MARKET PLACE
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Medical filters utilise different types of filter media depending on the nature of the filter process. Wet–wet or wet–solid filtration is often distinguished by depth or surface capabilities.
In depth filtration the filtrate is captured in the filter media. Filtration occurs as the particles pass through and become lodged in the tortuous path formed by the random fibre structure. Typical applications include gas and vacuum filtration as well as venting. In surface filtration the filtrate is captured on the surface of the filter media, typical applications include removal of particles or contaminants from liquids. Surface filters are often used in combination with a depth media as a prefilter to prevent clogging of the surface filter. The various media used in wet–wet and wet–solid applications are compared in Table I.
Membranes
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Table I: (click to enlarge) Comparison of media used in wet–wet and wet–solid filtration applications. |
Membranes provide a means of removing extremely small particles and achieving the highest purity possible. They exhibit high pore density and excellent filtration efficiency and are available in a range of materials. However, membranes exhibit comparatively low flow rates and high pressure build-up and are mechanically weak, leading to problems during manufacturing.
Monofilament fabrics
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Figure 1: (click to enlarge) Monofilament fabrics typically used in particulate filtration. |
For particle filtration, monofilament fabrics offer a good alternative to membranes (Figure 1). This type of filter media differs significantly from other filtration materials. The construction of a woven textile provides a combination of precise pore openings, high flow rates, strength and toughness unmatched by media of the same opening size range. Monofilament fabrics are generally used in surface filtration applications requiring removal of particles ranging in size from approximately 1000 µm down to 1 µm. The utilised raw materials are pure and do not require any binders or compounds. Monofilament fabrics are generally less expensive than membranes and are stronger, with higher flow rates, but lower pore density.
Depth media
A less expensive alternative to monofilament fabrics and membranes are depth media that include nonwovens such as needled felts and random fibre media. These products are generally used in applications requiring removal of particles of 10–200 µm. The advantages of depth media include low-cost, high dirt-holding capacity and high filtration efficiency. However, the pore size distribution is wide compared with woven filters or membranes, resulting in statistical filtration rather than precision filtration.
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Figure 2: New pleat designs improve surface area and flow rates, and are suitable for custom designs. |
Innovation
In the case of monofilament textiles for filtration and separation applications, innovations in raw materials, surface modification techniques and filter designs offer the best opportunities for new functionalities.
Raw materials. Standard materials for woven filters used in medical filtration are polyamide or polyester. However, several new polymers have recently been introduced that expand the operating environment of synthetic fabrics. Newer fluorinated polymers offer enhanced performance in certain critical environments. Polyetheretherketone (PEEK) fabrics are another recent example. The high temperature resistance and chemical stability of this polymer have allowed many uses previously inappropriate for synthetic textiles. In addition, PEEK is one of the most biocompatible materials available.
Surface modification. Woven fabrics can be modified to have low or high surface energies. Standard surface treatments include hydrophilic and hydrophobic coatings. New surface treatment technologies such as plasma grafting can permanently impart new properties on polymers that are foreign to the polymer such as adhesive-bondable fluoroplastics or biomedical implants with long-term medicinal properties.
Filter designs. Conventional medical filters such as arterial filters require a high surface area, which is usually achieved with standard pleated elements. New pleating designs increase the surface area and thereby improve the flow rates. The geometry of the pleated filter can be modified to fit any type of housing (Figure 2).
For more information, contact Dr Helen Vogt, Business Development, at Sefar AG, Filtration Division, Moosstrasse 2, CH-8803 Ruschlikon, Switzerland, tel. +41 44 724 65 11, e-mail: helen.vogt@sefar.ch, www.sefar.com.
