Biomedical Engineering Reference
In-Depth Information
Figure 12.5 (Left) Schematic of tubular braiding machine. Fibers and wire (indicated in gray) are spooled from
carriers that rotate on a circular track. Fibers may be braided around a center mandrel or other fibers in the core of
the braid. (Center) Arrow indicates path taken by one yarn carrier in maypole braiding pattern. (Right) Photograph
of tubular braiding machine at CEAM.
braid itself is a tough structure that protects elements woven into the outer sheath, as well as other
elements in the core. Thus functional elements (wires and/or perhaps sensors) are truly integrated
into the fibers of the host composite, rather than acting as inclusions in the matrix phase.
Furthermore, braiding allows fine control of the pitch and diameter of the wire coil such that the
electromagnetic properties may be tuned for desired performance. The sense of the coil, as left-
handed or right-handed, may also be varied in this process to address issues of chirality, as
discussed below (see Amirkhizi et al., 2003).
12.2.1.2.1 Chirality
The introduction of coil geometry not only affects the inductance of the medium and consequently
the overall dielectric constant, but also introduces different capacitative response than mere straight
wires. This capacitative response usually changes the overall magnetic properties of the medium,
although the inductive response still remains the dominant effect. Part of the magnetic response is
induced by the chirality effect which is discussed presently. However, a more careful and thorough
study is needed since the techniques that can be used to eliminate chirality do not necessarily
change the axial magnetic effects.
Of importance is the effect of the handedness of the coils on the EM field vectors. The geometry
of the coils requires that the current density in the conductors has a circumferential component, in
addition to the axial component which is the only component present in the case of the straight
wires. The oscillating circumferential component of the current enhances the magnetic field of the
propagating wave with a component parallel to the axis of the coils. Note that as the active
component of the electric field is parallel to the axis of the coils, the accompanying magnetic
field is normal to it. Therefore the enhanced magnetic field is normal to the external excitation.
Moreover the extra component is in phase with the current density and in turn with the external
electric field, whereas the external magnetic field and electric field are out of phase by a quarter of a
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