Recent innovations in interfacial strategies for DLP 3D printing process optimization

Abstract

Three-dimensional (3D) printing, also known as additive manufacturing, is capable of transforming computer-aided designs into intricate structures directly and on demand. This technology has garnered significant attention in recent years. Among the various approaches, digital light processing (DLP) 3D printing, which utilizes polymers or prepolymers as the ink, has emerged as the leading new technology, driven by high demand across diverse fields such as customized production, healthcare, education, and art design. DLP 3D printing technology employs cured slices as molding units and is recognized for its potential to achieve both high printing speed and resolution. Recent insights into the DLP printing process highlight its inherent interface transformations between liquid and solid states. This review summarizes key aspects of the printing process, speed, precision, and material diversity optimization, from the view of interfacial interactions between solid and liquid phases which are influenced by resin formation, curing surfaces and light source properties. These interactions include those at the liquid resin–UV pattern interface, the cured structure–curing surface interface, the liquid resin–curing surface interface, and the liquid resin–cured structure interface, each contributing to the unique characteristics of the printed results. Finally, this review addresses the current challenges and limitations of DLP 3D printing, providing valuable insights for future improvements and guiding potential innovations in the field.

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Wider impact

This article highlights recent innovations in digital light processing (DLP) 3D printing technology, with a focus on the involved interfaces and their corresponding regulations. The DLP 3D printing process is inherently cyclic, involving the curing of a thin layer of liquid resin on the curing surface under light irradiation, the separation of the cured structure from the curing interface, and the refilling of the next resin layer. This cycle continuously creates and replaces liquid–liquid, liquid–solid, and solid–solid interfaces, each of which significantly impacts the printing process and the final printed results. Therefore, this review summarizes the latest advancements in regulating key interfaces, such as the liquid resin–UV pattern interface, the cured structure–curing surface interface, the liquid resin–curing surface interface, and the liquid resin–cured structure interface, to optimize the printing speed, precision, process efficiency, and material diversity. Additionally, the challenges and limitations of DLP 3D printing are discussed, providing insights for future improvements and innovations in the field.

1. Introduction

Additive manufacturing, commonly known as 3D printing, has garnered significant attention from researchers and investors alike due to its wide range of processing materials, high dimensional freedom, and minimal waste production.^1–3 As 3D printing technology has matured, it has evolved from merely creating simple 3D models for inspection to fabricating complex structures with intricate geometries. This advancement has broadened its application across various fields impacting both industry and everyday life.^4–9 Unlike traditional manufacturing methods, 3D printing eliminates the need for moulds or dies, positioning itself as a sustainable manufacturing alternative.

3D printing includes various printing formats, as displayed in Fig. 1, such as 3D inkjet printing,^10,11 direct ink writing^12–14 (DIW), fused filament fabrication (FFF),^15,16 selective laser sintering¹⁷ (SLS), multiphoton polymerization^18,19 (2PP), stereolithography^20,21 (SLA), digital light processing^22,23 (DLP), etc., which are based on different printing mechanisms and printing units. Among them, printing formats of 3D inkjet printing, DIW, DLP, SLA and 2PP technologies employ a liquid material that can be solidified based on varied mechanisms as the initial printing material. Meanwhile, FFF technology employs solid filaments and SLS technology employs powder particles as the initial printing material. During printing, the initial printing materials are processed to a liquid state for all printing formats based on the corresponding construction mechanism but with different liquid states as the moulding unit. In detail, inkjet printing, SLS, SLA and 2PP employ voxels which are ink droplets and laser dots, DIW and FFF employ the printing routine-based liquid flow, and DLP employs the slice pattern as the corresponding moulding unit. With corresponding different printing units, different printing formats possess different printing speeds, precisions and application scopes.^24,25 DLP 3D printing avoids the addition of a moulding unit by directly using slices as the moulding unit rather than dots. With a simplified printing procedure, fewer influencing factors are involved during the printing process, making it one of the most potential methods to realize high printing speed and printing precision simultaneously among various printing formats.

Fig. 1 Overview of different 3D printing formats along with the corresponding raw materials and the moulding unit.

In this review, we first introduce two different printing configurations, the printing process and the involved interfaces of DLP 3D printing technologies. In the following sections, we summarize the recent scientific attempts for the printing process, speed, precision and material diversity optimization of DLP 3D printing based on the regulation of the interfaces including the liquid resin–UV pattern interface for printing speed and configuration optimization, the cured structure–curing surface interface for printing continuity optimization, the liquid resin–curing surface interface for the printing process, results and property regulation, and the liquid resin–cured structure interface for printing results and material complexity optimization. These regulations influence the printing process, the apparatus configurations and the morphological and physical properties of 3D printed structures. Finally, we present the future challenges and advancements of the DLP 3D printing methodology.

2. The involved interfaces

DLP 3D printing constructs 3D structures through layer-by-layer stacking of slice patterns. The slice patterns are prepared through layer UV polymerization or curing of liquid resin films under light source illumination. Traditionally, a standard DLP 3D printing system is composed of a UV source for projecting slice patterns, a resin vat for loading liquid resin and for providing the curing surface where UV polymerization mainly occurs, a supporting plate where 3D structures are formed on and the moving platform system for the positioning and motion control of the supporting plate.²⁶ A DLP UV projector employs a microelectron-mechanical system-based display device²⁷ whose main part is the micromirror device that can accept a digital video and transmits to the eye an analog signal that the eye interprets as a colour image in real time. Slice images with a selected portion exposed simultaneously across the entire plane can thus be realized, whose resolution is related to a pixel dimension. Printing precision is thus cooperatively related to the precision of the DLP UV projector, the stepping precision of a moving platform, and the photopolymerization precision of liquid resin under light source illumination.

