New assembly methods for PP dialyser filter cartridges

As manufacturers of dialyser filter cartridges increasingly move away from using PC, Didier Perret, medical business development manager, Branson Welding and Assembly at Emerson evaluates new assembly methods that best suit the alternative materials.

Patients suffering from end-stage renal failure rely on haemodialysis (or simply dialysis) for survival. A machine known as a dialyser acts like an external, artificial kidney, removing blood from the body, filtering out harmful impurities and returning the blood to the body. All such machines include a cartridge containing extremely fine filtration media encased in a plastic housing (Figure 1). The industry, however, is undergoing a major shift in the type of polymer used to make these cartridges, which is affecting all players in the market, from contract manufactures to brand owners. 

Historically, these tubular housings have been commonly made of polycarbonate (PC), an amorphous thermoplastic polymer with excellent dimensional stability, high strength, and good heat and cold resistance. Yield strength is high for this polymer (in the 58–70 MPa range), and its coefficient of friction is approximately 0.40. These properties are advantageous when mechanical assembly methods (screw-type fasteners) are used. Properly tightened screws will not loosen for up to 48 months. The surface energy of PC (46 dynes/cm) is also high among plastic materials, so gluing — using a range of affordable glue types — has also been a common bonding technique. Until recently, these assembly techniques and materials have been used by most of the major filter-cartridge manufacturers, but that situation is changing.

Not only is PC a relatively expensive polymer, costing two to five times more than polypropylene (PP), it is also more difficult to process because it must be dried before it can be moulded or extruded. Even more concerning to manufacturers and brand owners in the kidney dialysis market is that PC contains bisphenol A (BPA) — a chemical that has been linked to adverse health effects. BPA-containing plastics have not been banned primarily because normal kidney function easily eliminates low levels of this compound. According to recent studies, however, serum BPA levels accumulate as renal function decreases, and are highest in individuals with chronic kidney disease who are on haemodialysis. The safety of BPA in the general population, therefore, cannot be extrapolated to the dialysis population. 

The PP alternative

Given the issues with PC, dialyser filter manufacturers are increasingly turning to alternative materials. In some cases, acrylonitrile butadiene styrene (ABS), polyphenylene sulphide (PPS), polysulfone (PSU), poly(phenylene sulfide−phenyleneamine) (PPSA) or Polyethylene (PE) are used, but polypropylene (PP), in most cases, is the preferred choice. PP is nontoxic, tasteless, low density, unaffected by humidity, and relatively inexpensive and easy to process. Unfortunately, this thermoplastic polymer becomes brittle at low temperature, has a low coefficient of friction and low yield strength (approximately 12–40 MPa), all of which makes it unsuitable for screw-type fasteners. Even under compression, a correctly tightened assembly will loosen within a relatively short time.

Likewise, PP is a poor candidate for gluing, as its surface energy is so low (30 dynes/cm). Making the gluing process effective requires plasma or other expensive and time-consuming surface treatment. The best solution for PP assembly, therefore, is welding, for which there are at least three good options: ultrasonic welding, laser welding and spin welding. Each method has advantages and disadvantages. By considering the full picture and the best investment/benefit ratio, manufacturers can receive the best technology to satisfy their needs.

Ultrasonic, laser and spin welding

Ultrasonic welding has been used for more than 70 years to join thermoplastic parts that would be too complex or prohibitively expensive to mould in one piece. The process is fast, flexible, and economic, making it ideal for large series production, and it is easily controlled and automated. The latest systems set a new industry standard for outstanding weld quality, process reliability, intelligent process controls and data gathering. 

Ultrasonic welding does require a one-time, upfront investment in the necessary equipment and application-specific tooling to precisely hold the various plastic components in place during the welding process, which usually takes one second or less. Other than power, there are no incremental consumable or assembly costs, regardless of production volume, so return on investment is normally predictable and rapid. Ultrasonic welding can also positively influence a product’s sustainability rating due to the lack of solvents and improved energy efficiency.

Spin welding can be used to bond both ends of a filter cartridge. It can provide a strong bond with simple joint designs (Figure 2) but is can create some undesirable particulates.

The newest trend in dialyser filter assembly is laser welding, which requires a higher capital investment for equipment compared with ultrasonic welding. Components are pre-assembled before welding, and no vibration or movement is required to produce clean, particulate-free welds. In operation, multiple laser beams apply energy along the full length of weld surface. One surface freely transmits the laser energy (without itself being affected) through to the second (laser-absorbing) surface where laser energy is converted to heat that is conducted across the interface, creating the weld. Usually, the transmissive surface is more or less clear, and the absorptive layer is darker, but that does not always have to be the case. Using an over moulding method or various coatings and additives, an otherwise laser-transparent material can be made to absorb laser radiation, making it possible to create clear-on-clear assemblies.

Other applications in kidney dialysis systems

Filtration cartridges are just one component in a complex system of pumps, flow meters, valves, a ‘bubble trap’ (which prevents any air bubbles going back to the patient’s body), as well as controls and proportioning systems for dialysate solution, which pulls toxins from the patient’s blood. These devices are made of polymeric material and are assembled effectively using the aforementioned welding processes. Figure 3 shows these devices and indicates which process(es) are commonly used to make them. 

Similar technologies may be used to produce the fibre filtration elements housed in the filtration cartridge. The fibres may be cut to length using ultrasonics, and the cut ends may be cauterised using an Emerson IR emitter — a metal-foil heater that emits infrared wavelengths matched to the absorption characteristics of the polymer fibres.

Conclusion

Most dialyser filters are produced by contract manufacturers and then assembled into finished products by a machine builder who supplies them to the Market Authorisation Holder or brand owner/marketer. As the industry turns from traditional materials such as PC to cheaper, BPA-free PP, manufacturers can simplify their products, reduce costs, and improve performance by shifting from adhesives and fasteners to plastic welding methods.