# Multimaterial 3D printing of structured surfaces for increased hydrophobicity of biocomposite materials

Kayah St. Germain, Damien Marchand, Laurence Chocinski-Arnault, Hani E. Naguib, Fabienne Touchard.

► To cite this version:

Kayah St. Germain, Damien Marchand, Laurence Chocinski-Arnault, Hani E. Naguib, Fabienne Touchard. Multimaterial 3D printing of structured surfaces for increased hydrophobicity of biocomposite materials. Additive Manufacturing Letters, 2025, 14, pp.100286. 10.1016/j.addlet.2025.100286. hal-05348255

HAL Id: hal-05348255

https://hal.science/hal-05348255v1

Submitted on 5 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Multimaterial 3D printing of Structured Surfaces for Increased Hydrophobicity of Biocomposite Materials

Kayah St. Germain<fnref n="1" />,<fnref n="2" />, Damien Marchand<fnref n="2" />, Laurence Chocinski-Arnault<fnref n="2" />, Hani E. Naguib<fnref n="1" />* and Fabienne Touchard<fnref n="2" />*

**Key Words: 3D Printing, Multimaterial Printing, Surface Wettability, Roughness, Biocomposites, Adhesion**

**Abstract**

Material extrusion 3D printing is an up-and-coming additive manufacturing method that is continuously being explored for its many benefits including rapid prototyping, high degree of customizability, and low material waste production, among others. One of the most widely used materials in FDM 3D printing is polylactic acid (PLA) due to its ease of printability and bio-origins. Recently, new biofiller reinforced PLA biocomposite filaments have begun to be sold commercially, but the introduction of the biofiller creates problems of increased hydrophilicity and hygroscopicity. In this study, a possible solution to this problem was explored by using multimaterial 3D printing to add a thin, structured outer layer to the biocomposite in either pure PLA or TPU. This layer helps limit any external moisture from coming into contact with the underlying biocomposite by creating a barrier with increased hydrophobicity. A grid, triangle, and honeycomb pattern were each tested at 50%, 75%, and 100% pattern densities for each material. It was found that, along with the pattern that was printed, the filament deposition process created additional roughness that influenced the way the water droplets interacted with the surface. All the patterned surfaces displayed a water higher contact angle than the respective material when it was printed in a flat manner. Additionally, factors that influence the feasibility of using this outer structured layer to improve the surface hydrophobicity of biocomposite parts were explored, including material compatibility and adhesion.

<footnote n="1">¹University of Toronto, Mechanical and Industrial Engineering, 27 King's College Cir, M5S 1A1 Toronto, ON, Canada Email: kayah.stgermain@mail.utoronto.ca, hani.naguib@mail.utoronto.ca, Web Page: https://tsmart.mie.utoronto.ca/</footnote>

<footnote n="2">² Institut Pprime, Dpt Physique et Mécanique des Matériaux, CNRS-ENSMA-Université de Poitiers, 1 av. C. Ader, B.P. 40109, 86961 Futuroscope Chasseneuil, France Email: damien.marchand@ensma.fr, laurence.chocinski@ensma.fr, fabienne touchard@ensma.fr, Web Page: https://pprime.fr/en/home-pprime/</footnote>

<footnote>*Corresponding Author</footnote>
# 1. Introduction

In recent years there has been a large push for the increased integration of biocomposite materials into the manufacturing industry as an eco-friendly alternative to traditional manufacturing materials. Composite materials consist of two main parts; a continuous matrix, which serves to bind the overall material together, and a discontinuous reinforcing filler, supplying the composite with the preferred mechanical properties [1]. Biocomposites are composite materials whose polymer matrix and filler material are derived from natural or biodegradable artificial sources, making them sustainable to produce, easier to recycle, and better for the environment [2], [3]. Biocomposites hold a lot of promise but exchanging traditional manufacturing materials for these green counterparts also introduces some complications that were not previously present and now need to be addressed. The incorporation of plant-sourced materials, which are largely hydrophilic and hygroscopic, has a significant impact on the way the composite material interacts with water. The hydrophilicity of the biofillers [4] causes increased water uptake and, in turn, increased crack propagation, leading to decreased mechanical strength due to debonding between the filler and matrix [5], [6]. This fundamental change in the behaviour of the material could lead to increased complications, especially in high moisture environments. Bioplastics that serve as matrices for biocomposite materials include poly-lactic acid (PLA), poly-hydroxyalkanoates, thermoplastic starch, and poly-ethylene oxide (PEO), with some options for plant-based reinforcing fibres being hemp, flax, and jute [7]. PLA being both a bioplastic and one of the most widely used and studied materials for 3D-printing has led to the creation of PLA based biocomposite 3D printing filaments, which are currently in the process of being developed and sold commercially.

Much like biocomposite materials, 3D printing is an additive manufacturing technique that is currently gaining popularity and being extensively researched for wider incorporation into industry [8]. It holds significant advantages over traditional manufacturing processes including increased design flexibility, high levels of customization at low costs, minimal waste production, and the ability to fabricate complete multi-material systems in a single step [9], [10]. Fused deposition modelling (FDM) is a type of 3D printing that heats a filament feedstock then extrudes and deposits it through a nozzle in a layer-by-layer fashion onto a print bed to build a part from the bottom up [11]. There are a number of different printing parameters that can be changed to offer customizability beyond simply the design flexibility offered by computer aided design (CAD) modelling, including the print pattern, the infill type and density, and the layer thickness. Additionally, in FDM 3D printing, nozzles can be added to allow for parts to be fabricated from multiple materials in a single process, though multi-material 3D printing comes with its own set of challenges [12], [13]. Despite the advantages that come with the FDM fabrication technique that make it particularly suitable for this study, there are still a number of potential pitfalls to look out for including voids or poor adhesion between layers, reduced mechanical strength due to the interfaces between layers, and poor or non-uniform surface finish [14], [15].

This study focuses on a commercially available filament that consists of a PLA matrix with hemp fibre reinforcement (hPLA) and looks at tackling the issue of surface hydrophilicity by taking advantage of multi-material 3D printing. Limiting the interaction between water and the surface of the hPLA also reduces the chance of high levels of water absorption which was revealed to be a problem with hemp reinforced PLA in the past studies [16]. Additionally, water absorption is an already existing issue due to the nature of FDM 3D printing, which creates parts that contain micro-porosities [17], making them more susceptible to water infiltration than parts created using other manufacturing techniques [18]. The biocomposite material was printed as a base layer then a thin, structured layer was added to the top in one of two different materials, PLA or TPU, with each material presenting its own advantages and disadvantages. Three different structure types at three different densities were printed using each of the materials then the contact angles were
measured and compared. Tomography imaging was also used to closely examine the interface where the two materials met to evaluate feasibility of using multimaterial 3D printing in this way.

