The accurate control of the liquid motion in microfluidic networks is fundamental for many chemical and biological applications, and for producing more complex and integrated devices. A general method for on-off controlling the liquid flow in Microfluidics is the use of valves for closing or opening micro- or nano-channels. Different classes of on-off valves have been reported, including monolithic elastomeric switches, torque actuated elements, valves based on pH-sensitive, electrically stimulated, or optically controlled hydrogels, and on thermally induced expandable microspheres, electrorheological fluids, phase changes in aqueous solutions, and electrowetting. Using surface properties, and particularly changes in the wettability of the channel walls, also enables effective liquid handling. We exploit both these methods, producing monolithic elastomeric microvalves, and capillary surface functionalizations with chemical groups, including light-responsive molecules.   
For the realization of monolithic polymeric valves we follow the procedure first described by M. A. Unger, H.-P. Chou, T. Thorsen, A. Scherer, and S. R. Quake [Science 288, 113-116 (2000)]. The technique of choice is the “multilayer soft lithography”, that combines the standard replica molding procedure with the irreversible sealing between two elastomeric elements. The first step for obtaining an elastomeric valve is the realization of two layers, each containing a microfluidic channel (typically 100 µm wide and 10 µm high). Then, one mold is placed on the top of the other, aligning the two capillaries perpendicularly to each other. After the bonding procedure, which is usually carried out thermally, a monolithic, three-dimensionally patterned structure, entirely composed by elastomer, is achieved. The elastomeric membrane at the junction between channels typically has an active valve area of 100×100 µm2, and is relatively thin (∼10-30 µm). When an external pressure is applied to the upper channel (control channel), the membrane is defected downward by hydraulic or pneumatic actuation, and the lower channel (flow channel) is closed. Thus, the motion of a liquid flowing into the bottom, controlled channel is stopped. Instead, upon removing the external pressure, the membrane goes back to its initial position, and the flow channel is open, allowing the fluid to continue its motion. The response time of this on-off valves is on the order of 1 ms, and the applied pressures are on the order of 100 kPa, corresponding to an actuation force on the order of 1 mN for a 100 × 100 µm2 area.
We developed also flow control methods based on suitable functionalizations of the involved surfaces by proper polar moieties for increasing their wettability. We employed microfluidic channels with one wall in Si, and three walls in PDMS. We cover the walls of the microfabricated capillaries with -OH functionalities, hydroxilating Si surfaces by immersion in piranha solution and exposing PDMS and other polymers to O2 plasmas. Some typical experimental filling curves by polar fluids are displayed in Figure 1. The time to completely fill the capillaries is decreased by over a factor of 2 by employing -OH terminated walls. We obtained better results by using -SH functionalities linked to the previously -OH terminated surfaces by adsorption of (3-mercaptopropyl)trimethoxysilanes.

 

 

For more information, please contact: Dr. Dario Pisignano ( This e-mail address is being protected from spam bots, you need JavaScript enabled to view it )

Publications:

 

[1] L. Caprioli, E. Mele, F. E. Angilè, S. Girardo, A. Athanassiou, A. Camposeo, R. Cingolani, D. Pisignano
Appl. Phys. Lett. 91, 113113 (2007).