There are two configurations according to the relative position of the light source and the supporting plate, which includes the top-down^28,29 configuration (Fig. 2(a)) with the UV source located above the supporting plate and the bottom-up³⁰ configuration (Fig. 2(b)) with the UV source located below the supporting plate. The typical process of printing one layer includes three steps. The first step is the UV exposure and curing of one layer between the curing surface and the cured structure. When printing the first layer, it is cured between the supporting plate and the curing surface. The second step is the separation of the cured layer from the curing surface. For the bottom-up configuration, the supporting plate should be lifted upward for separation, while for the top-down configuration, the supporting plate should be lowered down for separation. And, the third step is the refilling of the liquid resin to regenerate a new liquid layer for curing of the next layer. Essentially, the three steps are the generation of the solid–solid interface under UV illumination from the liquid–solid interface (step I), the separation of the solid–solid interface (step II) and the generation of a new liquid–solid interface (step III), which are in fact the alternation of involved solid and liquid interfaces. Therefore, the interfacial control will influence the alternation process of the involved interfaces and further influence the printing procedure and printing results. For both configurations, the printing process occurs between constrained interfaces; for bottom-up it is between the vat bottom and the cured 3D structure and for top-down it is between the top cover and the cured 3D structure. The regulation strategy is versatile for the two configurations, and the bottom-up configuration is mainly introduced in the following descriptions.

Fig. 2 A schematic diagram of the experimental setup of the top-down and bottom-up DLP printing process. (a) A scheme of the top-down DLP 3D printing configuration. (b) A scheme of the bottom-up DLP 3D printing configuration. (c) The involved interfaces during the DLP 3D printing process, which includes the four displayed interfaces among the UV pattern, the liquid resin, the cured structure and the curing surface.

As the three steps are in fact the in situ interfacial curing process at the interfaces among the UV pattern, the cured structure and the curing surface, from the interfacial view, the regulation of the involved interfaces concerning curing will influence the printing process and printing results. For the whole printing process, it involves the interfaces among the UV pattern, the liquid resin, the cured structure and the curing surface, as displayed in Fig. 2(c). Here, we thus mainly summarize the interfacial strategies to regulate the contacting interfaces of solid and liquid phases involving the solid curing surface, the solid cured structure and the liquid resin, which are influenced by the properties of the light source, the liquid resin and the curing surface. With the regulation of different solid–liquid interfaces, the curing or moving motion at the involved interfaces varies accordingly, leading to different printing processes or printed structure properties such as the printing continuity, the printing configuration, the printing efficiency and the printing process influenced functionalization or unique properties of the 3D printed structures.

3. Liquid resin–UV pattern interfaces

Liquid resin will be cured at the interface between the cured structure (the supporting plate for the first layer) and the curing surface under UV illumination (Fig. 3(a)). Therefore, the UV curing process involves the interfaces among the UV pattern, liquid resin and curing surface, where the curing surface here functions as the physical confinement to inhibit over-curing of liquid resin along the z axis. The regulation of the liquid resin–UV pattern interface can be realized through UV pattern regulation and liquid resin composition regulation. In detail, the regulation of the UV pattern can be conducted through the regulation of the UV projecting intensity distribution or the projecting mode based on the traditional UV projector, or through regulating the light path based on UV projector modification. For the regulation of the light intensity distribution without modifying the UV projector, as displayed in Fig. 3(b), it mainly influences the solidification degree difference inside the film structures based on frontal photopolymerization and photopolymerization-induced volume shrinkage, which can then be deformed into 3D morphology after post-treatment or under external stimuli.³¹ Frontal photopolymerization^32,33 is conducted inside a thick layer of liquid resin under UV illumination from one side, which leads to light attenuation and photopolymerization difference towards the non-illumination liquid side with the increase of interaction time. Along with the material volume decrease as a result of covalent bond formation between monomers and cross-linkers during curing,³⁴ nonuniform volume shrinkage is generated along with the moving front with the corresponding stress field and bending tendency. Further combing the light intensity^35,36 pattern complication which enriches manufacturing parameters along the xy direction, bending structures and 3D origami structures can be prepared through systematic printing procedure design.

Fig. 3 The regulation and corresponding optimization of the interface between the UV pattern and the liquid resin. (a) A schematic diagram of the interface between the UV pattern and the liquid resin. (b) The regulation of the UV pattern–liquid resin interface through light intensity distribution regulation along with frontal photopolymerization to realize an external stimuli induced 3D structure construction. (b_I) A schematic process of volume shrinkage-induced bending. (b_II) Optical images of rectangular samples with different irradiation time periods. Reproduced with permission.³¹ Copyright 2017, American Association for the Advancement of Science. (c) The regulation of the interaction mode of the UV pattern with the liquid resin through slicing and projecting mode regulation, in which the employment of the supporting plate can be avoided. (c_I) A scheme of the CAL system. (c_II) A sequential view of the CAL printing process. (c_III) Optical images of samples printed by CAL. Reproduced with permission.³⁷ Copyright 2019, American Association for the Advancement of Science. (d) The regulation of the UV pattern–liquid resin interface from 2D to 3D based on the superposition of patterned optical fields from multiple beams. (d_I) A scheme of the holographic volumetric 3D fabrication system. (d_II) Optical images of printed examples. Reproduced with permission.³⁸ Copyright 2017, American Association for the Advancement of Science. (e) The regulation of the UV pattern–liquid resin interface through introducing a multi-focus scanning apparatus to generate the patterned superposition of patterned optical fields from multiple beams. (e_I) A scheme of the multi-focus system. (e_II) SEM images of printed samples. Reproduced with permission.³⁹ Copyright 2023, Springer Nature. (f) The regulation of the UV pattern–liquid resin interface through cooperative regulation of resin formation and light source regulation. (f_I) A scheme of the printing zone and associated photoinduced reaction pathways. (f_II) SEM images of printed samples. Reproduced with permission.⁴⁰ Copyright 2020, Springer Nature.

