Trouble Making Non-Adsorptive CytoVu Assemblies

Last week I made a post about the troubles that a toluene-based PEGylation process presents to the PDMS in the CytoVu assembly. It was suggested that this issue might be overcome by replacing toluene with ethanol in the process, but I can report that unless I did something wrong (which is, I suppose, quite possible) replacing toluene with ethanol, milliliter for milliliter, does not result in a well-PEGylated surface on glass or PDMS. This is evidenced by wetting angle tests: when PEGylated with toluene, glass coverslips demonstrate larger contact angles than un-PEGylated coverslips, but when PEGylation is attempted with ethanol no significant contact angle difference is observed.

Another option was explored: the possibility that plasma-activated PDMS surfaces could form permanent bonds with unactivated, PEGylated SiN and glass surfaces. Unfortunately, this too has proven unfruitful — unactivated SiN and glass do not form permanent bonds with activated PDMS, whether they are PEGylated or not!

A few options remain. A mask could be employed to allow the PEGylated SiN and glass surfaces to retain their PEG coatings in specific areas during exposure to the plasma-activation process, but what type of mask would suit this application, I don’t know. It would have to be able to be applied to the SiN and glass surfaces very precisely in order to avoid a loss of bonding AND exposure of the fluid wells to un-PEGylated surfaces, and it would have to be effective enough to create a fairly sharp cutoff between plasma-exposed and -unexposed surfaces. Another option is to mortar-bond the PDMS surfaces to the glass and SiN surfaces, but I have no familiarity with the process and can’t speak to its plausibility.

A final option is the most obvious and perhaps least interesting of the bunch: simply not creating a permanent chemical bond between the PDMS and the SiN and glass. Due to the hydrophobic nature of PDMS and the hydrophilic nature of, well, water, solutions pipetted into a fluid well do not appear to leak out into unbonded PDMS-glass interfaces. Assuming no permanent bonding needs to be done, n-Dodecyl-β-D-maltoside coating (reference, thanks to Jim McGrath) could be used to prevent adsorption of proteins in solution to the PDMS while traditional toluene-based PEGylations could be performed on the chips and coverslips. The obvious drawback of this solution is that the assembly is no longer one single, inseparable piece and, if handled carelessly, could lose its water-tightness. However, properly-handled CytoVu assemblies shouldn’t show any real issues. The PDMS used for these gaskets is quite tacky and it requires deliberate effort to separate glass or SiN surfaces from their interfaces with it. Even tackier PDMS compounds may be a possibility, although I’m not personally familiar with the PDMS fabrication process.

Now that I have some training with the fluorescence microscope, I’d like to start to collect adsorption data and performing basic separation experiments. Assuming this current problem is solved soon, that’s the next step!