## 3 Rapid Electrokinetic Patterning
### Current Challenges of REP  
REP is a new form of particle manipulation technique that utilizes the simultaneous application of a uniform AC electric field and a focused laser. It has been steadily applied for concentration,  
Figure 10: Physics of the REP technique. (**a**) Various forces involved in REP process. (**b**) Dependence of REP critical frequency on particle size. Reproduced from [58], with the permission of American Chemical Society (2010).  
Figure 9: Visualization and particle image velocimetry (PIV) analysis of electrothermal microfluidic motion. Reprinted by permission from (the RightsLink Permissions Springer Customer Service Centre GmbH): (Springer Nature) (Microfluids and Nanofluids) [116], Copyright (2015).  
trapping, patterning, and separation of micro- and nano-particles ranging from 22 nm to 10 \(\upmu\)m, establishing its firm position in colloidal manipulation [25; 58; 61; 62; 63; 109; 127; 128; 129]. Recently, REP was also used for manipulation of swimming bacteria [130] and carbon nanotubes [131], extending its applicability in various fields including biotechnology. Despite its excellent capability, REP faces two challenges [25; 58; 61; 62; 63; 127; 128; 129; 130; 129; 131]: (1) the use of an expensive laser system as a heat source and (2) low manipulation efficiency of particles suspended in a high conductivity solution. These two problems should be overcome to find novel applications of REP for on-chip bioassays.  
A 1064 nm wavelength NIR laser has been often used to create thermal gradients in a fluid in REP process. However, the high price and a large volume of the laser system affect the economics and portability of the REP technique. Therefore, finding an alternative heat source will enable various REP-based manipulations of particles at low cost. This issue is currently being resolved. [PERSON] et al. employed a micro-patterned thin film resistive heater instead of the optical laser [132]. The use of the heater during REP manipulation offered numerous benefits such as efficient supply of thermal energy to induce a non-uniform temperature field in a fluid. As previously mentioned, the other challenge is that it is very difficult to control particles suspended in a high conductivity electrolyte using REP [24; 25; 64; 130]. A high conductivity solution produces strong Joule heating even in an external electric field with moderate intensity [72; 133]. This causes the adhesion of particles on an electrode surface during REP manipulation. Because of that issue, REP has mostly been used to manipulate colloids suspended in low-conductivity solutions. However, it raises another problem in biotechnology that the low electrical conductivity of a suspending media hinders the viability of cells. To find wider biological applications, REP should also be adapted for manipulating particles in high conductivity electrolytes.  
[PERSON] and [PERSON] contributed equally to this work as co-corresponding authors.  
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2016R1D1A1B03934976).  
The authors declare no conflict of interest.