Page 201 - Flexible Robotics in Medicine

188 Chapter 7

the technique requires prolonged activation and cooling time (on orders of seconds), making
it too slow for most practical manipulation applications [13,14,16].

There are two types of jamming techniques, granular/particle and layer jamming. In both
types of jamming, changes in air pressure are used to modulate the relative shear stress
experienced between particles and layers that are enclosed by an elastic membrane [17].
When the air pressure difference inside and outside of the elastic membrane increases, the
instrument will be more rigid. In granular jamming, an increase in air pressure difference
inside and outside of the elastic membrane will squeeze the granules together, increasing
the rigidity of the granular system [9]. Both techniques are found to be able to generate
drastic stiffness increases without a significant change in external volume. Furthermore,
there has been a study on using jamming as dynamic haptic force feedback for the surgical
robot, in addition to its application in variable stiffness. Layer jamming, on the other hand,
is substantially more complicated to manufacture.

7.6.1 Design thinking framework

An effective variable stiffness system for our surgical robot will ideally have short
activation time, appropriately broad range and magnitude of stiffness, simple to
manufacture, and the ability to be scaled down to meet the size constraints of our surgical
robot. Table 7.1 shows how each of the stiffening method fares for different criteria.
Even though granular jamming ranks higher than the use of thermal phase-change
materials, we decided to work on thermal phase-change materials and layer jamming due to
two reasons. First, we decided to place more weight on criteria five as compared to other
criteria, as it is the main challenge that we face in miniaturizing our surgical robot and
instrument channels. Taking this into account, the use of thermal phase-change materials
offers more potential over the use of granular jamming. Furthermore, as our structures are
small in scale, only a small volume of thermal phase-change material is needed for the

Table 7.1: Comparison of various stiffening mechanisms.
Concept variants
Thermal
Tunable-stiffness phase-change Granular/particle Layer
Selection criteria materials materials jamming jamming
1 Length of activation time 0 2 1 1 1 1 1
2 Range of stiffness 0 2 1 1 1 1 1
3 Magnitude of stiffness 2 1 0 1 1 1 1
4 Complexity of 0 1 1 0 2 1
manufacturing
5 Scalability to small sizes 2 1 1 1 0 1 1