Liquid for Fused Deposition Modeling Technique (L-FDM)—A Revolution in Application Chemicals to 3D Printing Technology: Color and Elements

Featured Application: The L-FDM technique enables the direct introduction of chemicals, dyes, radioactive substances, pesticides, antibiotics, nanoparticles, trace elements, fertilizers, phos-phors, monomers for polymerization, proteins, peptides, and active ingredients in the direct printing process from a polymer material with a typical FDM printer. With the proposed technology, it is now possible to introduce chemical substances into polymer ﬁlaments that were previously impossible to apply due to undergoing physical or chemical transformations during previous processing processes. This article discusses methods that eliminate the need for costly and energy-consuming processing equipment. These methods can be utilized in any laboratory by users without access to specialized devices. Abstract: This article presents a novel 3D printing technique called L-FDM (liquid for fused deposition modeling), which is based on the deposition of molten thermoplastic material. The new method allows for the direct introduction of chemicals and polymer ﬁlament modiﬁcations during the printing process. In contrast to traditional incremental methods, L-FDM eliminates the need for extra granulating, extrusion, and processing equipment, making it possible to introduce chemical additives to the polymer matrix directly. This opens up exciting possibilities for chemical laboratories to test and experiment with new and known chemicals through 3D printing. The article discusses the technical aspects of L-FDM and its potential applications and provides practical examples of direct ﬁlament modiﬁcations using the technique. The results of these modiﬁcations were veriﬁed using a colorimeter, electron microscopy (SEM/EDS), and optical microscopy.


State-of-the-Art
Since the 1980s, there has been a rise in contemporary additive manufacturing (AM) technologies, commonly referred to as 3D printing. These technologies enable the use of a wide range of materials, including polymers, metals, ceramics, food, and biomaterials [1]. Three-dimensional printing provides several benefits, such as the ability to produce objects directly without the need for a complete technological infrastructure, the capacity to manufacture products with complex geometries, design flexibility, cost-effective use of materials, and eco-friendliness [2]. Among the various AM techniques, the most significant methods are stereolithography (SLA), selective laser sintering (SLS), and fused deposition This study explores the potential of directly introducing dyes or chemical compounds to the polymer matrix during printing introducing. A diagram of the conventional FDM additive technology is depicted in Figure 2. This research is based on using a 3D printer as a laboratory tool to introduce various chemical substances, e.g., dyes, solvents, organic compounds, and organosilicon compounds, into a polymer matrix. Most studies on polymer modification rely on traditional techniques that involve adding additives during the liquid polymer state. These methods include extrusion, mixing, rolling, and kneading in mixers [16] or mixing the modifier with a polymer solution in a volatile solvent and the subsequent removal [8]. This article discusses methods that eliminate the need for costly and energy-consuming processing equipment. These methods can be utilized in any laboratory by users without access to specialized devices. The proposed method of introducing chemical substances into the polymer matrix consists of surface modification of the filament through immersing in a solution (see Figure 3).
The filament is passed through a reservoir containing a modifier (a liquid chemical substance or its solution) and then through a drying or excess substance removal system. During the next stage, the filament that has been coated with the modifier is fed directly This study explores the potential of directly introducing dyes or chemical compounds to the polymer matrix during printing introducing. A diagram of the conventional FDM additive technology is depicted in Figure 2. This study explores the potential of directly introducing dyes or chemical compounds to the polymer matrix during printing introducing. A diagram of the conventional FDM additive technology is depicted in Figure 2. This research is based on using a 3D printer as a laboratory tool to introduce various chemical substances, e.g., dyes, solvents, organic compounds, and organosilicon compounds, into a polymer matrix. Most studies on polymer modification rely on traditional techniques that involve adding additives during the liquid polymer state. These methods include extrusion, mixing, rolling, and kneading in mixers [16] or mixing the modifier with a polymer solution in a volatile solvent and the subsequent removal [8]. This article discusses methods that eliminate the need for costly and energy-consuming processing equipment. These methods can be utilized in any laboratory by users without access to specialized devices. The proposed method of introducing chemical substances into the polymer matrix consists of surface modification of the filament through immersing in a solution (see Figure 3).
The filament is passed through a reservoir containing a modifier (a liquid chemical substance or its solution) and then through a drying or excess substance removal system. During the next stage, the filament that has been coated with the modifier is fed directly This research is based on using a 3D printer as a laboratory tool to introduce various chemical substances, e.g., dyes, solvents, organic compounds, and organosilicon compounds, into a polymer matrix. Most studies on polymer modification rely on traditional techniques that involve adding additives during the liquid polymer state. These methods include extrusion, mixing, rolling, and kneading in mixers [16] or mixing the modifier with a polymer solution in a volatile solvent and the subsequent removal [8]. This article discusses methods that eliminate the need for costly and energy-consuming processing equipment. These methods can be utilized in any laboratory by users without access to specialized devices. The proposed method of introducing chemical substances into the polymer matrix consists of surface modification of the filament through immersing in a solution (see Figure 3). millions of different chemical compounds are discovered worldwide, and the properties and potential uses/applications of most of them have not yet been explored. Most additives introduced into the polymer matrix serve functional purposes, such as improving product durability under different atmospheric conditions or enhancing antistatic properties, increasing the strength and usability, changing the thermal properties, and modifying the color, among others [17][18][19][20]. 2)). The filament modification presented in the diagrams in Figure  4(2.3,2.4) also involves passing the filament through a reservoir containing a modifier and a drying system, with the difference that, in the next stage, it does not go directly to the print head but is rewound onto the spool. This method also allows for multiple coatings of the filament surface with a chemical substance or the use of several different substances that dissolve in different solvents by winding the modified filament back onto the spool and passing it through the system again ( Figure 4(2.3)) or by using multiple reservoirs with different modifiers (Figure 4(2.4)). This feature enables the filament to be utilized at a different location or time than the modification procedure itself. The filament is passed through a reservoir containing a modifier (a liquid chemical substance or its solution) and then through a drying or excess substance removal system. During the next stage, the filament that has been coated with the modifier is fed directly to the 3D printer head. Once there, it is melted and extruded through a nozzle, forming a threedimensional object layer by layer. This method enables the testing of a large number of substances with high efficiency and throughput for creating previously unknown functional materials that open up entirely new application possibilities. Currently, tens of millions of different chemical compounds are discovered worldwide, and the properties and potential uses/applications of most of them have not yet been explored. Most additives introduced into the polymer matrix serve functional purposes, such as improving product durability under different atmospheric conditions or enhancing antistatic properties, increasing the strength and usability, changing the thermal properties, and modifying the color, among others [17][18][19][20].
Coating a filament's surface with a liquid modifier can occur either during printing ( Figure 4 2)). The filament modification presented in the diagrams in Figure 4(2.3,2.4) also involves passing the filament through a reservoir containing a modifier and a drying system, with the difference that, in the next stage, it does not go directly to the print head but is rewound onto the spool. This method also allows for multiple coatings of the filament surface with a chemical substance or the use of several different substances that dissolve in different solvents by winding the modified filament back onto the spool and passing it through the system again ( Figure 4(2.3)) or by using multiple reservoirs with different modifiers (Figure 4(2.4)). This feature enables the filament to be utilized at a different location or time than the modification procedure itself. In this article, we demonstrate the feasibility of incorporating particular substances into the polymer matrix during the printing process through the use of L-FDM technology. The obtained modifications were validated by a colorimetric analysis, optical microscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS). In this article, we demonstrate the feasibility of incorporating particular substances into the polymer matrix during the printing process through the use of L-FDM technology. The obtained modifications were validated by a colorimetric analysis, optical microscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS). (PET-G) pellets type Select BD 110 was purchased from Selenis (Portalegre, Portugal) and extruded into the filament. PLA WOOD filament was purchased from Spectrum Filaments, WOODFILL filament from ColorFabb (Belfeld, The Netherlands), and acrylonitrile butadiene styrene (ABS+) filament and acrylonitrile styrene acrylate (ASA) filament from Devil Design (Mikołów, Poland).

