Optics-Free, Non-Contact Measurements of Fluids, Bubbles, and Particles in Microchannels Using Metallic Nano-Islands on Graphene.

Despite the apparent convenience of microfluidic technologies for applications in healthcare, such devices often rely on capital-intensive optics and other peripheral equipment that limit throughput. Here, we monitored the transit of fluids, gases, particles, and cells as they flowed through a microfluidic channel without the use of a camera or laser, i.e., "optics-free" microfluidics. We did this by monitoring the deformation of the side walls caused by the analyte passing through the channel. Critically, the analyte did not have to make contact with the channel walls to induce a deflection. This minute deformation was transduced into a change in electrical resistance using an ultrasensitive piezoresitive film composed of metallic nano-islands on graphene. We related changes in the resistance of the sensor to the theoretical deformation of the channel at varying flow rates. Then, we used air bubbles to induce a perturbation on the elastomeric channel walls and measured the viscoelastic relaxation of the walls of the channel. We obtained a viscoelastic time constant of 11.3 ± 3.5 s-1 for polydimethylsiloxane, which is consistent with values obtained using other techniques. Finally, we flowed silica particles and human mesenchymal stem cells and measured the deformation profiles of the channel. This technique yielded a convenient, continuous, and non-contact measurement of rigid and deformable particles without the use of a laser or camera.