An Investigation of Integrated Multiscale Three-Dimensional Printing for Hierarchical Structures Fabrication

Many material systems in nature exhibit outstanding properties not found in artificial systems. The exceptional performance of natural materials, such as the structural color of butterfly, the drag force reduction of fish scales, the superhydrophobic effect of Salvinia Paradox, and water collection of desert beetles, benefit from hierarchical structures over a large range of scales from macroscale to nanoscale [1,2]. Such multiscale structures in nature inspire composite material design and show promising multifunctional applications in mechanical, optical, thermal, and electrical fields [2–4]. Meanwhile, structures found in nature possess comprehensive complexity in geometry, hierarchy, and material, setting challenges over the fabrication of such structures with traditional manufacturing approaches. Additive manufacturing (AM), as a solution, can fabricate objects with complex shapes by depositing material layer by layer [5,6]. Due to such unique fabrication capability, recent advances in AM technology have shown progressive achievements in a wide variety of areas, including the fabrication of bio-inspired structures and materials [7,8]. However, most AM technologies have the capability of fabricating three-dimensional (3D) structures at a single scale. For example, selective laser sintering/melting (SLS/M) is used to fabricate objects with the geometric features at the macro- or meso- scale using metal, ceramic and plastic powders [9]. Similarly, fused deposition modeling (FDM) is developed to build mesoscale objects by the extrusion of constant filament [9]. Besides, both inkjet printing and polyjet printing are used to fabricate objects with features limited to microscale using polymer or hydrogel [9].

Recently, a few 3D printing approaches were developed in an attempt to achieve manufacturing at multiple scales [10]. For example, the multiscale biomimetic blood vessel was printed by injecting biocompatible material into revisable gel through a microscale nozzle head [11–13]. Biomimic multiscale shark skin was designed and fabricated to achieve high hydrodynamic performance, using a microscale laser-based printing process [14]. To increase the fabrication resolution and efficiency of the laser-based stereolithography process, Mao, etc. Proposed an optimized multiscale printing process by integrating multidimensional shaped laser beam and selecting the laser beam with appropriate dimensions based on the fabrication requirements [15]. Also, Zhou et al. Proposed a stereolithography (SL) based multiscale manufacturing approach by integrating hybrid light sources including laser and digital projection light [16]. Similarly, a hybrid-light-source SL process was developed to fabricate macroscale objects with microscale textures [17]. Besides, advanced multimaterials devices in micro-and nanoscales can be printed by electrohydrodynamic printing, although the fabrication time will be significantly longer due to the printing principle [18].

Overall, most of the current AM fabrication procedures for the printing of multiscale hierarchical structures are associated with experimental complexity and limited flexibility, especially when the desired object has an overall dimension at the macroscale level yet with micro- and nanoscales features in detail. Therefore, the development of multiscale AM processes with the fabrication capability ranging from macroscale to nanoscale is highly valuable and necessary for the fabrication of bio-inspired devices for functional applications in different engineering fields. In general, each type of AM process is considered to have its most appropriate printing scope, in which the optimized resolution is determined by its intrinsic printing principle. For example, the overall dimensions of the structures in macroscale fabrication are from centimeters to millimeters with feature sizes of the order of 1000 s of microns, and the smallest feature size is hundreds of microns. While the fabrication dimension of the microscale process is 10 times smaller than the one in macroscale fabrication, and the resolution of the microscale process can be tens of microns. In nanoscale fabrication, the printing capability is from hundreds of microns to submicron. It is desired to fabricate features at different scales using the 3D printing processes whose scope best suits the purposive fabrication [19–21]. For example, based on the natural architecture of gecko's feet where macro, meso, micro, and nanofeatures are presented [22], the fabrication process of the design should be synchronized as the features scale down, in a hierarchical manner that macro- and meso- structures being built by the macrolevel process, followed by the fabrication of micro-and nanoscale features through each of their scale-matching processes (Fig. 1(a)).