Solid Chemicals
The solid chemicals were purchased from the following source: silver nitrate, iron (III) chloride, lead acetate, and rubidium sulphate were from Merck (Darmstadt, Germany).

Dyes and Pigments
The dyes and pigments were purchased from the following sources: rhodamine B from Warchem (Warszawa, Poland); alkaline blue, methylene blue, methyl orange, eosin, fluorexone from Merck; transparent dye from Ag-Bet (Drzewoszki Wielkie, Poland); fabric dye from Biel i Kolor (Gieczno, Poland); transparent red dye from Moldoo; and printer ink 545 Black PIXMA 545 from CANON (Tokyo, Japan).

Devices and Methods
Test samples were obtained using the L-FDM method described in Section 1.2. Three printer models were used: (1) For printing brittle materials, as well as materials with high flexibility, i.e., PLA WOOD, WOODFILL, a TPU Original Prusa i3MK3S+ from Prusa Research a.s. was used. A printer with a direct extruder was used ( Figure 5). (2) For printing samples from materials with high thermal shrinkage, i.e., ABS and ASA, a printer with a thermal chamber FlashForge Guider IIS (from FlashForge) (ABS, ASA) was used. (3) PLA and PET-G filaments were printed on Creality Ender 5 (Creality 3D) with a Bowden extruder. Table 1 presents the printing parameters. Two types of 3D models were printed: bars (i.e., cubes of dimensions of 4 mm by 10 mm by 80 mm) and cylinders with a diameter of 20 cm. Detailed technical specifications of the printers used are included in the Supplementary Materials.

