Page 463 - Flexible Robotics in Medicine

Thermo-responsive hydrogel-based circular valve 457

of the flowing sterile solution. The perfect sealing quality allows the proper functioning of
the valves. Despite the inherent ability for hydrogels to absorb water, once the hydrogel has
been sufficiently hydrated, no aqueous media can enter it. In terms of valve performance,
flowing media would not be absorbed by the hydrogel and will only pass through gaps
created or present in the hydrogel valve region. The flowing of media through these gaps
causes friction between the hydrogel valve and the media [7], adding a layer of flow rate
control by the hydrogel valve. Thus the perfect sealing quality validates the use of hydrogel
valves in the CBI process, where flowing media (sterile solution) is present. In this
application, it was found that such hydrogels exhibited significant amounts of force. The
forces exhibited are in the order of 10,000 pertaining to the weight of the hydrogel [8]. The
hydrogel performed its function by changing its lumen size in response to temperature
variations. Such variation in lumen size with temperature fits the ideal application as a
thermo-responsive hydrogel valve (TRHV) in the CBI process. The variation in lumen size
regulates the area of flow in response to temperature. The hydrogel exhibits the capability
to exert large amounts of force, which is needed to overcome the force of the flow of sterile
solution in the CBI process during volume transition. Coupled with the qualities of
hydrogels: perfect sealing and ability to withstand tremendous pressures, the hydrogel
used in this application seems to be translatable into the application as a valve in the
CBI process.

The shape memory alloy actuated hydrogel valve (SMAHV) was tested for two designs,
SMA wire-based SMAHV and SMA spring-based SMAHV with different wire diameters.
When SMA undergoes a series of thermal cycles during which high-temperature shapes and
low-temperature shapes are imposed on the SMA, the SMA acquires a “trained” shape.
Acquiring a “trained” shape [9,10] means that when SMA is heated, it changes its
conformation to that which it has been trained (austenite phase). When the SMA is cooled,
it transforms to the martensite phase, which is flexible and takes the shape of the
encapsulation material (hydrogel matrix). Once SMA has been shape-trained, these
conformations can be repeatedly obtained by heating and cooling the SMA. This allows for
use as a valve as the intended shape when heated cooled (ON OFF) can be trained and
reproduced. SMA demonstrates a high work output ratio compared to other types of
actuators [11]. Shape memory actuators [12] typically consist of an actuator element, a bias
force, an electrical control unit, and a means to fix the actuator to the intended system that
it will actuate in. In the CBI application, SMA was chosen as the actuator element. The
component providing the bias force would ideally store the force applied when the SMA is
heated and assume its “trained” shape and release that stored energy when the SMA is
cooled [9,10]. Thus if SMA was embedded in the sodium alginate and acrylamide hydrogel,
it would theoretically be able to actuate, and the hydrogel would provide the bias force.
Thus, using SMA embedded hydrogel valve for regulating the CBI process is a good
option. The high temperature produced during the actuation of SMA is trapped by the