3D/4D Printing of Polyurethanes by Vat Photopolymerization

Vat photopolymerization relies on the light‐induced polymerization of liquid photopolymer to produce the targeted structure. Among all the polymers prepared by vat photopolymerization, due to their numerous properties polyurethanes have recently gained great attention in the industrial and academic fields. In this review article, recent progress in printing techniques, resin compositions, and applications related to 3D/4D printing by vat photopolymerization of polyurethanes are highlighted.


Introduction
Although being one of the oldest polymers, with the first synthesis dating back to the early 1930s from Bayer, polyurethanes (PUs), and particularly thermoplastic polyurethanes (TPUs), continue to play an essential role in a variety of industries such as automobiles, sporting goods, medical equipment, construction, electronics and so on. [1]On top of that, because of their biocompatibility and biodegradability, they have been the most often used synthetic polymers in biomedical applications over the last 50 years. [2]These reasons explain that the worldwide polyurethane market was about 72.82 billion USD in 2021 and is expected to increase at a 4.3% annual rate until 2030. [3,4]This projection is mostly driven by the car industry's efforts to develop lighter materials to reduce fuel consumption.
Polyurethane is a multi-segmented copolymer formed by the polyaddition of polydiols and diisocyanates with a diamine or a diol as chain extenders (Figure 1a). [5]This results in a polyurethane composed of a linear polymer with a soft segment (SS) (due to the polydiol) and a hard segment (HS) (rigid DOI: 10.1002/admt.202300366diisocyanate moieties combined with the chain extenders).Because of phase segregation, the soft and hard segments form distinct domains.Due to the intermolecular hydrogen interactions between urethane groups, the HS domain operates as a physical cross-linking component, imparting elastic characteristics to the materials (Figure 1b).Because numerous synthesis parameters, such as the ratio, molecular weight, and chemical type of the hard and soft segments, can be changed, a myriad of materials can be produced, explaining the vast range of attributes of TPUs. [6]igure 2a depicts the most common diisocyanate structures.They are classified as aliphatic or aromatic diisocyanates, as well as symmetric or asymmetric structures. [5,6]The mostly used aromatic isocyanate is 4,4′-diphenylmethane diisocyanate (MDI).As expected, TPUs made of symmetric diisocyanates lead to better microphase separation and crystallinity than TPUs prepared with asymmetric structures.Aromatic isocyanates often have high T g for the HS, but reduced light stability, resulting in a yellowish tint after light exposure.Because of their ability to generate aromatic amines during degradation, such materials are thought to be more hazardous.
Polydiols (Figure 2b), which are typically oligomers ranging in size from 1000 to 5000 Da, can be classified based on their type of backbone.Polydiols based on polyethers, polyesters, and polycarbonates are the most common polymers.
Chain extenders (Figure 2c) are diols or diamines with low molecular weight.The structure of a chain extender might affect the HS domain by playing with the hydrogen bonding ability of the urethane group but also by modifying the mechanical properties of the backbone.
As previously noted, the careful selection of the three components has a significant influence on the properties of the materials.Since polyurethanes have been employed in so many different applications due to their highly adjustable properties, their usage in 3D printing has received a lot of attention.TPUs may be melted at high temperatures and hence processed for 3D printing using fused filament fabrication (FFF) first named FDM and invented by the company Stratasys, Inc. Specific TPU grades were created in that instance.Miller and co-workers [7] studied the mechanical characteristics and fatigue performance of three polycarbonate-based polyurethanes by varying hard and soft segment contents, which were manufactured by injection molding and FFF.They found that FFF is a very effective processing method for these materials with an apparent insensitivity of tensile monotonic properties and tensile fatigue performance due to the presence of a small percentage (<1%) of voids.FFF samples equaled or outperformed injection molded samples in terms of monotonic tension, compression, shear, and tensile fatigue, which was likely owing to favorable printing conditions.On the other side, given the benefits of 3D printing by vat photopolymerization technology such as high dimensional precision, superior surface polish, and strong material resilience, combining these advantages with polyurethane chemistry seems like a natural fit. [3,8,9]However, the chemistry of vat photopolymerization and conventional polyurethanes differs significantly, as industrial polyurethanes are thermoplastics while materials printed by vat photopolymerization are often thermosets exhibiting different macroscopic properties.As a result, a strong effort is focused to develop printable resin leading to PU like materials by vat photopolymerization.In this context, the choice of resin formulations, the possible printing techniques and the scope of applications are already broad and needs interdisciplinary competences.This review aims at offering a comprehensive overview of each of these details for a general audience.To the best of our knowledge, this is the first review of this topic and we believe that it will facilitate the reader to better understand the field, to select the most suitable formulation/printing technique for a targeted application and also inspire further developments the field in vat photopolymerization of polyurethanes.

3D Printing Methods
Vat photopolymerization is a type of 3D printing technology that uses a liquid resin that hardens when exposed to ultraviolet light via a photopolymerization process.The photopolymerization process involves using a light-curable resin, called a photopolymer, that is stored in a vat and exposed to either visible or UV light.When the curing light hits the resin, it triggers a polymerization reaction that results in the formation of chains of polymers or crosslinks, leading to the formation of a solid resin.The photopolymer mixture typically includes monomers, reactive diluents, photoinitiators, and sometimes fillers, as shown in Figure 3.When the photoinitiators are exposed to the curing light, they release reactive species that catalyze the chain   formation process among the monomers and reactive diluents.This chemical-thermal process is irreversible, meaning that the resulting prototypes cannot be changed back to liquid form.
By repeatedly applying this process, successive layers of resin are gradually built up to form a complete 3D object, based on a sliced STL file.Vat photopolymerization is known for its ability to produce high-resolution, detailed prints, but can be slower and more expensive than other 3D printing methods.2][13][14]

Stereolithography
The first vat polymerization system called stereolithography (SLA) (Figure 4) was introduced by Hull and Arcadia in 1985. [15] laser beaming in the UV range is used to polymerize a liquid resin in this layer-by-layer process, while a plate moving on the zaxis dives into the vat after each layer is produced.The main benefit of this technology is the high resolution facilitated by the laser beam focus.However, stereolithography looks to be a sluggish printing method because resin viscosity hinders the deposition of a fresh uncured layer of liquid polymer on top of the printing sample.The size of the produced item is also constrained by the depth of the vat and the amount of resin available. [16]

