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Carrier-mediated drug delivery has emerged as a powerful methodology for the treatment of various pathologies. The therapeutic index of traditional and novel drugs is enhanced via the increase of specificity due to targeting of drugs to a particular tissue, cell or intracellular compartment, the control over release kinetics, the protection of the active agent or a combination of the above. Nanoparticles (NPs) were proposed as drug carriers over 30 years ago and have received growing attention since, mainly due to their stability, enhanced loading capabilities and control over physicochemical properties. The unique pathophysiology of solid tumors allows passive accumulation of NPs at these sites upon intravenous injection. Furthermore, stealth NPs with long circulation times are more efficient in reaching tumor tissue. In addition to systemic administration, localized drug release may be achieved using macroscopic drug depots close to the target site. Among various systems considered for this approach, in situ-forming biomaterials in response to environmental stimuli have gained considerable attention, due to the non-invasive character, reduction of side effects associated with systemic administration and better control over biodistribution. This thesis focuses on the design, preparation and in vitro characterization of polymeric, hydrogel nanoparticles with thermoresponsive properties. Inverse emulsion polymerization was selected for their fabrication via cross-linking of acrylate derivatives of poly(ethylene glycol) (PEG) and poly(ethylene glycol)-bl-poly(propylene glycol)-blpoly( ethylene glycol) (PEG-PPG-PEG) copolymers, also known as Pluronics®. This polymerization technique allows for control over size, is versatile in respect to initiation and composition, and proceeds to full double-bond conversion in relatively short times. Incorporation of functional comonomers in the polymeric network additionally offers the possibility of further modifications, as is demonstrated by fluorescent labeling of the colloids. Moreover, hydrogel NPs of 100-500 nm are stable against aggregation as aqueous dispersions and as freeze-dried solid powders. The particles we discuss here, may be visualized as nanoscale, three-dimensional, polymeric networks consisting of PPG-rich, hydrophobic domains surrounded by a hydrophilic, PEG-rich matrix. The permanence of domains similar in hydrophobicity to Pluronic micellar cores, but insensitive to dilution under the critical micellar concentration, allows the accomodation of poorly water-soluble drugs through hydrophobic interactions, as was experimentally shown using the anticancer agent doxorubicin. A fast and efficient solvent evaporation technique was developed in order to physically encapsulate the drug. Doxorubicin is thus partially protected from degradation and diffuses out of the NPs without a burst, over one week under sink conditions in vitro. Thermosensitivity of nanoparticles is manifested as a size reduction of non-interacting colloids in dilute dispersions and as a macroscopic, fluid-to-solid, physical transition of concentrated samples. The driving force of these phenomena is an entropically-driven deswelling of the hydrogel NPs with increasing temperature, which leads to their hardening. At concentrations above which there is physical contact of neighboring particles, this intraparticulate event results in the dynamic arrest of particles within a 'cage' formed by their neighbors. This mild and reversible transition occurs at a clinically-relevant temperature range (25-30°C), with no syneresis or by-product formation, and is compatible with living cells. Upon dissolution in body fluids, the colloidal macroscopic drug depot will give rise to a colloidal dispersion; however, it is notable that the processes of encapsulated drug release and dissolution are independent and may be tailored on a case-to-case basis. In vitro cell culture studies revealed that nanoparticle cytotoxicity was negligible even at high concentrations. Interactions with macrophage-like cells, intended to model cells of the mononuclear phagocyte system, showed limited colloidal uptake which is not influenced by the presence of serum, but is energy dependent to a considerable extent (approx. 30%). We believe this low association stems from the hydrophilic, protein-repellent nature of the materials employed and suggests a stealth character. In conclusion, the nanoparticles presented here are well suited for certain drug delivery applications, including cancer therapy and in the prevention of post-operative adhesions, both in the form of injectable dilute dispersions or as in situ gelling thermoresponsive biomaterials.