poly(lactic acid)-PLA
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azeotropic dehydration
copolymers
polymerization based on D,L-lactic acid
composites of PDLLA
stereo polymerization of D,L-lactic acid
Poly(lactic acid) (PLA) is one of the main bio-based polymers sold worldwide. It is considered a hydrolytically degradable, compostable, biocompatible and bioabsorbable polymer, which makes it attractive for applications in the biomedical area (tissue engineering, drug delivery system, sutures, etc.), in agriculture, ecology, packaging, among others. It is a thermoplastic polyester that has a processing temperature in the range of 170 to 230 °C, and can be processed by extrusion, spinning, biaxial stretching and blown injection, thus covering a wide and dynamic field of applications, including food packaging, textile, and non-textile fabrics (such as curtains and wet wipes, respectively), toys, among others. When mixed with other polymers or in the form of composites, some properties of PLA can be modified and improved, such as flexibility, impact resistance and heat stability, allowing its application as flexible films and engineering plastics. Nevertheless, despite being often classified as a biodegradable plastic, PLA, especially the high molar mass (>100 kDa), is hardly biodegradable under ambient conditions, requiring very specific composting conditions (e.g., temperature above Tg (glass transition temperature), specific microorganism, adequate humidity, and aerobic environment for its complete decomposition into COand HO)
Poly(lactic acid) or polylactide (PLA) is a bio-based aliphatic polyester produced from renewable resources such as wheat, straw, corn, cellulose, starch, sorghum etc.,. In the early 1800s, Pelouze used a lactic acid distillation process to remove water (polycondensation), thus obtaining a low molar mass PLA. Later, in 1932, Wallace Carothers, a DuPont scientist, synthesized PLA by heating lactide under a vacuum, in a process known as ROP (Ring-Opening Polymerization). The PLA obtained so far had characteristics that limited its potential for application, such as low molar mass and instability in a humid atmosphere. In 1954, a lactide purification method was developed by DuPont and made it possible to obtain high molar mass (HMW) PLA. However, it was from 1966 onwards, with the studies by Kulkarni et al., about the biodegradation and non-toxicity of PLAthat it and its copolymers started to be applied in the biomedical field as sutures, prostheses, matrices for drug delivery systems, scaffolds among others
[4][9][16][17]. The joint venture Total and Corbion (Amsterdam, The Netherlands) and Hisun (Wuhan, China) also produce PLA on a large scale, with a production capacity of 75,000 and 10,000 metric tons/year, respectively [4][8][9][17][18]. Other manufacturers produce PLA on a smaller scale and have been listed in the references [4][19][20].The advancement and dissemination of PLA production and processing technology have caused its field of applications to expand significantly in recent decades, especially after the creation, in 2002, of the NatureWorks (USA) industrial large-scale PLA production plant, which currently operates with a production capacity of 150,000 metric tons/year. The joint venture Total and Corbion (Amsterdam, The Netherlands) and Hisun (Wuhan, China) also produce PLA on a large scale, with a production capacity of 75,000 and 10,000 metric tons/year, respectively. Other manufacturers produce PLA on a smaller scale and have been listed in the references
High molar mass PLA can be obtained from three main synthetic routes: direct condensation polymerization, azeotropic dehydration condensation and lactide ring-opening polymerization (ROP)
[22]. Water readily reacts with the formed oligomers, shifting the equilibrium towards the reactants and making it difficult to obtain a high molar mass polymer [4][23][24][25]. Removing condensed water from the reaction medium is quite complicated, as the increase in the concentration of oligomers leads to an increase in the viscosity of the medium [4][8][23]. This requires the use of high temperatures, in the range of 150–200 °C, pressure below 5 torr and a long reaction time in the presence of a chain coupling agent [3][11][15][19][21][26][27], or in some cases, an azeotropic solvent [19]. Other alternatives for the synthesis of high molar mass PLA by polycondensation such as solid-state polycondensation (SSP) were discussed by Masutani [19].Direct polycondensation takes place from the dehydration of lactic acid with simultaneous esterification of the monomers and the release of water for each acid unit added by condensation. Water readily reacts with the formed oligomers, shifting the equilibrium towards the reactants and making it difficult to obtain a high molar mass polymer. Removing condensed water from the reaction medium is quite complicated, as the increase in the concentration of oligomers leads to an increase in the viscosity of the medium. This requires the use of high temperatures, in the range of 150–200 °C, pressure below 5 torr and a long reaction time in the presence of a chain coupling agent, or in some cases, an azeotropic solvent. Other alternatives for the synthesis of high molar mass PLA by polycondensation such as solid-state polycondensation (SSP) were discussed by Masutani