The idea of integrating several scale-exclusive 3D printing methods is intuitive; however, it has put forward two major demands for the process development: first is to allocate each subprocess at the best of their advantages, and second is to investigate how to integrate these processes seamlessly. Based on our previous work, the mask image projection-based stereolithography (MIP-SL) process is highly efficient in building objects with features that are at macro- and meso- scales [23–29]. In MIP-SL, the illuminated light is reflected by the Digital Micromirrors Device (DMD), where the light intensity can be changed based on the angle of the micromirror in the DMD [23]. After going through optical lenses, the focusing image irradiates at the resin surface; accordingly, the resin will be cured after a suitable exposure to light (Fig. 1(b)). Since the DMD has millions of mirrors and each mirror represents one pixel in the focusing image, the projected mask image can achieve high-resolution light exposure [23]. Using different optical lenses designs, the mask images can achieve different projection sizes, ranging from several millimeters to several hundreds of millimeters [23–27]. Based on optimized pixel blending, the accuracy and surface finish of macroscale features printed by MIP-SL were improved dramatically [23,24]. A macroscale part with mesoscale features can be printed within minutes instead of hours that are typically required by using either two-way movements or mask video projection [25,26]. Besides, different types of materials including hydrogel, nanocomposite, and polymer, can be fabricated to construct a solid object with desired material distributions using MIP-SLA [29–31]. Compared with other 3D printing processes, the MIP-SL process is highly efficient in building objects with features that are in macro- and meso- scales, since each layer of material can be cured by one-time exposure using a high resolution two-dimensional (2D) light beam [23–25]. However, when dealing with microscale and even submicron features, the MIP-SL shows its drawbacks. The fabricated features tend to be less sharp due to the limited pixel resolution and the light accumulation effect among adjacent pixels that may distort the desired shape [28]. Considering the above pros and cons, we have selected MIP-SL as a suitable candidate for the fabrication of macroscale and mesoscale features [23–27].

In terms of nanofabrication, two photon polymerization (TPP) is one type of maskless lithography that enables the fabrication of submicron structures with features smaller than 100 nm using photocurable polymer [32–36]. Since the TPP was developed based on the laser writing process, and the photosensitive material is not accumulated in a layer-based manner, there are no topological constraints of geometrical shape that can be printed by TPP [33,34]. Moreover, the crosslink of photocurable polymer is caused by the nonlinear optical light absorption of multiple photons in TPP, hence TPP is capable of fabricating microscale and nanoscale structures with sharp features, which is impossible to be fabricated by using most AM approaches [34]. Due to its special fabrication capability, TPP is extensively used in the fabrication of 3D micro-and nanoscales structures. However, TPP is not ideal for building macro- and meso- scales structures, since there are several challenges when using TPP in making structures on such large scales. First, it is very time-consuming to fabricate macro- and meso- scales structures by TPP with the nanoscale size laser beam, since the laser beam needs to scan the entire volume to construct the 3D object. It took a couple of hours to fabricate 2 mm mesh-shaped structures with a thickness of 500 μm, which only cost 5 min to fabricate by using other microscale printing approaches [37]. Second, the flatness of the object is a critical factor in the TPP process during the fabrication process. Because the laser beam should be focused at the same horizontal position, the macro- and meso- scales fabrication increases the difficulty of the perfect flatness of the printed object [37]. Finally, material cracking may happen due to the shrinkage of the material during/after fabrication [36]. Considering the strength and drawbacks, TPP is an appropriate method to fabricate nanoscale features at high resolution and accuracy [30–34].

To fabricate geometric features in macro-, micro-, and nano- scales, a critical bridge that can connect MIP-SL with TPP is necessary to integrate. Immersed surface accumulation (ISA) process is developed to build microscale features on the surface of pre-existing macro- and meso- scales objects [37–39]. Due to the unique light guide tool, the ISA process shows flexibility in building microscale features with complex geometric shapes on the surface of a large-scale object, and both inside and outside surfaces can be decorated with desired microscale structures [37]. The above key features enable ISA to be a promising microscale solution to integrate with MIP-SL and TPP processes for the fabrication of multiscale structures. Hence, in the present research work, we investigated a 3D hierarchical printing approach with a focus on integrating the MIP-SL and TPP processes with the ISA process, for the fabrication of an object with multiscale structures with a dimensional span ranging from macroscale to nanoscale (Fig. 1). To achieve the multiscale fabrication, we design the hardware for integration, developed the vision-assisted alignment for integration, identified the design strategy for the transition between multiple processes, investigated the material fabrication, and conducted process planning capability for multiscale fabrication. Based on our proposed hierarchical printing approach, we demonstrated the potential applications in the fabrication of the multiscale biocompatible scaffold and the bio-inspired optical filter. Furthermore, considering the efficient fabrication strategy of our method, the proposed hierarchical printing is potentially useful in the future development of biomimetic functional devices with multiscale hierarchical structures [2,7].