Furthermore, regulating the projecting mode of the UV pattern can also influence the interaction mode of the UV pattern with the liquid resin, which regulates the curing mode of liquid resin, and further regulates the construction process of 3D structures. Traditionally, UV patterns are projected unidirectionally along the z axis according to the bottom-up or top-down configuration of the DLP printing apparatus. Furthermore, investigators have found that projecting light patterns rotationally can also realize the construction of 3D structures. Inspired from the mathematical framework of CT reconstruction, computed axial lithography (CAL),^37,41 which is also well-known as volumetric additive manufacturing, based on the rotational slicing mode of 3D models is proposed with the improvement of the slicing manner to avoid the employment of the curing surface, or sometimes without the confinement of the supporting plate with the construction process conducting completely inside the liquid resin, as displayed in Fig. 3(c). The viscosity of the liquid resin should be high enough or an auxiliary structure should exist with attachment to the cured part to maintain the position stability of the 3D structure. As the curing surface in traditional DLP 3D printing can to some extent act as a physical blocking to suppress light diffusion and broadening of polymerization, the printing precision problem originated from the uncontrolled free liquid resin–cured structure interface occurs. Thus, a feedback system, which is an observing camera at a 90° angle relative to the UV illumination direction, is further implemented to increase printing precision through the enhanced control of UV curing or photopolymerization.⁴²

The modification of the UV source and the corresponding light path is another way to regulate the interface between the liquid resin and the UV pattern. The current interaction interface between the UV pattern and the liquid resin is usually two-dimensional (2D), which requires further stacking of 2D slices into 3D structures. With the realization of the contact surface from 2D to 3D, the moulding unit can be 3D through which the construction speed and yield can be significantly increased. Thus, the superposition of patterned optical fields from multiple beams³⁸ is proposed to realize the one-step 3D construction process. Through the projection of multiple beams into bulk liquid resin, 3D structures can be prepared in one-step as displayed in Fig. 3(d), which increases the printing speed and decreases the layering effect. Furthermore, with the realization of the development of light sources that can realize 3D imaging with the 3D energy distribution, rather than the 3D imaging^43–47 that makes 2D images seem like 3D based on the human eyes’ visual characteristics, it is capable to realize one-step volumetric moulding, which increases the manufacture speed and can realize the high throughput 3D construction. While due to the limitation of the patterned optical fields, relatively simple structures are available. Holography is further introduced to the light source system to scale the patterned optical fields and to further improve the printed structure complexity, as displayed in Fig. 3(e). In detail, the light path acted on the liquid resin can be regulated by introducing a multi-focus scanning apparatus that splits a laser beam into multiple points,³⁹ which expands the interaction sites between the UV pattern and the liquid resin. Light can be controlled in the xy plane and the parallel pattern with up to 2000 individually programmable laser foci, and complex 3D structures with 90 nm resolution can be fabricated. Under the prerequisite that the polymerization kinetics is under a low-repetition-rate regenerative laser amplifier, large-scale meta structures and optical devices of up to a centimetre scale can be fabricated, which provides an effective solution for scaling up of the interaction sites of the UV pattern and liquid resin.

Furthermore, in addition to the UV source interaction and configuration regulation, a liquid resin composition can also be regulated to control the interaction of liquid resin with the UV pattern. Traditionally, the liquid resin is composed of monomers/oligomers, photoinitiators, crosslinkers and additives. Among them, monomers/oligomers mainly determine the properties of printed results. To face the urgent need of functionalization, development of new or modified monomers/oligomers and introduction of additives⁴⁸ into the resin system are usually conducted, such as the investigation of 3D printable poly(dimethylsiloxane) based materials with commercial Sylgard-184 properties,^49–51 the development of recyclable liquid resin,^52–55 and the capability to realize unique optical,^56–58 bio/chemical,^59–61 and mechanical^62–65 properties. Photoinitiators function to absorb the energy of a light source during printing and initiate the photopolymerization reactions of monomers/oligomers^66,67 and crosslinkers. Different photoinitiators require different energy sources, which are extended from traditional UV to visible and near-infrared light with the development of photoinitiators.⁶⁸ Meanwhile, for printing process optimization, rather than employing one photoinitiator, the employment of multi-photoinitiators and corresponding multi-light sources has been proposed to increase the influencing factors of DLP 3D printing. Through combining liquid resin formation regulation and UV projection mode regulation, xolography,⁴⁰ which employs liquid resin with photoswitchable photoinitiators along with two light sources, is thus proposed, as displayed in Fig. 3(f). After the introduction of multi-photoinitiators into the rotational slicing and printing system, intersecting light beams of different wavelengths can induce local polymerization inside a confined monomer volume upon linear excitation, through which printing resolution can be increased by ten times. The construction rate of the same 3D structures can be increased to 4–5 orders of magnitude higher than the two-photon photopolymerization technology. Therefore, the regulation of the liquid resin–UV pattern interface can be referred as the regulation of light intensity distribution, light path regulation and liquid resin composition along with UV source modification individually or cooperatively to control the liquid resin curing process under UV illumination, which influences the printing process and leading to the controlled moulding unit, speed and efficiency.

4. Cured structure–curing surface interfaces

After liquid resin being polymerized into the cured structure under the illumination of the UV pattern, the cured structure should leave or separate from the curing surface to make room for the generation of the next liquid layer for layer curing and stacking for 3D construction. For traditional DLP 3D printing, the involved three steps to print one layer of the liquid resin film introduced in the 2nd section are conducted separately and successively because of two reasons. The first reason is that complete separation should be realized for the next layer, while insufficient separation often occurs due to large adhesion between the cured structure and the curing surface, with sticking of the cured structure and the curing surface as the common phenomenon. The second reason is the insufficient resin refilling due to the slow liquid spreading speed between the cured structure and the curing surface. It will slow down the speed to generate the next layer of curing, thereby decreasing the printing speed and leading to defects and failure of the printing process. The regulation of the cured structure–curing surface interface thus mainly influences the separation process of the cured structure from the curing surface and the successive curing of the liquid resin at the curing surface (Fig. 4(a)). The optimized situation is the realization of the simultaneous processing of the three steps, which will in fact realize printing continuity at first and then will also optimize the printing precision.

