Thread-based microfluidic channels: Fabrication and application in organic semiconductor crystalline needles

Abstract

Thread, which is a twisted bundle of monofilaments, can function as microfluidic channels. This application of thread can be attributed to the capillary action occurring in the narrow space between the monofilaments through which fluid can pass without the use of pumps. The microfluidic channels were fabricated by sandwiching thread between two glass slides. As expected, fluid spontaneously passed through the thread. However, the fluid was flooded into the space between the glass slides. This was also due to the capillary action because the contact angle of the fluid on the slides was smaller than 90°. This drawback was overcome by using a reservoir, a mechanism designed by considering the total surface free energy of the system. As an application of the channels made of thread, a method for the regular arrangement of crystalline needles of an organic semiconductor (9,10-dibromoanthracene) was developed. One of the glass slides sandwiching the thread was coated with polydimethylsiloxane, whose surface was oxidized through UV-ozone treatment. In the area of the polydimethylsiloxane surface adjacent to the thread, regular surface undulations were created by the organic solvents used as fluids. When dissolved in the solvent, the organic semiconductor precipitated as crystalline needles along the undulations. Because normal microfluidic channels are tunnels created in solid materials, only the space inside the channels can be used for applications. In contrast, the application developed in this study uses the space outside the channels, a novel methodology unique to thread-based channels.