Rapid Fabrication of Membrane-Integrated Thermoplastic Elastomer Microfluidic Devices

Abstract

Leveraging the advantageous material properties of recently developed soft thermoplastic elastomer materials, this work presents the facile and rapid fabrication of composite membrane-integrated microfluidic devices consisting of Flexdym^TM polymer and commercially available porous polycarbonate membranes. The three-layer devices can be fabricated in under 2.5 h, consisting of a 2-min hot embossing cycle, conformal contact between device layers and a low-temperature baking step. The strength of the Flexdym^TM-polycarbonate seal was characterized using a specialized microfluidic delamination device and an automated pressure controller configuration, offering a standardized and high-throughput method of microfluidic burst testing. Given a minimum bonding distance of 200 μm, the materials showed bonding that reliably withstood pressures of 500 mbar and above, which is sufficient for most microfluidic cell culture applications. Bonding was also stable when subjected to long term pressurization (10 h) and repeated use (10,000 pressure cycles). Cell culture trials confirmed good cell adhesion and sustained culture of human dermal fibroblasts on a polycarbonate membrane inside the device channels over the course of one week. In comparison to existing porous membrane-based microfluidic platforms of this configuration, most often made of polydimethylsiloxane (PDMS), these devices offer a streamlined fabrication methodology with materials having favourable properties for cell culture applications and the potential for implementation in barrier model organ-on-chips.

Keywords: delamination testing; membrane-based cell culture; microfluidic device; rapid fabrication; thermoplastic elastomer.

Figure 1

Schematics of the composite membrane-integrated cell culture device fabrication workflow starting from (A) a pre-extruded Flexdym^TM sTPE sheet and a microfluidic mould. Fabrication consists of (B) a 150 °C hot embossing cycle of the sTPE sheet atop a microfluidic mould, (C) cutting of the micropatterned sTPE to appropriate device size and punching access holes, (D) layering of the micropatterned sTPE layers with an off-the-shelf porous polycarbonate membrane and (E) baking at 80 °C to achieve device bonding resulting from the mobility of the intrinsically adhesive “soft” block polymer chains. Devices of this configuration used for cell culture contained channels of cross section 800 µm × 110 µm (width × height) and 27 mm length. The durations of each fabrication step are included.

Figure 2

(A) Expanded view of the FD-PC-FD microfluidic chip design for delamination tests, consisting of two disconnected channels separated by a gap of varying distances. The inlet channel is increasingly pressurized, with no flow occurring until the delamination of the PC membrane from the FD gap structure occurs, at which point fluid crosses the gap into the outlet channel. (B) And (C) respectively show cross sections of the gap portion of the device before and after delamination. (D) Schematic of the automated delamination testing setup utilizing flow and pressure sensors and a valve matrix in series with a water-filled reservoir pressurized by a pressure controller. Continuous data logging and sensor feedback allowed the sequential testing of the pressure capacities of up to 10 microfluidic devices with no user monitoring.

Figure 3

(A) FD-PC and FD-FD bonding evaluation through pressure delamination testing of devices with gap distances from 100 to 1000 µm. FD-PC devices show reduced bonding strength compared to FD-FD bonding but reliably withstand pressures of 500 mbar at gap distances of 200 µm and above. (B) Pressure delamination testing of FD-FD and FD-PC devices (fixed 400 µm gap distance) at 1, 7 and 14 days after fabrication. An additional set of FD-PC devices was aged in high humidity, 37 °C incubation conditions (abbreviated “Inc.” in the graph), which revealed no significant impact on the device sealing due to time post-fabrication or incubation conditions (n = 5 devices per dataset; error bars represent one standard deviation).

Figure 4

(A) Flow-pressure correlation in FD-PC devices from tests measuring the flow rate in a straight microfluidic channel (of width 200, 400 or 800 μm) and the corresponding pressure at the channel inlet. Within 500 mbar of pressure applied at the device, flow rates of up to approximately 150 μL/min can be reached. (B) Wall shear stresses that can be achieved in each of the example devices, as calculated from the flow rate data in (A), depending on the pressure applied. Shear stresses of up to approximately 140 dyne/cm² can be generated with pressures of 500 mbar and below.

Figure 5

(A,B) Representative images of human dermal fibroblasts (HDFs) cultured in FD-PC-FD devices over the course of 7 days. HDFs presented a primarily spindle geometry, commonly seen when HDFs are cultured to high confluency, due to higher density of cells. HDFs were cultured on top of the polycarbonate membrane for 7 days prior to being fixed and stained with 488-Alexa Fluor^TM 488 Phalloidin (staining for F-actin, green) and DAPI (nuclear, blue), to demonstrate cell adhesion and maintained presence in static culture within devices over the course of 1 week. Scale bars = 150 μm.