Variable-Size Injectable Dialysis Chambers

We required a disposable dialysis chamber of approximately 100 μL internal volume to fit within a small reaction vial. Since our application demanded the highest possible rate of diffusion into and out of the chamber, the surface area-to-volume ratio had to be as high as was practical. Injectable dialysis chambers are commercially available, e.g., the Slide-A-Lyzer® Cassette (Pierce, Rockford, IL, USA). However, they are relatively expensive and the outside dimensions too bulky (7 × 40 × 40 mm) for our application. The device described here uses washers cut from silicone rubber, with dialysis membrane glued to both faces. While the devices are more complicated to construct than other small dialysis chambers described recently (1), they afford an increased flexibility in size. Washers were cut from 1.6-mmthick translucent silicone rubber, of high tensile strength (1100 psi) and medium hardness (McMaster-Carr, Chicago, IL, USA) with handheld punches. No smoothing of cut edges was required. The washers had an external diameter of 15.9 mm and an internal diameter of 9.5 mm. A small amount of Super Glue was spread over one side of a washer, which was then glued onto a sheet of Spectra/Por® Dialysis Membrane (MWCO: 12 000– 14000; Spectrum, Laguna Hills, CA, USA) (Figure 1, left panel). Even pressure applied over the entire washer (with, in our case, the base of a handheld 7-mL plastic scintillation vial) for 10–15 s results in a good seal. The resulting half-finished devices were cut from the dialysis membrane, and the gluing process was repeated on the other side to create a chamber with an internal volume of approximately 110 μL. Construction was completed by cutting each chamber from the sheet. Dialysis chambers can be constructed to any size, using a variety of thicknesses of silicone rubber and diameters of the punches. With the punches available to us and 1.6-mm silicone rubber, we can easily make dialysis chambers with up to 900 μL internal volume. This could be increased by cementing two half-finished dialysis chambers together or by using thicker silicone rubber. The width of the glued surface of the rubber needs to be sufficient to provide satisfactory bonding. We found that a width of 3 mm was sufficient to ensure a good seal. Flat sheets of dialysis membrane were used not only for ease but also to avoid the creases in the membrane found in tubular dialysis membranes, which are difficult, if not impossible, to glue across sufficiently tightly to prevent failure of the seal. The glue was not affected by the Tris buffers that were used, and the chambers maintained their integrity overnight in small vials in a shaking water bath. Samples (100 μL) were injected into the chambers using a 1-cc insulin syringe with micro-fine IV needle (Becton Dickinson Labware, Bedford, MA, USA). The fine-gauge needles were required to penetrate the very thin chamber walls (1.6-mm thickness). Some stretching of the membrane results if the sample is introduced prior to evacuating the air in the chamber. To avoid this stretching, it is possible to evacuate air from the chamber before injection of the sample by carefully drawing back on the syringe plunger. To completely exclude air bubbles from the chamber, it is necessary to remove the needle from the chamber after evacuation, expel the air from the syringe needle,