Development of Reversible Hydrophilic/Hydrophobic Surfaces, 01-R9507

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Principal Investigators
(Nitin Nitin)
Richard K. Suzuki
Neal K. Vail

Inclusive Dates:  10/01/04 – 04/01/2006

Background - There is a growing scientific and industrial interest in the ability to control the wetting of surfaces. Super-hydrophobic surfaces are of particular interest because they have numerous applications including self-cleaning materials and protein purification. Concurrently, there is interest in surfaces capable of switching between hydrophobic and hydrophilic states. Thermal changes can cause certain polymers to reversibly switch their hydrophobicity. Surfaces with thermally responsive, reversible hydrophobicity have potential applications in various areas such as protein purification, controlled cell adhesion, and selectively repellent coatings.

Approach - Layer-by-layer self-assembly of charged polymers uses the consecutive deposition of alternating cationic and anionic polymers onto a surface. This straightforward and simple method allows the build-up of multiple layers on surfaces. The driving force for build-up of the layers is the ionic attraction between opposite charges in the alternating polymer layers. Not all thermally responsive polymers are charged, and ordinarily these polymers could not be used in a layer-by-layer self-assembly process. This limitation was overcome in this project by developing co-polymers that contained both thermally responsive monomer units and charged monomer units. Because proteins adsorb on to hydrophobic surfaces, the surfaces coated with the thermally responsive co-polymers were examined as a means to extract proteins from solution.

Accomplishments - Charged co-polymers were successfully synthesized. These co-polymers were comprised of thermally responsive monomer units and either cationic monomer units or anionic monomer units. The co-polymers were deposited onto various surfaces including glass, quartz, and stainless steel. The coatings adhered very well to all surfaces and were physically robust. Different methods could be used to deposit the co-polymers such as spin-coating, dip-coating, and coating via flow-chamber. An automated coating system was also successfully developed and built. This system allowed the time-consuming, layer-by-layer deposition process to run under computer control. 

The co-polymer coatings demonstrated the ability to extract fluorescein isothiocyanate labeled albumin from solution during a change in temperature. The absorbed albumin could be reversibly released from the surface with another change in temperature, as shown in the movie. These experiments demonstrate that the charged co-polymers developed through this project possess the same thermally controlled hydrophobic/hydrophilic properties of native thermally responsive polymers.

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