1. Auckland Bioengineering Institute, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand e-mail:
2. BioInstrumentation Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 e-mail:
3. Department of Engineering Science, Auckland Bioengineering Institute, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand e-mail: