Page 211 - Flexible Robotics in Medicine

198 Chapter 8

8.2.3.2 Telescoping precurved superelastic tubes
This mechanism is like the “snake-like” slave; however, unlike the “snake-like” slave, it does not
use tendon wire to bend, but from elastic potential induced in the elastic backbone tubes. The
steering of the tool is by translating and rotating precurved tubes. Miniaturization is possible for
this design, but the trade-off would be that there will be a reduction in stiffness. Medical nitinol
tubes have unique superelasticity and shape memory effect [10 13]. This material is further
biocompatible [14 16] as surface treatment through electro-polishing and passivation.

8.3 Bending tube mechanism with groove cutting designs

8.3.1 Bending tube design

The main point is to have a steerable drill that can have a 6 90-degree bend at the drill tip.
Having this bending angle means that it should be kink-resistant and must be able to resist
permanent deformation under a certain amount of strain. Moreover, this part must be
flexible enough to return to its original position after the bending.

The requirements for this steerable drill are that it must be in the range of 2 4mm in
diameter and should be able to support the bend for drilling to occur. Drill tip should be
able to extend out to keep contact force with the drilling surface.
As this is medical equipment, the material that is being used should be biocompatible. The
following section will discuss the concepts that are being considered and how effective each
idea is in fulfilling the requirement of the prototype.
The development of medical equipment using nitinol has been prevalent over the last 20 years
[13]. Making use of nitinol superelasticity property, we design cutting patterns so that the tube
will be able to bend. Two designs are introduced in this section, as shown in Fig. 8.1.

8.3.2 Simulations

8.3.2.1 Nitinol tube cutting fishbone-like design 1
To simulate how different degrees of pulling force will affect the nitinol tube cutting
fishbone-like design 1, we use the Solidworks Simulation Study advisor to study how
varying amounts of force will affect the static and fatigue of this design.

We used a varying amount of pulling force, from 2 N all the way to 10 N, to run this
simulation. Nitinol mechanical properties (from Johnson Matthey Medical Components) are
shown in Table 8.1. Since nitinol’s yield strength material property is over a range in
martensite state, we applied the minimum and maximum property for our simulation.