Page 432 - Flexible Robotics in Medicine

426 Chapter 19

being rigid and stable enough to support a drill when tightened and locked. The shape and
angle formed when all the segments are interlocked together have been predetermined by
previous tests to ensure a 90-degree drilling angle to the trachea. The cross section of each
segment reveals a large central lumen for the drill, and four outer holes for each of the
tendon wires to pass through. This ensures that the tendon wires do not get entangled with
each other and reduce the friction due to the drill rotation. The tendon wires are made of a
material with properties that can transmit enough force and withstand cyclic loading. In
addition, the rounded surfaces of the bending segment help reduce tissue damage while
navigating through the trachea.

19.3.4 Cut design selection in terms of interface sliding and detachment

The shape of cut designs significantly affects the stability of Goldrill (refer to design
verification section). An ideal cut design requires minimum size, minimum obstruction
when interfaces detached, maximum stability when interfaces intact. Initially, we intended
to keep a uniform cut design for easier fabrication. However, we found that because the
force distribution and moment arms vary among different positions, and their effects are
much more significant when dimensions of the cross section are reduced, it is better to have
different cut designs to accommodate different stability force requirements.

Interface sliding and detachment will result in frontal/dorsal instability of the entire solid
body during drilling. The black arrows in Fig. 19.4 show a typical interface motion of our
first cut design with the presence of force F y . When both segments are intact (wire pulled),
0
0
point A/A and B/B coincident, respectively. Because F y introduces a moment on the right
0
segment, A tends to slide upward on curve AB, while the upper part completely moves
away from the left segment. In both E1/E2 versions, the wires are positioned in lateral
position in an elliptical cross-sectional design, so they do not prevent this
sliding detachment motion. On the other hand, this design provides maximum flexibility.
Therefore, we retain that in our later design but just applying at segments closer to the tip
where maximal flexibility is required.
We introduced a step design BC as well as a semi male female extrusion curve to limit the
sliding detachment motion (Fig. 19.5). The new design only helped to alleviate the
problem to some extent (with the presence of the red arrow “stopping,” the relative motion
indicated by black arrows reduced). We attribute this to a low friction coefficient for or 3D-
printed material, which causes sliding even when interfaces are intact. Besides, we also
attempted to reduce the dimension of cross section, which not only raises the difficulty of
stabilization but also makes the extruded part in the interfaces very prone to breaking.
Therefore, we want to bring in an extra stabilizing mechanism by changing the wire
placement from lateral to frontal/distal.