Photopolymerization-based additive manufacturing of ceramics: A systematic review

Conversion of inorganic-organic frameworks (ceramic precursors and ceramic-polymer mixtures) into solid mass ceramic structures based on photopolymerization process is currently receiving plentiful attention in the field of additive manufacturing (3D printing). Various techniques (e.g., stereolithography, digital light processing, and two-photon polymerization) that are compatible with this strategy have so far been widely investigated. This is due to their cost-viability, flexibility, and ability to design and manufacture complex geometric structures. Different platforms related to these techniques have been developed too, in order to meet up with modern technology demand. Most relevant to this review are the challenges faced by the researchers in using these 3D printing techniques for the fabrication of ceramic structures. These challenges often range from shape shrinkage, mass loss, poor densification, cracking, weak mechanical performance to undesirable surface roughness of the final ceramic structures. This is due to the brittle nature of ceramic materials. Based on the summary and discussion on the current progress of material-technique correlation available, here we show the significance of material composition and printing processes in addressing these challenges. The use of appropriate solid loading, solvent, and preceramic polymers in forming slurries is suggested as steps in the right direction. Techniques are indicated as another factor playing vital roles and their selection and development are suggested as plausible ways to remove these barriers.


Introduction 
Ceramics are the materials with appreciable refractory properties [1]. This has made them relevant for a wide developed to date. Examples are tape casting, dry pressing, casting, and injection molding [4,5]. Despite the effectiveness of those approaches, the use of molds generally inflates the cost of production. More importantly, when it comes to the production of highly complex structures, limitation arises with the use of conventional methods.
Based on the above, research attention has been widely shifted to the use of additive manufacturing (AM) techniques for the fabrication of ceramics. This is due to fact that AM offers (a) highly flexible design of complex structures, (b) integral manufacturing of complex components, (c) material waste minimization, (d) cost-effective production, and (e) short product lead time [6]. It is worth noting that there are different types of AM techniques depending on their energy sources and material requirements [6]. However, from the operation principle and material requirement perspectives, AM techniques can be generally classified as extrusion and jetting-based, photopolymerization-based, and powder sintering/melting-based techniques. They offer different advantages and benefits as discussed elsewhere [6]. For example, selective laser sintering (SLS) has been recently demonstrated for printing Al 2 O 3 ceramics with poly-hollow microspheres [7,8]. It was claimed that the technique offers structures with porosity of about 72.41%, making them suitable for catalytic and bio-engineering applications.
Nonetheless, photopolymerization-based additive manufacturing (photopolymerization-based AM) techniques are highly versatile in equipment setup and material requirements, and are able to produce parts with the highest resolution [9]. They are extensively studied and also considered the most promising AM technology in ceramic fabrication and hence become the focus of this review. In this review, a number of major photopolymerization-based AM techniques used for ceramic production are discussed and the relevant working principles, material, and technical requirements are compared in detail. Key challenges and problems associated are also summarized with possible solutions raised.
Photopolymerization-based AM is a series of techniques in which application of light (mostly in ultraviolet range) is used to trigger photochemical reaction and shaping [10]. Among the various types of AM techniques, photopolymerization-based AM is the first AM technique invented dated back to the 1980s, which was originally used for rapid prototyping of 3D polymeric components [11]. Only since the 1990s, the techniques then were applied in the photopolymerization of ceramicpolymer suspension mixtures into ceramic structures [6]. Examples of the photopolymerization-based AM techniques are stereolithography (SL), digital light processing (DLP), two-photon polymerization (TPP), liquid crystal display (LCD) printing, continuous liquid interface production (CLIP), multi-jet printing (MJP), holographic 3D printing technology, and their derivatives [6,[12][13][14]. These approaches normally offer structure with high feature resolution and surface quality. The photopolymerization-based AM-derived components are often found superior to that of other kinds of AM techniques (e.g., fused deposition modeling (FDM), direct ink writing (DIW), inkjet printing (IJP), selective laser sintering/melting (SLS/SLM), laser engineered net shaping (LENS) in term of structure engineering [15][16][17][18][19][20][21]. This is due to the ability of the photopolymerization-based AM techniques to produce materials with high resolution and complex structures.
From the material processing perspective, photopolymerization-based AM techniques are mainly used to process slurries containing photoinitiator and ceramicpolymer matrix as feedstock [6]. The efficiency of most of these approaches (i.e., SL, TPP, DLP, LCD, CLIP, MJP) primarily relies on the photosensitivity of the light-curable polymer matrix and other parameters related to the light intensity of the techniques [22]. Nonetheless, it is worth noting that the printing process plays a vital role too. In general, printing process and material precursor properties (e.g., particle size) may be compromised, and hence enhances the performance of sintered samples [23]. The strategy may aid high mass loading of solid content to avoid mass loss of finished ceramic parts too [24,25].
Despite the advances that have been made by compromising printing parameters, only about 50 vol% of the ceramic solid content in the slurries is normally found suitable for incorporating the stated properties (e.g., pore reduction, high density, and low shrinkage) into the finished parts [26,27]. In fact, high solid loading (≥ 50 vol%) can decrease flow ability, increase viscosity, cause inhomogeneity, and hence reduce suitability of the slurry for efficient polymerization [10,28]. It may hinder workability on the slurries, and hence reduce the fineness of the final structure. Mismatch between the material (slurry) refractive index and light from the 3D printing techniques is another issue normally causing setbacks. This is because that phenomenon (irradiation-material refractive index mismatch) has tendency to reduce curing depth and cause particle coarsening [29]. It may in turn result in the incident light scattering and hence give rise to structure with poor resolution, severe cracks, low mechanical performance, and surface roughness [29]. These are some of the challenges currently faced by researchers in this research area. To address these, for example, Ding et al. [30,31] managed to prepare gray-colored SiC ceramic slurries suitable for DLP 3D printing by increasing the exposure energy.
Based on the above, several advances have been recently made in order to fabricate parts with complex structures and superior mechanical performance, among which, the use of appropriate material-technique correlation has so far shown interesting results in tackling those eminent setbacks associated with the use of photopolymerization-based AM techniques for the fabrication of ceramics [10,[32][33][34][35]. The approach has in fact resulted in the fabrication of advanced ceramic structures ( Fig. 1), and also led to surge increase in article publication in this research area. Several interesting review articles have been published too, in which working principles, advantages, and usefulness of various photopolymerization-based AM techniques are indicated [6,[36][37][38][39]. However, among those review articles, only few unraveled the significance of materialprinting process correlation for achieving defect-free structures with complex geometries and superior mechanical properties.
From literature survey, one may note that printing process and precursor property engineering are one of the current research ideas which have shown appreciable results in using photopolymerization-based AM tech-niques for the fabrication of ceramics. Therefore, a timely review of the recent advances made in the field of photopolymerization-based AM is important for further development in this research area, and hence aids the researchers' effort. Based on the above, in this review, the profound understanding of the photopolymerization-based AM techniques is provided. The relevance of these techniques for the fabrication of ceramics is discussed. The main discussions include the following aspects: (1) chemistry and materials behind photopolymerization of different ceramic feedstocks, (2) polymer-derived ceramics that are obtainable through photopolymerization-based AM, (3) different photopolymerization-based AM techniques of relevance to 3D printed ceramics (in each of the techniques, insight, advantages, and disadvantages are given on the materialtechnique correlation), (4) ceramic functionalities that are derivable via photopolymerization-based AM techniques. The open challenges, way forwards, and prospects associated with the photopolymerization-based AM techniques are discussed too.

