The fiber fabrication method starts with the assembly of a macroscopic object called a preform31, which serves as a template for the porous fiber we wish to produce. The preform acts here as a reservoir with a well-defined architecture that is filled with a polymer solution before the draw. During the drawing process, the preform is heated in a furnace above the glass transition of the preform materials and above the phase separation temperature of the polymer solution, such that the solution remains homogeneous in the furnace. The preform is drawn into a fiber by constant pulling at a controlled speed, and the fiber cools down to room temperature upon exiting the furnace (Fig. 1a). Provided the polymer solution demixes between the drawing temperature and ambient temperature, the solution phase separates in the fiber leading to a porous polymeric core whose pores are filled with a solvent-rich phase (Fig. 1c), much like in a conventional solution extrusion technique. Phase separation in TIPS processing can either occur through liquid&#;liquid (L&#;L) demixing15, where the two phases initially formed are liquid and the polymer-rich phase solidifies at a lower temperature; or solid&#;liquid (S&#;L) demixing in which case the polymer crystallizes out of the solvent directly14. In this work, we exploit both mechanisms. As an illustrative example, Fig. 1b shows a typical phase diagram for a solution which phase separates via a L&#;L demixing mechanism15, 32. After the draw, the dense cladding illustrated in Fig. 1c may be dissolved with an appropriate solvent and the liquid phase subsequently evaporated, resulting in porous fibers.
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Fig. 1Fiber fabrication method and cross-sections. (a) General illustration of thermal drawing process, with the associated temperature profile. The dashed line denotes the phase separation temperature for the polymer solution in the core. (b) Schematic phase diagram for a generic polymer solution. State 1 is the homogeneous state in the furnace and state 2 is the phase separated state at room temperature. (c) Illustration of a section of drawn fiber with the dense cladding surrounding the porous core. (d, e) Cross-sectional SEM images of porous fibers made of (d) polyvinylidene fluoride and (e) polycaprolactone (PCL) obtained after cladding dissolution
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We demonstrate the process using a cyclic olefin copolymer (COC, TOPAS from TOPAS Advanced Polymers) preform cladding as a cylindrical reservoir. COC is an amorphous thermoplastic displaying excellent chemical resistance to many polar organic solvents, and can be thermally drawn between 150 to 280&#;°C. Before the draw, a cylindrical hollow reservoir is filled with a 1&#;:&#;4 wt. solution of polyvinylidene fluoride (PVdF, Kynar 761, Arkema) in propylene carbonate. This solution has a S&#;L demixing temperature around 35&#;°C33, so that it is homogeneous during the draw process but phase separates in the fiber core as it cools down to ambient temperature. Porous PVdF fibers are obtained by selectively dissolving the COC cladding in toluene. The scanning electron microscopy (SEM) micrograph of the fiber cross-section (Fig. 1d) shows a morphology consisting of interconnected PVdF spherulites separated by voids. Such morphology is common for S&#;L phase separated PVdF membranes14, 32, and arises from nucleation and growth of highly semicrystalline PVdF spherulites33.
This method can be leveraged to generate porous fibers of different materials. As another example, a solution of poly-ε-caprolactone (PCL, Mn&#;80&#;k, Sigma-Aldrich) in a 1&#;:&#;1 wt. mixture of propylene carbonate and triethylene glycol was added to a COC-clad preform. These two miscible solvents act as a latent solvent for the polymer and a phase separation of the solution also occurs in the fiber as it cools down to room temperature. A SEM micrograph of the porous core is shown in Fig 1e. It displays a very different morphology than that of the porous PVdF fibers, with a broad distribution of cellular pores. Supplementary Fig. 1 shows the result of a dye diffusion experiment proving the existence of a continuous diffusion path along the pores&#;the details of the experiment are outlined in Supplementary Note 1. This morphology is indicative of a L&#;L phase separation occurring through nucleation of a solvent-rich phase followed by subsequent solidification of the polymer-rich phase15, 32 and has also been reported previously in literature for PCL membranes34.
