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Polymers are a group of macromolecules used in formulations of pharmaceuticals, one example being the delivery system DC Bead™. Further, some of the most abundant and for drug delivery important constituents of the subcutaneous tissue are charged polymers (polyelectrolytes), e.g. collagen, hyaluronic acid, and chondroitin sulfate. The interactions between these subcutaneous polyelectrolytes and drug molecules are believed to heavily affect the transport and absorption of subcutaneously injected drugs. To increase the understanding of how the interactions between subcutaneous polymers and drug molecules affect the pharmaceutical behavior in subcutaneous tissue, we developed a new microfluidic-based platform. The platform is used to study interactions between polyelectrolytes and drug molecules, and can beyond the investigation of subcutaneous interactions be used to develop polyelectrolyte-based microgel formulations. In this thesis, the microfluidic method denoted “Microfluidic chip for interactions studies” (MIS) is presented, and the design, validation, and several examples of usage are described. The method which is based on microfluidic instrumentation, utilizes spherical microgels created using different types of polymers/polyelectrolytes. These hydrogels collapse when experiencing attractive interactions with drug molecules making it possible to investigate drug binding by studying the volume change of the microgels. We prove that the interactions are strongly affected by charges both on the gel networks and the drug molecules. Further, the aggregation behavior of drugs in a polyelectrolyte-rich environment is studied in detail. Results show that both a strong aggregation behavior and a high charge on the drugs may affect the transport through a network of polyelectrolytes. The behavior of drugs in subcutaneous polyelectrolyte-rich environments such as hyaluronic acid networks, can partly explain bioavailability and absorption rates of the drugs in vivo. Several potential drug delivery systems in the form of microgels were investigated together with both small amphiphilic molecules and larger peptides exhibiting a wide range of physicochemical properties. The results indicate a possibility of delivering large amounts of drug in low volumes of microgel suspensions but with varying release times, ranging from seconds to days. The MIS was able to provide information about the interactions in a large number of polyelectrolyte-drug systems. The studies were performed in a highly efficient and cost-effective way, with experiments being mostly automated. This makes it a suitable method for rapid screening experiments in the development of new microgel formulations, and as part of larger studies utilizing several different methods to better understand and predict the behavior and absorption profiles of potential subcutaneously administrated drugs.