Acknowledgments
Y. D. and A.T. are PhD fellow of FRIA (Fund for Research Training in
Industry and Agriculture). The CAREM (Centre for Applied Research and
Education in Microscopy-ULiege) is acknowledged for the microscopy
facilities. We thank Erwan Plougonven from ULiege for the micro-CT scan
of the spike and its base, used to prepare Figure 1C.