A comprehensive review on realization of self-cleaning surfaces by additive manufacturing

Abstract

Self-cleaning surfaces revolutionizing the technology world due to their novel property of cleaning themselves, and its multi-functional self-cleaning surfaces exhibit at least one or more functional properties (transparent, conducting, anti-bacterial, anti-corrosion, etc.) This review article focuses on the fundamentals of wettability, material parameters controlling surface wettability and three different paths to realization of self-cleaning surfaces, i.e., (i) super-hydrophobic, (ii) super-hydrophilic and (iii) photocatalytic. The subsequent part of the article mostly focuses on the super-hydrophobic path towards realizing self-cleaning surfaces. In the super-hydrophobic path, the objective is to make the surface extremely repellent to water so that water droplets slide and ‘roll off’ from the surface. The next section of the review article focuses on the role of additive manufacturing in the fabrication of super-hydrophobic micro-structures. Amidst the different fabrication processes of self-cleaning surfaces, additive manufacturing stays ahead as it has the manufacturing capacity to create complex micro-structures in a scalable and cost-effective manner. A few prominent types of additive manufacturing processes were strategically chosen which are based on powder bed fusion, vat photopolymerization, material extrusion and material jetting techniques. All these additive manufacturing techniques have been extensively reviewed, and the relative advantages and challenges faced by each during the scalable and affordable fabrication of super-hydrophobic self-cleaning surfaces have been discussed. The article concludes with the latest developments in this field of research and future potential. These surfaces are key to answer sustainable development goals in manufacturing industries.

Graphical abstract

1 Introduction

Self-cleaning materials and coatings are key in addressing a world becoming more urbanized and plagued by pollution and dirt. The advent of new materials with self-cleaning properties and innovative coatings are engineered to maintain surface cleanliness without requiring intensive chemicals cleaning, providing a cost-effective solution to save the environment. Self-cleaning coatings have key benefits, (a) reducing corrosion and increasing the end of life (b) reducing the dependence on water for cleaning (c) reducing the dependence of toxic chemicals for cleaning (d) minimize the level of air pollution containing compounds detrimental to human health and the environment. As the self-cleaning properties depends on understanding interfacial interaction at liquid–solid interface, as it plays an important role in many areas of condensed matter physics, surface chemistry, material science and engineering applications such as self-cleaning surfaces [1], advanced smart coatings, channel-less microfluidics [2], chromatography, and biochemical reactions [3]. The key parameter behind the behavior of liquids on solid surfaces and have a significant impact on determining wetting, adhesion, and stability of suspensions and emulsions. Wettability refers to a liquid's capacity to maintain contact with a solid surface, a phenomenon governed by the equilibrium between intermolecular interactions involving adhesion and cohesion [4]. When water droplet rests on perfectly flat, smooth, non-porous, chemically stable, horizontal solid surface it forms an inverted semi-circular meniscus (Fig. 1a). Three interfacial surface tension forces $\left( {\gamma_{sv, } \gamma_{lv } , \gamma_{sl } } \right)$ and gravitational force maintain stability of the droplet. Vertical component of the inclined force $\gamma_{lv}$ gets balanced by the gravitational force whereas the force equilibrium equation in the horizontal direction is defined by the Young’s Equation (Fig. 1a).

$$\gamma_{lv } \cos \theta = \gamma_{sv } - \gamma_{sl}$$

(1)

Fig. 1

a Schematic representation Young’s Contact Angle (YCA) when liquid droplet is resting on a perfectly smooth, non-porous, horizontal surface. Schematic showing water contact angle for b Wenzel state and c Cassie–Baxter state roughness [5]

The angle denoted as θ in Young's Equation is termed the Young's Contact Angle (YCA) or Contact Angle (CA) for short. In the case where the liquid is water, it is specifically referred to as the water contact angle (WCA).

According to the behavior of contact angle of water, we can classify the surfaces as super-hydrophilic (WCA < 10°), hydrophilic (WCA < 90°), hydrophobic (WCA > 90°), and super-hydrophobic (WCA > 150°). Similarly, if the liquid is oil, surfaces can be classified as super oleophobic, oleophobic, oleophilic, and super oleophilic. When a surface exhibits a contact angle exceeding 90° or 150° with all types of liquids, it is categorized as omniphobic [1].

Self-cleaning surfaces are gaining interest because of their ability to remove dirt and clean themselves, which reduces the high cost of maintenance of hard-to-reach surfaces, which we would like to keep clean continuously (from building façade to bathroom tiles to solar cell panels etc.) [7]. Different types of self-cleaning surfaces including superhydrophobic, super hydrophilic, and photocatalytic have different cleaning mechanisms. The “lotus effect” is thought to be the mechanism through which superhydrophobic surfaces accomplish self-cleaning [8]. The hierarchical roughness of the surface reduces the droplet's adhesion, allowing water to collect dirt particles as it moves across the surface. While in the case of super hydrophilic surfaces, water spreads out and removes the fouling debris and dirt particles. Photo-catalytic self-cleaning utilizes some hydrophilic surfaces with an additional property where it can induce catalytic breakdown of complex organic impurities using sunlight and subsequently remove them via the hydrophilic cleaning method [9].

Additive manufacturing, also known as 3D printing, is the state-of-the-art twenty-first century approach for layer-by-layer style of advanced manufacturing requiring less material, time and manual labor [10]. This technology has the potential to revolutionize the world in different fields such as science and technology, food, fashion, biomedical applications, etc. [11]. It is popular in the manufacturing sector because of the low cost, less complexity and high efficiency. The basic concept of 3D printing is that it joins the material and produces it according to a computer-aided design. Additive manufacturing includes different technologies such as stereo-lithography, selective laser sintering, digital light processing, fused deposition modeling, 2-photon polymerization, direct metal laser sintering, material jetting, electron beam melting, etc. By creating different microstructures, properties of a surface layer can be changed such as hydrophilicity or hydrophobicity. Additive manufacturing usher a new era of advanced industrial manufacturing capacity as it is highly challenging to create controlled low-cost patterns on different types of hard and soft surfaces. The effect of topology of such 3D printed structures on mechanical properties such as compression strength, tensile strength and energy absorption have discussed before [12]. Large efforts have been made to print complex topologies like schwarzites [13], tubulanes [14], and sea shells [15]. Their mechanical properties, structural behaviors and mechano-stabilization properties were also investigated [22,23,24,25]. The process of fabrication of microstructures becomes less complex and chemical-free with the advent of 3D printing. In this review article, we present an overview of various 3D printing techniques available recently, to design multifunctional self-cleaning surfaces. Different applications of superhydrophobic surfaces reported till now such as self-cleaning, antistick, oil/water separation, and microdroplet manipulation are discussed in this report. This review article ends with a comparative discussion of advantages and challenges faced by these techniques.

2 Review methodology adopted

A systematic literature search was conducted using the Elsevier, Springer, Wiley, and Emerald databases on the topic 3D printed self-cleaning surfaces. The keywords of the search request were organized to two categories (1) Fabrication technology: rapid prototyping, additive manufacturing, 3D printing (2) properties: self-cleaning, oil–water separation, anti-icing. We opted for using general terms across all categories to create a more comprehensive search query. Specific terms such as particular additive manufacturing technologies, specific instrument types, and specific intervention names were deliberately omitted. To mitigate any potential bias, consistent criteria for assessing quality, inclusion, and exclusion were applied throughout the search process. The study encompassed research papers, renowned conference proceedings, and book chapters. Subsequently, records were filtered based on citation counts, with preference given to the most cited publications. A thorough examination of the abstracts, conclusions, and materials and methods sections yielded the most valuable insights. Furthermore, the results section was scrutinized for numerical data. This review takes into account studies that:

• Different AM technologies

• Fabrication of self-cleaning/superhydrophobic surfaces

• Study the properties achieved

The above classification fits with the topics defined in the introduction section, paths to realization of self-cleaning surface, and overview of additive manufacturing.

