dramatically because of their clustering and confinement into endosomal compartments.10 Recently, our group compared three commercial SPIONs with different particle size, zeta potential, particle arrangement and solution relaxivity. Despite the differences in chemistries, the SPIONs were shown to have very similar T 2 cells.10 relaxivity when internalised into stem This emphasised that when designing SPIONs as MRI contrast agents for stem cells, it is more important to maximise SPION uptake without altering cell viability or phenotype, rather than focussing on the se.10, 11 structural and magnetic properties of the SPIONs per Optimising uptake of SPIONs into stem cells can be difficult because of factors.11 multiple influencing SPIONs usually require sufficient colloidal stability in aqueous conditions for storage and transfer to physiological medium. Colloidal stability in the physiological medium used to label stem cells may not be mandatory, toxicity.11 but is relevant as sedimentation can impact uptake and Polymers are commonly used as coatings for SPIONs as they give long-lasting colloidal stability repulsion.11-13 through steric or electrostatic However, not all polymer coatings result in the internalisation of SPIONs into stem cells. Common hydrophilic polymer coatings for SPIONs such as polyethylene glycol and dextran are known to lead to low uptake and require the use of a transfection agent or post-modification step for cells.14-16 sufficient uptake into stem This was the case for the two formerly clinically approved products Endorem™ and Resovist®, which contained a dextran coating and often required the use of transfection agents to achieve a significant cellular loading of cells.14 iron oxide into stem Endorem™ and Resovist® have often been used for cell 2009.13, 14 tracking off-label; however, they were both removed from the market in There is renewed interest in developing SPIONs which can be directly taken up by stem cells without the need for extra chemical additives, thus simplifying labelling and agents.11, 13 avoiding potential toxicities from transfection Altering the surface charge of SPIONs has become a popular strategy for increasing uptake of SPIONs into stem cells, particularly with cationic polymers such polyethylenimine17 poly-L-lysine.18 as or Another approach involves taking neutral pullulan19 dextran16, 20 polymers such as or and functionalising with amines to give an overall positive charge. We have previously shown how manipulation of the surface charge of SPION coatings can have an effect on both uptake and MRI contrast in cells.16 mesenchymal stem This was achieved by altering the amount of amine- containing dextran in the co-precipitation reaction used to form the SPIONs. It is still not well understood if the higher uptake of cationic SPIONs is facilitated through their binding to the negatively charged cell membrane or via the incorporation of specific proteins into the protein corona, although most studies suggest the protein corona uptake.21, 22 formed around nanomaterials in medium controls cellular Varying the polymer concentration in a co-precipitation reaction can yield relaxivity23 particles with different core sizes, polymer coverage and solution but, as noted above, SPIONs with different solution relaxivity can have very similar cell relaxivity.10 It is still not well established or understood how the integrated physicochemical properties of SPIONs such as core size, hydrodynamic diameter and amount of polymer in the particle shell affect uptake of SPIONs or their interaction with the physiological medium. Although such information is not known for SPIONs, al.24 Freese et reported that gold nanoparticles with different core size and surface charge can have significantly different uptake in human dermal and human brain endothelial cells: a larger core size resulted in increased uptake of overall incubation.24 nanoparticles after 24 hours Here, we demonstrate for the first time that the amount of polymer used in SPION synthesis is an important factor in tailoring not only core size and magnetic properties, but also stem cell uptake and the associated MR contrast.