[3][8][11][15][19][21]. The main disadvantages of this synthesis lie in the use of diols and diacids as solvents, catalyst residue and low yields [3][22].In condensation by azeotropic dehydration, first the LA is distilled under a vacuum for about 3 h to remove most of the condensation water. Then, the azeotropic solvent (diphenyl ether) and the catalyst are added to the LA solution, with the solvent being refluxed and returning to the reaction flask after passing through a molecular sieve, without the need for chain extenders or adjuvants to obtain high molar mass PLA. The main disadvantages of this synthesis lie in the use of diols and diacids as solvents, catalyst residue and low yields
[22] and generally occurs in a two-step process. The first step consists of obtaining lactide with high optical purity; the second consists of the lactide polymerization promoted by an initiator or catalyst [11][15][21][28]. The catalysts commonly used in this synthesis are metallic catalysts, such as Zn and Sn oxides, zinc and tin chlorides or tin octoate [22][28][29][30].The most widespread and industrially employed route in PLA synthesis is lactide ring-opening polymerization (ROP). ROP is a propagation process of cyclic monomers initiated by different ionsand generally occurs in a two-step process. The first step consists of obtaining lactide with high optical purity; the second consists of the lactide polymerization promoted by an initiator or catalyst. The catalysts commonly used in this synthesis are metallic catalysts, such as Zn and Sn oxides, zinc and tin chlorides or tin octoate
[19][22][31]. ROP can be classified in terms of the reaction mechanism as: anionic polymerization, cationic polymerization, and coordination-insertion mechanism [19][21][22][26][31][32]. The most popular catalyst used in this synthesis is tin(II) bis-2-ethylhexanoic acid (tin octoate), due to its solubility in molten lactide, low product racemization, high conversion and catalytic activity, and for providing PLA of high molar mass (≥1000 kDa) [13][26][30]. Another relevant aspect of ROP is that it makes it possible to control the microstructure of the polymer, including the order of insertion of monomers in the polymer chain based on their stereochemistry, as well as the combination of reaction time, temperature, type, and concentration of catalyst [26][28][32].Compared to direct polycondensation, ROP can be performed under milder conditions, such as a reaction temperature of 130 °C and a shorter reaction time. ROP can be classified in terms of the reaction mechanism as: anionic polymerization, cationic polymerization, and coordination-insertion mechanism. The most popular catalyst used in this synthesis is tin(II) bis-2-ethylhexanoic acid (tin octoate), due to its solubility in molten lactide, low product racemization, high conversion and catalytic activity, and for providing PLA of high molar mass (≥1000 kDa). Another relevant aspect of ROP is that it makes it possible to control the microstructure of the polymer, including the order of insertion of monomers in the polymer chain based on their stereochemistry, as well as the combination of reaction time, temperature, type, and concentration of catalyst
PLA presents a great structural diversity based on its enantiomeric constitution. The enantiopure monomers of L-lactic acid and D-lactic acid (or their lactide analogues) lead to the formation of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA), respectively. These homopolymers are semi-crystalline and have the same thermal properties, such as melting temperature (T) ranging between 170–180 °C, glass transition temperature (T) around 55–60 °C and crystallinity around 35%. PLLA with a percentage of L-isomer above 90% in its composition tends to be semi-crystalline, while PLA with a content lower than this tends to be amorphous
[12][17][36]. PDLLA has no Tm but has a Tg around 60 °C [19][33]. Due to its amorphous nature, PDLLA shows a faster degradation rate than stereoregular PLA, making it preferred for applications as a drug delivery vehicle and as a low-strength scaffolding material for tissue engineering [17][37]. PDLLA may show some crystallinity when synthesized by stereocontrolled ROP, through the action of a catalyst/initiator [19][26]. Aluminum alkoxide catalysts, Schiff bases and other single-site complexes have been used in the synthesis of stereoregular PDLLA [26].Polymerization of the racemic mixture of lactic acid, rac-lactide (rac-LA) and meso-lactide (m-LA) results in poly(D,L-lactic acid) or PDLLA, a random copolymer of D and L-lactic units, with an irregular and completely amorphous structure. PDLLA has no Tbut has a Taround 60 °C. Due to its amorphous nature, PDLLA shows a faster degradation rate than stereoregular PLA, making it preferred for applications as a drug delivery vehicle and as a low-strength scaffolding material for tissue engineering. PDLLA may show some crystallinity when synthesized by stereocontrolled ROP, through the action of a catalyst/initiator. Aluminum alkoxide catalysts, Schiff bases and other single-site complexes have been used in the synthesis of stereoregular PDLLA
