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Production of Medical Hollow Fiber Membranes from Synthetic Polymers
by Dr. Thomas Mueller, General Manager FilaTech Filament Technology u. Spinnanlagen GmbH
Market Situation
The main use of hollow fiber membranes (HFM) in medicine is for blood dialysis and blood filtration. It is estimated, that in 2004 about 120 million dialyzers and hemofilters are consumed, 70 million of them are containing HFM made of synthetic polymers, mainly polysulphone (PSU) and polyethersulphone (PES). The remaining quantity uses cellulosic HFM. An average dialyzer contains about 3 km of HFM as a bundle of several thousand fibers of around 280 mm length. That means that actually 210 million km of HFM from synthetic polymers are produced.
The market for blood dialyzers is growing at about 8% per year worldwide. The requirement for new spinning capacity can be estimated to at least 8% or 29 million km of the total HFM consumption including cellulosic dialyzers, as the annual growth is realized only with synthetic HFM. Patients with renal failure in western industrial countries and Japan are treated by dialysis as standard method, the health care systems in many other countries are not so much developed and only a part of the patients can be treated. The countries with highest population belong to this group. For example, only 4 to 5 million dialyzers have been used in China last year at a potential of 60 million pieces per year.
Spinning Process
The typical spinning process of synthetic HFM can be described as “Air Gap Wet Solution Spinning”. In this process, a viscous spinning solution is continuously extruded from a special nozzle downwards into an air gap. At the same time the so called “bore fluid” is injected into the core of the hollow fiber (see figure 1). The fiber is coagulated below the air gap in a spin bath below, followed by a washing process to remove the solvent, which is typically organic. The solvent free HFM is wound on a reel in wet stage or after in line drying. Actually processes with in line drying become more dominant, as this process allows the crimping of the HFM.. The use of crimped HFM is improving the dialyzer performance compared to use of straight HFM. The final product of the “dry” process is a fiber bundle, which can be directly transferred to the dialyzer assembly plant, whereas the product of the “wet” process, a wet fiber bundle, has to undergo a batch drying stage before the assembly of the dialyzer is possible. The spinning process allows maximum spinning speeds between about 30 m/min (with in line drying) and 60 m/min (wet winding).
 Figure 1: Face of FilaTech HFM nozzle
The key steps of the process are the formation of the hollow fiber at high dimensional accuracy and the adjustment and optimization of the membrane function of the hollow fiber. The blood dialyzing membrane is typically formed as a thin layer on the inner surface of the hollow fiber wall by the contact of bore fluid with the liquid hollow fiber when both materials are leaving the nozzle. By diffusion of non-solvent from the bore fluid into the extruded hollow fiber shaped spinning solution and diffusion of solvent from the spinning solution into the bore fluid, the membrane will be formed. By accurate control of the diffusion related temperature and solvent concentration of bore fluid and spinning solution, the uniformity of the HFM performance over time and different nozzles will be guaranteed. A minimum variation of the HFM dimensions is reached by accurate metering of spinning solution and bore fluid to the nozzle.
Economy
More than 22,000 nozzles are necessary to produce 210 million km of synthetic HFM per year at an average speed of existing spinning lines of 25 m/min (mix of “dry” and “wet” process with majority of “dry”, operated 8000 hours per year at 80% of maximum speed and at 80% productivity). A minimum of 3,000 to 3,500 new nozzles have to be put into operation every year in the next years. The big HFM and dialyzer producers have increased or are increasing at the moment their capacity in order to be prepared for the bigger markets resulting in an overall overcapacity. The actual global market prices for dialyzers are considerably lower compared to a few years ago. As the cost portion for the HFM in the dialyzer is increased consequently, the producers are also focusing now on the economy of the HFM production. Only multi-nozzle spinning lines with minimum operation cost and production efficiency with respect to fiber breaks, waste production, down time etc. can match the economic requirements of HFM production. High product quality and uniformity at maximum performance of the HFM are further requirements for multi-nozzle spinning lines. Only the big global players in the market like FMC, GAMBRO and some Japanese producers own this kind of spinning technology.
FilaTech HFM Spinning Technology
FilaTech, in general specialized on delivery of custom made machines and plants to synthetic fiber and polymer industry like pilot spinning lines and cleaning equipment for filters and spin packs, has developed multi-nozzle state of the art synthetic HFM spinning lines. Actually FilaTech is the only company world wide, which is offering independent turn key production technology to the medical industry including spinning lines of up to 1.500 nozzles per line for “dry” HFM production and the necessary water and solvent recovery technology for treatment of up to more than 7 metric tons of wash liquid per hour. The production capacity per line can reach up to more than 15.5 million km of product. The scope includes the complete machinery with spinning nozzles especially calibrated for highest uniformity of the produced HFM, multi-nozzle spin blocks (32 nozzles per block for the standard lines, 48 nozzles per block for the “+50%”-lines), crimping systems including the new micro crimp technology (see figure 2), reel winding systems for highest HFM yield etc. The line design is very compact for most efficient usage of energy, utility and personnel with a fiber to fiber distance to as low values as 1.2 mm for the standard lines and down to 0.75 mm for the “+50%”-lines (see also figure 3).
Figure 2: Crimped HFM from FilaTech spinning line (upper standard crimp, lower micro crimp)
Automatic bundle winding systems with higher yield and reduced personnel requirement are actually under development at FilaTech in order to improve the production cost for the HFM further.
Figure 3: FilaTech compact HFM spinning line
For more information please contact FilaTech, Tel. +49-2224-9435-14, email: sales@filatech.de
Represented in North America by Frankl & Thomas, Inc. 800/832-7746 X 12 Al Thomas info@frankl-thomas.com |