Microfluidic Platform with Multiplexed Electronic Detection for Spatial Tracking of Particles
Summary
We demonstrate a microfluidic platform with an integrated surface electrode network that combines resistive pulse sensing (RPS) with code division multiple access (CDMA), to multiplex detection and sizing of particles in multiple microfluidic channels.
Abstract
Microfluidic processing of biological samples typically involves differential manipulations of suspended particles under various force fields in order to spatially fractionate the sample based on a biological property of interest. For the resultant spatial distribution to be used as the assay readout, microfluidic devices are often subjected to microscopic analysis requiring complex instrumentation with higher cost and reduced portability. To address this limitation, we have developed an integrated electronic sensing technology for multiplexed detection of particles at different locations on a microfluidic chip. Our technology, called Microfluidic CODES, combines Resistive Pulse Sensing with Code Division Multiple Access to compress 2D spatial information into a 1D electrical signal. In this paper, we present a practical demonstration of the Microfluidic CODES technology to detect and size cultured cancer cells distributed over multiple microfluidic channels. As validated by the high-speed microscopy, our technology can accurately analyze dense cell populations all electronically without the need for an external instrument. As such, the Microfluidic CODES can potentially enable low-cost integrated lab-on-a-chip devices that are well suited for the point-of-care testing of biological samples.
Introduction
Accurate detection and analysis of biological particles such as cells, bacteria or viruses suspended in liquid is of great interest for a range of applications1,2,3. Well-matched in size, microfluidic devices offer unique advantages for this purpose such as high-sensitivity, gentle sample manipulation and well-controlled microenvironment4,5,6,7. In addition, microfluidic devices can be designed to employ a combination of fluid dynamics and force fields to passively fractionate a heterogeneous population of biological particles based on various properties8,9,10,11,12. In those devices, the resultant particle distribution can be used as readout but spatial information is typically accessible only through microscopy, limiting the practical utility of the microfluidic device by tying it to a lab infrastructure. Therefore, an integrated sensor that can readily report particles' spatiotemporal mapping, as they are manipulated on a microfluidic device, can potentially enable low-cost, integrated lab-on-a-chip devices that are particularly attractive for the testing of samples in mobile, resource-limited settings.
Thin film electrodes have been used as integrated sensors in microfluidic devices for various applications13,14. Resistive Pulse Sensing (RPS) is particularly attractive for integrated sensing of small particles in microfluidic channels as it offers a robust, sensitive, and high-throughput detection mechanism directly from electrical measurements15. In RPS, the impedance modulation between a pair of electrodes, immersed in an electrolyte, is used as a means to detect a particle. When the particle passes through an aperture, sized on the order of the particle, the number and amplitude of transient pulses in the electrical current are used to count and size particles, respectively. Moreover, the sensor geometry can be designed with a photolithographic resolution to shape resistive pulse waveforms in order to enhance sensitivity16,17,18,19 or to estimate vertical position of particles in microfluidic channels20.
We have recently introduced a scalable and simple multiplexed resistive pulse sensing technology called Microfluidic Coded Orthogonal Detection by Electrical Sensing (Microfluidic CODES)21. Microfluidic CODES relies on an interconnected network of resistive pulse sensors, each consisting of an array of electrodes micromachined to modulate conduction in a unique, distinguishable manner, so as to enable multiplexing. We have specifically designed each sensor to produce orthogonal electrical signals similar to the digital codes used in code division multiple access22 (CDMA) telecommunication networks, so that individual resistive pulse sensor signal can be uniquely recovered from a single output waveform, even if signals from different sensors interfere. In this way, our technology compresses 2D spatial information of particles into a 1D electrical signal, permitting monitoring of particles at different locations on a microfluidic chip, while keeping both device- and system-level complexity to a minimum.
In this paper, we present a detailed protocol for experimental and computational methods necessary to use the Microfluidic CODES technology, as well as representative results from its use in analysis of simulated biological samples. Using the results from a prototype device with four multiplexed sensors as an example to explain the technique, we provide protocols on (1) the microfabrication process to create microfluidic devices with the Microfluidic CODES technology, (2) the description of the experimental setup including the electronic, optical, and fluidic hardware, (3) the computer algorithm for decoding interfering signals from different sensors, and (4) the results from detection and analysis of cancer cells in microfluidic channels. We believe that using the detailed protocol described here, other researchers can apply our technology for their research.
Protocol
Representative Results
Discussion
Multiple resistive pulse sensors have previously been incorporated into microfluidic chips28,29,30,31,32. In these systems, resistive pulse sensors were either not multiplexed28,29,30,31 or they required individual sensors to be driv…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This work was supported by National Science Foundation Award No. ECCS 1610995. The authors would like to thank the Institute of Electronics and Nanotechnology and the Parker H. Petit Institute for Bioengineering and Bioscience staff for their support in using shared facilities. The authors also would like to thank Chia-Heng Chu for his help in preparing the manuscript.
Materials
| 98% Sulfuric Acid | BDH Chemicals | BDH3074-3.8LP | |
| 30% Hydrogen Peroxide | BDH Chemicals | BDH7690-3 | |
| Trichlorosilane | Aldrich Chemistry | 235725-100G | |
| NR9-1500PY Negative Photoresist | Furuttex | ||
| Resist Developer RD6 | Furuttex | ||
| Acetone | BDH Chemicals | BDH1101-4LP | |
| SU-8 2015 Negative Photoresist | Microchem | SU8-2015 | |
| SU-8 Developer | Microchem | Y010200 | |
| Polydimethylsiloxane (PDMS) | Dow Corning | 3097358-1004 | Sylgard 184 Silicone Elastomer Kit |
| Isopropyl Alcohol | BDH Chemicals | BDH1133-4LP | |
| RPMI 1640 | Corning Cellgro | 10-040-CV | |
| Fetal Bovine Serum (FBS) | Seradigm | 1500-050 | |
| Penicillin-Streptomycin | Amresco | K952-100ML | |
| Phosphate-Buffered Saline (PBS) | Corning Cellgro | 21-040-CM | |
| PHD 22/2000 Syringe Pump | Harvard Apparatus | 70-2001 | |
| HF2LI Lock-in Amplifier | Zurich Instrument | ||
| HF2TA Current Amplifier | Zurich Instrument | ||
| Eclipse Ti-U Microscope | Nikon Corporation | ||
| DS-Fi2 High-Definition Color Camera | Nikon Corporation | ||
| v7.3 High-speed Camera | Phantom | ||
| PCIe-6361 Data Acquisition Board | National Instruments | 781050-01 | |
| BNC-2120 Shielded Connector Block | National Instruments | 777960-01 | |
| PX-250 Plasma Treatment System | Nordson MARCH |
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