This work investigates a new novel way of addressing the increased hydrophilicity of biocomposite materials. Previous research focussing on increasing hydrophobicity of biocomposite surfaces have found success in surface modification of the biofiller itself [19], [20] and the addition of an external coating or surface treatment [21], [22]. Research that intersected water repellent surfaces and 3D printing explored directly fabricating water repellent surfaces using different additive manufacturing techniques such as stereolithography, material jetting, or custom 3D printing set-ups [23], [24], [25], or using 3D printing to print molds to fabricate the surfaces from a different material, such as polydimethylsiloxane [26], [27], [28]. This research differentiates from previous that has been done on similar topics because it looks at using FDM 3D printing to directly create a high hydrophobicity surface to be directly applied in a single step fabrication to address the hydrophilicity of biocomposite materials without needing any pre or post treatment. If successful, this method of addressing the hydrophilicity of biocomposite materials could have significant advantages such as eliminating the need for a hydrophobic coating, many of which still contain harmful chemicals such as PFAS, being able to complete the part in a single fabrication process, and having a large amount of design options to choose from depending on the needs of the application.

## 2. Experimental Method

### 2.1 Materials and Procedures

Three types of filaments were used in this study, hPLA, PLA, and TPU, whose material properties can be found in Table 1. hPLA was chosen as the biocomposite base material because it is an excellent candidate to replace synthetic polymers [29]. hPLA has improved stiffness in comparison with other more commonly used plastics for additive manufacturing [30]. Moreover, the utilization of hemp in PLA bio-composite is considered as a strong substitute for wood-based fillers because of its higher cellulose fraction [31] and its superior sustainability, as hemp can be harvested every 4 months [32]. PLA filament was chosen as one of the structuring materials because the lack of filler means it is still more hydrophobic than the hPLA, it is highly compatible to be printed alongside the hPLA due to them both having the same base material, and both are smart materials meaning the shape memory properties may still be preserved. TPU was chosen, despite being a difficult material to incorporate into multimaterial printing, for being one of the most naturally hydrophobic printing materials available. All 3D printing was done using an Ultimaker S5 FDM multimaterial printer and the CAD files were prepared for printing using the corresponding Ultimaker Cura slicing software.

<table><caption>**Table 1:** 3D Printer Filament Properties</caption><thead><tr><th>Material</th><th>Filament Diameter (mm)</th><th>Tensile Stress at Yield (MPa)</th><th>Print Temperature (°C)</th><th>Build Plate Temperature (°C)</th><th>Printing Speed (mm/s)</th><th>Hemp Weight %</th><th>Supplier</th></tr></thead><tbody><tr><td>hPLA</td><td>2.85</td><td>16</td><td>210</td><td>60</td><td>100</td><td>≤10</td><td>3D-Fuel</td></tr><tr><td>Tough PLA</td><td>2.85</td><td>37</td><td>225</td><td>60</td><td>25</td><td>0</td><td>Ultimaker</td></tr><tr><td>TPU</td><td>2.90</td><td>No Yield</td><td>220-235</td><td>70</td><td>25</td><td>0</td><td>Ultimaker</td></tr></tbody></table>
The three different structure layer types that were studied were a grid structure, a triangle structure, and a honeycomb structure, printed at three different pattern densities 50%, 75%, and 100%. Changing the pattern density affected the size and number of repeating pattern units present on the surface. Figure 1a) shows the 9 different surface structure patterns as depicted in the slicing software. As the density increased, the repeating pattern units became smaller and more tightly packed. The added structured layer was 1mm thick (corresponding to 5 deposited filament layers) and printed on top of a thin plate of hPLA (Figure 1b)). The complete printer specifications for each material can be found in Table 2. No additional post processing or modification was done to the samples post-printing. For comparison, a reference sample of PLA on hPLA and TPU on hPLA without any structuring was printed in the same multi-layer way to identify the influence of structuring on the water contact angle. The layer of secondary material added to the hPLA substrate for the reference samples was printed in a standard line pattern using a 0.4mm nozzle.

<table><caption>**Table 2:** Sample Printing Parameters</caption><thead><tr><th>Specification</th><th>hPLA</th><th>PLA and TPU</th></tr></thead><tbody><tr><td>Z resolution (layer thickness)</td><td>0.2mm</td><td>0.2mm</td></tr><tr><td>XY resolution (nozzle diameter)</td><td>0.8mm</td><td>0.4mm</td></tr><tr><td>Infill Pattern</td><td>Lines</td><td>Grid, Triangles, Tri-Hexagon</td></tr><tr><td>Infill Orientation</td><td>90 degrees</td><td>90 degrees</td></tr><tr><td>Infill Density</td><td>100%</td><td>50%, 75%, 100%</td></tr></tbody></table>

For the tomography imaging, used to examine the material interfaces, samples that consisted of an inner 10mmx10mmx10mm hPLA cube covered in a thin 0.4mm outer layer in either PLA or TPU (Figure 1c)) were printed using the same Ultimaker S5 FDM multimaterial printer. In these samples, the outer layer was not structured and fully covered the face of the hPLA cube, similar to the way the reference sample was printed. These cube samples allowed for the different interface combination to be easily observed.

## 2.2 Sample Analysis

A Zeiss optical microscope was used to examine the structured surface layer. Microscope imaging revealed the effect of the FDM fabrication process on the surface finish, giving a clear image of the macroscopic surface pattern and the secondary pattern created by the filament deposition. Additionally, tomography X-ray imaging was done on the cube samples using an UltraTom CT scanner from RX Solutions to examine the interface between materials for hPLA/PLA, PLA/hPLA, hPLA/TPU, and TPU/hPLA. A voxel resolution of 12µm was used for the CT scanning.

## 2.3 Contact Angle Measurement

Contact angle measurements were conducted using an Ossila Contact Angle Goniometer (Figure 1d)) at approximately 20°C and 40% humidity. Prior to taking the measurement, the surface was cleaned using ethanol to remove any dirt or surface contamination then left to evaporate to ensure that no ethanol remained to influence the measurement. Distilled water was exclusively used as the liquid for the measurements. A 25µL droplet was placed on the surface and given approximately 10 second to equilibrate before the measurement was taken. This helped mitigate the external effects that could influence the measurement over time such as vibrations, or local evaporation, or condensation. Measurements were taken from two
different directions, 90° from one another, to check for difference in the measurements that could arise from surface anisotropy.