Analyses-Instrumental Methods and Measurements
Colorimetric measurements were performed using an EnviSense NR60CP colorimeter for the L*a*b* parameters (also referred to as CIELab). The diameter of the conical cap was 4 mm, and the detector was a silicon photodiode.
The morphology and microstructure of the prepared composites were observed by scanning electron microscopy (SEM). The imaging was performed in three SE modes. The surface observations were analyzed using a Hitachi SU70 scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) for the chemical analysis.
Light microscopy images of the surface and fractures of the composites were taken using a KEYENCE VHX-7000 digital microscope (Keyence International, Mechelen, Belgium, NV/SA) with a VH-Z100R wide-angle zoom lens at 100× magnification. The images were taken with a depth composition and the aid of 3D imaging built-in software (System Ver. 1.05).

Analyses-Instrumental Methods and Measurements
Colorimetric measurements were performed using an EnviSense NR60CP colorimeter for the L*a*b* parameters (also referred to as CIELab). The diameter of the conical cap was 4 mm, and the detector was a silicon photodiode.
The morphology and microstructure of the prepared composites were observed by scanning electron microscopy (SEM). The imaging was performed in three SE modes. The surface observations were analyzed using a Hitachi SU70 scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) for the chemical analysis.
Light microscopy images of the surface and fractures of the composites were taken using a KEYENCE VHX-7000 digital microscope (Keyence International, Mechelen, Belgium, NV/SA) with a VH-Z100R wide-angle zoom lens at 100× magnification. The images were taken with a depth composition and the aid of 3D imaging built-in software (System Ver. 1.05).

Colour
According to the research conducted, L-FDM technology enables the incorporation of coloring substances and elements. The feasibility of dyeing the filament during the printing process was investigated, using commercially available dyes such as rhodamine B, alkali blue, methylene blue, methyl orange, eosin, fluorenone, transparent red dye from Moldoo, and black printer ink from Canon (as shown in Figures 6-9 and listed in Table 2). Initially, bar-shaped objects were printed, and then, a colorimetric and microscopic analysis was conducted to evaluate their characteristics. Further analysis was performed on the bars printed using L-FDM technology, utilizing a colorimeter and optical microscope for the structural analysis.
printing process was investigated, using commercially available dyes such as rhodamine B, alkali blue, methylene blue, methyl orange, eosin, fluorenone, transparent red dye from Moldoo, and black printer ink from Canon (as shown in Figures 6-9 and listed in Table 2). Initially, bar-shaped objects were printed, and then, a colorimetric and microscopic analysis was conducted to evaluate their characteristics. Further analysis was performed on the bars printed using L-FDM technology, utilizing a colorimeter and optical microscope for the structural analysis. Figure 6. L-FDM-printed colored bars. Short sample labels were placed above the bars. The photo shows polylactide bars. The first one from the left is the reference sample, while the following ones are colored samples with (from the left): alkaline blue, methylene blue, methyl orange, eosin, fluorexone, transparent red dye by Moldoo, and black printer ink by Canon. Table 2. Samples presented in Figure 6 and their markings.

Marking of the Sample Full Sample Name/Filament and Solution Used in Printing L-FDM Samples
PLA + alkaline blue dissolved in methanol 1. 2 PLA + methylene blue dissolved in methanol 1. 3 PLA + methyl orange dissolved in methanol 1. 4 PLA + eosin dissolved in methanol 1. 5 PLA + fluorexone dissolved in methanol 1. 6 PLA + transparent red dye by Moldoo 1. 7 PLA + printer ink 545 Black PIXMA 545 by CANON To accurately examine the color intensity of L-FDM composites, we printed cylindershaped objects. The design of these objects (shape and structure) allows for the observation of color spread in a single layer. For this experiment, we used PLA, as it is a transparent polymer with favorable processing parameters and minimal thermal shrinkage. Table 3 provides a summary of the assay.  Table 3. Samples presented in Figure 7 and their markings.  commonly used filaments have been tested. PET-G, which is more durable and flexible than PLA, was chosen for testing, as well as the ABS copolymer, which has a higher degree of flexibility and an opaque, matte surface. Both materials exhibit higher thermal shrinkage and require additional bed heating during the printing process. The filament dyeing process for these materials was similar to that used for PLA-based composites. Figure 8 illustrates the results obtained, and Table 4 provides a list of the symbols and sample names.  Table 4. Samples presented in Figure 8 and their markings. We created cylindrical PLA-based composite samples that incorporated fibers and wood dust as a filler, resulting in a porous structure and high absorbency. Our goal was to assess the efficacy of L-FDM technology modifications using a polymer with increased porosity and higher absorptivity compared to unmodified PLA. To conduct our research, we utilized various organic solvents and their mixtures containing a solution of rhodamine B. Figure 9 provides details on the dyes employed during the process, while Table 5 summarizes the sample compositions.    Table 5. Samples presented in Figure 9 and their markings.

Marking of the Sample
Full Sample Name Ref. 5 WOODFILL reference 5.1 WOODFILL + rhodamine B dissolved in water (proportion of 50 mg per 1 mL)  Table 2. Samples presented in Figure 6 and their markings.