Fig. 4 The regulation and corresponding optimizations of the interface between the cured structure and the curing surface. (a) A schematic diagram of the interface between the cured structure and the curing surface. (b) The regulation of the cured structure–curing surface interface through an O₂ permeable curing surface which introduces a “dead zone” between the cured structure and the curing surface. Reproduced with permission.⁶⁹ Copyright 2015, The American Association for the Advancement of Science. (c) The regulation of the cured structure–curing surface interface through a lubricant infused curing surface which introduces a slippery layer between the cured structure and the curing surface. Reproduced with permission.⁷⁰ Copyright 2018, the authors of The American Association for the Advancement of Science. (d) The regulation of the cured structure–curing surface interface through a mobile curing surface which introduces a flowing inert liquid layer between the cured structure and the curing surface. Reproduced with permission.⁷¹ Copyright 2019, The American Association for the Advancement of Science. (e) The regulation of the cured structure–curing surface interface through employing a highly deformable hydrogel membrane as the curing surface. Reproduced with permission.⁷² Copyright 2021, Springer Nature. (f) The regulation of the cured structure–curing surface interface through cooperative regulation of resin formation and light source regulation. Reproduced with permission.⁷³ Copyright 2018, the authors of The American Association for the Advancement of Science.

In 2015, research studies propose the continuous liquid interface production (CLIP) strategy through the employment of oxygen permeable films as the curing surface to enhance the cured structure separation process,⁶⁹ as displayed in Fig. 4(b). Typically, oxygen will either quench the photoinitiator during the UV curing process or create peroxides through combining with the free radicals, leading to the incomplete cure of liquid resins and the surface tackiness of printed structures.^74,75 Through introducing the oxygen permeable film as the curing surface, a dead zone that is intrinsically an oxygen-containing uncured liquid resin layer is generated between the cured structure and the curing surface, resulting in an easy separation process of the cured structure from the curing surface and a continuous printing process. With dead zone thicknesses on the order of tens of micrometres, a simple relationship can be described with the liquid resin curing process and printing resolution. While the dead zone is just at the interface where UV curing occurs, the location of the oxygen inside the dead zone should be maintained stable so that diffusion of oxygen or quenched free radicals to the bulk liquid resin can be suppressed; otherwise, defects in 3D structures will inevitably occur, which in turn decrease the printing precision, especially the situation of high-speed continuous printing of microscale structures. Also, the oxygen inhibition is effective with free radical polymerization resins, and whether it influences the composition and stability of the liquid resin should also be monitored for the long-term printing process.

The above regulations can be concluded as to modify the curing surface through introducing a liquid layer on the upper curing surface between the cured structure and the curing surface, which is similar to the function of the slippery surfaces in nature to reduce the separation adhesion and ease the separation process. In detail, insects ‘aquaplane’ on the peristome surface of the pitcher plant due to a water layer.⁷⁶ Inspired from this phenomenon, researchers have proposed a low-adhesive DLP 3D printing process through employing the lubricant infused surface as the curing surface, as displayed in Fig. 4(c), which will not influence the resin properties.⁷⁰ The investigation and preparation of biomimetic slippery surfaces are mature and systematic^77–83 and have been proven to shield the underlying solid substrate from adhesion by other liquids or solids.^84–86 Therefore, using the inert liquid overlayer swelled inside a polymer frame as the curing surface to reduce the sticking phenomenon during 3D printing, the adhesion between the cured structure and the curing surface can be decreased regardless of the liquid resin type. Also, the slippery properties of the curing surface can endow the interface with faster liquid resin spreading properties, leading to a higher liquid resin refilling speed between the cured structure and the curing surface to generate a new liquid resin layer for continuous curing, partially solving the second obstacle of the resin refilling phenomenon and endowing a more general printing process.

Rather than employing a static inert liquid overlayer above the curing surface, high-area rapid printing (HARP) technology that employs a flowing inert liquid layer as the curing surface can also realize a continuous printing process, as displayed in Fig. 4(d), which can at the same time take away the heat generated by photopolymerization and decrease the heat accumulation at the curing surface.⁷¹ As UV polymerization is an exothermic reaction, the temperature at the curing region between the cured structure and the curing surface will gradually increase due to the accumulation of released heat along with curing. With the increase of printing time, heat localizing will further influence the polymerization kinetics and printing stability, which will be even serious during a high-speed continuous printing process. Through introducing a microfluidic chamber inside the curing surface, a floating fluorinated oil layer can be confined above the curing surface. Liquid resin can be floated on a bed of the flowing immiscible liquid layer, on which the interfacial adhesion can be decreased to realize printing continuity. The speed of the mobile liquid interface should be maintained as a laminar flow, which generates a slip boundary between the cured structure and the curing surface. The slip boundary allows the cured structure to be continuously retracted from the curing surface, which can realize continuous printing, decrease the heat accumulation and enlarge the available printing volume.^87,88

In addition to the introduction of an inert liquid layer between the cured structure and the curing surface to ease the separation process, the employment of a deformable curing interface^89,90 is another way to regulate the cured structure–curing surface interface. For the traditional deformable curing surface, printing continuity is hard to be realized due to a much larger separation distance needed to separate the cured structure from the curing surface than the single layer thickness. To address this problem, researchers have proposed and proved that a highly deformable hydrogel curing surface is capable of realizing a continuous printing process,⁷² as displayed in Fig. 4(e). Compared with the traditional deformable curing surface, the deformation and recovery velocity of the hydrogel curing surface along the printing direction are significantly increased, which further reduce the adhesion between the cured structure and the curing surface so that the separation difficulty is compromised. According to the investigation, the hydrogel composition and film thickness will together influence the corresponding separation distance and separation adhesive force, which also determines the hydrogel mechanical properties and printing continuity. With a certain composition and thickness, this deformation is less than single layer curing, and printing continuity can be realized accordingly. Therefore, the regulation of the curing surface mechanical properties and relative mechanical differences between the curing surface and the cured structure can be another regulating way for printing continuity optimization.⁹¹ A cohesive zone model based on the bilinear traction–separation law⁹² is the common theoretical model to theoretically investigate the separation process between the cured structure and the curing surface, and it can also guide the selection of the liquid resin, cured structure and curing surface with suitable physical or chemical properties to optimize the DLP 3D printing process.⁹³