3 Fundamental of wettability

Wettability refers to the capacity of water or any liquid to either spread out or adhere to a surface. This property holds significant importance across various scientific and engineering domains, including applications like coatings, oil recovery, and microfluidics. The wettability of a solid surface is dictated by both its surface chemistry, involving intermolecular interactions, and its surface texture, which pertains to surface roughness. There are many plants and animals in the nature, which utilize the wetting properties such as lotus leaf, gecko foot, and butterfly wings [16, 17].

Lotus tree, which is renowned for its self-cleaning leaf properties. When water droplets land on lotus leaves, they retain their round shape and effectively sweep away impurities as they roll off. Lotus leaves have a static contact angle of approximately 164° and a dynamic contact angle of approximately 3° for water droplets. The leaf surface introduced hierarchic roughness using two levels of structures: micro-pillars with base diameters around 8 μm, heights of 10 μm, and pitches ranging from 7 to 30 μm. Additionally, there are nanopillars within the micro-pillars, measuring 400–700 nm in height and 70–130 nm in base diameter [8]. Water striders [18] and mosquito (Culex pipiens) eyes [19] are among the biological structures that exhibit superhydrophobicity due to their hierarchical structures. Likewise, the surfaces of duck feathers and butterfly wings exhibit superhydrophobic characteristics. This is attributed to their textured surfaces, which create indentations capturing air pockets that hinder complete wetting by water [20].

3.1 Parameters controlling surface wettability

Surface wettability is primarily governed by two key factors: surface chemistry and surface roughness. The wettability of a surface is significantly impacted by its chemical composition. The existence of polar functional groups on the surface generally enhances wettability, while nonpolar functional groups tend to diminish it. Additionally, the surface energy of a material, which is influenced by its chemical composition, plays a vital role in determining how liquids interact with the surface. Materials with high surface energy tend to attract liquids and are typically more hydrophilic, while those with low surface energy tend to repel liquids and are more hydrophobic. The chemical composition of a surface can be altered by introducing different functional groups or coatings to the surface, which can bring major changes in wettability.

Surface roughness also affects the wetting properties of a surface. There are two types of surface roughness configuration, one is Wenzel (Fig. 1b) and Cassie Baxter (Fig. 1c). In the Wenzel state, the water droplet achieves full contact with the surface, whereas in the Cassie–Baxter state, the liquid droplet resides atop the surface protrusions [21]. In the Wenzel state, heightened increasing surface roughness enhances the hydrophobicity of a hydrophobic surface, and for a hydrophilic surface, greater roughness amplifies its hydrophilic nature [21]. For a superhydrophobic surface, low contact angle hysteresis (CAH < 10°) is crucial along with higher contact angle, which can be only achieved in the Cassie–Baxter type roughness (Fig. 1c) [22]. Super-hydrophobicity increases with increasing surface roughness either in Cassie–Baxter or in Wenzel type roughness and maintaining a steady interface with air voids positioned between the solid and liquid components aids in achieving a reduced Contact Angle Hysteresis (CAH) [22, 23]. The CAH is the difference between the advancing contact angle (ACA) and receding contact angles (RCA). These properties of low CAH & high WCA values can be achieved through hierarchical roughness, generally found in natural superhydrophobic surfaces like lotus leaf (Nelumbo nucifera) (Fig. 2a, b) [6] and shark skin [24]. Hierarchical roughness is obtained in natural or artificially manufactured surfaces when nano-scale protrusions are created on ordered micro-structures created naturally (lotus leaf, shark skin etc.) or artificially (normal or additive manufacturing) (Fig. 2c) [25].

Fig. 2

a The lotus plant (Nelumbo nucifera); b Super-hydrophobicity of lotus leaves, c The schematic diagram showing hierarchical roughness [6]

To incorporate the influence of surface roughness, various models and theories have been developed to compute the contact angle of liquids on uneven surfaces. Among these are the Cassie–Baxter equation, the Wenzel equation, and the theory of super hydrophobicity. The Cassie–Baxter state materializes when a liquid droplet perches upon an uneven surface, anchored by air pockets entrapped between the liquid and the surface [26]. This state is characterized by an elevated contact angle, signifying strong non-wettability of the droplet, and the effective contact angle is determined using the Cassie–Baxter equation:

$$\cos \theta_{eff} = \varphi \cos \theta + 1 - \varphi$$

(2)

In this context, φ represents the proportion of the surface area of the solid engaged by the liquid, while θ signifies the water contact angle. Conversely, the Wenzel state emerges when the droplet completely wets the surface irregularities, resulting in a diminished contact angle and an expanded contact region. For this condition, the effective contact angle is computed using the Wenzel equation, which takes the form:

$$\cos \theta_{{\text{eff}}} = r \cos \theta$$

(3)

Here, r denotes the roughness factor, characterized as the ratio between the real surface area of the solid and its projected area. The surface roughness can be measured using metrics like the root-mean-square (RMS) roughness, which measures the mean deviation of surface height on average, or the fractal dimension, which characterizes the surface's self-similarity at different length scales [27]. In general, Wenzel's model pertains to homogeneous surfaces while Cassie–Baxter's model is applicable to heterogeneous surfaces. While the Wenzel and Cassie–Baxter models prove valuable in explaining prevalent wetting phenomena, they do not encompass certain vital parameters essential in practical applications. Typically, two distinctive types of CAH effects are observed. The initial one is termed the "petal effect," observed when the surface showcases a substantial contact angle, notable CAH, and robust water adhesion. Conversely, the lotus leaf manifests the converse effect, featuring an elevated contact angle, minor CAH, and minimal water adhesion. The distinction between the advancing and receding angles, denoted as θ_adv and θ_rec respectively, can be mathematically expressed as:

$$\theta_{{\text{adv}}} - \theta_{{\text{rec}}} = \left( {\frac{8V}{{\sigma R_0 }}} \right)^{1/2} h\left( \theta \right)$$

(4)

The CAH value is affected by various factors, including the initial radius of the spherical droplet (R_o), the potential barrier (V) that the droplet must overcome during displacement and the surface tension (σ). Another important factor is the geometrical factor [h(θ)], which influences the magnitude of CAH. Micro-structures with sharp edges have the ability to anchor the contact line between air, liquid, and solid, leading to an increase in the contact angle hysteresis (CAH). A higher CAH results in a reduced tendency of droplets to roll on the surface. Conversely, pillars featuring rounded edges are better suited for enhancing self-cleaning properties [27].

3.2 Paths to realization of self-cleaning surface

There are three ways to create self-cleaning surfaces—super hydrophilic, superhydrophobic and photocatalytic. The precise mechanisms how these surfaces work has already been discussed in the introduction. Among these three types of self-cleaning surfaces, multiple processes are available to fabricate superhydrophobic self-cleaning surfaces such as chemical vapor deposition, electro chemical etching, laser texturing, self-assembly and electrospinning. However, these methods can be complex, expensive, and not easily scalable, and often lead to an unpredictable morphology [9, 28, 29].

Developing hierarchical roughness by mimicking natural surfaces is a way to solve this issue. additive manufacturing offers a promising approach to fabricate surfaces with hierarchical roughness. By stacking layers of material to create astonishing prototypes with intricate geometries, 3D printing methods can be utilized to enhance the self-cleaning properties of surfaces. Hierarchical structures have features in multiple levels, from microlevel to nanolevel. These together trap air molecules and reduce the area of contact between solid and liquid. Multi-functional smart surface shows more than one functional property, one of which is self-cleaning property, while others can be properties such as transparent, conducting, anti-bacterial, anti-corrosion, anti-icing, drag reduction etc. By using AM techniques, it is possible to make surfaces with similar hierarchical roughness, facilitating the fabrication of new materials with increased functionalities.