Digital Light Processing
Digital light processing (DLP) is another layer-by-layer photopolymerization process (Figure 5).Unlike SLA, DLP employs a whole layer pattern light source that is projected onto the sample.When the plate sinks into the resin, a projector placed beneath the vat creates this pattern.The light source can be a standard lamp or a light-emitting diode (LED) with a lateral resolution of up to 10 μm.As compared to SLA, this approach greatly reduces printing time, and the polymerization is less susceptible to oxygen inhibition. [16]DLP printing is also beneficial since it is printed upside down, requiring a smaller vat and less resin.If the light penetration is high, thicker layers may be produced, lowering printing time even more.Nevertheless, the vertical resolution suffers as a result.By integrating light-absorbing compounds into the resin, light penetration may be controlled. [17]By trapping the filler particles within the crosslinked polymer network, DLP may even be utilized to print resin with ceramic and metal fillers. [18]esides these benefits, the DLP approach has certain disadvantages.Since light must be projected over the entire sample,  printing can be less precise than in other stereolithography procedures.Moreover, light projection might result in shadows and reflections, which can degrade print quality.

Continuous Liquid Interface Production
Unlike SLA and DLP, Continuous Liquid Interface Production (CLIP) is not a layer-by-layer process.There is no need for recoating between each layer since an oxygen-permeable film limits polymerization in the area near the light source, allowing the process to be developed continuously (Figure 6).While the timeconsuming feature of SLA and DLP was the recoating phase, the CLIP process looks to be significantly faster than the layerby-layer approaches with the same resolution. [19]According to the following equation, the thickness of the "dead zone" of resin is a function of the photoinitiator absorption coefficient ( PI ), photon flux (Ф 0 ), a constant (C), and resin curing dose (D C0 ), which is an indicator of the reactivity of the monomer with the photoinitiator: [20] Dozen dead thickness = C This method invented by DeSimone is mainly commercialized by Carbon Inc., Redwood City in California with improved resins and hardware that can create samples with a large mechanical property range.

Two-Photon Polymerization
Two-photon polymerization also known as multiphoton polymerization, is a microscale 3D printing technology that enables resolutions of less than 30 nm. [21] Polymerization takes place only in the focal volume, or voxel, of a laser with a pulse.These laser beams are required to provide a high photon density for the resin to absorb the two photons.The size of the samples is determined by the scanning speed and laser power. [22]Two-photon polymerization offers high precision and the ability to create complex structures, but it can be time-consuming, expensive, and cannot be used to print objects on a large scale.

Other Techniques
New vat photopolymerization processes have recently emerged.Hot lithography (HL) is a vat photopolymerization variation that employs heat to speed up the resin curing process.Traditional vat photopolymerization cures the resin by exposing it to UV light, which may be a long process.HL may cure the resin more quickly by adding heat to the process, enabling quicker print rates and higher-resolution items and mechanical properties. [23,24]Volumetric 3D printing is a variation of vat photopolymerization in which 3D items are created by exposing a volume of resin to light rather than being built layer by layer.Because the items may be formed from all directions at once, it is possible to create objects with more complex shapes and internal features. [25]irculating vat photopolymerization enables the printing of objects using resins including high-weight fillers.To prevent filler sedimentation, a pump mechanism is employed to circulate the resin. [26]Lastly, Direct Ink Writing is a type of 3D printing technology that uses a process similar to extrusion to create objects layer-by-layer.In this process, an "ink" or "paste" made up of a material such as a photosensitive material and/or a ceramic is extruded through a small nozzle and deposited onto a substrate.When exposed to light, the resin undergoes photopolymerization that crosslinks the material.The nozzle is then moved to create the desired shape of the object being printed.Direct Ink Writing has the advantage of being able to print complex geometries with high precision and can also incorporate multiple materials into a single object. [27]

Composition of the 3D Printing Resin
The vast majority of the resins used in 3D-printing vat photopolymerization are composed of a reactive oligomer/polymer, a reactive diluent, a photoinitiator, and various additives such as fillers, dyes, or photosensitizers.It has also to be mentioned that instead of using a reactive oligomer/polymer vat photopolymerization of polyurethanes can be obtained by mixing previously prepared PU in presence of monomers and photoinitiators to lead to an interpenetrated polymer network.The specific role, preparation, and usage of each resin's component will be described in the following sections.

Synthetic Strategies for the Preparation of Reactive Oligomers/Polymers
One strategy for preparing polyurethane-type materials via vat photopolymerization is to use acrylate/methacrylate end-capped oligo or polyurethane as one of the 3D-printable resin components.Many of these compounds are already commercially available.Typically, they are prepared following an isocyanate or nonisocyanate route.It has also to be highlighted that vat photopolymerization of polyurethanes can be obtained by direct polymerization of isocyanate and alcohol precursors using photobase generators.Besides, another strategy for preparing polyurethanetype materials is to employ a dual cure system, as proposed by Carbon.Some representative examples for each synthetic strategy are presented below.
Farzan et al. [29] described the synthesis of PU/polycaprolactone (PCL) and/or polyethylene glycol (PEG) di-acrylates as key resin components for fabricating 3D-printed scaffolds by stereolithography.To do so, PU/PCL and/or PEG di-acrylate were synthesized in two steps.PCL-diol and/or PEG were initially reacted with hexyldiisocyante (HDI) in the presence of dibutyltin dilaurate (DBTDL) as a catalyst.After obtaining the prepolymer, it was end-functionalized using 2-hydroxyethyl methacrylate (HEMA) to introduce a methacrylate moiety at each chain end (Figure 8).
Huang and co-workers also synthesized polycaprolactonebased polyurethane acrylates by reacting previously prepared isophorone-based acrylate and PCL-diol (Figure 9). [30]The isophorone-based acrylate was made by reacting isophorone diisocyanate with 2-hydroxyethyl acrylate.The polyurethane acrylates were then mixed with propylene glycol (PPG), poly (ethylene glycol) diacrylate (PEGDA), and TPO as a photoinitiator to obtain the desired printable resin.
Peng et al. [31] prepared also several 3D-printable resins composed of polyurethane acrylates (PUA).The PUA was created in two steps (Figure 10).To commence, an isocyanate-terminated prepolymer was obtained by reacting isophorone diisocyanate with a diol.PUAs were then generated by reacting the previously created prepolymer with 2-hydroxy ethyl acrylate (HEA).
Li et al. [32] demonstrated that the DLP 3D printing technology could be utilized to create shape memory items from polyurethanes matrices having reversible Diels-Alder junctions (PUDA).PUDA was created in two steps (Figure 11).Initially, the Diels-Alder reaction between N,N′-4, 4′-diphenylmethanebismalemide and furfuryl alcohol produced a diol (DA-diol).

