# A Review on Optoelectrokinetics-Based Manipulation and Fabrication of Micro/Nanomaterials
## 6 Conclusions and Prospects  
As discussed above, the OEK technique has been widely used by the microfluidic community to make a list of research achievements in terms of the manipulation and fabrication of micro/nanomaterials Although some of these achievements are also accomplished by other competing techniques, OEK is superior to the others in that it consumes ultra-low power and offers a programmable, flexible and versatile approach for parallel and multi-scaled micromanipulation. OEK has also been found to be capable of detecting cellular properties and statuses and measuring drug concentration in a label-free and dynamic manner, without relying on any other techniques [88; 89; 90; 91; 92; 93; 94]. These advantages suggest that OEK is well-positioned to promote the use of microfluidic tools for micro/nanomanipulation. Nonetheless, challenges remain for OEK to transform from a lab-based technique to one that is widely useful in practical applications. Here is a summary of these challenges. Firstly, the 3D manipulation mechanism of OEK should be further explored. Early studies on the OEK technique were mostly focused on 2D manipulation schemes, which proved to be more effective than other competing techniques. With the evolvement of OEK chips, however, researchers have realized the necessity of shifting their focus to 3D manipulation. One example of this shift in focus is a two-layer a-Si:H based OEK chip, in which the top ITO glass is replaced with a-Si:H to enable spatial and adsorption-free manipulation of beads using a negative ODEP force [95]. However, this OEK chip cannot manipulate micro/nanoparticles using a positive ODEP force and requires using other OEK forces. Besides, the manipulation is liable to be affected by dynamic fluid flow and electrothermal motion, which is because the manipulated micro/nanoparticles tend to be located in the center of the chip. The poor light transmission can significantly compromise the quality of observation. A lateral-field OEK device was developed to enable parallel single-cell manipulation, which can also assemble nanowires and integrate with other microfluidic components [96]. Nevertheless, the fabrication process is complicated and involves high costs. By dynamically projecting a series of optical patterns, a layer-by-layer approach was built to create 3D microstructures in a mask-free manner [81; 84]. However, these incident light patterns are only suitable for fabricating hydrogel microstructures and are insufficient for manipulating nanoparticles and assembling nanowires into functional devices. All the above problems warrant continued efforts to further enhance capabilities of OEKs in 3D manipulation. Secondly, to facilitate the adoption of OEK-based microfluidics, it is important to develop an integrated chip that is ready for a complete and conventional laboratory-level process. This requires the integration of OEK with conventional microfluidic systems and/or components. A number of attempts have been made on such integration to move OEK further beyond its typical functions such as manipulation, separation, assembly, and fabrication of micro/nano-entities. For instance, to enhance the detection performance of OEK, a pair of optical fibers were embedded into the microchannel to rapidly and accurately count the number of particles and cells [97; 98]. A lens-free holographic microscope was incorporated into the OEK chip to allow observing microparticles and cells in a large field of view [99]. This approach offers the ability to rapidly capture the holographic images of microparticles and cells across an ultra-large area of up to hundreds of square millimeters. In addition, on-chip continuous medium exchange and electroporation of cells [100] were achieved by incorporating a conventional microchannel bonded onto a top ITO glass layer. The combination of an OEK chip and surface acoustic wave elements was utilized [101; 102] to provide functions such as concentration, guiding, focusing, trapping, and sorting of polystyrene beads and cell lysis.