In order to integrate three different printing process, a key problem is how to build new features on the prefabricated objects from other processes. For a macroscale object by MIP-SL, both ISA and TPP processes have the flexibility of building around inserts so that microscale and nanoscale features can be deposited on the object. Another key problem to be addressed is how to avoid possible misalignment issues between different AM processes. Accordingly, we added a vision system in both ISA and TPP processes to closely monitor the fabrication process and to provide the alignment of structures that are fabricated on different scales. In the following section, the hardware design and implantation of vision-assisted alignment for the integration of the MIP-SL, ISA, and TPP processes will be discussed in detail.

The ISA process with a bottom-up frame was developed to fabricate 3D microscale structures with an emphasis on building around insert [37]. In ISA, a light guide tool with the DMD-based imaging system was used as the accumulation tool, which was immersed inside the liquid resin to build microscale features on a prefabricated object by continuously projecting the 2D patterned light beam from bottom. To generate the 2D patterned light beam, the ultraviolet (UV) light reflected by the DMD chip went through the collimating lens and finally focused on the end surface of the light guide tool by the 4x objective lens. Since the DMD chip with millions of micromirrors can generate complex and diverse 2D patterned light beams, ISA has the capability of fabricating microscale structures with complex geometric shapes [37,38]. In order to integrate ISA with MIP-SL, in situ visualization module was designed to detect the MIP-SL built object and determine the relative location of the light beam with respect to the MIP-SL built geometry (Fig. 2(a)). The in situ vision system contained a beam splitter, a focusing convex lens, and a complementary metal-oxide-semiconductor (CMOS) camera (Fig. 2(a)). The light beam illuminating the prebuilt object traveled back through the objective lens and beam-splitter and was focused by a convex lens. To observe the image of the MIP-SL built object and the focused light beam, a CMOS camera was setup behind the focusing convex lens, such that the focused light can be captured by the CMOS camera, and a clear image of the fabrication region was obtained. The size of the view field of CMOS camera is 10 mm x 10 mm and the resolution of the image is 1280 × 720. Besides, the light-emitting diode (LED) emits visible light to illuminate the previously MIP-SL built part (Fig. 2(a)). The captured detection image was displayed on a computer monitor with 50× magnifications. Since the light should be able to penetrate through the prebuilt part where the nanoscale features will be printed using TPP process, a glass slide instead of an aluminum block was used as the platform, on which the part was printed using MIP-SL, ISA, and TPP. A pair of magnetic fixed blocks were used to mount the glass slide in both MIP-SL and ISA, which facilitates the transition between multiple processes.

The experimental setup of TPP with top-down frame is shown in Fig. 2(b). In TPP, the near-infrared light is applied to trigger the photopolymerization of the liquid resin, where the phase of photosensitive polymer is changed from liquid to solid [34]. To achieve multiphoton absorption, ultrashort and fast pulsed laser and high numerical aperture lenses are used in TPP. The polymerization of photosensitive resin is only excited by the absorption of two or more photons, and the photopolymerization of polymer only occurs at the place where the light intensity of the laser beam reaches the critical value, which is determined by the photosensitivity of the resin [34]. Compared with single-photon absorption, like MIP-SL or ISA, TPP requires much higher energy to solidify the material [34]. The exposure dose of the focused laser beam, which is proportional to the square of the laser intensity, can be modulated by controlling the power of the laser beam, and this nonlinear light intensity modulation allows the fabrication of 3D features that are smaller than the wavelength of light [34]. The dimension of the laser beam, of which the wavelength is 800 nm, can be controlled as small as 100 nm to fabricate nanoscale features [34]. To integrate the ISA and TPP, we developed an in situ monitoring module including a beam splitter, a focusing convex lens, visible LED, collimating lens, charge-coupled device (CCD) monochrome microscope camera. The fabrication area was illumined by the collimated visible light emitted from LED, and the illumination light didn't initiate the photopolymerization of material which only be activated after the absorption of two photons under the exposure of 780 nm laser. The light from the fabrication area will be reflected by the beam splitter and focused on the CCD camera with the resolution of 1920 x 1080. Using the designed in situ monitoring module and the accordingly obtained image, the relative location of the prebuild object and fabrication plane can be determined. Besides, the whole fabrication process can be monitored to improve the printing quality.