4 Overview of additive manufacturing

Additive manufacturing is an innovative manufacturing process that allows the creation of complex three-dimensional objects with unparalleled design freedom and precision. In contrast to conventional manufacturing techniques that necessitate material removal from a larger block, additive manufacturing entails the meticulous deposition of material layer by layer, under the guidance of a digital model. The process starts with a digital design created using sophisticated computer-aided design (CAD) software. This design is then transformed into a set of instructions that guide the additive manufacturing machine to add material layer-by-layer, building up the object until the final product is achieved. Additive manufacturing is a true engineering marvel, offering several advantages over traditional manufacturing processes. Firstly, it allows for intricate shapes and geometry to be created, which would be otherwise impossible using conventional techniques. This means that additive manufacturing can produce objects with complex internal structures and shapes, which opens up new avenues for design and innovation. Some of the 3D printing techniques like 2-photon polymerization and stereolithography have resolutions ranging from 100 nm to few microns (Fig. 3).

Fig. 3

Classification of additive manufacturing technologies with different material processing techniques and their resolutions in creating micro-nanostructures

The materials used in additive manufacturing are diverse and include thermoplastics, photopolymers, metals, ceramics, composites, and biomaterials (Fig. 4). Polymers are widely used in stereolithography and digital light processing techniques. Metals are generally used in powder bed fusion, ceramics in binder jetting, and composites in fused filament fabrication or continuous fiber reinforcement. Biomaterials, which are compatible with biological systems, are used in medical applications such as tissue engineering, implant design, and drug delivery.

Fig. 4

Materials used in different additive manufacturing processes

Additive manufacturing is incredibly efficient with minimal material wastage. It employs precise amount of material required to build an object as opposed to traditional manufacturing that leads to excess raw material wastage. This makes it more cost-effective and environmentally friendly. It can produce objects with unique properties and characteristics which are hard to accomplish or cannot be realized through alternate methods. This is because the additive manufacturing process can utilize an extensive variety of materials and can create objects with graded structures, enabling the optimization of material properties.

Additive manufacturing has become an effective tool for creating smart multifunctional surfaces with tailored micro- and nanostructures that exhibit desirable properties such as superhydrophobicity, self-healing, and anti-corrosion. Fabrication of superhydrophobic surfaces by different AM techniques, using different microstructures and the maximum contact angle achieved has listed in the Table 1. A technological pathway showing the inventions of different AM techniques is discussed in Fig. 5. These surfaces are built by precise layer-by-layer addition of materials guided by digital models. Through additive manufacturing, intricate surfaces can be produced that traditional manufacturing techniques cannot achieve, providing superior performance and protection in a range of environments. These surfaces hold the capacity to revolutionize many industries, including aerospace, oil and gas, and consumer products.

Table 1 Showing the maximum water contact angle achieved by superhydrophobic surfaces fabricated by different AM techniques

Fig. 5

Diagram showing the technological pathway of additive manufacturing—inspired from [30]

4.1 Vat photopolymerization

Vat photopolymerization (VP) stands as the pioneering 3D printing process, employing a light source to initiate polymerization reactions within photosensitive materials. This method is categorized based on the light source and polymerization mechanism employed. Numerous lab-scale and commercial technologies, adhering to the same fundamental principle, have subsequently been introduced like stereolithography (SLA), two-photon polymerization (2PP), digital light processing (DLP), and volumetric 3D printing. Various industrial sectors, encompassing mechanical, medical sciences, electrical/electronics, etc., have embraced these techniques to produce functional components.

4.1.1 Stereolithography

Stereolithography is one of the most extensively used vat photopolymerization technique [44]. It is based on the solidification of liquid resin taken in a reservoir by electromagnetic radiation or laser beam. Laser beam scans over the resin and starts the photopolymerization layer by layer. Through the chemical crosslinking process resin in the liquid form transforms into solid form [45]. SLA system mainly consist of the substrate which is staged in a tray of vat or resin and the laser beam. The laser cures the photopolymer layer by layer according to the information given by the computer aided design (CAD) data. Once the first layer completed, it lower the platform and the next layer may then be scanned [46]. The SLA techniques can be classified according to the motion of the build platform and laser. The SLA process allows fabrication of complex prototypes precisely, makes it an ideal 3D printing process. A major hindrance faced by the SLA is the poor thermo-mechanical performance of the printed part, because of the layer-by-layer nature of printing. However, SLA has been widely applied now to make superhydrophobic and omni phobic surfaces, as it can print microstructures of few microns. Oleophobic and superhydrophobic surfaces gain attention due to their functional abilities like self-cleaning and antifouling. Some methods reported for the fabrication of micro/nanostructures with super hydrophobicity and superoleophobicity are either time-consuming or expensive [47,48,49].

Inspired from the nature, Caterina et al. fabricated stable hydrophobic and oleophobic surfaces by applying stereolithography technique using perfluoropolyether (PFPE) [31]. It is the first work reported on micro structuring of PFPE by SLA method. The main parameters affecting the printing resolution are critical energy E_c and on the penetration depth D_p of the formulation [50]. They printed arrays of cylindrical pillars with different pillar spacing constant height and diameter (Fig. 6). They compared the hydrophobicity of these surfaces with the SL prints of a commercially available stereolithographic high performing resin. The contact angle for bare DL260 and PFPE-DUTA surfaces are < 90° and ~ 110° respectively. After the microtexturing by SL all DL-260 samples exhibited WCA of 115°–125°. For the arrays with spacing ≤ 250 µm, the water droplet existed in Marmur state, while it transferred from Marmur to Wenzel with increasing spacings. Marmur state is a metastable arrangement where droplets exhibit partial wetting on the lateral surfaces while also partially resting on air pockets [51]. For the arrays of 200 µm spacing, PFPE-DUTAs textured surfaces exhibited WCA of 151° and lipophobicity with CA values 120° (Using Nujol Oil as the liquid). The trend of decreasing contact angle with increasing spacing was also established by the authors.

Fig. 6

(Inspired from [34])

CAD images of micropillars with 100 µm diameter and 400 µm height with a spacing of 200 µm tilted view, b spacing of 200 µm top view, c spacing of 250 µm tilted view, and d spacing of 250 µm top view

The fabrication of complex 3D PTFE geometries by digital ultraviolet (UV) lithography is reported by Yangxi Zhang et al. The PTFE nanoparticles mixed with PEGDA (polyethylene glycol diacrylate) aqueous dispersion were printed by layer-by-layer UV photopolymerization process [32]. They could achieve high resolution of 6 µm in 2D patterns and ~ 46 µm in 3D microstructures (Fig. 7). Along with the micro-scale texturing, the nano-scale roughness leads to super hydrophobicity. They also investigated the effect of sintering temperature, water content and mixing ratio of PEGDA/PTFE on the surface texturing.

Fig. 7

CAD images of the 3D-printed microstructures: a micro-honeycomb structure; b zigzag micro-beam array; c 3D triangular-frame; d micro-ball array; e micro-mushroom array; f suspended micro-cross array. (Inspired from [32])

Yang Liu et al. 3D-printed biomimetic superhydrophobic surfaces with a projection micro stereolithography (PμSL) 3D printing system [33]. They reported 3D printed superhydrophobic surface with petal-shaped microstructures, which draws inspiration from the droplet pinning effect, that have been observed in nepenthes peristome. They examined and refined factors such as the quantity of petals, the ratio of petals, and the gap distance to enhance water repellency, which was evaluated by droplet bearing capacity. The results of their study illustrate that the surface resembling petals achieves its highest droplet-bearing capability when there are four petals, a spacing distance of 100 µm, and a petal proportion of 50%. Compared to the typical mushroom microstructures, the optimized petal-like microstructures yield a Peak enhancement rate in load-bearing capacity of 58.3%.