Chemistry and material requirement for photopolymerization-based AM
Photopolymerization is the formation of long chain macromolecules through continuous reaction of small molecules (monomers) in the presence of light exposure [41]. The reaction is normally initiated by the photoexcited reactive species [36]. The reactive species are normally incorporated into the monomer units, and hence cause polymerization reaction in the form of www.springer.com/journal/40145 long-chain reaction [36]. Generally, photopolymerization rate and efficiency rely on the intensity and wavelength of the light exposed and generated by the AM techniques [42,43]. In fact, literature survey suggests that different light sources with wavelength in the range of 200-700 nm have been so far exploited for the polymerization of the photosensitive liquid resins [44][45][46]. Although this often depends on the photopolymerization-based AM techniques specified, the property of the photosensitive polymers normally is a key [47]. Specifically, the use of appropriate photoinitiator to generate and propagate reactive species is essential whereas the properties of the monomers and ceramic precursors play vital roles in carrying-out cross-linking reaction process (linear chain reaction of the monomers) [36]. It shows that material-printing process correlation is crucial to the photopolymerization-based AM-derived ceramics with appreciable performances. Based on the above, different kinds of precursors (both conventional and advanced materials) of relevance to the formation of ceramics through photopolymerization-based AM will be briefly discussed in this section. Here, ceramic precursors that are consisted of powder ceramic-polymer matrix and preceramic polymers are regarded as conventional and advanced ceramic precursors, respectively.

1 Ceramic-polymer matrix
Ceramic-polymer matrix is regarded as conventional precursor in this review. It is normally in suspension/ slurry forms made up of powdered ceramics, monomers, and photoinitiator [6]. The shaping process in fact involves firstly the photopolymerization of organic contents of the matrix. Then the polymerized content forms interconnected networks to encase or bind the ceramic particles in the slurry matrix to become selfsustaining solid parts. The examples of the powdered ceramics which have been so far used to form con-ventional ceramic precursor are alumina (Al 2 O 3 ), zirconia (ZrO 2 ), silica (SiO 2 ), hydroxyapatite (HA), cordierite, zirconate titanate oxides (PZT), silica carbide (SiC), and silicon nitride (Si 3 N 4 ) [6,36,48,49], to name a few. The powdered ceramics of about 40-50 vol% in the ceramic-polymer matrix are usually recommended to maintain suitable viscosity and to avoid too much shrinkage in the final ceramic structures [36].
It is worth noting that ceramics themselves are highly brittle and non-photosensitive in nature [36]. This often worsens the processing of the conventional precursors during the 3D printing especially by using photopolymerization-based AM techniques. Nonetheless, the various research advances which have been so far conducted have shown that ceramic-polymer matrix can be easily printed and shaped into complex structures via photopolymerization-based AM techniques depending on the formulation approaches [29,50,51]. Specifically, monomer/oligomer-photoinitiator-powdered ceramic combination and correlation are of high significance for forming efficient and photocurable precursors for ceramics [29,50,51]. It makes the use of monomer/ oligomer and photoinitiator crucial for the forming of ceramic-polymer matrix as precursors in the field of photopolymerization-based AM. The typical steps involve for the formulation of ceramic-polymer matrix, and its use for 3D printing of ceramics via photopolymerizationbased AM techniques is illustrated in Fig. 2. As shown in Fig. 2, each of the constituents of the ceramicpolymer matrix has a significant role during the photopolymerization process as discussed subsequent sections.

Photoinitiator
Photoinitiators (PIs) are the photo-reactive species [45]. They are normally added to the liquid resins (ceramicpolymer matrix) as an initiator of photopolymerization process [52]. Specifically, PIs are molecules that can easily absorb photon of light, and hence form reactive www.springer.com/journal/40145 species [52]. The reactive species will then attack the functional group of a monomer in the liquid resin, decomposes the alkyl functional group (i.e., C-C), and hence generates more reactive species [53,54]. This normally multiplies the reactive species in the reaction system, and therefore increases the reaction chain [36]. It will result in the formation of polymer structures. The polymer formation will then give rise to the covalent bond between the adjacent functional groups with bonding strength stronger than that of monomers [36]. It will generate solid structures (printed body) consisting of the binder and ceramics [47]. The printed (green) body may have different chemical and structural properties when compared to that of liquid resins (i.e., the ceramic-polymer matrix) to indicate structural transformation (i.e., photopolymerization) [52].
Photoinitiator can be categorized into free-radical and cationic reactive species [55]. The free-radical PI produces free-radical after absorbing photon of light [56]. Free-radical PIs are useful for the photopolymerization of liquid resins consisting of acrylates and methacrylate monomers (i.e., free-radical photopolymerization) [56]. However, cationic reactive photoinitiators are the PIs producing acid after absorbing photon of light (e.g., UV light) [55]. They are useful for the photopolymerization of liquid resins with functional groups of epoxides, vinyl ethers, propenyl ethers, siloxanes, cyclic acetals, and furfural [57]. All of these show the significance of PI for the photopolymerization of conventional ceramic precursors into ceramics via photopolymerization-based AM techniques.

Monomer
Monomers are organic molecules which are responsible for binding the ceramic particles during the photopolymerization process [46]. It constitutes the major part of the ceramic-polymer precursors. It also forms large portion of the printed or green ceramic structures [36]. Monomers are essentially necessary to be thermally volatile in order to drive their easy removal during the post-thermal treatment (debinding) of the printed structures at temperature of about 500-550 ℃ [36,58,59]. It suggests that analysis of thermogravimetric property of the monomer is essential before being employed as binder. The analysis may aid sintering (heating at much higher temperature of > 1000 ℃) of the final structure too, since complete removal of the binders will give rise to defect-free and highly dense structures [58,59]. Similarly, it is important that monomer has high solubility to make it serves as reactive diluents for ceramic particles [46,60]. Monomers have to be engineered to generate photosensitive liquid resins with low viscosity. This will in turn result in the formation of slurry that has rheological properties (e.g., flow-ability, photosensitivity, and viscosity) matching well with different kinds of photopolymerization-based AM techniques. The typical examples of monomers that has rheological property suitable for the various types of photopolymerization-based AM are acrylates, methacrylate epoxides, vinyl ethers, propenyl ethers, siloxanes, cyclic acetals, and furfural [47,56,57].
In the field of photopolymerization-based AM, oligomers and monomers have similar functions; however, the former (oligomers) are consisted of two or more structural units of the later (monomers) [45]. It thus makes oligomers to exhibit larger molecular structures and higher viscosity. It may be otherwise referred to as prepolymers or macromonomers. Oligomers have also been found relevant as binders for photopolymerization of ceramic particles [36]. However, they are less attractive when compared to the monomers. This is because they have inferior workability due to its high viscosity [36]. This normally reduces light penetration through the liquid resins, and hence hinders the curing depth [36]. In fact, most work is being conducted using monomers. Brief but coincided discuss of photopolymerization of monomers is therefore given in the subsequent section.
(1) Chemical mechanism for photopolymerization of monomer Acrylate based resins (e.g., methylacrylate) with RH (R = aryl group or CH 3 ) are the monomers which have been widely used for 3D printing of ceramic parts [61]. The monomers are highly reactive in the presence of suitable photoinitiator under UV light [62,63]. However, it is worth noting that other factors such as temperature, atmosphere, light irradiation rate, and photoinitiator type may influence the monomer polymerization efficiency too [63]. For example, temperature in the range of 70-90 ℃, and oxygen inhibition atmosphere has been reported as factors favored photopolymerization of acrylatebased resins [63]. The role of photoinitiator is crucial as well. In fact, the type of photoinitiator is normally used to classify photopolymerization process into cation/anionic and radical [64].
Based on the existing literature, radical photopolymerization is the most common one [64], and the www.springer.com/journal/40145 formation of its photoinitiating systems is normally caused by carbon-carbon bond cleavage of PIs (e.g., aromatic carbonyl compounds). The typical mechanism involved is illustrated in step 1 of Fig. 3. This is normally regarded as Type I photoinitiating process [63]. In that mechanism, benzoyl radical is the primary photoinitiator or radical (PI*) for propagating the polymerization of monomers [64]. However, the secondary (alkyl) radical may influence the photopolymerization process too, depending on its concentration [64]. The common examples of Type I photoinitiators are benzyl dimethyl ketals, a-amino ketones, acylphosphine oxides, hydroxyalkylphenones, and benzoin ether derivatives [64,65].
It is worth noting that Type II photoinitiating system has been widely reported too. The chemical mechanism involves formation of aromatic ketone coupled with main radical (i.e., radical of hydrogen donor) for propagating the photopolymerization of monomers [64]. The mechanism is illustrated in step 2 of Fig. 3. During the photopolymerization process, the aromatic ketone radical will be disappeared through chemical coupling reaction after being used by the hydrogen donor (RH) [64]. This approach can be also referred to as hydrogen abstraction mechanism [64,65]. Aromatic diketones, benzophenone, phenylglyoxalates, thioxanthones, methylphenylglyoxylate, and camphorquinone are common Type II photoinitiators whereas amines, ethers, and alcohols are used as hydrogen donors [66,67]. Hydrogen abstraction from monomer or another photoinitiator has also been reported as shown in steps 3 and 4 of Scheme 1 [63].
One should note that when excessive amount of PI is used, the efficiency of photopolymerization may be Chemical mechanism for the formation of photoinitiating system [63]. "hv" represents UV light, and "·" represents radical. Reproduced with permission from Ref. [63], © The Society of Polymer Science, Japan 2019. compromised. For example, Sanai et al. [63] recently demonstrated the use of methyl phenylglyoxylate (MP) as photoinitiator for photopolymerization of acrylate-based resin. In their research, initiation and determination pathways were proposed using Types I and II processes. It was realized that acrylate-based resins could exhibit both intermolecular and intramolecular hydrogen abstraction as shown in Fig. 4. Specifically, when MP initiates polymerization of acrylates, and abstracts hydrogen from the monomer particularly at its alkyl group, efficient photopolymerization process is derived [63]. This is due to the direct formation of acryloyl groups which enables high crosslinking ability. It in turn results in substantial amount of gelation (steps 1 and 2 of Fig. 4). This suggests that when appropriate amount of PI (particularly < 5%) is used, efficient photopolymerization of ceramicpolymer matrix may be obtained [68,69]. Furthermore, when MP abstracts hydrogen from another MP (i.e., due to excessive PI), benzoylcarbonyloxy group will be formed [63]. It may in turn give rise to dead polymers as shown steps 3 and 4 of Fig. 4. All of these show that efficient photopolymerization of ceramicpolymer matrix relies on precursor formulation.
In summary, conventional precursors (ceramicpolymer matrix) for photopolymerization-based AM can be formed using powdered ceramics, organic monomers (e.g., acrylate and epoxy), and PIs. Other materials like fillers (carbon, silica glass, etc.) can be added too, in order to increase the mechanical properties of the final ceramics. The ceramic-polymer matrix with about 40-50 vol% of the ceramic powder has been found useful for the fabrication of highly dense ceramic structures. That composition can be used to avoid excessive shrinkage and delamination during post-thermal treatment, and hence gives rise to final ceramic structures with appreciable performance.