Marking of the Sample Full Sample Name/Filament and Solution Used in Printing L-FDM Samples
Ref. To accurately examine the color intensity of L-FDM composites, we printed cylindershaped objects. The design of these objects (shape and structure) allows for the observation of color spread in a single layer. For this experiment, we used PLA, as it is a transparent polymer with favorable processing parameters and minimal thermal shrinkage. Table 3 provides a summary of the assay. Table 3. Samples presented in Figure 7 and their markings.

Marking of the Sample Full Sample Name Filament and Solution Used in Printing L-FDM Samples
Ref. The L-FDM method of modification has been researched extensively, and other commonly used filaments have been tested. PET-G, which is more durable and flexible than PLA, was chosen for testing, as well as the ABS copolymer, which has a higher degree of flexibility and an opaque, matte surface. Both materials exhibit higher thermal shrinkage and require additional bed heating during the printing process. The filament dyeing process for these materials was similar to that used for PLA-based composites. Figure 8 illustrates the results obtained, and Table 4 provides a list of the symbols and sample names. Table 4. Samples presented in Figure 8 and their markings.

Marking of the Sample Full Sample Name
Ref. We created cylindrical PLA-based composite samples that incorporated fibers and wood dust as a filler, resulting in a porous structure and high absorbency. Our goal was to assess the efficacy of L-FDM technology modifications using a polymer with increased porosity and higher absorptivity compared to unmodified PLA. To conduct our research, we utilized various organic solvents and their mixtures containing a solution of rhodamine B. Figure 9 provides details on the dyes employed during the process, while Table 5 summarizes the sample compositions. Table 5. Samples presented in Figure 9 and their markings.

Marking of the Sample Full Sample Name
Ref. 5 WOODFILL reference 5.1 WOODFILL + rhodamine B dissolved in water (proportion of 50 mg per 1 mL) 5.2 WOODFILL + rhodamine B dissolved in water and isopropanol mixed in a 1:1 ratio (proportion of 50 mg per 1 mL of solvent) 5.3 WOODFILL + rhodamine B dissolved in water, isopropanol, and glycerine mixed in a 1:1:1 ratio (proportion of 50 mg per 1 mL of solvent) Ref. 6 PLA WOOD reference 6.1 PLA WOOD + rhodamine B dissolved in methanol (unsaturated solution) 6.2 PLA WOOD + Transparent dye by Ag-Bet in order starting from the bottom: green, purple, and blue turquoise.
6.3 PLA WOOD + rhodamine B dissolved in water and dissolved in water and isopropanol mixed in a 1:1 ratio

Elements
The L-FDM technique allows for the direct introduction of metal ions and nanoparticles into the polymer matrix during the 3D printing process. Metal salts are dissolved in organic solvents such as methanol or a solvent mixture. These selected compounds serve as both the source of the element and the determinant of the final product's color, which is derived from the metal dispersed in the polymer matrix. To examine the distribution of ions within the polymer matrix, we utilized molecular structures and conducted SEM microscopy with EDS mapping (Section 3.5). The results confirm that this innovative printing technique is an effective way to incorporate modifiers into the polymer matrix ( Figure 10). Table 6 shows the polymers used together with solutions of metal salts. compounds serve as both the source of the element and the determinant of the final product's color, which is derived from the metal dispersed in the polymer matrix. To examine the distribution of ions within the polymer matrix, we utilized molecular structures and conducted SEM microscopy with EDS mapping (Section 3.5). The results confirm that this innovative printing technique is an effective way to incorporate modifiers into the polymer matrix ( Figure 10). Table 6 shows the polymers used together with solutions of metal salts.   Table 6. Samples presented in Figure 10 and their markings. The same process was applied to PLA filaments that had dust and wood fibers added as a filler. It was anticipated that using a more absorbent and porous material would result in components with a more vivid color, such as when dyes are absorbed by the filament. Figure 11 shows the described effect is presented below, while the composition of the compositions obtained in the tests is presented in Table 7.

9.2
ABS + lead acetate dissolved in methanol (saturated solution) The same process was applied to PLA filaments that had dust and wood fibers added as a filler. It was anticipated that using a more absorbent and porous material would result in components with a more vivid color, such as when dyes are absorbed by the filament. Figure 11 shows the described effect is presented below, while the composition of the compositions obtained in the tests is presented in Table 7.  Table 7. Samples presented in Figure 11 and their markings.

Marking of the Sample
Full Sample Name Ref. 6 PLA WOOD reference 6.4 PLA WOOD + silver nitrate dissolved in water and isopropanol mixed in a 1:1 ratio (saturated solution) 6.5 PLA WOOD + iron (III) chloride dissolved in water, isopropanol, and glycerine mixed in a 1:1:1 ratio (saturated solution)

Colorimetric Analysis
Color change tests on samples using a colorimeter based on the CIELab system were conducted. The L*, a*, and b* represent each of the three values the CIELab color space uses to measure objective colors and calculate color differences [21]. The a and b axes are placed at right angles to each other and indicate the color tone. Parameter a* ranges from green (negative values) to red (positive values). In contrast, negative b* represents blue, and positive b* corresponds with yellow. The L parameter denotes luminance (brightness) and describes the color in the range from black to white. A negative value indicates a  Table 7. Samples presented in Figure 11 and their markings.

