, thus limited applications. However, PLA can benefit from the incorporation of various polymer components with complementary properties to yield improved physical properties. The fabrication of fully bio-based nanocomposites such as that of PLA and cellulose is of particular interest to research scientists. Incorporating organic fillers such as nanocellulose in a PLA matrix gives rise to improved composite properties. Because plant cell walls comprise semicrystalline cellulose molecules embedded in matrices, the crystalline portion of the stiff chains extracted from medicinal plants is currently exploited for potential filler roles. In this review, the use of medicinal plants in bio-based composite applications is covered. Several medicinal plants as sources of cellulose, different cellulose extraction methods, as well as the resulting fiber properties, are discussed. Although tremendous progress has been made in developing biocomposites, a lot of research still needs to be carried out.

Keywords

Poly(lactic acid)cellulose medicinal plants biocomposites

Introduction

It is estimated that about 367 million tons of plastics are produced across the globe annually, and its consumption increases by 5% every year.¹ Increased consumer demand and dependence on plastic consumables result in the continuous production of plastic. From the total solid plastic waste generated, 58% is being disposed in landfills across the globe, 18% is recycled while 24% is incinerated.² Most commercial plastics are very stable in the environment and do not breakdown into simple compounds by microbial action, which then leads to their accumulation when disposed. As a result, the effects of plastics persist for decades, long after their disposal, thus posing health and environmental concerns.^2–4 Moreover, the use of non-renewable sources such as fossil fuels to produce plastics also threatens energy security.^2,3,5,6

Managing plastic pollution is a pervasive and persistent challenge for everyone, hence the adoption of numerous plastic waste handling strategies by local and international governments. These strategies include decomposing in landfills, recycling, incineration, microbial degradations, and conversion into useful materials. However, the same strategies meant to remediate the undesirable effects of plastics are in most instances costly, labor-intensive, time-consuming, less effective, and tend to leave footprints in the environment through the emission of toxins during processing. Thus far, mitigation strategies have only been able to reduce the pressure exerted by plastic waste on the environment but are unable to handle the amount of plastic that is being disposed or reverse its detrimental effects. This has prompted various stakeholders to revise existing policies and innovate technologies that will be both economically feasible and environmentally friendly for plastic waste management.^3–5,7

According to Seltenrich et al.,⁸ the next generation of plastic materials should be designed in such a way that they are readily biodegradable commodities to reduce their potential accumulation in the environment. Nature constitutes a great source of organic materials, and it is the only sustainable source of fuels and materials available to humans. Biopolymers are abundant in nature, and their sources include plants, animals, and microorganisms.^9,10 Thus, biopolymers can be isolated from these sources and developed into commercial plastics using different techniques depending on the type of the raw material (Figure 1). Typically, the processes include (i) polymer extraction from microorganisms;¹¹ (ii) natural polymer extraction and modification from plant material;¹² and (iii) polymerization of bio-based monomers.¹³ Converse to petrochemical polymers, polymers that are fully derived from natural sources degrade completely, and their degradation products are simple compounds like water and carbon dioxide that can be reused by other organisms. Again, most polymers from natural sources are common in all living matter and therefore neither harmful to humans nor the environment. Cellulose, lignin, hemicellulose, starch, protein, and modified compounds, as natural polymers, have been used extensively for the fabrication of biopolymers and composites and for other applications including food, cosmetics, and medicine.^9,10,14,15

Amongst the most used biopolymers, poly(lactic acid) (PLA) gained a lot of attention because it is usually derived from starch-based plants like corn, potatoes, and cane through fermentation.^16,17 PLA possesses good processability, high mechanical strength, and thermoplastic characteristics. However, its poor impact resistance and relatively high cost make it inferior to those of conventional petroleum-based polymers and limit its use to certain medical and packaging applications.^15,18,19