2 Advanced preceramic precursors
In this review, the polymers that do not involve ceramicpolymer matrix and can be directly photopolymerized and pyrolyzed into ceramics are regarded as advanced precursors (i.e., preceramic precursors (PCPs)). These polymers are normally prepared from monomers containing silicon [70]. They are commonly used to fabricate polymer-derived ceramics (PDCs) via photopolymerization-based AM techniques [70,71]. The selection and properties of the preceramic precursors are known to normally influence the performance and Chemical mechanism for the formation of photoinitiating system [63]. "·" represents radical, and "n" represent number of polymerization steps. Reproduced with permission from Ref. [63], © The Society of Polymer Science, Japan 2019.
properties of the final ceramics (i.e., PDCs) as discussed in Section 3. Examples of these polymers are polycarbosilane, polysilazane, and polysiloxane compounds. It is worth noting that these polymers generally exhibit different photopolymerization mechanisms and hence give rise to PDCs with varying degree of performance [72,73]. Specifically, preparation of the photosensitive organosilicon polymers is the primary challenge before the subsequent photopolymerization. For instance, addition of acrylate or methacrylate to organosilicon polymers (i.e., preceramic precursors) is normally used to enhance carbon-carbon cross-linking ability of precursors, particularly during photopolymerization via stereolithographic technique [73]. However, that approach normally suffers from volume shrinkage, and it is highly sensitive to oxygen contaminant [73]. As a result of that, the use of thiol-ene free radical reaction (i.e., click reaction) has thus become popular. This is a reaction where photopolymerization is based on cross-linking reaction between sulfurhydrogen and carbon-carbon group within preceramic precursors. The reaction can give rise to highly yield ceramics, fast photopolymerization rate, little or no volumetric shrinkage, and it is insensitive to oxygen and water contaminants [74].
In addition, it is well known that preceramic polymers with high-molecular weight, high solubility, and high cross-linking ability have possibility of producing PDCs with appreciable performance [70]. The bonding between the Si and carbon, nitrogen, or oxygen on the organosilicon polymer plays a vital role too [70]. It shows that the properties of the preceramic precursors are one of the key factors determining the photopolymerization efficiency. Therefore, this section will be used to discuss various preceramic precursors (advanced precursors) of relevance to the photopolymerizationbased AM of ceramics.

Polycarbosilane
Polycarbosilane is organosilicon useful for the fabrication of the PDCs containing Si-C bonds. Polycarbosilane is often prepared using cross coupling reaction between molecules of polysilanes (e.g., polydimethylsilane, polydimethylmethylphenylsilane, and polydimethyl-methylsilane), and hence generates multiple carbon-carbon bonds [73]. The photopolymerization process of the polycarbosilane into SiC is illustrated in Fig. 5. Polysilane (e.g., polydimethyl-methylsilane) normally exhibits similar photopolymerization process when it is being used directly for the SiC preparation [70,75,76]. Post-thermal treatment (i.e., pyrolysis at about 1000-1100 ℃) of the green PDCs obtained from the polycarbosilane or directly from the polysilane is crucial in order to obtain dense PDC structures [73]. It normally enhances S-C cross-linking reaction. However, it is worth noting that the PDCs (SiC) derived from the photopolymerization of the polycarbosilane normally exhibits superior performance in term of high ceramic yield, density, and tensile strength [77]. This is because polycarbosilane contains high molecular weight of carbon and superior cross-linking ability [70,78]. It shows that the use of polycarbosilane as preceramic precursor benefitted the SiC production regardless of the photopolymerization-based AM techniques [73].
Its photopolymerization process may give rise to stable and defect-free thermoset ceramic structures, hence preserving the final shape after pyrolysis [73]. This performance is somehow better than that of SiOC/SiCN and SiC and other ceramics (α-Al 2 O 3 ) prepared from other precursors and/or fabricated via different routes (e.g., powder-based approaches) [79,80].
In summary, polycarbosilane is useful preceramic precursor for the fabrication of SiOC or SiC ceramics via photopolymerization-based AM techniques. Similarly, polysilane can be directly polymerized into SiC too. However, it is worth noting that polymers with high molecular weight and carbon-carbon cross-linking ability favor high yield of final ceramics. It has the tendency to improve the density and tensile strength too.

Polysilazane
Polysilazane is another type of the advanced precursors for ceramic 3D printing. It is mainly used for the preparation of nitrogen-containing PDCs. The examples of such PDCs that can be obtained from the polysilazane preceramic precursors are silicon nitride (Si 3 N 4 ), silicon carbonitride (SiCN), Si 3 N 4 , and silicon oxynitide (SiON) [81,82]. Polyborosilazane is a chemical family of the polysilazane, and it is mostly relevant for the preparation of silicon borocarbonitride (SiBCN) [83]. The photopolymerization process of the polysilazane preceramic precursors is shown in Fig. 6. The polymers have general molecular structure of [-R 1 R 2 Si-N=] n where R and n are the alkyl group and the number of polymerized chains, respectively [73].
It has been reported that polysilazane can be efficiently polymerized into SiCN even in the presence of ordinary visible light irradiation [84,85]. For instance, a liquid mixture consisting of polysilane, photoinitiator (PI, e.g., 2,2-dimethoxy-2-phenyl acetophenone) has been photopolymerized into PDCs in the presence of ordinary visible light irradiation with wavelength of > 400 nm [84]. It has also been found that the use of polysilazane for the fabrication of SiCN structures using AM techniques can give rise to materials with appreciable thermal resistance [84]. This is due to the high carbon-carbon cross-linking ability coupled with nitrogen diffusion into the C-C, and hence reinforces the bonding structure. All of these have thus proven suitability and the efficient photopolymerization of the polysilazane for AM of PDCs particularly SiCN.
In summary, polysilazane is a good preceramic precursor for the fabrication of nitrogen containing PDCs. It has efficient photopolymerization process due to the high C-C cross-linking ability. However, pyrolysis of the green PDCs under nitrogen atmosphere at a temperature range of 1000-1100 ℃ can aid formation of SiCN material with highly dense structures. The approach normally favors tensile strength of the final PDCs too [86].