The design and manufacturing process of biomimetic surfaces that mimic the characteristic dimensions and morphology of rice leaf riblets were reported using an SLA printer recently [34]. Microchannels were designed using AutoCAD software, and a Form 2 SLA 3D printer was employed to fabricate flat samples and biomimetic surfaces. The surfaces were then modified with TiO₂ nanoparticles using a dip-coating technique, making them hydrophobic. The coating process was repeated twice, and the samples were washed and dried between each layer. Finally, the samples were stored in a dark environment due to the photocatalytic reactions reported for TiO₂ nanoparticles. On flat printed surfaces, the filaments create a difference of 35° in the directionality of the surface, resulting in a hydrophobic quality (measured by contact angles greater than 95°) perpendicular to the filaments, and a hydrophilic behavior (measured by contact angles less than 74°) parallel to them. All microchannel surfaces achieved a superhydrophobic state with the TiO₂-HTMS coating, exhibiting contact angles greater than 160° [34]. The 100 µm coated microchannels were found to have the most effective biomimicry, with an advancing contact angle of 165°, contact angle hysteresis lower than 9°, and a contact angle difference of 5°, all of which meet the requirements for a superhydrophobic surface inspired by a rice leaf. To prove it theoretically, they did numerical simulations considered water flowing through a closed rectangular channel with biomimetic microchannel structures (riblets) on the bottom wall, a hydrophilic top wall with no-slip boundary conditions, and a periodic lateral boundary conditions to replicate the experimental conditions. The flow is laminar, driven by a pressure difference, and stationary, with specific no-slip and shear force boundary conditions at the solid-water, solid-air, and air–water interfaces.

Mayoussi et al. introduced an innovative method named 'Fluoropor,' in which a photosensitive fluorinated resin is blended with a mixture of porogen. This mixture is then 3D printed employing a stereolithography (SLA) process, yielding superhydrophobic micro/nanoporous membranes displaying static contact angles of 164° [35]. By modifying the porogen ratio within the mixture, the membranes' pore dimensions can be tuned between 30 and 300 nm. The study showcases the practical utility of these printed membranes in tasks such as oil/water separation, the creation of Salvinia-like layers to reduce drag in marine transportation, and preventing bio-fouling [35].

The SLA process employs photopolymer resins like acrylate-based resins, epoxy-based resins, polyurethane-based resins, etc. These resins are a class of materials widely used in protective coatings, adhesives, and anti-corrosive applications. When utilized for SLA printing, they provide excellent finished surface, decreased volumetric shrinkage, increased production capabilities, and larger build volumes. Despite having all these advantages, it faces major challenges in the application scenarios which require mechanical robustness due to its inherent brittleness. Another major concern is the redundant structural layers created during printing of the prototypes, which is due to the layer-by-layer nature of printing. This anisotropic nature causes varying mechanical properties along different axes. Yuewei Li et al. reported the use of core–shell particles to increase the mechanical properties of the photocurable resins [52]. They made core shell particle with polybutadiene-co-polystyrene as the core and shell with polymethyl methacrylate, polystyrene, and poly glycidyl methacrylate. In addition to these photocurable resins, ceramic resins can also be used in SLA printing process. The review article by Be et al., gives a comprehensive overview of the latest research in the field of photopolymerization of ceramic resins by SLA process [53]. They also discussed the potential of this technology to create high-precision ceramic parts which can be employed in different industrial areas such as automotive, electronics, biomedical, aerospace and defense applications etc. The diverse resins used in SLA printing have been tailored to specific application requirements. These printing resins are available commercially in different categories such as standard, hard, durable, heat-resistant, rubber-like, dental, and moldable variants. The selection of the most fitting material is crucial, whether it is for creating prototypes, crafting robust components, simulating engineering plastics, fashioning flexible objects, fabricating dental models, or generating patterns for casting.

An innovative strategy of combining high-resolution stereolithography and replica molding could make macroscopic devices with microscopic features without any limitations on the materials used. Even though SLA 3D printing is a popular and versatile technique for creating complex 3D structures, like any technology, it is not without its challenges. The materials used in SLA 3D printing are typically specialized resins that are formulated for the process. This can limit the selection of materials available for use in the printing process and make it difficult to achieve certain properties, such as high strength or heat resistance. And the synthesis of functionalized resins is complex and expensive.

4.1.2 Two-photon polymerization

Two photon polymerization (2PP) is a high resolution maskless vat photopolymerization technique. 2PP is known for its ability to create nano- and microstructures. The single-photon polymerization in SLA takes place at the surface of a photosensitive resin, allowing for the construction of 3D structures one layer at a time. Conversely, in two-photon polymerization (2PP) utilizing near-infrared (NIR) femtosecond (Fs) laser pulses, both photons are simultaneously absorbed by the photo-initiator. This simultaneous absorption enables them to function as a single photon, initiating polymerization. This capability facilitates the direct writing of any desired 3D model into the volume of photosensitive materials that are transparent in the NIR and highly absorbent in the UV spectral range [54, 55]. By the radical quenching mechanism, people have achieved a ~ 100 nm resolution. There are many applications for the 2PP process including creating micro–nano-hierarchical structures for superhydrophobic surfaces. Fractal structures are also gaining interest in making superhydrophobic surfaces. Yang Lin et al. could achieve super hydrophobicity by creating fractal tetrahedron and pyramids arrays on glass substrates (Fig. 8). Subsequently, they coated it with hexamethyldisiloxane (HMDSO) by a chemical vapour deposition (CVD) technique to increase the hydrophobicity [36]. Also, by fabricating micro–nano-structures on a plastic film, they could make flexible superhydrophobic surfaces. But all the surfaces before HMDSO coating were either hydrophilic or hydrophobic. This shows that the structures along with the coating caused super hydrophobicity.

Fig. 8

Diagrams of Sierpinski tetrahedron and corresponding Computer-aided design (CAD) models of the microstructure array. a–c Diagrams of stage-0, stage-1, and stage-2 Sierpinski tetrahedron, respectively. d–f CAD models of stage-0, stage-1, and stage-2 Sierpinski tetrahedron array. The length for each Sierpinski tetrahedron unit is 20 µm and the center-to-center distance between units is 30 µm. The printed surface area is roughly 3 × 3 mm. (Inspired from [36])

A mushroom-like micro- and nanostructures created on a hydrophilic surface by direct laser writing through two-photon polymerization could achieve a water contact angle of 128◦. They could fully reproduce the computer-aided design by this technique (Fig. 9) [37]. The “mushroom” structures were created with a supporting underside (resembling the leg of a mushroom) and a decorated top side (resembling the hat of a mushroom) featuring micro- and nanostructures in a ripple-like pattern. The upper surface featured pillar formations resembling mushrooms, which encompassed a spectrum of lengths, ranging from several µm for micro-structured mushroom-like pillars (MMP) to tens of nm for nanostructured mushroom-like pillars (NMP). These distinct micro- and nano-structures displayed hydrophobic characteristics, showcasing contact angles of 127° and 128°, respectively. Conversely, the flat polymer surfaces exhibited hydrophilic traits, with a contact angle measuring 43°.

Fig. 9

Computer-aided designs of microstructured (MMP) and nanostructured mushroom-like pillars (NMP) a, d close, top views of single MMP and NMP; b, e close, tilted view of single MMP and NMP, respectively; c, f large, top views of MMP and NMP areas, respectively.(Inspired from [37])

Separate investigations by Tricinci et al. [38] and Yang et al. [56] delved into experiments simulating Salvinia molesta. They revealed that specific design parameters including diameter, height, number of eggbeater arms, and spacing between stalks significantly impact the attainment of optimal solid–liquid and air–liquid fractions. These factors ultimately dictate the extent of super hydrophobicity, contact angle, and contact angle hysteresis. An absence of arms on the head (N = 0) yielded a Wenzel state, while N = 2 led to a Cassie–Baxter state with170° contact angle, and N = 4 resulted in a contact angle of 152°. However, due to the hydrophilic nature of the printing material, water droplets partially infiltrate the eggbeater arms, resulting in a hybrid Cassie–Wenzel state for N ≥ 2.