[19][38]. PLA stereocomplexes (sc) have a melting temperature (Tm) about 50 °C higher than that of homopolymers, varying around 230 °C [39][40]. However, these sc-PLA can be formed concomitantly with PLA homocrystals (hc), as the kinetics of homocrystal formation is favored in high molar mass PLLA/PDLA mixtures (weight-average molecular weight, Mw > 40 kDa) [41], thus limiting the exclusive production of sc-PLA. One way to circumvent this problem is the production of stereoblock PLA (sb-PLA), which is a copolymer containing isotactic sequences of PLLA and PDLA [42]. The sc-PLA and sb-PLA can be obtained from the stereoselective ROP (rac- or m-LA) using a chiral catalyst [43][44], as can be seen in Scheme 1, which has been described in the literature [19]. In addition to stereoblock synthesis, other alternatives to improve PLA stereocomplex are melt blending, the addition of nucleating agents and polymer blending [39]. Several review articles have been published on the synthesis, structure, crystallization behavior, other properties, and applications of these sc-PLA [45][46][47][48][49].In 1987, Ikada et al., reported, for the first time, that a blend of PLLA and PDLA in equal proportions (1:1) produced stereocomplex crystals, which had different properties from pure homopolymers. PLA stereocomplexes (sc) have a melting temperature (T) about 50 °C higher than that of homopolymers, varying around 230 °C. However, these sc-PLA can be formed concomitantly with PLA homocrystals (hc), as the kinetics of homocrystal formation is favored in high molar mass PLLA/PDLA mixtures (weight-average molecular weight, Mw > 40 kDa), thus limiting the exclusive production of sc-PLA. One way to circumvent this problem is the production of stereoblock PLA (sb-PLA), which is a copolymer containing isotactic sequences of PLLA and PDLA. The sc-PLA and sb-PLA can be obtained from the stereoselective ROP (rac- or m-LA) using a chiral catalyst, as can be seen in, which has been described in the literature. In addition to stereoblock synthesis, other alternatives to improve PLA stereocomplex are melt blending, the addition of nucleating agents and polymer blending. Several review articles have been published on the synthesis, structure, crystallization behavior, other properties, and applications of these sc-PLA
Scheme 1. The variety of PLA microstructures and synthetic routes.
Some common desired properties of PLA, such as rigidity, permeability, crystallinity and thermal stability, and hydrophobicity/hydrophilicity can be improved by processes such as copolymerization, blending and production of polymer composites. The copolymerization process consists of the simultaneous polymerization of two or more monomers that interact via chemical reactions and produce PLA copolymers. These copolymers can be sequenced alternately, in blocks or grafts, but this considerably compromises their biocompatibility. However, biocompatibility can be improved by the copolymerization of lactic acid with other hydrophilic monomers or polymers. For example, block copolymers of lactic acid and polyethylene glycol (PEG) are hydrophilic and some of them are even water-soluble
[50], producing blends. That is the case of PLLA/PDLA, PLLA/PDLLA, PLA/starch blends, PLA/PHB (polyhydroxybutyrate) and others. In the literature [34][51][52] several possibilities of PLA blends were revised, considering systems that include mixtures with hydrophobic and hydrophilic polymers, other polyesters and so on. A recent review article by Hamad et al., discussed the production of PLA modified by polymer blending techniques to achieve suitable properties for some applications, such as medical, packaging, battery and semiconducting [53]. They pointed out that biodegradable polymers have been mostly utilized due to their environmental advantages and high toughness compared with pure PLA, but other polymers still show low cost, better mechanical properties, high thermal stability, and processability. For instance, they verified in the literature that PLA/TPS (PLA-thermoplastic starch) blend could be useful in general packaging applications by means of compatibilizers such as PLA-g-MA (PLA-g-maleic anhydride), GMA-g-PEO (glycidyl methacrylate-g-poly(ethylene oxide)), TPS-g-MA (thermoplastic starch-g-maleic anhydride), PLA-g-TPS (PLA-g-thermoplastic starch). The PLA blend improved the biodegradation rate, but this process promoted weakness and low elastic modulus. Some PLA blends also have a higher flexibility than pure PLA, which make this material more appropriate for the fabrication of 3D scaffolds. Furthermore, PLA blends can be used to produce hierarchically porous materials for biomedical applications, since micropores can enhance the tissue ingrowth and the smaller pores (submicrometer scale) can provide the cell differentiation. Given this scenario, the authors proposed that research on PLA blends should preferably focus on their application. In addition, significant effort is required to improve the biodegradability of PLA-containing systems, after the material has fulfilled its specific role in the application.The blending process consists of two or more different polymers mechanically mixed and connected through physical interactions, producing