Marking of the Sample Full Sample Name
Ref. 6 PLA WOOD reference 6.4 PLA WOOD + silver nitrate dissolved in water and isopropanol mixed in a 1:1 ratio (saturated solution) 6.5 PLA WOOD + iron (III) chloride dissolved in water, isopropanol, and glycerine mixed in a 1:1:1 ratio (saturated solution)

Colorimetric Analysis
Color change tests on samples using a colorimeter based on the CIELab system were conducted. The L*, a*, and b* represent each of the three values the CIELab color space uses to measure objective colors and calculate color differences [21]. The a and b axes are placed at right angles to each other and indicate the color tone. Parameter a* ranges from green (negative values) to red (positive values). In contrast, negative b* represents blue, and positive b* corresponds with yellow. The L parameter denotes luminance (brightness) and describes the color in the range from black to white. A negative value indicates a change to a darker shade, while a positive value means a change to a lighter shade. For black, its value is 0, while, for white, it is equal to 100. Using the CIELab system, differences between the color shades are determined. It is the distance between two points in a three-dimensional space, which can be written as [22]: (1) The assessment of the color change is made on the basis of the change of the ∆E parameter, the ranges of which are listed in Table 8. Very small difference, only obvious to a trained eye 2 < ∆E < 3.5 Medium difference, also obvious to an untrained eye 3.5 < ∆E < 5 An obvious difference ∆E > 5 A very obvious difference Figure 12a shows the color changes (∆E) of samples printed with the L-FDM method, in which dye solutions were used as modifiers, i.e., alkaline blue, methylene blue, methyl orange, eosin, fluorexone, transparent red dye by Moldoo, and black printer ink by Canon. The values on the Z-axis corresponded to the markings of the tested samples. The filaments were passed through a reservoir containing solutions of said dyes in methanol and printed in the form of trabeculae. The names and compositions of individual compositions were collected and presented earlier in Table 2. Figure 12b shows the color changes for cylindrical samples modified with commercially available dyes and solutions of rhodamine B in a mixture of water and organic solvents (water:isopropanol and water:isopropanol:glycerine) (see Tables 3 and 4 for a detailed explanation of the determinations). Figure 12c shows the ∆E values for composites containing fibers and wood dust with both color modifiers (5.1-5.3) and metal salts (6.1-6.3, sample designations Tables 5 and 7). Figure 12d shows color changes for the composites based on PLA and PET-G modified with metal salts (silver nitrate and iron III chloride) ( Table 6). Among the tested PLA composites (Figure 12a), the least intense coloration was observed for samples modified with a methanol solution of eosin and fluorexon (samples marked 1.4 and 1.5), while the most intense coloration and the highest value of delta E showed samples marked 1.6 and 1.7. Differences in the degree of coloring resulted from the compatibility of a given dye with the polymer matrix, its solubility in the solvent, and Among the tested PLA composites (Figure 12a), the least intense coloration was observed for samples modified with a methanol solution of eosin and fluorexon (samples marked 1.4 and 1.5), while the most intense coloration and the highest value of delta E showed samples marked 1.6 and 1.7. Differences in the degree of coloring resulted from the compatibility of a given dye with the polymer matrix, its solubility in the solvent, and the molar absorption coefficient, which is characteristic of a given dye structure. In the case of the cylindrical samples (Figure 12b), the analysis of the color changes showed that, for the PLA polymer, the highest changes were characterized by 2.2, 2.7, and 2.10, which were water:isopropanol solutions in a 1:1 ratio. The analysis of the collected data indicated that the factor determining the degree of saturation of the material with the dye solution was the solvent. The collected data for the rhodamine solutions indicated ∆E = 5.83 for sample 2.6, in which the solvent was water, ∆E = 63.04 for sample 2.7 in a H 2 O:iPr 1:1 solution, and ∆E = 24.71 in H 2 O:iPr:Gly 1:1:1 (sample 2.8); the obtained results indicated that the water:alcohol solution penetrated the pores of the material, enabling its better penetration and more intense coloring, and the addition of glycerin increased the density and viscosity of the solution. Therefore, its ability to penetrate the composite structure decreased, the wettability of the polymer with water was limited compared to other solutions, and the smallest color change was observed between the reference and the modified sample. Analogous tests were carried out for rhodamine saturated solutions in water, H 2 O:iPr 1:1, and H 2 O:iPr:Gly 1:1:1 (samples 2.9, 2.10, and 2.11, respectively). The smallest color change ∆E < 3.5 was observed for samples 2. These composites had a porous structure due to the presence of a filler in the form of fibers and wood dust. The result was a better ability to absorb solutions with liquid dyes and modifiers into the material, thanks to which, the L-FDM prints were characterized by an increased content of dye in the polymer matrix and, thus, more intense coloring. Modification of the polymers was also carried out in the presence of solutions containing metal salts, which could be used to introduce specific elements into the polymer matrix and, thanks to the molecular structure, significantly simplify the method of confirming the ongoing modification using microscopic methods and, additionally, intensively stain the material. Cylinders modified with silver nitrate and iron (III) chloride were characterized by ∆E > 5, which indicated a high saturation with ions both on the surface and in the internal structure of the polymer (Sections 3.4 and 3.5). The color change analysis carried out confirmed the effective introduction of dyes and chemical compounds into the polymer matrix. The saturation and, thus, the intensity of the color depended on the solvents used, the concentration, and the microstructure of the polymer surface. Figures 13 and 14 show the surfaces and sections of the cylinder-shaped samples obtained by L-FDM. Filaments based on PLA and PLA with a wood filler were passed through a container containing a dye solution: rhodamine B in various organic solutions, navy blue fabric dye, and transparent blue turquoise dye are presented in Figure 13 and PLA, PLA WOOD, and PET-G after passing through solutions containing metal salts ( Figure 14). Iron (III) chloride and silver nitrate were used as dyes and element sources because of their distinctive colors. All samples were printed in the vase mode, which means that the cylinder wall had a width similar to the diameter of the nozzle and was made of one layer. It was decided to choose such a model object so that the observation of the color change was as beneficial as possible, and the effect was not multiplied by thicker walls of the printouts. In the cross-sectional images, only the outline was visible, with no fill. Pictures of the surfaces of the dyed cylinders collected in Figures 13 and 14 allow assessing the intensity of the color and the uniformity of the dye distribution in the polymer structure for each of the layers arranged on top of each other. Based on the analyses carried out, it was observed that the cylinder models showed a constant level of color intensity throughout the volume. Although slight agglomerations of the dye particles were observed in the microscopic images, their distribution on all layers of the printed model seemed to be even. The cross-sectional images proved that the dye not only covered the surface of the print but also penetrated deep into the polymer structure, which confirmed the conclusions formulated for the colorimetric measurements. In the case of using a polymer filled with fiber and wood flour (PLA WOOD and WOODFILL), more strongly colored areas could be observed in the microscopic images, which was evidenced by the greater color intensity (the presence of darker spots) ( Figure 14N,O). This effect can be attributed to the greater absorption of the dye solution by the porous filler. Figure 13E,F show the WOODFILL fiber dyed with rhodamine B solutions. The cross-section of the print layer showed stronger coloring on its outer edge and at the interface of individual layers. The intensity of the color depended on both the type of solvent used and the amount of dissolved dye.

