# A Review on Optoelectrokinetics-Based Manipulation and Fabrication of Micro/Nanomaterials
## 5 Fabrication and Assembly of Hydrogel-based Micro/Nanostructures  
Mask-free and non-UV based polymerization and prototyping of high-aspect-ratio 3D hydrogel microstructures, such as poly (ethylene glycol) (PEG)-diacrylate (PEGDA), was demonstrated in an OEK chip [80; 81; 82; 83; 84; 85]. A laser or UV-based method was proposed to obtain PEGDA-based micro/nanostructures [86; 87]. The abovementioned OEK-based mask-free method could meet the same purpose without using any UV light sources or lasers [80]. Figure 14 shows the experimental results with controlled sizes, shapes and thicknesses of PEGDA-based structures. Parallel micro/nanostructures were flexibly patterned onto the chip using optical images. Furthermore, by projecting a series of custom-designed and dynamic optical patterns that served as digital masks, 3D microstructures were fabricated rapidly  
Figure 12: Synthesis of various silver nanostructures by optically-projected patterns in OEK chip. (**a**) Silver crystal nanoparticles. (**b**) Stacked silver hexagonal nanoplates. (**c**) Crystallized silver octahedra and hexagonal silver nanoplates. (**d**) An array of silver nanoparticles-based micropillars (reproduced from Ref. [77]). Figure 13: SEM images of MoS\({}_{2}\) thin-film transistors fabricated with Au (**a**) and Ag (**d**) electrodes; (**b**,**e**) are the enlarged view of the rectangle areas in (**a**,**d**), respectively; (**c**,**f**) are the AFM characterization results of the heights of MoS\({}_{2}\) thin-film transistors (reproduced from Ref. [78]). in a layer-by-layer manner, with their thickness varying from tens of nanometers to hundreds of micrometers [81]. In addition, hollow and circular tubes were fabricated with a length, diameter, wall thickness, and high-aspect-ratio tuned by the exposure time, as shown in Figure 15 [82]. These tubes could manipulate and trap polystyrene beads when they grew longer as the exposure time increased. Then, the PEGDA-based tube continued to elongate and the beads were trapped and moved away from the initial position. Arrays of hydrogel micropillars were also fabricated. These micropillars were capable of serving as micro-scaled cavity molds for casting polydimethylsiloxane, demonstrating potential of finding applications in microfluidics-related fields. The abovementioned results indicate this technology is well-suited for the rapid fabrication of microfluidic chips. Assembly of PEGDA-based microstructures was also realized, followed by an application of bottom-up functional tissue construction and engineering [83]. Arbitrary PEGDA-based microstructures were polymerized in a high-throughput and on-demand manner by a DMD. Then, the fabricated microstructures were transferred into an OEK chip for assembly, which was achieved by using microfluidic flow. Figure 16 illustrates the assembly of PEGDA-based microstructures with different sizes and shapes. Before moving into the OEK chip, the microstructures were either fabricated individually or patterned on a per-array basis. Then, microstructures were aligned into horizontal lines under the action of an ODEP force. Figure 16A-G present the dynamic process of translating microstructures into a single structure for further biomedical and tissue applications. Furthermore, microstructures with the same shape could be assembled into the same layer, as shown in Figure 16H-L.  
Figure 14: SEM images of poly (ethylene glycol) (PEG)-diacrylate (PEGDA)-based micro/nanostructures fabricated using optically-projected spot patterns with different exposure times. The heights in (**a–f**) measured by AFM were 233 nm, 455 nm, 687 nm, 950 nm, 1.34 μm, and 1.62 μm, respectively (reproduced from Ref. [80]). Figure 15: (**a**) PEGDA-based hydrogel tube with a length of \(\sim\)20 μm, an outer diameter of \(\sim\)10 μm, and an inner diameter of \(\sim\)5 μm. (**b**) PEGDA-based hydrogel tube with a length of \(\sim\)35 μm, an outer diameter of \(\sim\)25 μm, and an inner diameter of \(\sim\)20 μm (reproduced from Ref. [82]).