Harnessing catalytic pumps for directional delivery of microparticles in microchambers

Abstract

The directed transport of microparticles in microfluidic devices is vital for efficient bioassays and fabrication of complex microstructures. There remains, however, a need for methods to propel and steer microscopic cargo that do not require modifying these particles. Using theory and experiments, we show that catalytic surface reactions can be used to deliver microparticle cargo to specified regions in microchambers. Here reagents diffuse from a gel reservoir and react with the catalyst-coated surface. Fluid density gradients due to the spatially varying reagent concentration induce a convective flow, which carries the suspended particles until the reagents are consumed. Consequently, the cargo is deposited around a specific position on the surface. The velocity and final peak location of the cargo can be tuned independently. By increasing the local particle concentration, highly sensitive assays can be performed efficiently and rapidly. Moreover, the process can be repeated by introducing fresh reagent into the microchamber.

Microfluidic Pump Driven by Anisotropic Phoresis

“…Conversely, the gradient can generate the motion of a fluid at a fixed solid-fluid interface, which is usually referred to as phoretic osmosis, and in the case of a fluid-fluid interface, which is known as phoretic capillary [17]. Catalytic surfaces and related chemical gradients have shown a large potential in microfluidic applications [29][30][31][32], while thermal gradients are relatively less exploited. Thermal gradient-driven motion has though promising prospectives since it works equally well in charged and neutral solutions, and it is pollution-free due to the absence of surfactants or chemical fuels, which enables the way to bio-compatible applications [33][34][35].…”

Enzymatically Powered Surface‐Associated Self‐Motile Protocells

“…For self‐phoresis, an asymmetric distribution of reaction products in vicinity of the enzyme creates corresponding shear or osmotic flows. Sen and Balazs and co‐workers have presented a theoretical model for micropumps driven by surface bound enzymes . When substrate‐loaded microparticles release substrate near an enzyme patch, the subsequent enzymatic reaction leads to spatial variation in the fluid density that causes convectional flow, which in turn induces a directional motion of substrate‐loaded microparticles.…”

Temperature Gradients Drive Bulk Flow Within Microchannel Lined by Fluid–Fluid Interfaces

“…Traditionally, mechanical pumps are utilized for generating the pressure gradients necessary for driving fluid flow. However, other attractive methods have arisen due to their efficiency at small scales and controllability given certain signals, including the use of enzymatic reactions, capillary effects, sound, light, and electric and magnetic fields …”

“…Here, we present a proof of concept: a microfluidic channel with side walls lined with fluid‐filled cavities that introduce fluid–fluid interfaces ( Figure ). By inducing temperature gradients on the order of degrees Celsius per centimeter across the interfaces using external heating and/or cooling, we pumped water with flow speeds on the order of tens of micrometers per second, greatly exceeding the flow speeds exhibited by other surface‐driven flows like enzymatic reactions …”

“…Previous studies have reported other transport methods that operate without the use of mechanical parts, including the use of thermophoresis and enzymatic reactions . For thermophoresis, large temperature gradients on the order of 10 4 °C cm −1 were required to achieve velocities on the order of 0.1 μm s −1 .…”