Regardless of the scale of the filtration process, the filter is housed in a device so that the device can:
Provide mechanical support for the filter, without which most filters would rupture under the forces they endure during use
Incorporate a mechanism to seal the edge of the filter so that liquid flow is through the filter. If the liquid flows around the edge of the filter, the filtrate is contaminated.
The method used to seal the filter is also an important consideration in filter selection.
There are several ways to seal a filter. Reusable devices can be constructed of glass, plastic, or stainless steel. The filter is placed into the base of the unit with a gasket or o-ring that covers its perimeter. The top of the unit is screwed or clamped into place, and the pressure of the closure seals the edge of the membrane. Assembly of the unit needs to be done carefully to prevent movement or distortion of the filter.
For fabricated devices, the filter is most commonly sealed with:
The adhesive is applied to the perimeter of the filter to provide an impenetrable barrier between the housing and the filter. To some degree, the adhesive needs to flow into the porous structure of the filter. An advantage of an adhesive is that it allows the bonding of different types of filters into a single device. One disadvantage is that the adhesive must be given adequate time to cure and form the seal.
In direct bonding, the filter is fused to the housing. Ultrasonic welding or heat sealing is used to bond a polymeric filter directly to the plastic housing in a single step. Because this method involves localized melting of the housing and/or filter, it is best suited to membrane filters and thin, nonwoven filters. The polymer used in the filter and the housing have to be selected for compatibility with each other so that a strong, integral bond can be formed. The advantage of direct bonding is that the seal is formed immediately without any requirement for further processing. One disadvantage is that thick filters and non-polymeric filters can't be sealed by this technique.
Overmolding is a simplified process for sealing a filter into a device. Plastic is molded around the filter and other parts of the housing to form an integral seal and a complete device in one step. As the liquid plastic fills in the air spaces between the parts of the housing, it also penetrates into the edge of the filter. Conditions are controlled so that the plastic does not intrude into the effective filtration area of the membrane. Regardless of the method use to seal the filter into a device, the seal must be integral, meaning nothing can pass through it. Also, the sealing process must not cause holes in the filter.
The efficacy of the sealing process can be assessed in an integrity test. This method involves pressurizing a finished device on one side of the filter and measuring the rate of liquid or air passage on the downstream side. A defective device with a flaw in the seal or a breach in the filter will exhibit very high flow rates compared to a normal device.
Flaws and Breaches
When a filter is sealed into a device, the fraction of membrane surface area used to form the seal will be excluded from the filtration stream. It is undesirable to lose any additional surface area, as this will reduce the filtration capacity of the device. It is also undesirable to cause a breach in the membrane during sealing. Beyond the seal, the structure of the filter should remain unaffected. Potential problem areas include:
Pressure: When applied as part of the sealing process, pressure causes compression of the filter; too much can result in the filter being compressed at the edge of the seal and, in extreme cases, cracked.
Adhesives: Used as part of the sealing process, the amount of adhesive used should be limited to what is necessary and sufficient to produce the seal. Excess adhesive can migrate across the surface of the membrane and clog the pores.
Ultrasonic energy or heat: When these sealing methods are used, the porous structure of the filter normally collapses as the polymers and housing melt together. If too much energy is applied or the energy is applied imprecisely, the filter’s porous structure can collapse in areas away from the seal. In extreme cases, holes may form.
For many applications, chemical contamination of the filtrate is a major concern. Materials used for the housing and the filter are selected for a low potential to introduce contaminants into the filtrate. Nonwoven filters, depending of the specific nature of their construction, may shed fibers into the filtrate. Because of the additional handling required during fabrication and the potential for introduction of contaminants, finished devices are often tested for the release of chemicals in the intended application. This is always the case when the filtrate is to be used in a medical application.