For brain, heart, kidney and lung tissues no large differences were observed in titanium concentration for tissue samples from mice which had been subcutaneously injected with TiTannic gels compared to tissue samples from mice which had not been exposed to TiTannic gels. The titanium concentration in these tissues were below 50 ng Ti per g of tissue (Figure ). This is in agreement with previously reported results for unexposed rodents. A background level is expected as titanium is abundant in our environment, especially in the form of TiO2 (e.g. in pigments, sunscreens, toothpaste, paints) with median daily adult intake in the UK being approximately 2.5 mg. For patients with titanium-containing implants, soft tissue Ti concentrations of around 1 × 10 6 ng Ti per g tissue have been reported directly adjacent to the implant, and concentrations around 6,500 ng Ti per g for tissues collected 3 cm from the implant. In the present study, increased Ti concentrations were observed for liver and spleen tissues at the longer time points (Figure ), with concentrations of around 50-100 ng Ti per g of tissue. The overall low titanium concentrations observed is in agreement with the low gel disassembly observed both in vitro and in vivo (Figure , Figure and Figure ). Titanium concentration in blood and urine samples collected from the animals were also determined using mass spectroscopy (Figure ). At earlier time points (3 days and 1 week) a 5-10fold increase in Ti concentration was observed for the blood samples obtained from animals subcutaneously injected with TiTannic gel compared to blood obtained from unexposed animals: from around 1 µg Ti per L of blood to 5-10 µg per L. Basal concentrations on the order of 1 µg Ti per L of blood is routinely reported for unexposed rodents and patients. Ti concentration in urine samples was low (close to limit of detection) for all samples studied (Figure ), as expected as the metal is not readily excreted in the urine. Taken together with the observed Ti accumulation in the liver (Figure ), this suggests that a possible excretion mechanism may instead be through the feces. Feces were not collected and analyzed in the current study, but may be of interest for future studies. TiTannic gels exhibit more sustained drug release compared to Pluronic F127 gels.