Page 490 - Handbook of Biomechatronics

484 Ahmet Fatih Tabak

discussed here. Furthermore, it must be stressed out that genetic manipula-
tion is not discussed within this context to steer clear of the exhaustive
debates on the ethical implications of the field.

3.1 Manufacturing Techniques
Some micro-swimmers have no distinct head to be towed; they are rather on
the side of proof of concept studies to demonstrate the manufacturing tech-
nique or application value. Some micro-swimmers are, although are not
micro in dimensions so better to call them mock-ups to be correct, are com-
plete with onboard power packs and control circuits sealed within their
head. Similarly, some devices listed in this section are manufactured in a
series of subsequent steps borrowed from microfabrication and are assembled
together with different techniques. It seems possible to use “top-to-down”or
“down-to-top” approaches depending on the application.
Micro-robotic devices are fundamentally a member of a much larger
family known as microelectromechanical systems (MEMS) and, therefore,
MEMS manufacturing techniques with semiconductors and soft metals
are often cast, especially when the dimensions are smaller than 1 mm. Fur-
ther, it is safe to say that MEMS manufacturing techniques are improved
hand in hand with the research on micromachines as new methods and
approaches are realized based on sheer need. A perfect recent example is
given by Zhang et al. (2009): The authors presented a helical tail
manufacturing method based on the self-folding ribbon of InGaAs/GaAs/
Cr with a total thickness of 42nm, deposited and etched on a GaAs (001)
wafer with molecular beam epitaxy and electron-beam (E-beam) evapora-
tion with the help of a sacrificial layer of AlGaAs. This very ingenious layer
combination is what forces the ribbon to fold into a helix after it is freed from
the substrate surface. The helical structure itself is not magnetic and thus to
that effect, before the self-folding step, authors deposited a Cr/Ni/Au layer
arrangement of a total thickness of 200nm, at one end of the ribbon as a
square patch of 4.5μm 4.5μm in dimensions. Finally, authors freed the
ribbon from the van der Waals forces on the substrate surface with a micro-
manipulator. The total length of the ribbon is around 50μm and is actuated
with a set of three Helmholtz coil pairs, that is, one pair for each axis.
At this juncture, it is important to emphasize that the numbers (001)
emphasized in parenthesis are known as Miller indices (Hsu, 2002) denoting
the orientation of the atoms in the wafer after it is sliced to a thin and smooth
cross-sectional layer of single crystal. This direction is important for wafer-
based micro-manufacturing techniques such as the epitaxy, for acoustic