Research advances in the functionalization of hollow plant fibers and their application as thermal insulation fillers

3.1. Morphological structure

Hollow plant fibers, including Willow Catkins, possess a distinct hollow lumen structure. The fiber diameters are approximately 10 times the thickness of their cell walls, resulting in a hollowness ratio reaching as high as 90% [14]. Under brief compression, hollow plant fibers retain intact cell walls, and the collapsed lumens recover readily when released. Kapok fiber is acknowledged as one of the finest natural cellulose fibers, with an average length ranging from 14.5 to 27.5 mm and a diameter of 15-35 μm (lumen diameter: 13.2-16 μm) [26]. The high aspect ratio of these fibers facilitates the formation of additional air layers between them, effectively reducing heat conduction. Furthermore, their non-crimping characteristic contributes to maintaining the structural stability of filling materials, preventing fiber aggregation and ensuring high loftiness, which enhances thermal insulation performance. Scanning electron microscopy (SEM) observations reveal that Kapok fibers exhibit a cylindrical hollow morphology, with both ends sealed: one end is bulbous, while the other tapers gradually. The fiber surface is completely covered by a wax coating, leading to a smooth texture without noticeable roughness [27]. Brunauer-Emmett-Teller (BET) multilayer adsorption analysis indicates that Kapok fibers possess a relatively large specific surface area of 2.99 m²/g [10].

3.2. Wall layer structure

The cell walls of Kapok and similar fibers are approximately 1 μm in thickness, predominantly consisting of helically arranged cellulose microfibril bands, with the helical seams filled with lignin and hemicellulose [28]. Analogous to wood fibers, hollow plant fibers feature a complex, layered cell wall structure. As depicted in Figure 2, both cross-sectional and longitudinal views distinctly reveal multiple layers: the cuticle (S), the cell wall layers (W3, W2, W1), and the inner layer (IS) [29]. The S-layer, serving as the protective outermost layer, demonstrates the highest packing density and the smallest thickness (40-70 nm). The IS-layer, slightly thicker than the S-layer, possesses a looser structure, enabling microfibrils to detach easily and disperse into the lumen. The W1-layer comprises an interwoven network of microfibrils (∼200 nm thick), whereas the W2- and W3-layers consist of axially aligned microfibrils (∼500 nm thick). The W1 and W3 layers are more densely packed compared to W2, and transition zones with low packing density exist between adjacent layers [30]. Notably, the cell walls of hollow plant fibers contain parallel, continuously open pores, with 80% of the total pore volume comprising pores ranging from 2-40 nm in diameter [10]. The presence of abundant nanopores (2-40 nm) in the cell walls, which are nearly two orders of magnitude larger than water vapor molecules (0.33 nm), creates an interconnected capillary network that enables exceptionally rapid moisture diffusion. This unique porous architecture, combined with the gradient density distribution from the dense outer cuticle to the loose inner layer, not only endows hollow plant fiber-based fabrics with superior breathability and moisture-wicking properties but also maintains structural integrity during repeated wetting-drying cycles, making them ideal for sustainable textile applications requiring both comfort and durability.

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Figure 2.

Morphology, cell wall structure, and composition distribution model of Kapok fiber [26].

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3.3. Molecular structure

Hollow plant fibers, exemplified by willow catkin fibers, are predominantly composed of cellulose, hemicellulose, and lignin [8], along with a minor amount of waxy substances [30]. As illustrated in Table 1, the precise composition of these components varies across different types of hollow plant fibers. Cellulose, as the primary constituent, exhibits superior thermal insulation properties due to its low thermal conductivity, making it highly suitable for heat preservation applications [31]. Additionally, certain hollow plant fibers, such as Kapok fibers, contain approximately 3% waxy substances, which play a critical role in conferring hydrophobicity, antibacterial characteristics, and pest resistance to the fibers [13,32].

Beyond chemical composition, hollow plant fibers share a conserved molecular architecture. Figure 2 illustrates the multi-assembly structure of Kapok fibers, where cellulose molecules aggregate into microfibril bundles via hydrogen bonding, aligning parallel to the fiber axis to form the structural framework. Hemicellulose, characterized by its short-branched chains, fills the interstitial spaces between microfibrils. Additionally, the hydroxyl groups on lignin’s phenyl rings form hydrogen bonds with the hydroxyl groups of cellulose glucose units, cross-linking as lignin-carbohydrate complexes (LCCs) to reinforce the fiber cell wall [26].