EDS Analysis-Elements Mapping
The EDS mapping of the surfaces and cross-sections of L-FDM-printed cylinders ( Figure 15) illustrated the distribution of the elements introduced with the L-FDM technique in the polymer matrix. The surfaces of the cylinder walls of the samples printed from polymers and copolymers such as ABS, ASA, PLA, and PET-G were characterized by a uniform distribution of the elements in the matrix ( Figure 15A-C,E). On the other hand, in the cross-section of the printing layer (see also Figure S1 in the Supplementary Materials), a higher content of lead was visible on its outer edge and at the contact point of the individual layers. The distribution of iron and rubidium in the EDS images of unfilled fiber samples was more homogeneous (Figure 15H,I,J). The large clusters visible in the EDS images of the PLA WOOD prints with silver nitrate and ferric chloride were probably the result of increased absorption of the modifier solution by the porous filler, which consisted of wood fibers and dust. When comparing the EDS images of the L-FDM prints with lead acetate dissolved in methanol (saturated solution), differences in the lead dispersion could be observed. Based on this, it could be concluded that, with the printing parameters used, the material from which the filament was made had either a better or worse ability to mix with the modifier used. This dependence was visibly better on the EDS images of the cylinder cross-sections ( Figure 15A and Supplementary Figure S1A-C). As could be seen, the ASA polymer showed the lowest ability to penetrate the polymer matrix, while ABS and PLA WOOD showed the highest concentration of lead on the outer edges and at the interface of the individual layers. PET-G exhibited the most homogeneous lead dispersion in the polymer matrix using the L-FDM technique with lead acetate dissolved in methanol. The color intensity on the EDS maps was higher for the PLA WOOD polymer. This effect occurred due to better wetting of the filament surface (rough structure of the filament surface) and its greater absorbency caused by the presence of wood fibers and dust. The EDS analysis provided evidence of elements in the polymer matrix of the L-FDM prints. This finding confirmed the possibility of using this technique as a tool for introducing various chemical substances into the polymer matrix.
be attributed to the greater absorption of the dye solution by the porous filler. Figure 13E,F show the WOODFILL fiber dyed with rhodamine B solutions. The cross-section of the print layer showed stronger coloring on its outer edge and at the interface of individual layers. The intensity of the color depended on both the type of solvent used and the amount of dissolved dye.