In addition to the regulation of curing surface properties, the adjustment of the liquid resin composition can also lead to the introduction of an inert liquid layer between the cured structure and the curing surface.⁹⁴ Recently, different from the traditional liquid resin which employs a photoinitiator with the employment of a UV projector for real-time polymerization, researchers have developed a resin system composed of multiphotoinitiators with different excitation wavelengths and complementary absorbance spectra,^73,95 as displayed in Fig. 4(f). Along with the employment of multiple light sources including 365 nm UV light and 458 nm visible light, a visible light photoinitiator whose excitation can be inhibited by UV light and the other UV photoinitiator that can be inhibited by visible light are employed. Through controlling the projection time of the UV and visible light sources along with designed slices, an inhibited liquid resin layer can be generated between the cured structure and the curing surface, which eases the separation process and enables the continuous printing process. The regulation of the liquid resin composition can thus expand the applicable liquid resin species, introduce new properties or new regulating parameters for enriched functions of 3D structrues,^96–104 and can regulate the interfacial phenomenon at the interface between the cured structure and the curing surface along with the light source adjustment. Recently, the regulation of initiators can expand the light source from UV to heat,¹⁰⁵ which further expands the raw materials suitable for 3D structure construction. Therefore, current investigations of the cured structure–curing surface interface mainly solve the adhesion problem between the cured structure and the curing surface to ease the cured structure separation process and for printing process optimization.

5. Liquid resin–curing surface interfaces

Along with the separation process of the cured structure from the curing surface, liquid resin should refill the newly generated gap between the cured structure and the curing surface, which then generates a new liquid layer for next layer curing and stacking. As stated above, this insufficient resin refilling due to the slow liquid spreading speed on the curing surface or between the cured structure and the curing surface acts as one of the obstacles to realize the continuous printing process, where insufficient resin refilling will slow down the printing process and lead to defects and failure of the printing process.¹⁰⁶ The regulation of the liquid resin–curing surface interface thus mainly influences the resin refiling speed and tendency, which will influence the printing continuity and further influence the material distribution inside the refilling layer and then the whole 3D structure. The regulation induced resin refilling motion is introduced first, which mainly focuses on the refilling improvement of high viscosity liquid resins to realize continuous printing of composite resin materials.

To realize functionalization, the incorporation of additives inside the liquid resin is a common way which can combine the advantages of different compositions.^107–109 Meanwhile, the liquid resin viscosity inevitably increases and becomes a side effect which increases the processing difficulty. Therefore, the critical lies in the interfacial ways of increasing the mobility of the high viscosity liquid resin at the curing surface or between the cured structure and the curing surface. Though printing continuity can increase the viscosity range due to the printing continuity induced pressure difference and the suction force for resin refilling, further increased resin viscosity will hinder the implementation of continuous printing. Accordingly, researchers have introduced a heating module to the curing surface,⁹³ as displayed in Fig. 5(b), which is similar to the introduction of the heating module around the nozzle in inkjet printing technology and direct ink writing technology to enlarge the viscosity range of ink.^110–114 Interface temperatures can thus be increased, through which the apparent viscosity of liquid resin can be decreased with increased liquid resin fluidity.¹¹⁵ Liquid resin that cannot be continuously printed becomes printable under the function of the heating module. Therefore, strategies that are capable of increasing the fluidity of high viscosity liquid resin can be employed to regulate the interfacial properties of liquid resin around the curing surface and to optimize the printing continuity. Furthermore, acoustophoretic liquefaction¹¹⁶ is employed to enhance the fluidity of high viscosity shear-thinning liquid resin, as displayed in Fig. 5(c). Through introducing a surface transducer to the curing surface, the in situ rheological control of the shear-thinning polymer–particle resin system can be realized. With the increase of shear rate, the apparent viscosity of liquid resin decreases and transforms to higher fluidity under sufficient shear. With the surface transducer operating in the absence of UV illumination, the vibrations simultaneously promote resin recirculation and minimize the adhesion between the cured structure and the curing surface without influencing the curing process.