The organization of fibrils varies across different wall layers [33]. In the W1 layer, cellulose molecules form a network-like structure; in the W2 and W3 layers, cellulose molecules are aligned at varying angles relative to the fiber axis. This intricate arrangement significantly enhances the mechanical strength of the cell wall. Within the fibrils, cellulose molecular chains exhibit a tendency to self-assemble into crystalline regions, while the inter-fibrillar regions consist of disordered chain segments that constitute amorphous regions [34].

The molecular assembly mechanism of hollow plant fibers demonstrates nature’s sophisticated material design strategy. The oriented arrangement of cellulose microfibrils combined with the interfacial crosslinking of hemicellulose and lignin forms a multiscale reinforced architecture, while the alternating distribution of crystalline and amorphous regions achieves an optimal balance between rigidity and flexibility. This hierarchical organization, spanning from molecular-level hydrogen bonding networks to supramolecular LCCS, and finally to microscale layered wall structures, not only explains the fibers’ exceptional thermal insulation and mechanical properties but also provides a theoretical framework for biomimetic material design. By precisely controlling the alignment of cellulose microfibrils and interfacial interactions, it becomes possible to engineer sustainable materials with tailored functionalities.

4.1. Thermal insulation property

Hollow plant fibers possess a remarkable thermal insulation property due to the well-developed lumen structures and abundant mesopores, which effectively immobilize air. The trapped air forms stagnant layers that act as thermal barriers, isolating the human body from cold environments and significantly reducing heat transfer via conduction. Moreover, the cellulose component within hollow plant fibers, characterized by inherently low thermal conductivity, demonstrates robust capabilities in reflecting and scattering thermal radiation. The hollow structure further amplifies these effects by increasing the surface area available for thermal radiation reflection and scattering, thereby minimizing heat loss. As illustrated in Figure 3(a), human body heat primarily dissipates through radiation and convection [37]. For batting materials positioned between fabric layers, heat transfer occurs through two media (air and fibers) via three pathways: direct air conduction, fiber-to-fiber contact conduction, and interfacial conduction at the air-fiber boundary.

The study by Chen et al. [38] highlights the superior thermal insulation potential of poplar catkins, attributing this to their high fill power and thermal resistance. Fill power, a key metric for insulation capability, indicates the volume occupied by a unit mass of fiber. Poplar catkins exhibit a high fill power of 9,340 cm³/28.35g, outperforming down (7,210 cm³/28.35g) and wool (4,588 cm³/28.35g). This is due to their fine diameter and hollow structure. Thermal resistance, another critical indicator, shows poplar catkins have an R_ct value of 0.316 m²°C/W, comparable to wool (0.331 m²°C/W) and polyester (Hollofil II, 0.310 m²°C/W), though slightly lower than down (0.354 m²°C/W). Using a thermal manikin, the study found poplar catkins match the thermal insulation performance of common natural and synthetic fibers. These results position poplar catkins as a promising material for warm clothing and textile products. Cui et al. [39] conducted a systematic study on the performance of kapok/down blended wadding and found that the thermal insulation properties of the blended material improved significantly with increasing Kapok content. When the Kapok content in the blended wadding increased from 0% to 50%, the thermal conductivity of the wadding decreased from 3.79×10^-2 to 3.09×10^-2 W/m·°K, while the heat retention rate increased by approximately 13%. Additionally, Kapok fibers exhibited superior bulkiness (172.2 cm³/g) and compression resistance compared to down fibrils, though repeated compression led to performance degradation due to lumen collapse. The study recommended using a blend of Kapok and whole down in non-repeated compression scenarios to balance thermal insulation, resource sustainability, and antibacterial properties. Figure 3(b) illustrates the research findings regarding composite nonwovens that integrate kapok and milkweed fibers, revealing a consistent reduction in thermal conductivity as the proportion of hollow fiber content increases [40]. These findings provide an important theoretical basis for developing lightweight, high-performance bio-based composite insulation materials.

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Figure 3.

(a) Thermal insulation mechanisms of clothing [37]. (b) The impact of fiber blending ratio on the thermal insulation of Kapok/cotton and Calotropis gigantea/cotton non-woven fabrics [40].