This preform-to-fiber TIPS processing methodology is characterized by a number of unique attributes. The first advantage is that we can easily control the fiber cross-sectional geometry, unlike in fiber extrusion-based processes which requires complex spinneret engineering as well as optimized cooling and solidification conditions35. The preform acts here as a template for the porous fiber, and by machining the internal geometry of the preform into arbitrary shapes, we can produce porous fibers with complex external geometries such as triangular, or cross-shaped (Fig. 2a&#;d). The ability to do so is a consequence of the large viscosity contrast between the cladding and the solution during the drawing process. At the high draw temperature, the cladding material exhibits a viscosity close to 104&#;Pa&#;s&#;1, roughly four to five orders of magnitude higher than that of the polymer solution (see Supplementary Note 2 and Supplementary Fig. 2). This high viscosity of the cladding kinetically prevents the capillary rounding of sharp features to occur, whereas the low-viscosity solution fills the whole accessible volume and phase separates into a porous fiber of the same shape (cf. Supplementary Methods and Supplementary Figs 5 and 6 for more details on the drawing process). The external geometry of the porous fiber is therefore only limited by our ability to machine a preform of the matching shape, and does not require complex spinneret engineering and optimization of spinning conditions. Even with advanced spinneret design and spinning processes, obtaining geometries that significantly deviate from equilibrium still presents significant challenges35. Controlling the external geometry could enable control over the mechanical properties of the fibers, as was shown for natural jute fibers for instance36.
Fig. 2Control of external geometry and architecture. (a) Schematic illustration of a preform with a cross-shaped reservoir for PVdF solution and (b) SEM micrograph of the cross-shaped porous PVdF fiber after drawing and cladding dissolution. (c) Schematic illustration of a preform with a triangle-shaped reservoir for PVdF solution and (d) SEM micrograph of the triangle-shaped porous PVdF fiber after drawing and cladding dissolution. (e) Schematic illustration of a cyclic olefin copolymer preform with a cylindrical reservoir for polycaprolactone (PCL) solution lined with a thin LDPE wall and (f, g) SEM micrographs of the final porous PCL core/dense LDPE shell fiber after drawing and cladding removal
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The second and most important key benefit of the preform-to-fiber TIPS is the ability to combine multiple materials adjacent to a porous domain. In solution extrusion-based methods, this requires the design of advanced concentric co-extrusion nozzles. Here, multimaterial structural control is achieved by initially designing and assembling a preform template with the desired materials. We demonstrate this by producing a porous PCL core/dense low-density polyethylene (LDPE) shell fiber, using our preform-to-fiber TIPS approach (Fig. 2e&#;g). First, a preform is constructed consisting of a hollow core lined by a shell of LDPE of 200&#;µm, embedded in a large centimer-size COC cladding (Fig. 2e). Second, the PCL solution is introduced into the hollow core and the multimaterial preform is drawn into a fiber. Finally, the COC cladding is selectively dissolved using cyclohexane and the solvent is dried out of the core to leave only a thin shell of LDPE surrounding a porous PCL core (Fig. 2f&#;g). Here, the in-fiber LDPE shell wall thickness is 10&#;µm, but in principle it can be made thinner by faster drawing speeds. This type of concentric core-shell cylindrical structure could be useful for gas separation fibers with dense selective barriers2, 3.
In addition to external geometry and materials distribution, the preform-to-fiber TIPS method also allows some degree of control over the microstructure of the porous domain through control of the quenching temperature, specifically for L&#;L demixing solutions. Figure 3 demonstrates the effects of fiber quenching temperature on the average pore size of a PCL porous core fiber, estimated from an image analysis of the cross-sectional micrographs. For this study, fiber samples were reheated post-draw to T reheat&#;=&#;150&#;°C in order to rehomogenize the solution, and quenched for a fixed time Δt&#;=&#;5&#;min in a water/ethylene glycol bath at set temperatures T bath. Three fiber samples were post-processed per quenching temperature, and a total of 6 SEM micrographs were analysed per cooling condition. The results indicate that fibers quenched in colder baths exhibit smaller average pore sizes, with an apparent transition in the behaviour of pore size vs. quenching temperature around 40&#;°C.
