Biomedical Engineering Reference
In-Depth Information
teacher's demonstration of how hydrating pea or bean seeds easily and effectively crack glass bottles
and animal skulls by what must be considered a slow, controlled, and silent demolition (Brauner and
Bukatsch, 1980). Although consisting of many smaller colloidal subunits (storage cells) pea seeds
are large, compact bodies that hydrate and expand as one unit. A hydration motor operating with
such compact swelling bodies rather than solute molecules has a huge advantage over an osmotic
design. It no longer needs a fragile membrane with fine pores but can operate with a sturdy
and inexpensivemetallic or ceramic sieve with pores small enough to retain the dehydrated swelling
bodies.
However, pea seeds are designed to hydrate slowly and for that reason less than ideal motor
material. Two factors keep their rate of hydration low: (i) the mechanical resistance of seed shells to
expansion and (ii) the limited accessibility of the internal seed space to water (Larson, 1968). To
become technically attractive one needs swelling bodies with an increased access for water or other
solvents. The access to internal parts of a swelling body depends on the rate of diffusion and this
parallels the surface to volume ratio or
specific surface
(Levitt, 1969). The specific surface
S
v
is the
total area of all surfaces (
S
) of a body divided by its volume (
V
):
S
v
¼
S
=
V
:
In the case of a sphere (pea) with radius
r
this equation simplifies to
S
v
¼
3/
r
and shows that with
increasing radius of round swelling bodies their
S
v
and with it accessibility to water decreases.
Large storage organs, such as potato tubers, fruits, and seeds, have very densely packed cells and
with no internal surfaces; that is, their accessibility or exchange of matter is restricted. The swelling
rate of such compact bodies can be increased by a reduced size. It is for this reason that the breaking
of seeds into smaller particles (grits) accelerates their hydration process (Figure 19.1).
Alternatively, hydration can be sped up by using particles with a more elaborate internal surface
as in plant organs like leaves where large intercellular airspaces provide an internal surface that is
10 to 32 times larger than their external surface (Turill, 1936). Exchange rates for leaves are many
orders of magnitude higher than for seeds. Rapidly swelling bodies can be produced by packing
colloid particles inside a porous and elastic shell. Such rapid swelling bodies would allow to
construct a simple and robust sieve-based colloid motor as shown in Figure 19.2.
A last consideration for the efficiency of colloid hydration is based on the fact that the diffusion of
water molecules is by 2 orders of magnitude faster in the gas phase than in liquid. The use of water-
saturated air rather than water in the motor shown in Figure 19.2 would considerably increase its
Hydration by water/humid air
Dehydration by dry air
Retracting
springs
Piston
Seeds
colloids
swelling bodies
Perforated tube
Figure 19.2 Schematic view of a membrane-free colloidal hydration motor-based on macroscopic swelling
bodies like seeds and biomimetic colloid clusters. Water or water-saturated air (fog) initiates the power stroke by
expanding the swelling bodies. Dry air reverses the expansion with springs pushing the piston back into the initial
position.
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