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Tendon routing and anchoring for cableriven single-t surgical manipulators 173

• Anisotropic rigidity: In such devices, there exist materials that have relatively higher
rigidity as compared to other expandable areas. When pressure is applied, areas with
lower rigidity will be enlarged more than the areas with higher rigidity. This difference
in enlargement will then cause the bending of the device.

7.2.3 Smart material actuation

Smart materials comprise of a group of substances such as piezoelectric actuators, SMAs,
and ionic polymer-metal composite, among many others [7]. These substances undergo
deformation under the action of external stimuli such as heat, potential difference, or
magnetic fields. For our design, we have only considered SMAs due to their extensive
biocompatibility. SMAs are a group of materials that fall under the category of shape
memory materials, which “remembers” its original undeformed shape. After undergoing
deformation from an external force, the material can return to its original shape with the
application of heat due to the restructuring of crystal phases. SMAs can be used both as a
force transmission device as well as a standalone actuating material by itself [3,8 10].

Owing to superior properties such as corrosion resistance, magnetically neutral, and
biocompatibility, SMAs can be used in a wide variety of medical applications. Additionally,
research has shown that it has high strength and work density and is hence capable of
exerting a considerable force despite its small size. Furthermore, SMAs can exist in various
forms, such as plates, tubes, coils, rings, or wires. There are, however, certain disadvantages
of using SMAs in applications requiring high-frequency actuation or when the strains
required are substantial. Additionally, the control of such actuators is difficult as the models
used to capture its motion and deformation are not fully understood, and there is currently a
lack of understanding of the hysteresis characteristics. Since SMAs can involve significant
heats, insulation of current and temperature is required, which might otherwise pose a
safety issue to humans under operating situations.

7.2.4 Design considerations and evaluating actuation methods

To focus on one type of actuation principle, we evaluate our options by considering their
miniaturization potential under lumen constraints, reliability, and repeatability. Compared to
TRM, TSM allows for more straightforward mechanical design and has lower bulkiness,
energy needs, and mass. In cable-driven actuation, the use of TSM provides a more
accurate application of the force by sheath constraints and hence offers more
predictable control than TRM. On the other hand, smart material actuators and pneumatic
actuators usually either require a bulky setup, are challenging to operate in narrow spaces,
or may pose safety issues. After evaluating based on these criteria, a tendon-sheath cable-
driven actuation mechanism is used as the primary principle for actuation.