Design and characterization of a package-less hybrid PDMS-CMOS-FR4 contact-imaging system for microfluidic integration

Abstract

We demonstrate a hybrid "package-less" polydimethylsiloxane (PDMS)-complementary-metal-oxide-semiconductor (CMOS)-FR4 system for contact imaging. The system embeds the CMOS image sensor directly in a PDMS layer instead of the standard chip package to support microfluidic structures much larger and more complex than those in prior art. The CMOS/PDMS layer is self-aligned to form a continuous, flat surface to provide structural support for upper microfluidic layers. The system consists of five layers of PDMS implementing fluid channels, valves, chambers, and inlets/outlets. A custom CMOS image sensor with integrated signal conditioning circuits directly captures light from sample fluid for high optical collection efficiency. Owing to the flexibility afforded by the integration process, the system demonstrates, for the first time, integrated valves in contact imaging. Moreover, we present the first direct comparison of the optical performance of a CMOS image sensor and a photomultiplier tube (PMT) in identical contact-imaging conditions. Measurements show that our CMOS sensor achieves 17 dB better signal-to-noise ratio (SNR) compared to a commercial PMT across a broad range of integration times, with a maximum SNR of 47 dB. Chemiluminescent testing successfully shows signal detection for different analyte concentrations and integration times. The contact-imaging system demonstrates a detection limit of 25 μM of a 9,10-diphenylanthracene-based solution.

Keywords: CMOS image sensor; PDMS; contact imaging; hybrid microfluidic; integrated valves.

FIG. 1

(a) Cross-sectional view of a conventional contact-imaging system using a chip package with limited area for the microfluidic device and (b) our contact-imaging system using a PDMS base which can be arbitrarily scaled to support any microfluidic device size.

FIG. 2.

Our contact-imaging microfluidic system employing five PDMS layers, a CMOS image sensor, and an FR4 PCB (bond wires come out-of-plane, not shown).

FIG. 3.

(a) Layout of microfluidic device showing channels, valves, and inlets/outlets. (b) Cross-section views of the microfluidic device. (c) Microfluidic valve before and after closing.

FIG. 4.

(a) PDMS device fabrication steps beginning with 1. photoresist patterning, 2. mold development, 3. PDMS deposition, 4. PDMS detachment, and 5. PDMS layers bonding. (b) CMOS image sensor held by a needle to a wafer before PDMS is poured into the PVC ring to form the base layer.

FIG. 5.

(a) Assembled microfluidic device on the PCB. Channels have been filled with dyed water for contrast. (b) Another view of the microfluidic device showing the electrical connections from the embedded CMOS sensor to the PCB via bond wires.

FIG. 6.

(a) Micrograph of the fabricated CMOS sensor. (b) CMOS sensor circuit schematic.

FIG. 7.

(a) Circuit schematic of the folded-cascode op amp. (b) Measured op amp gain.

FIG. 8.

(a) Contact-imaging system test setup diagram. (b) Close-up view of the test setup.

FIG. 9.

(a) Valve testing by injecting fluid through inlet IN1 with valve V4 closed and (b) with V4 open.

FIG. 10.

(a) CMOS sensor output showing four integration cycles and (b) PMT output for the same test conditions.

FIG. 11.

(a) SNR vs integration time for the CMOS sensor and PMT. (b) CMOS sensor SNR vs incident optical power. (c) CMOS sensor output voltage vs dye concentration of a 9,10-diphenylanthracene-based chemiluminescent solution for different integration times. The dashed lines denote the dark signal floor.