Polycaprolactone Composites/Blends and Their Applications Especially in Water Treatment

Abstract

Biodegradable poly(ɛ-caprolactone) (PCL) and its composites or blends have received a lot of attention in the last decade because of their potential applications in human life and environmental remediation. Greater efforts have been made to develop biodegradable chemical materials as adsorbents that do not pollute the environment in order to replace traditional materials. Among the numerous types of degradable materials, PCL is currently the most promising, the most popular, and the best material to be developed, and it is referred to as a “green” eco-friendly material. Membranes and adsorbents for water treatment, packaging and compost bags, controlled drug carriers, and biomaterials for tissues such as bone, cartilage, ligament, skeletal muscle, skin, cardiovascular and nerve tissues are just some of the applications of this biodegradable polymer (PCL). The goal of this review is to present a brief overview of PCL, syntheses of PCL, its properties, PCL composites, and PCL blends and to provide a detailed investigation into the utility of PCL/PCL-based adsorbing agents in the removal of dyes/heavy metal ions. Overall, it can be confirmed that PCL blends and composites were found to be significant competitors to other well-known adsorbents in the treatment of wastewaters, necessitating a thorough investigation of their manufacture.

1. Introduction

In recent years, various polymers have been employed to create composites, blends, and membranes for water treatment and remediation [1]. However, some of these polymers are not biodegradable and, since they are not properly disposed of in the environment, this is another issue for waste management [2]. Biodegradable polymers, which decompose easily, may be the best alternative for these materials. Biodegradable polymers include both natural polymers and synthetic polymers, as classified in Figure 1 [3,4,5]. PCL is a widely recognized biodegradable and biocompatible petroleum-based polymer with numerous applications in packaging, scaffolds, prosthetics, sutures, drug delivery, films, carry bags, pouches, trays, reusable dishes, membranes, and other fields. It can be degraded without polluting the environment by hydrolysis of ester bonds or by microorganisms [6]. It is also easily blended with other polymers and is soluble not in water but in a number of different organic solvents [7]. In addition to the applications mentioned above, PCL can be used in water treatment plants as it is a biocompatible, biodegradable and environmentally friendly polymer.

However, reinforcing fillers are required to improve the performance of the PCL matrix by increasing its mechanical, chemical, and thermal properties, as well as to broaden its application to other fields [8]. As a result, PCL was investigated by interacting with various loading agents. In particular, studies of mixtures of cellulose, chitosan, alginate, metal oxides, zeolites, and biodegradable polymers to obtain blends or composites to resolve some of the noted drawbacks of PCL have been reported [9]. Besides the fillers added to PCL, PCL itself can be used as filler. Some properties are also expected of PCL reinforcing fillers. For instance, low-cost, biodegradable, renewable resources and high strength, low density, and easy-to-process materials are desired. It is also desirable to have a homogeneous mixture or powerful interplay between the polymer matrices and the reinforcing agents. Because of their dissimilar properties, these materials can be difficult to interact with each other. To give an example of this dissimilarity, cellulose fibers are difficult to dissolve and disperse in the PCL matrix because of the hydrophilicity of cellulose fibers and the hydrophobicity of PCL [10]. In order to improve the cellulose fiber dispersion in PCL, chemical treatments are applied to the fiber surface to increase its chemical and physical interaction with the PCL matrix. This problem is solved by applying chemical treatments to the fiber surface to improve the cellulose fiber dispersion in PCL. This improvement in compatibility provides better interface interaction that improves composite mechanical performance [11]. In short, when PCL is combined with other polymers, it changes their biodegradability and mechanical properties [12]. The majority of literature reviews have focused on PCL applications, varying from sutures [13] to wound dressings [14], artificial blood vessels [15], nerve regeneration [16], bone engineering [17], food packaging systems [18], membrane [19] and drug delivery devices [20]. One of the biggest issues and challenges in the modern world is how to completely clean wastewater without endangering the environment or other living things by leaving any organic or inorganic dyes or heavy metal ions behind. Even in extremely small amounts, toxic heavy metal ions and dyes in wastewater have a harmful impact on living things. It is unavoidable to employ PCL blends and composites that are safe for the environment, do not produce waste after usage, and provide efficient removal outcomes. There is, however, no systematic and up-to-date review of PCL blends and composite applications for heavy metal ions’ and dyes’ removal from wastewater.

Several studies have taken place over the last decade to explore the possibility of improving PCL properties through the use of other biopolymers and metal oxides for removing pollutants from aqueous solution. Keeping the preceding discussion in mind, the present review article has been written to comprehensively summarize recent applications of PCL blends and composites, especially in dye and heavy metal removal. Besides water remediation, PCL blends and composites are inevitably important in biomedical applications, such as tissue engineering, drug release and wound healing. Therefore, some of the applications in this field are also mentioned. Meanwhile, the synthesis, degradation, mechanical properties, and some other applications of PCL will be briefly mentioned.

2. Chemical Synthesis of PCL

PCL is a non-hazardous and non-biobased aliphatic polyester [21]. Commercially, PCL is produced from crude oil via two ways: (i) polycondensation of 6-hydroxyhexanoic acid (C₆H₁₂O₃) [22] and (ii) ROP of ε-CL (C₆H₁₀O₂) monomer with tin octoate, transition metal alkoxides or their derivative catalysts [23]. If the catalyst does not act as the initiator, low molecular alcohols are employed to help regulate the molecular weights that define the polymer’s properties [24]. For instance, PCL with a molecular weight (M_w) of around 80,000 Da has plastic properties, whereas PCL with M_w of 10,000 Da is inherently very hard and brittle [25].

PCL is synthesized in the first method through polycondensation reactions of 6-hydroxyhexanoic acid in the presence of suitable catalysts and continuous removal of water under vacuum. The synthesis reaction is typically completed in a few hours at temperatures ranging from 80 to 150 °C. This synthesis method typically yields a low molecular weight with a broad polydispersity index and low quality polymer (Scheme 1) [22].

ROP, on the other hand, is preferred in industry for PCL production because it provides controllable polymerization, a higher molecular mass, and a lower polydispersity index polymer. Anionic, cationic, and coordination-insertion catalysts are the three types of catalysts used in the ROP of ɛ-caprolactone. Because these three types of catalysts work in different ways, they result in the formation of polymers with varying structures and sizes.

As catalysts for ɛ-caprolactone anionic ring-opening polymerization, various alkali metal salts (t-BuOK, i-PrOK, KH, PhLi, and i-Pr₂NLi, among others) were used [26]. The ring opens solely in the acyl-oxygen position in anionic polymerization of ɛ-CL, and the species produced by the growth are alkoxide anions. The major disadvantage of this process is the presence of significant intramolecular transesterification in the final stage of the polymerization, also known as “backbiting”. If polymerization is not stopped before backbiting, the resulting polymer has a smaller molecular weight (M_w) or a cyclic structure. Anionic polymerization, on the other hand, allows for the controlled synthesis of high M_w polymers in a polar solvent.

Cationic ROP includes the creation of a positively charged species that is attacked by the monomer’s carbonyl oxygen. The attack causes the positively charged species to ring-open via SN₂-type mechanisms [27,28]. Cationic polymerization is a difficult method to control and frequently results in low molecular weight polymers or oligomers. Cationic polymerization can be carried out using catalysts such as cationic hetero-scorpionate aluminum complexes, mono- and bis(phosphino-phenolate) aluminum cationic complexes [29], Trityl tetrafluoroborate (Ph₃CBF₄) [30], and iron (III) chloride (FeCl₃) or FeCl₃/nH₂O systems [31].

The last mechanism is coordination-insertion ROP, which is a quasi-anionic ROP [32]. As illustrated in Scheme 2, the ɛ-caprolactone monomer first binds to the metal center through the carbonyl (C=O) oxygen atom. The catalyst’s nucleophilic alkoxide group is then coordinated to the activated carbonyl group of ɛ-caprolactone, resulting in cleavage of the acyl–oxygen bond. Following ɛ-caprolactone monomers bind to the metal center and insert into the metal–alkoxide bond, resulting in PCL formation. Some of the most commonly used catalysts in ROP can be listed as tin octoate [33], metal alkoxides including aluminum [34], tin [35], zirconium [36], or titanium [37], rare earth metal alkoxides and their derivatives containing coordinated Lewis bases (Ln(OR)₃(L)_n) (L = 2,2′-bipyridyl, n = 2, 1,10-phenanthroline, n = 1, 18-crown-6 ether, n = 1) [38], transition metal(TM)-carboxylates as in zinc (3,5-dimethylpyrazole)carboxylates [39], TM-phenolates like titanium compounds (LTiX₂) bearing a sulfide-linked bis(phenol) ligand (L = OSSO type ligand) [40,41], TM-Schiff base complexes such as (LLa(OC₆H₂-2,6-^tBu₂-4-CH₃)(THF)), (LLaO^tBu(THF)), [LLaOⁱPr]₂, (LLaOBn)₂, and (LLaOEt)₂ (L = NH(CH₂CH₂N=CHC₆H₂-3,5-^tBu₂-2-O)₂) [42]. ɛ-CL was recently initiated by exfoliated oxidized biochar (Eoxbc). The surface hydroxyl groups (OH) on Eoxbc are enough to start and promote the ROP of ɛ-CL using stannous octoate, bases or enzymes as catalysts [43].

Some spectroscopic properties of PCL can be summarized as in

Table 1. Spectroscopic data related with PCL [37].

1H-NMR results (in CDCl3)	δ(ppm): 4.1 (t, J = 6.7 Hz, εCH2O), 2.3 (t, J = 7.5 Hz, αCH2), 1.6 (m, J = 7.3 Hz, β,δCH2), 1.4 (m, J = 7.3 Hz, γCH2)
13C NMR results (in CDCl3)	δ(ppm): 174 (C=O), 64 (εCH2O), 34 (αCH2), 28 (δCH2), 26 (βCH2), 25 (γCH2). [O=C-αCH2βCH2γCH2δCH2εCH2O-].
FTIR results (cm−1)	2936 (CH, asym str), 2865 (CH, sym str), 1723 (C=O, asym str), 1472, 1418, 1394, 1366 (C=O, sym str) 1288, 1240 (C–O–C str vib), 1162 (C–O str vib), 1100, 1046, 1018 cm−1 etc.

.

Of all the synthesis methods, the coordination-insertion ROP method is the most common and suitable process for the polymerization of ɛ-caprolactone. PCLs obtained by this method have high molecular weights and narrow polydispersity indices.

3. Degradation of PCL

PCL biodegrades depending on the amount of crystallinity, polymer molecular mass, and other degradation parameters, such as environment, temperature, pH, and salinity [25]. Because there are more hydroxyl groups present at higher pH, the rate of degradation will therefore be higher. However, the existence of hydrophobic CH₂ units as the recurring moiety in PCL significantly hinders its degradation kinetics, leading to a notably protracted degradation timeline, typically spanning over 2–3 years.

PCL undergoes degradation through two common pathways: (i) enzymatic degradation (also called surface erosion mechanism) and (ii) hydrolytic degradation (also known as bulk erosion mechanism) (Figure 2) [25,44].

Surface enzymatic erosion degrades PCL with significant mass loss but very little change in molecular weight. Surface erosion of PCL refers to the hydrolytic degradation of the polymer backbone solely at the PCL surface, wherein the speed of hydrolytic chain scission and the synthesis of monomers and oligomers are faster than the speed of water penetration within the polymer bulk, leading to polymer thinning without observable impact on its molecular weight. In other words, surface erosion is expected if bond hydrolysis happens faster than water diffusion.