Synthesis of Polymerizable Oligo or Polyurethane via a Non-isocyanate Route
Isocyanate-based polyurethanes are recognized to have possible health risks related to the isocyanate functional group, such as respiratory sensitivity, asthma, and skin irritation.Nonisocyanate polyurethanes (NIPUs), on the other hand, are free of these risks, making them a safer and more environmentally friendly choice.Moreover, the utilization of NIPUs enables the synthesis of polyurethanes utilizing a broader range of starting materials, such as alcohols and carbonates, which can result in reduced costs and greater sustainability.
In recent years, there has been increased interest in the use of NIPUs as an alternative to standard isocyanate-based polyurethanes for 3D printing applications.For example, DLP 3D printing was used to create complex polymeric structures from NIPUs utilizing a photopolymerizable blend of bis(allyl urethane) monomers, multi-thiols, and photoinitiators.Bis(allyl urethane) monomers were synthesized by heating cyclic allyl carbonates with, for example, cadaverine (Figure 12). [8]igure 8. Preparation of PU/PCL and/or PEG di-acrylate.Reproduced with permission. [29]Copyright 2020, Elsevier.
Mülhaupt et al. [33] described a nonisocyanate approach for obtaining different liquid hydroxyurethane methacrylates (HUMA).Glycerol carbonate methacrylate (GCMA) is typically made by reacting glycidyl methacrylate (GMA), 2,6-di-tert-butylcresol, and tetrabutylammonium bromide under heating and 30 bars of carbon dioxide.After the reaction of GCMA with the appropriate di or triamine, hydroxyurethane methacrylates were produced.Interestingly, using a DLP printer, several 3D objects with complex geometries were created from the synthesized hydroxyurethanes (Figure 13).
Later, Mülhaupt et al. functionalized urethane methacrylate hydroxyl groups with 2-isocyanatoethyl methacrylate (IEMA), methacrylic anhydride (MAA), or acetic anhydride (AA) (Figure 14).IEMA and MAA allow methacrylate groups to be introduced through urethane and ester functions, respectively, whereas AA turns the OH group into the equivalent ester without altering methacrylate functionality.Surprisingly, as compared to non-modified HUMA, the chemical transformation of the OH group resulted in a decrease in viscosity after reactions with MAA and AA and an increase in viscosity after reactions with IEMA.These findings were attributable to the lack of hydrogen bonding when MAA and AA were utilized, and the existence of supramolecular interaction when IEMA was used due to the formation of urethane junctions.The modified HUMA was mixed with 4-acryloylmorpholine as a reactive diluent and phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide as a photoinitiator to create 3D printable resins.UV-curable resins were produced with a DLP-based printer to construct 3D objects with tunable mechanical properties, depending on the resin composition.
Pyo et al. [35] reported the synthesis of diurethane methacrylates utilizing several biobased amines and an isocyanatefree method.To generate a printable mixture, such diurethanes were combined with ethanol and lithium phenyl-2,4,6trimethylbenzoylphosphinate as a photoinitiator.Using an inhouse built continuous optical 3D printing system, cytocompatible 3D structures with high resolution were created from the latter.Initially, diurethane methacrylates were produced by reacting mono-methacrylated trimethylolpropane cyclic carbonate (TMP-MAC) with putrescine, cadaverine, spermidine, and spermine in the absence of a catalyst and a solvent (Figure 16).

Interpenetrated Networks
Interestingly, Naficy et al. [36] reported the preparation of 3D hydrogels allowing reversible shape deformation upon hydration and temperature as a stimulus.To do so, the group developed a printable resin composed of commercially available polyether-based polyurethanes (PEO-PU), 2-hydroxyethyl methacrylate, and N-isopropyl acrylamide (NIPAM), N,N′methylenebisacrylamide (BIS) as a crosslinking agent and ketoglutaric acid as UV initiator (Figure 17).The rheological properties of the inks were adjusted by the PEO-PU molecular weight.According to the authors, the interpenetrated polymer Figure 9. Synthesis of polycaprolactone-based polyurethane acrylates. [30]twork nature of the polymer matrix was responsible for specific mechanical properties.
Bae et al. [37] investigated the synthesis of a waterborne polyurethane and (multi)acrylate resin for 3D object preparation using the 3D DLP printing technique.The waterborne polyurethane was typically made in three steps (Figure 18).To begin, a 530 g mol −1 polycaprolactone diol was allowed to react with 4,4′-methylene dicyclohexyl diisocyanate (H 12 MDI), resulting in the corresponding prepolymer.This was combined with dimethylolbutanoic acid (DMBA) and H 12 MDI to create a PU with a soft and hard backbone structure.Finally, the polymer was neutralized with triethylamine to produce the corresponding isocyanate-terminated prepolymer, which was then dispersed in water.(Multi)acrylate monomers were then used to tune the mechanical properties and print the final material.

Direct Polymerization from Isocyanate and Alcohol Precursors
Interestingly, Zivic et al. [38] showed direct 3D printing of polyurethane using photopolymerization of multifunctional alcohols and isocyanates in the presence of thioxanthone-based photobase generators (Figure 19).
Photobase generators were synthesized in three steps (Figure 20).Initially, a bromo acetate derivative was allowed to react with 2-hydroxythioxanthone.After deprotection, the appropriate salt was obtained.Finally, the neutralization of the salt by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) resulted in the expected photobase generators.The formulation's application in 3D objects was obtained using an extrusion printing technology that combines fused deposition modeling and stereolithography procedures.Typically, the resin is put in a cartridge to be deposited as a thin layer and then photopolymerized with a LED irradiation source at 365 nm.The repetition of this procedure enabled the creation of 3D items (Figure 21).