Polysiloxane
Polysiloxane is mostly used for the formation of silicon oxycarbide (SiOC) [70,87]. Among the preceramic precursors which have been exploited for the PDCs printing via photopolymerization-based AM techniques, polysiloxane is the only oxygen containing polymer. This makes polysiloxane mostly suitable for the fabrication of SiOC ceramics. It can be used for the printing and fabrication of ceramics with appreciable mechanical and thermal resistance too [88,89]. This is because polysiloxane has low inter-molecular force on its silicon-oxygen-silicon skeleton, and it has high cross-linking ability too [70,87]. This normally aids high yield of the final SiOC ceramics. The preceramic polymer is normally prepared using sol-gel methods (i.e., hydrolysis and condensation of silicon alkoxide) [90][91][92]. The material photopolymerization process is illustrated in Fig. 7.
The common example of the polysiloxane is polysil-sesquioxane found relevant for 3D printing of SiOC nanoceramics [93]. Generally, the 3D printed PDCs from polysiloxane precursor normally have carbon content of > 20 wt%, favorable to high thermal and chemical stability of the final ceramics [93]. Similarly, it has been demonstrated that secondary element intercalation (Al, Ti, and B) into the polysiloxane is possible by using sol-gel preparatory approach [93,94]. This may enable the fabrication of PDCs with structure phase like SiCNO, SiBCN, SiBCO, SiAlCN, and SiAlCO [93]. However, the high viscosity of the polysiloxane containing secondary elements is a major challenge hindering its implementation for photopolymerization-based AM techniques. It is a research area that needs more attention.
www.springer.com/journal/40145 (mainly polymers) are normally printed based on computer designed (CAD) models [95]. The printed polymers are essentially necessary to be pyrolyzed at temperature of ≥ 1000 ℃ under inert atmospheres in order to generate stable and dense structures [96]. It is worth noting that PDCs can be sub-categorized into binary, ternary, and quaternary as detailed in the subsequent sub-sections. This depends on the phase composition of the final ceramics. However, it has been widely demonstrated that the selection of precursor plays a vital role in the formation of particular phases [70]. In short, various forms of PDCs including complex phases have been recently fabricated through the use of photopolymerization-based AM techniques [70].

1 Binary polymer derived ceramics
These are PDCs consisting of two elements. The common examples of these materials are SiC and Si 3 N 4 . When compared to other AM techniques, fabrication of these ceramics (SiC and Si 3 N 4 ) from the preceramic precursors using photopolymerization-based AM often enables good shaping process, and it can also reduce time-and energy-consumption. SiC can be derived from polycarbosilane [70]. The pyrolysis of the printed polycarbosilane at temperature of > 1000 ℃ can result in SiC structures with appreciable mechanical strength and chemical stability [78].
It has been reported that modification of the polycarbosilane is essential to obtain SiC with appreciable properties. For instance, the mixture of polycarbosilane and epoxy containing molecules (e.g., epoxytetrahydrofuran) has been used to fabricate highly porous SiC at the expense of low density [97]. It shows that incorporation of epoxide into the polycarbosilane backbone is useful for the fabrication of 3D-printed SiC scaffolds for bioengineering application. Similarly, polystyrene has been used to modify polycarbosilane in which SiC/SiO 2 core shell is obtained after pyrolyzing the photopolymerized sample at 1200 ℃ [98]. All of these show that preceramic precursor properties can be compromised in order to prepare binary SiC with remarkable performance.
Nonetheless, it is worth mentioning that pyrolysis temperature may influence the performance of final ceramic structures too [78]. It has in fact been discovered that the use of high pyrolytic temperature (≥ 1000 ℃) can lead to substantial phase transformation of the preceramic resins into SiC ceramic structures without clogging [70,78,99]. Specifically, Li et al. [99] have revealed that there is possibility of having amorphous and crystalline phases at temperature of 600-800 ℃, and 1000 ℃ (Fig. 8), respectively. It shows that ceramic parts with appreciable properties can only be obtained by pyrolyzing printed preceramic resins at temperature of ≥ 1000 ℃ (Fig. 8 (c)) [99]. Similarly, Si 3 N 4 has also been reported using carbonfree polysilazane as precursor via DLP [100]. The yield of the Si 3 N 4 was normally improved by pyrolyzing the printed samples under N 2 atmosphere at temperature of > 1200 ℃ [70,[101][102][103]. However, only few reports are currently available on the 3D printed Si 3 N 4 using preceramic precursors. This is due to the fact that polysilazane (the common precursor for the Si 3 N 4 ) contains carbon, and hence the formation of SiCN is usually unavoidable. It is a challenge causing setbacks for the preparation of pure form of Si 3 N 4 using photopolymerization-based AM techniques.
In summary, photopolymerization of preceramic precursors such as polycarbosilane and carbon-free polysilazane using AM techniques can give rise to SiC and Si 3 N 4 , respectively. The performance and properties of the final ceramic have been so far improved by compromising the preceramic properties. The use of appropriate post-thermal treatment (i.e., pyrolysis at temperature range of 1000-1500 ℃) is essential too. All of these have been demonstrated as factors determining the mechanical strength, chemical stability, and porosity of the final components [104,105]. However, it is worth noting that the pure form of these ceramics, particularly Si 3 N 4 is difficult to prepare due to the lack of suitable precursors. SiC is prone to surface oxidation, and thus is less popular when compared to other types of PDCs. These are areas where further research studies are needed.

2 Ternary polymer derived ceramics
These are ceramics containing three elements. The common examples are SiCN, SiCO, SiOC, and SiON. They are often derived using polysiloxane, polycarbosilane, and polysilazane. It is worth noting that among the ternary ceramics, SiOC is extremely popular due to its strong oxidation and corrosion resistance ability [106,107]. It has appreciable chemical stability, and thermal durability too [104,108,109]. Eckel et al. [71] reported photopolymerization of polysilozane compound into SiOC ceramics. In their work, the UV-curable preceramic polymer (polysilozane) was mixed with photoinitiator, and hence polymerized the mixture ( Fig. 9(a)). The polymerized samples were then pyrolyzed at 1000 ℃ ( Fig. 9(b)). The approach gave rise to various forms of defect-free cellular structures (Figs. 9(c)-9(f)).
Similarly, de Hazan and Penner [104] used polycarbosilane polymer as precursor for the fabrication of SiOC ceramics. The photopolymerization of the preceramic polymer was performed in the presence of acrylate resin using SL technique. The materials exhibit non-porous surface which aids the material flexibility and its engineering towards different geometric shapes. Nonetheless, the SiOC ceramics with high porous surface have also been fabricated using different types of photopolymerization-based AM techniques [110][111][112][113]. It shows that properties of SiOC derived via photopolymerization-based AM techniques can be engineered towards catalytic and bioengineering applications.
In brief, the properties of the PDCs especially the ternary forms can be easily tailored towards particular application using photopolymerization-based AM techniques. However, precursor selection plays a vital role on the emerging properties of the final PDCs. This is the research area where more attention is needed in order to fabricate ternary PDCs with appreciable property for advanced technological applications.

