Nano-Bio Integration - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 237

Fig. 9.2 Nano-bio flagella
with controlled direction of
movement

dsDNA blood cell

magnetic particle

Artificial flexible flagella able to propel nanoscale cargoes have been constructed
( Dreyfus et al. 2005 ). The flagellum consists of a linear chain of streptavidin-
coated micrometer-sized superparamagnetic colloids linked by 107-nm-long flex-
ible dsDNA with biotin molecules at each end, and the cargo is a red blood cell, as
shown in Fig. 9.2 . A uniform magnetic field aligns the flagellum, while an oscillating
transverse field actuates it. Reversible displacements do not move the cargo since the
micrometer-scale fluid dynamics is governed by viscous and not by inertial terms.
Therefore, swimming schemes that break the time-reversal invariance are needed to
propel the blood cell. An oscillating magnetic field B with a static and a time-varying
sinusoidal components with comparable amplitudes, B s and B osc , respectively, acts
as fuel and drives the flagellum, which follows the instantaneous magnetic field
direction, bending, and reorienting in the process. A propelling force is generated
for suitable flagellum flexibility and magnetic field, the artificial swimmer moving
toward the free extremity of the flagellum, in the opposite direction compared
to spermatozoa. The average velocity is oriented in the direction of B s and its
maximum value is of about one blood-cell diameter per second.
Self-propelled bacteria such as Escherichia coli , when placed in asymmetric
environments, can impart a rotary motion to nano- and microstructures immersed
in a bacterial bath, as shown in Di Leonardo et al. ( 2010 ); the imparted motion
is random in symmetric environments. Suspensions of motile cells from bacteria
form a strong nonequilibrium fluid with an intrinsically irreversible dynamics.
Collective action of these flagellar motors generates a propelling force that pushes
against the drag force of the fluid, resulting in a nonconservative external force
field. An asymmetric microstructure such as that represented in Fig. 9.3 a, with a
diameter of 48m, can be rotated by a swimming cell, which aligns and slides
parallel to the microstructure wall in a direction that depends on the contact angle,
due to the repulsive contact force between cell and microstructure. In particular,
bacterial population in a concentrated suspension tends to concentrate and order at
the concave corners of the microstructure, generating a sufficient large torque to
rotate the microgear at an average rotation frequency of approximately 1 rpm when
the bacterial concentration becomes 10 10 mL 1 ; note that bacteria move at a speed
of 20ms 1 . In contrast, symmetric microgears, as those in Fig. 9.3 b, rotate in a
fluctuating direction, with a negligible average rotation angle. These cell-propelled
microstructures rotate as long as nutrients are kept in sufficient supply and harmful
metabolic by-products are removed. The number and shape of teeth do not influence
significantly the rotation frequency, the configurations in Fig. 9.3 a, c, displaying
almost the same maximum angular speed in the bacterial bath. The angular speed
increases, tough, with the bacterial concentration. The directional motion that arises