1. Department of Electrical and Electronic Engineering, Advanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7HX, United Kingdom
2. Centre for Biomedical Engineering, Department of Mechanical Engineering Sciences, University of Surrey, Guildford, Surrey GU2 7HX, United Kingdom
3. Theiss Research, La Jolla, California 92037, USA
4. Materials Science and Engineering Division, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899, USA