In recent years, additive manufacturing techniques (AM), also known as three-dimensional printing (3DP), have been intensively researched and are increasingly finding industrial applications as part of Industry 4.0 initiatives [1]. These technologies allow designing and fabricating objects with specific shapes and sizes that would be difficult, and sometimes impossible, with traditional manufacturing techniques. However, the range of available materials, resins, and inks available for AM is limited, thus hindering the growth of the field. The development of formulations tailored specifically for 3DP that can add advanced functionality to the parts fabricated is a promising avenue. In this regard, polymeric macromolecules have been widely employed for printing smart materials, including materials that respond to external stimuli from the environment in a reproducible, reliable, specific, and useful way [2], which is known as 4D printing ^[3][4][5][6][3,4,5,6]. For instance, hydrogels are being evaluated as potential materials, since they are excellent candidates for biomedical applications given their biocompatibility ^[7][8][9][7,8,9]. Furthermore, emerging polymers with greatly varied chemical structures and properties have been developed for AM, including covalent organic frameworks (COFs) [10], polypeptides [11], ionogels ^[12][13][14][12,13,14], and many more [15]. Recently, polymeric ionic liquids (PILs) have gained prominence as intelligent materials [16] capable of responding to temperature [17], light [18], solvents [19], pH [20], and CO₂ [21]. Ionic liquids (ILs) and polymeric analogues are a fascinating field of research, due to the possibility of controlling their macroscopic properties by changing the formulation (cation/anion) ^[22][23][22,23]. These polymers can be specially designed to be printed by 3D techniques allowing the design and manufacture of the high-resolution multi-material devices essential for the development of future technologies [24].

This research emphasizes the state of the art of 3D-printable PIL-based materials and their most common applications, as found in the literature. The countless cation–anion combinations (it is theoretically feasible to generate 10¹⁸ distinct ILs) possess unique physicochemical properties that can be tuned easily, without sacrificing 3D printability and resolution ^[25][26][25,26]. Moreover, PILs are ideal matrices to stabilize molecular and nanostructured materials, due to their ionic and supramolecular interactions. These polymeric compounds can encapsulate smart molecules, directly translating their advanced properties to the macro scale through additive manufacturing [27]. The development of printable PILs is an expanding area, although it is still in its infancy.

The recent growth of AM techniques is attributed to several advantages, including the rapid translation of fundamental molecular properties, fabrication of complex geometries with high precision and resolution, unprecedent flexibility in design, waste minimization, and personal customization, which enables the fine control of structural features ^[28][29]. Furthermore, it enables distributed production, as opposed to the logistically complex production lines with conventional production methods. Whereas traditional fabrication techniques have limitations in controlling the geometry and architecture of macroscopic components, without compromising the performance, 3DP techniques are an efficient strategy to engineer well-controlled functional materials across scales (from nano to macroscale) with accurate control over the geometry (dimension, porosity, and morphology) and structure. Moreover, 3DP expands the boundaries of materials science and provides an exciting opportunity for interdisciplinary research, as it allows the incorporation of multiple nano-materials in the same printing process, to obtain multi-functional devices.

The main feature of AM is to design an object by adding material in a layer-by-layer method until the model is completed. The first step is to generate data from a three-dimensional computer-aided design (3D CAD) software, in a simple machine avoiding complicate production chains or process planning. Then, the digital CAD model is translated into computer readable format as an STL file (an industry-standard stereo-lithography format), which slices cross-sections into codes that allow the machine to build the object before printing. This is a fast, reliable, and facile way to design prototypes and devices, and additionally, this 3D CAD file can be easily shared and modified, with large online databases of designs shared under creative commons licenses ^[29][30].

Additive manufacturing is a booming area. According to the Smithers Group—a provider of strategic market research reports—billions of dollars are invested in the 3DP market and annual growth is expected until 2027, potentially reaching $55.8 billion. Nevertheless, in 2021, the AM industry growth was not as high as expected (growing only 7.5%, which was quite low compared to the average growth of 27.4% observed over the previous 10 years), due to the pandemic situation that affected all countries ^[30][31].

