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Author to whom correspondence should be addressed: Hyun S. Kim; E-Mail: kevin.kim@yale.edu; Tel.: +1-203-785-6938; Fax: +1-203-785-3024.
Hyun S. Kim conceived the idea and supervised the project.
Johannes M. Ludwig performed literature search and prepared the manuscript.
Juan C. Camacho, Nima Kokabi, and Minzhi Xing contributed substantially by providing pictures, editing, and finalizing the manuscript.
The authors declare no conflict of interest.
© 2015 by the authors; licensee MDPI, Basel, Switzerland.