The enzymatic degradation is primarily carried out by the enzyme lipase (which can be found in both fungi and bacteria) at chain ends, folds, and edges of crystals, in which chain mobility is greater [25]. The enzyme works by cleaving PCL ester bonds. Khan et al. investigated the PCL degradation adequacy of three different Lactobacillus lipases and discovered that PCL degradation depends on enzyme concentration and type [45]. Aris et al. investigated the hydrolysis of PCL (M_n: 10,000 Da) by Candida antarctica lipase B (CALB) in organic solvent and discovered that rising PCL concentration decreased degradation rate while increasing reaction temperature from 30 to 50 °C increased it [46]. Ma et al. used Candida antarctica lipase to degrade PCL films with three distinct molecular weights, and the findings indicated that lipase activity was unaffected by the molecular weights of the PCL films. However as the molecular weight increased, more pores appeared in the degraded PCL films, and as the degradation time lengthened, the pores grew deeper [47].

Hydrolytic degradation (bulk erosion) reduces the molecular weight of PCL while causing minor mass loss. This degradation mode involves the penetration of water throughout the entirety of the polymer bulk, instigating hydrolysis and consequent random hydrolytic chain scission that eventually leads to a reduction in molecular weight [48]. Since the cleavage occurs in the non-crystalline region, the crystallinity of the polymer increases. The bulk of the polymer is diffused out by the monomers or oligomers, causing slower polymer erosion and more pronounced bulk degradation. As a result, internal polymer degradation is faster than surface polymer degradation. In other words, bulk erosion occurs when the speed of water diffusion surpasses the speed of bond hydrolysis. Bulk erosion has been reported in aliphatic polyesters such as PCL. Although mass loss is low in this type of erosion, cracks form in the sample, which threaten mechanical functionality [48].

Under controlled conditions, the biodegradability of pure PCL, its blends and nanocomposites were investigated in different enzymes, pure microorganism (fungi), organic manure, Ganges water, and basic buffer solution. The rate of PCL biodegradation in nanohybrids has increased significantly and is highly dependent on the working environment and the chemicals used [49]. The pH of the system affects the rate of degradation. PCL degrades faster in an alkaline environment than in an acidic environment [48]. These enzymatic and hydrolytic degradations produce polymers with a lower molecular weight (3000) and 6-hydroxycaproic acid, resulting in smaller particles. As a result, polymer biodegradation produces fragmentation compounds and is considered complete when H₂O and CO₂ are produced by microbial action, as seen in Figure 3 [49,50].

Because of its hydrolytic nature, PCL’s slow degradation rate can be accelerated by adding other polymers or degradation catalysts. Polymers such as lactide, glycolide, succinic acid, and other natural fibrous materials can be employed to improve the degradation rate [51,52].

4. Material Properties of PCL

Polycaprolactone (PCL) is a semicrystalline aliphatic polyester which has glass transition temperature (T_g) of −60 °C, a melting point between 58 and 64 °C depending on its crystalline structure, and degradation time of approximately 2–3 years [53]. PCL is extremely tough because of the presence of amorphous regions in the rubbery state. The low melting temperature of PCL and its depolymerization at high temperatures (>200 °C) are the main disadvantages for its utilization, which can be overcome by cross-linking or blending with the other polymers [54]. PCL can be graded according to molecular weights and shows different physical and mechanical characteristics depending upon molecular weights. For examples, PCL with a molecular weight of about 40,000 Da is hard and extensible, while PCL with a molecular weight of between 10,000 Da (PCL-300) and 5000 Da (PCL-150) is pretty hard and brittle. In the industrial setting, PCL-700 is preferred for blending because the end functional groups play a significant role in affecting blendability and compatibility when the molecular weight is reduced [25,55]. The crystallinity of PCL tends to decline with increasing molecular weight. PCL’s great solubility in some organic solvents, low melting point and exceptional blend-compatibility has sparked in-depth investigation into its potential use in the biomedical area [56].

PCL’s mechanical, physical, thermal, biodegradable, and biocompatible properties are entirely determined by its crystallinity and molecular weight. Because of its superior rheological and viscoelastic properties, the Food and Drug Administration (FDA) has approved its use in a variety of long-term degradable medical appliance and implants. The medical device industry was searching for biodegradable PCL to replace metal implants (such as nails, screws, plates, etc.) [57]. PCL crystallization and morphologic properties are extremely important. PCL is a relatively hydrophobic polymer with a variety of properties, as shown in

Table 2. A list of PCL’s mechanical and physical characteristics [53,58,59].

Properties	Values
Molecular weight, Mn, g/mol	3 × 103 to 8 × 104
Density, g/cm3	1.07 to 1.20
Melting temperature (Tm), °C	56 to 65
Glass transition temp. (Tg), °C	−65 to −60
Decomposition temp. (Td), °C	350
Degradation time, years	2–3
Inherent viscosity, cm3/g	100–130
Intrinsic viscosity, cm3/g	0.9
Tensile strength, σm, MPa	4–785
Tensile elastic modulus, Et, MPa	250–440
Yield stress, σy, kN/m2	8.2 × 103–1.1 × 104
Young modulus, MPa	210–440
Elongation at break, %	20–1000
Conformation	Extended and zigzag
Degree of crystallinity	Semicrystalline (50%)
Water vapor permeability, g/m2·day	783
Oxygen transmission rate, m3/m2·day	486

[53,58,59]. PCL’s lower glass transition (T_g) and melting (T_m) temperatures make it easy to form at low temperatures. In some organic solvents, such as benzene (C₆H₆), carbon tetrachloride (CCl₄), chloroform (CHCl₃), cyclohexanone (C₆H₁₀O), methylene chloride (CH₂Cl₂), isonitro-propane (C₃H₇NO₂), and toluene (C₇H₈), PCL is quickly soluble at RT. However, it dissolves poorly in solvents like acetone(C₃H₆O), acetonitrile (C₂H₃N), 2-butanone (C₄H₈O), dimethyl formamide (C₃H₇NO), and ethyl acetate (C₄H₈O₂) [44,60,61]. Heating to 50 °C can improve its solubility in these solvents. PCL is insoluble in water (H₂O), ethyl alcohol (C₂H₆O), diethyl ether (C₄H₁₀O) and petroleum ether (C₆H₁₄) [60].

When the mechanical values in Table 2 are examined, it can be seen that PCL exhibits noticeably low tensile strength and modulus, but a high elongation at break. Because of its physical and mechanical characteristics, miscibility with many different polymers, biodegradability, and ability to form composites, PCL is an important bioplastic polymer. It can be combined with other plastic polymers and oxide materials to improve adhesion, dyeability, and stress crack resistance. As seen in Figure 4, bioplastics are divided into three categories. Of these classifications, biopolymers that are both petroleum-based and biodegradable are poly(ɛ-caprolactone) (PCL), poly(butylene-adipate-co-terephthalate) (PBAT), and poly(vinyl alcohol) (PVA). Bio-based and biodegradable ones are poly(lactic acid) (PLA), poly(hydroxy alkanoate), and (PHA), poly(butylene succinate) (PBS). Non-biodegradable examples of bio-based and petroleum-based are polyethylene (PE), polyamide (PA), poly(trimethylene terephthalate) (PTT) and polypropylene (PP), poly(ethylene terephthalate) (PET) [62,63]. Some polymers, such as polyethylene, can be both bio-based and petroleum-based.

5. PCL Blends/Composites

5.1. PCL Reinforced with Other Polymers

Polymer blends have been employed as a substituent because of the low mechanical properties, low thermal stability, low adsorption capacity, and hydrophobic properties of homo-PCL [64,65]. Numerous studies have shown that combining PCL nanofibers with other polymers, such as cellulose, nitrocellulose, cellulose butyrate, chitosan, chitin, alginate, and pectin, improves its physical properties. Cellulose is a low-cost, widely available material that is both renewable and biodegradable. The researchers looked into how cellulosic filler type and ingredients affected the mechanical characteristics of PCL-based composites [66]. The mechanical characteristics of PCL composites were found to be modulated by multiple interdependent factors, including chemical suitability, interfacial strength, filler dispersion, and padding aspect ratio [67]. Because of the hydrophilic surface of the cellulose-based nano reinforcement particles, the interaction between the template and the padding is poor. Chemical suitability is critical in managing dispersion and adhesion between them. As a result, weak padding–template interactions are common when hydrophilic paddings, like cellulose nanocrystal (CNC), cellulose nanofiber (CNF), and bacterial nanocellulose (BC), are added to hydrophobic templates (PCL, PLA, etc.). These issues, however, can be mitigated by a variety of surface adjustments [53].

In terms of thermal stability and mechanical properties, PCL nanocomposites strengthened with cellulose nanocrystals (CNCs) (surface modified by covalent grafting or physisorption) performed better than unfilled PCL and nanocomposites with unmodified CNCs. PCL strengthened with grafted CNCs had a higher degradation temperature than micelle-modified CNCs which had a higher degradation temperature than latex-adsorbed CNCs with a long PBMA chain length. In terms of thermal and mechanical characteristics of the final nanocomposite, the finding obviously show that covalent grafting outperforms physisorption. This one-of-a-kind study will be extremely useful in the future design of PCL/CNC-based composites with customized features [68]. The morphological analyses performed revealed a reasonable distribution of cellulose nanofibers (CNFs) within the PCL template, as well as high reliability among the fabricated scaffolds. Remarkable impacts on surface wettability and thermal features have been observed for scaffolds. In terms of mechanical features, reinforced scaffolds had higher scaffold stiffness [69].

The cellulose acetate nanofiber (CANF) and the cellulose nanofiber (CNF) were produced as reinforcing materials for PCL. CANF was prepared by electrospinning and CNF was obtained by the deacetylation of the electro-spun CANF. The CANF and CNF were then compressed separately into PCL as reinforcing compounds to yield CANF/PCL and CNF/PCL composites. The fracture-surface SEM micrographs showed that CANF is more compatible with the PCL matrix than CNF. The mechanical features of CANF/PCL and CNF/PCL were investigated, and it was discovered that both the fracture stress and the Young’s modulus were extremely improved over pure PCL. Pure PCL had a Young’s modulus (E) of about 250 MPa. More specifically, the fracture stress and Young’s modulus of PCL increased by factors of 1.9 and 3.0 (E: 750 MPa), respectively, when 35 wt% of CANF was added to PCL. By adding the same amount of CNF, they became 2.6 and 4.5 (E_CNF/PCL: ~1100 MPa) times higher, respectively, by comparison to those of pure PCL, due to the extremely high Young’s modulus of CNF [61]. Successful ring-opening copolymerization was performed by adding 4-dimethylaminopyridine as a catalyst to tetrabutylammonium acetate/dimethyl sulfoxide solvent mixtures, and microcrystalline cellulose grafted PCL (MCC-G-PCL) was produced. By adding graphene oxide (GO), a new UV-shielding film based on MCC-g-PCL was created. This study offers a framework for creating UV-shielding polymers based on cellulose and for better comprehending the UV-shielding mechanism [70].

