Page 187 - Flexible Robotics in Medicine

174 Chapter 7

7.3 Constructing the flexible backbone

7.3.1 Tendon-driven spring backbones and tendon fixation

We conceptualized three different means of tendon fixation to a spring, which will serve as
the stem backbone of our surgical robot. The first was a tensile spring with outer routed
tendon, while the second and third were compressive springs with inner routed and
crisscross routed tendons, respectively. Here, we only focus on the second method, and
more information can be found in ref. [11]. In our final design of compressive springs with
inner routed tendons, the guide system is placed on the inside circumference of the spring
stem. The tendons are constrained within the sheath of the spring, so there is no worry of
damaging the surrounding tissue. Although this method works well, it was challenging to
find a process to manufacture guides smaller than 3 mm in diameter. For this, we created
our customized tools and drill set, which could drill up to 0.3 mm in diameter. A piece of
2
1mm aluminum sheet metal was used to make the guides for this method. The engineering
of the device posed a significant challenge due to the strictly physical design constraints of
the device. The overall diameter of the individual channels (3 mm) warranted an aluminum
sheet guides with guide holes ranging from 0.1 to 0.4 mm in diameter. A custom-made drill
was used to meet the design needs and keep the production cost low. DC motor with high
RPM was coupled with a geared chuck and drills bits to create a custom drill.

This part focuses on flexible actuation using tendon routing, which is especially challenging
as we are imposed with strict geometrical constraints with an allowance of only 2 3mm
for the individual channels. We begin by broadly considering different existing actuation
principles and consider the possibility of adapting them to our device. A design-centric
approach is taken to focus on one actuation principle called the tendon-driven mechanism
and conceptualize ways to implement it with a 3 mm spring backbone. With the tendon-
driven principle, an actuation method is proposed, which allows miniaturization of the tools
up to needle size channels (1 mm). Using this method, three different tendon fixation
methods are prototyped and evaluated based on an evaluation matrix. Finally, a
mathematical model is developed, which allows the new robot to be precisely controlled
using our system architecture.

We now turn toward different methods to fix and route the tendons with the spring backbone.
Different guiding methods are prototyped and tested, and its pros and cons are discussed.

7.3.2 Anchors to guide the tendons

Actuation and manipulation of compressions springs are achieved using the tendon-driven
mechanism. Regardless of how the tendons were routed onto the spring backbone
(inner/outer routed), four major problems had to be addressed before implementing the