Frontal Photopolymerization for Microfluidic Applications

Abstract

Frontal photopolymerization (FPP) offers numerous advantages for the rapid prototyping of microfluidic devices. Quantitative utilization of this method, however, requires a control of the vertical dimensions of the patterned resist material. To address this fundamental problem, we study the ultraviolet (UV) photopolymerization of a series of multifunctional thiolene resists through a combination of experiments and analytical modeling of the polymerization fronts. We describe this nonlinear spatio-temporal growth process in terms of a "minimal" model involving an order parameter phi(x, t) characterizing the extent of monomer-to-polymer conversion, the optical attenuation T(x, t), and the solid front position h(t). The latter exhibits an induction time (or equivalent critical UV dose) characterizing the onset of frontal propagation. We also observe a novel transition between two logarithmic rates of growth, determined by the Beer-Lambert attenuation constants mu(0) and mu(infinity) of the monomer and fully polymerized material, respectively. The measured frontal kinetics and optical transmission of the thiolene resist materials are consistent with our photopolymerization model, exhibiting both "photodarkening" and "photoinvariant" polymerization. This is apparently the first observation of photodarkening reported in FPP. On the basis of these results, multilevel fluidic devices with controlled height are readily fabricated with modulated illumination. A representative two-level microfluidic device, incorporating a chaotic mixer, a T junction, and a series of controlled flow constrictions, illustrates the practical versatility of this fabrication method.

Combinatorial/High Throughput Methods for the Determination of Polyanhydride Phase Behavior

“…A release layer of aluminum foil was used instead of polydimethylsiloxane (PDMS Sylgard 184 Base and PDMS Sylgard 184 Cure (Dow Corning, Midland, MI)) employed by other researchers. 9,11,13,18 PDMS films require a cure time of 12 h and often result in uneven features, which have a detrimental effect on the uniformity of the resulting thiolene device.…”

“…The biggest challenge in the implementation of combinatorial methods is library fabrication and high throughput screening. In the present study, we retrofit previously described sample preparation and characterization tools [9][10][11][12][13] for the study of polyanhydride phase behavior. A modification of the solution based flow-coating method developed by Meredith et al 12 was used to create continuous * To whom correspondence should be addressed.…”

Parallel Synthesis and High Throughput Dissolution Testing of Biodegradable Polyanhydride Copolymers

“…The substrate arrays were designed and fabricated using a recently developed rapid prototyping technique based on the contact lithography of multifunctional thiolene resins. 20 The photomask was designed with a standard graphics package and printed on a transparency with a 1200-dpi resolution laser printer. It consisted of a 10 × 10 grid (with 300-µm line width) and three additional (control) squares, with dimensions commensurate with a glass slide ( Figure 1).…”

“…The patterned height (and therefore, the well volume) was determined by the administered UV dose in this frontal photopolymerization process. 20 Selective solvents (ethanol and acetone) washed away the unpolymerized material. A feature height of 400 µm was fabricated with a 12.0 mJ/cm 2 dose, resulting in 2-µL microwells patterned on glass.…”

Water‐Triggered Frontal Polymerization

“…[2] Pojman et al [1,3,4] have since extensively examined the phenomenon. In recent years, three classes of FP such as: thermal FP, [5] isothermal FP [1,6] and UV initiated (photo) FP [7,8] have emerged.…”