Page 248 - Flexible Robotics in Medicine

Soft-bodied flexible bending mechanism with silent shape memory alloys 237

be poured in the mold and cured at room temperature for 4 8 hours (can be accelerated by
curing at higher temperatures). The cured silicone elastomer is soft, durable, and highly
stretchable [33]. The steps of mixing of the two parts of the silicone and the pouring in the
mold are shown schematically in Fig. 10.4.

10.2.4 Fabrication of the tentacle

The fabrication of the tentacle is done in steps, as shown in Fig. 10.4. Universal Mold
Release (de-molding agent) from Smooth-On, Inc. (Pennsylvania, United States) is sprayed
on the inner walls of the sealed 3D-printed mold to facilitate easy removal of the silicone
elastomer once the curing is completed.

Once the silicone elastomer is removed, the SMA wires are assembled in their respective
channels. Since single strands of SMA wires were unable to produce enough force to bend the
whole tentacle to its respective direction, each SMA wire is passed through the bottom opening
of the channel and the taken from the top and then routed back to the adjacent channel from
the top and exiting from the bottom, and connected to the controller. Thus, three SMA wires
are routed through two channels each, utilizing all the six channels. The SMA wires are
aligned such that the programmed shape is toward the periphery of the tentacle. This alignment
pushes the tentacle to bend outwards when SMA is actuated, which helps in achieving higher
bending angles compared to the bend aligned inwards if the tentacle. The assembly of the
SMA wires in the channels of the tentacle is shown schematically in Fig. 10.4.

10.2.5 Control setup

A simple electronic system is implemented to control the tentacle structure with a joystick.
An Arduino Uno microcontroller with ATmega328 is programmed to control the direction
of the tentacle structure with the movement of the joystick. The bending angles of the
tentacle structures depend on the voltage applied to the SMA wires in the tentacle structure
and the period of application. SMA wires require a specific current in order to work
optimally; a 250 μm diameter SMA wire is recommended to be operated at 1 A for best
results. The Arduino Uno microcontroller provides only 0.5 A, which is insufficient, and
hence external batteries (3 V) are used to supply power.
The circuit diagram of the setup is shown in Fig. 10.5A. The real-time photo of the setup
during the operation of the joystick to control the tentacle structure is shown in Fig. 10.5B.
According to the manufacturer’s datasheet, an SMA wire of 250 μm diameter has a
resistance of 18.5 Ω/m, and requires 1060 mA of current for contraction. Based on that and
the application intended, the length of the wire is selected. Due to the use of 3 V external
batteries, the total length of the SMA wire should have a resistance of 2.857 Ω (R 5 V/I).
Based on the resistance of the SMA wire per unit length, the SMA wire length for actuating