3 Complex polymer derived ceramics
These are PDCs with more than three elements. They can be referred to as quaternary PDCs. Typical examples are silicon borocarbonitride (SiBCN), and silicon aluminocarbonitride (SiAlCN). Although only few reports are currently available on this material, they are normally prepared using polyborosilazane and aluminosilazane. It may be worth noting that the performance and yield of quaternary/complex ceramic is normally superior to that of other ceramics. Example of quaternary ceramics recently reported through photopolymerizationbased AM is SiBCN where polyborosilazane and photosensitive acrylate were used as precursors [83]. It was claimed that the SiBCN ceramic exhibited appreciable thermal stability and resistance in air at high temperature of about 1500 ℃ [83]. Furthermore, the use of polyborosilazane as precursor is favorable for the high yield (58%-78%) of the SiBCN ceramic [114]. When polyborosilazane is used as precursor, the ceramic yield is about 1.7 times higher than that of polysilazane [115].
Similarly, it has been found that SiBCN can be obtained by polymerizing a mixture of UV-curable borazine ring and acrylate [116,117]. The borosilazane precursor can give rise to SIBCN ceramics with defect-free and highly dense structures [116,117]. It was also found that the use of the borosilazane precursor can aid photopolymerization process in which a complete reaction may be obtained within 5 min of UV irradiation.

Photopolymerization-based AM techniques currently used for ceramic fabrication
Photopolymerization-based AM techniques are the 3D printing techniques suitable for the transformation of polymers or their matrix into ceramics [36,118]. The techniques can be used to fabricate ceramics with complex geometric structures. They are the approaches using light-activated spatial and temporal control for irreversible polymerization of monomers into polymers [36,118]. It can successfully transform liquid resins containing ceramic particles into solid mass of ceramicpolymer composite via photoreaction. Through the photopolymerization process, the techniques can be used to shape liquid resins into solids with complex geometric structures. Once again, the common and recent examples of the photopolymerization-based AM techniques are stereolithography (SL), digital light processing (DLP), two photon polymerization, liquid crystal display (LCD) printing, continuous liquid interface production (CLIP), and multi-jet printing (MJP). From the material requirement and operation perspective, each of the photopolymerization-based AM techniques differs. They offer different kinds of benefits, and the associated setbacks are different too. For example the two major prominent examples of the photopolymerization-based AM techniques are SL and DLP, and they have different light sources. Specifically, SL uses light spot with wavelength of about 355 nm, and it is useful for the photopolymerization of nonsemiconducting materials [62,119]. In contrast, DLP uses digital micromirror device (DMD) with wavelength of about 405 nm, and it is mainly used for the photopolymerization of semiconducting materials [9]. Similarly, LCD is a recent technique with strike resemblance to the DLP; however, the two techniques have different light intensity [120]. In short, DLP and LCD are designed for different printing processes. Therefore, this section will be used to discuss different 3D printing techniques of relevance for the photopolymerization of both conventional and advanced ceramic precursors. For each technique, emphasis will be placed on the printing process-material correlation, and the current advances made in solving the eminent challenges associated with the techniques.

1 Stereolithography (SL)
Stereolithography is one of the earliest AM techniques [6]. The photopolymerization is carried out by directly exposing the UV-light to the resin or monomer tank via top-down (i.e., the curing light is above the resin) or bottom-up (the curing light is beneath the resin) fashions [10,121]. This normally gives rise to solidification of the liquid resin. A layer by layer patterning of the printed material can be done by controlling the movement of the laser beam [29]. It is worth noting that SL is the best approach for printing large size models due to its wide range of laser beam movement (along XYZ direction) as illustrated in Fig. 10(a) [36]. Although, SL has slow printing rate due to long range movement of its laser beam, it has the ability to produce structures with wide range of sizes (e.g., dental, toys, molds, automotive, aerospace field, and so on).
The typical examples of ceramics fabricated through this approach were reported by Chen et al. [13] in 2012. In that research, effect of printing parameters (i.e., laser scanning speed, layer thickness setup, hatch spacing, and laser spot compensation) was investigated on the performance of the final ceramic parts. After compromising several parameters, it was discovered that hatch spacing, laser scanning speed, and sliced layer thickness of 0.15 mm, 1400 mm/s, 0.15 mm, and 0.35 mm respectively favored the performance of final silica-based ceramics (Figs. 10(b) and 10(c)). Effect of sintering temperature in the range of 10001400 ℃ was investigated too, and found that optimum performance with bending strength of ~10 MPa could be obtained at 1000 ℃ at heating rate of 150 ℃/h for 1 h. However, a higher temperature of 1200 ℃ was found suitable for incorporating porosity of 35% into the porous functional parts of the ceramics (Fig. 10(b)).
Before the above works were conducted, the same research group investigated the amount of silica content suitable for forming suspension to realize ceramic structures with appreciable performance [122]. Silica content of about 50 vol% was used to form suspension containing acrylamide (monomer), methylenebisacrylamide as cross-linker, and glycerol as solvent. The commercial silica materials were found having a refractive index, density, and mean particle sizes of 1.50, 2.65 g/cm 3 , and 1.553-9.343 μm, respectively. It was discovered that the volume and mean diameter of the silica particles influenced properties of the final ceramic parts. Specifically, by increasing the concentration of monomer the curing depth of the printed sample could be enhanced. This is because silica particles possess a higher refractive index. It thus indicates that high loading (e.g., above 50 vol%) of the silica content may reduce curing depth due to its high refractive index. High ratio of monomer to solid content in a slurry can thus benefit curing depth, and hence reduce curing duration.
In addition, Tian et al. [123] have revealed efficient post-treatment for the SL-derived ceramic structures which successfully minimized linear shrinkage. It was discovered that with the use of vacuum-freezing drying process the linear shrinkage of the green structures obtained via SL could be minimized to approximately 2.0%. It shows that the mass loss and cracks can be somehow avoided during sintering process by using vacuum-freezing drying for the green structures. All of these indicate that the precursor properties (i.e., slurry rheological property), printing process as well as post-thermal treatment play vital roles during the use of SL for ceramic fabrication. It suggests that the use of appropriate material-printing process correlation is crucial for the fabrication of 3D printed ceramics with appreciable properties.
In addition to the above, several PDCs have been reported to be printed via stereolithography approach. Examples are SiOC, SiC, and SiBCN [71,104,108]. It www.springer.com/journal/40145 is worth mentioning that with the use of SL for the fabrication of PDCs, the selection of preceramic precursors determines the properties of the final PDC parts.
Specifically, the composition of the preceramic polymer normally influences the yield, properties, and performance of the final PDCs. For example, it has been proven that siloxane with Si-O-Si backbone is mainly useful for the preparation of SiOC [71]. Similarly, the preceramic polymers consisting of siloxane and silazane mixture have been used to prepare SiOCN using SL technique [71]. It shows that silazane with Si-N-Si backbone is mainly used to prepare SiCN and SiOCN ceramics. In addition, the yield of the final PDCs can be substantially improved using preceramics with high carbon-carbon crosslinking density [71]. In that regard, infiltration of crosslinking agents (e.g., divinyl benzene) into the preceramic precursor has been found helpful [71]. It has the possibility to increase the carbon content in the final PDCs, and hence enhances the material performance. Incorporation of other kinds of elements (e.g., B, Zr, Fe, Co, Ni, Cu, Pd, Pt) into the preceramic precursors to tailor final PDCs towards particular applications (thermal membrane, magnetic and catalytic application) has also been found possible [124]. In short, different kinds of the ceramics fabricated through SL by compromising precursor formulation are shown in Table 1. Therein, the performance of the final ceramics is indicated which is somehow depended on the precursor formulation. Generally, the tensile strength increases with solid content volume up to about 50 vol% indicating the optimum solid content loading.
In summary, SL has widely shown promises for the fabrication of ceramics with appreciable resolution and large size models. The approach is suitable for the transformation of both conventional and advanced ceramic precursors into ceramic structures with promising properties.