When multiple processes were being integrated, it was necessary to determine the relative location of the light beam to the previously built geometry. To address the issue, optical vision assisted in situ modules were designed and added to both microscale ISA and nanoscale TPP prototype machines, allowing one to observe the location of the curing light and the prebuilt object for the alignment. In terms of the initial position in X/Y plane, assume the center portion of the light beam (1920 × 1080) was used to build microfeatures. The movements of the stage in the X/Y directions were required for the alignment of the light beam and the prebuilt object. The X and Y stages moved both the tank and the prebuilt object in the corresponding direction. The target of the alignment of different processes was to get the relative position of the curing light and the prebuilt object. With the hardware designs of in situ visualization modules in ISA and YPP, the vision-assisted alignment coordinating the prebuilt objects with different projection lights is used to integrate multiple processes, as presented in Fig. 3(b).

The specific alignment process has the following steps.

1. Step 1: project a full-size rectangular light beam. Then mark four corners P₀ ∼ P₃ of this projection image in the alignment imaging system, and calculate the resolution of each pixel of the captured image in the alignment imaging system based on the real size of the light beam.

2. Step 2: mount the pre-built object and move the X and Y stages to allow four corners of the pre-built object to be captured inside the alignment imaging system. Mark four corners M₀ ∼ M₄ of the pre-built object in the alignment imaging system.

3. Step 3: move the X and Y stages with corresponding distances $X′$ and $Y′$ that were calculated based on Eq. 1 to make the center point C of the fabrication region closer to the center point P of the projection image.

4. Step 4: Repeat above process to reduce the alignment error. Before the printing process, project the fabrication light beam on the fabrication region of prebuilt object, where the printed feature with smaller scale will be attached to, and capture the image to further improve the alignment precision.

After calibrating the relevant position of projection beam and prebuilt object in X/Y plane, the photocurable resin was added to the printing area and the relative position of the light guide tool and the prebuilt object in the printing direction was determined by the color level of resin in the captured image in ISA (Fig. 3(c)) [37]. Based on the captured images and the platform mounted with the glass slide with prebuilt object was moved to the initial position in the Z direction for the fabrication of microscale structures [27]. Similarly, TPP system had a vision-assisted alignment process to observe the prebuilt microscale object. Before the fabrication of submicron features on the prebuilt object, the calibration of the relative position between the focused printing laser and prebuilt object in the TPP process was the same as the aforementioned calibration process between the macroscale MIP-SL and the microscale ISA. By adjusting the position of the glass slide in the Z direction using Z linear actuator, the fabrication region, where the submicron features were to be printed, was focused with the help of the vision-assisted system. At the same time, the printing laser was focused on the same X/Y plane, so that the two-photon cured features were attached on the surface of the prebuilt microscale object. Based on the developed vision-assisted system, the MIP-SL, ISA, and TPP setups can be seamlessly integrated to fabricate various multiscale hierarchical structures ranging from macroscale to nanoscale.

During the transition between consecutive processes, e.g., fabricating subsequent features at the same layer of the prebuilt object, the attachment between different scales of features at the transition region is a critical challenge. Particularly, for the overhanging feature with no solid support from the structure built by the previous larger scale AM process, the newly cured feature only attaches to the sidewall of the prebuilt structure with a limited adherence. Without any support structures from the bottom layer of the prebuilt object, the newly cured feature maintains its shape by the bending strength at the endpoint of each side. The bending force of these features is minor due to the limited contact area A and the tensile stress of the material $στ$ so that the deformation caused by the material self-gravity load results in the damage of microscale structure when the new features are written on it. To solve the problem, the area under the overhanging feature should first be printed with a certain amount of transparent polymer that may reinforce underneath for the microscale features to attach (Fig. 4). Compared with original contact area A, the deformation will be largely eliminated with straight support from the layer of material underneath, and the micro-and nanoscales features can be further built on the top surface of the transparent polymer (Fig. 4(b)).