Fig. 5 The regulation and corresponding optimizations of the interface between the liquid resin and the curing surface. (a) A schematic diagram of the interface between the liquid resin and the curing surface. (b) The regulation of the liquid resin–curing surface interface through introducing a heating module to the curing surface to enhance the liquid resin mobility on the curing surface. Reproduced with permission.⁹³ Copyright 2020, John Wiley and Sons. (c) The regulation of the liquid resin–curing surface interface through introducing a surface transducer to the curing surface to enhance the liquid resin mobility on the curing surface. Reproduced with permission.¹¹⁶ Copyright 2022, John Wiley and Sons. (d) The regulation of the liquid resin–curing surface interface through regulating the refilling properties of additives inside the liquid resin to complete refilling. (d_I) Schematic illustration and optical samples of the ink refilling induced assembly. (d_II) Surface and cross-sectional SEM images of the printed sample. Reproduced with permission.¹¹⁷ Copyright 2022, the authors of Springer Nature. (e) The regulation of the liquid resin–curing surface interface through regulating the refilling properties of additives inside the liquid resin to complete non-refilling. (e_I) A schematic configuration of size-dependent resin refilling induced additive manufacturing based on the continuous DLP 3D printing system. (e_II) The top-angled cross-sectional SEM image of the printed sample with a surface roughness. Reproduced with permission.¹¹⁸ Copyright 2020, the authors of Springer Nature. (f) The regulation of the liquid resin–curing surface interface through employing the polymerization-induced phase separation liquid resin to regulate the material distribution. Reproduced with permission.¹¹⁹ Copyright 2021, Springer Nature. (g) The regulation of the liquid resin–curing surface interface through introducing a magnetic field along the curing surface to regulate the material distribution. (g_I) A scheme of the 3D magnetic printing configuration. (g_II) Schematic of the 3D magnetic printing process. (g_III) Reinforcement micro-architectures printed from 3D magnetic printing. Reproduced with permission.¹²⁰ Copyright 2015, Springer Nature. (h) The regulation of the liquid resin–curing surface interface through introducing an electric field along the curing surface to regulate the material distribution. (h_I) Illustration of the electrically assisted printing configuration. (h_II) SEM images of the printed nacre structure. Reproduced with permission.¹²¹ Copyright 2019, The American Association for the Advancement of Science. (i) The regulation of the liquid resin–curing surface interface through introducing a microfluidic device at the curing surface to increase the material diversity. (i_I) Illustration of the DLP printing platform and workflow. (i_II) Optical images of the printed 2D and 3D samples. Reproduced with permission.¹²² Copyright 2021, John Wiley and Sons. (j) The regulation of the liquid resin–curing surface interface through introducing a receding TCL at the curing surface to increase the material utilization efficiency and printing stability. (j_I) Schematic and optical images of the one-droplet printing process. (j_II) Characterization of printing stability. Reproduced with permission.¹²³ Copyright 2020, the authors of Springer Nature.

With the enhanced liquid resin fluidity of the composite resin system, it has been further found that the moving tendency of the additives inside the composite resin along with the refilling resin flow is an additive property and printing parameter related, which further influences the additive distribution inside the liquid resin or the cured 3D structure. For the influence of additive properties, a particle dimension plays a critical role in the motion of additives after analysing current investigations. Current results can be concluded in two extreme situations including additive complete refilling^124–127 and complete non-refilling along with the printing process. For the first situation, the additives can move along with the refilling resin with the same tendency and can be solidified inside the cured structure, and the additives are often soluble or can be uniformly suspended inside the liquid resin, with the additive dimension much smaller than the interval dimension between the cured structure and the curing surface which can be regulated by the moving speed of the supporting plate. For example, researchers have added a kind of porogen into the liquid resin system, which is miscible with the liquid resin but is immiscible with the cured structure.¹²⁸ The porogen is uniformly distributed inside the cured structure after printing due to the homogeneous moving character of porogen along with the refilling resins. After selectively removing the porogen, 3D structures with homogeneous nanoscale porous structures and superhydrophobic wetting properties can be prepared.¹²⁹ Also, for additives that are not miscible with the liquid resin, it should be uniformly suspended inside the liquid resin system as a prerequisite so that liquid resin can maintain a stable composition during printing. For example, in order to stabilize latex particles with surface carboxyl groups, researchers have designed a water-based hydrogel resin with acrylamide as the monomer,¹¹⁷ as displayed in Fig. 5(d). The amide group of the monomer can form hydrogen bonds with the surface carboxyl groups of the suspended latex particles, which leads to the uniform bonding of monomers on the surface of latex particles, promotes the stable dispersion of the composite liquid resin and realizes the even distribution of latex particles inside the 3D printed structures. Finally, 3D structural colour can be prepared with controlled morphology and angle-dependent outer appearance. Through adjusting the particle diameter and the printing speed, the displayed structural colour can be regulated. In addition, the multi-structural colour with clear boundaries can also be prepared. Furthermore, with high printing precision, a light propagation structure with a smooth sidewall can be prepared, through which the colour, shape and direction of the output light can be controlled, which extends the application scope of DLP 3D printing. The complete refilling and uniform distribution of additives inside the composite resin can also be extended to other additives,^130–134 which realizes the functionalization and further extends the application scope of 3D printing.

The second situation is that particles cannot completely move along with the refilling resin flow, which is left outside the printed structure and is distributed in the liquid resin or on the cured structure surface after printing. Researchers have added micrometre scale sodium citrate particles into the liquid resin system to form the composite resin,¹¹⁸ as displayed in Fig. 5(e). The sodium citrate particles are not capable of refilling along with the liquid resin during the continuous printing process, which can only be solidified on the surface of the 3D printed structure after printing instead. After selectively removing the salt particles cured on the surface, micropores and surface roughness are thus achieved only on the 3D structure surface. The 3D structure with the surface roughness can thus be acquired. Also, through the employment of the liquid resin that is based on polymerization-induced phase separation (PIPS), material distribution control towards the surface of the 3D printed structure can also be realized,¹¹⁹ as displayed in Fig. 5(f). This kind of resin system mainly exploits concomitant changes in the thermodynamics of mixing that occur during UV curing, as well as spatiotemporal variations in monomer to polymer conversion.^135–137 Through balancing the kinetics of polymerization, crosslinking density and diffusion rates of different components, the flux of silver particles is controlled towards the surface of the 3D printed structure, enabling the fabrication of surface conductive silver features, such as dipole antenna array, strain sensors, and objects with antibacterial surfaces. Therefore, through the regulation of liquid resin composition properties, the refilling phenomenon along the interface between the liquid resin and the curing surface can be regulated, resulting in different material distributions and corresponding printing results.