2 Digital light processing (DLP)
Although DLP has some similarities with SL, the major difference is that photopolymerization process of the DLP is mainly relied on image projectors (DMD) in which light with wavelength of ~405 nm can be produced [6]. The projector is a mask-like device responsible for the projection of the image through the photosensitive liquid resin. In fact, the need for mask as light projector makes DLP more expensive when compared to SL [132,133]. DLP can be used to produce high resolution image with minimum size less than 50 µm [6]. The technique favors small size structures like jewelry casting and dentistry. The small  structures are normally produced with high precision and resolution in accordance to the CAD models [134]. DLP is mostly used for photopolymerization of radical-free photosensitive resins since it uses light source with longer wavelength [6]. Schmidt et al. [135] recently fabricated bioactive glass-ceramic scaffolds with complex structures using DLP. Polymer precursors for the scaffold were prepared by adding glass particles into photoactive polymer resins (i.e., tripropylene glycol diacrylate). The glass material has composition of 53.98 wt% SiO 2 , 34.58 wt% CaO, and 11.7 wt% MgO. The ceramic-polymer matrix was thereafter printed and polymerized using DLP with light having wavelength in the range of 400-500 nm. In that research, the use of photo-absorber was avoided since the photopolymerization process is performed using light with longer wavelength. The printed structure was thus sintered at 1100 ℃ to remove the polymer resin, and hence final scaffold with complex structure was obtained (Figs. 14(a) and 14(b)). It was claimed that the final structure exhibited porosity and compressive strength of about 83% and 3 MPa respectively. It shows the suitability of the DLP approach for ceramic structure engineering towards bio-technology application, such as tissue engineering.
In another report recently given by Liu et al. [32], it was revealed that structures as well as porosity of the ceramic fabricated via DLP can be substantially controlled by balancing technique-ceramic precursor properties. The research demonstrated the photopolymerization of a resin-based ceramic slurry consisting of Li 4 SiO 4 and trimethyl-propane-triacrylate. In their work, 1,6-hexanediol diacrylate, 2,4,6-trimethylbenzoyl-diphenyl oxide phosphine, and fluorocarbon polymer solution were used as reactive diluent, photoinitiator, and dispersant, respectively. The steps for the slurry preparation are shown in Fig. 15(a). With the use of DLP technique, ceramic units suitable for tritium breeding were printed, and it has tritium breeder units (Figs. 15(b) and 15(c)) demonstrating its suitability for potential fusion application.
In that research, it is discovered that light exposure time can be used to solve the problem of structure deformation which often arises during sample detachment from the building platform of the DLP [32]. Specifically, two sets of light exposure time were used in which the initial 15 s exposure time was used to form the base for the structure while only 2 s light exposure time was used for the printing of the main structure. The approach aids the development of defectfree structures even after sintering treatment (Fig.  15(c)). More recently, Zhang et al. [34] also compared the elementary building units prepared for resin, and ceramics through SL and DLP techniques, which help the understanding of ceramic printing process and the dimension retention performance (Fig. 16).
He and Song [136] prepared photocurable Al 2 O 3 slurry, and the slurry consists of aluminum oxide powder with varying loading of 2-70 wt% in a photocurable acrylate resin containing phosphate ester as dispersant for printing ceramic structures. As shown in Figs. 11(b) and 11(c), the viscosity and shear stress increase with Al 2 O 3 loading. It in turn reduces curing depth during UV-irradiation in an SL technique. However, one may say that optimum curing depth normally achieved at 50 vol% of the solid content loading according to the above discussions and data reported by He and Song (Figs. 11(d) and 11(e)) [136]. It suggests that optimum photopolymerization of the Al 2 O 3 -polymer matrix can be obtained at 50 vol% or wt% of the solid loading.
With the use of the suspension-enclosing projectionstereolithography (SEPS) as shown in Fig. 11(a), and the conventional ceramic precursor (alumina paste), He and Song's research shows that different geometric structures (Figs. 12(a)-12(h)) can be designed without directly attaching to the SL building platform [136]. The detachment of the printed samples was relatively simple and safe without deformation. Based on its simplicity, one may suggest that SEPS have possibility of solving the problem of structure deformation which normally occurs during detachment of printed polymers from the SL building platform.
Fu et al. [137] have recently demonstrated that porous ZrOC structure can be successfully printed using homemade precursor. The preceramic precursor is prepared by mixing zirconium n-propoxide, methylacrylic acid, and trimethylolpropane triacrylate together with diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide as photoinitiator ( Fig. 13(a)). The printing process and pyrolysis were performed as shown in Fig. 13(a). Although the structure exhibited linear shrinkage and mass loss of 79% and 42% respectively, high porosity of about 55.49% was obtained after pyrolyzing the green sample at temperature range of 600-1200 ℃ (Fig. 13(b)). It favors the realization of ultralight and ultrastrong structures (Figs. 13(c) and 13(d)). Such performance is partly attributed to the use of precursor with low viscosity and high photosensitivity [137].   Furthermore, it may be noted that several other advanced ceramic precursors have been recently used for the fabrication of both PDCs and traditional ceramics via DLP (Table 2). However, the properties and performance of the final ceramics strongly rely on the precursor formulation. For instance, the use of solid content with small particles favors properties of the final ceramic parts as demonstrated in Table 2. This is because small particles have possibility of minimizing light scattering which may in turn increase the curing depth and width.     www.springer.com/journal/40145 In summary, DLP is an approach suitable for the fabrication of structures with high resolution and precision. It has faster printing rate when compared to SL. However, the approach is not suitable for the fabrication of material with large size. The use of mask-like devices for image projection contributes to the printing cost, and hence makes DLP slightly more expensive when compared to SL.

3 Two-photon polymerization (TPP)
TPP is a 3D printing technique in which photopolymerization of ceramic precursor takes place by absorbing two photons of light at wavelength of near infrared (~780 nm) or green laser beam (515 nm) [6]. With the use of this technique, the photopolymerization normally occurs within a spatially localized focal spot in the photosensitive resin ( Fig. 17(a)) [153]. Therefore, this technique is mostly suitable for preceramic precursor resins than conventional ceramic-polymer matrix as the latter is generally opaque to the incident light. The approach can offer ceramics with nanosized structures making it relevant for the printing of ceramics for nanotechnology application. In fact, TPP has been widely used for the printing of objects with high resolution of < 200 nm which seems not obtainable through other techniques that are based on single photon photopolymerization process [154,155].
It is worth noting that TPP technique is mostly useful for the printing and fabrication of polymeric materials [6,156,157]. However, the need for nanostructured ceramics particularly in the field of nanotechnology has expanded the use of this technique to the ceramic fabrication. Recently, Brigo et al. [158] used TPP technique to investigate the influence of printing process and precursor formulation on the nanostructures of the SiOC. In that research, acrylate siloxane and bis(dimethylamino)benzophenone (BDEBP) were mixed and used as preceramic precursor for the printing of SiOC. The printing process was performed using radical photopolymerization approach with light source having wavelength of ~365 nm [158].
Brigo et al.'s research revealed that SiOC structures with high resolution and height in the range of 100 µm -1 mm (Fig. 17) can be fabricated depending on the printing process. It is discovered that a layer-bylayer structure growth is possible through a bottom-up printing system with laser beam focusing on lower glass-resin interface. The structures were then grown along the z-coordinated direction. The printing process was performed by placing precursor droplets between two glass substrates separated by polydimethylsiloxane (PDMS) membrane as gasket (Fig. 17(b)) [158]. The laser beam was then scanned at the upper glass-resin interface which in turn gave rise to layer-by-layer growth of the structure in decreasing pattern along the Z-coordinate [158]. The approach gave rise to structures Fig. 17 Schematics of (a) the "standard" approach with writing direction: up, and (b) "inverted" configuration with writing direction: down for 3D printing of SiOC ceramics. The complex 3D structures of the SiOC produced using the preceramic resist: before pyrolysis (c, d) Pisa tower, (e, f) Colosseum, (g-i) the pyrolyzed Kelvin cell on support base. Reproduced with permission from Ref. [158]. © The Authors 2018. with complex geometries (Figs. 17(c)-17(f)). Even though the structure underwent a slight deformation depending on the height of the support pillars (inset of Fig. 17(g)), a crack-free surface was observable after pyrolyzing at 1000 ℃ (inset of Fig. 17(h)).
In addition, TPP has also been widely used as an auxiliary approach to support other techniques, and hence photopolymerized ceramics in the range of micro to nanosize were produced [6]. Oftentimes, strategies are aimed to develop porous scaffolds or hollow structures. With the use of this approach, polymer matrix containing SiO 2 , TiN, Al 2 O 3 were printed into 3D hollow structures [159][160][161]. For instance, Meza and Greer [160] demonstrated the use of TPP coupled with direct laser writing, atomic layer deposition (ALD), and O 2 plasma etching (Fig. 18) for the fabrication of TiN ceramic with nanolattices structures (Figs. 18(b)-18(e)). The final material exhibits high von Mises stress and tensile strength of 2.50 and 98 GPa, respectively [160]. With a similar approach, SiO 2 microlattice and Al 2 O 3 nanolattice with hollow-structures have been reported too [159]. All of these show the relevance of the TPP for the photopolymerization of ceramic precursors into ceramics with micro-nanosizes.
In summary, one may note that among the photopolymerization techniques relevant for the fabrication of ceramics, TPP is the most suitable for submicron scale PDC fabrication. The approach is suitable for incurporating porous surface into ceramic structures too. This thus makes the approach relevant for the production of 3D printed ceramics for nanotechnology applications.

