Droplet manipulation and microparticle sampling on perforated microfilter membranes

This paper describes droplet manipulation and microparticle sampling where droplets driven by the electrowetting-on-dielectric (EWOD) principle are transported on perforated microfilter membranes and pick up microparticles in their path. Three designs of microfilter membranes that have different hole shapes (rounded rectangle versus circle holes), sizes (≥6 µm) and opening area ratios (5, 9 and 20%) are microfabricated and tested along with a commercial membrane (rounded rectangle holes of 60 µm × 128 µm, 17% opening area ratio). All the tested perforated membranes are embedded with a linear array of electrodes for EWOD actuation. Reversible EWOD actuations and droplet transportations are successfully achieved on the perforated filter membranes. Particle sampling is examined against glass particles (8 µm diameter and ~14° contact angle) and polystyrene particles (8 µm diameter and ~66° contact angle). It is demonstrated that as droplets are moved on the microfilter surfaces by EWOD actuation, they efficiently pick up the microparticles in their path, showing high sampling efficiencies: over 95% for the glass particles and over 85% for the polystyrene particles. This particle sampling method uses a small liquid volume (microliters or smaller) and has the capability of fully automatic handling. Thus, it is expected to be highly compatible and easily integrated with lab-on-a-chip systems for follow-up biological and chemical analyses.