Additionally, quantitative life cycle analysis (LCA) data shows that the process of AM reduces environmental impacts compared to other existing manufacturing technologies. Indeed, the additive nature of these technique minimizes the amount of material employed, and reduces the energy required and the resources demanded. Furthermore, it is estimated that the implementation of printing processes might lower carbon dioxide emission intensities by about 5% by 2025 ^[31][32].

The first rapid prototyping system, “stereolithography apparatus” (SLA), was patented by Charles Hull in 1986 ^[32][33]. Since then, many different types of AM techniques have been developed and commercialized. Depending on the form of the starting material and the way the materials are deposited and assembled in the additive fashion, 3DP techniques can be divided into seven categories, opening new pathways for creating devices with outstanding performance at fine resolutions ^[33][34]. These techniques cover a broad range of scales, from micro- to macroscale devices, and materials (Table 1). All of them have been widely studied in various literature reviews, in which the materials, benefits, drawbacks, and main application areas of each printing method were specified ^{[34][35][36][37]}[35,36,37,38]. According to the technique employed, the parts may be post-processed to improve their strength or surface quality. This work mainly focuses on vat polymerization, as further explained above.

Over recent years, different methods such as Inkjet ^[39][40] and FFF ^[40][41] have been employed for 3D manufacturing of PIL materials; however, vat polymerization has become the preferred technique for printing PILs. Both the DLP ^[41][42][42,43] and SLA ^[41][42][42,43] methods have been widely used in this area.

3. Polymeric Ionic Liquids

3.1. Properties of Ionic Liquids

ILs are organic molten salts in which the ions are poorly coordinated and the salts melt below 100 °C, or even at room temperature. The first IL, ethanol ammonium nitrate, was reported in 1888 ^[43][82], but it was few decades ago when the vast potential of these compounds was noticed. The renewed interest in them was due to their unique characteristics: chemical and thermal stability, high viscosity, wide electrochemical windows, low vapor pressure, specific solvating ability, good tunable solubility, wide solid to liquid range, and high ionic conductivity. The number of publications related to ILs has been increasing sharply every year for the last two decades, indicating the relevance of these materials ^[44][45][46][83,84,85]. Initially, these materials were mainly used as solvents for organic synthesis and catalysis, since they were more desirable than conventional volatile solvents ^[47][48][49][86,87,88]. Indeed, due to their low flammability, negligible vapor pressure, low melting point, and non-volatility properties, ILs were considered “green solvents” ^[50][51][89,90], although some concerns about their toxicity have appeared ^[52][53][54][91,92,93]. Nevertheless, these ionic compounds can contribute to making chemical production and usage more sustainable ^[55][94]. Furthermore, they can create a biphasic system, having the catalyst solubilized, while the products are insoluble in the IL ^[56][57][95,96], a feature that has also been widely used in analytical chemistry for separation procedures ^[58][97]. Progressively, the uses of ILs have broadened, being used in CO₂ capture and separation systems ^[59][98], in electrocatalysis ^{[60][61][62][63]}[99,100,101,102], in pharmaceutical industries ^[64][103], and even in space technology ^[65][104]. In particular, these chemicals have shown a lot of promise in the development of green catalytic technologies ^[66][105]. ILs can play the role of solvent in catalytic reactions, since they are able to dissolve a wide range of organometallic and inorganic compounds. Furthermore, the ionic behavior of these materials can stabilize the catalytic species and intermediates ^[67][68][106,107]. Moreover, the ILs themselves have been used as the catalyst in a variety of reactions, from biodiesel production, to CO₂ conversion ^[69][70][71][108,109,110]. In some studies, ILs acted as both a solvent and a catalyst simultaneously ^[72][111]. Likewise, they have been extensively used in electrochemical applications as their charge density is much higher than that of a traditional salt solution ^[73][74][112,113]. Electrolytes based on ILs offer several benefits over aqueous and organic electrolytes, making them attractive for use in flexible and stretchable energy storage systems ^{[75][76][77][78][79]}[114,115,116,117,118], high temperature supercapacitors ^[80][81][82][119,120,121], lithium-ion batteries ^[83][84][122,123], and electrochemical CO₂ reductions ^[85][86][124,125]. Doping IL into polymers is another process that has recently been investigated, yielding significant changes in both conductivity and overall mechanical properties ^[87][88][126,127]. Pure polymer electrolytes generally have a limited ionic conductivity, thus adding an ionic moisture can be very beneficial ^[89][90][128,129]. The addition of ILs can benefit, not only neutral polymers, but also conductive polymers ^[91][92][130,131]. Despite ILs being very popular in both the academic community and industry, due to their well-known advantages ^[93][132], some downsides have been highlighted. The main problematics in using ILs are related to processing difficulties. For example, in catalysis or as a solvent the employment of ionic moisture can complicate the purification procedure of the product, and it is almost impossible to recycle them. Moreover, ILs cannot be used in fixed bed reactors, which, along with certain toxicity concerns, are critical challenges to overcome for future scale-up. Additionally, these compounds are liquids, which means that they can liquify and leach over time. An initial approach to overcoming these disadvantages was to use supported ionic liquid phases (SILPs) prepared by immobilizing the ILs onto solid supports ^[94][95][96][133,134,135]. In many cases the performance was improved, since the properties of ILs were transferred to the solid support, and simultaneously working with a solid material offered additional benefits ^[96][97][98][135,136,137]. Further methods have been developed to prepare IL-based polymers such as in situ polymerization of neutral vinyl monomers with non-polymerizable ILs or directly polymerizing an ionic monomer of IL structure ^{[99][100][101][102]}[138,139,140,141]. A variety of polymeric IL systems, including polycation-type ILs ^[102][141], polyanion-type ILs ^[103][142], copolymer ^[100][139], and poly(zwitterion) ^[102][141], have been reported, as summarized in Figure 1.

