Biomaterials for MEMS

190 K.Yu,G.GaoandJ.N.Kizhakkedathu

Figure 8.18. Illustration for the process of surface grafting via ATRP from the silanized

glass surface covered with initiators. Images adapted from Ref. 103.

for the statistical copolymer brushes formed from a 1:1 homogeneous reaction

mixture of the two oppositely charged monomers.

8.4 POLY(METH)ACRYLATE BASED NON-FOULING SURFACES

Genzer et al. used assemblies of surface-bound poly(2-hydroxyethyl methacrylate)

(PHEMA) brushes with gradual variation of molecular weight and/or grafting

density to vary the amount of adsorbed fibrinogen.

109

They achieved a gradient

of adsorbed fibrinogen on the surface by changing the molecular weight and graft

density with least fibrinogen adsorption found at maximum PHEMA coverage.

Cell culture studies using MC-3T3 E1 cells on these surfaces showed that cell

morphology and density can be varied as function of fibrinogen (and hence

PHEMA) coverage (Fig. 8.20).

Ignatova and J´erˆome synthesized copolymer brushes by a two-step “grafting

from” method that consisted of a) electrografting of poly(2-phenyl-2-(2,2,6,6-

tetramethyl-piperidin-1-yloxy)-ethylacrylate) onto stainless steel b) the nitroxide-

mediated radical polymerization of 2-(dimethylamino ethyl)acrylate and styrene

or n-butyl acrylate initiated from the electrografted polyacrylate chains.

110

The

grafted copolymer brushes were quaternized to make them anti-bacterial. Quartz

crystal microbalance experiments have proven that fibrinogen adhesion was lower

on the hydrophilic quaternized films compared to the nonionic counterpart. Also,

the quaternized copolymer surfaces exhibited significant antibacterial activity

against the gram-positive bacteria S. aureus and the gram-negative bacteria E. coli

8.5 POLY(METH)ACRYLAMIDE BASED NON-FOULING SURFACES

Poly(N-isopropylacrylamide) (PNIPAAm) is a water-soluble thermo-responsive

polymer. PNIPAAm has a lower critical solution temperature (LCST) at about

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Polymer-Based Biocompatible Surface Coatings 191

Figure 8.19. Cell culture on different surfaces: (a) control glass, (b) glass grafted with

polySBMA, and (c) glass grafted with polyCBMA. Images adapted from Ref. 104.

32

◦

C in an aqueous solution. It assumes a random coil structure (hydrophilic state)

below the LCST and a collapsed globular structure above the LCST (hydrophobic

state).

111−115

Because of this unique property, surfaces grafted with PNIPAAm

were used for controlled cell adhesion studies. Kang et al. synthesized well-

defined functional polymer-Si hybrids, consisting of covalently tethered brushes

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192 K.Yu,G.GaoandJ.N.Kizhakkedathu

Figure 8.20. Left: contour plots of a) dry thickness of PHEMA in a MW-σ orthogonal

PHEMA gradient (scale in nm); b) dry FN thickness in a MW-σ orthogonal PHEMA/FN

gradient (scale in nm), the scales depicting the position on the substrate in parts (a) and

(b) are in cm. Right: fluorescence microscopy (10x) images of fluorescently labeled MC3T3-

E1 cells (nucleus: DAPI blue, cytoskeleton/actin: phalloidin red) cultured on PHEMA/FN

gradient substrates. Images were taken at positions on the PHEMA/FN gradient sample

marked with the numbers in part (b). The error bars associated with the thickness measure-

ments (≈ 0.1 nm) are smaller than the thickness increments in the contour plots. Images

adapted from Ref. 109. For color reference, see page 271.

of POEGMA, PNIPAAm and poly (NIPAAm-OEGMA) copolymers and block

copolymers on silicon surfaces via surface-initiated ATRP.

116

Cell adhesion was

evaluated by culturing the cell line 3T3-Swiss albino on the hybrid surface. As

to PNIPAAm grafted silicon surface, at 30

◦

C (above the lower critical solution

temperature of NIPAAm), the seeded cells adhered, spread, and proliferated on

the NIPAAm graft polymerized silicon surface. Below the LCST, the cells detached

from these surfaces spontaneously. Incorporation of POEGMA units into the

NIPAAm chains of the Si-g-PNIPAAm surface via copolymerization resulted in

more rapid cell detachment during the temperature transition.

Our group utilized aqueous ATRP to graft a series of cleavable hydrophilic

PNIPAAm homopolymers and block copolymers of substituted acrylamides from

polystyrene latex to give brushes of controlled MW and surface density.

117

Expo-

sure to radiolabeled single proteins or to whole plasma and subsequent analysis

by SDS-PAGE showed that PNIPAAm brushes decreased protein adsorption rel-

ative to the latex surface or other substituted polyacrylamides. The PNIPAAm

brushes exhibited a second-order phase transition around 30

◦

C as reflected by a

decrease in the hydrodynamic thickness of the brush at higher temperatures. Total

plasma protein adsorption was increased at 40

◦

Ccomparedto20

◦

C but there was

significant differential adsorption behavior among the proteins detected by gel-

electrophoresis analysis.

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