High Throughput Techniques for the Investigation of Cell–Material Interactions - Biointerfaces: Where Material Meets Biology - page 292

however can be charged groups such as anions (carboxylates, sulfates, sul-
fonates, phosphates) or cations (amines) or polar uncharged groups such as
alcohols. There are several methods that have been employed to create
wettability gradients including plasma polymerisation, 9 self-assembled
monolayers (SAMs), 175 corona discharge, 8,208 and photodegradation 209 just
to name a few, with a comprehensive review on this subject by Ruardy et al. 22
Choee et al. 8 produced a wettability gradient on polyethylene (PE) through
oxidation of the polymer with increasing power via corona discharge. The
resulting carbon-based radicals including metastable peroxides and
hydroperoxides reacted further to produce various oxygen-containing
functionalities such as ketones, aldehydes, hydroxyls and carboxylic acids
thataremorehydrophiliccomparedtoPE. 210 A resulting gradient with
contact angles ranging from 951 to 181 was then exposed to NIH/3T3
fibroblast cells. It was found that the fibroblasts grew best on a moderately
hydrophilic surface (contact angle

d n 3 r 4 n g | 7

551). Wei et al. 9 and Yoshinari et al. 10
have prepared hexamethyldisiloxane (HMDSO) coatings, displaying a
hydrophobic surface, via plasma polymerisation. Subsequent O 2 -plasma
treatment with varying duration across the surface gave a gradient of
wettability. The adsorption of Fn increased with increasing wettability,
whilst albumin adsorption occurred preferentially on more hydrophobic
regions of the gradient.

B

10.2.3 Biological Density Gradients
The ECM changes comprises many proteins including collagen, vitronectin,
Fn, elastin and laminin. These proteins have been used extensively
in the investigation of cellular responses to surfaces through either
deliberately adsorbing a particular protein to the surface or in many cases
the inadvertent deposition of proteins (e.g.,

.

from serum in culture

media). 1,3,15,82,123,211,212
Gradient surfaces displaying a gradually decreasing density of Fn were
prepared by Bhat et al. 176 PHEMA gradients were prepared via atom transfer
radical polymerisation (ATRP). 202 Fn adsorption was found to decrease
across a gradient displaying a linear increase in PHEMA density. For very low
thicknesses of PHEMA, a thick layer of Fn was adsorbed. However, the Fn
thickness decreased rapidly as the thickness of the PHEMA increased until
no Fn was adsorbed approximately half way along the gradient. Fn serves as
an anchor for the attachment of osteoblastic cells used in this study. 31-33
Therefore as expected, the concentration of the cells decreased as the
thickness (density) of the PHEMA increased (i.e., as the density of Fn de-
creased). Variances in cell morphology was also observed along the gradient.
Well spread cells were observed on the thin PHEMA region whereas rounded
cells were observed on the thick PHEMA. In an PHEMA/Fn gradient prepared
by Mei et al. 213 a sigmoidal response was observed for both Fn adsorption
and cell attachment as the PHEMA density increased with maximum cell
attachment observed at a Fn density of 50 ng cm 2 .