Page 200 - Flexible Robotics in Medicine

Tendon routing and anchoring for cableriven single-t surgical manipulators 187

miniaturization that we managed to achieve, which to our knowledge has not been
previously done. From the tests, the device was able to grab an object, position itself, and
burn the object safely.

7.6 Attempts at variable stiffness mechanisms

The flexible spring body of our surgical robot can provide the necessary flexibility for
configuration control while minimizing tissue injury, while it is being maneuvered inside
the body. However, it is not well suited for applying force to the tissue. The lack of rigidity
makes it difficult for the instruments to maintain their position against external force within
the body. From Fig. 7.16, we can see that when our instruments are in an actuated state,
external forces can still cause deformation, making it difficult for us to apply substantial
forces to the tissue.

Rigid tools are required to ensure accurate positioning and adequate contact force of the
instrument tip with the tissue during surgical procedures [10,12]. Integrating appropriate
variable stiffness into our surgical robot can allow the instruments to change their bending
stiffness to suit the force output requirements [9]. Various variable stiffness mechanisms in
soft robotics have been documented in past literature. These mechanisms can be categorized
as tunable-stiffness materials, phase-change materials, and jamming techniques. Tunable-
stiffness materials are often field-activated [13,14], and commonly used materials include
magnetorheological and electrorheological fluids. However, there are complexities
associated with the sealing and ensuring the homogeneity of the fluid [12].

Stiffening using thermal phase-change materials is often a solder-based or wax-based
locking mechanism that requires thermal activation [9,14]. However, phase-change stiffness
is limited in their ability to varying the level of stiffness and can usually only achieve
binary stiffness where the instrument is either sufficiently rigid or soft [9,15]. Additionally,

Figure 7.16
(A) A surgical instrument is actuated to the desired configuration. (B) Upon application of light
external force, the spring backbone is deformed to another position despite still being under an
actuated state.