Another study by Marco et al., shows the fabrication of sub micrometer range three-dimensional (3D) structures employing a new multifunctional perfluoropolyether-based resist, which is inherently water repellent and chemically resistant [57]. In this study, they have formulated and synthesized a fluorinated resist, which was effectively utilized to create woodpile structures across different experimental settings. This marks the pioneering instance of successfully producing three-dimensional structures featuring sub-micrometer precision and customizable geometries, all while incorporating them with hydrophobic properties and robust chemical resistance.

Zheqin Dong et al. developed a technique for producing 3D structures with micrometer accuracy using a combination of macroscopic 3D printing like digital light processing, two-photon lithography (2PL), and material-independent discontinuous dewetting [58]. First, they produced inherent superhydrophobic objects by DLP, which are then subjected to 2PL to create hydrophilic micropatterns on their surface. Taking advantage of the discontinuous wetting phenomenon, they have successfully demonstrated the selective deposition of functional material solutions forming microscopically hydrophilic areas on the surfaces of 3D structures. This process in-turn results in high resolution and excellent design flexibility using 2-photon polymerization techniques. Significantly, this approach transcends material limitations and enables micro-patterning of diverse functional materials suspended within aqueous solutions, encompassing polydopamine, silica, and silver nanoparticles.

Another recent study by Bunea et al. reported the successful one-step fabrication of superhydrophobic surface by 2 photon-polymerization micro 3D printing [39]. They generated an array of hexagonal pillars, each characterized by a distinctive “micro-hoodoo” morphology—incorporating a reentrant cross-sectional profile. This innovative pillar design enhances its resilience against complete wetting, a phenomenon where a shift from a non-wetting Cassie–Baxter state to a Wenzel wetting state is observed. The fabrication process involves the rapid creation of 4 × 4 mm² arrays through a single-step approach utilizing two-photon polymerization direct laser writing. This technique employs a commercially available resin, IP-PDMS, derived from polydimethylsiloxane (PDMS), renowned for its hydrophobic properties. Despite the relative simplicity of the structures—featuring a single reentrant surface the application of this hydrophobic resin for creating intricate surface patterns enables the attainment of superhydrophobic behavior. Among the various micropatterns investigated, five distinctly showed superhydrophobic attributes, attaining an impressive static contact angle with water of up to 158.1°. Stephan Milles et al. evaluated the self-cleaning efficiency of laser-textured surfaces fabricated by Direct Laser Writing (DLW), Direct Laser Interference Patterning (DLIP) and a combination of both. The Al surfaces textured by DLW + DLIP showed a maximum contact angle of 161° [59]. Mechanically durable super-repellent surfaces have been fabricated by 2-photon polymerization using fluorinated SiO₂ nanoparticle coating [60]. The re-entrant microcell/nanoparticle structures showed static contact angles for water and ethylene glycol oil of 167.3° ± 1.8° and 159.5° ± 2.5°, respectively. The CAH values for water and ethylene glycol were 2.5° ± 1.5° and 4.5° ± 2.0°, respectively. After the abrasion test, the microcells exhibited static water contact angles of 152.0° ± 3.5°, with CAH of 7.1° ± 2.5°. They have done the theoretical studies as well, and Fig. 10 shows detailed simulation results. Figure 10a shows the locations, water may be pinned and Fig. 10b shows static contact angle values obtained by the simulation studies. Although micropillars could theoretically achieve higher static contact angles and breakthrough pressure compared to microcells, they suffered from a significant drawback of poor mechanical stability, which was experimentally validated [60].

Fig. 10

Simulations. a Schematic showing the three locations where the water may be pinned b static contact angle θ_CB and breakthrough pressure P_b for different contact line pinning locations. Snapshots showing the c static contact angle and d the breakthrough pressure simulation e STATIC structural finite element simulation results showing equivalent stress in MPa under 1.2 kPa normal compressive pressure at the top surface and tangential force corresponding to a coefficient of friction of μ = 0.7 for (i) micropillar and (ii) microcell [60]

2Photon polymerization can create structures with sub-micron resolution, making it ideal for applications that require high precision and accuracy. Since 2PP is a direct-write technique (not requiring any lithographic mask), it permits the creation of complex three-dimensional configurations that are challenging or unattainable using alternative production techniques. Also, this technique applies to an extensive variety of materials, encompassing photopolymer resins, composites, and biocompatible materials. Additionally, due to its non-invasive nature it does not require any heat or pressure, making it suitable for printing delicate or sensitive materials.

4.1.3 Digital light processing

Digital light processing (DLP) is a faster additive manufacturing technology compared to SLA. Compared to the other vat photopolymerization techniques like SLA and 2PP, it uses a projected light to print objects. It flashes the first layer to be printed, and each layer gets printed according to the image from the projector. DLP printers are used to make objects with fine details like dental and jewellery moulds rapidly. Kaur et al. reported the fabrication of superhydrophobic objects by DLP printing [40]. They developed an innovative ink formulation by blending non-fluorinated acrylates and Hydrophobic Fumed Silica (HFS) (Fig. 11). They investigated the influence of HFS concentration and pillar arrangement design on achieving superhydrophobic characteristics. The superhydrophobic attributes were assessed through measurements of water droplet contact and rolling angles on the surface. They demonstrated the potential of increased buoyancy through super hydrophobicity by showing that printed SH objects floated in water, even after being forcefully submerged, while their non-SH counterparts did not. This provides evidence in favor of the concept of using super hydrophobicity to increase buoyancy.

Fig. 11

a Diagram depicting DLP printing along with an explanation of ink composition, Visual representations of b a printed array of micropillars; c a cube featuring micropillar arrays on its surfaces; (Inspired from [40])

A similar approach of fabricating superhydrophobic objects by DLP printing has been reported recently [41]. They designed a new ink composed of (meth)acrylate monomers having hydrophobic properties and porogen solvents and 3D printed SH complex structures (Fig. 12) and microfluidic devices which are liquid impermeable but gas permeable due to its porous structure. Also, they demonstrated a 3D-printed oil-absorbent with hierarchical structure.

Fig. 12

Additive manufacturing of superhydrophobic objects with bulk nanostructure. a Illustrative picture showcasing the DLP 3D-printing process employing a phase-separating ink. b During this process, the homogeneous ink undergoes phase separation and generates inherent nano-porous structures through photopolymerization, which ultimately results in bulk super hydrophobicity (Inspired from [41])

Zhipeng Jin et al. successfully fabricated a superhydrophobic ceramic object with controllable gradient pore structures, which can be used for oil–water separation at high temperatures using a digital light processing technique [42]. They used a ceramic slurry (Al₆Si₂O₁₃) as the precursor material. Subsequently, the porous configuration underwent sintering at approximately 1450 °C. Following this, a modification phase was executed, wherein SiO₂ was introduced through a sol–gel methodology, leading to the development of a superhydrophobic/super-oleophilic SiO₂/GPCS ensemble with remarkable contact angles (CAs) of up to 162.1°. The SiO2/GPCS composite, synthesized in this manner, exhibited significant efficacy in executing diverse oil/water separation tasks encompassing kerosene, hexadecane, dichloromethane, diesel oil, among others. Across all tested oils, the SiO₂/GPCS consistently demonstrated exceptional separation efficiencies exceeding 96.5%. Notably, the controllable parameters of pore size and taper angles enabled fine-tuning of separation efficiency and permeation flux. Beyond this, the SiO₂/GPCS exhibited commendable mechanical and chemical stability under ultrasonic treatment and in varying solution conditions. The capability for oil/water separation persisted even after multiple separation cycles, showcasing substantial durability. Additionally, the SiO₂/GPCS showcased proficient oil separation from water, even in elevated-temperature settings, further extending its utility. They also characterized surface morphology and structure using different characterization techniques such as TEM, XPS, XRD, and SEM.