3.2. Properties of Polymeric Ionic Liquids

Polymeric ionic liquids/poly(ionic liquids) (PILs) are the union of multiple anionic or cationic monomers, forming a polymer backbone with its counterion species bonded to it. These high-molecular-mass compounds combine attractive IL properties with polymer properties (i.e., durability, low toxicity, and easy polymer processing). In other words, during the development of PILs, it is possible to preserve the specific features of ILs and improve other ones by taking advantage of the polymer’s nature, including mechanical robustness, thermal stability, hydrophilicity, rapid manipulation, optimum assembly, and stabilizing properties ^[104][143]. Another unique feature of PILs is that the properties of the polymers can be controlled by changing the corresponding ions. Depending on the ion pairs it is possible to vary the polymer’s solubility, fusibility, hydrophilicity, ionic conductivity, molecular mass, glass transition temperature, heat resistance, and thermal stability in a wide range. Most PILs are the nitrogen-based polycationic type, namely imidazolium, pyridine, pyridinium, quaternary ammonium, and cholinium; although phosphorous-containing cationic PILs are gaining popularity (Figure 2a). While ILs can only undergo structural variation through ion metathesis or cation/anion exchange, PILs can be modified by selecting cations, anions, side chains, and functionalities. Even when using the same ionic species and the same number of monomers, different PILs can be built by only changing the architecture: linear, hyperbranched, or star-shaped polymers; obtaining an unlimited library of species (Figure 2b) ^[105][144]. Homopolymerization is an important route to prepare PILs; however, other types of polymers can be added covalently to the PIL to form PIL block copolymers. In an initial approach, PILs were considered for improving the properties of ILs. The tailorable physicochemical properties of PILs have provided the basis for the further design of smart materials, implementing this new class of polyelectrolytes in fields from material science and chemistry to medicine; with applications related to energy and environment, analytical chemistry, electrochemistry, biotechnology, bioelectronics, catalysis, and surface science, among others. Thus, these multifunctional polymers have been recognized as a separate class from ILs, and extensive work has been done in this direction ^{[105][106][107]}[144,145,146]. Solid electrolytes are very desirable in many electrochemical devices, in order to avoid leakage issues and, additionally, they can improve certain electrochemical properties, such as energy density, rate capability, and cycling stability. In this regard, PILs are often less fragile and less hygroscopic than traditional polyelectrolytes. Moreover, electrochromic devices and actuators using PIL-based electrolytes have a better performance, cyclability, speed, and long-term stability ^[22][108][22,147]. The ionic polymers are both permanent and strong polyelectrolytes owing to the IL species’ solvent-independent ionization state. The unique dielectric properties of PILs makes them potentially useful, not only as polymer electrolytes, but also as sorbents and dispersing agents ^[109][110][148,149]. Several groups have investigated PIL-based electrolyte membranes for use in alkaline fuel-cell operations, where the anion exchange membrane acts as an electrolyte to transport anions from the cathode to anode and is one of the key components of alkaline fuel cells ^{[111][112][113]}[150,151,152]. The use of PILs as binders for electrodes and/or electrolytes in the design of solid-state supercapacitors is also remarkable ^[114][115][153,154]. The addition of PILs into graphene electrodes, for instance, improves the contact between the conductive carbon and the liquid electrolyte, facilitating the diffusion of ions into the electrode and increasing the specific capacity of the supercapacitor by 130% ^[116][155]. Moreover, the addition of PIL improved the cyclability of a supercapacitor, keeping the specific capacitance almost unaltered after multiple cycles. In addition, PILs are ideal materials to stabilize and solubilize molecular and nanostructured materials, as a result of their ionic and supramolecular interactions; hence, these polymeric compounds are extensively used to stabilize smart materials ^[117][118][156,157]. In the fabrication of composite materials using light-based 3DP techniques, it is crucial to avoid phase separations, to avoid the presence of turbidity that will undesirably scatter light and thus reduce the printed resolution. Generally speaking, ionic polymers are able to homogeneously distribute a broad range of additives in their matrices, thus minimizing these problems. For example, some reports demonstrated the excellent capabilities of PILs to immobilize and stabilize metal