1. low temperature prevent cells from active transport.

2. coated particles could be linked to selecting ligand via DNA bridge or other molecules

3. after separation linkage could be broken enzymatically or by other method and particles could be removed

Result: negligible level of particles uptake and virtually no activation.

Depending of cells (MSC f.e.) positive selection could be based on binding to integrin molecules thus binding not harmfull for cells moreover even protective sometimes. Addition of FBS to media also introduce a lot factors which might signalize to cell about "friendly surround".

Hi,

Sometimes we need to focus in details of how things are done (e.g. cellular uptake of particles, wether nano or micro), to prevent the action. Keeping this in mind, you may find the following articles useful:

The role of surface charge in cellular uptake and cytotoxicity of medical nanoparticles

E Fröhlich - Int J Nanomedicine, 2012 - academia.edu

Effects of nanoparticle size on cellular uptake and liver MRI with polyvinylpyrrolidone-coated iron oxide nanoparticles

J Huang, L Bu, J Xie, K Chen, Z Cheng, X Li, X Chen - ACS nano, 2010 - ACS Publications

Tailor-made magnetic Fe3O4@ mTiO2 microspheres with a tunable mesoporous anatase shell for highly selective and effective enrichment of phosphopeptides

WF Ma, Y Zhang, LL Li, LJ You, P Zhang, YT Zhang… - Acs Nano, 2012 - ACS Publications

Porous hollow Fe3O4 nanoparticles for targeted delivery and controlled release of cisplatin

K Cheng, S Peng, C Xu, S Sun - Journal of the American Chemical …, 2009 - ACS Publications

Targeted temperature sensitive magnetic liposomes for thermo-chemotherapy

P Pradhan, J Giri, F Rieken, C Koch… - Journal of Controlled …, 2010 - Elsevier