Twin-screw extrusion was used to create PCL/CNC nanocomposites at various CNC contents. Using a microcellular injection molding technique and CO₂ as the blowing agent, the nanocomposites were foamed. FTIR and Raman measurements revealed that the PCL matrix and CNCs inter-reacted, creating a robust interface between the two. In the foaming process, the CNCs productively served as nucleating agents, resulting in smaller pores and higher pore density with a higher CNC ingredient. PCL/CNC nanocomposites with 1% CNC showed higher yield stress (16.50 MPa for PCL and 17.40 MPa for 1% CNC), tensile modulus (~184 MPa for PCL and ~200 MPa for 1% CNC), complex viscosities, and higher storage moduli because of the strengthening impact of CNCs. The adding of CNCs resulted in a decrease in the decomposition temperature (T_d, 411.4 ± 0.4 for PCL and 409.3 ± 0.7 for 1% CNC) but an increase in the glass transition temperature (Tg, 2 °C higher than that of pure PCL), crystallization temperature (Tc, from 34.2 ± 0.4 to 35.6 ± 0.9 °C), and crystallinity (χ_c, from 39.0 ± 0.8 to 40.6 ± 1.0%) of PCL. The findings suggested that, at 0.5% and 1% CNC contents, the nanocomposites were appropriate for cell growth, while at 5% CNC contents, cell death increased, primarily because of the sulfonic acid groups on the CNCs. The biocompatibility of the nanocomposites would be enhanced by further CNC modification [71].

Hashiwaki et al. used hot-press molding to create thermoplastic blend films made of PCL and chitin butyrate (ChB) (50:50) [72]. The film properties changed depending on how much chitin was butyryl substituted. They tested various degrees of butyryl substitution with chitin, which had a 5% degree of deacetylation, and found that high degrees of substitution led to improved miscibility with PCL. The PCL–ChB ratios can be changed to alter the mechanical stability of the films utilized as thermoplastic scaffolding substance. Especially, the partially miscible blend displayed the highest ductility attributable to a balance in the size and dispersion of the PCL microcrystals and the domain size of the blend components. This blend structure strengthens the physical and chemical features of the two materials by combining them [73].

PCL and chitosan (CS) are the two polymers most frequently encountered in biomedical applications. Sani et al. (2021) proposed graft work to solve the problem of finding co-solvents, which is the biggest challenge in PCL/CS blending studies in their research. PCL/CS graft polymer instead of blend was synthesized and used in the production of bone tissue engineering scaffold. In addition, hydroxyapatite nanoparticles (nHAP, 1, 3, 5 and 10% w/w) were added to the PCL/CS graft polymer to enhance the bioactivity and mechanical features of the scaffold. Elongation at break decreased significantly from 546 ± 24% for 1% nHAP to less than 100% for specimens including 3, 5, and 10% w/w nHAP. Tensile test in bulk and fiber state revealed that the porosity, fiber diameter and distribution had an important consequence on mechanical features of graft polymer. Besides, the mineralization and deterioration of the scaffold immersed in simulated body fluid (SBF) and phosphate-buffered saline (PBS) are accelerated by both CS-g-PCL and nHAP [74].

The thermoplastic waxy starch (TPS) phase mixed with PCL, 70/30 (w/w), was combined with cellulose nanocrystals (CNCs) as reinforcing agents by higher shear rate co-rotating extrusion. In terms of TPS mass, nanocomposites were created using 3.25%, 6.5%, and 13% by weight CNCs. In comparison to the TPS/PCL matrix, the nanocomposite with only 3.25 wt% CNCs increased the tensile strength by at least three times and the elastic modulus by more than six times, reaching values close to 12 MPa and 1.0 GPa, in order. Furthermore, the lower number of CNCs ensured the thermal stability of the TPS/PCL nanomaterials (250 °C), with a 20 °C increase in T_onset compared to the neat matrix [75].

Peterson et al. carried out the synthesis of block polymer beads by PVA(aq) suspension polymerization to produce PCL-b-poly(styrene-co-divinylbenzene) beads, which can be electively etched under basic environments to obtain mesoporous polymer microspheres with constant pore size. The resulting microphase separation method produced a nanostructured dual-continuous morphology. The particle size (from 60 to 300 μm) by stirring speed and pore size (from 6 to 11 nm) by the molar mass of the PCL block polymer were independently tunable: BET measurement showed that the beads’ surface area was approximately 300 m² g⁻¹ when using a PCL macro-chain transfer agent (macroCTA) of 13 kg mol⁻¹. The beads formed were all on the micrometer length scale and did not have an impermeable skin layer. They also succeeded in incorporating functional pore walls into the beads utilizing multiblock starting polymers. These functionalized beads showed high interest for organic dyes such as toluidine blue and methyl red in aqueous solution and captured dye from solution at rates faster than that of mercantile ion exchange resins [76].

Electrospinning was used to create a PCL super-amphiphobic perfluorooctyl acrylate (PCL-b-PTFOA) nanofiber film with a hierarchical porous surface. Because of its hierarchical pore morphology and low surface energy composition, the PCL-b-PTFOA nanofiber membrane could repulse a variety of solutions comprising water, coffee, and oil solutions. When compared to the PCL nanofiber membrane, the elongation at break of the PCL-b-PTFOA increased from 152.97% to 678.15%, the tensile strength increased from 4.11 to 5.78 MPa, and the membrane’s super amphiphobicity did not end up losing, with a strain of up to 600% [77].

5.2. PCL Reinforced with Metal Oxides

Membrane separation processes have played very important roles in water treatment plants. Membranes were made of biocompatible, non-toxic, and environmentally friendly materials, such as PCL and some natural polymers. Nano or microfiber (2–6 µm) electro-spun membranes combining PCL (15% w/v) with zeolite (20 w/v) were fabricated by using the multilayer electrospinning technique and used for water treatment processes. The resulting membrane (PCL/zeolite) had an average contact angle of 119.53 ± 5.24 degrees. Angles greater than 90 degrees indicate that the membrane has a low wetting tendency, i.e., it is hydrophobic. PCL membranes are expected to retain their mechanical entirety as PCL for many years before they fully degrade [78].

Sustainable PCL composites were produced by using exfoliated oxidized biochar (Eoxbc) as a bio-based filler. In the existence of both Eoxbc and Sn(Oct)₂, the conversion of ɛ-caprolactone to PCL increased from 55% to 88%, but the molecular weight of PCL decreased from 31,400 Da to 13,000 Da under the same reaction conditions. When compared to pure PCL, the novel materials created here have higher percentages of crystallinity and stiffness. At a comparatively high maximum temperature (T_max) of 288 °C, neat PCL only exhibits one primary degradation step. But compared to the neat polymer, PCL/Eoxbc composites had lower T_max and were consequently less thermally stable. In this study, the researchers presented the first attempt to enhance PCL’s degradability and mechanical qualities through the use of a sustainable strategy that incorporates biochar [43].

Huang et al. fabricated a new clay-enhanced PCL/chitosan/curcumin (PCL/Clay/CS/Cur) composite film and measurements with instrumental techniques revealed successful clay dispersion. The morphology of clay-fortified Cur-loading films was discovered to be porous and spongy with discrete structures. The tensile strength was found to be significantly increased by the addition of nano-clay, from 1.94 to 11.81 MPa. Additionally, compared to membranes without clay, the drug-loading films with clay showed better controlled-release profiles than curcumin. Skin disinfection tests and SEM measurements revealed that almost no bacteria were present on the wound covered with a PCL/Clay/CS/Cur film, proving that the film was successful in preventing bacterial (Staphylococcus aureus) infection [79].

Karimian et al. synthesized the PCL/cellulose nanofiber nanocomposite containing ZrO₂ nanoparticles (PCL/CNF/ZrO₂) in three different ZrO₂ amounts (0.5, 1, 2%). TGA, DTA, and DSC results showed that the nanocomposite has more thermally stable (T_d changed from 367.69 to 377.83 °C for 1% ZrO₂) and strength properties than PCL. This nanocomposite was used for wound dressing bandage and exhibited moderate to good antimicrobial function opposed to all tested bacterial (S. aureus and E. coli) and fungal strains (C. albicans) with an inhibition zone between 6.5 and 9 mm [80].

Electro-spun PCL membranes with an average diameter of 1.02–1.76 μm were created by Ramirez-Cedillo et al. by adding microparticles or nanoparticles, like Ag, TiO₂, and Na₂Ti₆O₁₃. These membranes improved bacterial resistance, cell adhesion, and proliferation. Compared to the membranes of PCL/TiO₂ (60.8 °C) and PCL/Na₂Ti₆O₁₃ (57.8 °C), the PCL/Ag membrane showed the lowest endothermic peak, indicating the melting point (Tm) at 55.57 °C. Strength of PCL/TiO₂ increased from 0.6 MPa to 6.3 MPa compared to PCL without particles. One could argue that the Na₂Ti₆O₁₃ electro-spun composite is better suited for tissue regeneration processes [81].

PCL composite membranes reinforced with TiO₂ nanoparticles (NPs) were produced using two different solvents, acetic acid and a mixture of trichloromethane (CHCl₃) and N,N-dimethylformamide (DMF). The PCL/TiO₂ membranes prepared with CHCl₃-DMF had better surface homogeneity than those prepared with acetic acid, according to experimental measurements of their physicochemical properties. It was discovered that the compound crystallization changed as the TiO₂ NPs content increased. Thus, the pore structures of the PCL membranes as well as their physical characteristics, including strength, ductility, surface roughness, and contact angle (80–103°), were affected by the concentration of TiO₂ NPs and the solvent’s characteristics. The membrane samples developed with acetic acid had the highest roughness values (lateral resolution ~11 μm for acetic acid, 6 μm for CHCl₃-DMF solution). Therefore, it can be said that PCL and TiO₂ nanoparticles interact well with the CHCl₃-DMF solvent [82].

Scaffaro et al. reported a rapid method for designing multi-scale nanohybrid structures (GO-CNT@PCL) that include a 3D fibrous PCL network encircled by graphene oxide (GO) layers that contain carbon nanotube (CNT) brushes with π–πstacking. The process was based on the electrospinning of PCL solutions onto a suspension of GO and CNTs in ethyl alcohol. The hierarchical architecture and nano-patterned surface allow the initial characteristics of PCL, GO, and CNTs to be aggregated into ultra lightweight (99% porosity), but robust (1575% hardness enhancement) and amphiphilic (both hydrophilic and hydrophobic) monoliths. Some physical properties of this nanohybrid material such as surface tension (28.6 mN/m) and electrical conductivity (29.3 μS/cm) were better than pure PCL (27.3 mN/m, 0.7 μS/cm). These nanohybrid monoliths removed MB and MO from aqueous solution in an approximately 100% yield [83].

6. Applications of PCL Blends/Composites

6.1. Pollutant Removal Applications of PCL Blends/Composites

6.1.1. Dye Removal by PCL Blends

Some polymers, particularly those with efficient functional groups that are insoluble in water, have been employed as adsorption agents for the removing of dyes and heavy metal ions. Polymers’ ability to remove pollutants is determined not only by their active functional groups, but also by their structure, surface area, and porosity. Because of its limited adsorption capacity, low mechanical qualities, and water-insoluble hydrophobic features, pure PCL is ineffective as an adsorbent. Because of these poor qualities, polymer blends or composites of PCL have been employed as excellent adsorption agents for the elimination of dyes from aqueous solution in place of pure PCL. Blending a hydrophobic polymer (PCL) with functional polymers, such as cellulose, chitin, chitosan, lignin, and alginate, at varying ratios is significant for enhancing adsorption active sites and lowering costs. When compared to other polymer classes, polymer nanofibers offer unique features, like high water permeability, high porosity and surface area. As a result, converting ɛ-caprolactone polymers into PCL fibers can be another successful approach for dye removal [84].

