Emerging Technologies Inspired by Plants - Bioinspired Approaches for Human-Centric Technologies - page 122

similar to the actual pool of osmolytes active in plant tissue, we can use a more
general equation, such as a virial expansion, which is normally used to account for
the deviation from the ideal polymer solution. An example relation is given by

OM 3 :

c 2 M 2 ʠ c 1 M þ þ ð 4 : 3 Þ

This simple model can be used to calculate the pressure of an osmotic solution
from the concentration of the osmolytes (Atkins and de Paula 2006 ). From the
osmotic pressure calculation, the net water flux through the osmotic membrane,
maximum force of the actuator, and energy density can be calculated as well as the
characteristic time, which is described below.
Osmotic pressure causes the flow of water from a dilute to a concentrated
solution according to the following equation:

q _ ¼ S OM ʱ OM ʠ þ ð p d p c Þ , ð 4 : 4 Þ

where ʱ OM denote the

surface area and permeability of the osmotic membrane, respectively; and p d and p c
denote the pressure of the dilute and concentrated solution, respectively.
In actual osmotic phenomena, the total flux q tot is also influenced by external
concentration polarization phenomena (ECP) and internal concentration polariza-
tion phenomena (ICP). The ECP results in a decrease in the osmotic pressure due to
the formation of a bilayer of ions on the face of an osmotic membrane. The ICP
results in a decrease in the osmotic pressure, due to the formation of a double layer
of ions on different faces of the membrane.

q is the solvent flux across the osmotic membrane; S OM and _

4.4.1.3 Bioinspired Osmotic Systems

The osmotic principle, which is used by plants for their movements, has been
considered for actuating artificial systems. Most of the related studies were devel-
oped in a biomedical context (particularly for controlled drug release). Starting
from a study by Theeuewes and Yum ( 1976 ), numerous small pumping systems
were developed based on forward osmosis with the goal of achieving a constant
drug release rate over a prolonged period (Herrlich et al. 2012 ). Most of these
systems were developed for intracorporeal applications, and their miniaturization
strongly relies on a simple design based on the osmotic principle, because of the
availability of water (as a solvent) in bodily fluids and because no external power
source is required for operation. Two types of devices can be identified, depending
on whether the drug is used as an osmotic agent or not. If the drug is used as an
osmotic agent, then it is possible to maximize drug storage, but the solubility of the
drug strongly affects the release performance, and it is difficult to use an osmotic
pump with different drugs. If the drug is not used as an osmotic agent, then a release
rate is achieved independent of the drug properties by introducing an additional
compartment that stores the osmotic agent and a movable wall that pushes the drug