To overcome these limitations and expand its scope of application, current advances in polymer science include the deployment of cellulose in PLA infusions as complementary materials. A composite of PLA and cellulose nanocrystals may exhibit a synergistic effect compared to individual components. Cellulose is another naturally derived polymer that is commonly used as a filler to address limitations suffered by other polymers because of excellent mechanical properties, surface functionalization, and renewability. Cellulose is the major constituent of cell walls in plants, and it is potentially attractive in terms of cost effectiveness and non-toxicity.^15,18,20,21

Several researchers have studied the potential reinforcement properties of PLA by using cellulose extracted from various medicinal^14,22 and non-medicinal plants.^23–25 Although tremendous research has been made for cellulose-based materials derived from non-medicinal plants (i.e., agricultural waste: sugar cane bagasse and maize stalk) as potential materials for fabricating PLA composites, cellulose extracted from medicinal plants is also interesting. This is because medicinal plants are used as additives in pharmaceutical, neutraceutical, toxicology, and other chemical industries, not only for the treatment of diseases but also as potential materials for maintaining good health and environmental conditions. Consequently, current trends in biotechnology and polymer science include the employment of indigenous knowledge acquired from the traditional use of medicinal plants and their functional activities as a guide to discovering polymeric material with potentially new characteristics, unique to the source material.²⁶ Therefore, this contribution explores various medicinal plants as sources of cellulose for the fabrication of PLA composites for advanced applications.

Medicinal plants as cellulose source materials

Medicinal plants are the richest source of biomaterials, and polysaccharides are the major active ingredients, which are responsible for many pharmacological activities in these plants. Biomacromolecules found in kingdom Plantae facilitate a lot of biological signals, such as cell to cell communication, immune recognition, and mitogenesis. These compounds possess prominent therapeutic properties like immunostimulatory, anti-tumor, and antioxidant activities. As such, polysaccharides can be used in pharmaceuticals, biomaterials, foodstuff and nutrition, and biofuels among others. For instance, various indigestible plant polysaccharides were defined as the dietary fiber by the Food and Agriculture Organization (FAO) such as cellulose, hemicelluloses, pectin, oligosaccharides, and gums.

Among the indigestible plant polysaccharides, cellulose and hemicellulose stimulate bowel movement. Furthermore, cellulosic materials have been vastly used in biomedical field. Lastly, lignocellulose biomass is one of the world’s leading biofuel resources with the greatest potential. These milestones in the development and commercialization of plant-based products have prompted a lot of research interest toward biomolecules from various plant species.^26–28

Biomolecules present great potential in many applications because of their biocompatibility, biodegradability, high stability, and non-toxicity. As such, a number of studies reported on the polymeric material extraction from medicinal plants whose biological activities are mainly influenced by their chain conformations and chemical structures. In order to address and advance the sustainable development and economic growth of mankind, the chemical structures have been correlated with biological and/or functional properties. In most instances, the traditional medicinal use of these plants has been used as a guiding thread to survey their potential.^26,27,29 Previous studies have demonstrated the extraction of cellulose fibers with different dimensions from various medicinal plants by employing acid hydrolysis and/or mechanical treatment.^30,31 The obtained cellulose was analyzed using morphology techniques such as optical, scanning electron, and atomic force microscopies, as well as physicochemical techniques like infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. The results illustrated the successful extraction of cellulose from the herbaceous plants with a percentage yield ranging from 40 to 90%.³² The percentage yield depends on the raw material and the process of extraction. From Table 1, it can be seen that the cellulose obtained using the alkali and bleaching treatments revealed the highest cellulose yield and percentage crystallinity. The high cellulose content suggests the possibility to use the cellulose obtained from medicinal plants in the development of biodegradable composites with enhanced properties.

Table 1.

Summary of the medicinal use, methods of extraction, and percentage yield and crystallinity of the cellulose extracted from medicinal plants.