For instance, Guo et al. fabricated novel composite fiber adsorption materials with different mass ratios consisting of PCL and beta-cyclodextrin-based polymer (PCD) by electrospinning. The fiber materials obtained showed exceptional elective adsorption for MB and 4-aminoazobenzene (AAB) solutions. Addition of a PCD constituent to composite fibers improved the mechanical resistance (breaking stress 3.41 MPa for PCL/(50%)PCD) of membranes and altered the sorption uptake because of the molecular structure of the cavity through host–guest interaction. The specific surface area values rose substantially along with increasing of PCD constituent in the composite fibers, reaching 11.5 m²/g for PCL/(50%)PCD composite fibers (7.50 m²/g for neat PCL fibers), suggesting the development of additional anchoring sites aiding the subsequent adsorption of dye compounds. The dye removal efficiency reached from 7.44 mg/g to 24.1 mg/g for AAB and from 3.89 mg/g to 11.2 for MB. Because of the excellent stability and selectivity of the adsorption process, the currently produced beta-cyclodextrin-based composite fibers (average diameter of 200–400 nm) have shown potential large scale uses in dye capture and wastewater remediation [85].

Many investigations are being conducted to find effective and ecologically acceptable techniques for degrading phenols and their derivatives. In order to create novel types of biocatalytic systems capable of removing bisphenol A, an attempt was made to generate PCL-chitosan electro-spun materials for tyrosinase immobilization. SEM scans validated the shape of the fibers and enzyme deposition. After enzyme deposition, fiber diameter increased significantly from 344 ± 121 nm to 689 ± 365 nm, demonstrating enzyme adhesion to the fiber surface. The constructed system was used in batch tests including the biodegradation of bisphenol A under different remediation circumstances. Over 80% of the pollutant was eliminated after 120 min of processing at temperatures ranging from 15 to 45 degrees Celsius and pH levels ranging from 6 to 9, using solutions containing up to 3 mg/L. After 30 days of storage, the immobilized biomolecule tyrosinase retained around 90% of its original activity (60% for free enzyme) and was still capable of removing more than 80% of bisphenol A, even after 10 repeated usages [86].

Chitosan-based nanofibrous membranes for dye adsorption from solution were created using hybrid electrospinning of chitosan (CS) and PCL or PCL-block-poly(ethylene glycol) (PCL-b-PEG or abbreviated as P) amphiphilic copolymers. PEG segments were shown to be capable of controlling the secondary fiber and core-shell structure. The addition of PEG component enhanced the compatibility of PCL and CS, decreased CS loss because of intermolecular hydrogen bonds formed between CS and PCL-b-PEG copolymer, and increased membrane stability. The adsorption studies revealed that the anionic CR dye adsorption process on CS-based membranes fits well with the pseudo-second order and the Langmuir isotherm models. The CS-based membranes (CS/P_5K, diameter of ~807.89 nm) with a PEG molecular weight of 5 kDa had high reusability, with highest adsorption capacity of 291.55 mg/g computed using the Langmuir model. The CS/P_5K removal effectiveness was maintained at 90.18% and dropped to 80% after 10 repetitions, demonstrating that the CS/P_5K nanofibrous membrane can be reused for dye removal. As a result, CS/PCL/PEG nanofibers appear to be promising membranes for water purification [87].

Amphiphilic PCL-PEG-PCL/Bentonite A2 nanocomposites were synthesized by in situ ROP of ε-CL in the existence of PEG chains, catalyzed by an intercalated Algerian Bentonite tetrabutylammonium hydrogen sulfate (TBHSA) (A1). For this purpose, sodium cations in bentonite were exchanged by tetrabutylammonium cations [(C₄H₉)₄N⁺]. The XRD and transmission electron microscopy (TEM) measurements revealed that the silicate sheets in the PCL-PEG-PCL/layered silicate nanocomposites were partly exfoliated. The nanocomposite (A2) was employed to efficiently remove MB dye from aqueous medium. As the mass of the nanocomposite rose, so did the dye’s adsorption capacity, which reached an ideal value of 0.13 g adsorbent at pH = 6.8. Calculating an adsorption capacity (q_max) of 600 mg/g within 90 min, the Langmuir isotherm provided the best match [88].

Karagöz et al. designed and manufactured electro-spun PCL nanofibers (NFs) adjusted with TiO₂ and Ag NPs. The as-prepared PCL/TiO₂-Ag NFs mats were created using one-step electrospinning and used for three separate applications: (a) reusable surface-enhanced Raman spectroscopy (SERS) substrate for non-qualitative analysis to track pollutants, (b) photocatalyst for organic pollutant degradation, and (c) antibacterial agent to kill bacteria. For the detection of MB dye, PCL/TiO₂-Ag NFs performed as a highly efficient SERS platform with a detection limit of 10 nM (MB). The photocatalytic degradation of the probe analytes MB under UV irradiation was completed in less than 180 min with degradation efficiency 93–95% using a PCL/TiO₂-Ag NFs nano-catalyst [89].

A membrane having a greater hydrophilic nature is predicted to be more fouling resistant. The biodegradable nature of PCL and the anti-fouling property of TiO₂ were combined in a composite and used in the construction of water purification membranes. TiO₂ nanoparticles’ strong association boosted the stiffness of the supramolecular polymeric chain and improved the thermostability of the composite membranes. Porosity and pore size must also be evaluated to establish the membrane’s appropriateness for separation applications. To promote pore development in the membranes, PEG is supplemented to the polymeric solution. The tensile strength of the membranes (2.5 N/mm²) was increased by combining PEG with PCL and including TiO₂ nanoparticles. The optimal PEG and TiO₂ compositions were 6.24 and 1.0 wt% in PCL-TiO₂-PEG membrane, respectively. After bovine serum albumin filtering, the anti-fouling property of TiO₂ gave rise to roughly 90% flux recovery for the PCL-TiO₂-PEG membrane when compared to the PCL-PEG membrane. For membranes in water remediation uses, the PCL-TiO₂ combination would be a superior option [90].

Immobilization of particular bacteria onto electro-spun nanofiber webs of PCL was used to create innovative bio-composite materials. After adequate numbers of bacteria were immobilized on electro-spun nanofiber webs, equivalent web samples were used to evaluate their ability to remove the Setazol Blue BRF-X dye. Both PCL/bacteria and PLA/bacteria webs eliminated the Setazol Blue BRF-X dye within 48 h at each dose investigated (50, 100, and 200 mg/L), and their removal efficiency (87.88% and 89.57% for 50 mgL⁻¹ of dye concentration) were extremely similar to free bacterial cells. The samples’ Q_eq values were in the same order: 119.56 mg·g⁻¹ for free bacteria, 109.75 mg·g⁻¹ for PCL/bacteria web samples, and 112.15 mg·g⁻¹ for PLA/bacteria web samples. The bacteria immobilized webs were then evaluated for five reuses at starting dye concentration of 100 mg/L and found at the end of the test to be potentially reusable (removal capacity of PCL/bacteria:95.36%), with higher bacterial count immobilization and faster removal of dyes. Overall, their findings indicate that electro-spun nanofiber webs are viable platforms for bacterial integration and that webs immobilized with bacteria can be employed as starting inoculants for the removal of textile dyes from an aqueous medium [91].

Polydopamine (PDA) nanoparticles have a high adsorption capacity and can be utilized as an adsorbent. On the other hand, nano-sized PDA adsorbents are prone to aggregation and so have severe limitations in the adsorption field. To address this issue, Wang et al. produced the PCL/PEO copolymer by electrospinning and blended it with PDA. They discovered that modifying the surface and porosity of the produced nanofibers enhanced their adsorption capability. The PCL/PEO@PDA composite illustrated a high number of active sites for MB and MO adsorption. It is important to note that the PCL/PEO@PDA-45 composite as an adsorbent showed a better matched adsorption capacity for the anionic dye MO (60.22 mg·g⁻¹) than that of the cationic dye MB (14.85 mg·g⁻¹). Furthermore, the PCL/PEO@PDA blends were reusable numerous times and performed well as adsorbents. The removal effectiveness of the PCL/PEO@PDA adsorbent on MO dye reduced from 99% to 93% after eight reuses, confirming the adsorbent’s outstanding stability and recyclability [92].

Self-assembly nanohybrid structures GO@PCL, CNT@PCL, and GO-CNT@PCL were produced by a wet electrospinning-aided method with a three-dimensional fibrous PCL network surrounded by graphene oxide (GO) layers on which carbon nanotube (CNT) brushes were stuck [83,93]. They studied the influence of surface chemistry on the sorption ability of fluffy scaffolds’ nanohybrid materials towards water soluble organic dyes, such as MB and MO. Given that such a cationic MB dye may interact with aromatic and oxygenated areas of composites, all of the compounds perform quite well in terms of MB removal. GO-CNT@PCL outperformed the other nanohybrid samples, with a sorption capacity of 400 mg·g⁻¹ and an elimination efficiency of almost 100%. In the instance of MO removal, GO-CNT@PCL demonstrated 100% effectiveness with more than 80 mg·g⁻¹ adsorption in 6 h, likely due to the combination of wettability by GO and the amount of active sites by CNTs [83].

Three PEG/PCL copolymers were prepared with various topologies but the same mol ratio of PEG to PCL. Different techniques were used to characterize these compounds, which included di-block (Di-PEG-PCL), tri-block (Tri-PEG-PCL), and multi-block (Multi-PEG-PCL) copolymers. The influence of topology on the efficiency of MB removal from aqueous medium was researched. The multi-block copolymer outperforms other topologies in terms of adsorption efficiency. The regression coefficient data showed that the Freundlich isotherm (R²= 0.9840) accurately describes the MB adsorption by copolymers [94].

6.1.2. Dye Removal by PCL/Metal Oxide Composites

Metal oxides have been added to PCL to improve dyeability, thermal stability, adhesion, and other properties. Because it is miscible with a wide range of metal oxides, flexible, biocompatible, and mechanically suitable, it has the exceptional ability to enhance the characteristics of other materials, which results in a wide range of applications.

The electrospinning method was used to create high surface area porous PCL membranes containing Fe₃O₄ nanoparticles. In the existence of H₂O₂ and UV light, they were tested as degradation catalysts in Fenton reactions to break down cationic MB dye. The results showed that the fiber catalyst system (fiber diameter: 713 ± 139 nm for 15% PCL) performed well in terms of MB removal (99% MB) and was functional after six cycles (96% MB). PCL membranes may interact strongly with organic compounds because of their hydrophobic surfaces. The excellent catalytic properties of the composites are attributable to the synergy of the porous surface (pore size spans from 2.43 to 6.56 μm) and the implanted Fe₃O₄ nanoparticles. It also has the benefit of being environmentally safe and readily separated without the use of an external magnetic field [95].

PCL nanofibrous membranes integrated with hydroxyapatite (HAP) adjusted with varying concentrations of Cu(II) ions were created. A variety of spectrometric, microscopic, and surface science methods were used to describe the architectures, morphologies, and surface roughness of the built (Cu-HAP@PCL) membranes. Examination of the surface morphology showed that these membranes present as web fibers with different diameters ranging from 0.45–1.5, 0.45–1.21, 0.21–1.5, 0.3–1.1 to 0.2–0.9 μm; however, the average roughness increased exponentially with increasing Cu(II) ion content. The maximum degradations of MB dye were achieved after 70 min of visible light irradiation utilizing the constructed Cu-HAP@PCL membranes, with values of 88.6%, 89.4%, 91.0%, 91.1%, and 94.3%. The degradation mechanism can be summarized by the following reactions.

[Cu_xCa_1−x(PO₄)₆(OH)₂] + hν → e⁻(CB) + h⁺(VB)

(1)

[Cu_xCa_1−x(PO₄)₆(OH)₂] + H₂O + 2e⁻ → 2OH⁻ + Cu

(2)

e⁻ + O₂ → O₂*⁻

(3)

e⁻ + O₂*⁻ + 2H⁺ → H₂O

(4)

2e⁻ + HO₂*⁻ + H⁺ → OH* + OH⁻

(5)

MB + OH* → MB(degenerated) + CO₂ + H₂O

(6)

Hydroxyl ions (OH*) and superoxide scavengers (O₂*⁻) are the most efficient ions in the destruction of MB dye. Because these ions have a strong chemical activity while interacting with MB molecules, small fragments of molecules may be formed [96].

