Populations of certain unicellular organisms, such as suspensions of yeast in nutrient solutions, undergo transitions to coordinated activity with increasing cell density. The collective behavior is believed to arise through communication by chemical signaling via the extracellular solution. We studied large, heterogeneous populations of discrete chemical oscillators (approximately 100,000) with well-defined kinetics to characterize two different types of density-dependent transitions to synchronized oscillatory behavior. For different chemical exchange rates between the oscillators and the surrounding solution, increasing oscillator density led to (i) the gradual synchronization of oscillatory activity, or (ii) the sudden "switching on" of synchronized oscillatory activity. We analyze the roles of oscillator density and exchange rate of signaling species in these transitions with a mathematical model of the interacting chemical oscillators.

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In situ tertiary amine-catalyzed thiol-acrylate chemistry was employed to produce hydrophilic microfluidic devices via a soft lithography process. The process involved the Michael addition of a secondary amine to a multifunctional acrylate producing a nonvolatile in situ tertiary amine catalyst/comonomer molecule. The Michael addition of a multifunctional thiol to a multifunctional acrylate was facilitated by the catalytic activity of the in situ catalyst/comonomer. These cost-efficient thiol-acrylate devices were prepared at room temperature, rapidly, and with little equipment. The thiol-acrylate thermoset materials were more natively hydrophilic than the normally employed poly(dimethylsiloxane) (PDMS) thermoset material, and the surface energies were stable compared to PDMS. Because the final chip was self-adhered via a simple chemical process utilizing the same chemistry, and it was naturally hydrophilic, there was no need for expensive instrumentation or complicated methods to "activate" the surface. There was also no need for postprocessing removal of the catalyst as it was incorporated into the polymer network. These bottom-up devices were fabricated to completion proving their validity as microfluidic devices, and the materials were manipulated and characterized via various analyses illustrating the potential diversity and tunability of the devices.

Spontaneous spatiotemporal wave activity occurs in groups of excitable particles for groups larger than a critical size. Experiments are carried out with particles loaded with the catalyst of the Belousov-Zhabotinsky reaction that are immersed in catalyst-free reaction mixture. The particles diffusively exchange activator and inhibitor species with the surrounding solution. All particles are nonoscillatory when separated from the other particles; however, target and spiral waves are exhibited in sufficiently large groups. A cellular particle model of the system also exhibits transitions from excitable steady state behavior to spatiotemporal wave activity with increasing group size.