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Tendon routing and anchoring for cableriven single-t surgical manipulators 189

stiffening mechanism. Hence, the expected time needed for heating to activation the phase-
change may be fast enough to be within acceptable ranges. Second, granular and layer
jamming employs similar concepts behind their working principles. Hence, by working on
layer jamming first, we can first ascertain the feasibility behind using differential pressure
as our stiffening mechanism. If the working mechanism has potential, but the
manufacturing of layer jamming proves to be too complicated, we can always attempt
granular jamming instead.

7.6.2 Thermal phase-change materials: wax

As mentioned earlier, wax and solder are the more often used materials in thermal phase-
change stiffening mechanism. Both wax and solder are inexpensive, commercially
available, and capable of considerable volumetric and shape changes [18 21]. In contrast
to wax, solder has a higher thermal diffusivity at a faster rate and hence a shorter response
time [22]. Additionally, Telleria et al. [22] identified a solder-based mechanism to have
advantages over the wax-based mechanism in terms of weight and strength, but are similar
when it comes to scalability to small sizes. However, wax allows for higher multiple and
cyclical usage as compared to solder [18]. Furthermore, since wax can melt and solidify at
relatively lower temperatures as compared to solder, it poses a lower safety risk when used
as a stiffening mechanism in our surgical robot. Table 7.2 shows a comparison of using
wax-based and solder-based stiffening mechanism in our surgical robot.
While there was a close tie between wax and solder, we used wax as the central medium to
demonstrate our proof of concept as sealing efficiency played an essential role in the
decision, and wax was more comfortable to seal using simple materials.
To test the feasibility of thermal phase-change stiffening activation, we designed a scaled-
up model. We filled up a tube with melted candle wax using a syringe and inserted a
nichrome resistance wire as our heating element into the tube. We then sealed both ends of
the tube and connected both ends of the resistance wire to a 12 V power source. To test for

Table 7.2: Comparison of wax-based versus solder-based stiffening mechanism.

Concept variants (thermal phase-change)
Selection criteria Wax Solder
1 Speed of activation 0 1
2 Weight 0 1
3 Strength 0 1
4 Scalability to small sizes 0 0
5 Sealing efficiency 1 0
6 Cyclical usage 1 0
7 Safety 1 0