1. Micro Engineering and Micro Systems LaboratoryDepartment of Mechanical EngineeringFaculty of EngineeringIbaraki UniversityHitachiIbaraki316‐8511Japan
2. Research Center for Ubiquitous MEMS and Micro Engineering (UMEMSME)National Institute of Advanced Industrial Science and Technology (AIST)TsukubaIbaraki305‐8564Japan