Microfluidic Valves Made From Polymerized Polyethylene Glycol Diacrylate

Abstract

Pneumatically actuated, non-elastomeric membrane valves fabricated from polymerized polyethylene glycol diacrylate (poly-PEGDA) have been characterized for temporal response, valve closure, and long-term durability. A ~100 ms valve opening time and a ~20 ms closure time offer valve operation as fast as 8 Hz with potential for further improvement. Comparison of circular and rectangular valve geometries indicates that the surface area for membrane interaction in the valve region is important for valve performance. After initial fabrication, the fluid pressure required to open a closed circular valve is ~50 kPa higher than the control pressure holding the valve closed. However, after ~1000 actuations to reconfigure polymer chains and increase elasticity in the membrane, the fluid pressure required to open a valve becomes the same as the control pressure holding the valve closed. After these initial conditioning actuations, poly-PEGDA valves show considerable robustness with no change in effective operation after 115,000 actuations. Such valves constructed from non-adsorptive poly-PEGDA could also find use as pumps, for application in small volume assays interfaced with biosensors or impedance detection, for example.

Keywords: membrane valve; non-adsorptive polymer; non-elastomeric polymer; pneumatic actuation; poly-PEGDA; valve characterization; valve response.

Figure 1

Schematic of a three-layer poly-PEGDA valve. (A) The left blue inset is a cross sectional view along the dashed blue line for an open or closed valve. Top-view images on the right show an open (top) and closed (bottom) valve with green dyed fluid added for contrast. Valve diameter (D) is 700 μm, pedestal width (P) is 30 μm, and the fluid channel width is 100 μm. (B) Top-view photomicrograph of a valve before filling with liquid. Interference fringes indicate that the membrane is deflected upward after the final bonding step. White scale bar is 200 μm. (C) SEM of a three-layer valve device cross-section along the dashed blue line in (B).

Figure 2

Overview of poly-PEGDA valve fabrication. The top box demonstrates fabrication of the polymerized control (top), membrane (middle), and fluidic (bottom) layers. Final assembly is shown in the bottom box. (A) Clean silicon wafers. (B) SU8 patterns define features. (C) Spacers define poly-PEGDA thickness. (D) Glass wafer forms top of mold. (E) Prepolymer is introduced. (F) Polymerization of poly-PEGDA. (G) Glass cover wafer is removed. (H) Finished poly-PEGDA is removed, diced, and cleaned; an input hole is cut into the control layer. (I) The just-released top surface of the membrane layer (G, middle) is bonded to the control layer (H, top). (J) The bonded control and membrane layers are removed and (K) bonded under vacuum to the fluidic layer using UV light, (L) resulting in a completed valve device.

Figure 3

Valve temporal response. Valve was actuated at 1 Hz and 30% duty cycle. Fall time (valve closure) was 0.019 s and rise time (valve opening) was 0.105 s. No vacuum was used to open the valve, and the fluid backpressure was negligible in these experiments.

Figure 4

Fluid pressure and volumetric flow rate as a function of time for a constant control pressure. Sensors in the fluid and control lines monitor pressures, and meniscus tracking on the fluid output allows for flow measurement. The flow rate increases rapidly once the fluid pressure exceeds the control pressure at ~240 s.

Figure 5

Calculated (line) and experimentally measured (circles) deflection via applied pressure, for a 45 μm thick circular membrane with an elastic modulus of 0.13 GPa, a 350 μm radius, and a Poisson’s ratio of 0.35. Calculated data are from Equation 2. Very low pressure (~9 kPa) is required for significant membrane deflection (>2 μm).

Figure 6

Valve performance after a number of actuations as a function of control pressure. (A) Initial valve testing shows a higher fluid pressure is required to open the valve for a given control pressure. After ~1500 actuations, the fluid pressure to open a valve decreases to match the control pressure. A circular valve with a 15 μm pedestal width was used for this test. (B) Valves maintain this linear fluid vs. control pressure relationship to at least 115,000 actuations. A different circular valve with a 30 μm pedestal width was used for this test.