Fig. 3Influence of quenching temperature on microstructure. (a) Average pore size for different quenching temperatures in ethylene&#;glycol&#;water bath. The error bar corresponds to the SD over the mean pore size in the six images analysed per quenching conditions. The inset displays an illustration of the possible phase diagram with a dashed line at the working concentration, highlighting both L&#;L and S&#;L demixing. (b&#;d) Associated cross-sectional SEM images for fibers quenched at &#;20, 20 and 60&#;°C
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These results can be rationalized considering the specifics of the PCL solution&#;s phase diagram, schematically represented in the inset of Fig. 3a. At our working concentration, the solution exhibits a L&#;L demixing at a relatively elevated temperature (&#;110&#;°C, observed visually by appearance of turbidity in the solution), followed by a PCL solidification at lower temperatures, around 40&#;°C (also observed qualitatively). Therefore, when quenched above 40&#;°C, we expect the L&#;L phase separated structure to exhibit coarsening. During coarsening, the system seeks to minimize the interfacial energy between the polymer-rich phase and the solvent-rich phase by minimizing the surface area and forming larger voids32, 37. This can happen either through diffusion of molecules from small to large droplets (Ostwald ripening), coalescence of droplets or hydrodynamic flow38, 39. All these processes result in scaling laws of the form:
$${\left\langle {R} \right\rangle}^\alpha = {\left\langle {{R_0}} \right\rangle ^\alpha } + {K_\alpha }\left( T \right){\rm{\Delta }}t$$
(1)
where α is 3 for Ostwald ripening or coalescence-based coarsening and 1 for coarsening through hydrodynamic flow, K α is a prefactor depending on temperature and on the exact mechanism involved, and R 0 is the average droplet nucleus size38, 39. Although the exact functional for K α is unknown for our system, all of the associated processes are thermally activated and thus their rates grow with increasing temperature&#;explaining the increase in pore size with temperature. For samples quenched in a bath below 40&#;°C, the PCL will rapidly crystallize, thus setting the structure and preventing further coarsening. The effective time of coarsening Δt eff, or time during which the structure is in a L&#;L state, is reduced as the sample is convectively cooled from T reheat to T bath. At deep enough quenching, coarsening is almost completely suppressed and the average pore size approaches the droplet nucleus size, setting a lower limit on pore size for L&#;L demixing solutions. For this reason, the pore size is nearly temperature independent for deep quenching as seen in Fig. 3a. The Supplementary Discussion gives more details on the coarsening behaviour.
In practice, this cooling step could be added in series with the drawing process itself to permit direct control of the pore size along arbitrary lengths of fiber. The coarsening mechanism varies between solution systems, specific concentrations, temperatures and time&#;but α and K α can be determined experimentally by performing measurements of the average pore size with time at fixed temperatures39, 40. This knowledge could then be used to determine specific cooling conditions required to obtain a desired pore size.
So far we have demonstrated the ability with our method to control the microstructure as well as incorporate multiple materials adjacent to a porous domain within a fiber. These features open up the possibility of introducing functionalities in porous fibers that go beyond conventional passive mass transport. To demonstrate this idea, we developed a fiber that can be used to measure the ionic conductivity of an ionically conductive liquid filling the porous domain in the core of a fiber.
In our process, when the fiber exits the furnace and cools down, the core consists of a porous polymer filled with a solvent-rich liquid phase. By initially introducing a soluble and thermally stable ionic liquid in the core solution, the pores can be filled with an ionically conductive electrolyte in a single step. We can then use ionic conductivity measurements to probe the transport properties of the fibers. We focus here on measurements performed in the transverse direction of the fiber, whereas experiments on axial measurements are presented in Supplementary Note 3 and shown in Supplementary Fig. 3.