blends. That is the case of PLLA/PDLA, PLLA/PDLLA, PLA/starch blends, PLA/PHB (polyhydroxybutyrate) and others. In the literatureseveral possibilities of PLA blends were revised, considering systems that include mixtures with hydrophobic and hydrophilic polymers, other polyesters and so on. A recent review article by Hamad et al., discussed the production of PLA modified by polymer blending techniques to achieve suitable properties for some applications, such as medical, packaging, battery and semiconducting. They pointed out that biodegradable polymers have been mostly utilized due to their environmental advantages and high toughness compared with pure PLA, but other polymers still show low cost, better mechanical properties, high thermal stability, and processability. For instance, they verified in the literature that PLA/TPS (PLA-thermoplastic starch) blend could be useful in general packaging applications by means of compatibilizers such as PLA-g-MA (PLA-g-maleic anhydride), GMA-g-PEO (glycidyl methacrylate-g-poly(ethylene oxide)), TPS-g-MA (thermoplastic starch-g-maleic anhydride), PLA-g-TPS (PLA-g-thermoplastic starch). The PLA blend improved the biodegradation rate, but this process promoted weakness and low elastic modulus. Some PLA blends also have a higher flexibility than pure PLA, which make this material more appropriate for the fabrication of 3D scaffolds. Furthermore, PLA blends can be used to produce hierarchically porous materials for biomedical applications, since micropores can enhance the tissue ingrowth and the smaller pores (submicrometer scale) can provide the cell differentiation. Given this scenario, the authors proposed that research on PLA blends should preferably focus on their application. In addition, significant effort is required to improve the biodegradability of PLA-containing systems, after the material has fulfilled its specific role in the application.
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[50][54]. When at least one of the phases of this composite has nanometric dimensions, it is called a nanocomposite [50]. Unlike blends, the constituents of a composite interact with each other through strong chemical and physical reactions [54]. A literature review by Murariu and Dubois highlights recent and current developments, results, and trends in the field of PLA-based composites [55]. In addition, a comprehensive review of PLA composites reinforced with synthetic and natural fibers has been published by Ashothaman et al. [56]. The authors mainly address some methods of manufacturing polymeric composites (especially using compression molding and injection molding methods) and reinforcement of PLA composites with different fibers, between natural fibers (treated or not) and synthetic fibers. They pointed out that PLA composites reinforced with synthetic fibers are more easily manufactured, since both are hydrophobic and have good compatibility, while natural fibers, due to their hydrophilic nature, have low adhesion or incompatibility with the polymer. PLA composites reinforced with bioactive glass fibers and magnesium, PLA biocomposites with cellulosic fibers treated by microwave and enzymatic treatment, PLA-based biocomposite reinforced with flex fiber with treated surface, PLA reinforced with maple wood flour, PLA reinforced with hydroxyapatite, among others were mentioned. The authors concluded, based on the cited studies, that it is possible to improve the mechanical strength, stiffness and crystalline behavior of PLA composites reinforced with these fibers.Polymer composites processes, in turn, are multiphase systems formed by two or more components, generally polymeric and one non-polymeric. When at least one of the phases of this composite has nanometric dimensions, it is called a nanocomposite. Unlike blends, the constituents of a composite interact with each other through strong chemical and physical reactions. A literature review by Murariu and Dubois highlights recent and current developments, results, and trends in the field of PLA-based composites. In addition, a comprehensive review of PLA composites reinforced with synthetic and natural fibers has been published by Ashothaman et al.. The authors mainly address some methods of manufacturing polymeric composites (especially using compression molding and injection molding methods) and reinforcement of PLA composites with different fibers, between natural fibers (treated or not) and synthetic fibers. They pointed out that PLA composites reinforced with synthetic fibers are more easily manufactured, since both are hydrophobic and have good compatibility, while natural fibers, due to their hydrophilic nature, have low adhesion or incompatibility with the polymer. PLA composites reinforced with bioactive glass fibers and magnesium, PLA biocomposites with cellulosic fibers treated by microwave and enzymatic treatment, PLA-based biocomposite reinforced with flex fiber with treated surface, PLA reinforced with maple wood flour, PLA reinforced with hydroxyapatite, among others were mentioned. The authors concluded, based on the cited studies, that it is possible to improve the mechanical strength, stiffness and crystalline behavior of PLA composites reinforced with these fibers.
