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.

Encapsulating cells within hydrogels is important for generating three-dimensional (3D) tissue constructs for drug delivery and tissue engineering. This paper describes, for the first time, the fabrication of large numbers of cell-laden microgel particles using a continuous microfluidic process called stop-flow lithography (SFL). Prepolymer solution containing cells was flowed through a microfluidic device and arrays of individual particles were repeatedly defined using pulses of UV light through a transparency mask. Unlike photolithography, SFL can be used to synthesize microgel particles continuously while maintaining control over particle size, shape and anisotropy. Therefore, SFL may become a useful tool for generating cell-laden microgels for various biomedical applications.

Stop‐flow lithography (SFL) is used for patterning colloidal building blocks into controlled structures (gears and other shapes) at rates that exceed 103 min−1 using an index‐matched system composed of silica microspheres suspended in a photocurable acrylamide solution as shown in the figure. These structures are dried and then transformed, in batch, at elevated temperatures into microcomponents composed of porous or glassy silicon oxide or porous silicon via magnesiothermic reduction.

Pi-conjugated polymers and oligomers show charge transfer (CT) absorption bands when mixed with electron acceptors in chloroform solution. This is attributed to the formation of (ground state) donor-acceptor complexes in solution. By varying the concentration of the donor and acceptor, the extinction coefficient for the CT absorption and the association constant of donor and acceptor are estimated. The spectral position of the CT bands correlates with the electrochemical oxidation potential of the pi-conjugated donor and the reduction potential of the acceptor.

Polymeric particles of complex shapes and chemistry have been used for a wide variety of applications in the materials and bioengineering fields. An interesting means of introducing complexity is through curvature. In this work, stop-flow lithography is used to generate concave/convex particles at high throughputs of 3x10(4) particles/h. These particles have finely tuned curvature in the plane orthogonal to the plane of projection of light. The shape in the plane of projection of light is determined by the mask shape. The chemical programmability of this technique is further demonstrated by creating multifunctional particles, i.e., patchy and capped particles. The directed assembly of these particles can find potential application in a variety of fields like biology, photonics, and liquid crystals.