## 5 Conclusions  
In this research, we proposed a dimple structure to realize the accurate digital manipulation of droplets using EWOD devices. The three-dimensional shape of the dimple structure was successfully formed by simple embossing. On a flat substrate, the success rate of the droplet motion was lower than 70.8%, but on the dimple structure, the droplets were able to be moved along the dimple structures correctly without unintended motion caused by several environmental conditions. In addition, it was confirmed using a sliding experiment that the adsorption energy, which is an indicator of the droplet stability, increased by 25%. As a result, it was proved that the dimple structure increased the controllability of the droplets.  
Stabilizing the droplet at the position of the dimple structure can reduce the manufacturing costs because the wiring path and the electrode tile can be formed on the same layer of a disposable sheet. In the near future, low-cost and highly accurate droplet manipulation may become possible by integrating EWOD devices with a dimple structure into basic wet-lab experiments. In other words, it is expected to achieve practical use as an automatic operation system that can greatly contribute to the biochemical industry.  
**Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/9/12/2406/s1](http://www.mdpi.com/2076-3417/9/12/2406/s1)]([http://www.mdpi.com/2076-3417/9/12/2406/s1](http://www.mdpi.com/2076-3417/9/12/2406/s1)), Video S1: Droplet manipulation with dimple, Video S2: Droplet manipulation without dimple.  
**Author Contributions:** Conceptualization, K.M., T.I. and T.N.; methodology, K.M., and N.T.; validation, K.M.; writing--original draft preparation, K.M.; writing--review and editing, K.M., and S.A.; funding acquisition, K.M. and T.N.  
**Funding:** This work was partially supported by RIKEN-AIST joint research fund, the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for young scientists (B) (No. 17K17715) and Leading Initiative for Excellent Young Researchers (LEADER) in FY 2016 of the Ministry of Education, Culture, Sports, Science and Technology, Japan.  
**Acknowledgments:** We would like to thank the 4 University Nano Micro Fabrication Consortium in Kawasaki, Japan ([[http://www.nano-micro.sakura.nejp/home/](http://www.nano-micro.sakura.nejp/home/)]([http://www.nano-micro.sakura.nejp/home/](http://www.nano-micro.sakura.nejp/home/))) who provided open facilities and experimental equipment for this work. We would like to thank Editage (www.editage.jp) for English language editing.  
**Conflicts of Interest:** The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.  
Figure 7: Results of sliding experiment. (**A**) Graph of the sliding angle for each condition. (**B**) Graph of adhesion energy calculated from the sliding angle. The volumes of the droplets were 5 \(\mu\)L, 10 \(\mu\)L, and 20 \(\mu\)L. The plotted points are the average of five data points, and the error bar shows the standard deviation. Dotted lines on the graph (**B**) represent average values of the three data of each surface.