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We start by employing the preform-to-fiber TIPS method to fabricate a fiber with the cross-section shown in Fig. 4a. The core is a porous domain prepared from a solution of 1&#;:&#;4 wt. PVdF and propylene carbonate, with the addition of a 10&#;3&#;m (mol&#;kg&#;1 of solvent) PYR13TFSI (Solvionic) ionic liquid in propylene carbonate providing ionic conductivity. We selected PYR13TFSI as an ionic liquid owing to its solubility in propylene carbonate and high thermal stability preventing degradation during the draw. A low ionic liquid concentration was intentionally selected to decrease the conductivity of the electrolyte and later be able to neglect resistive contributions from other elements in the system. Adjacent to the porous core are two carbon-loaded polyethylene electrodes (CPE), contiguous with two eutectic Bi-In (Indium Corporation) metal buses. The combination of a porous domain, composite electrodes, and metal buses within a single fiber is out-of-reach to standard extrusion-based processing methods for porous fibers. Here we are able to produce the fiber by initially combining the different materials in a prescribed architecture at the preform level, subsequently inserting a solution in the core, and drawing the preform into a fiber.
Fig. 4Transverse ionic transport measurement through impedance spectroscopy of ionic liquid-filled porous core fibers. (a) Optical micrograph of fiber sample displaying a porous core filled with a 10&#;3&#;m PYR13TFSI in propylene carbonate solution, adjacent CPEs and contiguous Bi-In metal buses. (b) Simple equivalent circuit expected from fiber samples. CPA refer to Constant Phase Angle elements. (c) Photograph of a connected fiber sample. (d) Impedance spectra for a fiber between 12 and 1&#;cm, sequentially cut by 10&#;mm decrements. Solid lines are fittings results with equivalent circuit. The inset shows the dependence of R Gel as a function of the inverse-length of the fiber and shows a linear relationship, as expected from geometric considerations
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In-fiber ionic conductivity was measured by AC impedance spectroscopy. This method consists of measuring the impedance of a sample over a wide frequency range and mapping the results to a physically motivated equivalent circuit41. In our case, the simplest equivalent circuit is presented in Fig. 4b, following typical analysis of electrolyte systems. For a given fiber length, the CPE electrodes are counted for their resistive contribution through R CPE, whereas the electrode/gel-electrolyte interface is modeled as a Constant Phase Angle element \(Z_{{\rm{int}}}^{{\rm{CPA}}}\). The bulk porous-electrolyte is modeled with a parallel circuit of a resistive component R Gel associated with ion-transport in the liquid region and a capacitive component \(Z_{{\rm{Gel}}}^{{\rm{CPA}}}\) in the form of a CPA element, which takes into account polarization effects in the dielectric electrolyte region. CPA elements, also known as Constant Phase Angle elements, are used in AC impedance models and data fitting to represent so-called imperfect capacitor behaviors41.
To perform the measurement, the fiber samples are cut to a specific length and the tips are sealed with wax to prevent solvent evaporation. Electrical connection to the fibers is established by mechanically exposing the metal buses on both sides and connecting them to the instrument (Solartron &#;A) using copper clips (Fig. 4b). The impedance is measured over the frequency range 1&#;MHz&#;100&#;Hz and gradually the fiber is cut by 10&#;mm increments. Nyquist plots for such samples are shown in Fig. 4d, with best fits from the equivalent circuit detailed above. From this we can extract the resistance of the porous electrolyte for various lengths&#;in practice, close to the diameter of the high frequency semicircle. The inset shows the plot of R Gel as a function of fiber inverse length, which follows a linear behavior expected from:
$${R_{{\rm{Gel}}}} = \frac{1}{{{\sigma _{{\rm{Gel}}}}}} \cdot \frac{t}{{w \cdot L}}$$
(2)
where σ Gel is the ionic conductivity of the porous electrolyte, t is the thickness and w the width of the electrolyte region. We deduce the effective ionic conductivity of 2.87&#;±&#;0.47&#;µS&#;cm&#;1 (n&#;=&#;5 samples) from geometric considerations, compared with 2.09&#;µS&#;cm&#;1 for the pure PYR13TFSI in PC electrolyte (see Supplementary Note 4 and Supplementary Fig. 4). We attributed the discrepancy to possible inhomogeneities in the ionic liquid concentration, as well as possible solvent evaporation during the drawing process.
