Page 312 - Flexible Robotics in Medicine

302 Chapter 13

Three stepper motors control the overall mechanistic motion of BEE. Two of the motors,
coupled with axles (Section 13.5), control the rotational motion of the flexible tip (one
motor is for rotation in the up and down direction, and one motor is for rotation in the right
and left direction), and the third motor, coupled with a gear (Section 13.5) and gear rack
(Section 13.5), controls the translational motion of the device into the nasal orifice. The
specifications for the rotational and translational motion of BEE can be found in
Section 13.5 (Table 13.1).

13.4.4 Material selection

We utilized both ABS and VeroClear plastic to perform rapid prototyping. ABS plastic is a
thermoplastic, which becomes soft and moldable when heated but hard and rigid when
cooled. It was only utilized for the macro-scale prototypes because it was unable to
achieve the high dimensional stability we required for the actual-scale prototypes.
VeroClear material, on the other hand, is a photopolymer which combines high
dimensional stability with surface smoothness and is ideal for fine-detail prototype
building. It was thus utilized for the rapid prototyping of Prototypes 4.0 onward. However,
neither ABS plastic nor VeroClear has ideal properties for use in the final desired
prototype. For example, neither ABS plastic nor VeroClear is medically approved materials
for implantation in the body.
What we ultimately desire for use in the device is a medically approved biocompatible
polymeric material that should be highly wear-resistant to prevent the generation of
particulate matter at any time within the nasal cavities during the operation of the device.
However, the material chosen can have physical properties close to that of VeroClear, as
VeroClear has been shown to be durable and capable of withstanding the forces
experienced by the device during its operation.

Therefore we propose the use of ultrahigh molecular weight polyethylene (UHMWPE)
in the final prototype. The UHMWPE hardness-61 has a comparable young’s
modulus, tensile strength, and percentage elongation at break compared to
VeroClear. Moreover, UHMWPE is known to have excellent wear resistance, which is
desirable since contact could occur between the device and the walls of the nasal
passage, and we do not want the particulate matter to be generated during operation of
the device.

In addition to the material selection for the body of the device, we also need to select a
material for the cable, which drives the rotational motion of the device’s flexible tip. We
thus similarly demonstrated that the nylon thread utilized in the prototypes could withstand
the forces (Section 13.6) experienced by the device during its operation. We utilized
UHMWPE and nylon as materials in the final prototype.