How to convince suspension cells to adhere more tightly?
I'm developing a cell-based assay in 96-well plates that requires adherent cells, as they need to be washed at least twice during the protocol. I'm using in-house strains of [HT1080](https://www.atcc.org/Products/All/CCL-121.aspx) cells (some overexpressing a certain protein of interest) that unfortunately have been selected for suspension growth in chemically-defined media (50/50 [CD-CHO](http://products.invitrogen.com/ivgn/product/10743011?ICID=search-product)/[CD-293](https://products.invitrogen.com/ivgn/product/11913019?ICID=search-product)). I can convince them to adhere by culturing in [DMEM](http://www.invitrogen.com/site/us/en/home/Products-and-Services/Applications/Cell-Culture/Mammalian-Cell-Culture/Classical_Media/dmem.html) with 10% [FBS](http://products.invitrogen.com/ivgn/product/16000036?ICID=search-product) and growing them in [CELLBind](http://www.corning.com/lifesciences/us_canada/en/technical_resources/surfaces/culture/corning_cellbind_polystyrene.aspx) flasks, but they still tend to pull away if there is very much shearing force, such as during the wash steps, or when they're in PBS during the assay wash and incubation steps (maybe 2 hours total for now, I'm still optimizing that).Unfortunately, the parameters of my experiments don't allow me to keep the cells in serum-containing media during the assay, as it would interfere with the uptake of my drug. Like I've said, when in PBS they start detaching, and I would assume the same in HBSS, which I don't really want to use anyway due to its low buffering capacity in CO<sub>2</sub> incubators. Might DMEM alone (without FBS) provide a little more impetus to stay attached during the assay? What would the effect be of increasing the amount of FBS to say 15 or 20% during culturing - would that promote stronger attachment? I'm a little hesitant to coat the wells with poly-*D*-lysine, as my readout is on a fluorescent imager, and I'm worried it would substantially increase background. I don't really have enough time to re-select the cells for adherent growth (although if I'd known about these issues from my predecessor 3 or 4 months ago I would have started right away...), so are there any other tricks I can try in the meantime? Are there any media supplements I could use to promote adherin expression or something like that? Any tips or advice at all would be appreciated.