To visualize the biodistribution of porous silica microparticles in vivo, known ligands are used to immobilize gadolinium in the pores for use as MRI contrast agents. MRI studies with rodents show that a fraction of the microparticles administered by intraperitoneal injection passed through the kidneys by an unknown mechanism and are, surprisingly, excreted via the urine. Analysis of the urine with SEM and energy dispersive spectroscopy corroborated the MRI results.
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Steinbacher et al. (2010) studied this question.
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