We present two routes of millifluidics-based synthesis of solid core-solid shell particles with submillimetersize. These particles, which behave as mechanical resonators, are key components oftargeted 3D locally resonant acoustic metamaterials working in the ultrasonic frequency range. Thefirst route involves the encapsulation of a solid core into a liquid shell which is then in-line solidifiedthrough gelation. The second route involves two successive co-flows followed by two solidificationstriggered by UV irradiation and complexation, respectively. Original solutions to several practicalissues are presented. The advantages and drawbacks of both routes are analyzed in the light of thetargeted specifications of the resonators.