Design and dynamic characterization of "single-stroke" peristaltic PDMS micropumps

Abstract

In this paper, we present a monolithic PDMS micropump that generates peristaltic flow using a single control channel that actuates a group of different-sized microvalves. An elastomeric microvalve design with a raised seat, which improves bonding reliability, is incorporated into the micropump. Pump performance is evaluated based on several design parameters--size, number, and connection of successive microvalves along with control channel pressure at various operating frequencies. Flow rates ranging 0-5.87 µL min(-1) were observed. The micropump design demonstrated here represents a substantial reduction in the number of/real estate taken up by the control lines that are required to run a peristaltic pump, hence it should become a widespread tool for parallel fluid processing in high-throughput microfluidics.

Fig. 1. Overview of “single-stroke” micropump (SSμP) fabrication and operation

(a) Fabrication of the PDMS micropump by soft lithography. Silicon masters (not shown) of the fluidic layer and control layer are fabricated by photolithography using SU-8 negative photoresist; replica layers are made by pouring PDMS polymer (10: 1 w/w, pre-polymer: crosslinker) onto silanized masters. (b) Assembly of fluidic layer, control layer, and PDMS membrane by O₂-plasma bonding. Thin PDMS membrane is spun onto clean, silanized silicon wafers, cured, and subsequently bonded to the control layer after O₂-plasma exposure; access ports are opened in the fluidic layer and bonded under vacuum to the control layer and thin membrane. (c) Top half: Schematic representation and stereomicrograph of a SSμP; bottom half: operation schematic of a microvalve. Orthogonal cross-section of the microvalve at the valve seat (i) and above the pumping element (ii) is depicted along with the overview of the microvalve (iii). At rest, the valve is partially open due to the gap between the valve seat and thin membrane in the fluidic channel; in this “leaky state”, the microvalve functions as a flow resistor that restricts large particles and reduces fluid flow. When positive pressure is applied, the thin membrane deflects upward, pressing against the raised valve seat, effectively closing the channel; however, only the raised valve seat is closed completely while the region with regular channel height is not, due to the rectangular geometry of the channel. The volume displaced by the deflecting thin membrane into the 35 μm tall region of the channel provides the bulk of fluid flow in pumping operations. When vacuum is applied, the thin membrane deflects downward, revealing a large opening for fluid passage beneath the valve seat; the vacuum stroke speeds up air evacuation from the pressurized state and also enables greater fluid filling.

Fig. 2. Flow rate characterization for eight designs of the single-stroke micropump (SSμP)

Three-element serial SSμP (a) design schematic and (b) flow rate measurement in (i) forward and (ii) backward pumping mode. Measurements were taken in the sequence of ascending frequency (solid lines) initially followed by a descending sequence (dotted lines) to test membrane hysteresis. Four-element parallel SSμP (c) design schematic and (d) flow rate measurement.

Fig. 3. Sequential activation of different-sized elements in serial and parallel single-stroke micropumps (SSμPs)

Frame-by-frame sequence of (a) three-element serial (Pump 3) and (b) four-element parallel (Pump 8 modified) SSμPs. Frames are obtained from video showing the pumping sequence of each pump (video is available in the ESI†) recorded at 30 fps and high definition with an SLR camera. Time of each frame is presented as shown in the actual video footage with the time format minute:-second.millisecond. The four element parallel pump used in the video is modified from the original design by removing the last three microvalves from the fluidic channel.

Fig. 4. Effect of control channel pressure on flow rate versus operating frequency

(a) Design of serial, 5-element SSμP used in the experiment; the smallest elements (a) and the largest elements (b) have lengths of 300 and 900 μm, respectively, with increment lengths (Δ) of 150 μm. (b) Flow rate measurement curve at selected operating frequencies for the 5-element micropump design with control channel pneumatic pressures of 6, 9, and 12 psi.