Porex - Advanced balancing act

Porex Corporation has developed a new class of self-sealing porous materials with aqueous and organic-solution-blocking capabilities for medical devices. These porous materials also provide high airflow and bacterial-filtration efficiency for better patient care.


Porous plastics can be formed by sintering polymeric particles into a continual omnidirectional open-cell 3D matrix. Sintered porous plastic media have advantages over traditional membrane porous media in mechanical strength, high pore size, high airflow and ease of assembly. Sintered porous plastic products have been widely adapted in applications that require wicking, venting, filtering and diffusing because of their uniquely engineered homogenous open-cell structure.

Sintered porous plastics can be made of many thermoplastic polymers, such as polyethylene, PTFE, polypropylene, ethyl vinyl acetate, nylon, polyethersulphone, polycarbonate, thermoplastic urethane, polyvinylidene fluoride, polystyrene, polyacrylates, polymethyl methacrylate and polyesters.

'Self-sealing' sintered porous plastic media are made with absorptive additives. When dry, air or gases freely pass through the porous media. However, when wet, the absorbent additives will block the liquids by forming a high-viscosity solution or gel, and retard the airflow. This property is called self-sealing. Compared with other porous media, such as commercial submicron hydrophobic membranes, sintered porous media have much higher pore size and airflow. Sintered self-sealing porous plastic media can be moulded into a variety of shapes that provide optimal performance and flexibility for device design.

Porex Corporation porous materials have been widely used as vents in medical devices and proven to be effective airborne bacterial barriers with high bacterial filtration efficiency - well over 99.9% (ASTM F2101).

The organic solvent and acidic solution challenge

As described, current sintered porous plastic self-sealing media use absorbents or modified celluloses as additives for forming a high-viscosity solution or gel in the porous polymer matrix and blocking liquid and gas flow when exposed to aqueous solutions. However, this type of polymeric additive with highly charged functional groups, such as sodium carboxylic and sodium acrylic acid, will quickly deteriorate in acidic solution or organic solvents due to decreased solution-ionisation capability. When a solution is acidic, the functional group of the polymeric absorbent cannot dissociate fast enough to form a gel or high-viscosity solution to prevent liquid passing through the porous media. Organic solvents decrease the polymer ionisation and significantly slow the self-sealing response. Current self-sealing products on the market could not block aqueous solution with over 40% organic solvents and acidic solutions.

A new class of self-sealing porous media

The new self-sealing media use a linear long-chain polyacrylic acid, which is not cross-linked or is very lightly cross-linked. Polyacrylic acids are not neutralised in most conditions. This new additive has the right balance of polarity, solubility and dissolution dynamic for a quick self-sealing response in a wide range of solution compositions. The sintered porous self-sealing media using this new additive could block water, acidic solutions and polar organic solvents.

The new Porex self-sealing porous materials have a broader blocking capability for polar organic solvents and acidic solution compared with the current self-sealing media. This innovative capability will help advance the design and improve the performance of medical devices for better patient care.

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BFE material map of Porex medical-grade PTFE.
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