GELTHINFILM

Polymer hydrogels with stimuli-responsive properties are interesting materials for several applications due to their large volume change by absorption or expulsion of water. The hydrogels can respond to various stimuli such as pH, salt concentration, temperature, light, electric or magnetic field. The reversibility of the volume transition is an additional advantage for the applications of these gels as responsive materials. However, the swelling/collapse phase transition of macroscopic hydrogels is a diffusion-limited process. The decrease of the feature size in thin films is an appropriate way to create structures with fast response times. Hydrogel thin films can be then rendered multifunctional and multiresponsive without compromising their mechanical stability, which is secured by a 3D cross-linked network structure. The hydrogel thin films we are interested to study are chemical polymer networks that are covalently linked to the surface. Their size can range from nanometers to hundred of microns. The advantages of gel thin films can be explored for the fabrication of miniaturized devices with fast response times. A stimuli-responsive gel material embedded inside a microfluidic channel can operate as a valve (functional gate) which opens or closes the microchannel for a water flow. “Smart” hydrogel valves eliminate the need of bulky external control and thus allow the creation of handy and autonomous “lab-on-a-chip” systems for analytics. The project fits precisely within that framework and focuses on three areas: 1) Development of the surface-attached hydrogel thin films. Among the possible external stimuli, we choose to focus on gels responding to temperature, light or electric field. The synthesis follows a straightforward approach which allows easily the variation of the chemical functions of the polymers and then of the responsive properties of the hydrogels. The principle is based on thiol-ene chemistry. The formation of surface-attached gel films will be achieved by adding bifunctional thiol molecules as cross-linkers to the ene-reactive polymers on thiol-modified surfaces. The objective is to obtain hydrogel films with a wide range of thicknesses and with the desired responsiveness nature. 2) Structure of the surface-attached hydrogel thin films. The structural investigation of these thin films tackles a fundamental aspect of confined and constrained systems. We will attempt to answer the following questions. What is the effect of the confinement on the swelling of the gel films? How does the attachment on surface affect the free surface of the gels? Three stages will be exploited: the linear swelling ratio of the hydrogel films in the direction normal to the substrate (using ellipsometry), the interfacial width (using neutron reflectivity) and the wrinkling aspects (using AFM) of the free surface of the hydrogels. 3) Microfluidic applications of the hydrogel thin films as microvalves. We will show the possibility to design surface-attached hydrogel thin films in microfluidic devices for the actuation of mechanical microvalves. It means that the surfaces functionalized by the hydrogel layers will be embedded in miniaturized systems and their function as microvalves will be tested. The use of light and electric field as stimuli parameters is innovative, providing microvalves easily activated with fast responses. The nature of the responsiveness is also compatible with patterning processes. This route is very promising for the development of more and more high-performance “lab-on-a-chip”. The strong point of the project is the complete investigation of thin films of stimuli-responsive hydrogels, starting from a controlled synthesis and a detailed characterization of the structure to a concrete application in microfluidics. This multidisciplinary project provides the possibility to assemble chemistry of polymers, fundamental aspects of physics on confined systems and pioneering concepts of microfluidics engineering.