Page 466 - Handbook of Biomechatronics

460 Ahmet Fatih Tabak

2 BACKGROUND

The concept of swimming inside the human body is more promising
for a micro-robot than other methods of commuting as human beings are
basically electrochemical machines with an intense and interconnected
web of ducts and channels filled with liquids of different properties. Purcell
put the conditions of micro-swimming in perspective with his famous
“Scallop theorem” providing calculations and demonstrating how conven-
tional methods of propulsion should endure accomplishing very little in
terms of controllable displacement. He also argued that the efficiency of
swimming in micro-realm is expected to be very low as the entire process
is rather dependent on continuous time-irreversible action to harness pro-
pulsive effect mainly from the shear resistance of the surrounding medium
instead of inertia of the swimmer (Purcell, 1977). This is indeed the condi-
tion on which bacteria and spermatozoa swim as an inspirational answer, to
high viscous forces dominating inertia, provided by nature. However, this
also means that one of the biggest challenges is to supply the energy in order
to sustain the continuous swimming as long as it is needed. Hence, that
requirement should be minimized with increased efficiency.
The natural micro-swimmers, such as bacteria and spermatozoa that
inspire us, are fully functional, “nonholonomic,” that is, velocities are not
independent of each other and the position is dependent on the total trajec-
tory history of the moving object (Kane and Levinson, 1985), and autono-
mous systems with onboard energy generation and steering capabilities.
They possess rudimentary survival instincts based on chemical reactions
and spatial gradients. They continuously search for nutrients with a series
of motions generally classified with the suffix-taxis; in a sense helping them
to decide their next move based on their interaction with the surroundings
as they produce the energy required to commute (Lighthill, 1976).
What makes them successful is that their ability to move in an environ-
ment not suitable to move with conventional propulsion methods mainly
relying on inertial forces. A helpful analogy would be thinking of a human
being trying to swim in a pool filled with honey. The repeated swimming
stroke that person will exploit in that viscous liquid will prove to be useless
since an action tracing its own steps back as the time progress will generate
only a minute amount of displacement with respect to the body length.
Indeed, the overall performance can be envisioned as a hysteresis behavior
with which almost all power is spent in terms of heat loss due to enormous