Elias et al. investigated the efficiency of a PCL–organically modified montmorillonite clay (Cloisite 10 A) nanocomposite membrane in the removal and degradation of rhodamine B from aqueous systems under VIS light (50 W). A nanocomposite containing nine by weight of Cloisite 10 A effectively converts rhodamine B into environmentally friendly products in less than 3 h. C10A’s adsorptivity and -OH groups on the surface promote electron-hole pair separation and improve photocatalytic activity [97].

The feasibility of immobilizing TiO₂ nanoparticles (NPs) on PCL beads and foams for the photodegradation of textile dyes in wastewater is discussed in this work. PCL foams were created by treating PCL beads in supercritical carbon dioxide in an ecologically friendly manner. Within 24 h of exposure to sun-like illumination, PCL beads and foams filled with TiO₂ NPs were utilized as photocatalysts to degrade the textile dyes C.I. acid orange 7 and C.I. basic yellow 28 from aqueous solutions. Unlike the PCL beads, the PCL foams completely discolored the dye solution after 24 h of illumination. PCL foams also had exceptional buoyancy that lasted longer than four weeks. Furthermore, their photocatalytic activity was sustained after three cycles of photodegradation, demonstrating that the buoyant photocatalyst outperformed the non-buoyant PCL beads in terms of photocatalytic activity. As a result, the PCL foams with immobilized TiO₂ nanoparticles have high photocatalytic activity and maintained floatability, making them a feasible choice for practical usage as a floating photocatalyst in textile industry wastewater management [98].

Geravand and colleague [99] developed a biodegradable membrane made of PCL/MXene nanosheets (Ti₃C₂(OH)₂). Using hydrophilic MXene as the nanofiller resulted in significant improvements in biodegradability, water permeability, and anti-fouling characteristics. The findings demonstrated that the PCL membrane combined with 4 wt% MXene displayed the maximum hydrophilicity and pure water permeance (PWP), which were nearly four times higher than those of the neat PCL membrane (5.99 vs. 1.43 L/m².h.bar). Furthermore, the membrane containing 1% MXene exhibited 98.92% rejection of crystal violet ((CV) = 50 ppm). All three mechanical features improved with rising the MXene concentration to 3wt% (elastic modulus from 19.87 ± 0.07 to 36.61 ± 0.52 MPa, tensile strength from 5.14 ± 0.27 to 10.51 ± 0.33 MPa, and elongation at break from 316 ± 6 to 359 ± 7%), but declined with a continual rise in MXene concentration. The mechanical performance of membranes was affected by a variety of parameters, including MX distribution uniformity, polymer aggregation, and augmented membrane porosity [99].

As photocatalytic materials, neat TiO₂ nanoparticles (TiO₂ NPs) have limited reusability. To achieve effective photocatalysis, certain high surface area supports are added to the TiO₂ loading. Porous electro-spun PCL fiber mats were utilized in the study to support the capacity and dispersion of TiO₂. The porous PCL mats can provide a broad contact surface area for excellent TiO₂ photocatalytic activity. Furthermore, the inclusion of rectorite (REC) might lower fiber diameters, induce stabilization of the anatase phase, and increase specific surface area, which could be favorable to photocatalysis. More notably, REC improved the performance of porous PCL/TiO₂ mats, resulting in high photocatalytic degradation. The findings suggested that porous PCL/TiO₂/REC mats might be good photocatalysts for the degradation of organic dyes like Rhodamine B. After 3 h, the crimson solution turned translucent while the mats became red. At the end of the experiment, the photocatalytic activity of TiO₂ NPs and fibrous mats all achieved 98%. In the first 45 min, the photocatalytic effectiveness of PCL/TiO₂/REC mats was greater than that of neat TiO₂ NPs, confirming that REC expedited the photocatalytic process [100].

The solution casting approach to create TiO₂ nanoparticle embedded polymer films from two biodegradable polymers such as cellulose acetate (CA) and PCL was used. The photo-degradation rate rose with rising content of TiO₂ NPs in CA and PCL films. TiO₂-PCL films had the highest photocatalytic efficiency of the two types of polymer materials tested. After 3 h of UV-A light exposure, PCL film combined with 5% TiO₂ had the greatest MB degradation rate (72%), followed by 5% CA film, with 53% dye degradation. As a result of the significant quantity of NPs around the pores and cracks on the film, the porous structure of TiO₂ in PCL-TiO₂ film could produce high MB degradation [101].

Solution casting was used to create the PCL and Fe₂O₃/PCL nanocomposites sheets/films. The morphological analysis revealed that the Fe₂O₃ nanoparticles (250 nm) were well disseminated and lodged inside the PCL matrix. The thermal stability of Fe₂O₃/PCL was poorer than that of pure PCL, which might be attributed to the activity of Fe₂O₃ as a catalyst in the thermal degradation of PCL. According to the DSC investigations, the crystallization temperature of the Fe₂O₃/PCL was somewhat higher than that of pure PCL. The polarized optical microscopy studies revealed that the size of Fe₂O₃/PCL spherulites was less than that of pure PCL. The UV-Vis spectra demonstrate that, as the irradiation period and the number of Fe₂O₃ nanoparticles in the nanocomposites increase, so did the photodegradation of rhodamine B (RhB) dye. Within 2 and 10 h, pure PCL photodegraded between 24 and 72% of the RhB dye, while Fe₂O₃ (6 wt%)/PCL composites degraded around 72 and 98% of the dye, respectively [102].

A simple hydrothermal approach was used to create hydroxyapatite (HAP) nanorods that were hydrophobically modified using lauric acid (LA). Pickering emulsion template technique was used to load HAP compound modified by LA (RHAP) and magnetite (m, Fe₃O₄) NPs as emulsifiers on PCL microspheres. The structure, constituents, and morphology of synthesized pure HAP nanorods (hydrophilic, water contact angle 11.7°) and m-RHAP-PCL microspheres (hydrophobic, water contact angle 111.3°) were analyzed by XRD, SEM, FTIR, and TGA. MG was used as a model dye to study the adsorption capability of m-RHAP-PCL microspheres. The highest adsorption capacity (q_max) of m-RHAP-PCL was 609 mg·g⁻¹, and its relative adsorption performance was greater than that of pure HAP, showing that it has better MG adsorption potential. The values of R_L (from 0.157 to 0.263, which were much smaller than 1) and 1/n (0.204–0.290, which were less than 0.5) suggested that the adsorption process had occurred on its own. Moreover, the adsorbent recovers easily in magnetic fields and its ability to remove MG remains unchanged over four cycles, demonstrating the reusable nature and stability of the m-RHAP-PCL adsorbent for MG sorption [103].

A chitosan/PCL-block poly(ethylene glycol)/SiO₂ aerogel@polydopamine (CS/PCL-b-PEG/SA@PDA) membrane was developed for the adsorption and removal of Congo red (CR) dye. Using simultaneous electrospinning electrospray technology, SiO₂ aerogel with nano-porous network architecture was immobilized into multicomponent polymer fibers, followed by PDA modification. The composite adsorbent exhibited a highest CR adsorption capacity of 598.8 mg·g⁻¹ and showed remarkable reusability. Because of the significant amount of NH₂, COO^-, and OH groups in their structures and amphiphilic characteristics, CS and PCL-b-PEG were chosen as adsorbent template substances in this research [104].

A mat of graphene oxide (GO) nanosheets was employed to dope fine powder of magnetite nanoparticles (MNPs) with varying amounts of copper ions (Cu(II)). These compositions (Cu_xFe_3-xO₄/GO) were fully integrated into PCL electro-spun nanofibrous membranes. The morphological characteristics of the membranes revealed that diameters were varied with Cu(II) ions variation, starting at 0.35–1.06 μm and 1.8–3.9 μm without Cu, being 0.19–0.45 μm and 0.75–1.42 μm for the greatest contribution of Cu, while GO scattered grains of 0.56–1.5 μm were observed. The toughness at the highest additional dopant was 4.69 ± 0.29 MJ/m³, while the tensile strength was adjusted to around 8.96 ± 0.45 MPa. These nanofiber membranes’ ability to capture MB dye from aqueous medium was also tested. The created nanofiber membrane could absorb 95.1% of the MB after 36 min of contact time, and the composition was still stable after five removals, with a performance of about 90.1% for the 0.8Cu-MNPs-GO@PCL [105].

The hydrothermal method was used to synthesize PCL/CA/MnWO₄ nanocomposite film. SEM, XRD, FTIR, UV-DRS, and XPS were used to investigate the morphology, chemical composition, optical, and molecular bonding properties of nanocomposite. Photocatalytic activities of MnWO₄ and CA/PCL/MnWO₄ nanocomposites on crystal violet (CV) dye were investigated and found to be 76.02% and 82.29%. Furthermore, these nanocomposite films showed significant efficiency and stability in the removal of CV dye, with only a small performance decline even after three cycles of repetitive use. The enhanced photocatalytic performance was attributed to the reduction in the band gap caused by the strong surface interaction between PCL/CA membrane and MnWO₄. These results indicated that PCL-based photocatalyst could be a promising candidate for photocatalytic dye degradation processes [106].

Ring-opening polymerization was used to create a PCL/Fe₃O₄ magnetic nanocomposite (PCL/Fe₃O₄ MNC) in this research. Then, as a new adsorbent, PCL/Fe₃O₄ MNC was utilized to capture remazol brilliant violet 5R (RBV 5R) from aqueous solution. The response surface methodology (RSM) was employed to maximize the adsorption procedure and define the optimum conditions. At a contact time of 168 min, a PCL/Fe₃O₄ MNC amount of 0.40 g and an RBV 5R concentration of 7.18 mgL⁻¹, 95.40% RBV 5R removal was obtained. ATR tests revealed that the chemical bonding in PCL/Fe₃O₄ MNC did not change significantly before and after the adsorption method [107].

PCL composite was used not only for dye removal but also for oil separation. By incorporating SiO₂ aerogel, exceptional superhydrophobic PCL membranes with an alternating hierarchical micro-nanometer structure were created. When the SiO₂ aerogel content in the PCL membrane was 0.5% (PCL/SiO₂-a0.5), the maximum water contact angle (WCA) of 166.8 ± 1.5° was acquired, which was greater than other published polymer-based membranes. The surface energy of the PCL membrane was reduced by SiO₂ aerogel. The internal structure of the PCL/SiO₂-a0.5 membrane, which was made up of micro-nano spheres and fibers, increased the porosity of the membrane, allowing for more adsorption area for water-in-oil separation. In the meantime, the PCL/SiO₂-a0.5 membrane demonstrated exceptional chemical stability, self-cleaning ability, and reusability [108].

Uzunok and Sönmez improved the hydrophobicity of PCL by incorporating different silane-based cross-linkers such as tris [3-(trimethoxy-silyl)propyl] iso-cyanurate, tetraethyl orthosilicate, and 1,8-bis(triethoxy-silyl)octane for the removal of oil from an aqueous system. The obtained sorbents showed high and quick absorption properties in the range of 5450–51,000 mg·g⁻¹. After 10 cycles of oil sorption, the sorbents demonstrated high oil sorption capacity and reusability without any loss [109].

