Attempts in Making PDMS Gaskets in CytoVu Assemblies Play Nicely with PEGylation

This summer I’ll be working on preliminary development of technology that might lead to SiMPore drug discovery assays. The goal is to use SiMPore membranes to create much more efficient and generalized assays, which represents a big opportunity for the company. However, several challenges need to be addressed in order to make such a device plausible, and the first one we’ve chosen to address is the non-specific protein adsorption properties of the three materials currently used for CytoVu assemblies: Glass, PDMS, and pnc-Si/SiN hybrid membranes.

CytoVu assemblies are being used as a model for assay development because they’re multi-welled yet don’t allow interaction between wells. While CytoVu products only have four membranes (and thus eight wells — a top well and a bottom well for each membrane,) getting a CytoVu assembly to behave on a small scale like an effective and efficient 384-well drug discovery assay would on a large scale is an important first step.

For obvious reasons, then, losing protein to the assembly via adsorption is problematic. In order to reliably determine whether a drug is effective in breaking apart a target structure (as is the goal of this type of assay,) researchers need to be sure that the solutions they put into the assay aren’t going to be modified by the assembly. Fortunately, this problem is easily solved for glass and SiMPore membranes — traditional PEGylation techniques should be effective in blocking adsorption (and this will be tested in the coming weeks.)

Unfortunately, the PDMS gaskets used in CytoVu assemblies don’t respond so amicably to PEGylation — the exposure to toluene during the process causes them to swell and often times come free of their bonds with chips and glass (which were previously permanent due to plasma treatment.) This article explores various common techniques for getting around this problem, but as best I can tell, none of them seem to be particularly well-suited to this application. Since the exact mechanism of this swelling isn’t abundantly well-understood, I ran some tests to see if this swelling could be reduced or negated by successive toluene baths. I weighed three PDMS gaskets before bathing them in toluene for an hour, drying them under a fume hood, and reweighing them. After the first toluene bath, the gaskets began to warp as shown:

Once dried, these gaskets displayed a loss of mass equivalent to about 2% of their original masses. After a second toluene bath, the gaskets began to swell considerably:

The gaskets continued to show a loss of mass, this time of  about 1% of their previous masses (those measured after the first bath.) I found similar results after a third bath, and gasket 2 broke at some point during it.

This rather clearly suggests that the toluene, while destructive to the structure of PDMS, doesn’t cease to cause swelling in PDMS which it has already acted upon (in retrospect, perhaps this should’ve been obvious,) and demonstrates that traditional, unmodified PEGylation techniques are not suited to this application.

Further, since the PEGylation doesn’t survive a plasma treatment, it won’t work to simply PEGylate the coverslips and chips before giving them the plasma treatment. On Jim McGrath’s suggestion, I will test whether or not it is possible to get a sturdy and even bond between a plasma treated PDMS gasket and an untreated, PEGylated chip and coverslip. If this fails, we will have to move towards changing the procedure involved in constructing CytoVu assemblies — promising options include mason bonding the PDMS to the PEGylated chips, and applying a mask to the chip before PEGylating it in order to provide a more bond-friendly unPEGylated surface for plasma activated PDMS.

These options will be explored when I return to work on the 17th. I’ll post updates to this issue as more information comes in.