Surface engineering of biomedical nanoparticles is of great importance to maintain nano-stability, resist nonspecific biomolecular adsorption so as to enhance in vivo circulation time. Cell membrane mimicking zwitterions can form superhydrophilic antifouling surface which helps to maintain colloidal stability and resist immune elimination via electrostatically induced hydration. The stealthy surface prolongs the blood circulation time to reach "passive targeting", further "active targeting" is also available if combined with stimuli-responsive or bioactive molecules. Hence "zwitteration" has been developed as a new surface engineering strategy of biomedical nanoparticles. This review mainly talks about surface engineering of inorganic nanoparticles with small molecular and polymeric zwitterions, zwitterionic polymeric assemblies as drug delivery system and zwitterionic polymeric prodrugs. Introduction of special properities and applications of mixed-charge materials is also involved.

Contents
1 Introduction
2 Surface design of inorganic nanoparticles with zwitterions
2.1 Small molecular zwitterions
2.2 Polymeric zwitterions
3 Zwitterionic polymeric assemblies for drug delivery
3.1 Drug non-covalently loaded assemblies
3.2 Polymeric prodrugs
4 Mixed-charged materials
5 Conclusion and outlook