EDS Analysis-Elements Mapping
The EDS mapping of the surfaces and cross-sections of L-FDM-printed cylinders ( Figure 15) illustrated the distribution of the elements introduced with the L-FDM technique in the polymer matrix. The surfaces of the cylinder walls of the samples printed homogeneous lead dispersion in the polymer matrix using the L-FDM technique with acetate dissolved in methanol. The color intensity on the EDS maps was higher for PLA WOOD polymer. This effect occurred due to better wetting of the filament sur (rough structure of the filament surface) and its greater absorbency caused by the pres of wood fibers and dust. The EDS analysis provided evidence of elements in the poly matrix of the L-FDM prints. This finding confirmed the possibility of using this techn as a tool for introducing various chemical substances into the polymer matrix.

Functional Features, Limitations, and Directions of Potential Applications for the L-FDM Method
The examples displayed in Table 9 demonstrated the diverse applications of the innovative L-FDM printing technique. In the pharmaceutical area, L-FDM printing allows producing personalized drugs, allowing the adjustment of doses and compositions tailored to the individual needs of patients. In addition, this technology enables the production of tablets of nonstandard shapes with the possibility of controlled release [23][24][25].
Three-dimensional printing presents a potential avenue for incorporating radioactive isotopes into polymer matrices in the realm of isotope technologies. With this method, the precision and customization are enhanced, allowing for the creation of printed targets that optimize the geometry. This ultimately enhances the irradiation efficiency and overall effectiveness. In the field of nuclear medicine, 3D printing is used to create personalized, anthropomorphic phantoms. These phantoms are important for planning radiotherapy, validating dosimetry, and studying medical imaging. Tailored treatment strategies for patients can be achieved through the use of custom phantoms, resulting in improved outcomes. L-FDM technology allows for the direct production of radioactive materials without the need for additional processing steps such as rolling, extrusion, or granulation. By eliminating these steps, the risk of contamination for various machines is greatly re-duced [26][27][28]. One crucial matter to consider is the incorporation of technology in basic chemical research. Utilizing new compounds can expedite the testing process for potential applications without requiring scientists to possess in-depth knowledge regarding plastic processing or complex equipment. The modification of a polymer with chemical compounds can be studied with ease by researchers immediately after laboratory synthesis. Apart from medicine and pharmacy, 3D printing has potential applications in the cosmetic industry. Specifically, this technique can help in printing skin delivery platforms such as patches and microneedles [29]. Printed cosmetic microneedles can be a delivery system for both hydrophilic and lipophilic active ingredients that will be introduced into the material during the L-FDM process [30]. According to scientific research, the use of 3D printing allows for the customization of dressings to fit the specific needs of each patient. By incorporating metals, nanoparticles, drugs, natural compounds, proteins, and peptides into the polymer matrix, there is the potential for these dressings to aid in wound healing, acne treatment, pain relief, and antiaging [31]. Table 9. Potential applications.

