Modeling of Oxygen-Inhibited Free Radical Photopolymerization in a PDMS Microfluidic Device

Abstract

Free-radical photopolymerization performed within PDMS microfluidic devices is now used for a variety of applications. We propose, through model and experiment, that atmospheric oxygen diffusing in through the porous PDMS is responsible for the presence, under UV light, of a thin, un-cross-linked film of oligomer abutting the walls of an all-PDMS device. After the advent of light exposure, an induction time τ i is required before the oxygen present in the oligomer is depleted, and cross-linking reactions can begin. A polymerized structure then grows from the center of the device outward, increasing sharply in height with time and leaving only a thin un-cross-linked film of thickness, δ i,c , close to the walls where oxygen can penetrate. Under suitable simplification of the reaction-diffusion model developed, scaling relationships were obtained for τ i (∼Da-1) and δ i,c (∼Da-1/2) as a function of a Damköhler number, Da. The relationships were successfully verified by comparison with both the full solution and experimental data. The analysis shows that control over particle height can be obtained more easily by changing initiator concentration, irradiation intensity, or channel height rather than exposure time.

Synthesis of colloidal microgels using oxygen-controlled flow lithography

“…We note that the expression for a differs from its counterpart in the original contribution by a factor of E, which denotes the energy content of a mole of photons at 365 nm (328 kJ mol À1 ). 25 This conversion factor was not explicitly stated previously. However, we do so here to maintain dimensional consistency.…”

“…Here, we adopt the convention that gelation occurs when monomer conversion rst reaches 2% (x c ¼ 0.98), similar to what was done previously. 25 Below 10% O 2 , the propagation reaction dominates the inhibition reaction between radical species and dissolved oxygen. As a result, particles grow from the channel center, where the dissolved oxygen concentration is at a minimum, outward.…”

“…As a result, particles grow from the channel center, where the dissolved oxygen concentration is at a minimum, outward. The 10% O 2 threshold satises the critical Da criterion for effective synthesis (Da $ 4) developed by Dendukuri et al 25 To conrm the validity of the simulations, we synthesized 10 mm discs in a 2 mm tall tiered microuidic device under a range of chamber oxygen concentrations. We imaged the resulting gel structures in situ.…”

A microfluidic-based cell encapsulation platform to achieve high long-term cell viability in photopolymerized PEGNB hydrogel microspheres

“…Chain-growth polymerization (PEGDA), however, is almost completely inhibited by the presence of oxygen 45 because the rate of radical quenching by oxygen is much faster than the rate of chain initiation and propagation 38,46 . Only once oxygen is locally consumed can polymerization occur.…”

Stop flow lithography in perfluoropolyether (PFPE) microfluidic channels

“…PDMS is optically transparent, compatible with standard photolithography techniques, and has 60 low-autofluorescence 21 . By having a large gas permeability 22 , PDMS devices allow sufficient diffusion of oxygen to inhibit radical polymerization near the surface 23 . Because of these advantages, PDMS devices have been predominantly used for SFL.…”