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4.2. Hydrophobicity and moisture management

Hollow plant fibers exhibit a unique combination of hydrophobicity and moisture management capabilities, attributable to their distinctive structural and compositional features. Kapok fibers, as exemplary hollow plant fibers, possess a natural wax coating primarily consisting of long-chain fatty acids and alcohols [11]. The nonpolar C-C and C-H bonds within these molecular structures preclude interaction with polar solvents such as water, thereby conferring inherent water repellency on the fibers. As shown in Figure 4, atomic force microscopy (AFM) observations disclose nanoscale surface wrinkles that form a micro-nano binary structure. This structure significantly enhances the hydrophobic properties of the fibers by inhibiting water droplet adhesion. The porous hydrophobic surface generates negative capillary pressure, effectively preventing liquid water penetration [41]. Upon contact with the fiber aggregate surface, water droplets experience repulsive forces that counteract gravitational effects, enabling them to retain spherical shapes and roll off swiftly without retention.

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Figure 4.

(a) Height image, (b) phase image, (c) 3D view, and (d) Water Contact Angle (WCA) test of the Kapok fiber surface [41].

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Remarkably, these fibers exhibit dynamic moisture regulation characterized by rapid water release and gradual absorption. Specifically, they demonstrate exceptional rates of moisture liberation [42]. Despite their surface hydrophobicity, which inhibits water droplet spreading, the internal lumen and pore structures are capable of absorbing and retaining a limited amount of moisture. Cellulose’s polar groups form hydrogen bonds with water, enabling humidity-dependent adsorption, while surface tension effects facilitate vapor diffusion into lumens [43]. Under conditions of high relative humidity, water vapor diffuses into the hollow cavities, showcasing unique moisture-wicking and absorption capabilities. This balanced moisture regulation enables hollow plant fibers to effectively modulate microclimate humidity, thus ensuring superior wearing comfort. Particularly noteworthy is the fibers’ ability to maintain rapid moisture liberation rates while achieving gradual absorption kinetics, a combination rarely found in synthetic textiles. These fundamental insights could inspire innovative designs for smart textiles requiring simultaneous water protection and breathability, such as sportswear, medical drapes, and protective clothing.

4.3. Mechanical properties

As presented in Table 2, hollow plant fibers, such as those of Kapok and Calotropis, exhibit relatively low tensile strength and bending stiffness [43]. This limitation in mechanical performance stems from their microporous structure and thin cell walls, which may constrain their applicability in spinning processes [44,45]. The variations in mechanical properties of hollow plant fibers can be ascribed to multiple factors, including fiber microstructure, processing techniques, and environmental conditions.

The cellulose content and the degree of orientation within the fine structure significantly influence the stiffness and strength of fibers [25]. Kapok fibers, characterized by a high crystallinity orientation index (COI) of 71.5%, tend to exhibit reduced flexibility, which may result in issues such as brittleness during spinning processes [46]. The processing of hollow plant fibers also has a notable impact on their mechanical properties. Mechanical forces exerted on these fibers disproportionately affect weaker regions. For example, the spiral pattern observed in Kapok fiber cell walls indicates axial weak connections, leading to non-uniform mechanical properties along individual fibers [34]. Environmental factors further contribute to the mechanical performance of hollow plant fiber batting. Wang et al. [46] noted that the compression elasticity of Kapok fiber assemblies performs more effectively under dry conditions, and reducing environmental humidity aids in preserving the hollow structure, thereby enhancing thermal insulation performance.

4.4. Additional properties

Beyond the aforementioned properties, hollow plant fibers such as Kapok exhibit remarkable antibacterial characteristics due to their high lignin and wax content, which effectively inhibits the growth of microorganisms responsible for decay [47]. Furthermore, Willow Catkin fibers display exceptional thermal stability, as evidenced by thermogravimetric analysis (TGA) revealing a thermal degradation temperature range of 201-360°C [12].

Nonwoven fabrics produced from coconut coir, sisal, and banana fibers exhibit superior acoustic damping performance, making them widely applicable in automotive interiors. The inherently low density, large lumen, and thin-walled structure of these fibers increase the friction between sound waves and the surface, thereby effectively converting acoustic energy into mechanical and thermal energy [48]. Comparative analyses indicate that Kapok fibers provide enhanced sound absorption capabilities compared to commercial glass wool and absorbent cotton fibers at reduced thickness levels [49].