This experiment not only demonstrates our ability to produce complex fibers with porous domains, composite polymers and metals, but also that we are able to build functionality into the fiber. In this case we can add electrical capabilities by introducing conductive materials adjacent to the porous core and use these capabilities to get information on the electrolyte contained in the core. This capability, for example, could be applied later to smart textiles with the ability to intake and electrochemically monitor sweat.
Finally, the preform-to-fiber TIPS method is not only a platform for production of porous fibers, but it can also be harnessed to produce porous microspheres. To this end, we build on previously established results regarding controlled capillary instabilities in thermally drawn fibers42,43,44. After the draw, fibers (with porous core and non-porous cladding in place) are reheated above the glass transition temperature of the cladding material and critical temperature of the solution. The solution homogenized and the cylindrical core evolves into a row of particles, under the effect of capillary forces. The timescale to complete breakup is a function of the core size, viscosity of both the cladding and the core, as well as the surface tension of the core/cladding interface45. Depending on the sizes and processing temperature, this timescale is on the order of tens of minutes to hours (cf. Supplementary Movie 1). After the complete formation of spheres, we could quench the sample and thus obtain a fiber whose core consisted of a series of discrete porous particles, which could later be extracted by cladding dissolution. A schematic representation of this general process is shown in Fig. 5a.
Fig. 5Porous microsphere production with controlled capillary breakup of porous core fibers. (a) General illustration of the capillary breakup process. Fibers are reheated above the phase transition temperatures of the solution and glass transition of the cladding. The core then evolves into a row of spheres under the effect of surface tension. Once breakup is completed samples are rapidly quenched. (b, c) SEM image and close-up of a polycaprolactone (PCL) porous microsphere and of (d, e) a polyvinylidene fluoride porous microsphere
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By applying this method to COC-clad fibers with cores filled with either PCL or PVdF solutions, we obtained porous spherical particles of both polymers. We show SEM micrographs of both types of particles in Fig. 5b&#;e. Porous particles such as these could find applications in solid phase extraction or chromatography46. Furthermore, one could combine breakup with multimaterial porous fibers to generate structured multimaterial porous microspheres.
This review introduces an innovative technology termed &#;Micro-Extrusion Foaming (MEF)&#;, which amalgamates the merits of physical foaming and 3D printing. It presents a groundbreaking approach to producing porous polymer fibers and parts. Conventional methods for creating porous materials often encounter obstacles such as the extensive use of organic solvents, intricate processing, and suboptimal production efficiency. The MEF technique surmounts these challenges by initially saturating a polymer filament with compressed CO 2 or N 2 , followed by cell nucleation and growth during the molten extrusion process. This technology offers manifold advantages, encompassing an adjustable pore size and porosity, environmental friendliness, high processing efficiency, and compatibility with diverse polymer materials. The review meticulously elucidates the principles and fabrication process integral to MEF, encompassing the creation of porous fibers through the elongational behavior of foamed melts and the generation of porous parts through the stacking of foamed melts. Furthermore, the review explores the varied applications of this technology across diverse fields and imparts insights for future directions and challenges. These include augmenting material performance, refining fabrication processes, and broadening the scope of applications. MEF technology holds immense potential in the realm of porous material preparation, heralding noteworthy advancements and innovations in manufacturing and materials science.
