Laser direct-writing to enable filtration in paper-based devices

We report the use of a laser-based direct-write (LDW) technique that allows the fabrication of porous barriers, which enable in-line filtration within a paper-based microfluidic device. The barriers were produced within porous substrates, namely nitrocellulose membranes, via local deposition of a photo-polymer that was subsequently polymerised by exposure from a laser source. Adjustment of the photo-polymer deposition parameters determines the porosity of the barriers, which, when carefully designed and integrated within a fluidic channel, can act as filters that enable either complete blocking, selective flow or controlled separation of particles of different sizes within a fluid travelling through the channel. We have successfully identified the fabrication parameters for the creation of barriers that allow the filtration of two different types of particles, Au-nanoparticles with sizes of 40, 100 and 200 nm and latex microbeads with sizes of 200 nm and 1 mu m, dispersed within an aqueous solution. We also report the use of a variable-porosity barrier for selective separation of latex microbeads from Au-nanoparticles, thereby showing the usefulness of this technique for enabling in-line filtration in such paper-based microfluidic devices.