<figure><img src="image_2.png" /><figcaption>**Figure 1:** a) Cura model of nine different surface structures types that were examined in the study b) 3D-printed sample showing the hPLA base layer with TPU structured top layer c) Multi-material 3D printed cube samples where hPLA inner cube was coated in a thin layer of either a white pure PLA or black TPU d) Contact angle goniometer set-up</figcaption></figure>

# 3. Results and Discussion

## 3.1 Surface Morphology

FDM 3D printing is an additive manufacturing process that operates by heating and depositing polymer filament from a nozzle to form a part layer by layer [11]. Because of the mechanics of this process, the part is not left with a smooth surface finish, as ridges and step patterns are typically formed by the layering and pulling of the filament. Surface wettability being primarily concerned with the interface created between the liquid and the solid surface and being heavily influenced by surface roughness, the variations in surface finish have a notable effect on the wettability. While the overall macroscopic pattern could be observed with the naked eye, optical imaging and confocal microscopy were used to examine the nuances of the surface. Another point that should be mentioned is that when FDM 3D printing thin samples, there is a chance that the samples warp and curve during or following the printing process. In the case of this study there was no visual indication of any warping of the samples having occurred during or after fabrication but if that is a concern then a slant/curve study can be performed.

A grid structure is completely symmetrical from all directions which implies that a uniformly structured grid surface should display a uniform surface finish and isotropic wettability. Observing the grid structured surfaces through the optical microscope, this was found to not be the case because of the influence of the 3D printing fabrication technique on the surface finish. With the printing path that was taken when creating the grid, all the parallel lines in one direction were deposited as a single layer before the bisecting
perpendicular lines were deposited as a second layer (Supplementary Video 1). In Figure 2a), it is visible that the two layers never completely meld together to form a flat surface at the junction where the lines intersect, meaning that the final deposited layer is resting on top of the one below. This results in a secondary line pattern being formed that consists of uninterrupted lengths of filament deposited in a single direction at the top of the surface. This layering of the filaments formed a step pattern in one direction and a line pattern in the other, imbuing the surface with a clear case of anisotropy.

In the triangle structured surface, there were still elevation differences that existed on the surface as a result of the filaments crossing over one another when being deposited, but there is a distinct difference when compared to how the grid pattern was printed. For the triangle pattern, the filaments crossed each other in a way that, at each junction, the topmost filament was pulled in a different direction (Supplementary Video 2) with no distinct single top layer existing. Figure 2d) shows the triangle structured surface at 50% pattern density where the variation in the deposition of the filaments and the way they intersect one another can be most clearly seen. This results in a surface that does not provide a distinct path in a particular direction for a droplet to spread, like on the anisotropic grid pattern, but also not a completely uniform and isotropic one. The varying intersections of the filaments means that depending on where a droplet is placed on the surface, it could result in different spreading behaviour.

For the honeycomb pattern, the printing path that was taken during the filament deposition changed depending on the density at which the structure was being printed. Because of the variation in printing path, this was the only pattern where both isotropy and anisotropy were present in the surface. At 50% pattern density, the pattern was printed more akin to the grid, with the layers showing a clear directionality (Supplementary Video 3). The top deposited layers formed distinct long diamond shapes that stretched along a single direction while the filament that crossed in the perpendicular direction were contained in a single layer underneath. A water droplet placed on the surface would be more heavily influenced to spread in the direction of the length of the triangles. Contrarily, when printed at the increased densities, the printing path was more liken to that of the triangle pattern. The filaments pulled horizontally and vertically were interspersed in the way they crossed over one another which, again, avoided creating distinct layers (Supplementary Video 4). Figure 2g) shows the honeycomb pattern at the lowest pattern density, 50%, printed anisotropically, while Figure 2h) and i) shows the honeycomb pattern at the higher densities, printed isotropically.

Further, the microscope images provided a closer look at the print quality as the pattern density increased and the printer was required to produce finer detail to accurately replicate the patterns. The final printed product can be compared to the CAD model that was given to the printer and evaluated on two important factors: the pattern shape accuracy and the pattern consistency across the surface. At 50% pattern density across all pattern types, the printed pattern accurately replicated the model, and the shape and dimensions of the structures were consistent. The 50% pattern density required the fewest fine details which resulted in the highest quality and a most accurate print. As the density increased, noticeable deviation developed between the computer model and the final printed sample. At 75% pattern density, deformation to the pattern shape and loss in resolution became noticeable, although, the pattern still appeared largely symmetrical, and a single repeatable patterned unit could be identified. At 100% pattern density the shape resolution was lost and there was a loss of consistency and repeatability in the pattern, with large variations seen across the surface. In the grid pattern, deformation could be seen in the square shape of the porosities at 75% pattern density (Figure 2b)). The 100% pattern density had the square shape completely lost, with the porosities present having an inconsistent size, and the surface lacking uniformity (Figure 2c)). The other two pattern types showed a similar deviation from the CAD model and loss of resolution with the increase in pattern density. In the triangle pattern, the 75% density remained mostly uniform and accurate (Figure
2e)) but at 100% density, the deposited filaments started to eclipse the pattern, leaving fully covered areas where filament deposition occurred too close together and the filaments met (Figure 2f)). The octagons that made up the honeycomb pattern lost definition at the corners and took on a circular shape with the increase in density, along with a loss in the small triangle detail that can be seen at 50% pattern density (Figure 2g)-i)).

<figure><img src="image_3.png" /><figcaption>**Figure 2:** a) PLA printed grid pattern at 50% density b) PLA printed grid pattern at 75% density c) PLA printed grid pattern at 100% density d) PLA printed triangle pattern at 50% density e) PLA printed triangle pattern at 75% density f) PLA printed triangle pattern at 100% density g) PLA printed honeycomb pattern at 50% density h) PLA printed honeycomb pattern at 75% density i) PLA printed honeycomb pattern at 100% density</figcaption></figure>

## 3.2 Material Contact Angles

The contact angles were measured on the reference TPU and PLA samples so that the influence of the structuring could be seen and compared. The contact angles on the reference samples were measured as 91.5° and 75.6° for TPU and PLA respectively, which is in line with the range that has been previously reported in literature [33-36]. Based on the fundamental principles of surface wettability, TPU having a contact angle over 90°, along with confirmation from prior studies [37], indicates that this material is an ideal candidate for this application. Deliberate structuring of an inherently hydrophobic material promotes the creation of a solid-liquid-air composite interface which, in turn, increases the contact angle [38]. Using the criteria derived from Young's thermodynamics-based contact angle equation, PLA having a contact angle less than 90° classifies it as a hydrophilic material. Looking to the fundamentals of surface wettability once again, this implies that the introduction of structuring should increase the hydrophilicity of the surface, the opposite of what is desired here. Despite this, there are alternate contact angle classifications that have been proposed more recently that say that the water contact for the base material only has to be greater than 65° for a material to be considered hydrophobic [39]. Additionally, there is literature that has been
successful in creating surfaces displaying a variety of contact angles using PLA depending on the
architecture of the surface [40,41].