Other potential photopolymerization-based AM techniques for ceramic fabrication
There are still a number of recently developed photopolymerization-based AM techniques which have been used for the 3D printing of polymeric materials but not yet employed for ceramic 3D printing. Three kinds of representative techniques including liquid crystal display (LCD) printing, continuous liquid interface production (CLIP), and multi-jet printing (MJP), are briefly presented and discussed here.

1 Liquid crystal display (LCD) printing
LCD is one of the latest and much affordable photopolymerization-based AM techniques with a bottom-up printing system [162]. The approach is similar to the DLP and SL but with different imaging system [162,163]. Specifically, LCD uses liquid crystal display for the imaging of the polymer resins [9]. Its fundamental principle is based on the use of electric field to cause spontaneous changes in the molecule rearrangement of liquid resins [9]. This normally results in monomer polymerization, and hence prevents further penetration of light [164]. LCD offers appreciable image resolution, but the precision is somehow lower than that of traditional DLP. This is because the molecules of liquid crystal normally undergo incomplete polymerization and/or rearrangement and hence causes light leakage [9]. Even though much attention has not been given to the use of LCD for the polymerization of ceramic materials, the approach is suitable for the photopolymerization of preceramic materials. This is because free-radical and cationic photopolymerization process is possible through this technique [9].
In summary, LCD is a cheap 3D printing technique, and it can be used for the fabrication of structure with good resolution. However, the technique has not received the needed attention for the printing of ceramics. It is a research that needs attention since LCD has potential to reduce the cost associated with material printing. Furthermore, the short-life span of the LCD screen is another issue hindering its use for ceramic fabrication. Therefore, one may suggest that increasing the light emitting capacity of the LCD screen may in turn give rise to its long-life span. This may enhance the possibility of emitting strong light intensity too, and hence make the technique relevant for wider range of materials.

2 Continuous liquid interface production (CLIP)
CLIP is a 3D printing developed by the Carbon 3D Corp. [14,165]. The technique is similar to DLP. However, it consists of a special module in which there is incorporation of an oxygen permeation membrane (Fig. 19). The specialization is normally used to inhibit free-radical polymerization. The membrane at the bottom can allow the in-flow of the light beam and oxygen. The oxygen is often expected to quench the photo-excited radical or formed peroxide by interacting with the free radical [166]. This results in the formation and retain of liquid resin at the bottom while the polymerization takes place on top [9]. The technique has been used for the printing of polymeric resins in which structures with appreciable resolution are obtained [14,165].
The printing rate of CLIP is about 25 times and above greater than that of DLP. CLIP offers structures with higher precision. This is because the continuous printing prevents the physical layer appearance on the side surface of the printed parts. Compared with DLP printing, no staircase effect is generated so that the properties of the printed parts can be more isotropic. All of these show the suitability of the approach for polymerization of ceramic materials, particularly for printing PDCs using low-viscosity preceramic precursors [167]. However, to the best of our knowledge, the technique has not yet been extended to the printing of ceramic, because the involvement of ceramic content extremely increase the viscosity of the slurries so that the flowing of the liquid within the dead zone would be highly challenging.

3 Multi-jet printing (MJP)
Multi-jet printing is an AM technique that consists of multiple nozzles [9]. The technique is often used to print structures with multiple colors (i.e., for decoration or demonstration purpose) since inks with different colors can be injected at a time. MJP is designed in such a way that its nozzles can spray photosensitive liquid resins layer by layer. With the use of the technique, individual layers can be printed and cured before adding the subsequent ones. In particular, the nozzles spray a layer on the building platform, and the spread layer will then be smoothened and flattened by the roller and followed by photocuring using a UV lamp [9]. The next layer will then be sprayed by the nozzles and treated in a way similar to the previous layer until the part fabrication is complete.
MJP technique can be used for photopolymerization of wide range of photosensitive materials of relevance to radical, cationic, and hybrid photopolymerization [9]. This is because it has a light source with a wide range of wavelength (355-405 nm). Similar to the inkjet printing, MJP needs inks with low viscosity in order to avoid blockage of the nozzles. This is because its nozzles are in pico-size nature [9]. All of these make MJP a relevant 3D printing approach suitable for the printing and decoration of ceramics. Currently, only little is known about this approach for the photo-polymerization of ceramic materials. However, it is worth noting that the technique has complex components, and the material preparation is complex too since low viscosity is a prerequisite. It thus makes the whole process complicated and expensive.

Ceramic functionalities obtainable via photopolymerization-based AM
The functionalities of ceramics derived through photopolymerization-based AM are normally dependent on the precursor composition regardless of the techniques. Nonetheless, it is worth noting that techniques have plausible contribution too since they are relevant for tailoring structural properties. As shown in Table 3, DLP and SL are mainly useful for dense and microporous structures while TPP is relevant for tailoring nanostructures. In addition, it is worth noting that polymerization-based AM techniques have shown possibility for printing ceramics for biomedical applications [168]. This is due to their ability to fabricate complex structures coupled with the relevance for internal structure engineering [169]. However, it is worth noting that due to the particle densification and coarsening of ceramics after thermal post-treatment of printed parts, the porosity of the final structures is normally reduced greatly [169], which to some extent hinders the suitability of the printed ceramics for bioceramic application as generally high porosity is more desirable [169]. Therefore, in this section, mechanical, catalytic, electrical, and optical/photonic applications of ceramics prepared through photopolymerization-based AM are the major functionalities that will be discussed. The efficiency of the functionalities depends however on the precursor compositions [170]. For example, fillers (e.g., carbon, silica etc.) have so far been used to Table 3