Recently some studies have shown that polylactic acid microplastics have a prominent impact on marine biota. Due to their compostable nature, polylactic acid microplastics cannot be degraded and are assimilated as all marine biota do not have the specific enzymes responsible for their degradation. In case of petroleum-based microplastics, it is well known that ingestion of these contaminants can cause a number of adverse effects. However, in the case of polylactic acid microplastics, very recently scientific community has shown an interest in their ecotoxicological evaluation. Very few studies have been conducted on the impact of polylactic acid microplastics on limited number of aquatic species which revealed that in some cases these contaminants can negatively affect the exposed individuals analogous to petroleum-based microplastics. The literature survey reveals that Green et al. (2016) is pioneer in assessing the toxic effects of polylactic acid microplastics on aquatic biota. After this, many research studies focused on the ecotoxicological evaluation of polylactic acid microplastics using a range of experimental model species (Table 2).
Sediment microbial communities consist of large number of the earth’s biodiversity and which play a key role in biogeochemical cycling of nutrients (Vincent et al. 2021) and ecological purification of pollutants (John et al. 2022). The presence of microplastics in the environment provides a new habitat to these microbes (McCormick et al. 2014), but on the other hand, the degradation of these polymers can produce toxic substances which will have negative effects on these microbes (Kong et al. 2018). A few species of bacteria will be benefited but the others will be negatively affected (Li et al. 2020), as different species of bacteria respond differently to the presence of microplastics (Wang et al. 2020). The impact of petroleum-based microplastics on microbial communities is well studied. However, studies focusing on the impact of polylactic acid microplastics on microbial communities are scarce.
Seeley et al. (2020) have conducted a 2-week microcosm experiment to investigate the effects of petroleum-based and bio-based microplastics on composition and function of sedimentary microbial communities. They reported a significant alteration in microbial communities exposed to petroleum-based microplastics. Surprisingly, polylactic acid microplastics were found to promote nitrification and denitrification. Based on the results, the authors suggested that the microorganisms might have utilized the microplastics as organic carbon for the energy which facilitated these phenomena. However, degradation and assimilation of polylactic acid microplastics in 2 weeks have not been reported in the literature.
Microalgae are the primary producer of aquatic ecosystems which play an important role in their functioning (Casado et al. 2013). Being a primary producer, microalgae account for 50% of net production (Barbosa 2009), and therefore, any alteration in microalgae population can have serious effects on food webs. Many studies have been carried out to investigate the impact of petroleum-based microplastics on several parameters of microalgae, including growth (Long et al. 2017), morphological changes (Mao et al. 2018), chlorophyll content (Prata et al. 2018), photosynthesis (Zhang et al. 2017) and gene expression (Lagarde et al. 2016). The results of all these studies reveal that petroleum-based microplastics have deleterious effects on microalgae ranging from individual to population level.
The impact of polylactic acid microplastics on microalgae has also been investigated. Su et al. (2022) investigated the effects of petroleum-based microplastics and bio-based microplastics including polylactic acid on marine alga Chlorella vulgaris. The authors reported that both types of microplastics inhibited the growth of microalgae. However, polylactic acid microplastics were reported to have severe effects on growth of algae with highest inhibition rate of 47.95%, as compared to other petroleum-based and bio-based microplastics. The authors attributed these effects to the physiochemical properties and chemical changes of microplastics. The authors also reported that microplastics can stimulate pigments content (chlorophyll a, chlorophyll b and carotenoid), attributed to the cellular defense against stress. These findings are very interesting in two domains. Firstly, in the context of adaptation and or defense mechanisms against microplastics pollution, secondly, in understanding the fact that polylactic acid microplastics appear to be more toxic as compared to petroleum-based microplastics.