**3.3 Structured Surface Contact Angles**

As previously discussed, there were three different surface patterns printed at three different pattern densities in both PLA and TPU, resulting in 18 different total configurations. Looking at the results for all the different patterns that were tested, a number of conclusions can be drawn. In all cases, the structured surfaces demonstrated a decrease in surface wettability and increase in contact angle when compared to the reference sample, showing that the introduction of surface structuring had the desired effect of increased contact angle. It was also noted that of all the different influencing factors included in this study: material, pattern type, and pattern density, the most influential was the pattern type coupled with the printing path. This is not an abnormal observation, and it has been previously seen that changing pattern density or spacing provided little to no significant change to the measured contact angle [42].

Each of the surfaces could be sorted using the measured contact angles into two different categories: anisotropic and isotropic. In the anisotropic case, when observed from different directions, the contact angles are significantly different. One direction typically displays a higher contact angle, with the droplet appearing highly spherical, and in the other, the droplet appears elongated due to increased spreading. In the isotropic case, the water contact angle appears the same from all observable directions. The water contact angles on the isotropic structured surfaces tended to fall between the two more extreme values of the anisotropic surfaces. When observed from a top view, a droplet on an isotropic surface will have a circular shape, on an anisotropic surface, it will appear oval. For a surface to be categorized as having anisotropic wetting, the difference in the contact angles from the two different directions needs to be at least 10° [42]. In analyzing the contact angles measurements, it was also observed that the standard deviations could become quite large, this can likely be attributed to the non-uniformity and variability in the surface structure caused by the printing process.

On the grid patterned surfaces, all the pattern densities and both materials displayed similar contact angles (Figure 3a)). When measuring the contact angle of the droplet on the surface in the direction parallel to the print path, the water contact angles ranged from 118° to 125°, and in the perpendicular direction, they ranged from 90° to 105°. As the pattern density increased, a slight increase could be observed in the contact angles in the parallel direction whereas there was a slight decrease in the contact angle in the perpendicular direction. Each grid structured surface showed a difference in contact angle between the parallel and perpendicular direction well over 10°, classifying them all as anisotropic. Relating back to the surface morphology, the direction in which the droplet experienced increased spreading, forming a lower contact angle, correlated to the direction that the filaments ran on the very top layer. This is because the water droplet was able to more easily advance along the uninterrupted lengths of filament. The higher contact angles correlated to the direction which contained the step pattern, where the increased roughness prevented the droplet from spreading. Figure 4a) shows the water droplet imaged from the two different directions.

The contact angles on the triangle structured surfaces had a more varied result which was somewhat expected given the surface morphology. The range across the three pattern densities were 107° to 101° and 99° to 95° for the TPU and PLA respectively with the contact angle decreasing with the increasing pattern density (Figure 3b)). As mentioned previously, the printing path for the pattern resulted in a high level of variation across the surface. The effects of this were reflected in the contact angle measurements. Depending on where the droplet was placed on the surface, some areas resulted in a higher contact angle, some lower contact angles, or a large hysteresis between the left and right-side angle (Figure 4b)). Whereas on the grid surface, the droplet presented in one of two ways depending on the direction from which it was viewed, the
behaviour of the droplet on the triangle surface could not be as easily attributed to one particular direction or placement. Nevertheless, even with the more variable behaviour presented by the droplets on the triangle pattern surface, the variation in the contact angles was not large or distinct enough to create a distinctly anisotropic effect. The increase in pattern density resulting in a decrease in the average contact angle can likely be attributed to the filaments on the higher density patterns being closer to one another. This resulted in a flatter and more uniform surface with less porosities, allowing the droplet to spread over the surface more easily and evenly. The triangle pattern was also one that showed a more significant contact angle difference between the surfaces printed from TPU and PLA.

For the honeycomb structured surfaces, both isotropic and anisotropic behaviour was present depending on the pattern density, caused by the different printing paths creating different surface finishes (Figure 3c). This was the only pattern type where changing the pattern density resulted in a change of printing path. At 50% pattern density, the surface presented as anisotropic. The contact angle in the parallel direction was measured to be approximately 120° for both materials with the contact angle in the perpendicular direction being approximately 100° for the TPU and 96° for the PLA. Hydrophobic structured surfaces typically exist in one of two states; a Cassie-Baxter state where the water droplet rests on top of the roughness without penetrating in, leaving small air pockets between the roughness and the droplet, or in a Wenzel state, where the droplet fully wets the roughness [43]. In our case, the contact angles for the honeycomb pattern at 50% pattern density were measured in the Cassie-Baxter state but it was observed that this state was highly metastable, and the droplet was susceptible to entering a Wenzel wetting regime. Because of the nature of the pattern and lack of nano roughness, when the droplet fully wet the roughness, the surface experienced increased wetting and a lower contact angle (Figure 4c)). This negates the usefulness of structuring as a tool to prevent water from coming into contact with the substrate material. This pattern type may have been particularly prone to transitioning to the Wenzel state due to the hexagonal shape of the pores, with more circular shapes tending to have weaker pressure resistance [44], the size of the porosities being the largest of all the tested patterns, and the increased distance between structures favouring the Wenzel state [41]. At the higher pattern densities, the droplet interacted with the surface in an isotropic manner, with the contact angle decreasing slightly with the increase in density due to increased material presence. At 75% pattern density the contact angle was 105° for the TPU and 100° for the PLA and at 100% was approximately 100° for both materials.
<figure><img src="image_4.png" /><figcaption>**Figure 3:** a) Contact angles for grid structure surfaces b) contact angles for triangle structured surfaces c) contact angles for honeycomb structured surfaces</figcaption></figure>

## 3.4 Surface Tailoring

One of the main takeaways from the contact angle measurements is that depending on the needs of the surface, different printing parameters can be chosen to tailor the final surface finish and behaviour. For the application that is being explored here, using the added structured layer to increase the surface hydrophobicity of biocomposite materials with multimaterial 3D printing, there are other factors to be considered to discern if this is a feasible solution that can be used in industry. These include the secondary material selection, the most suitable type of surface structuring, the strength of the interface between the biocomposite and the secondary material, and the stability of the Cassie-Baxter state of the surface.