As natural fiber materials, hollow plant fibers exhibit thermal insulation performance that equals or surpasses that of synthetic fibers and animal-derived down. Their distinctive structure provides exceptional advantages in hydrophobicity, breathability, and antimicrobial properties, which are challenging to replicate with alternative fibers. From an economic standpoint, Wang et al. [50] suggested replacing duck down with Kapok fibers in cold-weather apparel, highlighting comparable thermal resistance and evaporative resistance at only one-tenth of the material cost. Collectively, these attributes establish hollow plant fibers as a revolutionary material for the development of next-generation winter clothing systems.

5.1. Mechanical reinforcement

Compressive elasticity represents a crucial mechanical property for filler materials. Hollow plant fibers subjected to prolonged compression experience severe structural deformation, resulting in substantial loss of entrapped air within their lumens and thereby diminishing their thermal insulation performance. Consequently, improving the long-term resilience of hollow plant fibers to extend product lifespan has become a pivotal focus in the industry.

Rapid concentrated alkali treatment of flattened Kapok fibers induces swelling in the amorphous regions of cellulose and polymorphic transitions in crystalline domains, thereby facilitating targeted molecular rearrangement. This process successfully restores the hollow structure in 83% of kapok fibers [52]. Li et al. [53] systematically investigated the effects of alkali treatment on the surface morphology and mechanical properties of Kapok fiber. The alkali-treated fibers exhibited irregular etching patterns, which enhanced the surface friction coefficients and consequently improved fiber cohesion during spinning. Initially, the tensile strength of fibers increased; however, it decreased with excessive alkali concentration due to cell wall dissolution. An optimal concentration range of 8-10 g/L was identified for achieving balanced performance. While ethanol, amines, deep eutectic solvents, and ionic liquids exhibit cellulose-swelling properties [54], their utilization in fiber mechanical reinforcement modification has been rarely documented in the literature.

The challenge of insufficient compression elasticity of hollow plant fibers hinders their commercial potential. To address this, researchers have explored various reinforcement strategies. Saxena et al. [55] significantly enhanced the elasticity of the composite material by adding 7% nano calcium carbonate as a filler to the mixed system of banana fiber and sisal fiber, achieving a maximum elastic modulus of 20.61 GPa. This approach highlights the potential of nanofillers in improving the mechanical resilience of natural fiber composites. However, such methods often focus on external additives rather than optimizing the intrinsic structure of the fibers themselves, which may limit their sustainability and cost-effectiveness.

In contrast, fewer studies have explored the development of elastic properties by leveraging the inherent structure of hollow plant fibers. Recent advancements in cellulose-based materials, however, offer promising inspiration. As shown in Figure 5, Chen et al. [56] introduced a highly elastic cellulosic material fabricated via chemical treatment and freeze-drying processes. The modification partially removed lignin and hemicellulose, softening the wood cell walls and forming a honeycomb-like structure with an internal hydrogel network. This unique architecture endowed the material with remarkable compressive elasticity, allowing it to fully recover its original shape under a compressive strain of up to 70% and maintain structural stability even after 10,000 compression/release cycles. The success of this top-down approach suggests that similar strategies could be applied to hollow plant fibers.

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Figure 5.

Elastic enhancement of wood fiber via the ice templating method [56].

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5.2. Superhydrophobic modification

5.2.1. Plasma technology

Plasma treatment (Figure 6a) is a technique that employs high-energy particles to bombard material surfaces, thereby creating 3D micro/nano-scale structures through etching [57]. When applied to inherently hydrophobic materials, plasma etching can enhance surface roughness, which in turn increases water contact angles [58]. Yao et al. [59] reported an environmentally friendly method for the rapid production of superhydrophobic cellulose-based materials. By first utilizing plasma etching to increase the roughness of bagasse fiber surfaces and subsequently depositing trisilanol butyl to reduce surface energy, the water contact angle of the fibers was significantly improved from 2.4° to 152.9°, achieving effective superhydrophobicity.

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Figure 6.