Being primary producers, phytoplankton plays a key role in maintaining aquatic ecosystems. Due to their key role in food chains/webs, there is a serious concern about the impact of microplastics on phytoplankton (Koenigstein 2020). Any alteration or threat to the primary producers will have significant effects on the food chains/webs and consequently on the entire ecosystems, therefore, assessing the impact of emerging pollutants on primary producers is crucial. Literature survey revealed that petroleum-based microplastics have deleterious effects on various parameters of phytoplankton including photosynthetic capacity and growth (Sánchez-Fortún et al. 2021).
Studies on the impact of polylactic acid microplastics on phytoplankton are scare. Only a few studies have assessed their impact on phytoplankton. For example, Yokota and Mehlrose (2020) have assessed the impact of polylactic acid microplastics, originated from body wash scrub, on natural phytoplankton communities in a 7-days incubation experiment conducted in temperate mesotrophic lake. The authors reported that polylactic acid microplastics eliminated cryptophytes and increased chrysophytes, resulting in the alteration of taxonomic composition of the phytoplankton in the mesocosms. They suggested that chrysophytes contain a protective siliceous loricae against the polylactic acid microplastics whereas cryptophytes do not have any such protection and thereby got affected by the polylactic acid microplastics.
Zooplanktons are primary consumers and located at the base of food chains/webs, thereby channeling nutrients and energy from the primary producers to higher trophic levels. Most researchers investigating the effects of microplastics on aquatic biota have focused on the primary consumers. As compared to other species, zooplanktons are more prone to microplastics and therefore documented as potential microplastics consumers (Cole and Galloway 2015). Microplastics act as analogues of zooplankton prey (Gambardella et al. 2017) and can have negative impact on different ecological processes. Recently, many studies have reported adverse effects of petroleum-based microplastics on a range of zooplankton species. The results of those studies reveal that petroleum-based microplastics have adverse effects on survival rate and reproduction (Yu et al. 2020; Zhang et al. 2019a), feeding capacity and selectivity (Cole et al. 2019; Coppock et al. 2019) and behavior (Suwaki et al. 2020). However, studies investigating the effects of polylactic acid microplastics on zooplankton are scare and only a few studies are available.
Zimmermann et al. (2020) investigated that how polylactic acid microplastics affected the survival, reproduction, and growth of Daphnia magna in a 21-days experiment. They found that polylactic acid microplastics cause the mortality of 60% individuals exposed to 500 mg/L, while in control the mortality was 5%. The authors also reported decrease in reproductive output and body length in the exposed individuals, induced by the microplastics themselves rather than leachates or additives. Similarly, very recently Di Giannantonio et al. (2022) studied the effects of polylactic acid microplastics on uptake of microplastics, immobility, and behavior of two zooplankton species, the crustacean Artemia franciscana and the cnidarian Aurelia species (common jellyfish) in a 24 h experiment. The authors reported polylactic acid microplastics in the digestive system of A. franciscana and in the gelatinous tissue of Aurelia species exposed to 100 mg/L, with no effects on the immobility of both the species. However, significant alterations were reported in the swimming behavior (pulsation) of Aurelia species at all the exposure concentrations (1, 10 and 100 mg/L), attributed to the direct toxicity of polylactic acid microplastics. It is worthy to note that the concentration of microplastics used in majority of the ecotoxicological studies, to evaluate their potential effects on the exposed organisms, are much higher as compared to their concentrations found in the natural environment.
Annelids are invertebrates which play an important role in benthic ecosystems by serving as a link from primary producers to higher trophic levels and in the cycling of minerals (Rafia and Ashok 2014). They are the dominant invertebrates of the deep sea and mostly occupy sediments. Recent studies reported that petroleum-based microplastics can have deleterious effects on various parameters of annelids which include decrease in food intake (Wright et al. 2013), impairment of immune system, physical stress and even death (Browne et al. 2013), leading to a drastic impact on ecological processes (Green et al. 2016). To the best of our knowledge, only three studies have assessed the negative effects of polylactic acid microplastics on annelids. Klein et al. (2021) investigated the impact of polylactic acid microplastics (mixed and or layered on sediment surface) on freshwater worms (Lumbriculus variegatus) under laboratory conditions. The authors reported a significant reduction in the survival of the worms exposed to microplastics mixed with the sediments. However, they attributed the toxicity to the associated chemicals rather than to the polymer.