The surface structuring used can be chosen based on properties required in the final surface. If more uniform surface behaviour is desired, it would be preferable to use a surface structure pattern that results in an isotropic finish, such as the triangle pattern. If directional water shedding is desired, it could be beneficial to use a surface pattern that results in an anisotropic surface finish. Anisotropic surfaces are often seen in nature being used by plants, such as rice leaves [47,48], and animals, such as butterfly wings [45,46], to direct water that lands on the surface to roll off in a certain direction. It is possible to at least partially artificially replicate this effect using the anisotropic 3D printed surfaces. Additionally, because the experimental results indicated that, in most cases, changing the structure density had little effect on the resulting contact angle, the amount of the base material that is exposed and secondary material that is added can be tailored without effecting the hydrophobicity granted by the structuring. Adding a surface structuring at a higher density would cover more of the underlying base material, in this case the biocomposite, while a lower density would leave more of it uncovered and limit the amount of secondary material that is added. This could be important if there are limiting factors such as dimensional requirements, weight requirements, or limited amount of material.
<figure><img src="image_5.png" /><figcaption>**Figure 4:** Wettability of 3D printed structured surface samples a) images of water droplet on grid surface taken parallel and perpendicular to print direction demonstrating anisotropy of surface b) image of droplet on triangle structure surface showing hysteresis between left and right contact angle caused by surface morphology c) images of droplet on honeycomb structured surface i) parallel to printing direction in Cassie-Baxter state ii) perpendicular to printing direction in Cassie-Baxter states iii) in the Wenzel state</figcaption></figure>

Another consideration is one that is not clearly reflected in the experimental results but is present due to the chemistry of the materials. As mentioned previously, PLA is an inherently more hydrophilic material than TPU. This was reflected in the water contact angle measured on the reference sample. Although the contact angles measured on the structured surfaces for the two different materials were extremely similar to one another, this does not fully reflect the behaviour of droplets on the surface. Particularly regarding if the droplet is interacting with the surface in the Cassie-Baxter or Wenzel state. When a droplet is in the Cassie-Baxter state it may be metastable and transition to the Wenzel state depending on the geometry of the structuring and the energy barrier separating the two states [49]. The effect of surface structuring on the way a droplet interacts with a surface is very clearly demonstrated when comparing two famous plant surfaces, the lotus leaf and the rose petal. Both surfaces demonstrate a static contact angle of over 150° [50,51] but when examined closely, it is observed that a droplet on a lotus leaf only interacts with the tips of the nano-roughness, a Cassie-Baxter surface, whereas a water droplet on a rose petal fully wets the microstructure, a Wenzel surface [52,53]. From comparing their surface structuring, it can be concluded that a smaller micro-structure with nano-roughness will create a more stable, Cassie-Baxter surface [54] whereas larger microstructure with larger valleys will favour a Wenzel state [51,55]. Wetting transitions can happen almost instantaneously after the droplet encounters the surface as a result of the structuring or over time as it rests on the surface as it is exposed to external factors [56]. External factors can also force the transition, including evaporation, time, or vibration [57]. The concept of metastability was clearly shown with the honeycomb pattern at 50% pattern density which was discussed above. For the applications targeted here, it would be preferable for the surface to hold a stable Cassie-Baxter state for as long as possible since the goal is to keep the hPLA substrate out of contact with liquid. Because the PLA is a more hydrophilic material than TPU, the Cassie-Baxter state that the droplet exists in when first coming into contact with the surface is more susceptible to entering a Wenzel wetting regime.

When using multimaterial 3D printing, one extremely important factor that has a large effect on the quality, usability, and longevity of the final printed part is the compatibility of the materials used. In this study, there are two pairings of materials that are explored: PLA printed on hPLA and TPU printed on hPLA. Interfaces are constantly being created in the FDM 3D printing process due to the extrusion of the filament forming
layers instead of a single continuous piece of material. Parts manufactured using FDM 3D printing contain a large number of interfaces, increasing the number of areas of reduced mechanical strength throughout the part when compared to those that are fabricated using traditional manufacturing techniques [58]. This reduction in mechanical strength at boundary interfaces exists for parts printed from a single material and can be further exacerbated if two different materials being printed together do not have a chemical affinity for one another [59], resulting in areas of compounded physical and chemical weakening. Both PLA and hPLA are rigid polymers with the same base polymer matrix. This is the most ideal scenario for creating a strong interface during multimaterial 3D printing due to the inherent chemical compatibility [60]. TPU is a thermoplastic elastomer that is known for its elasticity and flexibility [61]. Due to being a soft polymer and hydrophobic, TPU is a material that can be difficult to 3D print both by itself and alongside other materials due to poor layer adhesion [62]. Previous research found that when printing the two differing materials together, soft TPU and hard PLA, it can be difficult to form flush layers during printing [63] and the mechanical strength of the interface is reduced when compared to an interface consisting of the same or similar materials [64,65].

Tomography was used to image the multimaterial hPLA/PLA and hPLA/TPU cube samples to examine the interfaces between the materials and further verify what was found in literature. Figure 5a) shows the cross sections on the CAD model where the multi-material interfaces occur, with the bottom of the cube being where the hPLA was printed onto the outer material and the top being where the outer material was printed onto the hPLA. Examining the material combinations both ways is important because the order in which the materials are printed can affect the success of the print. Looking at the PLA and hPLA interfaces in Figure 5b), in both cases the materials layered directly onto each other with no significant areas of imperfections or gaping, indicating good compatibility between the two materials. In these samples, the multi-material interface was only indicated by the visible change in filament direction caused by the change in printing pattern from one material to the next. When the hPLA and TPU were printed together there were more varied levels of success. When the TPU was printed onto the hPLA, it was visible where the change in material occurred, but the layers still appear to meet consistently along the entire length of the interface (Figure 5c)i)). When the hPLA was printed onto the TPU, the hPLA was not able to fully adhere to the TPU substrate, causing a large gap to form at the interface where the two materials met (Figure 5c)ii)). This indicates that there is a highest chance of print failure when hPLA is printed onto TPU. Previous studies have explored strategies to strengthen the interface between PLA and TPU when printed together by overlapping or weaving the two materials together [66]. When considering using 3D printing on an industrial scale, it would be most beneficial to use a material combination that has a low chance of failure during printing and creates a stronger interface to limit damage and premature part failure.
<figure><img src="image_6.png" /><figcaption>**Figure 5:** Tomography for cube sample a) Cura model showing the placement of the two multi-material interfaces at the top and bottom of the cube sample b) Tomography images of hPLA and PLA interfaces i) where the PLA is printed onto the hPLA and ii) where the hPLA is printed onto the PLA c) Tomography images of the hPLA and TPU interfaces i) where the TPU is printed onto the hPLA and ii) where the hPLA is printed onto the TPU</figcaption></figure>

