in fresh agarose and imaged with a Turbo RARE (Rapid Acquisition with Refocused Echoes) T -weighted sequence with the following parameters: field of view 30x30 2 mm, matrix size: 512 x 512, echo time: 30 ms, repetition time: 2200 ms, averages: 6, slice thickness: 0.5mm, scan time: 14 minutes 5 seconds. Effect of serum on particle properties. of serum on particle properties. To assess the adsorption of serum proteins by the particles and their effects on size distribution and zeta potential, SPIONs were mL-1 oC suspended in culture medium at a concentration of 50 µg and kept at 37 in a humidified incubator for 24 hours. After incubation, particles were separated from unbound protein using a magnetic-activated cell sorter (MACS). In brief, MACS columns (Miltenyi Biotec) were equilibrated with 1 mL PBS and 1 mL culture medium. Then, 1 mL of sample was run through the column, followed by a PBS wash and elution in 0.01 M NaCl. Control samples were generated by keeping SPIONs in water but allowing them to run through the MACS column. These were used to check any effects of magnetic field exposure alone on the size distribution of SPIONs.

## Acknowledgements

This work was supported by the UK Regenerative Medicine Platform (MR/K026739/1). Imaging data in this article were obtained in the Centre for Preclinical Imaging (CPI) of the University of Liverpool. The CPI has been funded by a Medical Research Council (MRC) grant (MR/L012707/1). MJR is a Royal Society Research Professor. DA thanks the EPSRC for a Fellowship (EP/L021978/1). George Miller is thanked for ICP measurements.

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