Dual Cure Systems
The main strategy to prepare a material by vat photopolymerization that is not usually polymerized by a radical process might be to utilize an acrylate/methacrylate resin containing the materials' precursor. [39]Once the 3D object is printed by classical acrylate/methacrylate chemistry, other stimuli such as heat are used to trigger the polymerization or preparation of the final materials from the precursor embedded into the 3D object.Qi and co-workers, for example, developed a hybrid ink, containing both a common photocurable acrylate-based resin and a second thermocurable epoxy-anhydride resin, that may be utilized for DLP 3D printing of high-performance epoxy thermosets using a twostage curing technique. [40]The moduli of the DLP printed materials ranged from a few tens to hundreds of MPa, preventing shape change during the transfer from the 3D printer to the oven; the second stage cure had a very little volume change, helping in the preservation of the 3D-printed shape.
Carbon 3D, a leader in 3D printing, developed numerous commercially available proprietary resins that might broaden the Figure 10.Synthesis of polyurethane acrylates. [31]riety of uses.They recently developed polyurethane material 3D printed through vat photopolymerization. [41,42] Their resins are composed of two components.The first component is a blocked or reactive blocked-isocyanate prepolymer with a reactive diluent (optionally) and the photoinitiator noted Part A, and the second component is a chain extender noted Part B (Figure 22).During the irradiating stage, the initial components react to generate a polymer scaffold containing blocked diisocyanates, which could be unblocked by heating or microwave irradiation during the second step to react with the chain extender.Figure 22 shows a dual cure system with a thermally cleavable end group.Just before the printing, Part A and Part B were mixed.
Following the UV-curing of the object due to the crosslinking of acrylate/methacrylate functionality of the compound initially located in Part A, the isocyanates groups could be unblocked thermally (Step 2) to recover/activate reactive species able to react with the compounds that were initially located in Part B. This produces a polyurethane/polyurea linkage within the original cured material or scaffold.This chemistry finally creates an interpenetrated network that could mimic the properties of common polyurethanes.According to the published patents, [41,42] chain extenders can be diols, diamines, triols, triamines, or their mixtures and the preferred chain extenders are ethylene glycol, 1,4-butanediol, methylene dicyclohexylamine, hy- It should be noted that the thermal cleavage described above is more a displacement reaction of the chain extender (often a diamine) with the hindered urea, than a true dissociation, resulting in the final polyurethanes/polyureas without the formation of isocyanate intermediates.
This type of blocked isocyanate is widely documented in the literature. [44]Among the many blocking technologies, an aldehyde-blocking agent might be used to block the diisocyanate or isocyanate-functional oligomer or prepolymer.The reaction product of such an aldehyde-blocking agent and an isocyanate has an advantage over TBAEMA blocked PUs in that urea production reduces hydrogen bonding, resulting in lower viscosity blocked isocyanates (Figure 23a).A second advantage is the elimination of free amines in the final product, which might oxidize and induce yellowness or deterioration.
The use of TBAEMA as a blocking agent (Figure 23b) has nevertheless one advantage, it is a commercially available product.When heated to a suitable temperature (for example, around 100 °C), the urea bond formed between the tertiary amine of TBAEMA and the isocyanate becomes labile, regenerating the isocyanate groups that will react with the chain extender(s) Figure 11.Synthesis of polyurethanes matrices having reversible Diels-Alder junctions (PUDA). [32]ring thermal cure to form high molecular weight polyurethanes.
The advantage of this dual system allows Carbon to propose the printing of objects with high tensile modulus (1700 MPa) and tensile strength (35 MPa) with an elongation at break that could reach 100%. [45]The influence of the UV curing conditions and thermal curing conditions was then investigated.First, Obst et al. [46] reported that a lower irradiation time increased the elongation at break.They also showed that tensile strength increased by more than 100% from the intermediate state to the final object, suggesting that the formation of the secondary polyurethane/polyurea network toughens the material.Bachmann and co-workers then investigated the influence of the temperature on the mixed resin to determine its impact on the printability and mechanical properties of the materials. [43]They observed that the viscosity of the resin was increased by preheating and also by the released reaction heat of the photopolymerization.3D objects could be obtained whatever the conditions.Neverthe-less, tensile tests showed slight to moderate differences in terms of tensile modulus, tensile strength, and elongation at break.