(a) Mechanism of plasma-induced superhydrophobic modification [59]. The * denotes reactive species in an excited state, where O represents excited oxygen atoms generated in the plasma, and Cell represents the activated radicals generated on the cellulose surface. (b) Sol-gel method for preparation of superhydrophobic cellulose paper. TEOS: Tetraethyl orthosilicate [62]. (c) Growth of ZnO nanorods on cotton fabrics via microwave hydrothermal method OTMS: Octadecyltrimethoxysilane [66]. (d) Enzyme etching in conjunction with chemical vapor deposition for the fabrication of robust and eco-friendly superhydrophobic fabrics [70].

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Compared to wet chemical modification, plasma technology not only eliminates industrial wastewater discharge but also enables automated processing, thereby gaining increasing prevalence in the textile industry. Phuong et al. [60] employed plasma etching as a pretreatment method, followed by simple dip-coating, to produce superhydrophobic cotton fibers with a water contact angle reaching up to 173°. The covalent bonds formed between the fiber surface and tetraethyl orthosilicate ensured exceptional wash durability for the treated cotton fabric. Similarly, Tao et al. [61] applied cold plasma technology to hydrophobically modify kapok fibers, exploring the correlation between microstructure and hydrophobicity. After etching, the enhanced surface roughness, characterized by grooves and cracks, expanded the fiber surface area, which improved water resistance and enhanced the capillary effect of the fabric.

5.3. Flame retardant modification

5.3.1. Gamma irradiation

Gamma rays, as a form of high-energy electromagnetic radiation characterized by short wavelengths (0.001-0.0001 mm) and high frequency, possess strong penetrating capability. Chung et al. [72] utilized gamma irradiation to develop flame-retardant Kapok fibers. Their study revealed that gamma irradiation effectively removed flammable lipid compounds from Kapok fibers while maintaining the integrity of their hollow lumen structure. Furthermore, the methoxy groups within lignin were cleaved by gamma irradiation, which enhanced lignin’s thermal stability and thereby slowed down the combustion process. During a 10-min flame resistance test conducted at 550°C in an electric furnace, the control samples were entirely reduced to white ash, whereas all gamma-irradiated samples exhibited incomplete combustion, turning black instead (Figure 7a). Within the radiation dose range of 0-1000 kGy, the flame-retardant performance was found to be directly proportional to the irradiation dose.

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Figure 7.

(a) Kapok fibers were burnt in an electric furnace at 550°C after gamma irradiation [72]. (b) Schematic diagram of the phosphorylation of KF with PA in the presence of urea [73]. (c) Flammability test of raw KF sheet and phosphylated KF sheet [73]. (d) Preparation of PEI/APP-coated ramie fabrics using the layer-by-layer spraying technique [78].

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6. Applications of hollow plant fibers in various fields

6.2. Sportswear

The evaporation of sweat plays a crucial role in athletic performance. Hollow plant fibers, such as those derived from Milkweed, exhibit superior moisture-wicking and breathability by effectively transporting perspiration from the skin to the outer surface of garments for rapid evaporation. Zarehshi et al. [84] developed a nonwoven fabric for sportswear by carding milkweed fibers to form fiber webs, followed by needle punching to enhance fiber cohesion and structural integrity. The capillary action within the lumens of Milkweed fibers facilitates efficient liquid transport driven by surface tension, enabling more effective sweat release compared to traditional cotton fibers. Permeability tests demonstrated that milkweed-based nonwovens exhibited a 46.58% higher water vapor transmission rate compared to cotton fabrics. This significantly reduces the sticky sensation associated with sweat accumulation in cotton-based sportswear, thereby enhancing physiological comfort during physical activity.

6.3. Medical protective apparel

The inherent antibacterial characteristics of hollow plant fibers render them exceptionally well-suited for medical protective products, providing sustainable alternatives to address the global supply chain challenges associated with petroleum-based materials that have been intensified by the COVID-19 pandemic. Mula et al.[85] developed nonwoven fabrics for surgical gowns utilizing Milkweed fibers as the primary matrix, reinforced with bio-based polylactic acid/polybutylene succinate (PLA/PBS) composite fibers through meticulously optimized carding and hot-pressing processes. Performance evaluations conducted in accordance with EN 13795 standards revealed that the milkweed fiber gowns exhibited superior tensile strength compared to commercially available options while preserving excellent mechanical properties even after repeated use. The remarkable hydrophobic nature of Milkweed fibers ensures dependable protection against bloodborne pathogens and wound exudates. Moreover, the gowns fulfill the stringent requirements for air permeability and moisture vapor transmission rate in surgical applications, with adjustable thermal resistance achieved via fiber ratio optimization to ensure optimal thermophysiological comfort during extended procedures (Figure 8b).

