Towards an Implantable, Low Flow Micropump That Uses No Power in the Blocked-Flow State

Abstract

Low flow rate micropumps play an increasingly important role in drug therapy research. Infusions to small biological structures and lab-on-a-chip applications require ultra-low flow rates and will benefit from the ability to expend no power in the blocked-flow state. Here we present a planar micropump based on gallium phase-change actuation that leverages expansion during solidification to occlude the flow channel in the off-power state. The presented four chamber peristaltic micropump was fabricated with a combination of Micro Electro Mechanical System (MEMS) techniques and additive manufacturing direct write technologies. The device is 7 mm × 13 mm × 1 mm (<100 mm³) with the flow channel and exterior coated with biocompatible Parylene-C, critical for implantable applications. Controllable pump rates from 18 to 104 nL/min were demonstrated, with 11.1 ± 0.35 nL pumped per actuation at an efficiency of 11 mJ/nL. The normally-closed state of the gallium actuator prevents flow and diffusion between the pump and the biological system or lab-on-a-chip, without consuming power. This is especially important for implanted applications with periodic drug delivery regimens.

Keywords: Hybrid MEMs; and phase-change; direct write; gallium; implantable; integration; micropump; microsystems; peristaltic.

Figure A1

Images of micro-contact mold process. (a) SU-8 patterned lithographically on Si substrate; (b) PDMS poured over positive SU-8 mold, cured, then lifted free; (c) PDMS mold pressed against molten gallium on Si substrate; (d) Gallium was solidify by cooling with ice pack; (e) PDMS mold pulled off leaving gallium on substrate; (f) Free standing gallium ingot; (g) Top view of micromold-printed gallium ingot; (h) side view of gallium ingot.

Figure A2

Illustration of cross-section of test fixture for testing diaphragm deflection. Visual inspection with Wyko interference microscope allows for correlation between actuation pressure and deflection at the center of the diaphragm for comparison to theory. (a) Top view showing pneumatic access, plenum, and underlying chamber; (b) Illustration of plenum/chamber arrangement, (pneumatic access shown out of position for clarity).

Figure A3

Membrane deflection images and graph. (a,b) From the Wyko interferometer of the pump diaphragm at rest, 0 psi, and at 3 psi; (c) Data from the Wyko showing the maximum deflection at the center of a 500-µm diameter diaphragm.

Figure A4

Diagram of parameters for diaphragm deflection. Large deflection (w₀ > h) of diaphragms of diameter D under trans-diaphragm pressure ΔP_D.

Figure 1

Family of curves showing Reynolds number vs. flow rate in rectangular channels for various flow channel cross sectional sizes. Passive valves are ineffective in the laminar flow regime requiring active valves for robust flow rectification.

Figure 2

Conceptual illustration of a four-chamber peristaltic pump (6.4 mm × 4.4 mm × 1 mm) for use with phase-change actuation, coupled to small diameter capillary tubing.

Figure 3

Gallium-based phase change actuator in the solidified (closed) state (a); and liquified (open) state (b). Inset: micro-ingot of Ga used for length-based resolution of sub-mg quantities.

Figure 4

Micropump fabrication process with direct write integration over classic Micro Electro Mechanical System (MEMS) structures. (Not to scale) (a) 1 µm Tetraethyl Orthosilicate (TEOS) patterned to define channels and chambers with 0.3 µm Al heaters and contact pads; (b) Photoresist, XeF₂ etch to 130 µm to define the interconnect channels; (c) Second XeF₂ etch to create channels and chambers (15 µm depth) and to etch the interconnect channels to their final depth of 145 µm; (d) Parylene-C (PCPX) deposited (3 µm) to coat channels and form diaphragms; (e) Cover glass secured over interconnect area; (f) Wafer diced to reveal interconnect openings; (g) Epoxy resin printed to define plena, capillary tubing (140 µm OD) captured with 40 µm of Parylene-C; (h) Capillaries connected to vacuum with nanotight fittings to drawn down the diaphragms, gallium ingots (412 nL) placed in each of the four plena, melted, and re-solidified to hold the diaphragms before removing vacuum; (i) A Cover glass was manually coated with UV curable resin and lowered onto gallium filled plena; (j) When heated the gallium withdraws from the pump chamber pulling in fluid. The device can be encapsulated in Parylene-C to provide a biocompatible external surface.

Figure 5

(a) Illustration of the long-term diaphragm deflection test set-up. Magnification shows a close up of the air/water interface examined on a weekly basis; (b) Set-up for gallium thermal actuation peristaltic pump characterization; (c) Photographic sequence of the output capillary over printed gradations for fluid movement measurement demonstrates 1.5 mm movement (14.8 nL) over 30 s for a pump rate of 29.6 nL/min.

Figure 6

SEM of in-situ formed Parylene-C encapsulated TEOS diaphragm over a micropump chamber. Inset: Close-up of diaphragm visualizing dimples in the Parylene-C surface corresponding to the etched holes in the underlying TEOS framework.

Figure 7

Photograph of pump with plena and capillary tubing. Visible are the interconnected capillary tubes, heaters, diaphragm chambers, plena, and heater contact pads. Heater miniaturization was limited by resolution of the printed transparency masks used in this work.

Figure 8

Volume of fluid pumped for the four pump chambers. n = 16, Average = 11.1 nL, σ = 0.35 nL.

Figure 9

Pump rate vs. actuation frequency for the micropump compared to theoretical rates based upon chamber volume design (n = 3). Lower flow rates are possible with duty cycle control, while higher rates can be achieved with larger chambers and scaling of the Ga.