nanoparticles, and acting as active catalysts in heterogeneous catalytic reactions ^{[119][120][121]}[158,159,160]. The nanoparticles can even be synthesized and immobilized simultaneously, forming the nanoparticles in situ during the polymerization process ^[122][161]. This approach is not limited to metallic nanoparticles, as a broad range of nanoparticles ^{[123][124][125][126]}[162,163,164,165], macrostructures such as polymers ^{[127][128][129][130]}[166,167,168,169], carbon-based materials ^[131][132][170,171], and enzymes ^{[133][134][135]}[172,173,174] have been successfully stabilized or immobilized by PILs. A wide variety of functional materials, including porosity-switchable membranes and coatings, ^[136][175] thermally responsive compounds ^[137][176], biosensors ^[138][177], flexible sensors ^[139][178], and reversible systems that switch hydrophobicity/hydrophilicity by exchanging the anion ^[136][175] and the cation ^[140][179], have been explored. Nevertheless, it is worth remembering that PILs have one of their charges polymerized, becoming a single-ion conductor. Therefore, the conductivity of neat PILs described in the literature up to now has usually been low and is still insufficient for practical use, due to the reduction of the ion mobility. Another factor that has a direct influence on conductivity is the increase of the glass transition temperature after polymerizing. Several times, it has been found that the ionic conductivity values of PILs are at least two magnitudes lower than the corresponding IL monomer unit ^[96][135]. Accordingly, the conductivity of PILs needs to be improved by doping with other materials such as ILs, metallic nanoparticles, conductive polymers, AMMs, etc.; taking advantage of the good stabilization properties of PILs. The preparation of mixtures of ILs with PILs has been explored for different applications ^{[114][115][141][142][143]}[153,154,180,181,182]. In this context, different ILs and PILs have been combined successfully without observing segregation into two phases, due to the similarity in their chemical structure. Completely compatible formulations have been used, for instance, as polymer electrolytes, enhancing their performance and conductivity compared to conventional polyelectrolytes ^[144][145][183,184], improving the device lifetimes ^[146][185], the solar cell efficiency ^[147][186], and the electromechanical performance of the actuators ^[148][187]. By mixing IL and PIL in different ratios, the electrolyte can be tailor-made. The ionic conductivity varies significantly (from 10⁻⁵ S/cm to 10⁻² S/cm) after increasing the IL content. In addition, the physical appearance of the polymer electrolytes changes from a transparent solid film (100% PIL) to a very viscous and sticky gel (low PIL/IL ratio), with intermediate states, where the polymer electrolytes are usually rubbery and transparent ^[22][146][22,185]. Another matter to be considered when using PILs and ILs is that the properties of these materials can be varied greatly just by changing the ions. This is helpful for designing customized PILs. However, in order to meet specific requirements, it is necessary to have a clear understanding of how chemical differences affect the final features. Depending on the chosen PIL, important properties such as the thermal stability can vary from 150 to >400 °C ^[22][149][22,188], measured as the onset of thermal degradation by TGA, and its electrochemical window from 2.5 to 5.0 V ^[150][189]. Solubility provides clear evidence of the variation across PILs. For instance, poly(1-vinyl-3-alkyl-imidazolium) with bromide anions is soluble in water as a conventional polyelectrolyte and, after substituting the halide anion with tetrafluoroborate or hexafluorophosphate, the polymer becomes soluble in methanol and polar aprotic solvents, but no longer in water. Then, changing the counter-anion for a fluorinated bis(trifluoromethanesulfonamide) anion or an hydrophobic anion, the polymer become soluble in non-polar solvents [23]. It is therefore essential to know all the characteristics of each PIL or IL before using them. In conclusion, to obtain the best performance from PILs, it is imperative to combine PILs with advanced materials, to produce smart composites. The ionic nature of PILs ensures their outstanding compatibility with other compounds, since the active ions across the polymer chains can promote interactions with other reactives ^[117][156]. The hybrid materials obtained after those unions are notable for having an enhanced performance, such as increased electrical conductivity, thermal stability, antimicrobial activity, and optical properties ^[116][155]. Moreover, PILs can be manufactured using 3D methods, which makes them promising candidates for developing next-generation devices. For all these reasons, 3D printable PIL nanocomposites are an exciting and promising research field.