Plant	Traditional medicinal use	Pharmacological property	Extraction method for cellulose	Type of cellulose	% yield of cellulose	% crystallinity	Reference
Helicteres isora	Treating of diarrhoeal infections	Not specified	Alkaline treatment, bleaching, acidic steam treatment, and homogenization	Nanofibrils	71	90	33–35
Retama raetam	Treating wounds, skin rash, kidney disorders, and microbiological infections	Not specified	Alkali and bleaching treatment procedures	Microfibers	52	78	32
Hibiscus sabdariffa	High blood pressure, liver diseases, and fevers	Anti-scorbutic, emollient, diuretic, refrigerant, and sedative	Oxalic acid hydrolysis and steam explosion	Microfibers	82	79	30,36
Syngonanthus nitens	Not specified	Anti-ulcerogenic, anti-oxidant, anti-bacterial, and anti-fungal activity	Alkali and bleaching treatments	Fibers	67	70–91	37,38
Phormium tenax	Production of woven mats and ropes	Not specified	Acid hydrolysis	Nanocrystals	61	Not specified	39
Leucaena leucocephala	Used to cure skin ailments and eliminate body hair	Anti-inflammatory, anti-diabetic, and anti-microbial	Alkali and bleaching treatment	Nanocrystals	90	71	40,41
Citrullus colocynthis	Diabetes and cancer	Anti-diabetic, anti-lipidemic, anti-oxidant, insecticide, cytotoxic, anti-microbial, and anti-inflammatory	Chemical extraction processes: deproteinization lipid extraction acid hydrolysis	Nanofibers	40	49–50	42–44
Kigelia africana	Treating of syphilis and rheumatism and cures wounds, abscesses, and ulcers	Anti-microbial, anti-bacterial, anti-fungal, anti-oxidant, and anti-inflammatory	Alkali treatment and bleaching treatment	Fibers	71	Not specified	45
Imperata brasiliensis	Diuretic and analgesic	Not specified	Sulfuric acid hydrolysis	Nanofibrils	44	56–64	46,47
Abelmoschus esculentus (Okra bamia)	Plasma replacement and blood-volume expander	Anti-spasmodic	Sulphuric acid hydrolysis	Nanocrystals	60–70	Not specified	48
Oil palm empty fruit bunch	Food sweetener for diabetic patients	Anti-cariogenic agent in tooth paste formulations	Chlorite bleaching, alkali treatment, and acid hydrolysis	Not specified	Not specified	Not specified	49
Hibiscus cannabinus (kenaf)	Improves oxidative stability	Anti-oxidant, anti-microbial, anti-inflammatory, and anti-thrombotic	Alkali treatment	Fibers	69	Not specified	50
Agave sisalana (sisal fibers)	Lowering blood pressure, anti-septic, and stops the growth of bacteria in the stomach and intestine	Anti-oxidant and anti-microbial	Sulfuric acid hydrolysis	Microfibers	Not specified	51	51–53
Humulus japonicus (HJ)	Heals pelvic inflammatory disease, relieves symptoms of heat, dampness, and toxicity	Anti-oxidative, anti-tumor, anti-tuberculosis, and anti-bacterial	Sulfuric acid hydrolysis	Nanocrystals	Not specified	Not specified	54
Panax ginseng C.A. Meyer	The regulation activities of metabolism and intestinal flora structure	Anti-tumor and anti-aging properties, inhibition of apoptosis, and immunogenicity	Enzyme hydrolysis	Fibers	68	Not specified	55
Camellia sinensis (green tea leaf)	Stimulant to promote urine excretion and stringent to control bleeding and wound healing	Anti-oxidant, anti-aging, anti-inflammatory, anti-bacterial, and astringent	Alkali and acid treatment	Nanocrystals	88	83	56,57

