Surface Treatment for Blood Dialysis Cartridges

In the construction of some types of hollow fiber membrane devices such as blood dialysis cartridges, the hollow fibers are terminated in tube sheets of cast plastic. As part of the manufacturing process, the tube sheets are cut to expose the inner hollow fiber passages to fluid headers. Such cutting and machining produce sharp edges at the surfaces where the hollow fibers emerge from the tube sheet intersections. Depending upon the cutting and machining method, the cut surface may contain rough edges, burrs, microfibers and other micro-size surface structures and sharp edges. This kind of micro-fuzzy surface has the same behavior as a microfiber filter which captures micro-particles from a fluid suspension at fixed sites having microradii.

In the operation of hollow fiber membrane devices with complex fluids having metastable suspensions of various particles such as animal blood, the precipitation of fluid components by the sharp edge mechanism produces particle deposits on the ends of the hollow fibers, and forms thrombi and emboli some of which become detached and block the hollow fibers internally or at fiber entrance points.

Specifically, in the operation of hollow fiber dialyzers as an artificial kidney for the treatment of blood, the sharp edge precipitation mechanism results in partial or complete blockage of the blood flow through the hollow fibers, depending upon the length of use and individual blood characteristics. The blocked hollow fibers decrease the membrane dialysis transfer area and thus degrade the performance of the artificial kidney.

The fiber filter effect and the sharp edge mechanism which causes precipitation of blood components on the tube sheet surfaces can be greatly reduced by eliminating micro structures of small radii at the tube sheet surface. Such sharp-edged surface structures have high surface energy and low heat conductivity and relax to edges of increased radii and lower particle attraction if the temperature is momentarily increased by the same mechanism as fire polishing of sharp edges in glass. The same high energy associated with the sharp edge mechanism enhances solution and chemical reaction effects at the sharp-edged surface structures when momentarily exposed to solvents or chemical reagents so as to increase the radii of the surface structures.

The sharp edge mechanism which captures particles from a suspension such as blood also is effective with a fluid varnish to increase surface structure radii by the same process by which the surface of wood is smoothed by coating it with varnish. The capillary tension effect of the fluid on the sharpe edges forms deposits of increased radii which become permanent on the setting of the varnish. In the varnishing process of improving tube header surfaces for reduction of blood component precipitation, loose and semi-loose surface debris are cemented in place and covered from contact with the blood.

The usual arrangement for hollow fiber membrane dialyzer devices comprises two tube sheets potted to the terminal ends of the hollow fiber bundle for access to the inner surfaces of the hollow fibers. If such a tube sheet surface accessing the hollow fiber interior passages were simply varnished, the hollow fibers would fill with varnish by capillary action. To prevent such tube blocking, a flow of gas out of the hollow fibers during the application and setting of the varnish keeps the varnish from entering and blocking the hollow fibers, but does not impede the rounding of the sharp edges at the hollow fiber entrance by the varnish. Such a gas flow can be introduced by pressurizing the outer surface of the hollow fibers with respect to the tube sheets or pressurizing one tube sheet while varnishing the opposite tube sheet surface.

Prior to the installation of the blood manifolds on the tube sheets, the hollow fiber bundle with machined tube sheet surfaces is placed in a fixture which temporarily manifolds an air supply to one tube sheet surface. With air blowing out of the exposed tube sheet surface, a polyurethane varnish is applied and set while the air flows through the hollow fibers.

The finished machined tube sheet surface of a hollow fiber bundle assembly is exposed momentarily to a high flux of infrared radiation from a heat lamp source to relax the energy of the sharp edges of the tube sheet surface and increase the radii of the surface.

The finished machined surface of a hollow fiber tube sheet is exposed to a combustion flame or a flow of electrically heated gas to flame polish the surface by surface energy effects and high local heat transfer rates.

The finished machined surface of a hollow fiber tube sheet is exposed to a sparingly applied solvent by a roller applicator or stamp pad so as to relax the sharp edge energy of the surface structure by momentary solution and edge radii increase.

The finished machined surface of a hollow fiber tube sheet is exposed to a chemical reagent to polish the micro structure of the surface by the selective chemical reaction of the reagent with the high surface energy sharp edges of the surface structure.