To allow bidirectional data communication to be realized in hearing aids which have only a restricted installation space, the earphone, i. The invention also provides a method of using the device to monitor leukocyte migration in the presence of physiological shear flow and therefore simulate physiological conditions of a blood vessel in vivo. In a preferred embodiment, the protruding parts of the frame and the wheels are embodied pivotable or displaceable in such a way that the axes of the brought together wheels coincide and the wheels are disposed oppositely to each other by the external faces thereof. One or more dampers for damping vibration of the fan track liner are positioned in each of the one or more pockets and are arranged so as to contact the annular casing element.