1 Mechanical and catalytic/electrical performance
Mechanical performance of ceramics prepared by photopolymerization-based AM normally exhibit substantial improvement by introducing fillers into their precursors (i.e., liquid resin-ceramic powder mixture). Fillers are useful for the textural reinforcement in order to reduce shrinkage during post-thermal treatment (i.e., sintering). This is because, during sintering, organic monomer removal will create voids which may in turn result in structural weakness. However, properties of filler-monomer-ceramic powder mixtures are needed to be well compromised in order to form printable slurry. Uniform distribution of fillers with the liquid resin-ceramic powder mixture is essential to ensure efficient printing process. This can be done by employing fillers with particle sizes that match well with that liquid resin-ceramic powder mixture. An example of the above is the research conducted by Pham et al. [182], where silica was used as filler for improving mechanical performance of SiCN ceramics printed via TPP technique. In their study, nano-to microstructures of SiCN ceramics was fabricated using polyvinylsilazane functionalized with 2-isocyanatoethyl methacrylate resin as precursor. The printed materials were pyrolyzed at 600 ℃ in N 2 atmosphere. The use of varying amount (20%-40%) of silica as filler resulted in shrinkage reduction with the increase in the silica content (Figs. 20(a)-20(f)). It is therefore suggested that the silica reduces the deformation and pore volumes during and after the pyrolysis. Moreover, it was discovered that silica with particle size of ~10 nm was compatible with, and well distributed in the ceramic precursor for efficient printing process [182]. It in turn improves the mechanical performance of the final SiCN ceramics.
It is worth noting that strategies similar to the above are being used to improve the electrical conductivity performance of final ceramic parts derive through photopolymerization-based AM. In that regard, the use of carbon and graphene as fillers to functionalize and modify ceramic precursors is common. The approach has also been found relevant for incorporating pores into photopolymerization-based AM derived ceramics. Nevertheless, the property and functional group of the introduced fillers are essentially necessary to be compatible with that of the liquid resin-ceramic powder or preceramic mixtures.
Mei et al. [183] recently demonstrated the use of DLP for the fabrication of photocatalytic molybdenum sulfide (MoS 2 )-modified Al 2 O 3 ceramics. In that research, carbon was employed as a pore former and reinforcement to functionalize ceramics containing MoS 2 (i.e., catalytic active material). The use of DLP for the printing of slurry consisted of Al 2 O 3 and resin also aids the formation of ceramics with complex microstructures [183]. The printed ceramics were thereafter sintered and functionalized with MoS 2 taking advantage of pores created by the carbon formed from pyrolyzed resin. It in turn enhances electron and hole conductivity of the final ceramics triggered by well exposed active sites in the structure. This favors the catalytic application of the prepared ceramics.

2 Optical/photonic performance
SL and DLP can also be used to print components for optical and photonic applications, particularly in macroscale since it is useful for the fabrication of complex and small size structures. In this process, ceramic powders (e.g., zirconium oxide (ZrO 2 ), titanium oxide (TiO 2 ), and silica (SiO 2 )) with refractive index of ≤ 1.7 are normally used in order to generate suitable photonic properties. The powders are essentially needed to be mixed with photocurable resin (e.g., acrylate based resins) for efficient printing process. For example, Kirihara and Niki [176] demonstrated the printing of photonic crystals from a mixture of liquid Al 2 O 3 -acrylate resin. The printed materials were then debinded and sintered at 600 ℃ for 2 h and 1500 ℃ for 2 h, respectively. The final photonic crystals of alumina with micro-lattice structures were obtained (Figs. 21(a)-21(c)). Similar to the above, Chen et al. [175] used SL for printing of photonic crystal of SiO 2 ceramics. They claimed that SiO 2 powders with particle size of about 100 nm were found suitable for the SL printing after mixing with acrylate based resins. This gave rise to photonic crystals of SiO 2 ceramics with diamond like structures. All of these show suitability of SL for printing of ceramics with photonic and optical properties. However, material-printing process correlation is crucial for achieving efficient printing process.
It is worth noting that TPP technique has been found useful for printing photonic crystals in sub-micrometer scales. For example, Duan et al.'s research [184] reveals that photonic crystals of (TiO 2 ) ceramics can be printed into micrometer scale. Titanium (IV) acrylate complexes were used as precursor wherein TiO 2 nanoceramics were generated in-situ after photopolymerization, hydrolysis, and sintering process. In short, one may summarize that photopolymerization-based AM techniques are useful for printing ceramics with desirable optical/photonic properties. Among them, SL and TPP have been widely used for printing photonic crystals in the macro-and microscale, respectively. Moreover, ZrO 2 , Al 2 O 3 , TiO 2 , and ZrO 2 are the potential materials currently showing appreciable functionalities.

Challenges, prospects, and summary
Although, photopolymerization-based AM techniques are suitable for the fabrication of ceramics with complex geometries, several barriers are hindering commercialization of a vast number of these techniques. For instance, nearly all of the photopolymerizationbased AM techniques require materials with low viscosity for efficient printing process. This often complicates the ceramic precursor preparation, and hence adds to the cost of the printing process. It makes the whole process cumbersome. Furthermore, liquid or preceramicbased resins with low viscosity are prone to high degree of molecular crosslinking. Generally, this further weakens the structural property of the final ceramics.
In addition, most photopolymerization-based AM techniques require multiple steps of post treatments (e.g., pyrolysis and sintering) of the printed ceramics. Such thermal treatments for the removal of the organic phases may result in the formation of a large number of voids and pores. It may hence weaken the structural performance. One may therefore suggest that researches on photopolymerization-based AM technique-material correlation are needed for further strengthening, in order to fabricate ceramics that are viable for advanced applications.
Specifically, the current barriers for the use of stereolithography for ceramic printings are:  Slow printing process delaying the delivery of the printed ceramic parts.  Multiple steps of post-processing of the printed ceramic parts adding to the cost of the final ceramic parts.  The use of toxic monomer resins making ceramic precursors preparation and post-treatment unfriendly to the human and environment.  Lack of suitable light intensity for engineering the internal structure of the printed ceramic parts restricting the applications of SL printed ceramic parts. DLP technique's main challenge is:  Limited projection area restricting the techniques to ceramic parts with small and medium sizes. For TPP, the identified challenges are as following:  The laser beam is only met for the curing of transparent polymeric materials limiting the number of ceramic materials that can be processed.  The technique is restricted to the fabrication of ceramic parts with small sizes at micro-nanoscales.  The printing rate is very slow due to ultra-fine resolution to be realized. Nevertheless, photopolymerization-based AM of ceramics offers substantial prospective value. Examples of prospective opportunities that may be derived through commercialization of photopolymerizationbased AM for the ceramics fabrication based on ceramic-monomer matrix or preceramic precursors are (a) production of load-bearing structural components with high resolution and precision, complex geometries, and zero defect, (b) fabrication of functional parts with micro-nanosize porous features for the applications in bio-engineering and energy storage/conversion. When the material-printing process correlation is well engineered, photopolymerization-based AM-derived ceramics may have the tendency to perform better than that are prepared through other kinds of 3D printing techniques.
For example, the use of hydrogels for the printing of bioceramic parts has shown promises in which bio-ceramics can be directly printed on the cells or bioactive molecules via photopolymerization-based AM techniques (e.g., DLP) [185]. It thus makes the production of highly flexible and bioactive ceramic parts possible [186]. Based on material-printing process compromising, particularly by using hydrogel to aid ceramic curing, a direct bioprinting of ceramics for bone regeneration that relies on natural body environment has been proposed too [186]. However, once again, slow printing process and the reliance on the toxic organic substances for the printing of bio-ceramics are still part of issues needed to be tackled. Therefore, development of non-toxic and printable bio-ceramic slurries with high curable ability at room temperature is thus suggested as a research in right direction to aid advanced tissue engineering via photopolymerization-based AM techniques. This is because in-situ bioprinting of ceramic scaffold for bone regeneration will be made possible without fear of further damage to the affected tissues.
In summary, photopolymerization-based AM offers a solution to the common problems (brittleness and crack) involved with machining of ceramics. The solution may bring about fabrication of ceramic components with high resolution and complex geometric structures. It may in turn increase the prospects of 3D printed ceramics across the mechanical, electronic, optical, biotechnical, and nanotechnical industries. The use of relevant material-printing process strategy is crucial for printing of ceramics with advanced properties. It is believed that the discussion given herein will offer deeper insights into the use of different photopolymerization-based AM techniques for the fabrication of advanced ceramics with superior properties and performance. The barriers and challenges to the use of photopolymerization-based AM techniques are outlined, and they are expected to benefit the researchers, scientists, and engineers whose interest matters most on the use of photopolymerization-based AM for the fabrication of ceramics for advanced applications. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.