While previous research shows dependence on chemical treatment for the extraction and modification of cellulose (as evident in Table 1 or the previous table), current trends in the extraction of cellulose from herbaceous plants show a slight shift from the traditional use of toxic solvents to greener technologies.^55–57 Green synthesis is another growing field whose aim is to develop and encourage the use of solvents that meet both technological and economical demands for biomaterial synthesis. This is because chemical solvents are toxic and often limit the application of their blends in food, toys, packaging, and biomedical sectors because they tend to leave undesired residues on the newly fabricated composites. However, green solvents are non-toxic, eco-friendly, and sustainable.^58–61 Supercritical and subcritical fluids, room-temperature ionic liquids, and organic solvents are the most promising approaches for current solvent innovations. Due to unique temperature-tunable properties of density, viscosity, dielectric constant, ionic product, diffusivity, electric conductance, and solvent ability, subcritical water has received the most attention.^62,63 The use of water exclusively as a reagent is a promising procedure not only because of its environmental benefits but also because of its low and cleaner effluent, low corrosion, and low reagent cost.^59–63

Subcritical water, also known as hydrothermal water hydrolysis, is a green alternative for dissociating biomacromolecules into primary and secondary products⁶⁴ (Figure 2). The process depends on critical water temperatures ranging between 100 and 374°C at a pressure higher than its vapor saturation pressure to maintain water in its liquid state. Contrary to ambient water, subcritical water behaves like an organic solvent due to its decreased polarity, surface tension, and dissociation constant. Pressurized hot water exhibits lower viscosity but higher diffusivity than water at room temperature, which favors the diffusion into the plant matrix and the release of compounds. The increase of temperature in pressurized hot water extraction can overcome the solute–matrix interaction caused by the van der Waals force, hydrogen bonding, dipole attraction of solute molecules, and active sites in matrix. Thus, essential oils, pectin, lactic acid, cellulose, and other metabolites have been extracted from medicinal plants using this method.^{62,63,65–67}

Apart from extracting compounds from medicinal plant biomass, subcritical water has also been employed to tune the properties of cellulose and other biopolymers, which include conversion to nanoscale.⁶⁴ One example is the efficient separation of acetylated cellulose from eucalyptus and its enhancement of the mechanical strength of polylactic acid.⁶⁸ Recent developments in the production of green composites involve functionalizing cellulose with aqueous solutions such as water at subcritical temperatures followed by typical processing through melt compounding, injection molding, and extrusion especially with PLA.^68,69 Also, the degradation of PLA by subcritical water is being investigated for composting to remediate the effects of PLA acid byproducts in soil due to its ability to liquefy biopolymers into simple sugars for the downstream fermentation of gasification.⁷⁰ In addition to bioprospecting suitable cellulosic materials and obtaining desirable yields, clean technologies for extracting, tuning, and processing are essential for successful green composite formation.

Fabrication of PLA composites with cellulose extracted from medicinal plants

The modification of PLA has become one of the most vital subjects in PLA research. To enhance the functionality of PLA whilst maintaining an environmentally friendly material, composite material formation using a reinforcement matter is a suitable approach. The combination of organic materials (i.e., cellulose), that are biodegradable,^71,72 and reproducible bioplastics can be made to suit the demands of specific applications. Furthermore, their exclusive properties may create new market development opportunities for biocomposites, in the era of green materials. This growing development has provided biopolymers with an opportunity to pose as a potential alternative to the petroleum-derived polymers which are harmful to nature and synthetic.^71–73 For sustainable development, ideal products are biodegradable composites where the matrix part is also biodegradable. Environmentally friendly composites from plant-derived fibers are novel materials of the 21^st century. These materials are of great importance to the materials’ world due to their potential to reduce the growing environmental threats. Furthermore, they present a possible resolution to the uncertainty of the petroleum supply.^74–76 Great emphasis remains on the development of composites based on fully bio-based thermoplastic matrices with biomaterials such as cellulose.

