We present a powerful and versatile technique that enables exquisite spatial and temporal control over local solution chemistry in microfluidic devices. Using a microscope and a UV lamp, we use projection lithography to photopolymerize thin (10-25 m) hydrogel membrane ''microwindows'' (HMMs) into standard microfluidic devices. These microwindows are permeable to solute and solvent diffusion and to electric fields, yet act as rigid walls from the standpoint of fluid flow. Reservoirs of solution may thus be rapidly imposed, switched, and maintained on one side of a HMM using standard microfluidic techniques, provoking changes in solution conditions on the other side without active mixing, stirring, or diluting. We highlight three paradigmatic experimental capabilities enabled by HMMs: (1) rapid dialysis and swapping of solute and/or solvent, (2) stable and convection-free localized concentration gradients, and (3) local electric permeability. The functional versatility of hydrogel microwindow membranes, coupled with the ease and speed of their fabrication and integration into simple microchannels or multilayer devices, will open a variety of novel applications and studies in a broad range of fields.

The effect on the gas permeance properties and structural morphology of the presence of methyl functional groups in a silica membrane was studied. Membranes were synthesized via chemical vapor deposition (CVD) at 650 °C and atmospheric pressure using three silicon compounds with differing numbers of methyl- and methoxy-functional groups: tetramethyl orthosilicate (TMOS), methyltrimethoxysilane (MTMOS), and dimethyldimethoxysilane (DMDMOS). The residence time of the silica precursors in the CVD process was adjusted for each precursor and optimized in terms of gas permeance and ideal gas selectivity criteria. Final H2 permeances at 600 °C for the TMOS-, MTMOS-, and DMDMOS-derived membranes were respectively 1.7 × 10−7, 2.4 × 10−7, and 4.4 × 10−8 mol∙m−2∙s−1∙Pa−1 and H2/N2 selectivities were 990, 740, and 410. The presence of methyl groups in the membranes fabricated with the MTMOS and DMDMOS precursors was confirmed via Fourier-transform infrared (FTIR) spectroscopy. From FTIR analysis, an increasing methyl signal in the silica structure was correlated with both an improvement in the hydrothermal stability and an increase in the apparent activation energy for hydrogen permeation. In addition, the permeation mechanism for several gas species (He, H2, Ne, CO2, N2, and CH4) was determined by fitting the gas permeance temperature dependence to one of three models: solid state, gas-translational, or surface diffusion.

The potential impact of nitrogen and ammonia exposure on hydrogen permeance through thin palladium membranes (1.3 µm to 4.1 µm thick) fabricated by electroless plating was studied. Additionally, a robust approach is introduced to quantify the pressure exponent which accounts for contributions to Knudsen flow through defects present in very thin membranes. In sharp contrast to previously published results, no flux inhibition was observed due to nitrogen or ammonia exposure. Studies included 24 h exposures to both pure gases and equimolar hydrogen/nitrogen or hydrogen/ammonia mixtures at trans-membrane pressures ranging up to 1.0 MPa and temperatures of 598 K to 773 K. One membrane did exhibit significant flux inhibition after helium exposure, but this was attributed to changes in surface microstructure associated with hydrogen departing the lattice. This apparent hydrogen flux inhibition behavior was permanently eliminated by air exposure which roughens the surface, and it is suggested that this surface structure mechanism is a more probable explanation for flux inhibition than adsorption of nitrogen-based species. Keywords: palladium (Pd) composite membrane; hydrogen (H 2) flux inhibition; ammonia (NH 3) adsorption; nitrogen (N 2) adsorption Additionally, in recent advances in ammonia production, water, rather than hydrogen, is used as the feedstock for electrolysis which can also be performed at much lower pressures than the

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