research activities in 2007 - CSEM

Stimuli-Responsive Surfaces and Smart Coatings
F. Montagne, R. Pugin
Due to their unique “switchable” properties, stimuli-responsive polymers have been attracting considerable attention in biotechnologies and
successful applications have already been demonstrated in sensing, intelligent textiles and bioseparation. As an illustration of CSEM activities in
the field of “smart” surfaces, presented here are MEMS compatible surfaces modified with poly(N-isopropylacrylamide) (PNIPAM), a thermoresponsive
polymer allowing the control of surface wettability.
Stimuli-responsive polymers, also referred to as “smart”
polymers, are a very interesting class of polymers since they
exhibit marked and rapid conformational changes in response
to external stimuli such as temperature, pH, electric field or
ionic strength. When grafted to surfaces, they confer to
materials unique surface properties as they have the ability to
control hydrophilic/hydrophobic balance, roughness, adhesion
or permeability.
In the frame of HYDROMEL European Project [1] , efforts were
particularly focused on thermally responsive polymers and
developed methods for modification of silicon and gold
surfaces with thin poly (N-isopropylacrylamide) (PNIPAM)
films. In water, free PNIPAM chains exhibit a very sharp
transition temperature, called LCST (Lower Critical Solubility
Temperature), at about 32°C. At temperatures lower than
32°C, PNIPAM chains hydrate to form expanded structures,
whereas they dehydrate and collapse at temperatures above
the LCST. It is a challenge to preserve these remarkable
hydration-dehydration changes when polymer chains are
covalently attached onto a surface in just a few nanometer
thick films. A first grafting method that has been developed
consists in the covalent immobilization of end-functionalized
PNIPAM under melt using reactive silanes as intermediate
coupling agents (ICA) (Figure 1). It is worth mentioning here
that the process can easily be adapted for the grafting of any
kind of functional polymers onto various types of reactive
surfaces (plane or colloidal).
Figure 1: Grafting process for covalent immobilization of tethered
PNIPAM on silicon surface and evidence of surface responsiveness
for a 6 nm thick PNIPAM film.
In the present case, responsive properties could be evidenced
by surface energy measurements showing an increase of the
water contact angle when the temperature is raised above the
LCST (Figure 1). Force measurements performed using
Atomic Force Microscopy in a liquid environment also attested
to a change in the profile of repulsive forces below and above
the theoretical value of LCST.
Thermally responsive micro-patterned surfaces have been
created using micro-contact printing (µCP). This technique,
also referred to as soft lithography, uses a PDMS stamp to
pattern molecules on surfaces. Briefly, the stamp is first 'inked'
with a solution of molecules, dried and then pressed onto the
surface to be patterned. The soft PDMS stamp makes
conformal contact with the surface and molecules are
transferred directly from the stamp to the surface within a few
seconds. As shown in Figure 2, µCP has been successfully
adapted for direct grafting of thiol-terminated PNIPAM chains
onto gold.
Figure 2: SEM picture of thermally responsive PNIPAM microdomains
printed onto gold surface. Size of the domains = 128
microns.
Thermally responsive surfaces are currently evaluated at
CSEM for reversible capture and release of cells. First results
show that cells adhere and proliferate on PNIPAM-modified
surfaces at 37°C (above LCST) and can then be released by
simply decreasing the temperature to 27°C (below LCST).
Based on these results, the year 2008 will see the creation of
patterned thermo-sensitive surfaces with tuned dimensions for
individual cell immobilization, as well as integration of these
components in automated system for cell transfection.
This work was partly funded by the OFES and the European
Community via the European project HYDROMEL. CSEM
thanks them for their support.
[1] HYDROMEL: Hybrid Ultra Precision Manufacturing Process
Based on Positional and Self-Assembly for Complex Micro-
Products – Sixth framework programme priority (NMP)
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