Biomedical Engineering Reference
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Fig. 9.8 The effect of temperature on the changing in hydrogen bonding and corresponding
conformation of PNIPAAm [ 169 ]
monolayer on patterned silicon wafer composed of square pillars. Different wetting
behaviors related to the pillar spacing on the silicon are reveal and the maximum
CA change was only about 66 ı basedon40 m spacing.
Alternatively, spiropyran is another kind of photochromic organic molecule.
Its closed nonpolar form with hydrophobicity and its highly polar open type—
merocyanine isomers with hydrophilicity can be reversibly tuned upon UV and Vis
irradiation by properly choosing photon energy. The process is reversible upon green
laser irradiation, and it possesses a light-switchable wetting property [ 156 , 163 ].
Obviously, the light-switchable wettability of films modified by spiropyrans can be
further amplified by introducing surface roughness [ 164 ].
9.4.2
Temperature and Dual Temperature/PH as External
Stimuli
Thermo-sensitive compounds, changing chemical compositions and/or surface
roughness under different temperature, have been considered as other promising
external stimuli to manipulate the surface wettability. Poly( N -isopropylacrylamide)
(PNIPAAm) is an prominent thermal responsive polymer favored by a lower critical
solution temperature (LCST) of about 32-33 ı C[ 165 - 168 ]. Below the LCST, the
intermolecular hydrogen bonding between PNIPAAm chains and water molecules
are predominant, resulting in an extended brush structure with hydrophilic ends.
Above the LCST, intramolecular hydrogen bonding in the PNIPAAm chains leads
to a compact and collapsed conformation showing hydrophobicity (Fig. 9.8 ). By
introducing the roughness into the surface, reversible switching between super-
hydrophobicity and superhydrophilicity has been achieve in a narrow temperature
range of about 10 ı C[ 169 ].
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