Due to the requirement of light penetration for the subsequent process, the material used for building the support should be transparent. Besides, the material used for building micro-and nanostructures on top of the support needs to be transparent to both visible and near-infrared photons. This is because the TPP process in this experiment used an up-right system, where transmission light microscopy was employed to identify predetermined locations (Fig. 4(a)). The light penetrable photocurable polymers, such as PEGDA and E-glass, were therefore chosen to fabricate the support to enable the micro/nanostructures to be fabricated on top of the macroscale object. The transparency of material is also critical in helping the alignment process of the fabrication region in the integration of two different processes. Meanwhile, a fabrication glass slide was used as the building platform in the integrated MIP-SL, ISA, and TPP processes for its easy transition and superior transparency. We first cured layers of transparent material at the thickness of 500 μm in total as the base that can prevent thin layers of cured polymer from slipping off the smooth surface of the glass slide. Furthermore, the microscale spacers with a height of 150 μm were designed on the multiscale structures and further printed using the microscale ISA process so that the printing resin doesn't need to cover the whole prebuilt object in nanoscale TPP printing system, which avoid significant material waste. With the help of ISA printed microspacers, the liquid bridge of resin was generated between the cover glass slide and the fabrication area of the prebuild object, providing enough resin for the fabrication of nanoscale features (Fig. 4(a)). Besides, the spacers provide a potential feeler gauge and help in positioning the focused laser spot on the fabrication area of prebuilt object in Z direction.

In the photopolymerization-based MIP-SL and ISA processes, the photocurable polymer is accumulated into 3D structures layer-by-layer after the initiation by the absorption of UV light. The vertical printing resolutions of MIP-SL and ISA processes are determined by the depth that light can penetrate in the photopolymerizable medium. To construct a multiscale 3D object using the proposed integrated process, the penetration of the projected light beam should be sufficiently deep to ensure the newly cured layer can be attached to the surface of the prebuilt object. However, if the light penetrates de-eply in the photocurable resin medium, it may not only solidify the next layer of the object at the surface but also results in unexpected polymerizations in previously polymerized layers of prebuilt object [40]. The cure depth of photopolymer $Cd$ is used to quantitatively represent the degree of the difficulty of light penetration [41]. In microscale fabrication, e.g., ISA, where the layer thickness only spans from 10$μ$m to 50$μ$m. Due to the microlevel geometry, the material with a high range of light penetration is prone to have over cure at the built regions. Also, the cure depth is sensitive to alter as it changes dramatically with the modification of the exposure time [42]. Therefore, the material curing performance of photocurable polymer should be studied in order to achieve integration of microscale ISA and macroscale MIP-SL. For the printing process of the microscale ISA and macroscale MIP-SL system, the layer thickness usually was 10–50 μm and 75–150 μm, respectively [24,37]. In consideration of solid attachment between the adjacent layers and the surface quality of the 3D printed part, the layer thickness is set to be half of the cure depth of the material in the macroscale fabrication [24,42]. To avoid the over cure, the cure depth of acrylate-based material was studied in this work.

where $α$ and $β$ is a constant for acrylate-based photochemical system, which is $1.38×10−5$ and $9.06×103$ respectively for E-glass resin; $γ$ is absorption coefficient of light absorber (317.46) when the concentrations of light-absorbing dye $Pd$ and photo-initiator $PI$ are 0.5% and 1% , respectively. In ISA process, the photocurable polymer solution was developed with the values of $PI$ and $Pd$ tuned for desired cure depth $Cd$ in order to tightly attach the surface of the prebuilt object.

When the part is fabricated using a traditional single photopolymerization-based printing process, the digital model will be sliced into a set of 2D layers to generate the projection images or tool path [1]. Since the proposed hierarchical 3D printing by integrating TPP, ISA, and MIP-SL can be controlled independently, separate geometry information at each scale was generated for the corresponding process. A sequential printing process coordinating macro-and meso scales 2D light beams and nanoscale laser beams with different printing resolutions was therefore developed. After being given an input 3D model, features, of which the cross-sectional area is bigger than 1 mm², were sliced to get the mask image and were further printed using the MIP-SL process. After that, microscale features with the area larger than 9 × 10⁻⁴ mm² were sliced and fabricated on the surface of prebuilt macroscale structures by using ISA. After the printing of macro-and microscales structures using MIP-SL and ISA, the printed object was fully cleaned using ethyl alcohol inside the ultrasonic bath for 30mins to remove the unpolymerized resin. Afterward, the printed object was thoroughly dried in a room environment to evaporate the remaining ethyl alcohol. Meanwhile, the submicron features were sliced into a set of voxels, of which the width d and height h were 500 nm and 500 nm, respectively. And then nanoscale features were printed in a voxel-by-voxel manner on the surface of the prebuilt microstructures. Finally, the cleaning of the final part after the printing of TPP was conducted. The sample was immersed in an ethanol bath for 20 min to wash away the residual liquid resin.