Field of Application
Potential Application Refs.
pharmaceutical drugs, customized drugs, and therapy [23][24][25] isotope technologies introducing radioactive isotopes into 3D prints [26] nuclear medicine manufacturing individualized anthropomorphic phantoms in many clinical applications and radiopharmaceuticals [27,28] personal care skin delivery platforms [29][30][31] oceanography coral restoration; slow-release materials, maintaining an alkaline pH level [32] chemical engineering chemical reactors, the introduction of some elements of catalyst (e.g., Pt and Rh) [33] material engineering imparting new properties to polymer materials and testing new modifiers [34] chemistry testing of newly obtained chemical compounds directly in the laboratory after synthesis [35] bioengineering implants, tissue engineering, and regenerative medicine [36,37] agrochemistry controlled release of micronutrients and pesticides [38][39][40] nanotechnology formation nanoparticles from chemical precursors in thermal decomposition directly during 3D printing [41] polymerization of materials polymerization of monomers during printing, e.g., methacrylates [42] In addition, the 3D printing method holds potential in the field of oceanography. With the harmful impact of ocean acidification on coral reef ecosystems, alkaline materials may be incorporated into 3D-printed objects to aid in the production of artificial coral replicas for reef restoration. The integration of nanotechnologies can aid in the creation of 3D-printed replicas of coral reefs that will promote a faster recovery. The printed replicas can incorporate slow-release materials that maintain an alkaline pH into the surrounding microenvironment, while soluble materials can be used to create nano/micropores that resemble natural porous corals. This facilitates rapid growth and enables live corals to penetrate the structures [32]. Artificial coral structures can be designed to provide a variety of microhabitats for different species of corals and other marine life.
The use of 3D printing in chemical engineering has proven to be highly valuable, particularly in the development of structural catalysts, mixers, and reactors. Through the use of 3D printing, complex and customized structures can be created, ultimately leading to an improved process efficiency and better results in a variety of chemical engineering applications [33]. A novel printing technology, capable of incorporating modifiers into polymer matrices, holds promise for advancement in materials engineering. Using the L-FDM method, scientists can use this as a tool for efficient, cheaper, and quick production of samples for research purposes and for imparting new properties to polymeric materials [34,35].
The L-FDM technique holds significant potential in the field of bioengineering, particularly in the areas of implants, tissue engineering, and regenerative medicine. The primary focus of tissue engineering lies in designing and producing scaffolds with specific properties, which can be achieved through the application of L-FDM. With the new method, it is possible to introduce ions found in the body's natural environment, resulting in biocompatible materials [36,37]. The use of agrochemicals with controlled-release systems is beneficial, as it enables the maximum effectiveness while minimizing nonspecific side effects. Moreover, it reduces the negative impact on the environment, human health, and other organisms. There are different ways to release substances, such as microcapsules, nanoparticles, delayed-release formulations, and polymer matrices. Using the L-FDM method, materials can be designed to have active ingredients that enable a controlled release by responding to factors such as humidity, temperature, and pH [38][39][40]. Currently, the known technologies on the market indicate the possibility of in situ thermal formation of nanoparticles (e.g., silver nanoparticles) during filament extrusion. The use of the L-FDM method allows the omission of several processing processes, such as the homogenization of materials, granulation, and extrusion, because it is possible to obtain nanoparticles from chemical precursors as a result of thermal decomposition directly during printing [41,42].
The aforementioned examples are just a glimpse of the vast application possibilities of the innovative L-FDM printing technique. Its potential and significant contributions to the field of science make it imperative to be explored and developed further.
During the research, problems were encountered during the L-FDM printing process; it was also noticed that it has some limitations, but some of them can be overcome/circumvented by using solutions, examples of which are listed in Table 10. The first problem is the relatively small amount of modifier possible to be introduced into the polymer, and another problem is with the precise determination of its contents. The proposed solution to the above-mentioned problems is, for example, increasing the total surface of the polymer absorbing the modifier by reducing the diameter of the filament, increasing the porosity, or changing the material from which the filament is made. The amount of the introduced substance can be controlled by a quantitative analysis of the finished product or inline spectroscopic measurements performed during the printing process with the use of colored substances. Another challenge posed by L-FDM printing is the appropriate selection of solvents for the material from which the filament is made and the chemical substance from which we want to make the solution. If there is a problem with the dissolution of the polymer in an organic solvent, the solvent or polymer from which the filament is made should be changed. Another solution to the problem of dissolving the filament is to control the temperature of the solvent in such a way that only the introduced chemical substance is dissolved. During the L-FDM printing process, to maintain the homogeneity of the obtained 3D elements (uniform color or even introduction of the modifier into the polymer matrix), it is important to evenly cover the surface of the filament with the solution. For this purpose, in addition to the appropriate selection of the polymer and solvent, the addition of a surfactant can be used. Depending on the viscosity of the modifier solution, the filament is covered with a thicker or thinner layer after immersion. The filament is covered with a thicker layer of the modifier solution if it has a higher viscosity.

Feature Limitations Solution
compatibility of the polymer with the modifier (low solubility, inertness, lack of reactivity) of the solvent with the printing material used, value measure (filament solubility < 0.1 g/L solution) the solubility of the filament material in the solvent used The amount of absorbed solution from the reservoir by the polymer depends, among other factors, on the duration of its interaction with the filament. This time is influenced by the desired printing speed, which is the rate at which the material is fed.

Conclusions
This paper presents an innovative direct method of introducing chemical substances such as dyes or chemical compounds into printouts in the FDM technique. Colorimetric tests (CIELab) and microscopic tests with EDS mapping confirm that plastics are modified directly during printing. The proposed approach described in this paper omits the use of complicated methods and expensive processing equipment and may potentially be a tool for obtaining materials with special applications. The research described is a small part of the technical possibilities for this technique. The proposed technique simplifies the preparation of new materials using thermoplastic polymers. The described solution makes it possible to expand the fields of application of the classic FDM printing technology, opening up new procedures and processes for research laboratories in the fields of chemistry, pharmacy, biotechnology, engineering, and many others. We propose to use the name liquid for fused deposition modeling (L-FDM) for the described technique. The authors of this work will continue research on the application of the developed method for the direct modification of filaments in the printing process.

Patents
The results of this publication have been patented with Polish patent application no. P.441923.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/app13137393/s1: Video S1: Process of liquid for fused deposition modeling; Table S1: FDM 3D printers used in this study-technical parameters; Table S2: FDM 3D printers used in this study-printing parameters.; Figure S1: EDS images of LFDM printed parts with metal ions introduced into the polymer matrix.; Figure S2: EDS images of LFDM printed parts with metal ions introduced into the polymer matrix. Images of the surface of the cylinder's side.; Figure S3: EDS images of LFDM printed parts with metal ions introduced into the polymer matrix. Images of the cross-sections.