Standard approaches to creating pores for a range of separation and other applications tend to create materials in which pore selectivity and gating are set by properties of the solid material. This limits the range of things that can be separated by any particular pore design. Now, a US team has taken a different approach that could open up new applications as well as precluding the issue of blocked pores caused by accumulation of materials and fouling.

Joanna Aizenberg of Harvard University and colleagues have developed dynamic pores that work in controlling flow in and out through a "fluid-based gating mechanism" and offer tunable, multiphase selectivity, taking a cue from the few known cases of natural cellular membrane pores. [Aizenberg et al., Nature, 2015; DOI: 10.1038/nature14253] The team's system involves a capillary-stabilized liquid acting as a reversible, reconfigurable gate that fills and seals pores in the closed state but makes a "non-fouling", liquid-lined pore when it is in the open state. The opening and closing can be tuned over a wide range of pressures, the team explains.

"The ability to selectively transport or extract materials is valuable for uses such as separating components of oil, gas and wastewater, for filtering blood and fluid samples, and broadly for 3D printing and microfluidic devices," explains Aizenberg. "Our new approach harnesses dynamic, highly sensitive/tunable/reversible control over multiply selective gating, which we can now apply toward many diverse applications." The team has demonstrated proof of principle with a range of materials including hydrophobic polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) and polypropylene , as well as hydrophilic nylon to create the necessary capillaries, which can then form an active porous membrane, each capillary being lined with liquid that has an affinity for the capillary material but be immiscible with a second carrier liquid.

"The fluid used in the gate is repellent and prevents any material from sticking to it and clogging the system throughout repeated and extended use," explains team member Xu Hou. "To accommodate different materials and desired extractions, operators of the system simply need to adjust the pressure to influence what substances will be allowed to flow through the fluid-filled gates."

"Next steps include both fundamental understanding of the fluid dynamics involved in gating and modulating continuous flow, and developing the system for a wider range of materials, e.g. to increase mechanical strength and enable use of a wider range of pore sizes and pressures," Aizenberg told Materials Today.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".