A mask planning testbed has been developed using the C++ language with the Microsoft Visual C++ compiler. The testbed integrated modules of geometry slicing, mask image generation, mask image projection, and motion controlling. To achieve multiscale fabrication, the testbed also synchronized the integration process based on the X/Y/Z movements. In addition, a matlab program including imaging processing and object detecting was developed for the vision alignment in the ISA process. The testbed including 2D and 3D digital data import, coordinate transform, and motion control was developed based labview platform for the TPP process. Based on the proposed method, the fabricated results of two hierarchical structures by the integrated multiscale 3D printing process are shown in Figs. 5 and 6, respectively.

Figure 5 demonstrated a bioscaffold with multiscale structures. Fig. 5(a) shows the mesoscale structure printed by the MIP-SL process using resin SI500 (EnvisionTEC US LLC, Dearborn, MI). It contained five inner holes with different shapes (pentagon, circular, Hexagon, rectangle, and heart shape), of which the longest diameter was 2 mm, and the thickness of the printed mesoscale structure was 1 mm. Then the ISA process was used to print microscale scaffold mesh matrix in the same layer of the MIP printed mesoscale structure with biocompatible PEGDA solution (Fig. 5(b)). Each microhole fabricated by ISA was 100 μm in diameter as shown in Fig. 5(c). The PEGDA-based biocompatible hydrogel solution was made by 60 wt% PEGDA (Mw 700, purchased from Sigma-Aldirich, Envision TEC Inc., Dearborn, MI). The UV light photo-initiator (Irgacure 819, BASF) was used at a concentration of 1 wt% to induce chain polymerization by the free radicals. To prepare the resin, the photo-initiator was first fully dissolved in the phosphate buffered saline (PBS); we then added 60 wt% PEGDA into the PBS solution. To uniformly mix the solution, the magnetic stirrer was used for global agitation for 2 h and the mixture was put into the ultrasonic bath with a power of 700 W, frequency of 20 kHz, for 20 min.

Finally, two different shaped submicron structures were printed using the TPP. The SEM images of the micro- and nano- scales features fabricated by the TPP process are shown in Figs. 5(d)–5(i). Figs. 5(d) and 5(e) show the SEM images of the submicron hexagon line printed on the surface of the PEGDA microscale mesh matrix. Another 3D submicron log-pile structure printed by the TPP on the surface of the microscale mesh matrix built by the ISA is shown in Figs. 5(f) and 5(g). Figs. 5(h) and 5(i) show the top view and side view of the log-pile structure printed by TPP. Such hierarchical multiscale structures have multiple potential applications for cell culture and drug screening [45,46].

In nature, some creatures show the dazzling color that draws much attention. For example, butterfly's iridescent wings, fish's glaring scales, and peacock's colorful tail features [1]. These splendid colors are produced by the optical interaction of light with nanoscale structures [46–49]. Such structural color has inspired the innovations of artificial optical systems [47]. In this test, an optical filter with nanoscale structures whose dimension is close to the wavelength of light was designed. In addition, the macroscale assembly feature was designed with a tapper hole to enable the insert and removal of an optical fiber whose size is 800 μm (Fig. 6(a)). The optical device inspired by the butterfly hierarchical structure was successfully fabricated by the developed hierarchical printing method (Figs. 6(d)–6(f)).

The mesoscale optical fiber holder with the gradually decreasing microscale inner hole was fabricated by MIP-SL process with the E-Glass 3SP resin mixed with multiwall carbon nanotubes (MWCNTs). The total height of the cylindrical holder was 1.8 mm and the Z stage moved up 100 μm/layer to maintain the same thickness of the newly refilling material. The radiation of each pixel in the 2D patterned light beam implied a Gaussian intensity profile [23]. We applied a nonlinear exposure time set to maintain the printing accuracy by adjusting the exposure time based on exposure area. Consequently, the optimal exposure time of the first layer and the last layer was set as 15 s and 8 s, respectively, and the time for the interim layers was obtained through interpolation in a nonlinear manner. The desired tapered inner hole was successfully built without over-cured features which are caused by the energy accumulation, and the nonlinear exposure planning significantly enhanced the finishing quality of the part.

