Page 493 - Handbook of Biomechatronics

Bioinspired and Biomimetic Micro-Robotics for Therapeutic Applications 487

deoxyribonucleic acid (DNA) chains with biotin: these two main compo-
nents form slender filaments by self-assembly under a homogeneous mag-
netic field for an optimized period of time. The diameter of the
superparamagnetic particles is 1μm, and the filaments are around 30μm
in length. The tail is attached to a red blood cell, representing a cargo which
will be towed by the magnetically actuated filament acting like the flagellum
of a sperm cell. Authors demonstrated the swimming characteristics with
respect to Sp and Mn e numbers.
A very similar method to generate active tails is to cast polymer solutions
mixed with magnetic particles in a mold to form a sheet. Once the polymer
solution is dried, the sheet can be rolled into a hollow ring and placed in a
powerful homogeneous magnetic field to form a predefined magnetization
profile, that is, remnant magnetization. This magnetization profile will later
help to generate plane wave propagation under oscillating EM fields (Diller
et al., 2014). The dimensions of the polymer sheets could vary from
mm-scale to cm-scale.
The aforementioned studies are examples of the down-to-top
manufacturing approach, that is, adding small amounts of material to the
micro-swimmer in one or several steps and giving them shape. Now, we
will review some of the top-to-down methods by which removing material
and large-scale assembly are common practices, although the sizes are mostly
in mm-scales. A suitable example is to manufacture the helical rigid tail via
removing material from a cylindrical solid using the electrical discharge
method (Mahoney et al., 2011): authors used a wire electrical discharge
machining (WEDM) system modified in-house with extra degrees of free-
dom to obtain a certain attack angle while cutting through a nitinol tubing
with an outer diameter of 1mm and length of 5mm to form the helical pro-
file with three full turns. The neodymium-iron-boron (NdFeB) grade N50
permanent magnet was then glued to the tail using epoxy. Grade Nxx is the
measure of energy potential that could be generated by a magnet. A much
earlier and simpler proof-of-concept as to demonstrate how such end-
effectors would be actuated presented in the literature is to coil copper wire
of 0.15mm into a rigid helical tail and gluing it to a cubic samarium-cobalt
3
(SmCo) magnet with a volume of 1mm (Honda et al., 1996), although the
overall dimensions are much larger than what is aimed for.
Although cm-scale robots do not really fit into the overall scheme of this
review of micro-manufacturing efforts, still there are three large-scale assem-
bly examples worth mentioning very briefly as in the future it might be pos-
sible to manufacture artificial systems of much smaller versions. Kim et al.