Page 225 - Flexible Robotics in Medicine

212 Chapter 8

Figure 8.19
Bending of 4 mm compression spring.

From Fig. 8.19, we can see that one bending radius of less than 1 mm could be achieved
with the 4 mm compression spring used. A total of three distinct types of wire were being
tested: 0.38 mm steel wire, thin fishing line, and a thick fishing line. These wires were
looped around the springs in a different configuration to test which ones could give a more
consistent bending.

When three of the same wires were tested, the spring bending orientation was not
consistent. This was because the wires tend to slip and move out of position. When the
flexible drill was being tested with it, the spring was not stiff enough to bend the drill, and
the spring just compressed linearly along the long axis instead of bending.

8.5.2 Polyimide medical tubing

Polyimide is a polymer thermoset plastic that has excellent thermal stability, chemical
resistance, and tensile strength. The tubes made of this material are further lightweight,
flexible, and resistant to heat and chemicals. Production of these tubing can be as thin as
0.025 mm, with the minimum inner diameter of 0.51 mm. These tubes are used for drug
delivery and other neurovascular applications.
Reasons to consider using these tubes are that first it is currently being used in the medical
industry. Second, it is mentioned to be flexible, which meets our needs. Third, the
dimension of the tubes can meet our requirements. Cutting patterns (as shown in
Section 8.3) can be done to increase flexibility.

A total of seven tubes of different dimensions are being tested. These seven tubes of about
1.2 mm in diameter are flexible, as can be seen in Fig. 8.20. However, upon further bending
to a smaller bending radius, the tube gives way, and plastic deformation occurs.