1. a Humboldt University, School of Medicine, Biomaterials Research Unit, Tuchholskystr. 2, 10098 Berlin, Germany
2. b Central Laboratory of Biophysics, Bulgarian Acadamy of Science, Acad. G. Bonchev, bl. 21, 1113 Sofia, Bulgaria
3. c Central Laboratory of Biophysics, Bulgarian Acadamy of Science, Acad. G. Bonchev, bl. 21, 1113 Sofia, Bulgaria