Nevertheless, many challenges are there in the widespread adoption of Digital Light Processing (DLP), as well as other Additive Manufacturing (AM) methodologies, that hinder the complete substitution of conventional techniques [61]. Among these challenges, the scale of production remains a limiting factor, including dimensions spanning ranging from microns to millimeters in scale. But Micro Projection Stereolithography shows encouraging prospects for micro feature fabrication, while aiming to optimize printing precision. However, a trade-off emerges between size and accuracy, a consequence of the restricted surface area as projected by the DLP source. Another challenge faced by DLP is the time-consuming process of separating cured layers. The quest for multi-material printing continues to be a pressing query, not solely restricted to DLP but also across diverse AM modalities. The meticulous cleaning during material swapping in multi-material printers to prevent cross-contamination and the absence of commercial availability of such printers, underscores the complexity of this challenge. These complications hinder the realization of densely structured solid objects using DLP technology. Additionally, the lack of significant research in metallic suspension-based manufacturing not only requires increased endeavors related to metal printing using DLP, and it also uncovers new doors for exploration in metallurgical domains. Addressing these various challenges demands a strategic approach and will inevitably catalyze further research and development initiatives aimed at advancing DLP technology.

4.2 Powder bed fusion

Powder bed fusion (PBF) is a form of additive manufacturing, where a heat source, typically a laser, is employed to sinter or fuse atomized powder particles. This process, akin to other additive techniques, is executed layer by layer until the final part is fully formed. Various powder bed fusion techniques include direct metal laser sintering (DMLS), selective laser melting (SLM), electron beam melting (EBM), selective laser sintering (SLS), and multi jet fusion. Among these SLS is known for its ability to work with a wide range of materials, including polymers, metals, and ceramics. The nature of different SLS powders affect the manufacturing measures during the process, which subsequently impact the structural characteristics of the end product [62]. The SLS procedure commences with a 3D CAD model, which is subsequently divided into slender cross-sectional layers via slicing software. A build platform is covered with a thin sheet of powdered material and the laser fuses the particles together selectively according to the sliced design. The platform moves down one layer, and this sequence is reiterated for the subsequent layer, building up the prototype from the bottom to up. One of the advantages that SLS printers have is that it doesn’t need any support structures. As a result, the fabrication of intricate internal structures within complex geometries would become more straightforward. Since SLS does not require support structures, it is possible to produce complex geometries and intricate internal structures that would be complicated or impossible to create with other manufacturing methods. SLS have utilizations across diverse sectors like aerospace, healthcare, automotive and consumer goods, as it utilizes wide variety of materials like polycarbonate, nylons, metals, ceramics and thermoplastic elastomers [63, 64].

In a study, Shushan Yuan et al. presented a two-stage approach for building a rugged, spongy, and micro/nanostructured ZIF-L coating on a membrane produced by SLS printing [43]. They achieved this by designing three-dimensional ZIF-L structures that feature a cross-hatched and multiscale configuration. Upon coating the membrane with PDMS, they observed that it exhibits an extremely low scrolling water contact angle of 1.56° and a high static water contact angle of 158.6°, resulting in extreme super hydrophobicity. Furthermore, when wetted with water, the membrane exhibited exceptional underwater superoleophobicity. Both membranes displayed excellent effectiveness in separating oil and water. They attributed the extremely water-repellent and highly oil-repellent underwater characteristics of the membrane to the structure that combines micro- and nano-level hierarchy created through 3D printing, ZIF-L structure design, and deposition [58].

Yuan et al. reported a successful method to fabricate a durable and hydrophobic polyamide-12 membrane through the sintering process of functionalized polyamide-12 powder [65]. The utilization of candle soot, a byproduct of incomplete candle combustion, has gained attention for constructing superhydrophobic surfaces, despite its conventional status as a pollutant. So, they used the candle soot to impart hydrophobic characteristics to polyamide-12 powder and could achieve a water contact angle of 140°. These membranes stayed the same even after being shaken in water for 30 min, soaked in a liquid for 7 days, and rubbed with sandpaper 10 times. They have the efficiency for separating mixtures of oil and water, with 99.1% of the oil being removed from the mixture. These 3D printed membranes are also very good at keeping oil away even when they are underwater. Of particular significance, this membrane exhibits the capability to effectively segregate immiscible organic mixtures attributed to its notable resistance to solvents.

Wu et al. recently reported (2023) generic approach of utilizing SLS 3D printing technology for the direct production of intrinsic superhydrophobic objects from various polymeric materials [66]. Illustrated by the example of a polypropylene (PP)/PTFE composite, the process involves laser manufacturing, where sintered PP powder contributes mechanical resilience and refining surface texture, while hydrophobic PTFE particles are introduced onto the surfaces and within the gaps of the PP powder, generating regions with water-repelling properties. By accumulating layers through sintering, the fabrication of inherent superhydrophobic 3D objects is achieved. The resulting products exhibit exceptional wear-resistant superhydrophobic properties, demonstrated by their resilience to 600 m sandpaper abrasion at 12.5 kPa pressure. This confirms their outstanding wear resistance. Furthermore, these single-step customized intricate 3D structures can be employed in designing platforms for droplet reactions and transmedia interfaces between water and air vehicles. Extending beyond PP, this method was successfully adapted for eight additional thermoplastic/thermoset polymers, encompassing polyamide, polyethylene, polyether block amide, polyethylene terephthalate, polymethyl methacrylate, polystyrene, phenolic resin, and epoxy resin. Fabrication of abrasion-resistant superhydrophilic surfaces has been reported by Zhenhua Wu et al. using SLS 3D printing of composites made from glass beads (GBs) and thermoplastic phenol–formaldehyde resin (PF) powder. A skeleton structure is formed during the printing process by GBs adhering to the surfaces and gaps, which creates several hydrophilic points [67]. SLS enabled printing of long interlayer micro-slits and intermittent pores with anisotropic structures, to achieve properties like anisotropic water transport capabilities. They could achieve a contact angle of 0° even after the 1000th abrasion. Congcan Shi et al. reported the fabrication of a Janus evaporator with tunable wettability, featuring a super hydrophilic bottom for saltwater transport and a superhydrophobic top for water redirection and steam transport using SLS 3D printing [68]. This unique structure achieved record-breaking long-term stability for solar evaporation in saturated brines.

An advantage of SLS is that parts produced can have high mechanical strength and can withstand harsh environments, rendering them suitable for functional prototype models, final-use components, and limited-volume manufacturing batches. However, SLS might incur higher costs compared to alternative 3D printing techniques and requires specialized equipment and expertise.