6.4. Advanced thermal insulation materials

Aldo et al. [86] successfully developed a novel ultra-lightweight natural fiber insulation board via hot-air processing (Figure 8c). This board is primarily composed of kapok fibers and employs polylactic acid (PLA) or bicomponent fibers as binders, achieving densities as low as 10, 15, and 20 kg/m³. In terms of thermal insulation performance, the board demonstrates exceptionally efficiency, with thermal conductivities of 0.0356 W/(m·K) (using PLA) and 0.03432 W/(m·K) (using bicomponent fibers) at 20°C, both significantly lower than the 0.04075 W/(m·K) of commercial glass wool. Furthermore, the board exhibits superior water-repellent properties due to the hydrophobic wax layer on the surface of kapok fibers and its high porosity. Specifically, it achieves a short-term water absorption rate of less than 1 kg/m² and a contact angle of up to 130°, far exceeding the performance of commercial glass wool.

Dong et al. [87] successfully fabricated a microtubule-based composite material composed of kapok fibers and hollow PET fibers. Through the application of a vacuum impregnation technique, a phase change material (PCM), consisting of myristic acid and tetradecyl alcohol (MA-TD), was effectively encapsulated within the microtubules of the Kapok fibers. Thermal property testing revealed that the PCM-encapsulated composite absorbed 76.8 J/g of latent heat within the comfortable temperature range of 24°C to 42.3°C and released 77.4 J/g of latent heat within the reverse temperature range of 29.7°C to 10.5°C. This material demonstrated both fundamental thermal insulation properties and dynamic thermal regulation capabilities, maintaining stability during repeated laundering cycles and exhibiting robust leakage resistance under continuous heating/cooling cycles, even under pressure.

He et al. [88] developed an eco-friendly wearable active-passive aerogel heater comprising calcium alginate, Kapok fiber, and carbon nanotubes (CNTs) (Figure 8d). The unique independent cavity structure of Kapok fiber substantially enhances the thermal insulation performance of the aerogel heater, demonstrating its low energy consumption and superior personal thermal management capability. Moreover, the incorporation of CNTs in the calcium alginate/Kapok fiber/CNT composite enables excellent solar radiation absorption and Joule heating, effectively warming the human body. Based on heat loss model analysis, the calcium alginate/Kapok fiber/CNT aerogel, when utilized as a wearable heater, can achieve up to 60% thermal energy savings compared to conventional building heating systems. In terms of thermal insulation properties, the material exhibits remarkably low thermal conductivity (0.024 W/(m·K)) and ultra-low density (0.027 g/cm³), significantly inhibiting heat transfer pathways and thus providing exceptional thermal insulation. Additionally, the composite material is capable of absorbing or releasing latent heat within the human comfort temperature range (24°C to 42.3°C or 29.7°C to 40.5°C), facilitating body temperature regulation and enhancing adaptability to cold environments.

Wang et al. [89] successfully developed an eco-friendly Kapok fiber/chitosan composite aerogel material through a simple yet cost-effective fabrication process that integrates wet-laid forming, immersion, and freeze-drying techniques. This material demonstrates exceptional thermal insulation properties, characterized by high porosity (98.32%–98.48%), excellent thermal resistance (0.072–0.079 m²·°C·W^-1), and ultralow density (0.019–0.022 g/cm³). These attributes make it a highly promising candidate for applications in packaging and building insulation. Furthermore, the Kapok fiber/chitosan aerogel exhibits superior amphiphilicity (oil-water affinity), effective UV-blocking capability, remarkable mechanical strength, and compressible resilience, which render it particularly suitable for outdoor environments. Additionally, owing to the inherent antibacterial properties of Kapok fibers, this material holds significant potential for use in food packaging and cold-chain logistics, where it can effectively inhibit microbial growth and significantly extend shelf life.

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Figure 8.

(a) The jacket filled with Kapok fibers [50]. (b) Schematic diagram of medical protective clothing [90]. (c) Kapok-PLA sample surface section [86]. (d) Conceptual illustration of the active-cum-passive wearable heater composed of CA/Kapok fiber/CNT for personal thermal management [88].

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