Due to its structural integrity, cellulose acts as a reinforcing material in nature by providing structural support to plants. Its potential as a reinforcing agent has been demonstrated by its many attractive properties. Regardless of source, cellulose is normally a high crystalline and high molecular weight polymeric material that usually forms highly crystalline fibers.^77–80 However, depending on the source and method of treatment applied during the extraction process and regeneration, the degree of polymerization, geometric features, crystallinity, and fibrillary organization differ. Thus, there exist different polymorphs, namely, celluloses I, II, III, and IV. Plant cellulose is commonly known as cellulose I or native cellulose, and the different polymorphs can be obtained from it through chemical or biological processes as outlined in Table 2. The elastic modulus of the crystalline regions is different for the different polymorphs of cellulose in the direction of the chain axis, which is an indication that the polymer skeletons of these polymorphs are entirely different from each other. This is due to the intramolecular hydrogen bonds and skeletal conformations that are altered during the crystallization transition.

Table 2.

Different cellulose polymorphs and their properties.

Type	Structure	Treatment	Young’s moduli (GPa)	References
I	Native (micro-crystalline)	None	138	72,79,81,82
II	Crystal structure	Strong alkaline solution or precipitation from solution (sodium hydroxide solution) mercerization	88	72,79,81
III_I	Crystal structure	Liquid ammonia	87	72,79,81,82
III_II	Crystal structure	Ammonia at −30°C	58	72,83
IV	Lattice structure	Hot bath under stretch heating (prepared from ammonia at −30°C) III with glycerol	75	72,79,81

Although modifications are feasible, the physical and mechanical properties can be selected by electing specific source materials. Different cellulose forms give rise to different properties. Cellulose extracted from various plant sources has an attractive combination of mechanical and thermal properties.^{78,79,81,84,85} Relative to PLA alone, PLA composites reinforced with suitable fillers exhibit physiological and mechanical properties that make them ideal for a variety of applications.^58,86–88 Plant-derived fibers with increased availability, superior properties, and environmental benefits such as cellulose are among the many materials that have been used to modify PLA. The benefits of improving the performance of PLA using such fillers include the retention of the biodegradability of the composite. Moreover, they also exhibit lower density, superior performance, and lower cost due to the abundant availability. Composites of PLA and cellulose extracted from agricultural plants or crops such as wheat straw, sugarcane bagasse, and maize stalk among others have been extensively investigated. However, these composites have displayed enhanced tensile modulus and reduced tensile strength in comparison to neat PLA. These shortcomings have been associated with weak interfacial interaction between the hydrophilic cellulose fiber and the hydrophobic PLA matrix and lack of fiber dispersion due to a high degree of fiber agglomeration.^22,89–91

Different procedures have been explored to improve the interactions between the PLA and cellulose matrices such as modifying the surface area of the cellulose. Improvements in cellulose properties and the development of better processing techniques are vital for the production of PLA/cellulose composites with enhanced properties. Furthermore, factors including preparation method, filler content, and interaction between the filler and matrix have been found to have important effects on the properties of the resulting PLA/cellulose composite materials. On the other hand, searching for alternative sources of cellulose from different plant materials with the intent to produce cellulose with varying features that may suit specific fields of application is of particular interest. Thus, the current interest in cellulose extracted from medicinal plants in PLA matrices.^22,90,91

Cellulose has been extracted from various medicinal plants and incorporated in PLA matrices to produce composites with improved mechanical and physiological properties using different techniques including solvent casting, compounding, and compression molding among others. Table 3 provides a summary of medicinal plants used for cellulose extraction used in PLA matrices, as well as the potential applications. Generally, the composites were prepared using melt compounding, solvent casting, and electric oven. The thermal, morphological, and mechanical properties of the composites were analyzed using differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM), and tensile testing. Irrespective of the fabrication technique, most of the properties of the biopolymers reinforced with cellulose nanoparticles have been improved, including crystallinity, compatibility, thermal stability, modulus, flexural strength, and tensile strength. The inherent properties of cellulose such as its extraordinary mechanical properties (high specific strength and modulus), large specific surface area, high aspect ratio, and environmental benefits are some of the reasons that improve the properties of the cellulose-reinforced polymers.^47,50 These results, therefore, show that cellulose extracted from medicinal plants has a potential to reinforce biopolymers and be used in packaging, building industry, car components, furniture, and medicinal applications.

