Received: 3 October 2018
Accepted: 20 December 2018
First Online: 10 January 2019
Compliance with ethical standards
: The authors declare that they have no conflict of interest.
: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The experiments carried out were approved by the Ethics Committee of the Christian-Albrechts-University Kiel, Germany (AZ.D417/09).
: Informed consent was obtained from all individual participants included in the study.