We present two routes of millifluidic-based synthesis of solid coresolid shell particles with submillimeter size. These particles, which behave as mechanical resonators, are key components of targeted three-dimensional locally resonant acoustic metamaterials working in the ultrasonic frequency range. The first route involves the encapsulation of a solid core into a liquid shell, which is then in-line solidified through gelation. The second route involves two successive coflows followed by two solidifications triggered by UV irradiation and complexation, respectively. Original solutions to several practical issues are presented. The advantages and drawbacks of both routes are analyzed in the light of the targeted specifications of the resonators. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013