Page 422 - Organic Electronics in Sensors and Biotechnology

Electrochemical Surface Switches and Electronic Ion Pumps Based on Conjugated Polymers 399

Water
q = 129°
H 4.1 μm θ = 44° 3.0 μm
N SU8 SU8
N
H n PPy PEDOT:PSS PPy PEDOT:PSS
(Oxidized state) (Reduced state)
FIGURE 11.4 Left: The chemical structure of PPy. A water droplet added to a
textured surface switch, composed of SU8 pillars and a PPy mesh with PPy in its
oxidized state (middle) and after switching to its reduced state (right). (Reproduced
with permission from X. Wang, M. Berggren, O. Inganas, “Dynamically controlled
surface energy and topography of microstructured conducting, polymer upon
electrochemical reduction, Langmuir 24(11):5942–5948 (2008) © 2008 American
Chemical Society.”)

11.1.3 Integration of Wettability Switches
in Microfluidic Systems
As dimensions of microfluidic systems are reduced, capillary forces
become increasingly dominant in controlling the flow of liquids, such
as aqueous samples. If the surface tension is changed along the ceiling,
floor, or walls of the microfluidic channel, the aqueous flow can be
controlled. 6, 12 EC P3HT-based surface tension switches were combined
with microfluidic systems made from PDMS (polydimethylsiloxane)
(Fig. 11.5). P3HT surface switches, individually addressable, were
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defined under each “branch” of PDMS channel Y-junctions. The P3HT
surfaces were then either switched to the oxidized state or left
unswitched in the neutral state. Then a water sample was applied to
the inlet at the channel “trunk” of the first Y-junction. The oxidized
P3HT surface exhibits a relatively higher surface tension which then
provides relatively higher capillary force acting on the aqueous sam-
ple, compared to channels including a reduced P3HT floor. We found
that water samples were guided considerably faster through the chan-
nel branches including a P3HT floor switched to the oxidized state
compared to branches including P3HT floors switched to the neutral
state. Electronic gating of fluids might open for active dispensing of
water samples possible to use in, e.g., lab-on-a-chip applications.

11.1.4 Electronic Control of Cell Seeding and Proliferation
Using Surface Switches
Eukaryotic cells are commonly cultivated and propagated in cell cul-
ture dishes. Soon after cells are seeded into the wells, they adhere to
the bottom surface of the well and eventually they start to prolifer-
ate. It is important to notice that cells do not adhere directly to the
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plastic surface of the cell culture dish. This is so because cells are usu-
ally handled in a suspension of cell culturing medium containing
large amounts of serum proteins. As these proteins rapidly diffuse in
the medium, they immediately adhere to the bottom of the well, thus