4. Conclusion

In the present study, multimaterial FDM 3D printing was used to add a thin outer structured layer in a
secondary material, PLA or TPU, to a hemp-PLA biocomposite base material to improve surface
hydrophobicity in a single step printing process. This additional layer helps overcome the increased
hydrophilicity and hygroscopicity that comes with the introduction of the plant-based biofiller to the PLA.
The experimental results show that all types of deliberate structuring resulted in an increase in contact angle
when compared to the reference samples. A direct link between the unique surface morphology that resulted
from using 3D printing process and how the surface and the water droplets interacted was also established.
Finally, factors that could be considered when choosing the printing parameters of the surface depending
on the needs of the specific application or part were discussed including the effects of the surface
morphology, the materials selection, the surface stability, and the material compatibility. There do exist
some limitation to this approach that should also be identified for the sake of further refining the research.
Because of the reliance on multimaterial 3D printing as the fabrication technique, the advancement of this
research is closely linked to the evolution of 3D printing technology. As printers become more advanced
and capable of finer resolution, it will be possible to print finer and more precise patterns of structures
which can lead to increased levels of hydrophobicity being achievable. Past that, new filaments with
different properties are constantly in the process of being produced. A filament that is naturally more
hydrophobic or one that contains a filler that could introduce a nano roughness to the surface would work
to further enhance the hydrophobicity if one of the presented patterns. Future studies on similar topics could
expand on this research in a number of directions including exploring the use of other materials or pattern
types for the structuring layer, looking at other types of biocomposites as the substrate material,
incorporating surface treatments or coatings to further increase the hydrophobicity, or studying the interface
strength between the two layers and techniques to improve material compatibility for more facile printing.
There are additional types of contact angle or wettability measurements that can be done such as CAH,
sliding tests, measurements over time, or droplet adhesion tests.
**CRediT authorship contribution statement**

Kayah St. Germain: Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Damien Marchand: Methodology, Investigation, Formal analysis, Data curation. Laurence Chocinski-Arnault: Methodology, Formal analysis, Data curation. Hani E. Naguib: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. Fabienne Touchard: Writing – review & editing, Supervision, Project administration, Funding acquisition.

**Declaration of competing interest**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Acknowledgements**

The authors would like to thank Institut Pprime (CNRS-ENSMA-Université de Poitiers) for the use of their research facilities and support from their personnel in conducting this research. The authors would also like to acknowledge the financial support provided by Mitacs and the University of Toronto for this research.

**Supplementary materials**

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.addlet.2025.100286.

**Data availability**

Data will be made available on request.

**References**

[1] R. Hsissou, R. Seghiri, Z. Benzekri, M. Hilali, M. Rafik, and A. Elharfi, "Polymer composite materials: A comprehensive review," Apr. 15, 2021, *Elsevier Ltd*. doi: 10.1016/j.compstruct.2021.113640.

[2] M. Nagalakshmaiah et al., "Biocomposites: Present trends and challenges for the future," in *Green Composites for Automotive Applications*, Elsevier, 2018, pp. 197–215. doi: 10.1016/B978-0-08-102177-4.00009-4.

[3] A. Mtibe, M. P. Motloung, J. Bandyopadhyay, and S. S. Ray, "Synthetic Biopolymers and Their Composites: Advantages and Limitations—An Overview," Aug. 01, 2021, *John Wiley and Sons Inc*. doi: 10.1002/marc.202100130.

[4] S. L. Schellbach, S. N. Monteiro, and J. W. Drelich, "A novel method for contact angle measurements on natural fibers," *Mater Lett*, vol. 164, pp. 599–604, Feb. 2016, doi: 10.1016/j.matlet.2015.11.039.

[5] P. Sahu and M. K. Gupta, "A review on the properties of natural fibres and its bio-composites: Effect of alkali treatment," Jan. 01, 2020, *SAGE Publications Ltd*. doi: 10.1177/1464420719875163.
[6] A. Le Duigou, A. Bourmaud, P. Davies, and C. Bailey, "Long term immersion in natural seawater of Flax/PLA biocomposite," *Ocean Engineering*, vol. 90, pp. 140–148, Nov. 2014, doi: 10.1016/j.oceaneng.2014.07.021.

[7] T. Manu, A. R. Nazmi, B. Shahri, N. Emerson, and T. Huber, "Biocomposites: A review of materials and perception," Jun. 01, 2022, *Elsevier Ltd*. doi: 10.1016/j.mtcomm.2022.103308.

[8] W. Bai et al., "Academic insights and perspectives in 3d printing: A bibliometric review," Sep. 01, 2021, *MDPI*. doi: 10.3390/app11188298.

[9] M. Attaran, "The rise of 3-D printing: The advantages of additive manufacturing over traditional manufacturing," *Bus Horiz*, vol. 60, no. 5, pp. 677–688, Sep. 2017, doi: 10.1016/j.bushor.2017.05.011.

[10] W. Gao et al., "The status, challenges, and future of additive manufacturing in engineering," *CAD Computer Aided Design*, vol. 69, pp. 65–89, Dec. 2015, doi: 10.1016/j.cad.2015.04.001.

[11] R. B. Kristiawan, F. Imaduddin, D. Ariawan, Ubaidillah, and Z. Arifin, "A review on the fused deposition modeling (FDM) 3D printing: Filament processing, materials, and printing parameters," Jan. 01, 2021, *De Gruyter Open Ltd*. doi: 10.1515/eng-2021-0063.

[12] D. Han and H. Lee, "Recent advances in multi-material additive manufacturing: methods and applications," Jun. 01, 2020, *Elsevier Ltd*. doi: 10.1016/j.coche.2020.03.004.

[13] S. Hasanov et al., "Review on additive manufacturing of multi-material parts: Progress and challenges," Feb. 01, 2022, *MDPI*. doi: 10.3390/jmmp6010004.

[14] J. D. Kechagias, S. P. Zaoutsos, N. A. Fountas, and N. M. Vaxevanidis, "Experimental investigation and neural network development for modeling tensile properties of polymethyl methacrylate (PMMA) filament material," *International Journal of Advanced Manufacturing Technology*, Oct. 2024, doi: 10.1007/s00170-024-14402-0.

[15] J. D. Kechagias, N. A. Fountas, I. Papantoniou, and N. M. Vaxevanidis, "Interlaminar bonding assessment in vertical-oriented filament material extrusion bending specimens," *International Journal of Advanced Manufacturing Technology*, Feb. 2025, doi: 10.1007/s00170-025-15124-7.

[16] A. Smoca, "Water absorption properties of hemp fibres reinforced PLA bio-composites," in *Engineering for Rural Development*, Latvia University of Life Sciences and Technologies, 2019, pp. 1079–1083. doi: 10.22616/ERDev2019.18.N522.

[17] A. Y. Al-Maharma, S. P. Patil, and B. Markert, "Effects of porosity on the mechanical properties of additively manufactured components: a critical review," Dec. 01, 2020, *IOP Publishing Ltd*. doi: 10.1088/2053-1591/abcc5d.

[18] D. Moreno Nieto, M. Alonso-García, M. A. Pardo-Vicente, and L. Rodríguez-Parada, "Product design by additive manufacturing for water environments: Study of degradation and absorption behavior of pla and petg," *Polymers (Basel)*, vol. 13, no. 7, Apr. 2021, doi: 10.3390/polym13071036.
[19] Z. Xiong et al., "Surface hydrophobic modification of starch with bio-based epoxy resins to fabricate high-performance polylactide composite materials," *Compos Sci Technol*, vol. 94, pp. 16-22, Apr. 2014, doi: 10.1016/j.compscitech.2014.01.007.