Diluents
Diluents and reactive diluents are required to adjust the resin viscosity and tune possibly the macroscopic properties of the printed item by acting as plasticizers or because they are covalently incorporated in the polymeric matrix.Poly(urethane) resins are particularly viscous due to the many hydrogen bonds formed by the urethane function.Yet, to achieve a properly printed material, the viscosity of the resin must be controlled.A viscosity of less than 2 Pa.s is suggested for 3D printing by vat photopolymerization to ensure good resolution of the manufactured item. [47]This is because the more viscous the resin, the less oxygen dissolves in it.Although the presence of oxygen is a constraint to photopolymerization, its presence is necessary to stop polymerization where Figure 12. Preparation of complex polymeric structures from non-isocyanate PUs utilizing a photopolymerizable blend of bis(allyl urethane) monomers and multi-thiols. [8]ere is no light, increasing the resolution of the printed item. [31]n top of that, lowering the viscosity of the resin improves its workability, enables rehomogenization under the tray, and allows for the incorporation of inorganic fillers into the formulation.The viscosity is also an important characteristic in the formulation of 3D printing resins since it affects both the printing speed of the printed object. [48]For health and environmental concerns, it is highly demanded to lower the percentage of volatile organic compounds.Therefore, in recent years, the selection of reactive diluents to lower viscosity has therefore become critical in the formulation of photopolymerizable resins. [49]Reactive diluents also have an essential effect on the final properties of the material [50] Figure 13.General synthesis of GCMA and HUMA from a 5-membered carbonate methacrylate (GCMA) and amines. [33]gure 14.Functionalization of urethane methacrylate hydroxyl groups with IEMA, MAA, or AA. [33]gure 15.Two-step non-isocyanate route for the synthesis of urethane-methacrylate monomers. [34]igure 16.General synthesis of diurethane methacrylates from a 6-membered carbonate methacrylate and amines. [35]gure 17.Synthesis strategy of 3D-printed hydrogels. [36]olor, T g , mechanical properties), making their selection critical in the resin formulation.Figure 24 depicts the most common reactive diluents used with PU resins.
For example, hydroxyethyl acrylate (e) is used as a reactive diluent in combination with polyurethane diacrylate to produce recyclable materials with shape memory via 4D printing. [32]Cheng et al. [51] investigated the effect of the diluent ratio on glass transition temperature and discovered that when hydroxyethyl acrylate percentage decreases, the glass transition temperature increases from 87 to 133 °C.This result was assessed to the used polyurethane diacrylate which is more rigid than the reactive diluent.Another research examines the effect of varying hydroxyethyl acrylate/hydroxyethyl methacrylate ratios on shrinking.
The shrinkage increases as the proportion of reactive diluent increases because the distance between molecules changes from the Van der Waals distance created by the urethane groups to the covalent bond distance created by the polymerization of the reactive diluent.The number of the reactive functional groups also increases when the ratio of reactive diluent increases.
The Mülhaupt group employed acryloyl morpholine (d) as a diluent with polyhydroxyurethane methacrylates to lower the viscosity of the printing resin while providing outstanding mechanical properties. [52,33]The process of addition of the reactive diluent can be a formulation key to obtain a suitable resin for 3D printing.Usually, the reactive diluent is added with the photoinitiator in the PU oligomer and mechanically stirred.Feng et al. [53] presented a formulation process of graphene-reinforced nanocomposite allowing to well-disperse graphene into a commercial polyurethane resin as an oligomer, trimethylolpropane trimethacrylate (TEGDMA) as a reactive diluent and phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide as a photoinitiator without bubbles (Figure 25).The resulting material has higher mechanical properties than the direct casting reference specimens.
Nonisocyanate urethane methacrylates are also interesting reactive diluents because they increase the amount of van der Waals interactions in the material and so improve its properties.
Three nonisocyanate urethane dimethacrylate reactive diluents, 2-(methacryloyloxy)ethyl 2-(methacryloyloxy)ethylcarbamate, (2-(methacryloyloxy)ethyl 3-(methacryloyloxy) propylcarbamate, and 1-(methacryloyloxy)propan-2-yl 3-(methacryloyloxy)propylcarbamate were synthesized by reacting a cyclic carbonate with an amino alcohol, followed by a reaction with methacrylic anhydride to make UV-polyurethane coatings. [54]hese reactive diluents were combined with an acrylated polyester (APE) oligomer and a free radical photoinitiator to make UV-curable polyurethane coatings with improved impact resistance and elongation at break.Meyer and co-workers demonstrated the good mechanical properties of a 3D printing resin using a non-isocyanate urethane methacrylate as a reactive diluent. [34]esearchers are increasingly trying to develop 3D printing resins without the need of diluents.Resins with low viscosity have been created utilizing low molecular weight PU polymers, with the low molecular weight chains acting as diluents. [55]Hot Figure 18.Preparation of waterborne polyurethane dispersion. [37]gure 19.Photopolymerization of multifunctional alcohols and isocyanates in the presence of thioxanthone-based photobase generators. [38]thography vat photopolymerization 3D printing may be an effective solution to avoid the use of diluent since it improves the mechanical and dimensional properties of the printed items. [24,23]

Initiators
Since most monomers do not create enough initiating radicals when exposed to UV radiation, a photoinitiator that effectively absorbs the incident light and produces radical or ionic species with a high quantum yield is required (the quantum yield of a photochemical reaction is defined by the ratio of the number of transformed molecules to the number of absorbed photons). [56]nce started, the chain reaction proceeds in the same manner as in conventional polymerization.There are two types of photoinitiators: Norish type I and Norish type II. [16]However, almost all photoinitiators used in poly(urethane) resins are type I.This can be explained by the faster curing rates and free radical   [38] Copyright 2020, American Chemical Society.
production of type I photoinitiators. [57]As a result, the majority of commercially available photoinitiators are type I. Table 1 summarizes examples of photoinitiators used in poly(urethanes) resin for vat photopolymerization.

Fillers and Dyes
Although polyurethanes have good properties for printed materials, their mechanical and thermal properties are limited due to their many crosslinking points, preventing their broad usage in industrial applications.It has been demonstrated that adding fillers to the resin is a helpful strategy for improving the final material's properties and reducing shrinkage. [76]The most frequent fillers for PU 3D printing resins are graphene and graphene oxide (Figure 26).In fact, the fillers' strong intermolecular interactions create chemical connections with the PU backbone, greatly increasing hardness and modulus.The use of these fillers can also improve the material's conductivity. [77,53]oo and Cho [78] printed conductive material using a polyaniline nanomaterial and graphene sheet in a PU matrix with several additional fillers.The printed materials' morphology, dispersion, and bonding structures were compared.To create a conductive and antistatic material, the formulas were improved.A PU resin containing 0.1 to 2.0 wt% graphene nanoplatelets enhanced tensile strength and Young's modulus up to 20%. [79]It is important to note that if the graphene ratio is too high, the mechanical properties of the printed material may suffer due to interruptions in UV curing and filler agglomeration.
Because of the existence of oxygenated bonds, graphene oxide is an easier filler to use in a 3D printing resin.This allows for greater solubilization into the resin. [73]Graphene functionalization has also been researched to increase the biocompatibility of graphene as a material. [80]Biodegradable and conductive nerve guiding conduits were printed with a PEGylated graphene oxide formulation with a PU resin with high tensile stress (3.51,0.54MPa) and strain (170%), conductivity (1.1 × 10 3 S.cm -1 ), and a low contact angle 72°. [29]nother way to increase the material's conductivity is to include multiwalled carbon nanotubes.To increase the dispersity of the multiwalled carbon nanotubes, a solution intercalation method using acetone was applied in a commercial urethane matrix, and a high electrical conductivity was found. [81]ue to the nature of certain fillers, they can interfere with light and sediment quickly, resulting in print failure or poor material homogenization.Copyright 2021, Elsevier.
For environmental concerns, the incorporation of biobased fillers into a photocurable resin is becoming increasingly popular.Lignin, the second most abundant natural polymer after cellulose, has been included in various PU resins for 3D printing due to its depolymerization and degradation properties.Unfortunately, this resin's printability has to be improved. [67,82,83]Sutton and co-workers achieved this aim by chemically modifying ligning by reduction acylation to minimize UV-Vis absorption of the resin during 3D [62] Müller and co-workers used anisotropic cellulose nanocrystals as biobased fillers to develop 3D printable materials with locally tunable mechanical properties in a single printing step of Direct Ink Writing.The materials consist of a polymer matrix with biocompatible photoreactive cinnamate derivatives and up to 30 wt% of anisotropic cellulose nanocrystals and can be further crosslinked upon illumination to adjust Young's moduli between 15 and 75 MPa, making them versatile for applications such as prosthetics and soft robotics. [27]ohan et al. [84] dispersed cellulose nanofibrils from oil palm empty fruit bunch fibers designed with polyethylene glycol and reduced graphene oxide into a UV-curable polyurethane-based resin.To investigate the mechanical properties, tensile speci-mens were created using a DLP 3D printer.As compared to unmodified PU, 3 wt% cellulose nanofibrils in the PU matrix enhances tensile strength by 37% with reduced graphene oxide and 24% with polyethylene glycol.The cellulose nanofibrils and reduced graphene oxide enhance toughness by 129%.However, at more than 3% cellulose nanofibrils, the mechanical properties were reduced due to cellulose nanofibril aggregation in the polymer matrix.To increase the mechanical properties of the same PU matrix, the same group added extracted organosol lignin and graphene nanoplatelets. [82]he addition of polyurethane as a filler was studied by Fang et al.They improved the toughness of an epoxy acrylate 3D-printed material by blending bifunctional and trifunctional polyurethane acrylate and hyper-branched polyesters (HBPs).Through an orthogonal experiment, the optimal formulation was determined and it was found that the addition of 10 wt% HBP improved the toughness of the photocurable system.However, this resulted in a loss of stiffness, elasticity modulus, and thermostability. [85]anosilica is another well-known filler that improves the durability of 3D-printed materials.To increase the mechanical  properties of the DLP printed material, Cheng et al. dispersed a nanosilica in a PU resin. [51]The same authors chose to use ultramarine dye and a photostabilizer to decrease the yellowing of the resin caused by the aromatic groups found in urethane moieties (Figure 28).This approach is highly interesting from a formulation aspect since most PU resins are readily yel-lowed after curing, which can be a critical problem for industrial applications.
Yang et al. [69] used nano alumina particles to create ceramic parts at a lower cost and with less technical difficulty.The formation of the 3D structure of ceramic products was achieved through self-made light-curing 3D printing technology Figure 25.Formulation process of graphene-reinforced nanocomposite into a commercial polyurethane resin Reproduced under terms of the CC-BY license. [53]Copyright 2019, The Authors, published by Hindawi.using a semi-solid ceramic precursor fluid composed of nano alumina particles, photocurable polyurethane acrylate, and isobornyl methacrylate resin.The solidification and formation of the ceramic material were achieved through secondary hightemperature sintering.

