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Ag and AgCu containing powdered silica fillers of polymers for medical applications have been synthesized using mechanochemical activation with a low content of solvent or dry powdered materials. The samples in the form of filled polymer films were obtained using polyurethane-urea and silica nanocomposites. The composites have been investigated by TEM, UV/Vis spectroscopy, XRD and microbiological analysis. It has been found that amorphous and nanosized structures of silver oxides and metallic Ag nanocrystallites are formed during thermal destruction of silver nitrate or ammonium complexes in silica matrix (fumed or precipitated silica). Sunlight exposure of nanocomposites contained Ag+ ions leads to the formation of Ag0 nanocrystallites. After UV irradiation, the decrease in surface plasmon resonance band intensity can be caused by the oxidation of Ag0. It is found that silica nanocomposites with the low Ag content (0.2 mmol g−1 SiO2) exhibited the highest antimicrobial activity, whereas the polymeric film with a filler content of 1 wt% exhibited the most efficient antimicrobial properties among tested polymeric composites. The observed transformations (Ag+ ↔ Ag0) of silver compounds do not suppress the biocidal activity of nanocomposites. [ABSTRACT FROM AUTHOR]