Specifically, the digital model of micro-and nanoscales optical filter is shown in Fig. 6(b). Using the 2D light beam projected from the calibrated mask image shown in Fig. 6(c), microscale filter support (Fig. 6(d)) was printed by ISA process with transparent E-Glass 3SP resin (EnvisionTEC Inc., Sigma Aldirich, St. Louis, MI) on the top surface of mesoscale optical fiber holder, which is designed with decreasing inner holes to ensure sufficient light going through during the TPP printing. As shown in Fig. 6(e), the microscale ISA processes were used to build the part and the layer thicknesses were set as 20 μm, respectively. The nanoscale features were built using a labview application capable of importing 2D and 3D digital files and transforming them into polyline coordinates, which will be read by the stacked XYZ stages used for the motion of the TPP process. The submicron optical filter was printed on the top surface of the hole of microscale filter support using acrylic-based resin (Fig. 6(f)).

The material systems in nature with exceptional performances benefit from hierarchical structures spanning over a large range of scales from macroscale to nanoscale. Such structures are usually associated with multiple scales of features with a large minimum to maximum ratio, which presents a significant challenge to existing manufacturing methods [2,9,49]. The aim of this research work is to investigate the integration of different photopolymerization-based AM processes ranging from macroscale to nanoscale to demonstrate the capability of fabricating multiscale functional material and hierarchical structures, in the hope to achieve numerous novel applications in different engineering fields. In this paper, a multiscale 3D printing process that integrates the macroscale MIP-SL, the microscale ISA, and the nanoscale TPP processes have been presented. The integration effort has been made for the fabrication of 3D objects with multiscale features. Taking advantage of the vision-assisted system, the sequential fabrication using the MIP-SL, ISA, and TPP systems can be integrated with satisfactory accuracy and resolution. Both hardware and software systems have been constructed to verify the developed multiscale fabrication approach. The experimental results based on two test cases demonstrated the effectiveness and feasibility of this integrated printing process in fabricating various multimaterial and multiscale hierarchical structures. This could open up novel hierarchical structure designs for various engineering fields in the future [50].

Authors acknowledge the Centre for Electron Microscopy and Micro-analysis (CEMMA) at USC for the use of microscopic measuring equipment.

• National Science Foundation (NSF) Division of Civil, Mechanical and Manufacturing Innovation (Grant Nos. 1151191 and 2114119; Funder ID: 10.13039/100000147).

A =

contact area

AM =

additive manufacturing

C =

the coordinates C₀ ∼ C₃ of four corners of the region

$Cd$ =

cure depth of material

CCD =

charge-coupled device

CMOS =

complementary metal-oxide-semiconductor

$Dp$ =

penetration depth of the exposure light

DMD =

digital micro-mirrors device

$Ec$ =

critical exposure of photocurable material

FDM =

fused deposition modeling

$Imax$ =

maximum light intensity

ISA =

immersed surface accumulation

LED =

light-emitting diode

M =

the coordinates of corners of the pre-built object

MIP-SL =

mask image projection-based stereolithography

MWCNTs =

multiwall carbon nanotubes

P =

the coordinates of corners of the calibrated projection light beam

$PI$ =

concentration of light photoinitiator

$Pd$ =

concentration of light absorber

PBS =

phosphate buffered saline

PEGDA =

poly(ethylene glycol) diacrylate

SEM =

scanning electron microscope (SEM) image

SL =

stereolithography

SLS/M =

selective laser sintering /melting (SLS/M)

TPP =

two-photon polymerization

UV =

ultraviolet

$T$ =

exposure time

$X′$ =

corresponding distance in X direction

$Y′$ =

corresponding distance in X direction

2D =

two-dimensional

3D =

three-dimensional

$γ$ =

absorption coefficient of light absorber

$στ$ =

the tensile stress of the material

$αβ$ =

a constant for acrylate based photo-chemical system