Similarly, Green et al. (2016) assessed the effects of polylactic acid and petroleum-based microplastics on lugworms (Arenicola marina) using concentrations of 0.02, 0.20 and 2% (wet sediment weight) in a 31-days mesocosm experiment with focus on health, biological activity and nitrogen cycling, in addition to the primary productivity of the sediments. The authors reported a significant impact of both types of microplastics on the health and behavior of the exposed individuals, as well as reduction in the primary productivity of the sediments they inhabited. Polylactic acid microplastics exposure not only reduced the feeding activity of the exposed individuals but also reduced the biomass of the algae on the surface of sediments. They also found that polylactic acid microplastics reduced the concentration of ammonia in pore water, which might be due to the potential of carbonyl and hydroxyl groups of polylactic acid to adsorb cations.
In another study, Green et al. (2017), while investigating the ecological impacts of polylactic acid and petroleum-based microplastics, high-density polyethylene, on the biodiversity and ecosystem functioning, found a difference in faunal invertebrate assemblages in the exposed groups, with less polychaetes and more oligochaetes, highlighting the potential of polylactic acid microplastics to affect ecosystem. These results are quite interesting in the context of species-specific response to microplastics (Bai et al. 2021) or other contaminants, as both the species were exposed to the same types and concentrations of microplastics but showed completely different responses.
Mollusks are a diverse group of filter feeders which can be found in a variety of aquatic habitats. They provide ecological services to a number of organisms ranging from habitat to food (Fernández-Pérez et al. 2018). Being filter feeders, mollusks can accumulate and transfer microplastics to higher trophic levels, which will have detrimental effects on their consumers including humans. Therefore, many ecotoxicological studies have used mollusks as bioindicators of pollution (Capillo et al. 2018). However, there are only a few studies available on the impact of polylactic acid microplastics on mollusks. Green et al. (2017) studied the ecological impacts of polylactic acid and petroleum-based microplastics on the biodiversity and ecosystem functioning of bivalve-dominated European flat oysters (Ostrea edulis) and blue mussels (Mytilus edulis) habitats in outdoor 50-days mesocosms experiment, using two different concentrations of 2.5 and 25 μg/L for each type of microplastics. The authors reported a significant reduction in filtration by M. edulis (exposed to 25 μg/L), while no effects were observed on ecosystem functioning or the associated assemblages of invertebrates. On the other hand, the authors reported a significant increase in filtration by O. edulis after exposure to 2.5 and 25 μg/L and decrease in the pore water ammonium and biomass of benthic cyanobacteria.
Khalid et al. (2021) also studied the effects of polylactic acid microplastics on blue mussels (M. edulis) using two different concentrations, 10 and 100 μg/L, in an 8-days experiment with biochemical endpoints. The authors found no significant effects of polylactic acid microplastics on M. edulis in terms of oxidative stress (catalase, glutathione-S-transferase, and superoxide dismutase activities), neurotoxicity (acetylcholinesterase), and immunotoxicity (lysosomal membrane stability and acid phosphatase activity). In contrast to these results, Green et al. (2019) found a significant alteration in the immunological profile of haemolymph of Mytilus edulis exposed to polylactic acid microplastics in a 52-days mesocosms experiment. However, the authors found no adverse effects of polylactic acid microplastics on the attachment strength of the exposed individuals.
Green (2016) investigated that how polylactic acid and petroleum-based microplastics at low and high concentrations (0.8 and 80 μg/L) affect the health and biological functioning of European flat oysters (Ostrea edulis) along with the impact on structure of associated macro faunal assemblages in a 60-days mesocosm experiment. They reported minimal effects on the exposed individuals, but the associated macro faunal assemblages were significantly altered which were ~ 1.2 and 1.5 times reduced as compared to the control. For instance, the biomass of Scrobicularia plana (peppery furrow shell clam), the abundance of juvenile Littorina sp. (periwinkles) and Idotea balthica (an isopod) were decreased 1.5, 2.0 and 8.0 times in groups exposed to either type of microplastics compared to the control.