Applications
Polyurethanes have been often described in the literature as suitable candidates for 3D printing by vat photopolymerization because of their versatility and excellent properties.As a result, the market for polyurethanes in 3D printing has recently received a lot of interest. [86]The many uses and commercial improvements that make polyurethanes promising materials for 3D printing by vat photopolymerization are highlighted in this section.

Biomedical
Since the mid-1990s, when 3D printing was initially used for surgical purposes, starting with anatomical modeling for bone reconstructive surgery, [87] polymeric implants, tissues, and scaffolds with complex shapes were developed. [88]This advancement in 3D printing enables the manufacture of specified dimensions and geometry, allowing for flexibility and reproducibility. [89]ecause of their outstanding biocompatibility, mechanical properties, and biostability, polyurethanes are excellent candidates for biomedical applications.Biocompatibility, or the capacity of a bioimplant to coexist with an organism without causing harm, has been widely investigated for polyurethanes.The biocompatibility of the associated materials is linked to the structure of the polyurethanes.Lyman et al., for example, demonstrated that blood biocompatibility is related to surface morphology. [90]oreover, hydrophobicity, crystallinity, urethane linkage, and surface free energy have been linked to alterations in lactate dehydrogenase activity and platelet adhesion. [91]ecause of their chemical composition and structure, these polymers are also sensitive to degradation.Due to the regularity of the synthetic polymer chain, crystalline regions reduce the polymer chains' susceptibility to degradative agents.This property has been studied for its potential use in tissue engineering and regenerative medicine. [92]ven though there have been fewer investigations employing polyurethanes for vat photopolymerization than for FFF 3D technology, researchers have maintained attention on this approach for biomedical applications. [93,94]n 2016, Jukka's group was among the first to publish a paper on SLA-made polyurethane materials for biomedical applications. [55]Elastomers' Young's modulus, tensile strength, and elongation at break were measured to be 2.5 MPa, 3.7 MPa, and 195%, respectively, which is equivalent to polymers for soft tissue engineering applications.The structure of the diol employed in polyurethane synthesis influences the hydrolytic degradation, viscosity, thermal, hydrophobicity, and mechanical properties of the PU-based material used to fabricate scaffolds.Because of their biocompatibility, degradability, and elasticity, these materials are good candidates for soft tissue engineering. [29]reen chemistry is an important topic of study for ecological and human health reasons.As a result, the biomedical field seeks to produce technologies that are more environmentally friendly and safe for patients.To increase the cytocompatibility of the final product, a waterborne polyurethane resin was coated on a 3Dprinted scaffold.The fabricated scaffold had no cytotoxicity and good cell adhesion.The final material's hydrophilicity was also improved. [95]able 1.Examples of photoinitiators in poly(urethanes) resin for vat photopolymerization.
Lithium phenyl-2,4,6trimethylbenzoylphosphinate I 275 nm,379 nm [35]   No initiator --- As described before, the aminolysis reaction between primary amines and cyclic carbonates is the most often used approach to avoid the usage of dangerous isocyanates.Primary amines are frequently easily extracted from biobased products.Pyo et al. used this approach to create photosensitive aliphatic polyhydroxyurethanes for 3D printing (Figure 29).The resulting materials demonstrated high optical transparency and cell viability. [35]he mechanical properties and biocompatibility of various materials have been demonstrated to be tunable depending on the structure of the diamine, dithiol, and the ratio of the reagents. [8]everal studies have employed biobased materials as fillers in 3Dprinted composites to improve performance and biodegradability.The inclusion of fillers allows for the reinforcement of the mechanical properties of printed materials.Employing biobased fillers prevents the emission of hazardous particles that are harmful to both human health and the environment.Furthermore, it may enable the recovery of biowaste such as oil palm empty fruit bunches. [84]Shie and co-workers made cartilage scaffolds with high cytocompatibility and mechanical properties using waterbased polyurethane with hyaluronic acid as a 3D-printed resin.Before printing, the water was evaporated, and the viscosity was adjusted with the comonomer hydroxy ethyl methacrylate and the photoinitiator (Figure 30).
The cell viability tests showed similar results for water-based thermoplastic polyurethane composites containing 0% or 50% water.Although this water-based PU is often used for coating rather than vat photopolymerization, it has demonstrated non-toxic and ecologically friendly properties.Moreover, the material provides cell adhesion, proliferation, and chondrogenic differentiation. [66]onic skins are flexible, transparent, and biocompatible materials inspired by human skin, composed of pressure sensors that transmit signals by ions rather than electronic skins that transmit signals via electrons.They are frequently made of hydrogels or ionogels, which are polymeric networks that have been swelled with water or ionic liquids.These ionic skins have attracted many researchers for the development of implantable or wearable electronics for biomedical applications such as soft robotics, prosthetics, artificial intelligence, and health monitoring. [96]A dual material was produced using two resins: a water-dilutable polyurethane acrylate and a hydrogel, using an alternative digital light processing method. [65]The hydrogel served as electrodes, Figure 27.Circulating vat photopolymerization (CVP).Reproduced with permission. [26]Copyright 2022, Wiley-VCH.
while the polyurethane served as dielectric layers.The hydrogel and polyurethane segments were chemically bound.This eliminates the ionic skin's signal drift during long-term usage and isolates the hydrogel from the air.

