1. Department of Chemical Engineering, Princeton University, Princeton, NJ 08544
2. Corresponding author: prudhomm@Princeton.edu
3. Elan Pharmaceuticals, One Research Way, Princeton, NJ 08540
4. Current address: BioDelivery Systems International, 4 Bruce Street, Newark, NJ 07103
5. Current address: Transave, 11 Deer Park Drive, Suite 117, Monmouth Junction, NJ 08852
6. Department of Chemistry, Rutgers, the State University of New Jersey, Piscataway, NJ 08854