Environmental Engineering Reference
In-Depth Information
Intelligent/Dynamic materials
(Triggered by changes in environment)
Active response
Passive response
Bulk control
Local control
Triggering molecule
Material after
response
Responsive
material
Δ pH, T, P
Triggering stimuli
-Δ pH, T, P
fIgure 22.1 environmentally responsive materials can be categorized into three different types: passive responsive materials, which
respond to the presence or absence of a specific molecule; active responsive materials, which can be reversibly triggered externally in
response to a stimulus, and among these active materials there are two categories. These are materials that respond to bulk changes in their
surroundings, such as pH and temperature, and materials that respond to local changes, such as light.
and applied voltage [23, 24]; and (4) locally triggered materials responsive to light [25-31] or the presence of a specific mole-
cule [32, 33]. In addition, several of these response polymers can be coupled or modified with specific functional chemical
groups to exhibit combinatory responses, such as designing a copolymeric material with both pH and temperature sensitivity.
22.1.1
bulk-triggered responsive materials (ph and temperature)
bulk-triggered responsive materials have found a series of applications in the delivery and precise release of chemical reagents,
such as drugs and growth factors, both in vivo and in vitro . some examples include hydrogels capable of responding to different
chemical and physical external stimuli (such as pH and temperature) that release a drug (protein or peptide) at specific sites in
the body [34]. In addition, the synthesis of systems with higher complexity has been designed to respond to specific enzymes
[35, 36] and chemical entities. This allows sophisticated cellular targeting and delivery mechanisms, such as the hydrogels
employed as insulin delivery systems, which are capable of responding to the presence of glucose [37]. These and other cleverly
designed delivery devices have improved therapies for a wide variety of diseases [37-39].
Additional applications, which this work is most interested in, are those in which these materials are used to deliver reagents
into in vitro microfluidic systems. The most successful attempts to fabricate valves have been accomplished using these bulk-
triggered responsive materials. These systems present several advantages, such as their capacity to externally control flows
(redirecting) and to introduce chemical reagents with fast response times by simply changing the bulk conditions of the system,
such as pH [40], pneumatic pressure [41], temperature [42], or chemical entities like glucose [16]. However, their design poses
a main problem for their use in cell culture systems, that is, their driven mechanical action is triggered by changing the bulk
conditions of the environment. Changing the bulk conditions of the system causes cells to be exposed to hostile environments
not apt for their survival. such challenges have been recently addressed by developing materials that present localized responses
to specific light wavelengths, [13, 43] or by implementing nanofabricated chemical reagent sources containing chemical
reagents that are dispensed to the system through pressure [44] or simple diffusion [45].
22.1.2
locally responsive materials (light)
optically responsive materials offer great advantages and present appropriate characteristics that enable them to be used in
mechanical actuators for controlling the flow and delivery of chemical reagents in cell culture systems. Although other cell
culture designs have been proposed for the delivery of chemical reagents without changing the bulk conditions of the system,
such as pressurized reagent sources [44], they require complicated circuitry and do not offer the capability of stimulating highly
specific sites within the system as easily as it is done using an external light source (laser) [14].
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