The Publisher regrets that this article is an accidental duplication of an article that has already been published in Adv. Drug Deliv. Rev. , 63 (2011) 352–366, doi: 10.1016/j.addr.2011.01.005 . The duplicate article has therefore been withdrawn.
This article originally published in Advanced Drug Delivery Reviews (2011), 352–366, Elsevier .
1 Both authors contributed equally to this manuscript.