PCL blends/composites have an important place not only in dye and heavy metal removal, but also in oil cleaning. The use of oil-soaked adsorbents in the recovery and clean-up of oil spills is gaining importance day by day. Recently, magnetic nanoparticle (MNP)-based absorbers have attracted interest as a new method both to treat oil spills and to reduce the amount of labor required. Eom et al. performed the synthesis of MNP embedded PCL adsorbent, which can be collected easily under a magnetic field and has oleophilic and environmentally friendly properties. MNP embedded PCL adsorbent (MNP/PCL) exhibited perfect Arabian light (AL) crude oil absorption capacity (45.7 g/g) and reduced the absorption time of oil-immersed sorbent because of its electro-spun structure. In the future, such sorbents may be applicable in large scale oil spill projects [110].

6.1.3. Heavy Metal Removal by PCL Composites/Blends

PCL is one of the promising compounds for manufacturing biocompatible membranes due to its outstanding structure and superior mechanical and physical features. PCL nanocomposites have also found usage in removing heavy metal ions from aqueous system. Bio-nanocomposite CD-PCL-TiO₂ was utilized as an adsorbent material in removing of Pb(II) ions from wastewater after a two-step process of sol gel synthesis of TiO₂ particles and polymer solution blending. The highest removal of Pb(II) ions was 98% at pH 9.7, concentration 10 ppm, and dosage of 0.005 g. This method has a significant advantage since it does not introduce any secondary pollutants into the treated water [111].

Liakos et al. mixed the solutions of 8.42% by weight PCL in dichloromethane with sodium alginate (SA) (5, 10, 15, 20, 25 or 30% by weight based on PCL) and the resulting films were placed in a heat extruder to form PCL/SA filaments.

The aim of this study was to develop new composite filaments based on thermo-plastic PCL polymers that are water-resistant and compatible with 3D printing technologies for use in environmental remediation by removing toxic metal ions. The interplay and stability of SA with PCL was because of the creation of hydrogen bonds between the former’s hydroxyl groups and the latter’s carbonyl groups. The resulting composite filaments with 30%SA had a highest adsorption capacity of around 90 mg·g⁻¹ for heavy metals such as copper(II) ions in aqueous system [112]. It was shown that the pure PCL filament did not absorb any Cu ions and all the SA-containing filaments showed Cu ions’ adsorption capabilities.

Electrospinning was used to create nanostructured membranes of cellulose acetate (CA) with varying PCL loadings (0%, 10%, 20%, and 30%) in removing of Pb²⁺ ions from aqueous medium. Using instrumental techniques revealed that the introduction of PCL into CA resulted in a finer fiber radius, increasing the membrane’s surface area and thus increasing the number of adsorption sites. According to the findings, Pb²⁺ ion adsorption capacity was increased from 43.96 mg·g⁻¹ of pristine CA membrane to 70.50 mg·g⁻¹ of CA/10%PCL filled membrane. Furthermore, the findings of this experiment agreed best with the pseudo second-order kinetics and Freundlich isotherm, both of which accurately described the adsorption process [113].

Coaxial electrospinning was used to create core-shell-structured CA-PCL/CS nanofibers with outstanding hexavalent chromium (Cr(VI)) removal performance. In an acidic environment, the influence of the core/shell ratio on the adsorption ability was investigated. The findings demonstrated that, when compared to the CS powder adsorbent, all core-shell-structured fibrous material demonstrated improved adsorption and durability. CA-PCL/CS fibrous material with a core/shell ratio of 0.442 absorbed 126 mg·g⁻¹ of Cr(VI) ions at RT. Adsorption kinetics showed that chemisorption was the rate-limiting step due to significant electron transfer, sharing or exchange between CA-PCL/CS nanofibers and Cr(VI) ions [114].

Benhacine et al. combined PCL with silver montmorillonite (Ag-MMT) to produce nanocomposite membranes. SEM analysis revealed that the synthesized membranes had homogeneous sponge microstructures. Gradual incorporation of nanoparticles (2, 3, and 5 wt%) into the PCL matrix resulted in a significant increase in membrane thickness. The authors of the current study administered the treatment to actual wastewater samples and documented a decrease in the concentrations of nitrates by 15.12% and sulphates by 45.61%, along with significant reductions in the levels of Pb, Zn, and Cd, by 41.38%, 53.57%, and 61.11%, respectively [115].

Irandoost et al. prepared nanofiber nanocomposite of PCL adsorbent modified by binary fillers such as nano-clay and zeolite clinoptilolite NPs to improve the adsorption properties. Lead removal studies have shown that each component in the nanofibrous adsorbent has a synergistic effect on the Pb(II) adsorption capacity (19.92 mg/g). In thermodynamic studies, negative ΔG values (between −0.173 and −3.003 kj/mol) indicated spontaneous Pb(II) adsorption at all temperatures and concentrations [116].

Using the electro-spinning method, a composite membrane based on PCL/cellulose nanofibers (CNF) with various ratios was fabricated with the goal of developing organic membranes with excellent mechanical features for removing impurities from tap water. The resulting green and environmentally friendly membranes exhibited structures containing nanometric porosity. Water quality variables evaluated after filtration with the PCL/CNF membranes revealed 100% turbidity removal, 100% conductivity and heavy metal removal ranging from 75% to 99% for iron and chromium [117]. Figure 5 outlines adsorption of dyes and heavy metal ions onto PCL blends/composites.

6.1.4. Adsorption Mechanism of Dyes/Metal Ions onto PCL Composites/Blends

Water-insoluble homo-PCL polymers are not very effective against environmental pollution, such as heavy metal ions and dyes, due to their low adsorption efficiency, low pore volumes, low pore sizes and high hydrophobic properties. Instead, PCL blends/composites are preferred as effective adsorbents for removing dyes and metal ions from an aqueous medium [118,119]. The blend of a polymer including functional groups with a hydrophilic polymer, like cellulose, chitosan, and chitin. in various ratios is crucial for enhancing hydrophilicity, raising adsorption active sites, and decreasing cost. Understanding the adsorption mechanism of dyes or heavy metal ions on PCL adsorbents is critical for optimizing the adsorption method and improving the efficiency of PCL nanofibers in dye removal. The adsorption of dyes or metal ions on the surface of nanofiber materials is controlled by solution conditions (such as pH and temperature), nanofiber nature (like porosity, area of surface, functional moieties, and surface morphology), and dye nature (like neutral, cationic, and anionic forms, and molecular size of dyes) [120,121]. The adsorption mechanism may be unclear due to the variety of factors influencing dyes or metal ions’ adsorption onto the surface of nanofibrous. It is essential to carry out isotherm, kinetic, thermodynamic, and spectroscopic studies, in addition to looking at the impact of pH, in order to gain a clear and comprehensive understanding of the adsorption mechanism. According to various studies [122,123,124], dyes’ or metal ions’ removal from effluents using pure PCL, PCL blends, and PCL composites are most likely accomplished through hydrogen bonding, van der Waals (VDW) forces, stacking, hydrophobic interactions, electrostatic interactions, and pore filling. The interactions are influenced by the type of functional moieties on the surface of PCL nanofibrous, their morphology, and the type of filler added to the nanofibrous. These interactions, which operate concurrently to varying degrees, can be utilized to explain the adsorption mechanism of dyes and metal ions [84,125,126,127].

Draoua et al. investigated the MB dye removal adsorption performance of a PCL–PEG–PCL/Bentonite nanocomposite [88]. They came to the conclusion that the sorption mechanism was dependent on the hydrophobic interaction between the dye and the copolymer’s hydrophobic block (PCL), as well as the electrostatic interaction between the surface (the SiO- and AlO-) and the edge moieties of bentonite and the dye’s sulfonic group.

Alrafai et al. [96] studied the degradation mechanism of MB on the surface of PCL nanofiber membranes associated with hydroxyapatite (HAP) doped with varying quantities of Cu(II) ions (Cu–HAP@PCL). The degradation efficiency of MB dye on the fabricated Cu–HAP@PCL membranes indicated that the maximum degradation of 94.3% was accomplished after 70 min exposure to visible light. They discovered that the degradation efficiency was primarily increased by increasing the Cu contribution and lengthening the irradiation time [96].

Chen et al., for example, used FTIR and XPS techniques to investigate the adsorption mechanism of CR dye on the surface of the CS/PCL-b-PEG/SA@PDA membrane. They came to the conclusion that dye adsorption was accomplished through a combination of interactions, like hydrophobic interactions, hydrogen bond interactions (between the adsorbent’s amine and hydroxyl groups and the CR’s sulfonate groups), and adsorption space on the porous membrane [104].

Pekdemir et al. suggested that there are two possible mechanisms for dye adsorption on the adsorbent surface, which are chemical and physical interactions between the dye compound (RBV-5R) and the polymer (PCL/Fe₃O₄) nanocomposite. The hydrogen bonding, π–π interactions, and VDW interactions between the adsorbing agent and the dye compound were thought to control the adsorption rate in the model [107].

Hussain et al. crosslinked tannin (TA)-reinforced 3-aminopropyltriethoxysilane (APTES) with PCL fabricated via electrospinning and investigated the colorimetric performance of membrane for Fe^2+/3+ ions. They discovered that Fe ions bind to the crosslinked composite membrane due to FeTA complexation to form PCL-FeTA-APTES. According to the results of adsorption filtration, PCL-FeTA-APTES membrane was reusable and had higher MB dye adsorption (32.04 mg·g⁻¹) than PCL-TA-APTES membrane (14.96 mg·g⁻¹). This finding showed that the adsorption mechanism is influenced by high adsorption sites on the PCL-FeTA-APTES membrane, which allows the weak π–πinteraction and electrostatic attraction between the cationic dye compounds and anionic adsorption sites of the membrane [128]. Figure 6 outlines the adsorption mechanisms of heavy metal ions and dyes onto PCL blends/composites.

Lee et al. prepared coffee/PCL composite filters from coffee grounds and PCL and utilized them to eliminate Cu(II) ions from an aqueous system [129]. They discovered that, for 50 and 100 M Cu(II) ions, the removal efficiency of coffee/PCL composites was 96.6% for 4 h (q_max = 25.91 mg/g). All the samples’ adsorption processes exhibited both monolayer and multilayer adsorption characteristics. The adsorption of Cu(II) ions was attributed to the strong binding or interaction between adsorbent and adsorbate.

In the study of Ramírez-Rodríguez et al., the hybrid membrane of PCL-whey protein isolate (PCL-WPI) adsorbed a significant amount of Cr in just under 60 min because of the abundance of adsorption sites on the adsorbing agent surface. However, the adsorption of Cr dropped significantly with time because of occupation of the active sites of the membrane, and adsorption equilibrium was reached after 3.5 h [130].

The dynamic elimination of arsenic from an aqueous medium with an adsorption membrane composed of an iron-intercalated montmorillonite-filled PCL matrix (Fe-MMT/PCL) was performed by Pena et al. [131]. According to parametric studies, longer breakthrough times were associated with a low flow rate (20 mL·min⁻¹), a low starting inlet concentration (2 ppm), and a thick nanofiber membrane (0.75 mm).

Composite membranes with functionalized fillers have being investigated for selecting adsorption. Carbon Quantum Dots-Polyacrylonitrile(PAN)/PCL nanocomposite membranes were prepared by electrospinning and were designed for Cu(II) adsorption. The maximum adsorption capacity of a nanofiber mat with respect to contact time was 63.45 mg·g⁻¹, with a highest adsorption efficiency of 90.74%. CQD in PAN and PAN/PCL membrane magnified the fiber size distribution from 50–100 nm to 150–250 nm, increasing the hydrophilicity of the PAN/PCL membrane. The physical adsorption method, which involves VDW forces and hydrogen bonds between adsorbent and adsorbate, governs the adsorption behavior [132].