[20] A. Ali et al., "Hydrophobic treatment of natural fibers and their composites—A review," May 01, 2018, *SAGE Publications Ltd*. doi: 10.1177/1528083716654468.

[21] C. V. Lang, J. Jung, T. Wang, and Y. Zhao, "Investigation of mechanisms and approaches for improving hydrophobicity of molded pulp biocomposites produced from apple pomace," *Food and Bioproducts Processing*, vol. 133, pp. 1-15, May 2022, doi: 10.1016/j.fbp.2022.02.003.

[22] A. Oberlintner, A. Vesel, K. Naumoska, B. Likozar, and U. Novak, "Permanent hydrophobic coating of chitosan/cellulose nanocrystals composite film by cold plasma processing," *Appl Surf Sci*, vol. 597, Sep. 2022, doi: 10.1016/j.apsusc.2022.153562.

[23] R. Jafari, C. Cloutier, A. Allahdini, and G. Momen, "Recent progress and challenges with 3D printing of patterned hydrophobic and superhydrophobic surfaces," *International Journal of Advanced Manufacturing Technology*, vol. 103, no. 1-4, pp. 1225-1238, Jul. 2019, doi: 10.1007/s00170-019-03630-4.

[24] Y. Liu, H. Zhang, P. Wang, Z. He, and G. Dong, "3D-printed bionic superhydrophobic surface with petal-like microstructures for droplet manipulation, oil-water separation, and drag reduction," *Mater Des*, vol. 219, Jul. 2022, doi: 10.1016/j.matdes.2022.110765.

[25] M. Barahman and A. M. Lyons, "Ratchetlike slip angle anisotropy on printed superhydrophobic surfaces," *Langmuir*, vol. 27, no. 16, pp. 9902-9909, Aug. 2011, doi: 10.1021/la201222a.

[26] J. Sung, H. M. Lee, G. H. Yoon, S. Bae, and H. So, "One-Step Fabrication of Superhydrophobic Surfaces with Wettability Gradient Using Three-Dimensional Printing," *International Journal of Precision Engineering and Manufacturing - Green Technology*, vol. 10, no. 1, pp. 85-96, Jan. 2023, doi: 10.1007/s40684-022-00418-y.

[27] S. Park, J. Sung, and H. So, "Three-dimensional printing-assisted all-in-one surfaces inspired by peristome structures for water-oil separation," *Surfaces and Interfaces*, vol. 29, Apr. 2022, doi: 10.1016/j.surfin.2022.101721.

[28] B. Kang, J. Sung, and H. So, "Realization of Superhydrophobic Surfaces Based on Three-Dimensional Printing Technology," *International Journal of Precision Engineering and Manufacturing - Green Technology*, vol. 8, no. 1, pp. 47-55, Jan. 2021, doi: 10.1007/s40684-019-00163-9.

[29] S. Guessasma, S. Belhabib, H. Nouri, Understanding the microstructural role of biosourced 3D printed structures on the tensile performance, *Polym. Test.* 77 (2019), https://doi.org/10.1016/j.polymtesting.2019.105924.

[30] R. Ambu, S.M. Oliveri, M. Cali, Neck orthosis design for 3D printing with user enhanced comfort features, *Int. J. Interact. Des. Manuf.* (2023), https://doi.org/10.1007/s12008-023-01507-1.
[31] X. Xiao, et al., Mechanical properties and microstructure of hemp hurd reinforced polylactide biocomposites for 3D printing, Polym. Compos. (2024), https://doi.org/10.1002/pc.28893.

[32] D. Jubinville, J. Sharifi, H. Fayazfar, T.H. Mekonnen, Hemp hurd filled PLA-PBAT blend biocomposites compatible with additive manufacturing processes: fabrication, rheology, and material property investigations, Polym. Compos. 44 (12) (2023) 8946–8961, https://doi.org/10.1002/pc.27749

[33] T. Paragkumar N, D. Edith, and J. L. Six, "Surface characteristics of PLA and PLGA films," *Appl Surf Sci*, vol. 253, no. 5, pp. 2758–2764, Dec. 2006, doi: 10.1016/j.apsusc.2006.05.047.

[34] O. Laput, I. Vasenina, M. C. Salvadori, K. Savkin, D. Zuza, and I. Kurzina, "Low-temperature plasma treatment of polylactic acid and PLA/HA composite material," *J Mater Sci*, vol. 54, no. 17, pp. 11726–11738, Sep. 2019, doi: 10.1007/s10853-019-03693-4.

[35] P. Alves, S. Pinto, H. C. De Sousa, and M. H. Gil, "Surface modification of a thermoplastic polyurethane by low-pressure plasma treatment to improve hydrophilicity," *J Appl Polym Sci*, vol. 122, no. 4, pp. 2302–2308, Nov. 2011, doi: 10.1002/app.34348.

[36] T. Douglas and H. J. Haugen, "Coating of polyurethane scaffolds with collagen: Comparison of coating and cross-linking techniques," in *Journal of Materials Science: Materials in Medicine*, Jul. 2008, pp. 2713–2719. doi: 10.1007/s10856-008-3393-6.

[37] A. A. Atwah, M. D. Almutairi, F. He, and M. A. Khan, "Influence of Printing Parameters on Self-Cleaning Properties of 3D Printed Polymeric Fabrics," *Polymers (Basel)*, vol. 14, no. 15, Aug. 2022, doi: 10.3390/polym14153128.

[38] F. Sotoudeh, S. M. Mousavi, N. Karimi, B. J. Lee, J. Abolfazli-Esfahani, and M. K. D. Manshadi, "Natural and synthetic superhydrophobic surfaces: A review of the fundamentals, structures, and applications," *Apr. 01, 2023, Elsevier B.V.* doi: 10.1016/j.aej.2023.01.058.

[39] E. A. Vogler, "Structure and reactivity of water at biomaterial surfaces," 1998.

[40] U. Modi and S. Prakash, "Wettability of 3D printed polylactic acid (PLA) parts," in *AIP Conference Proceedings*, American Institute of Physics Inc., Sep. 2019. doi: 10.1063/1.5123974.

[41] M. Amin, M. Singh, and K. R. Ravi, "Fabrication of superhydrophobic PLA surfaces by tailoring FDM 3D printing and chemical etching process," *Appl Surf Sci*, vol. 626, Jul. 2023, doi: 10.1016/j.apsusc.2023.157217.

[42] K. M. Lee, H. Park, J. Kim, and D. M. Chun, "Fabrication of a superhydrophobic surface using a fused deposition modeling (FDM) 3D printer with polylactic acid (PLA) filament and dip coating with silica nanoparticles," *Appl Surf Sci*, vol. 467–468, pp. 979–991, Feb. 2019, doi: 10.1016/j.apsusc.2018.10.205.