4.3 Material extrusion

Material extrusion is an extensively utilized low-cost additive manufacturing technique. This method encompasses two distinct types of material extrusion techniques. The first type involves solid feed extrusion, termed fused filament fabrication (FFF) or fused deposition modeling (FDM). In this approach, a solid material is extruded layer by layer to construct the desired object by a computer-aided design. The second type involves liquid feed extrusion, referred to as direct ink writing (DIW) or LDM (Liquid Deposition Modeling) 3D printing. In DIW, a liquid material is extruded to create the intended structure, providing flexibility in material selection and enabling the fabrication of intricate designs. The material comes through a hot nozzle and deposits along the XY plane on the platform. The print head moves up or the platform moves down, after the completion of one layer. The movement is in the z- direction and it is exactly one layer thickness [69, 70]. The time taken for the solidification of the extruded material is a key parameter in the FDM process. The fiber in the molten condition falls on the fiber which is solidified already. This makes a bond between these fibers. When the molten fiber solidifies quickly before making a bond with the other frozen filament, it creates voids. So, one of the disadvantages of FDM technique is the poor mechanical strength compared to SLA. The most used materials for the FDM technique are thermoplastics such as PLA, ABS, PET-G, TPU, PVA and their composite materials. Another limitation of FDM is that applying excessive pressure on the extruding material damages the print. This problem can be mitigated using high viscosity fluids which are resistant to deformation after solidification, however their extrusion is difficult due to their high viscosity. Alternatively, printing materials with low viscosity (in their liquid state) are easy for extrusion and patterning, although they have more chances of spreading, which makes printing difficult [71, 72].

Zhoukun He et al. reported a facile way to fabricate superhydrophobic surface by patterning PDMS filaments on a substrate using FDM technique [73]. An ordered and anisotropic grid pattern was printed with different printing speeds. Water contact angle perpendicular to the printing direction and parallel to the printing direction was different. Their dissimilarity showed the anisotropy of the porous PDMS film. The impact of geometrical parameters on the printed PDMS films were studied by adjusting speed of the printing and spacing of filament. They found the optimum speed of printing of 6.00 mm/s, that gives isotropic wettability [73]. A superhydrophobic surface with WCA of 155° was made, which exhibited resistance to thermal deterioration.

Fabrication of superhydrophobic surface utilizing PLA filament substance and applying a dip-coating technique with hydrophobic silica nanoparticles by FDM were reported (Fig. 13) [74]. They printed PLA flat surfaces with layer pattern and tool path pattern. Following the dip-coating process, the surface exhibited a patterned layer where the contact angle (CA) measured greater than 150° consistently. Other than the layer and tool path pattern, user defined grid pattern also showed super hydrophobicity after dip coating. The micro- and nanoscale hierarchical surface structures caused the superhydrophobicity. Ahmed Aldhaleai et al. reported a facile method for creating superhydrophobic surfaces by deposition of candle-soot on 3D printed microstructures [75]. He deposited PDMS-soot on a glass substrate by heating it at 350°, and a transparent superhydrophobic surface with water contact angle 169.7° was obtained. By printing microstructures and depositing candle-soot on it, they could achieve super hydrophobicity with CA of 159.4°. Even though the method is simple and cost-effective, the mechanical durability of the surfaces is poor. Changyou Yan et al. harnessed the inherent superhydrophilicity and underwater superoleophobicity of hydrogel to fabricate oil/water separation meshes through an integration of 3D printing and hydrogel-coated modification [76]. Specifically, Fe/PLA composites being fed into extruder and then utilized a 3D printer employing fused deposition modeling (FDM) to create orthogonal meshes through printing. These printed meshes were immersed in a solution of acrylic acid and acrylamide, initiating a process of hydrogel coating, bonding, and growth on the mesh surface through a redox reaction facilitated by Fe(II). Subsequent immersion in an inorganic salt solution facilitated the incorporation of the salt into the hydrogel coating, enhancing the de-emulsification properties for emulsions with oil dispersed in water. The mesh coated with hydrogel displayed remarkable super-hydrophilic and underwater super-oleophobic properties, demonstrating an underwater oil contact angle exceeding 150 °C and demonstrated low adhesion forces. By incorporating salts, the resulting super-hydrophilic and underwater super-oleophobic mesh (S-USM) functioned as a selective separation membrane, enabling water to pass through while repulsing oil droplets. To assess its separation efficiency, the S-USM was tested against four types of oil mixtures using simple homemade equipment. The experiments revealed that the S-USM achieved oil/water mixture separation with an efficiency reaching up to 85%. A method to improve the mechanical behavior of hydrogels for bone tissue engineering has been reported. They investigated the effects of infill percentage and strand diameter on the polycaprolactone/Chitosan/HA scaffolds 3D printed by LDM [77]. The hydrophilicity of the scaffolds was improved by employing chitosan. The research conducted further on this led to the addition of NaCl as a porogen for improving the porosity and mechanical properties of the scaffolds [78]. The impact of primary 3D printing parameters Infill Rate (IR), Printing Temperature (PT), Flow Rate (FR), Printing Acceleration (PA), and Printing Speed (PS) on the self-cleaning behavior of TPU (thermoplastic polyurethane) has been reported [79]. The results indicate that all the printing parameters have an almost equal influence on the fabric's wettability. Additionally, a validated and precise mathematical model for each cleaning attribute was established using RSM. This model could aid in advancing the application of FDM self-cleaning fabrics in the textile industry [79].

Fig. 13

Schematic image of sample preparation. (Inspired from [74])

One critical disadvantage additive manufacturing technology finds in the FDM process is the textured surface of printed components arises from the stepwise layering inherent in the printing process. But Beomchan Kang et al. reported that this can be used to make some hydrophobic surfaces [80]. He made 3D printed molds by an FDM printer with the material PLA and used these molds to cast PDMS hydrophobic surfaces. An array of waveforms was created on the PLA prints by varying the angle of printing. The tilting angle displayed a value nearly identical to that of the printing angle. The surfaces which were printed with an angle larger than 40° showed super hydrophobicity (Fig. 14).

Fig. 14

WCA values on each PDMS polymer surfaces cast from the 3D-printed PLA molds depending on the printing angle. It should be noted that WCA on surfaces with the printing angles larger than 40° exceeds 150° (Inspired from [80])

The same team reported the relation between four printing resolution and the hydrophobicity [81]. They printed inverted pyramidal shape having an area at the bottom of 10 mm × 10 mm with PDMS material using FDM printer. The spacing between consecutive stacked layers decreased when they varied the printing resolution as low, standard, high and hyper. The water contact angle (WCA) rose from 105° on the smooth PDMS surface to 143° on the surface derived from casting a 3D printed mold with low resolution. Furthermore, the PDMS material mold cast from the 3D prints, which was created using a printing angle of 90°, exhibited a water contact angle (WCA) of 160°. Consequently, they could create a superhydrophobic surface using a hydrophobic material cast from a 3D printed mold in comparison to other complex chemical processes like etching. Another work on PDMS with FDM were reported by Ahmed Aldhaleai et al. is a two-step process to create transparent superhydrophobic surface on glass and superhydrophobic surfaces on 3D-printed microstructures [75]. The authors present two methods: one involves heating polydimethylsiloxane (PDMS) solution at a 350 °C for 4 h and the PDMS soot is depositing on a glass substrate, resulting in a transparent, superhydrophobic surface without the need for additional chemical treatments. Another approach involves a combination of 3D printing and hydrophobic coating to create superhydrophobic surfaces. The glass surface treated with PDMS-soot exhibited a static water contact angle approximately around θ = 170° ± 2°. After that they coated the 3d printed micro structured surfaces with candle soot and octadecyltrichlorosilane solution respectively, and they could achieve static contact angles of 159.4° and 147.8° respectively.

The main attraction of material extrusion techniques is the affordability—FDM printers are relatively inexpensive compared to printers used for other 3D printing techniques. This makes it easier for hobbyists and small businesses to afford and use the technology. Another advantage is versatility, as it can use a variety of materials including ABS, PLA, nylon, and PETG, which offer different properties such as strength, flexibility and durability. But they have relatively lower resolution compared to other vat photopolymerization and powder bed fusion techniques (like SLS and DLP), resulting in lower quality prints. The layer lines may also be visible on the finished print. Moreover, it may have weaknesses along these layer lines which can affect their strength and durability. This can be mitigated by adjusting the printing settings, but it is another limitation of the technology.

