shows SEM images of the PMMA master, PDMS master, and PDMS device for the 40 fluidic interfaces integrated microdevice fabricated by the MMHSM process. shows the PDMS multi-compartment neuron co-culture device showing millimeter-scale compartments connected via arrays of 3 μm high microchannels. It can be seen that microscale channels and macroscale fluidic interfaces were successfully transferred to the final PDMS device.
Fig. 3 SEM images of (a) PMMA master, (b) PDMS master, and (c) PDMS device showing microchannels connected to millimeter-scale fluidic interfaces (Insets: Enlarged view of 500 μm wide channels). Scale bars, 200 μm. (d) Bottom-side of the PDMS (more ...)
The PDMS master was easily released from the PMMA master after the soft-lithography process without any surface treatment of the PMMA master. However, the final PDMS devices firmly adhered to the PDMS master after the curing process and could not be peeled off without damaging the microstructures when no coating was used. In order to facilitate the release process, the PDMS master was vapor coated with (tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilane (United Chemical Technologies, Inc., Bristol, PA) for 10 min and rinsed with IPA to remove excessive coating residues. Chemical treatment of the PDMS master solved the adhesion issue, but the high aspect ratio of the PDMS walls separating reservoirs and fluidic tubing interfaces still posed a challenge, and careful attention had to be paid when designing the reservoirs or fluidic interfaces on the PMMA master. When reservoirs or fluidic interfaces were too close to each other compared to the final thickness of the PDMS device, for example when the aspect ratio of the PDMS wall was 17.5: 1, the wall part of the PDMS was torn during the PDMS device replication process. For the multi-compartment neuron co-culture platform, the soma compartment and the axon/glia compartments were only 200 μm apart and it was challenging to replicate a 3.5 mm thick PDMS device from its master without damaging the device. Therefore, the design of the soma compartment was modified from a cylinder shape into a truncated cone shape with sidewalls tilted 20° toward the center ( lower left inset). This modified structure not only strengthened the PDMS walls separating the compartments, but also facilitated the release of the device. Using this method, no damage to the final PDMS devices was observed. In case of the PDMS device with integrated fluidic interfaces, the highest aspect ratio of the PDMS wall separating two macroscale structures was 1.06 : 1, so no damage was observed during the fabrication process. However, the bottom part of the 1.2 mm diameter pillars was enlarged to 1.8 mm to make it more mechanically robust ().
Fluidic interfaces and reservoirs on the PMMA master were engraved using a 0.8 mm square end mill (Roland, Irvine, CA). Only the area where macroscale structures, such as the fluidic interfaces or reservoirs, exist was engraved with the CNC milling machine. The rest of the PMMA surface was left intact to preserve the smooth and polished surface to obtain PDMS devices with smooth surfaces. Average surface roughness of the PMMA master and the imprint master before the hot-embossing process was 9.89±0.88 nm and 14.14± 1.06 nm respectively. After the hot-embossing process, surface roughness of the PMMA master changed to 14.19± 2.44 nm, showing that no significant changes had been made to the surface roughness during the hot-embossing process other than due to the imprint master surface roughness. This result matches well with recent results reported by Desai et al. that multiple self-replications, making a new master from a master fabricated by a soft-lithography process, have little effect on the surface roughness and dimensions of the final replica (Desai et al. 2009
). SEM images of the sample surfaces showed smooth surface profiles (), and the final PDMS devices did not show any fluidic seal failure between the device and polystyrene cell culture substrates even for reversible PDMS bonding. This was confirmed by assembling a PDMS multi-compartment neuron co-culture device on a poly-d-lysine (PDL) coated polystyrene culture plate after treating it with oxygen plasma followed by 30 min sterilization in 70% ethanol. The fluidic seal was tightly maintained through 4 weeks of neuron culture without any leakage. The imprint master used in this paper was fabricated by electroplating copper on a silicon wafer or by wet etching a glass slide, but any imprint master can be used. For example, a glass or silicon imprint master fabricated by a deep reactive ion etching (DRIE) process could also be used.
In order to confirm that the original geometry of the pattern was not distorted during the MMHSM fabrication process, average width/depth of microchannels from the PMMA master, the PDMS master, and the PDMS device were compared using SEM images and optical surface profile measurements (). The average width and depth of channels changed from 515.8±5.9 μm (PMMA master, n=10) to 515.7±2.7 μm (PDMS device, n=4) and from 21.3±0.8 μm (PMMA master, n=8) to 21.7±0.8 μm (PDMS device, n=6), respectively. The dimension changes were less than 1.88%, showing that almost no changes had been made during the replication process and that this process is very reliable.
Average height and width of the microchannels in the PMMA master, PDMS master and final PDMS device fabricated through the MMHSM process
Finally, 40 Teflon FEP tubings (Ø 1.58 mm) were inserted into the PDMS device with integrated fluidic interfaces (). Fluids were introduced through the 40 fluidic inlets and were successfully collected at two reservoirs without any leakage at the interface. To further investigate the robustness of the fluidic interface, a pressure threshold experiment was conducted. Schematic illustration of the experimental setup is shown in . A PDMS device with the integrated fluidic interfaces was bonded on a glass substrate after oxygen plasma treatment, followed by Teflon tubing insertion. Water droplets were applied around the inserted tubings to observe leakage while applying pressure to the fluidic interface. The fluidic interface did not show any leakage for applied pressure of up to 345 kPa. Although some fluidic connections using clamps provide stronger pressure tolerance (Bhagat et al. 2007
), most bio/medical microfluidic experiments do not require high pressure. The pressure tolerance of 345 kPa obtained through our fluidic interface is comparable to other widely used press-fit type fluidic interfaces (Christensen et al. 2005
; Thorsen et al. 2002
). The MMHSM fabrication process presented in this paper is not limited to fluidic interfaces or reservoirs, and can be widely used for a broad range of applications requiring both micro and macro scale structures in a single system.
Fig. 5 (a) A photographic image of a 3.5 mm thick PDMS microfluidic device (50×50 mm2) with 40 Teflon tubings connected to the integrated fluidic interfaces. Four different color dyes were used for visualization. (b) Schematic illustration showing the (more ...)