Emerging Technologies Inspired by Plants - Bioinspired Approaches for Human-Centric Technologies - page 123

outwards in response to an increase in volume of the osmotic agent compartment.
The design is more complex for the second type of system, which results in a
reduced amount of drug storage. In all of the cases, a constant release rate is attained
as long as the solution containing the osmotic agent remains saturated. Addition-
ally, the design of the drug outlet is important to guarantee that the drug release is
effectively driven by the osmotic process rather than by diffusive phenomena. This
also applies to the case in which tiny catheters are added to the outlet to achieve
localized release to targeted zones. Additionally, all of these systems were devel-
oped with biocompatible materials, which are suitable for implants even if the
biodegradable materials (degradation would be designed to occur after the opera-
tional time interval) would avoid the explantation phase. Additional devices that
include a solvent reservoir were proposed that were suitable for extracorporeal
usage. However, this increases the complexity of the system, and the performance
of these systems can be affected by additional factors such as environmental
temperature. Additional details on these types of osmotic pumps can be found in
Herrlich et al. ( 2012 ). Most of the previously discussed systems are commercially
available and are used in clinical development; however, they have not reached a
stable position within the biomedical market. Within the biomedical sector, a
miniature osmotic actuator was proposed in Li and Su ( 2010 ), which combines
drug release and mechanical actions that focused on bone distraction. In particular,
the distraction force (less than 10 N over nearly 200 h) was generated by an
osmosis-driven piston mechanism in which the design and fabrication steps were
carefully studied to avoid solution leakages.
Outside of the medical field, an osmotic actuator was proposed to steer the tip of
a mechatronic system inspired by the apex of the plant roots (Mazzolai et al. 2011 ),
which has applications in soil exploration and monitoring. This type of actuator was
based on electroosmosis because of its potential for reversibility. In particular, the
steering concept was based on three cells that were separated by pairs of semiper-
meable osmotic membranes and ion-selective membranes and individually coupled
with a piston mechanism. Technical issues primarily related to the degradation of
the ion-selective membranes during the electroosmotic process (lead acetate was
used as salt) encouraged the development of an alternative actuation strategy based
on forward osmosis. By adopting a bioinspired approach, a modeling study was
performed (Sinibaldi et al. 2013 ) to extract preliminary design guidelines based on
targeted performance metrics such as the characteristic time of actuation or the
maximum force. A dynamic model of the osmotic actuation concept was developed
based on the following key elements: an osmotic membrane, an actuation chamber
that contains the osmotic agent and both a rigid and a deformable boundary, and a
solute reservoir chamber. These elements should also be accounted for when
considering osmotic actuation in plants. The elastic deformation of the movable
boundary of the actuation chamber (which acted as a force transducer) was modeled
in more detail by assuming that energy storage occurred either through an external
elastic load (a spring-piston system) or membrane bulging (see Fig. 4.5 ).