[43] L. Feng et al., "Petal effect: A superhydrophobic state with high adhesive force," *Langmuir*, vol. 24, no. 8, pp. 4114–4119, Apr. 2008, doi: 10.1021/la703821h.

[44] M. Mail, M. Moosmann, P. Häger, and W. Barthlott, "Air retaining grids-a novel technology to maintain stable air layers under water for drag reduction," *Philosophical Transactions of the Royal*
*Society A: Mathematical, Physical and Engineering Sciences*, vol. 377, no. 2150, 2019, doi: 10.1098/rsta.2019.0126.

[45] L. Yang et al., "Fabrication of biomimetic anisotropic super-hydrophobic surface with rice leaf-like structures by femtosecond laser," *Opt Mater (Amst)*, vol. 112, Feb. 2021, doi: 10.1016/j.optmat.2020.110740.

[46] B. Barraza et al., "Superhydrophobic SLA 3D printed materials modified with nanoparticles biomimicking the hierarchical structure of a rice leaf," *Sci Technol Adv Mater*, vol. 23, no. 1, pp. 300-321, 2022, doi: 10.1080/14686996.2022.2063035.

[47] Z. Han et al., "Long-term durability of superhydrophobic properties of butterfly wing scales after continuous contact with water," *Colloids Surf A Physicochem Eng Asp*, vol. 518, pp. 139-144, Apr. 2017, doi: 10.1016/j.colSURFA.2017.01.030.

[48] Y. Zheng, X. Gao, and L. Jiang, "Directional adhesion of superhydrophobic butterfly wings," *Soft Matter*, vol. 3, no. 2, pp. 178-182, 2007, doi: 10.1039/b612667g.

[49] B. Wang, Y. Zhang, L. Shi, J. Li, and Z. Guo, "Advances in the theory of superhydrophobic surfaces," *J Mater Chem*, vol. 22, no. 38, pp. 20112-20127, Oct. 2012, doi: 10.1039/c2jm32780e.

[50] H.J. Ensikat, P. Ditsche-Kuru, C. Neinhuis, W. Barthlott, Superhydrophobicity in perfection: the outstanding properties of the lotus leaf, *Beilstein. J. Nanotechnol.* 2 (1) (2011) 152–161, https://doi.org/10.3762/bjnano.2.19.

[51] M. Chakraborty, J.A. Weibel, J.A. Schaber, S.V. Garimella, The wetting State of water on a rose petal, *Adv. Mater. Interfaces* 6 (17) (2019), https://doi.org/10.1002/admi.201900652.

[52] B. Bhushan, M. Nosonovsky, The rose petal effect and the modes of superhydrophobicity, *Philos. Trans. R. Soc. A: Math. Phys. Eng. Sci.* 368 (1929) (2010) 4713–4728, https://doi.org/10.1098/rsta.2010.0203.

[53] M. Zhang, S. Feng, L. Wang, Y. Zheng, Lotus effect in wetting and self-cleaning, *Biotribology* 5 (2016) 31–43, https://doi.org/10.1016/j.biotri.2015.08.002.

[54] S.H. Kim, Fabrication of superhydrophobic surfaces, *J. Adhes. Sci. Technol.* 22 (3–4) (2008) 235–250, https://doi.org/10.1163/156856108X305156.

[55] M. Jin, Q. Xing, Z. Chen, A review: natural superhydrophobic surfaces and applications, *J. Biomater. Nanobiotechnol.* 11 (02) (2020) 110–149, https://doi.org/10.4236/jbnb.2020.112008.

[56] E. Bormashenko, Progress in Understanding Wetting Transitions on Rough Surfaces, Elsevier, 2015, https://doi.org/10.1016/j.cis.2014.02.009.

[57] S. Parvate, P. Dixit, and S. Chattopadhyay, "Superhydrophobic Surfaces: Insights from Theory and Experiment," Feb. 27, 2020, *American Chemical Society*. doi: 10.1021/acs.jpcb.9b08567.
[58] B. Podsiadły, A. Skalski, W. Rozpiórski, and M. Słoma, "Are we able to print components as strong as injection molded?—comparing the properties of 3d printed and injection molded components made from abs thermoplastic," *Applied Sciences (Switzerland)*, vol. 11, no. 15, Aug. 2021, doi: 10.3390/app11156946.

[59] M. Ribeiro, O. Sousa Carneiro, and A. Ferreira da Silva, "Interface geometries in 3D multi-material prints by fused filament fabrication," *Rapid Prototyp J*, vol. 25, no. 1, pp. 38–46, Jan. 2019, doi: 10.1108/RPJ-05-2017-0107.

[60] L. R. Lopes, A. F. Silva, and O. S. Carneiro, "Multi-material 3D printing: The relevance of materials affinity on the boundary interface performance," *Addit Manuf*, vol. 23, pp. 45–52, Oct. 2018, doi: 10.1016/j.addma.2018.06.027.

[61] T. Xu, W. Shen, X. Lin, and Y. M. Xie, "Mechanical properties of additively manufactured thermoplastic polyurethane (TPU) material affected by various processing parameters," *Polymers (Basel)*, vol. 12, no. 12, pp. 1–16, Dec. 2020, doi: 10.3390/polym12123010.

[62] S. M. Desai, R. Y. Sonawane, and A. P. More, "Thermoplastic polyurethane for three-dimensional printing applications: A review," *Jul. 01, 2023, John Wiley and Sons Ltd.* doi: 10.1002/pat.6041.

[63] E. Brancewicz-Steinmetz, J. Sawicki, and P. Byczkowska, "The influence of 3d printing parameters on adhesion between polylactic acid (Pla) and thermoplastic polyurethane (tpu)," *Materials*, vol. 14, no. 21, Nov. 2021, doi: 10.3390/ma14216464.

[64] E. Brancewicz-Steinmetz, R. D. V. Vergara, V. H. Buzalski, and J. Sawicki, "Study of the adhesion between TPU and PLA in multi-material 3D printing," *Journal of Achievements in Materials and Manufacturing Engineering*, vol. 115, no. 2, pp. 49–56, Dec. 2022, doi: 10.5604/01.3001.0016.2672.

[65] M. Pivar, U. Vrabič-Brodnjak, M. Leskovšek, D. Gregor-Svetec, and D. Muck, "Material Compatibility in 4D Printing: Identifying the Optimal Combination for Programmable Multi-Material Structures," *Polymers (Basel)*, vol. 16, no. 15, Aug. 2024, doi: 10.3390/polym16152138.

[66] U. Altuntas, D. Coker, and D. Yavas, "Creating tougher interfaces via suture morphology in 3D-printed multi-material polymer composites by fused filament fabrication," *Addit Manuf*, vol. 61, Jan. 2023, doi: 10.1016/j.addma.2022.103359.