Electronics
Robot and electronics production frequently involves complex geometries and architecture, rendering standard manufacturing methods such as injection unsuitable.The advancement of 3D printing in microstereolithography, which provides micro/submicro printing resolution, enables the development of this type of technology. [97]Because of their high elongation and resilience, polyurethanes are excellent materials for flexible sensors and soft actuators. [98]atel et al. [60] introduced a variety of extremely stretchy polyurethane polymers for advanced electrical applications.A 3D printable resin was created by combining an epoxy aliphatic acry- Reproduced with permission. [35]Copyright 2017, American Chemical Society.
late and an aliphatic diurethane with 33 wt% isobornyl acrylate with TPO as an initiator.The ratio of epoxy aliphatic acrylate to diurethane was investigated to optimize mechanical properties for various electronic applications.The printed elastomers can be stretched up to 1100%, which is five times more than their commercial reference and competitive with commercial silicon rubber.The hydrogen bonds formed by the polymer's urethane groups account for this remarkable stretchability.Clear soft actuators and bucky ball electronics switches with high flexibility, conductivity, compressibility, and electric repeatability were successfully 3D printed with a DLP printer (Figure 31).
acrylate.A transparent piezoresistive strain sensor with high strength (6 MPa) and a wearable finger guard sensor with good conductivity was created by coating one of the 3D-printed polymers with an ionic hydrogel solution.A zinc oxide wearable UV photodetector was developed using FFF and a flexible thermoplastic polyurethane layer, followed by a PVA layer printed with precise control of the reflow to be ultra-flat.Finally, a Cu-Ag nanowire was printed on the substrate flowing by the zinc oxide layer.The photosensitivity of the electrode was investigated as a function of zinc oxide thickness.Although vat photopolymerization was employed, a flexible polyurethane was 3D printed to provide an easily made flexible photodetector.
Self-healing materials are another interesting component of electronics, soft robotics, and sensors.Li and co-workers described a 3D-printed polyurethane acrylate with disulfide bonds that is flexible and heals quickly (Figures 32 and 33). [10]

4D Printing
Skylar Tibbits at MIT's Self-Assembly Lab pioneered 4D printing in collaboration with the company Stratasys, Inc. [99] He added time as a fourth dimension to 3D printed materials that may change shape in response to stimuli such as light, temperature, water, pH, electromagnetic radiation, or acid.[102][103] Vat photopolymerization is easier to employ for 4D printing than FFF procedures because thermosets have superior shape memory performance and durability owing to structural stability.Polyurethanes are also good candidates due to their ease of tuning the chain structure.
Lantean et al. [101][102][103] describes the use of urethane acrylates and butyl acrylate to 4D print magnetoresponsive polymeric Figure 31.Conductive bucky balls working as an electric switch.a) A bucky ball completely coated with silver nanoparticles.b) A LED turns on after compressing the bucky ball.c) A bucky ball selectively coated with silver nanoparticles.d) A LED turns on after compressing the selectively coated bucky ball.e) Finite element simulation.f) Bucky ball's conductivity under 2000 times cyclic compressive testing.The scale bar is 10 mm.Reproduced with permission. [60]opyright 2019, American Chemical Society., c) a circular cone, d) a hollow cube, (2e) a Maya Pyramid and (2f-h) Self-healing experiment of the honeycomb structure.Reproduced with permission. [10]Copyright 2022, The Authors, published by Wiley-VCH GmbH.materials with tunable mechanical and magnetic properties.The magnetic response of the printed objects is adjusted by changing the loading of Fe 3 O 4 nanoparticles in the printing material.By using magnetic fields, the microstructure of the printed objects can be controlled, allowing for the creation of magnetoresponsive objects with complex functions, such as rolling, translation, stretching, and shape-shifting.The authors also present a systematic study of the magnetically driven self-assembly of Fe 3 O 4 nanoparticles into chain-like structures, which is used to produce a dataset to precisely program the microstructure during the printing step.By controlling the orientation and length of the magnetic chains in each printed layer, a desired microstructure can be obtained in a 3D printed piece.Finally, the authors demonstrated the use of the magnetoresponsive polymers to  [32] Copyright 2020, Elsevier.
create macroscopic remotely controlled hammer-like actuators with different stiffness.They also showed that magnetoresponsive gears can be combined with non-magnetic elements to create complex assemblies, such as gear-trains, linear actuators, and grippers that can be remotely controlled (Figure 34).
Zhao et al. printed a photopolymer with shape memory using polyurethane acrylate, epoxy acrylate, and isobornyl acrylate.During 16 consecutive tests, a fold-deploy test was used in water at various temperatures, and the form recovered in seconds. [74]üsgen's group also employed 4D printing to create flow chemical reactors.A non-swelling, UV-vis transparent, and chemically resistant flow reactor was printed using a hybrid resin produced from an isocyanate acrylate monomer.This reactor is suitable for photooxygenation as well as photoredox catalysis. [104]uang and co-workers created a combination of 4D printed and recyclable polyurethanes using Diels-Alder chemistry.According to dynamic mechanical analysis and IR data, the material after recycling is somewhat altered but still has strong mechanical properties.Shape memory cycle tests were conducted to measure the material's shape memory performance, and the results show that performance rises with temperature (only 8 s to recover at 80 °C for a flower petal shape) (Figure 35). [32]lthough 4D printing involves various types of stimuli, up to now the main stimuli used for 4D printing of polyurethanes are temperature or electromagnetic radiation, therefore in the next future polyurethanes sensitive to pH, light, water, or acids among others will be probably the subject of further investigations.