The surface chemistry of the cellulose acetate (CA)/PCL electro-spun fibers immobilized with the novel mercury-favored organic dye (NF06) sensor was discovered to effectively facilitate the sensor’s fluorescence resonance energy transfer (FRET). Following Hg(II) chelation, both mobile and immobilized NF06 carried out FRET via a ring-open mechanism. Since the thiorhodamine-6 G acceptor stayed in a cyclic spirolactam in the lack of Hg(II), the FRET process was found to be inhibiting. However, due to the favorable electrostatic interactions between Hg(II) and the N- and S-atoms of NF06, an NF06:Hg(II) complex could easily form once Hg(II) was present. Consequently, the thio-R6 GH evolved from a ring-closed to a ring-opened spirolactam version [133].

Bernhardt et al. [134] studied the ability of PLLA to enhance the adsorption capacity of PCL in removing Rose Bengal from an aqueous system. Solid samples of the PLLA-PCL-PLLA system were discovered to quickly eliminate over 90% of Rose Bengal from the aqueous medium, resulting in the full vanishing of the distinctive pink color. Rose Bengal was also effectively removed from water by solutions of the copolymers in CH₂Cl₂. Large inclusion formation constant (K_i) values ranging from 1.0 × 10⁵ to 7.9 × 10⁵ M⁻¹ were obtained, and an average adsorption capacity of 6.2 × 10⁻⁷ mol/g polymer was determined. The increased adsorption capacity was attributed to the hydrophilicity of the co-polymer because of the PLLA [134].

As a result, it was found that the formation of novel functional groups that increase the number of adsorption sites on the nanofibrous surface increases the adsorption capacity of polymer nanofibers when they are combined with one or more polymeric materials. Additionally, for describing the adsorption process of cationic, anionic dyes and heavy metal ions on the surface of blends and composites of PCL, the Langmuir and PSO models were the best isotherm and kinetic models.

Table 3. Removal of dyes from aqueous system using PCL blends/composites.

PCL Blends/Composites	Time	Dyes	Adsorption Capacity, mg·g−1	Ref.
PCL/(50%)PCD	2 h	MB	11.2	[85]
CS/P5K Nanofibrous Membranes	2 h	CR	291.55	[87]
PCL-PEG-PCL/Bentonite A2	1.5 h	MB	600	[88]
Bacteria/PCL Webs	48 h	Setazol Blue BRF-X	109.75	[91]
PCL/PEO@PDA-45	45 h	MO	60.22	[92]
GO-CNT@PCL	6 h	MO	80	[83]
Tri-PEG-PCL	2 h	MB	193.51	[94]
Multi-PEG-PCL	2 h	MB	256.01	[94]
m-RHAp-PCL Microspheres	55 h	MG	609.76	[103]
CS/PCL-b-PEG/SA@PDA-24	24 h	CR	598.8	[104]
PCL-FeTA-APTES	-	MB	32.04	[128]
PCL-BIX80	2 h	MB	79	[135]
PCL-BIX80	2 h	BG	254	[135]
PCL/PEI/TTL	8 h	MG	36.5	[136]

and

Table 4. Removal of heavy metal ions from aqueous system using PCL blends/composites.

PCL Blends/Composites	Time	Heavy Metal Ions	Adsorption Capacity, mg·g−1	Ref.
PCL/30%SA Filaments	30 days	Cu(II)	93.3	[112]
CA/10%PCL Membrane	6 h	Pb(II)	70.50	[113]
CA–PCL/CS	?	Cr(VI)	126	[114]
PCL/Clay/Zeolite	2 h	Pb(II)	19.92	[116]
Coffee/PCL	4 h	Cu(II)	25.91	[129]
CQD/PAN/PCL	?	Cu(II)	63.45	[132]
10%PCL/5%Clay Fiber	72 h	Cd(II), Cr(III), Pb(II)	29.59, 27.23, 32.88	[137]
PCL-CuHCF	40 min	Cs(I), Co(II)	178.7, 85.06	[138]
PCL/Mg micromotor	3 min	Ag(I)	0.635	[139]
Fe-MMt/PCL	6 h	Hg(II)	14.25	[140]
CMKC-coated PCL scaffolds	1 min	Ca(II)	2.186	[141]
TPCL	30 min	Pb(II), Cd(II)	10.27, 5.81	[142]

give some examples of PCL blends and composites for removing dyes and toxic metal ions from an aqueous system, but it is not easy to compare them, because the results vary greatly depending on experimental conditions, like temperature, initial concentration, adsorbent amount, contact time, and pH. As a result, PCL blends and composites with high adsorption capacity were prepared and studied.

6.2. Biomedical Applications of PCL Blends/Composites

Similar blends or composites of PCL have been used not only in dye and heavy metal removal, but also in tissue engineering, drug release, wound healing, and other fields. Since the hydrophobicity, low biocompatibility, slow degradation and low mechanical properties of PCL are disadvantages in biomedical applications, the preparation of its blend or composite is inevitable. Despite the aforementioned disadvantages, its rubbery properties, tunable biodegradability, and ease of formation into blends, composites, and copolymers make it a desirable material for utilization as a supporting device, particularly for hard tissue, a good scaffold material for tissue engineering, a desirable material for surgical sutures, and micro/nano vesicles for drug delivery [73]. Therefore, it is worth emphasizing the importance and place of PCL blends and composites in biomedical applications.

6.2.1. PCL Blends/Composites for Tissue Engineering

PCL’s surface roughness and hydrophilic adjustability supplies preferred surface and interfacial features for the tissue. The degradation of materials utilized in the manufacture of scaffolds for tissue engineering implementations is an important property. PCL degradation is affected by molecular weight, shape, residual monomer content, autocatalysis, and other factors. Generally, it takes 2–3 years for PCL to completely degrade in the tissue fluid, which is constantly changing in the biological environment. In tissue engineering, the rate of material degradation is expected to be similar to the rate of tissue regeneration. When degradation is slower than the rate of tissue regeneration, it impedes tissue growth; when it is faster, it causes the linkage between tissue and scaffold to be lost, delaying the healing process [143].

Das et al. described a novel method for producing a PCL/chitosan structure including membrane products with double-porosity (macro=voids with interconnected microporous network) by chemically optimizing the solvent-containing and non-solvent phases and using a customized phase transition method. The existence of chitosan in the chitosan/PCL structure improved hydrophilicity significantly. The cytocompatibility of the chitosan/PCL scaffold was also investigated and found non-toxic. In the future, the blend membrane with dual porous morphology can be used for small radius vascular bypass uses. Surface macro=voids (20–90 μm) may be beneficial for three-dimensional cell adhesion and incrementation, while an interconnected microporous spongy network (7–20 μm) is anticipated to transport vital nutrients, oxygen, and growth factors between the macro-voids and the supernatant. All of these findings indicated that the chitosan/PCL nanofibrous structure would be a fantastic system for bone and skin tissue engineering [144].

Antioxidants are essential for the accomplishment of neural tissue regeneration and biomaterials with antioxidant action may be beneficial for peripheral nerve repair. To create an antioxidant bio-scaffold for nerve regeneration, Wang et al. used solvent-free ROP to create lignin–PCL copolymers. The lignin/PCL copolymers were then combined with PCL and designed into nanofiber scaffolds to support neuron and Schwann cell growth. The inclusion of lignin–PCL improved the mechanical features of PCL nanofibers while also providing good antioxidant characteristics (up to 98.3 ± 1.9% inhibition of free radicals within 4 h). These findings proposed that nanofibrous material containing lignin copolymers stimulated cell proliferation in both BMSCs and Schwann cells, increased Schwann cell myelin basic protein expression, and stimulated neurite outgrowth in dorsal root ganglion neurons. Overall, these long-lasting, naturally antioxidant nanofibrous materials could be a promising candidate for nerve tissue engineering uses [145].

The primary goal of Rezaei et al.’s work was the creation of a biologically and mechanically proper 3D printed scaffold for lung tissue engineering using a chitosan/PCL bioink. Design-Expert software was used to investigate different compositions for 3D printing. The scaffolds were tested for MRC-5 cell line growth, incrementation, and migration using chemical, biological, and mechanical methods. According to the findings, the average radius of the chitosan/PCL strands was 180 mm. Changes in PCL content had no effect on printability, whereas chitosan concentration did. The scaffolds demonstrated excellent swelling, degradation, and mechanical manner, but they can be altered by adjusting the PCL ingredient. In vitro, the scaffolds also demonstrated significant cell biocompatibility, non-toxicity, low apoptosis, high increment, and cell adhesion. To summarize, scaffold 3 (chitosan/PCL ratio: 4:1) demonstrated superior MRC-5 cell culture activity. As a result, this scaffold may be an excellent choice for lung tissue engineering [146].

There have been several attempts to combine biopolymers with inexpensive natural micro/nano particles, such as lignin, alginate, and gums to create new substances with improved features. The technique of electrospinning (ELS) was used to create nanocomposites of lignin and PCL scaffold. Nanocomposites containing (0, 5, 10, and 15% wt%) lignin were produced by adding lignin powder to the PCL solution with stirring at RT. When compared to pure PCL, the scaffold with 10% lignin had suitable porosity, biodegradability, minimal fiber radius, ideal pore size, and improved tensile strength and elastic modulus. Degradation tests using samples immersed in phosphate-buffer saline revealed that the addition of lignin could accelerate degradation of PCL composites by up to 10% [147].

6.2.2. PCL Blends/Composites for Wound Healing

PCL blends/composites are of great importance in the development and design of wound dressings that have antibacterial effects, absorb blood leakage and provide tissue regeneration in the rapid healing of wounds. By film casting, Kotcharat et al. created a PCL/bacterial cellulose (BC) composite with a specific ratio of 1%, 5%, 10%, 20%, 30%, and 50% v/v of BC. Alkaline purification was used to extract BC from nata de coco. The percentage of crystallinity decreased with increasing BC content. Porosity was discovered in the microstructure of the BC/PCL composite. It was noteworthy that the thermal decomposition manner remained stable up to 400 °C. When BC was added to DI water and phosphate-buffered saline (PBS) solution, there was no significant change in swelling manner. Swelling and degradation features were studied for 6 h and 4 weeks, respectively. As a result, PCL and BC-based composites can benefit significantly as exceptional candidates for wound dressing applications [148].

From a blend solution of PCL, cellulose acetate (CA) and dextran, an electro-spun nanofibers mat for use in wound dressing was favorably created. The composite mat’s antimicrobial activity, blood clotting capacity, cell attachment, and cell increment were all improved by the addition of a small quantity of the antibacterial medicine tetracycline hydrochloride. The composite’s antimicrobial activity was tested using zone inhibition opposed to gram-positive and gram-negative bacteria, and the results show that it has high antibacterial activity. As a result, the synthesized composite fiber has good properties for wound dressing and skin engineering uses [149].

Kahdim et al. prepared biocompatible scaffolds using PCL, chitosan (CS) and Nigella sativa (NS) seed extract and investigated their antibacterial activity against Escherichia coli and Staphylococcus aureus. The prepared PCL/CS/NS scaffold had better antibacterial properties, with an inhibition zone of 8.00 ± 0.22 mm for S. aureus and 7.4 ± 0.16 mm for E. coli compared to PCL and PCL/CS [150]. It was shown that the larger the diameter of the inhibition zone, the greater the antibacterial activity of the scaffold; thus, scaffolds containing PCL/CS/NS were more effective against S. aureus.

