(531d) Fabrication of Tailored Hydrogel Particles By Controlled Oxygen Inhibited Photopolymerization

Synthetic hydrogels of poly(ethylene glycol) diacrylate (PEGDA) have been widely used in tissue scaffolding, drug delivery, and single cell analysis applications. Microfluidic devices, in conjunction with in situ photopolymerization, are used to produce homogeneous and monodisperse micro- and nano- sized hydrogel particles. These devices are typically constructed of polydimethylsiloxane (PDMS), which has a high permeability to oxygen and other gases. Acrylated monomers undergo radical polymerization which is intrinsically vulnerable to inhibition by molecular oxygen, resulting in an incomplete cure and the formation of peroxyl radicals. In hydrogel droplets, this inhibition is manifested as an oligomer shell around a polymerized core as a consequence of a persistent steady state region of high oxygen concentration. While undesirable in some applications, we can take advantage of this phenomenon by washing away the oligomer shell and obtain a surfactant-free hydrophilic surface which can be chemically modified. Additionally, we can fabricate hydrogel particles that have a lower size boundary than the one determined by fluid dynamics within the device. Physical approaches such as purging with an inert gas and using higher light intensity are used to reduce oxygen inhibition and thus change the oligomer layer thickness. We have developed a time dependent model that compares photopolymerization kinetics to mass transport rates to predict the unpolymerized shell thickness under different operating conditions. We validated this model empirically, and have established the trends observed from varying inert gas pressure, purging time, and optical intensity. Using these results, we can easily modify the operating parameters to obtain polymerized particles with desired diameters from arbitrary-sized droplets.