PDMS-based microfluidic capillary pressure-driven viscometry and application to Drosophila melanogaster's hemolymph

A low-cost and easy-to-fabricate microfluidic PDMS-based capillary pressure-driven viscometer is introduced after addressing the hydrophobicity and gas permeability challenges of PDMS using polyvinyl alcohol (PVA) coating. Then, the device was applied to measure the viscosity of 3rd instar Drosophila larval hemolymph for the first time. Drosophila melanogaster, the fruit fly, larvae are widely used model organisms for early-stage genetic and disease studies. Knowing the viscosity of Drosophila larvae's hemolymph, a fluid analogous to blood in vertebrates, and that of other insects is essential in a wide range of studies from physiology and hemorheology to aerospace engineering, for characterization of insect residue on aerodynamic leading edges. Yet, hemolymph viscometry is challenging due to the larvae's small size and low hemolymph volume. Microfluidic devices have been developed for measuring the fluid viscosity of small samples among which the capillary pressure-driven viscometers are simple without the need for active or moving parts. However, they rely on surface wetting properties of hydrophilic materials such as glass which dictates a complex and relatively expensive fabrication procedure. PDMS is a dominant material in the fabrication of microfluidic devices. However, its hydrophobicity and gas permeability has hindered its use in the capillary pressure-driven viscometers, which was addressed in this paper. Our PDMS viscometer was successfully validated with standard liquid viscosities ranging from 1 to 20cP with < 12% error. The larval hemolymph viscosity was measured to be 1.34 & PLUSMN; 0.07 cP. The measurement technique and the reported viscosity of Drosophila hemolymph can be used in hemorheology and physiology studies as well as modeling the insect hemolymph drop impact on airfoils in aerospace engineering.

Automated summary

A low-cost and easy-to-fabricate microfluidic PDMS-based capillary pressure-driven viscometer was developed by addressing the challenges of PDMS hydrophobicity and gas permeability using PVA coating. It was then used to measure the viscosity of Drosophila larval hemolymph for the first time. The successful validation of the PDMS viscometer and measurement of Drosophila hemolymph viscosity have important applications in various fields.