Stimuli-responsive materials as sensors and actuators in microfluidic devices

Abstract

The integration of stimuli-responsive materials into microfluidic systems can provide a means for external control over fluid flow and can reduce the overall complexity of microfluidic devices. Herein we present several approaches for introducing fluid movement and sensing using stimuli-responsive materials. The first approach comprises the use of adaptive nanostructured coatings for direct sensing of flow in continuous flow mode. For this, the inner walls of micro-capillaries and micro-channels were coated with polymeric materials that can be used to detect a variety of target species. Two types of adaptive coatings will be discussed. The first one is based on the conductive polymer polyaniline (PAni) [1,2] while the second consists of polymeric brushes based on spiropyran [3,4]. Using the “grafting” approach homogeneous coatings were obtained on the micro-channel/micro-capillary surface that retained their inherent nano-morphology. The optical proprieties of these coatings change in response to a variety of target analytical species (divalent metal ions, solvents of different polarities, ammonia, H+) passing through the microfluidic device in continuous flow mode. The grafting approach can provide nanostructured to microstructured coatings that combine small diffusion paths with relatively thick optical pathlengths, thereby providing sensitive and fast optical responses to the target analytes. The second approach comprises the use of porous photo-actuated hydrogels as photo-controlled micro-valves in microfluidic systems for repeatable ON/OFF flow modulation in flowing streams over a wide range of pH values (acidic to ca. pH 7.0). Incorporation of such stimuli-controlled structures in microfluidic devices offers unprecedented versatility and external flow control. We envision using these systems to create a new generation of sustainable, low-cost, photonically-controlled and self-reporting fluidic systems