Page 106 - Flexible Robotics in Medicine

Flexible steerable manipulator utilizing complementary configuration 91

4.3.5.2 Stability of the prototype (tip, forceps)
Forceps and bending sections must move when they are intended to move but remain
stationary when they are not supposed to move. This is controlled by the proximal control
system described earlier. One specific property of the stepper motor is the holding torque.
This means that a continuous current is drawn from the power source in order to ensure that
the stepper motor’s shaft is held stationary and will not turn when subjected to external
forces. The stepper motor used in the prototype had a holding torque of 9.81 N cm . This
ensures that when the operator applies a force on the forceps during the biopsy, the holding
torque will prevent any uncoiling of the spools, thereby ensuring that the distal tip is held in
place. This assumes that the distal tip will not move if there is no movement of the control
cables. Further discussion of this assumption can be found in the verification section of this
report.

4.3.5.3 Strength of jaw (pull out)

The jaws of the forceps must not be detached from the entire distal tip during the whole
biopsy procedure. For that matter, none of the parts of the entire prototype should ever
come loose, be detached, or generate debris or small particles that are pyrogenic or cause an
inflammatory response. Hence, the pivot of the forceps is made of metal in order to
accommodate cyclical loading and resist fracture. Note that the pull-out strength
measurement was not conducted because it is a destructive test, and there was only one
functional prototype. Hence, it was not feasible to conduct a pull-out test since it would
have tested the prototype for failure.

4.3.5.4 Strength of the distal tip
The individual segments within the distal tip must be able to take the compressive stresses
from the coupling cables (via the grooves), which terminate at the proximal-most segment,
as well as the tensile stresses from the control cables, which terminate at the proximal
control. The strength of the distal tip is largely dependent on the mechanical properties of
the material used. The height of the individual cylindrical segments has been reduced to the
minimum (to just sufficiently fit the spherical ball bearing-like structures) in order to reduce
the chance of failure by shear stress, given that the walls of the cylinder are quite thin.

4.3.5.5 Strength of forceps

Forceps must be able to take high stress in terms of tensile, compressive, and shear stresses.
This is to ensure that the biopsy sample is severed from the host cavity completely. Hence,
316L stainless steel was chosen for its mechanical properties and because it is known to be
biocompatible with appropriate surface treatment.