Porous materials featuring precisely defined and interconnected porous structures have attracted considerable attention across diverse fields owing to their distinctive properties and versatile applications [1,2,3]. The distinctive nature of porous structures arises from their fundamental characteristics, including a high surface area-to-volume ratio, interconnected pore networks, and a controllable pore size distribution [4,5]. Within the realm of porous polymer materials, porous fibers and porous parts emerge as two distinct morphological entities characterized by different dimensional attributes, yet they are inherently interconnected. Porous fibers denote materials composed of fibers exhibiting a porous structure, whereas porous parts constitute integral components or materials with pores or porous structures throughout their overall composition [6]. Porous fibers highlight the pore structure at the scale of individual fibers, whereas porous parts exhibit greater versatility, serving as integral components of diverse shapes, and find application across a broader spectrum of uses. This association stems from the broader understanding that porous fibers can be considered a subset of porous parts. This categorization is justified by the transformation of porous fibers into the latter category through processes such as stacking, weaving, and combining. Serving as significant subcategories, both porous fibers and porous parts exhibit extensive potential for diverse applications [7]. They exhibit exceptional functional properties, including superior adsorption [8,9], permeation separation [10,11,12], thermal insulation [13,14], hydrophobic properties [15,16,17], and catalytic properties [18,19,20,21]. Consequently, they find widespread utilization in various domains such as supercapacitors [22], drug delivery systems [15,23], wastewater purification [5,24], thermal insulation materials [25], sound absorption materials [26,27], sensors [28,29], and tissue engineering [30,31], as shown in . Significantly, these subclasses face shared challenges, including the regulation of pore structure, material selection, and fabrication methods. These challenges play pivotal roles in the entire preparation process of porous materials. Through a comprehensive grasp and application of the principles governing the functional applications of porous structures, researchers can adeptly customize and regulate characteristics such as porosity, pore size, and pore distribution to meet specific requirements [32,33,34,35]. This enables a deeper exploration and refinement of the design, fabrication, and utilization of porous materials.
Various fabrication methods have been developed and applied in the preparation of porous structures to cater to different application requirements. A fiber is a slender material with a high aspect ratio, which can be derived from natural sources, including plant fibers (such as cotton and linen) or animal fibers (like wool and silk) or artificially synthesized fibers, as in the case of synthetic fibers (such as nylon and polyester) [42]. Porous fibers are characterized by the presence of numerous small pores or voids within the fiber material. These pores can exhibit various scales, ranging from microscopic and nanoscale to macroscopic at the millimeter level [43]. The manufacturing methods of porous fibers have been closely associated with spinning techniques [44,45,46,47,48]. In , Yang et al. [49] prepared porous polyamide (PA) fibers by dry spinning using a solution of PA dissolved in a formic acid/chloroform co-solvent. The mechanical properties of the resultant fibers were investigated, revealing that the development of the porous structure could be attributed to the evaporation of the low-boiling-point solvent during the spinning process. In recent years, advancements in equipment and deepening research have given rise to new methods for fabricating porous fibers, garnering significant attention. Notably, high internal phase emulsion template methods [50,51,52], coaxial wet spinning [25,53], freeze spinning [6,13,54], and microfluidic spinning [37,55] have gained prominence. Among them, wet spinning refers to a molding method where the spinning solution is extruded into a coagulation bath through a syringe, resulting in the solidification of the polymer into porous fibers through a double-diffusion process. These methods share a common strategy for fabricating porous fibers, involving the initial formation of fibers in a solvent, followed by post-processing to remove the second phase and obtain the desired pore structure. This implies that their fabrication processes involve the use of significant amounts of organic solvents, challenging conditions for controlling the reaction, stringent environmental requirements, and complex post-processing procedures. Recent studies propose the integration of electrospinning with gas dissolution foaming to achieve porous fibers at the micrometer or even nanometer scale [56,57]. This approach harnesses the benefits of spinning technology while minimizing solvent usage. Porous parts share similar concepts of pore formation with porous fibers. The frequently used preparation methods include the high internal phase emulsion template method [58,59], solvent etching [60], freeze-drying [61,62], and thermally induced phase separation [63]. Foaming technology stands out as a commonly employed method for the preparation of porous materials, accomplishing lightweighting and the formation of closed-cell structures by introducing gas bubbles into the material [64,65]. Nevertheless, these methods are limited to generating sheet- or block-shaped porous materials and are accompanied by environmental concerns and complexity. In general, the preparation and production of porous fibers and parts encounter substantial challenges concerning environmental sustainability and economic viability. The efficient and continuous execution of production operations, as well as the creation of complex three-dimensional porous parts, have prompted researchers to explore novel methods that are both simple and environmentally friendly while maintaining high operability.
