1. E. Evans and A. Leung, Departments of Physics and Pathology, University of British Columbia, Vancouver, British Columbia, Canada V6T 1W5.
2. H. Bowman and D. Tirrell, Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003, USA.
3. D. Needham, Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA.