Microfluidic Generation, Encapsulation and Characterization of Asymmetric Droplet Interface Bilayers

Our research focuses on creating smart materials that utilize synthetic cell membranes assembled at liquid interfaces for autonomic sensing, actuation, and energy conversion. Unlike single membrane assemblies, systems featuring many membranes have the potential to offer multi-functionality, greater transduction sensitivity, and even emergent behaviors in response to environmental stimuli, similar to living tissue, which utilizes networks of highly packed cells to accomplish tasks. Here, we present for the first time a novel microfluidic platform capable of generating a stream of alternating droplet compositions, i.e. A-B-A-B, and sequentially capturing these droplets in precise locations to enable the spontaneous formation of synthetic lipid bilayers between droplets of different compositions (i.e. A and B) in an enclosed substrate. This platform preserves a key feature of the droplet interface bilayer (DIB) method, which allows asymmetric conditions within and across the membrane to be prescribed by simply using droplets containing different species. In this work, we demonstrate the ability to assemble bilayers consisting of asymmetric lipid compositions and, separately, show that alternating droplets containing the same lipid type can also be used to control the direction of ion channel insertion. In the first study, A and B droplet types contain liposomes comprised of different lipid types, which are used to establish an asymmetric composition of the leaflets that make up the lipid bilayer. This asymmetry results in a dc, non-zero membrane potential, which we measure via membrane capacitance versus bias voltage. In the second study, alamethicin peptides are included in only one of the droplet types, which enable voltage-dependent insertion to occur only at one polarity. Cyclic voltammetry measurements are performed to confirm the direction of insertion of alamethicin channels in bilayers. Also, these results show the ability to perform simultaneously electrical measurements on multiple DIB, which increases the experimental capacity and efficiency of a microfluidic approach. The ability to produce alternating droplets in a high throughput manner with electrical access provides a system to investigate the effects of lipid asymmetry on the function of membrane proteins in a controlled model system.

The effects of Bi4Ti3O12 interfacial ferroelectric layer on the dielectric properties of Au/n-Si structures

Au/n-Si metal-semiconductor (MS) and Au/Bi4Ti3O12/n-Si metal-ferroelectric-semiconductor (MFS) structures were fabricated and admittance measurements were held between 5 kHz and 1 MHz at room temperature so that dielectric properties of these structures could be investigated. The ferroelectric interfacial layer Bi4Ti3O12 decreased the polarization voltage by providing permanent dipoles at metal/semiconductor interface. Depending on different mechanisms, dispersion behavior was observed in dielectric constant, dielectric loss and loss tangent versus bias voltage plots of both MS and MFS structures. The real and imaginary parts of complex modulus of MFS structure take smaller values than those of MS structure, because permanent dipoles in ferroelectric layer cause a large spontaneous polarization mechanism. While the dispersion in AC conductivity versus frequency plots of MS structure was observed at high frequencies, for MFS structure it was observed at lower frequencies.

Tunnel Barrier Roughness Dependence of Magnetic Tunnel Junction with Synthetic Antiferromagnetic Pinned Layer

MTJs of structure Si/SiO2/Ta/Ru/IrMn/CoFe/Ru/CoFe/Al-O/CoFe/NiFe/Ru with different surface roughness of bottom electrode were prepared by sputtering, and it was investigated that the dependence of TMR ratio and resistance-area product (RA) on plasma oxidation time, thickness and surface roughness of tunnel barrier, and the tunneling characteristics of junction devices through I-V curves. To get resistance of below RA 10 kΩμm2, oxidation time of 10 s for 8 Å thick Al layer was required. In this case, thickness of Al2O3 barrier layer was 12.5 ~ 14 Å. For the 13 Å thick Al2O3 tunnel barrier, TMR ratio of optimized MTJ with uniform tunnel barrier was about 45% at bias voltage of 100 mV. Also the barrier height and the barrier width fitted to Simmon's relation were 12.3 Å and 3.07 eV, respectively, and these values agreed with that of MTJ within error range. The I-V curve and TMR ratio versus bias voltage curve of MTJ with rough tunnel barrier were linear and asymmetric, respectively, but in case of MTJ with uniform tunnel barrier, these curves were non-linear and symmetric, respectively. It was confirmed that the smooth surface of bottom electrode was a basic requirement for MTJ.

Design, Analysis, and Testing of Electrostatically Actuated Micromembranes

This paper reports a numerical design analysis of electrostatically actuated micromembranes. We systematically compare membrane performance in terms of natural frequencies, pull-in voltage (the bias voltage at which the membrane contacts the base electrode) and the effects of variable leg lengths for a given membrane size. Some experimental data on membrane deflection profiles versus bias voltage is included along with some experimentally determined pull-in voltages. Polysilicon micromembranes were successfully fabricated using the low cost MUMPs process that limits the user to three structural layers. The devices are designed with an emphasis on the response of the membrane to applied DC bias voltage to allow for variable stiffening. Circular membranes with diameters ranging from 60 to 160 μm, suspended 2 μm over square back plates of side lengths varying from 60 to 140 μm are investigated for voltages up to 90 volts. Three-dimensional electromechanical finite element simulations have been performed. Pull-in voltage values from simulations compare favorably with the measured results. It was observed that, for maximum deflection of the membrane upon application of DC bias voltage, the optimal dimensions for back plate and top membrane should fall within the ranges 80–120 μm and 80–140 μm, respectively.