Over the past three decades, 3D printing, also recognized as Additive Manufacturing (AM), has made substantial advancements and is currently deployed across diverse fields [66,67]. For the manufacturing of structurally complex components, AM provides an economical production solution instead of traditional processes [68,69]. Fused Deposition Modeling (FDM) is one of the most widely used 3D printing technologies, employing a layer-by-layer deposition approach [70]. The most commonly used implementation process in this technology is the filament extruder [71]. When printing and forming, thermoplastic material is extruded in a filament from a nozzle and is deposited layer by layer on the build platform. Each layer is rapidly cooled and solidified to achieve precise printing, as illustrated in . Once a layer is printed, the construction platform will descend, and subsequent layers will be deposited and adhered to the top of the previous layer [71]. Researchers have previously integrated 3D printing with fabrication methods for porous parts, leading to the emergence of innovative manufacturing processes [72,73,74]. For example, Choi et al. [41] combined the chemical foaming method with FDM technology to achieve in situ foaming and the one-step formation of lightweight (polylactic acid) (PLA) parts. This approach simplifies the fabrication of graded porous parts and provides high shape freedom for porous structures. Furthermore, Li et al. [75] have recently introduced a novel method for preparing porous fibers by combining foaming technology with the FDM process. This approach has demonstrated success while concurrently addressing environmental protection considerations and enhancing production efficiency. In the last decade, driven by increasing environmental awareness, foaming technology has undergone a shift toward adopting green and sustainable practices. Technologies such as water-based foaming and supercritical fluid foaming have gained prominence, reducing reliance on organic solvents and minimizing environmental pollution. These advancements present new avenues for the manufacturing of porous fibers and porous parts. MEF belongs to environmentally benign physical foaming techniques. It has emerged as a new method in recent years to expand the manufacturing capabilities of porous fibers and porous parts, becoming a hot research trend in the foaming field.
In response to environmental concerns, challenges related to complex processing, and low fabrication efficiency in the preparation of porous polymer fibers and parts, a novel solution has been proposed and developed: Polymer High-Pressure Fluid MEF technology. In contrast to alternative processes and technologies, this method achieves a green, environmentally friendly, highly efficient, and scalable preparation process. MEF stands out as a novel method for the preparation of porous fibers and parts, amalgamating the benefits of physical foaming while drawing inspiration from FDM technology. This technique involves impregnating a compressed gas into the polymer melt and inducing cell nucleation and growth during the melt extrusion process, resulting in porous structures. The aim of this review is to furnish a comprehensive overview of the process principles underlying MEF technology, coupled with a synthesis and outlook on recent research developments. The content is structured into four main segments. Firstly, the review introduces the overall process of MEF technology, covering its classification, front-end preparation, and fundamental principles. Secondly, two distinct sections are dedicated to the discussion of options for fabricating end products, specifically porous fibers and porous parts. Lastly, the review outlines the prospects for future research concerning the preparation of porous fibers and parts via MEF, as depicted in .
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