Commercially Available Products
Several 3D printing firms are developing their own materials.Evonik Industries AG, Arkema S.A., and BASF SE are among the most successful companies in the 3D printing material industry.
Carbon has distinguished itself from competitors with its CLIP (Continuous Liquid Interphase Printing) technology, which allows objects to be produced up to 30 cm h -1 quicker than with DLP technology.The company has also differentiated itself via the mechanical properties of its materials.As described in Section 3.1.5., these materials are typically two-component resins that are thermally and UV cured.
107] Some companies, such as Allnex USA Inc. and Arkema S.A. offer photopolymerizable oligomers that may be added to resin formulations to improve the mechanical properties of the final materials.Allnex's EBECRYL series, made of urethane methacrylate, and Sartomer's N3xtDimension liquid resins are two examples among others.

Conclusion
3D printing of polyurethanes by vat photopolymerization is an important and multidisciplinary approach that requires various skills including organic synthesis, polymer chemistry, material characterization, formulation, and system engineering.Research efforts have focused on the formulation of the resin, the study of diluents, and the addition of fillers to improve the properties of the printed materials, providing a broad library of printable resins.While many commercial resins are already proposed there is still plenty of room to develop innovative chemistry allowing faster polymerization kinetics, higher resolution, and more environmentally friendly approaches.This review highlights the numerous applications of 3D-printed polyurethanes, particularly in the field of biomedical applications.Nevertheless, the mechanical properties of the printed polyurethanes by vat photopolymerization need to be improved, especially in terms of their durability and strength.Compared to thermoplastic polyurethanes, printed polyurethanes exhibit lower mechanical properties, which can limit their practical applications.Another challenge is the toxicity of the resin and isocyanates used for the preparation of the oligomer precursors.Exposure to isocyanates can cause health problems, and there is a need to develop safer alternatives such as bio-based or non-isocyanate PU resins.Moreover, the inability to recycle thermoset-printed polyurethanes can lead to the accumulation of plastic waste and have adverse environmental effects.To address this issue, researchers could develop recyclable polyurethanes by introducing degradable moieties or printing with thermoplastic polyurethanes.Finally, research on vat polymerization of polyurethanes in some fields like aerospace, bioprinting, or 4D printing is still in its early stages and will be probably further investigated.However, the potential of this approach is enormous.The ongoing research and development in this field promise to unlock new possibilities for the future of manufacturing.

Figure 1 .
Figure 1.a) Preparation of polyurethane via a two step process.b) nano-organization of polyurethane in soft segments (SS) and hard segments (HS).

Figure 21 .
Figure 21.a) Scheme representing the extrusion and UV curing3D printing technique: a) thermostat, b) compressed air, c) formulation, d) LED, and e) 3D object.b) Photo of the specimen that has been printed.Reproduced with permission.[38]Copyright 2020, American Chemical Society.

Figure 23 .
Figure 23.a) Aldehyde blocking agent to prepare reactive polyurethane.b) TBAEMA as a blocking agent to prepare reactive polyurethane.[44]

Figure 29 .
Figure 29.Optical printing of intricate structures.a) Schematic process showing software-aided 3D reconstruction from grayscale patterns.b) Layer-by-layer printing procedure of 3D structure guided by computer.c) SEM image of the printed pattern.d) Complex contour biomimetic structures of shark skin.e−i) Complex contour functional structures of log-pile, squid, and bone.Scale bars: c) 200 and d−i) 500 μm.Reproduced with permission.[35]Copyright 2017, American Chemical Society.

Figure 28 .
Figure 28.a) Curing picture of different contents of photostabilizer, b) Curing picture of different contents of dye.Reproduced with permission.[51]Copyright 2020, Wiley-VCH GmbH.

Figure 30 .
Figure 30.a) The schematics of the manufacturing process of the water-based polyurethane-based photosensitive materials; The Raman spectra of the b) water-based light-cured polyurethanes and c) water-based thermoplastic polyurethanes with or without water removal processes; d) The images of the printed scaffolds; e) The images of the designed (left) and printed (right)porous lattice structures.Reproduced under terms of the CC-BY license.[66]Copyright 2017, The Authors, published by MDPI.

Figure 32 .
Figure 32.a) Optical microscopy recorded during the scratch healing process of the elastomer prepared from PUA; b) Images of the Olympic Rings pattern jointed by cylindrical samples.c) Images of the colored PUA samples cut into two pieces, connected, and healed.Reproduced with permission.[10]Copyright 2019, American Chemical Society.

Figure 33 .
Figure 33.PUA sample.3D objects fabricated with the photopolymer PUA by DLP 3D printing: a) the abbreviated name of the ICCAS, b) a honeycomb structure, c) a circular cone, d) a hollow cube, (2e) a Maya Pyramid and (2f-h) Self-healing experiment of the honeycomb structure.Reproduced with permission.[10]Copyright 2019, American Chemical Society.

Figure 34 .
Figure 34.a) Dimensions of the 3D printed rigid magnetic hammers (mm) and b) orientation of the microstructure within each hammer.c-h)Time evolution of rigid magnetoresponsive hammers having different orientations of the microstructure (indicated by the arrows) as a function of the applied magnetic field.Reproduced under terms of the CC-BY license.[103]Copyright 2022, The Authors, published by Wiley-VCH GmbH.

Figure 35 .
Figure 35.Fold-deploy experiment results for the printed samples with PUDA-40: a) recovery angle of the printed sample with time in a water bath at different temperatures; b) the recovery process of the sample in an 80 °C water bath;c-e) visual demonstration of shape recovery processes of the printed petal, lamp-chimney and gripper in a water bath or with a dryer.Reproduced with permission.[32]Copyright 2020, Elsevier.

Table 2 .
Examples of poly(urethanes) resin commercially available for vat photopolymerization.