The wires of the IDTs in PDMS from the three devices we use to calibrate are cut and then the samples are used for x-ray micro-CT (SKYSCAN 1272, Bruker, Germany). To obtain a tomographic image, a source current of 100 µA and a source voltage of 100 kV with a 0.11 µm copper foil as filter is applied on the cathode. An isotropic voxel size of 4 µm and a total field view of 16.128 x 10.752 mm was set and 1800 sequential projections on a 360° rotation are collected with a respective exposure time of 2414 ms. The radiographic projections are automatically post-corrected for alignment, ring artifacts and beam hardening using the Software NRecon from Bruker. The visual reconstruction is taken using the software CTVox supplied by Bruker.