Analyzing threshold pressure limitations in microfluidic transistors for self-regulated microfluidic circuits

Abstract

This paper reveals a critical limitation in the electro-hydraulic analogy between a microfluidic membrane-valve (lMV) and an electronic transistor. Unlike typical transistors that have similar on and off threshold voltages, in hydraulic lMVs, the threshold pressures for opening and closing are significantly different and can change, even for the same lMVs depending on overall circuit design and operation conditions. We explain, in particular, how the negative values of the closing threshold pressures significantly constrain operation of even simple hydraulic lMV circuits such as autonomously switching two-valve microfluidic oscillators. These understandings have significant implications in designing self-regulated microfluidic devices. Electric circuit analogy is widely used in microfluidic circuit design and analysis. For example, electric resistors correspond to microfluidic channel resistances and capacitors to flexible membranes. The analogy is based on the similarity in equations between these circuit components. Since the theory and simulation methods of electric circuits are wellestablished, they greatly facilitate the design and analysis of various microfluidic circuits. 1 Recently, analogy has been drawn between electronic transistors and microfluidic membrane-valves (lMV), which are used for self-regulated microfluidic circuits such as frequency-specific flow regulators, 2 digital logic circuits, [3][4][5][6] and oscillators. [7][8][9] Like an electronic circuit that operates itself with only a power source and thus minimize the use of its external controllers, the self-regulated microfluidic circuits have the potential to greatly reduce reliance on expensive and complex external controllers, which are barriers to broader use of microfluidic devices. A lMV is a crucial component that enables operation of self-regulated microfluidic devices through its on-off switching.Similar to the electronic transistor, the lMV's on-off switching is determined by the relative difference between the source (P S ) minus gate pressure (P G ) versus the threshold pressure ( Figure 1). As depicted in Figure 1(a), when P S is sufficiently greater than P G , the membrane of the lMV deflects down and the lMV is on (open). In other words, the lMV is on when P S À P G is greater than opening threshold pressure (P th-open ). To turn off (close) the lMV, P S À P G is less than closing threshold pressure (P th-close ). In typical silicon transistors, 10,11 the difference between on and off threshold voltage is negligible, thus providing a large parameter space for the design and operation of large-scale integrated circuits. Pneumatic lMVs also exhibit small differences between opening (P th-open ) and closing (P th-close ) threshold pressures. 5,12 However, in self-regulated microfluidic devices, the detailed characteristic of lMV's threshold pressure in the context of other microfluidic parameters such as fluidic resistor and inflow rate is largely unknown.Here, we report that P th-open and P th-close are significantly differe...

Predictable Duty Cycle Modulation through Coupled Pairing of Syringes with Microfluidic Oscillators

“…As C is proportional to the size of a given syringe, it is consequently proportional to Qin, which increases with the size of the syringe used. This finding is in agreement with previous results reported for four-way valves, where an increase in volumetric inflow rate through one valve increases calculated Pth for the opposite valve [21]. From Equation (5), we infer that increasing Pth in conditions with lower Qin, will produce higher toff; and that as the asymmetry between the flow rate across each valve increases, toff will increase for the valve with a lower inflow rate, producing larger oscillation periods.…”

“…A portion of this outflow is then diverted from its drain terminal to the gate terminal of valve 2, as the outflow from valve 1 had been diverted previously, and supplies the gate pressure necessary to force the accumulation of fluid upstream of valve 2, until the difference between PS2 and PG2 has exceeded Pth2 (Figure 1a,b). The coordination of these processes, resulting in the anti-synchronized opening and closing of both valve units, produces an oscillatory outflow (described in greater detail in previous work [2,21]). (a).…”

“…The challenge associated with modifying volumetric inflow rate lies in the effect this may have upon the threshold opening pressure of each valve [21]. Due to the complexity of the relationship between volumetric flow rate and duty cycle, predicting the duty cycle resulting from a change in volumetric flow rate is not trivial.…”

Analyzing the transition pressure and viscosity limit of a hydroelastic microfluidic oscillator

“…10 Based on their similarity, relevant theory and methods of electric circuits can be applied to analyze such microfluidic devices and networks. 5,11 From another perspective, the microfluidic design containing dynamic elastic features falls into the category of dynamic hydroelasticity problems. It involves complex interactions among the hydrodynamic, elastic, and inertial forces.…”

Elastomeric microfluidic valve with low, constant opening threshold pressure

“…[1][2][3] Because such applications require sophisticated regulation of ow timing and routing, elastomeric microvalves are crucial components in them. [16][17][18][19][20][21][22][23][24][25] Self-controlled microuidic circuits have received research attention because their operation mechanism is analogous to that of electronic circuits, thus providing possibilities of greatly reducing the need for off-chip controllers. Out of these two microvalve types, NC valves are becoming increasingly important for their application to selfcontrolled microuidic circuits, including digital logic circuits and oscillators.…”