Table 3.

Summary of PLA/cellulose (extracted from medicinal plants) composites.

Composite name	Fabrication technique	Improved properties	Target application	Reference
PLA/cellulose from Cannabis sativa (hemp)	Compound molding	• Increased PLA transcrystallinity	Automotive and building construction	90
PLA/cellulose from Phormium tenax (New Zealand flax)	Compounding using a twin-screw microextruder	• Increased chain mobility, resulting in reduced glass transition temperature of PLA	Active packaging	91
PLA/cellulose from Boehmeria nivea (ramie)	Heat pressing technology compression molding and compounding	• Improved mechanical properties than pure PLA	Aircraft’s interior structures	92
PLA/cellulose from Hibiscus sabdariffa L. Linne Malvaceae (kenaf)	Electric oven	• Improvement in strength and modulus was achieved	Various applications	93
PLA/cellulose from Linum usitatissimum (flax)	Micro compounding and hot compression molding	• Excellent tensile properties	Flame retardant applications	94
PLA/cellulose from Posidonia oceanica	Solvent casting	• Optical transparency of the PLA matrix is retained	Food packaging	95
PLA/cellulose from Hibiscus cannabinus (Kenaf)	Melt blending and compression molding	• Flexural strength and modulus were improved	Building industry, car components, furniture, and food packaging	52

Although to some extent there have been improvements when using medicinal plant-sourced cellulose in PLA matrices, their performance is still inferior in some applications. This further warrants continuation for the search for suitable cellulosic material from these poorly explored but promising ecosystems. Apart from herbs, other plant materials such as woody trees and agricultural crops have been used as sources of cellulose for reinforcing polymers since times-time in memorial. However, trees and agricultural crops are the most exploited in biocomposite preparations to date. Medicinal plants have been used selectively for this purpose, and only a few species have been excessively exploited such as flax, ramie, hemp, kenaf, jute, cotton, and sisal.⁹³ As a result, the use of most medicinal plant-derived compounds is equitably documented in biomedical and pharmaceutical journals, but there is limited information on their use in biopolymer matrices such as PLA. For example, the use of compounds derived from medicinal plants has been solely limited to pharmaceutical products and therefore it is worth exploring its potential as a possible source of cellulose to develop new green composites and broaden its practical applications.

Conclusions and future recommendations

Using natural polymers from plants, animals, or microorganisms could be a solution to preserve the diminishing oil-based resources currently exploited in the production of commercial plastics for various applications. The incorporation of cellulose into biopolymers offers countless opportunities to develop novel bio-based composites for various industries. PLA/cellulose nanocomposites, which are currently the subject of extensive research due to their outstanding mechanical properties comparable to traditional polymers such as poly(ethylene terephthalate) (PET) and polystyrene (PS), which are commonly used in durable applications, are the most promising candidates. However, regardless of their success in the medical and packaging fields, concerns over poor compatibility between PLA and organic fillers remain a major drawback. As such, their application remains limited. The mechanical properties, crystal structure, and morphology of cellulose fibers were shown to depend on the choice of the extraction method. Moreover, the different pharmacological properties observed were dependent on the type of medicinal plant used. It is therefore essential for researchers to continuously observe and exploit novel and sustainable sources. The development and practical application of medicinal plant-derived compounds are currently limited to pharmaceutical products; hence, there is a need to explore their potential in other industries. This may potentially aid in the development of new green composites with a broader application spectrum.

Footnotes

Acknowledgments

The National Research Foundation (NRF) for postgraduate scholarship and Black Academics Advancement Programme (BAAP), as well as the Central University of Technology (CUT) postgraduate student funds are acknowledged by S.T. Sikhosana and T.P. Gumede for financial support of the project.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the South African Agency for Science and Technology Advancement, 129433.

ORCID iD

Thandi P Gumede

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