4.4 Material jetting

Material Jetting is yet another additive manufacturing technology which works on the same principle as an inkjet printer. Here the liquid photoreactive material deposit on a built platform from a print head layer by layer. Like stereolithographic technique, the liquid solidifies using UV light. The printing technique is particularly responsive to the chemical properties of the substrate employed to uphold the structures. There are some additional parameters which affect the printing like the velocity and height at which the drops fall. One benefit of employing the material jetting 3D printing method is that we can print with more than one material, if the printer has multiple print heads [82].

The different textures of cylindrical protrusions and pyramidical micropattern were created using Multijet 3D printing to get superhydrophobic surfaces (Fig. 15) [83]. But directly after the printing, they could make a hydrophobic object with water contact angle ~ 148°. Hence after printing they dip-coated it with SiO₂ nanoparticles and Hexafor 644-D, leading to an elevation in the contact angle to 160°. The dipping with silica nanoparticles could achieve hierarchical structures which made the surface rougher.

Fig. 15

a Diagram illustrating Multi Jet 3D Printing b Cylindrical patterns and c Pyramid patterns. (Inspired from [83])

Superhydrophobic objects having helical structures and porous membranes were fabricated with standard resins using a commercially available MultiJet printer. A viscous Room Temperature Vulcanizing (RTV) silicone and an UV curable silicone used for making the posts. To make the mixture less viscous fumed silica was dispersed into it. They made pyramidal posts with pitches of 450–700 µm, and all of them showed super hydrophobicity. They subsequently proved that the direction of the dispensing tip motion control the shape of posts [84]. Using an ink having cellulose acetate, poly (viny1alcohol) (PVA), and SiO₂ nano-particles, super hydrophilic and underwater superoleophobic membranes were fabricated by direct inkjet writing technique [85]. They could achieve a contact angle of 159.14° ± 0.59° for oil underwater and a low contact angle of about 18.14° for water in air. The membrane could be utilized for oil–water separation with an efficiency of 99%. Coating of CA/PVA/Si on steel meshes could achieve a membrane with high mechanical durability and stability.

Material jetting can produce very high-resolution prints with fine details and smooth surfaces. This makes it ideal for creating intricate designs or small parts. One of the big advantages that material jetting has is the ability to use multiple materials to fabricate a single object. It can use multiple materials to create a single object. This allows for the fabrication of prototypes with different properties or colors. The ability to produce objects with high accuracy and repeatability, making it suitable for applications where precision is critical, can also be achieved. Coming to the disadvantage—material jetting has a small built volume compared to other 3D printing techniques. This restricts the size of products that can be printed. Moreover, it is a relatively expensive 3D printing technique, which may make it hard for small businesses or individuals to afford. Furthermore, it often requires post-processing steps, like cleaning or curing the printed objects, which may consume a significant amount of time and add to the overall expenses [36, 86, 87].

5 Environmental and practical implications

In investigating the environmental impacts of AM processes, energy efficiency is crucial, with electrical energy being the primary environmental concern in additive manufacturing. AM processes are less energy-efficient than injection molding in mass production, since AM consumes more electricity [88]. Several factors affect the energy consumption in AM processes: material used, as different materials require different amounts of energy due to their densities and heat capacities; build volume, with printers capable of parallel manufacturing being more energy-efficient; layer thickness, where lower thickness increases energy consumption due to more layers and slower speeds; and process speed, with faster printing reducing overall energy consumption. In different AM processes, various factors should consider while comparing energy consumption. In powder bed fusion, powder density has a major effect on energy consumption. So, the sustainability of AM depends on different parameters, and a general statement on energy consumption cannot be made. Utilizing an LCA (life cycle assessments) database is mandatory for further research.

In AM techniques, waste management involves using recycled materials and recycling waste generated during printing, both of which can substantially reduce environmental impacts. Various polymers like LDPE, HDPE, polypropylene, PLA, and ABS can be recycled into filaments, saving millions of dollars annually [89].

While self-cleaning surfaces hold great promise, their effectiveness can diminish over time in real-world environments [90]. Unlike traditional methods like sol-gel processes and chemical vapor deposition (CVD) that often rely on surface coatings, AM can directly fabricate micro- and nano-structures that are inherently part of the material. This approach can potentially improve durability as the self-cleaning features are not simply a surface layer susceptible to wear. However, studies have shown that even AM-fabricated micro/nano-structures can be compromised by external factors. Similarly, chemical exposure and UV radiation can degrade the surface properties or the material itself [90]. Durability data is often limited, and further research is necessary to understand how these surfaces maintain self-cleaning properties over extended periods under real-world conditions, especially considering the combined effects of multiple environmental factors.

6 Conclusion

6.1 Current progress

In this review, we presented current works on the realization of self-cleaning surfaces by additive manufacturing techniques. Several ways to achieve self-cleaning surfaces alongwith different wetting models have been discussed. Different AM technologies like SLA, DLP, 2PP, FDM, LDM, SLS/M, and MJ have discussed and their advantages, limitations, applications in various fields and unique features pertinent to self-cleaning surfaces are summarized in Table 2. With the help of these 3D printing technologies researchers have created self-cleaning surfaces mimicking the self-cleaning surfaces fabricated by mother nature through billions of years of evolution. A range of micro-structures like pillar, pyramid, re-entrant, fractal and eggbeater configurations can be produced using various printing methods with high resolution. Applying superhydrophobic coatings to printed objects or printing with inherently superhydrophobic materials achieves anti-wetting properties, with the latter offering notable stability and resistance. Also, development of self-cleaning surfaces by the fabrication of superhydrophilic and photocatalytic surfaces have also been discussed. The fabricated objects have applications in liquid manipulation, oil/water separation, drag reduction, and creation of anti-icing surface.

Table 2 Summary of the discussed manufacturing techniques, along with their advantages, drawbacks, typical applications, and unique features pertinent to self-cleaning surfaces

6.2 Challenges and outlook

Creating the hierarchic roughness (sequentially combining micro- and nano-patterns) with higher resolution in the submicron level/nanometer-scale still remains as a challenge for the 3D printing techniques available today. When the resolution increases, the cost of the printer also increases (i.e., 2PP printers). Also fabricating superhydrophobic surfaces on a big scale is restricted as most of the high-resolution printers have building plates of limited size. Mixing low surface energy materials within the 3D printing process can help in creating superhydrophobic self-cleaning surfaces. Also functional nanoparticles can be incorporated in the bulk material to create nanoscale roughness and develop multi-functional self-cleaning smart surfaces.

Even though there are variety of materials for different printing techniques, most of them do not possess low surface energy, thus it demands an extra coating on 3D printed structures to develop super hydrophobic self-cleaning surfaces. However, these coatings have a high chance of degradation by abrasion, and the hydrophilic nature of substrate will be revealed eventually Moreover, in many cases these coatings are not uniform and might not conformally coat the nanoscale protrusions.

A key development is the printing of bulk materials with inherent super hydrophobicity, facilitated by systems like photopolymerization-induced microphase separation (PIMS) using reversible addition-fragmentation chain transfer polymerization. This approach offers tunable structures and mechanical stability. Moreover, the concept of 4D printing introduces dynamic, stimuli-responsive superhydrophobic materials, with applications from shape-memory materials to adaptable designs. As 3D printing scales up and customization advances through techniques like selective laser sintering/melting (SLS/M), the practical deployment of these materials becomes viable.

Researchers are keen to find the new applications of 3D printing and to see the wonders it can make in various fields of science and technology. To take advantage of the novel and powerful 3D printing techniques in the field of developing self-cleaning surfaces needs more developments and inventions. It is a timely demand that we (at least partially) solve both the problems of rapid urbanization and water scarcity by developing and deploying self-cleaning surfaces in a scalable, cost-effective and environmentally friendly manner.