1. THIN FILM BARRIER FORMATION IN MICROCAVITIES
Asif Riaz1, Ram P. Gandhiraman1, Ivan K. Dimov1, Lourdes B. Desmonts1, Antonio J. Ricco1, Jens Ducrée1, Stephen Daniels1, and Luke P. Lee1,2
1Biomedical Diagnostics Institute, National Centre for Sensor Research, Dublin City University, IRELAND. 2Biomolecular Nanotechnology
Center, Berkeley Sensor and Actuator Center, Department of Bioengineering, University of California, Berkeley, CA, USA
Abstract: In microfluidics, the formation of glass-like thin-film barriers to conceal surface chemical properties of fabrication material such as plastics or elastomers is important for a range of applications in biology, analytical assays, and chemical synthesis1. The fabrication of PDMS microfluidic devices is simple and rapid,2 but the use of PDMS is limited because of its unpredictable and poorly controllable surface properties,3 and particularly because of its permeability to a wide range of organic compounds as well as water. We report a novel method of thin film formation in deep microcavities/channels using a plasma-enhanced chemical vapor deposition (PECVD) technique. Highly reactive species like Si and O were formed by the fragmentation of hexamethyldisiloxane (HMDSO) and O2 in a plasma environment. Diffusion of Si and O deep into microcavities and their deposition resulted into formation of thin film of SiOx on the walls. The thin film of SiOx acted as a barrier to block the penetration of small molecules like Rhodamine B (RhB) into PDMS.
Motivation
Characterization & Experimental Results
Blocking of small molecule penetration into PDMS
Accomplishing SiOx thin film inside of cavity
Rapid formation of SiOx via HMDSO and O2 in plasma
Reaction controls by PECVD method
Insulator formation for reliable CE performance
Elastomer surface engineering (i.e. glass like surface)
Reproducible batch processing on the cavity of wall
Figure 2. (a) A long meandering microfluidic channel (50 mm x 30 mm x 20 cm), exposed to PECVD conditions as shown in Figure 1, for 120 min. (b & c) Schematic representation of thin-film deposition. (d) Region of channel showing a good barrier to RhB diffusion. (e) Region of channel showing poor barrier to RhB diffusion.
Formation of SiOx Barrier
Figure 4. Effect of reaction time (a), and the concentration of HMDSO (b), on the thickness and penetration depth of SiOx thin film. In this experiment, PDMS-coated channels (50 mm x 100 mm x 10 cm) were bonded to a Si wafer instead of a glass substrate, and after thin film deposition the device was disassembled. The Si wafer was used to measure the thickness of SiOx film mechanically using a profilometer.
Figure 1. Formation of thin-film SiOx barrier on the walls of PDMS channel and characterization by Rhodamine B diffusion. (a) Native PDMS, (b) PDMS channel with SiOx barrier. Scale bar: 50 µm. The channel was filled and stored with RhB solution (10 mM) and after 3 hours it was microscopically examined. Reaction conditions: RF 300 W, O2: HMDS 500 : 16 sccm, 270 mTorr, 40 min, 50 ºC.
Figure 5. A 40-channel device (10 are shown) of 50 µm x 100 µm x 20 cm channels with various geometries and bends (I). The effective lengths of the barriers are shown with arrows from either side. Inset-II shows regions of the channels where the transition from an effective barrier layer to a poor barrier is revealed by an expansion of the RhB fluorescent regions along the channels; i.e., the channels appear much wider than they actually are due to RhB permeation into the PDMS. Inset-III shows the effective barrier film lengths (n = 3) vs. reaction time. Scale bar: 10 mm; SiOx barrier formation conditions as in Figure 1.
CONCLUSIONS: We report a novel method of thin film formation in deep microcavities/channels using a plasma-enhanced chemical vapor deposition (PECVD) technique. The field of gas-phase reaction is relatively unexplored for surface modification within assembled microfluidic devices. We have demonstrated for the first time a gas-phase chemical process for creating glass-like surface in assembled PDMS microfluidic channels. Our aim is to further investigate such thin-films in assembled microfluidic devices for surface bio-functionalization and capillary electrophoresis applications.
REFERENCES
[1] V. Barbier, M. Tatoulian, H. Li et al., Langmuir 22 (12), 5230 (2006).
[2] H. Y. Chen, Y. Elkasabi, and J. Lahann, J. Am. Chem. Soc. 128 (1), 374 (2006).
[3] Y. Y. Huang, P. Castrataro, C. C. Lee et al., Lab Chip 7 (1), 24 (2007).
Figure 2. Fluorescence of RhB vs. time in SiOx-coated channels of various dimensions (a). Effect of cross-sectional area of channels on efficiency of the thin film formation (b). Solid line shows the exponential fitting of the data points (y = yo + A1e(-x/t1)), yo = , A1 = 702.5, and t1= , R2= 0.987). Schematic illustration of thin film formation with cross section of the channels. Reaction conditions as in Figure 1.