Photopatterned materials in bioanalytical microfluidic technology

Abstract

Microfluidic technologies are playing an increasingly important role in biological inquiry. Sophisticated approaches to the microanalysis of biological specimens rely, in part, on the fine fluid and material control offered by microtechnology, as well as a sufficient capacity for systems integration. A suite of techniques that utilize photopatterning of polymers on fluidic surfaces, within fluidic volumes, and as primary device structures underpins recent technological innovation in bioanalysis. Well-characterized photopatterning approaches enable previously fabricated or commercially fabricated devices to be customized by the user in a straight-forward manner, making the tools accessible to laboratories that do not focus on microfabrication technology innovation. In this review of recent advances, we summarize reported microfluidic devices with photopatterned structures and regions as platforms for a diverse set of biological measurements and assays.

Figure 1

Typical photopatterning processes. (A) Modifications to channel surfaces by photopatterning. (i) Side view of previously fabricated microchannel made from light-permeable materials such as glass or transparent plastics. (ii) Channel is loaded with solution containing photoreactive species. (iii) Specific channel regions are exposed with UV light using photomasks. (iv) Photo-activated species react with channel walls and become immobilized. (v) Channel is washed to remove unreacted molecules. (vi) Previous steps can be repeated to pattern additional regions. (B) Creation of structures in channel volumes by photopatterning. (i) Side view of previously fabricated microchannel made from light-permeable materials such as glass or transparent plastics. (ii) Channel is loaded with solution containing photoreactive polymer precursors. (iii) Specific channel regions are exposed with UV light using photomasks. (iv) Photopolymerization occurs in regions exposed to UV light. (v) Channel is washed to remove unreacted precursor molecules. (vi) Previous steps can be repeated to pattern additional regions. (C) Formation of microfluidic channels by photopatterning. (i) Device cross section shown. Materials such as glass and silicon are typically used as substrates. (ii) Substrate is coated with photosensitive polymers, resins, etc. (iii) UV exposure of the photosensitive material through a photomask leads to a chemical reaction and the definition of features in the substrate. (iv) Exposed (or unexposed depending on material) material is removed with developing, washing, or etching steps. (v) The formed microchannels are closed by bonding top layer.

Figure 2

Spatially controlled photo-removal of MPC exposes discrete regions for cell attachment in microchannel. (A) Phase contrast images of MC-3T3 E1 cell aggregation in the microchannel after 2 h of incubation. (B) Patterned cells flown away after 24 h of incubation owing to cell aggregation. (C) Micro-patterned cells after 24 h of culture. (D) Phase-contrast images of micro-patterned endothelial cells after 24 h of culture, (E) 1 day of culture, and (F) 14 days of culture. (Jang et al 2010) - Reproduced by permission of the Royal Society of Chemistry.

Figure 3

Photopatterned membrane allows for sample processing steps such as preconcentration prior to separation assay. (A) Fluorescence micrographs show the distribution of labeled protein at different times of the preconcentration process as they are driven against the size exclusion membrane. (B) The electric field across is reversed driving proteins away from the membrane. (C) Protein size-based separations by SDS-PAGE. Reprinted with permission from (Hatch et al 2006). Copyright (2006) American Chemical Society.

Figure 4

PPM are photopatterned at regularly spaced intervals. Reaction-mixing efficiency is increased of on-chip labeling reaction compared to open channel and continuous monolith. (A) SEM images demonstrating periodic placement of monoliths in channel cross-section and 100 micrometer patterning resolution. (B) Fluorescence intensity and fluorescence ratio for the different channel configurations. (Mair et al 2009) - Reproduced by permission of the Royal Society of Chemistry.

Figure 5

Sequential biomolecule photo immobilization steps enable serial enzyme linked regions for multi-stage on-chip synthesis. Consecutive patterning of GFP patches on a polymer monolith. The top images show the location of immobilized GFP and the bottom graph shows the fluorescence intensity along the monolith. Reprinted with permission from (Logan et al 2007). Copyright (2007) American Chemical Society.

Figure 6

Different PA gel regions (loading, separation, blotting) are photopatterned in the device enabling automated separation and blotting. (A) Brightfield image of the glass device. (B) Magnified view of the chamber containing the different PA gel regions. The loading gel is shown in blue, the PAGE separation region in red, and the antibody functionalized blotting region in green. (C) Protocol steps overlaid on device micrographs. (D) Inverted grayscale image demonstrated band stacking at loading-separation interface. μ is the mobility and i indicates direction of electric current. Reprinted with permission from (He and Herr 2010) Copyright (2006) American Chemical Society.

Figure 7

Microfluidic channels and free standing structures are fabricated on PPS. PPS also enables substrate electrode integration, is biocompatible, and has low-autofluorescence. (A–C) Schematics and bright field image of PPS structure aligned to an aluminum layer. Scale bar = 500 micrometers. (D–F) Bright field image, schematics, and fluorescent images showing alignment and operation of simple dielectrophoretic structure. Scale bar D = 200 micrometers. Scale bar E, F = 50 micrometers. Reprinted with permission from (Desai et al 2008). Copyright (2008) American Chemical Society.