3.4 Passive, Capillary Force-Driven Modalities
Passive fluid handling has been popular because the integration of active actuation
of liquids using electric force, pumping by peristaltic action and pressurized valving
directly contribute to the complexity and cost of the system. Passive microfluidic
systems offer portability, ease of operation and power-free operation. The most
common and versatile passive liquid driving force is capillary force, which allow
for passive fluid flow, thereby avoid the requirement for external pumping system,
tubing and connectors [ 78 – 85 ]. In particular, capillary microfluidic systems can
perform autonomous fluid delivery [ 83 , 84 ] and allows the use of transparent
materials including PDMS, PMMA and glass, providing higher flexibility in
assay design and detection methods.
Gervais et al. [ 85 ] developed a one-step capillary-based microfluidic immuno-
assay platform comprised of sample collector, delay valves, flow resistors, detec-
tion antibody deposition zone and a reaction chamber with immobilized capture
antibody. The capillary-force was generated by capillary pumps with vents. The
microstructure of the capillary pumps, delay valves and flow resistors determined
the overall flow resistance and capillary pressure, which controlled the flow rate of
the device [ 79 ]. In this system, the delay valves consolidate the liquid flow from the
sample collector before reaching the flow resistors, thereby minimizing the problem
of air bubble trapping. The flow rate can be modulated by adding flow resistors.
Capture of the analyte was achieved by capture antibody patterned on the surface of
the reaction chamber. All other reagents were stored on-chip and redissolved as
sample solution flowed through during the assay. In this one-step design, the
addition of blood sample into the capillary circuit initiated the capillary-driven
Fig. 9.3 Concept of a centrifugal microfluidic platform incorporating sedimentation-based immu-
noassay for analyte detection from whole blood sample. (a) Schematic of the immunocomplex on
microbeads. (b) Fluorescent immunocomplex on microbeads were separated from the matrix of
whole blood through a density media. Taken from [ 77 ]. Copyright©2014 American Chemical
Society
9 On-Chip Immunoassay for Molecular Analysis 235