Adding extra fields, including a magnetic field,^138–142 an electric field,^143–148etc. along the interface of liquid resin and the curing surface can also regulate the distribution or arrangement of different components inside the liquid resin and the 3D printed structure. The magnetic field can arrange functional components into a predetermined alignment. 3D magnetic printing is thus proposed through introducing the magnetic field along the curing surface,¹²⁰ as displayed in Fig. 5(g). During printing, each layer of the liquid resin film composed of magnetized alumina particles is aligned through a designed magnetic field before polymerization under UV illumination. The applied field strength, liquid resin viscosity, nanoparticle coverage and the microparticle geometry cooperatively determine the treatment time of the magnetic field. With the successive magnetic alignment and UV curing layer by layer, 3D structures with 3D reinforcement architectures that with enhanced material performance can be 3D printed. Also, through the introduction of the electric field, an electrically assisted 3D printing method is proposed and is proved through using aligned graphene nanoplatelets in the liquid resin to build nacre-inspired hierarchical structures,¹²¹ as displayed in Fig. 5(h). The 3D printed nacre structure is lightweight, which also possesses comparable specific toughness and strength to the natural nacre and can also be employed to sense damage with a hesitated resistance change. Furthermore, acoustophoretic liquefaction introduced before to enhance liquid fluidity can also be employed here to control the interfacial orientation of additives inside the liquid resin and their solidification location.¹⁴⁹ Therefore, introducing the extra field along the interface between the cured structure and the curing surface, the arrangement and distribution of functional components can be controlled uniformly, leading to designed 3D structures and corresponding structure-related properties.

In addition, introducing a microfluidic device at the curing interface can also regulate the liquid resin quantity and species that are involved during printing of a single layer, which can realize multi-material printing. With the advancement of DLP 3D printing, the integration of different materials into a 3D structure is the new investigation trend facing the demand for functionalization.^150,151 Traditionally, for DLP 3D printing, the strategy of sequentially changing resin vat loading with different kinds of liquid resins is employed to change the employed liquid resin for printing the current layer and to realize multi-material printing along the z axis direction,¹⁵² which can be applied in sensors^153–155 and metamerials.^156–158 Meanwhile, the integration of the microfluidic chaotic mixer-linked system at the interface between the liquid resin and the curing surface can avoid the changing of resin vat and is potential to realize multi-material printing along the xy plane, as displayed in Fig. 5(i).¹²² Each layer is prepared through mixing different liquid resins with the chaotic flow and then can be cured under UV illumination. With the fine control of the input amount and number of liquid resins, sophisticated 3D structures, either continuous or discrete, can be fabricated in a precisely designed way. Cell gradients, chemical gradients, mechanical property gradients, and porosity gradients along with multiple gradients are realized through this chaotic mixer-linked printing platform, which extends the structural and material complexity of DLP 3D printing.

Furthermore, rather than regulating the existed interface between the liquid resin and the curing surface, introducing of a new interface, which increases the number of involved interfaces during printing, is another regulating way to influence the printing process. Traditionally, the liquid resin needs to cover the whole tank with an excessive quantity before the printing process. Compared with the structure to be printed, most resin is wasted due to unavoidable adhesion, leading to a low material utilization efficiency. Moreover, the light afterglow in the non-patterned region due to the apparatus limitation of the UV projector, will also be cured after long term utilization, which influences the printing precision and stability. To solve this problem, researchers proposed to introduce a new interface among air, liquid resin and curing surface to this system,¹²³ as displayed in Fig. 5(j). The involved interfaces increase from two to three. With the regulation of the interfacial adhesion, it is capable of employing the droplet morphology as the resin reservoir rather than vat. With the continuous lifting of the supporting plate, the liquid resin is consumed due to the curing process, during which the three-phase contact line (TCL) of the droplet reservoir receded simultaneously.^159–163 It can realize one droplet moulding and increases the material utilization efficiency. The droplet reservoir also endows the system with high freedom and enhances the printing stability under high UV intensity and at high printing speed. Extra curing induced protruding or stepped sidewalls under high printing speed, which requires high UV intensity can be prevented.^87,88 The one droplet printing strategy can realize zero waste, a stable printing process, and real on demand 3D fabrication. Therefore, through the regulation of the liquid resin–curing surface interface based on the regulation of the liquid resin composition, curing surface properties and the introduction of the extra field individually or cooperatively, the printing process and material distribution inside the liquid resin or the 3D printed structure and the related structural properties can be controlled.

6. Liquid resin–cured structure interfaces

The regulation of the liquid resin–cured structure interface can be divided into two situations, such as at the outer structural surface and the curing region around the curing surface. The first kind of interface mainly influences the liquid resin covering process onto the outer surface of the cured structure during printing or after printing during the post-treatment process to regulate the outline of printed structures and the printing procedure of DLP 3D printing, as displayed in Fig. 6(a). A step effect is inevitable due to the layer-by-layer slicing and manufacturing mode of the DLP 3D printing process, which limits the vertical printing precision, surface finish and isotropic mechanical properties^164,165 of 3D printed structures. For vat polymerization, the pull out of the cured structure from vat liquid resin is a necessary step, which is conducted simultaneously with the printing process for the bottom-up configuration while is successive after printing for the top-down configuration. A resin film will thus leave on the cured structure after printing. Traditionally, the adhered liquid resin should be washed off during the post-treatment procedure.^166,167 Recently, researchers have proposed to employ the curing of the covered liquid film after reaching meniscus equilibrium in the post-treatment process to increase the surface smoothness of printed structures,^168,169 as displayed in Fig. 6(b). The meniscus equilibrium post-curing method controls the meniscus formation to provide a transition between the adjacent layers to fill the stepped printed surface. However, the maximum range of the meniscus is constrained by the wetting angle and fails to cover the larger plateaus, leading to limited step elimination. Along with the employment of grayscale polymerization which regulates the interface between the UV pattern and the liquid resin cooperatively, subvoxel-scale precision and deep subwavelength surface roughness can be realized.¹⁷⁰ During printing, the implementation of the grayscale photopolymerization method is within the individual layer curing process, and fine control of the curing depth can be realized to fabricate smooth surface-relief structures. Meanwhile, the liquid film covering process is conducted separately after printing during the post-treatment process, leading to uncontrollable liquid films in the location and distribution, as well as decreased manufacture efficiency. To address this, researchers have found that through cooperatively regulation of the interface between the liquid resin and the curing surface, surface smoothness can be increased simultaneously after printing,¹⁷¹ as displayed in Fig. 6(c). Based on the one droplet printing process with the droplet morphology as the resin reservoir, the liquid film adhered on the cured structure can be regulated through the droplet diameter. Along with the regulation of resin viscosity, the contact lens structure with high surface finish and high optical transmittance can be acquired, which is helpful for the further promotion of 3D printing in optometry.