4. Applications of 3D-Printable PILs

At present, the development of polymers especially designed to be printed using 3D techniques is of huge interest. At the same time, in both academic research and industry, there is a need to create polymeric systems with controlled precision in their architecture, domain size, functionality, polarity, solubility, and reactivity. In this context, poly(ionic liquids) combine the tunable properties of ILs with the intrinsic features of polymers, becoming an invaluable material for additive manufacturing. However, the employment of polymerizable ILs as inks for AM techniques is an underexploited area of research. In 2014, the first study using printable ionic polymers was reported. 4-vinylbenzyl trioctyl phosphonium bis(trifluoromethanesulfonate)imide mixed with diacrylates was photopolymerized by mask projection micro-stereolithography with a low UV light intensity and high digital resolution. The IL monomers were polymerized during the printing process together with other comonomers, in the presence of a cross-linker and a photo-initiator. The resulting electro-active membranes showed high thermal stability, optical clarity, and ion-conductivity ^[41][42]. In that example, the IL monomers were viscous, hence the formulation was not appropriate for all printing techniques. A totally different strategy was proposed for an inkjet method, generating a variety of low viscosity printable polycationic materials with finely tuned mechanical, thermal, and superficial properties. The post-polymerization and modification of the polymer allowed improving the mechanical properties of the printed parts ^[39][40]. On the other hand, an important trend in the development of new materials is towards being lightweight, ultrathin, and flexible; the combination of different materials to obtain superior properties in multifunctional devices being necessary. Some required features, including higher stability, durability, catalytic activity, and a lower cost, are crucial for new types of materials used in medical, electronic, and environmental fields. In this context, smart materials such as nanomaterials and carbon-based materials are promising alternatives to meet these requirements, since they can improve the mechanical and electrical properties of polymer matrices. Over the past 20 years, a tremendous expansion in the development of nanostructured composite systems has emerged and been employed in a wide range of nanomaterial applications ^[151][190].