A disposable smart microfluidic platform integrated with on-chip flow sensors

Highlights

• Multiple on-chip flow sensors are integrated into the microfluidic platform.

• The number and location of the on-chip flow sensors can be freely customized.

• The platform including the on-chip flow sensors can be fully automated.

• Self-diagnosis functionality can be realized by the on-chip flow sensors.

• The microfluidic platform is disposable owing to the film-chip technique.

Abstract

Microfluidic devices are powerful tools for biological, biomedical, chemical, and pharmaceutical applications, but their commercialization is still hindered by the lack of methods to automatically control fluid flow in a low-cost, simple, accurate, and safe manner. This study introduces a disposable smart microfluidic platform (DIS-μChip), which can be fully automated and utilized for a wide range of applications. On-chip microfluidic flow sensors are integrated with the platform and placed at all inlet and outlet channels, thereby allowing the DIS-μChip to be fully automated with a pressure control system. Furthermore, these confer a self-diagnosis function through monitoring of all the input and output flow rates. The DIS-μChip consists of a disposable polymeric microchannel superstrate and a permanent multifunctional substrate, which could be assembled and disassembled using only vacuum pressure. The superstrate was fabricated by combining a polydimethylsiloxane microchannel structure with a polyethylene terephthalate (PET) thin film. The substrate contains sense electrodes for the on-chip-integrated flow sensors and functional components for creating an energy field, which can penetrate the PET thin film and manipulate the fluid in the microchannels of the superstrate. Owing to the film-chip technique, the superstrate was disposable and could prevent biological cross-contamination, which cannot be realized with conventional flow sensors. The usefulness of the DIS-μChip was demonstrated by using it to isolate circulating tumor cells from the blood of patients with pancreatic cancer and to obtain cancer-specific genetic information from them with droplet digital PCR.

Introduction

Precise flow control in microfluidics is critical for reliability and reproducibility. Currently, syringe (Li et al., 2014) and pressure-driven (Martino et al., 2016) pumps are commonly used for microfluidic flow control. In particular, syringe pumps are the most widely used owing to their ease of use and rapid results. However, ripples in microfluidic flows occur because of friction between the plunger and syringe barrel (Korczyk et al., 2011) as well as the stepping of the stepper motor (Li et al., 2014). In addition, when air is trapped in the syringe, it is not easy to remove it. The smaller the sample, the more serious the issues become, thereby requiring careful sample loading into the syringe. If the pumping process takes more than 5 min, dense substances in the sample are precipitated, resulting in the injection of a non-uniform concentration of the sample and the loss of many substances trapped in the flow volume between the syringe and microfluidic device. In addition, the syringe pump sometimes generates high pressure in the microchannel with excessive fluid resistance, thereby bursting the microfluidic device.

The pressure-driven pump can be used as an alternative method to address these problems of syringe pumps. It is comprised of an electro-pneumatic pressure control system that regulates air pressure to drive the fluid within a microchannel, and flow sensors that monitor flow rates (Frank et al., 2016). Based on a proportional-integral-derivative (PID) control algorithm, the pressure control system and flow sensors can be used to set a wide range of flow rates (from a few nanoliters to microliters per hour) quickly and accurately. In addition, sample loading is straightforward, and the problems of bubble trapping and precipitation of dense substances are not significant.

For biomedical and pharmaceutical applications, all parts directly in contact with biomaterials must be sterilized or discarded to prevent biological cross-contamination and enhance patient safety after a single use. For this reason, in the case of the pressure-driven pumping method, all parts that are in direct contact with biomaterials, including flow sensors, must be sterilized after use or discarded. However, the sterilization process is not usually possible for conventional flow sensors because all the electronic components are housed in a single package, which is vulnerable to typical sterilization processes. Furthermore, owing to their high cost, current commercially available flow sensors are not considered disposable, limiting their utility for biomedical and pharmaceutical applications. Commonly used calorimetric thermal flow sensors absorb biological substances toward the surface of the sensing area due to heat (Kuo et al., 2012; Nguyen, 1997), which contributes to the measurement of flow rates, resulting in reduced sensitivity and biological cross-contamination, even though the devices are cleaned after each use.

Several microfluidic flow sensors (Etxebarria et al., 2016, 2017; Kim et al., 2019; Tanaka et al., 2009) have recently been developed in a disposable format. Tanaka et al. (2009) and Etxebarria et al., 2016, 2017 used 100-μm-thick glass and cyclic-olefin polymer films as microchannel substrates, respectively, while heating and sensing electrodes were patterned on the reverse side of the films. Kim et al. (2019) introduced a disposable flow sensor consisting of a disposable microchannel superstrate and a reusable sensing substrate, which was fabricated by employing a silicone-coated 12-μm-thick polymer film. Although the reported flow sensors can be developed at low prices for single use, such individual flow sensors still have limitations, such as flow volume and bubble trapping between flow sensors and microfluidic devices, as well as inconvenient tubing connection steps. These main hurdles make it difficult for microfluidic devices to be developed in a fully automated format. Thus, the majority of the current commercially available microfluidic devices are developed to be insensitive (Hvichia et al., 2016; Renier et al., 2017; Warkiani et al., 2016) to variations in flow rates or are operated by passive flow control (Balasubramanian et al., 2017; Harb et al., 2013; Joanicot and Ajdari, 2005) using only the flow resistance of microchannels, without detection of flow rates. However, passive flow control methods are restricted to applications where simple-structured microchannels are used, whereas the recently developed advanced microfluidic devices (Fachin et al., 2017; Klein et al., 2015) usually involve complex microchannel structures, thereby requiring precise active flow control methods equipped with functionalities for flow rate detection.

In this paper, we introduce a disposable smart microfluidic platform, termed “DIS-μChip,” in which flow sensors and microfluidic functions are on-chip integrated with a disposable film-chip technique (Cho et al., 2017). Owing to the on-chip integration, the flow volume between the flow sensors and microfluidic functions is significantly reduced, and tubing connections are not necessary. In order to prove the usefulness of the DIS-μChip as a test vehicle, herein, it was applied to the isolation of circulating tumor cells (CTCs). To analytically evaluate the isolation efficiency of the DIS-μChip, MCF7 cancer cells added into healthy blood were isolated at various flow rates, controlled by a fully automated electro-pneumatic pressure system. To verify the clinical applicability of the DIS-μChip, CTCs were isolated from the blood of patients (n = 10) with pancreatic cancer and used for genetic analysis with droplet digital PCR (ddPCR).