In a study by Ahmed et al., ε-PCL scaffolds were loaded with various amounts of silver-Fe₃O₄ nanoparticles. The silver nanoparticles in the nanocomposite acted as antimicrobial agents against E. coli and the healing process took place without any damage to the dermal and epidermal tissues of the skin [151].

Wang et al. prepared PCL electro-spun nanofiber membranes containing different concentrations of pomegranate peel extract (PPE). The diameter of the inhibition zone of PCL/10PPE against E. coli and S. aureus reaches 19.15 ± 0.76 and 22.81 ± 0.29, respectively. These superior antimicrobial activity results can be explained by the high levels of polyphenols in PPE, which inhibit microbial biofilm formation, thereby reducing their growth and proliferation [152].

Teodoro et al. prepared a low-cost, multifunctional therapeutic wound dressing by loading curcumin (CC) into PCL nanofibers. This PCL/CC dressing was successful in fulfilling various functions, such as controlled release of CC, colorimetric monitoring of wound conditions, and barrier formation against microorganisms. In addition, a study of the proposed wound dressing (PCL/CC) on mice showed a reduction of bacteria present in the wound [153].

Hui et al. synthesized PCL/PAA/PEI-CMC Janus membrane to be used as a wound dressing to prevent adhesion of external bacteria and promote wound healing. When this membrane was exposed to bacterial solution, a large number of bacteria adhered to the hydrophilic side of the membrane (PCL/PAA/PEI-CMC), while no bacteria were found on the hydrophobic PCL side. This result demonstrated that the Janus hemostatic membrane was able to resist the invasion of external bacteria and thus prevent wound infection [154].

6.2.3. PCL Blends/Composites for Drug Delivery

The micro- and nanofiber structures of PCL blends/composites have a high surface area/volume ratio and therefore have high drug loading and release potential. El Gohary and colleagues used Oil/Water emulsion solvent evaporation method to fabricate PCL/Fe₃O₄ microspheres with magnetic properties. This prepared magnetic PCL was effective and successful in the delivery of Lornoxicam to the knee joint [155].

The advancement of a PCL electro-spun composite coated with pectin/polyaniline (PANi) that was elucidated and had a use for drug delivery was described in the work of Hamzah et al. The SEM micrograph showed a powerful interaction between the bilayer structures and the interlinked pores of uniform fibers. The composition pectin(12%)/PANi(3%) was discovered as the maximized composition with a high tensile strength of 55.48 ± 0.65 MPa and a modulus strength of 63.30 ± 0.43 MPa. This combination of desirable mechanical properties will make the double-layer composite a dynamic and versatile scaffold for drug delivery [156].

Gizaw et al. produced microfibers at various blend ratios by adding acetylsalicylic acid (ASA) to blend fibers consisting of 15 wt% PCL and 4 wt% CS using an electrospinning method [157]. Drug release was studied in PBS buffer solution and 30% ASA was released up to 2 h. In ASA drug release, the amount of release increased as the amount of CS increased, which was attributed to the surface wettability property of CS.

Another investigation involved the development of solvent cast films containing different ratios of PCL:Poly(lactide-co-glycolide) (PLGA). Formulations with higher concentrations of PLGA and triethyl citrate as plasticizer showed a lower burst effect, while higher 5-Fluorouracil (5FU, an anticancer drug) loading (2%) resulted in an increased release rate [158].

In Ahani and Mianehro’s study, PCL/PEO nanofibers were loaded with Acyclovir (ACV, an antiviral medication) to develop a vaginal delivery system to reduce the prevalence of herpes simplex virus 2 (HSV-2) infection in women. PEO/PCL nanofibers released ACV continuously for two weeks. Due to their controlled drug release properties, PCL/PEO nanofibers would be ideal candidates for drug delivery [159].

6.2.4. PCL Blends/Composites for Other Applications

Other biomedical applications include fixation of broken bones in the form of plates, pins, screws and wires, as well as optical sensors and biosensors [160]. In addition to these biomedical applications, PCL blends have important uses in biodegradable packaging as described in the following two articles.

Zhang et al. presented novel, high-performance lignin-PCL-based polyurethane bioplastics. The PCL was added to the lignin as a biodegradable soft segment via a hexamethylene diisocyanate (HDI) bridge with long flexible aliphatic organic chains and high activity. Investigation was carried out into how the NCO/OH mole ratio, lignin content, and PCL molecular mass affected the characteristics of the resulting polyurethane plastics. It is crucial to note that the polyurethane film retained high tensile strength (19.35 MPa), elongation at break (188.36%), and tear strength (38.94 kN/m) when the lignin content reached 37.3%; additionally, it was remarkably stable at 340.8 °C and displayed exceptional solvent stability. An efficient method for enhancing lignin processability and creating sustainable bioplastic materials, like sealing compounds, biodegradable packaging films, and bio-adhesives, is the accomplished production of lignin-based polyurethane bioplastics [161].

According to Gorrasi et al., a new thermoplastic biodegradable compound (m-PCL-Pectin) was produced using a solvent-free method by altering natural pectin with m-PCL, a compound with pendants made of glycidyl methacrylate and maleic anhydride. Films were produced by processing the obtained material while it was still molten. The structural, thermal, mechanical, and barrier features to water vapor of pure pectin and modified PCL films were evaluated and compared. The suggested green method presents an excellent chance to produce fruit waste in order to obtain flexible and completely biodegradable polymers for packaging uses, as an intriguing replacement for conventional thermoplastics that are not biodegradable. According to analysis of mechanical properties, pectin’s high rigidity and stiffness, as shown by stress at break and elongation at break point, severely restrict its use in the flexible packaging field. However, thanks to this approach, the elongation at break is significantly increased in the presence of PCL, which eliminates the limitation in pectin [162]. Figure 7 outlines some common applications of PCL blends and composites.

Table 5. Medical applications of PCL blends and composites.

PCL Blends/Composites	Biomedical Application	Results	Ref.
PCL/PGS/KGN	Cartilage tissue engineering	Coaxial PGS/PCL aligned nanofibers supported the controlled and sustained release of small molecule kartogenin	[163]
PCL/Col-HA	Peripheral nerve regeneration	Fibrous composed of PCL and Col-HA showed similarity to rat sciatic nerve in terms of mechanical properties	[164]
PCL/CS	Musculoskeletal tissue engineering	Studies using adult human chondrocytes show that the biocompatibility of the scaffolds does not degrade in cell culture in 28 days and can be used in cartilage repair.	[165]
PCL/MgO/ Mesenchymal stem cells	Bone Tissue Reconstruction	Mesenchymal stem cells-MgO/PCL nanofibers may be a suitable bio-implant for use in bone regenerative applications.	[166]
PCL/PHBV	Tissue engineering	Air-jet spun PHBV/PCL scaffolds are compatible with animal tissues and can be used in many tissue engineering applications.	[167]
PCL/PHB	Bone medical applications	Mechanical properties of blend (strength ~40 MPa, modulus ~2.5 GPa) are comparable to human trabecular bone.	[168]
PCL/PLA or PLLA	Tissue engineering	The addition of 20 wt% PLA or PLLA enhances cellular activity by increasing the biodegradability of PCL surfaces.	[169]
PCL/INU-PLA	Bone repair and tissue-engineering	Scaffolds showed mechanical performances compatible with the human trabecular bone, significantly improved surface properties, swelling ability, and biodegradation profiles.	[170]
PCL/Carbopol/Cur/ CS/PVA	Wound healing	The designed scaffold groups accelerated the wound-healing process compared with the control group.	[171]
PCL-b-PEG	Wound healing	PCL-b-PEG hydrogel formulations showed high antibacterial activity against E. coli and S. aureus.	[172]
PCL/CS/BRO/Ag	Wound healing	While BRO and Ag NPs incorporated into PCL/CS did not affect the toxicity of the resulting wound dressing, antibacterial activity against E. coli and S. aureus bacteria was significantly affected.	[173]

PGS: Poly(glycerol sebacate), Col: Collagen, HA: Hyaluronic acid, Cur: Curcumin, CS: Chitosan, PHBV: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHB: Poly(3-hydroxybutyrate), PEG: Polyethylene glycol, BRO: Bromelain.

summarizes the results of other recent studies and the areas in which they have been used. In PCL-based applications, PCL should form blends or composites with different chemicals depending on the area where PCL is used. For example, in water remediation, there is no need for chemicals that increase the degradation rate of PCL, whereas in tissue regeneration, chemicals that increase the degradation rate, such as polyethylene glycol or polylactide, are needed. Therefore, PCL blends and composites should be used where necessary, taking into account their chemical composition.

As a result, some common uses of PCL and PCL blends/composites can be summarized as shown in Figure 8, according to their chemical components, structures, the functional groups they contain, active binding sites, environmental friendliness, cost-effective preparation, and physical and mechanical properties [25,73,139,174,175,176,177].

7. Conclusions

PCL is a biodegradable polymer commonly employed commercially in biomedical applications. In addition, it provides many advantages in the development of adsorbents for the removal of dyes and heavy metals. Over the last decade, there has been a significant increase in the use of PCL as an adsorbent in research, indicating its acceptance. This review surveyed recent advances in the fabrication of various forms of PCL blends and composites, and their applications in wastewater treatment. The removal of anionic/cationic dyes and heavy metal ions can be excellent options for producing low-cost PCL-based adsorbents with improved mechanical properties. Compared to their counterparts and components, PCL blends and composites were much more effective in the removal of dyes and heavy metal ions as adsorbents. The adsorption capacity of PCL blends and composites against dyes and heavy metal ions is quite high (~98%) and it is also important to be regenerated and reused 3–10 times without degradation in aqueous media. Based on the data analyzed and discussed in this review, we can reach the conclusion that PCL and its composites/blends provide an exciting and viable platform for the long-term generation of biodegradable adsorption materials. This work offers a novel perspective, understanding and motivation for the manufacture of PCL blends and composites to be used as adsorbents, as they contain various electronegative functional groups, such as oxide (O²⁻), sulfur (S²⁻), carboxylate (COO⁻), amine (NH), amide (C(=O)NH) and hydroxyl (OH). Furthermore, PCL can be a perfect choice for actively used adsorbents, particularly for eco-friendly antimicrobial adsorbents that require further investigation. The properties and use of PCL can also be radically changed in a promising way and improved by nanotechnology techniques. In comparison to other conventional techniques, it can be said that electrospinning processes are one of the most popular ways to prepare PCL for use in blend and composite production.

Despite significant efforts to create and enhance the unique structures and functions of PCL blends and composites used for wastewater treatment, many limitations remain. The parameters of the reaction medium and the surrounding environment have a large influence on the features of PCL, PCL blends, and PCL-composites. As a result, the properties of the fabricated PCL blends and PCL-composites, like their environmental friendliness, degradability, processability, non-toxicity, mechanical and thermal features, as well as availability, long-term durability and cost-effectiveness, should be taken into account and further optimized in order to remove the other drawbacks. In addition, how to make more use of suspended PCL-blend and PCL-composites after dye and heavy metal removal processes to reduce overall costs should be investigated. The synthesis of new and renewable PCL blends and PCL composites with specific properties and mechanical properties as well as strong abilities in the purification of different pollutants from wastewater are becoming increasingly important. In biomedical applications, PCL has shown low bioactivity towards tissue due to its high hydrophobicity. This problem was addressed by mixing PCL with other polymers that increase hydrophilic and surface functionality. However, the development of bio-interactive materials, especially for biomedical applications, is still an area that needs to be solved.

In conclusion, this study provides a novel perspective, analysis and recommendation for the production and application of PCL blends and composites for adsorbent use, as well as biomedical applications.