1. Department of Chemical and Biological Engineering, BioFrontiers Institute University of Colorado, Boulder Boulder CO 80309 USA
2. Present address: McKetta Department of Chemical Engineering The University of Texas at Austin Austin TX 78712 USA
3. Department of Bioengineering University of Pennsylvania Philadelphia PA 19104 USA
4. Material Measurement Laboratory National Institute of Standards and Technology Boulder CO 80305 USA
5. Howard Hughes Medical Institute University of Colorado, Boulder Boulder CO 80309 USA