Beside the mussels, other filter feeder organisms were also used for the ecotoxicological evaluation of microplastics. For example, Anderson and Shenkar (2021) investigated the impact of polyethylene terephthalate and polylactic acid microplastics on the biological and ecological features of a solitary ascidian (Microcosmus exasperatus). The authors reported that both polylactic acid and petroleum-based microplastics had similar impact on the exposed individuals; for example, both types of microplastics reduced the fertilization rates in the exposed individuals.
Fish are a good source of unsaturated fatty acids and proteins; therefore, their consumption is recommended in human diet (Ali et al. 2017). Therefore, assessment of microplastics and its consequent impact on fish is of major environmental importance. Many ecotoxicological studies have used fish as bioindicator of water quality and ecosystem health. Fish have the potential to accumulate and magnify pollutants which may have potential impacts on their consumers including humans. Many studies have reported the ingestion and accumulation of petroleum-based microplastics in a range of fish species, while studies on polylactic acid microplastics are very few.
Recently, Chagas et al. (2021) studied the bioaccumulation of polylactic acid microplastics, at a concentration of 2.5 and 5 mg/L, in adult zebrafish and its consequent impact on behavioral, biochemical, and morphological parameters in a 30-days experiment. The authors reported the accumulation of microplastics in the liver, brain, gills, and carcass of the exposed group in addition to behavioral and morphological changes. The reported behavioral and biochemical changes were shoals predictive of co-specific social interaction and an anti-predator defense response defect, attributed to cholinergic changes inferred by an increase in the activity of acetylcholinesterase and redox imbalance whereas the morphological changes were alteration In the pigmentation pattern. However, in contrast to de Oliveira et al. (2021), no locomotor damages or anxiety-like behavior was observed in the exposed individuals. The possible reason might be difference in the life stages of the test organism as early life stages are more sensitive to different contaminants.
Most studies, while investigating the effects of various contaminants on aquatic organisms, have focused on early life stages of fish, for example larvae and embryo (Mu et al. 2022), because of their sensitivity to different contaminants (Schweizer et al. 2018), which are critical on individual and population health's point of view. For instance, zebrafish (Danio rerio) has been widely used as a biological model and/or as a representative of fish group by many researchers to investigate the toxicological impact of microplastics. de Oliveira et al. (2021) investigated the effects of polylactic acid microplastics (3 and 9 mg/L) on zebrafish larvae in a 5-days exposure experiment with behavioral and biochemical endpoints. The authors reported a decrease in the swimming speed and distance of the exposed individuals in open field test. The authors attributed these outcomes as a consequent impact of microplastics on fish locomotor and exploration activities. They also reported anxiety like behavior and accumulation of microplastics, which inhibited the activity of acetylcholinesterase leading to the reinforcement of neurotoxic action in the exposed group.
Similarly, another study also focused on the impact of polylactic acid microplastics (virgin and degraded) on zebrafish larvae (Zhang et al. 2021). They found a slower efflux and detoxification of degraded polylactic acid, mediated by ABC transporters and P450 enzymes, leading to increase in bioaccumulation of microplastics and thereby inhibiting the skeletal development of larvae. They also pointed the higher toxicity of degraded polylactic acid microplastics by identification of crucial mechanisms, for example, mitochondrial structural damage by oxidative stress, apoptosis, depolarization, and fission inhibition. However, no effects were reported on the hatching rate of larvae when exposed to both types of polylactic acid microplastics. The authors attributed these outcomes to the fact that the size of microplastics was larger as compared to the chorionic pore canals and the resistance of the chorionic barrier to polylactic acid microplastics.
Very recently, Duan et al. (2022) compared the accumulation and toxicity of polylactic acid and poly(ethylene terephthalate) microplastics using zebrafish (Danio rerio) as a model organism. The authors reported 170 times higher polylactic acid microplastics in the fish as compared to poly(ethylene terephthalate) microplastics resulting in intestinal epithelial tract damage followed by affecting the diversity of intestinal microbiota. The authors attributed these results to the depolymerization of polylactic acid in the digestive tract of fish, which decreased the intestinal pH and changed the carbon source structure. These results are quite interesting in understanding the toxicity of polylactic acid microplastics. These findings strongly support the concept that polylactic acid microplastics will have severe effects, similar to petroleum-based plastics, on the exposed individuals.
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