Fig. 6 The regulation and corresponding optimizations of the interface between the liquid resin and the cured structure. (a) A schematic diagram of the interface between the liquid resin and the cured structure. (b) The regulation of the liquid resin–cured structure interface by employing the liquid film covering after reaching meniscus equilibrium in the post-treatment process to increase the surface smoothness of the printed structure. Reproduced with permission.¹⁶⁹ Copyright 2016, Elsevier. (c) The regulation of the liquid resin–cured structure interface by employing the liquid film covering during the continuous process to increase the surface smoothness of the printed structure. Reproduced with permission.¹⁷¹ Copyright 2021, the authors of John Wiley and Sons. (d) The regulation of the liquid resin–cured structure interface by employing the charge programmable liquid resin to increase the material diversity. Reproduced with permission.¹⁷² Copyright 2020, Springer Nature. (e) The regulation of the liquid resin–cured structure interface by employing the centrifugal supporting plate to increase the boundary precision of the multi-material printed structure. Reproduced with permission.¹⁷³ Copyright 2022, Springer Nature.

For the interfacial regulation of the liquid resin–cured structure interface around the curing region, it can extend DLP 3D printing with multi-material compatible properties without introducing extra devices.^174,175 Traditionally, similar to the increase of the number of nozzles for 3D inkjet printing and direct ink writing,^176–183 increasing the involved number of liquid resins can increase the material range and diversity of 3D structures. However, the boundaries among different materials are hard to control, leading to relatively low boundary precision. Researchers have thus proposed a charge-programmed 3D printing strategy for preparing multi-material electronic devices,¹⁷² as displayed in Fig. 6(d). The importance lies in controlling the surface charge between the liquid resin and the cured structure. Through varying the pendant reactive groups present in the pre-polymer and monomer, liquid resin systems with different charges can be prepared. These reactive groups can be classified as positive (such as ammonium and phosphonium), negative (such as phosphate and carboxylate) or neutral (such as ethylene and ether groups). The charged liquid resin maintains distinct charge polarity after being UV cured into a cured structure. Through carefully selecting the materials for patterning with liquid resin switching, the arrangement of different charge distributions can be patterned in an individual layer on which patterned electrostatic mosaic of distinct positive, negative and neutral distributions can be acquired. Through stacking the patterned substrate based on electrostatic interactions between the current cured layer and the liquid resin of the next liquid layer, designed 3D structures with arbitrary 3D morphologies and 3D distribution of different components can be fabricated in a single step.

Furthermore, the complete removal of each adhered liquid resin is another way to increase the boundary precision. For current multi-material DLP 3D printing, the adhered liquid film on the cured structure is hard to be completely removed during the vat switching process, leading to severe material contamination of different materials, low controllability on the spatial arrangement of each material and low function integration.^184,185 Recently, researchers have proposed to introduce the rotating motor to the supporting plate, through which the centrifugal multi-material 3D printing method is established,¹⁷³ as displayed in Fig. 6(e). During printing of different materials, after a layer of one liquid resin is cured, the supporting plate is lifted up from the resin vat. Then, the rotating motor spins the supporting plate to remove the residual resin adhered on the cured structure until complete removal. Furthermore, the supporting plate falls inside the other vat of liquid resin for the next layer printing. Through this modification, material contamination can be significantly decreased during the multi-material switching process so that each unit composed of different materials can be printed with clear boundaries. Therefore, the regulation of the liquid resin–cured structure interface, printing process, printing results and 3D structural complexity can be controlled.

7. Conclusions

In conclusion, through the regulation of liquid resin, curing surfaces, UV sources and extra field introduction, the involved interfaces during DLP 3D printing can be regulated, leading to the corresponding printing procedure, speed, precision and material diversity optimization. The DLP 3D printing process is an inherently cyclic process involving the cure of a thin layer of liquid resin at the curing surface under the irradiation of the light source, the separation of the cured structure from the curing interface, and the refilling of the next thin layer of resin at the curing interface, which is intrinsically the generation and replacement of liquid–liquid, liquid–solid and solid–solid interfaces. The involved interfaces and corresponding interfacial regulations can thus significantly influence the printing process and the final printed results. The regulation of the involved interfaces can also extend beyond DLP to other 3D printing formats. For instance, the introduction of surface transducers to the interface between the liquid resin and the cured structure of the volumetric 3D printing strategy can facilitate intricate pattern formations within the resin.¹⁸⁶ During the alternation of the involved liquid and solid interfaces, the variation of one component will inevitably influence the other components or interfaces. Therefore, the combination of different involved interfaces together can also lead to combined improvement to optimize the printing process and printing results, such as through the cooperative regulation of the interfaces of the liquid resin–cured structure and the cured structure–curing surface, high material utilization efficiency and surface smoothness can be realized together. Furthermore, to increase the material and structural complexity of the moulding unit, the development of a stereoscopic light source which can project the 3D image with 3D energy distribution and the multi-photo responsive resin formation regulation along with the adjustment of UV source tuning can be combined, through which one step 3D printing of multi-material structures can be realized. Finally, the printability expansion of high-performance materials with specific properties^187–193 will further expand the processing and application field of 3D printing.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We acknowledge funding from the National Natural Science Foundation of China (grant no. 52173051), the Young Elite Scientist Sponsorship Program by CAST (2022QNRC001), and the Youth Innovation Promotion Association (2021031).

Notes and references

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