Yael Hanein

Controlled multibatch self-assembly of microdevices

A technique is described for assembly of multiple batches of micro components onto a single substrate. The substrate is prepared with hydrophobic alkanethiol-coated gold binding sites. To perform assembly, a hydrocarbon oil, which is applied to the substrate, wets exclusively the hydrophobic binding sites in water. Micro components are then added to the water, and assembled on the oil-wetted binding sites. Moreover, assembly can be controlled to take place on desired binding sites by using an electrochemical method to deactivate specific substrate binding sites. By repeatedly applying this technique, different batches of micro components can be sequentially assembled to a single substrate. As a post assembly procedure, electroplating is incorporated into the technique to establish electrical connections for assembled components. Important issues presented are: substrate fabrication techniques, electrochemical modulation by using a suitable alkanethiol (dodecanethiol), electroplating of tin and lead alloy and binding site design simulations. Finally, we demonstrate a two-batch assembly of silicon square parts, and establishing electrical connectivity for assembled surface-mount light emitting diodes (LEDs) by electroplating.

Electro-chemical and biological properties of carbon nanotube based multi-electrode arrays

A novel class of micro-electrodes was fabricated by synthesizing high density carbon nanotube islands on lithographically defined, passivated titanium nitride conductors on a silicon dioxide substrate. Electrochemical characterization in phosphate buffered saline of these new electrodes reveals superb electrochemical properties marked by featureless rectangular cyclic voltammetry curves corresponding to a DC surface specific capacitance and a volume specific capacitance as high as 10 mF cm(-2) and 10 F cm(-3), respectively. These electrodes are also characterized by a slowly varying impedance magnitude over the range of 1 Hz to 20 kHz. High fidelity extracellular recordings from cultured neurons were performed and analysed to validate the effectiveness of the fabricated electrodes. The enhanced electrochemical properties of the electrodes, their flexible and simple micro-fabrication preparation procedure as well as their bio-compatibility and durability suggest that carbon nanotube electrodes are a promising platform for high resolution capacitive electrochemical applications.

Carbon nanotube-based multi electrode arrays for neuronal interfacing: progress and prospects

Carbon nanotube (CNT) coatings have been demonstrated over the past several years as a promising material for neuronal interfacing applications. In particular, in the realm of neuronal implants, CNTs have major advantages owing to their unique mechanical and electrical properties. Here we review recent investigations utilizing CNTs in neuro-interfacing applications. Cell adhesion, neuronal engineering and multi electrode recordings with CNTs are described. We also highlight prospective advances in this field, in particular, progress toward flexible, bio-compatible CNT-based technology.

Process entanglement as a neuronal anchorage mechanism to rough surfaces

The organization of neurons and glia cells on substrates composed of pristine carbon nanotube islands was investigated using high resolution scanning electron microscopy, immunostaining and confocal microscopy. Neurons were found bound and preferentially anchored to the rough surfaces; moreover, the morphology of the neuronal processes on the small, isolated islands of high density carbon nanotubes was found to be conspicuously curled and entangled. We further demonstrate that the roughness of the surface must match the diameter of the neuronal processes in order to allow them to bind. The results presented here suggest that entanglement, a mechanical effect, may constitute an additional mechanism by which neurons (and possibly other cell types) anchor themselves to rough surfaces. Understanding the nature of the interface between neurons and carbon nanotubes is essential to effectively harness carbon nanotube technology in neurological applications such as neuro-prosthetic and retinal electrodes.

Highly Efficient and Broadband Wide-Angle Holography Using Patch-Dipole Nanoantenna Reflectarrays

We demonstrate wide-angle, broadband, and efficient reflection holography by utilizing coupled dipole-patch nanoantenna cells to impose an arbitrary phase profile on the reflected light. High-fidelity images were projected at angles of 45 and 20° with respect to the impinging light with efficiencies ranging between 40-50% over an optical bandwidth exceeding 180 nm. Excellent agreement with the theoretical predictions was found at a wide spectral range. The demonstration of such reflectarrays opens new avenues toward expanding the limits of large-angle holography.

The Metalliclike Conductivity of a Two-Dimensional Hole System

We report on a zero magnetic field transport study of a two-dimensional, variable-density, hole system in GaAs. As the density is varied we observe, for the first time in GaAs-based materials, a crossover from an insulating behavior at low-density, to a metallic-like behavior at high-density, where the metallic behavior is characterized by a large drop in the resistivity as the temperature is lowered. These results are in agreement with recent experiments on Si-based two-dimensional systems by Kravchenko et al. and others. We show that, in the metallic region, the resistivity is dominated by an exponential temperature-dependence with a characteristic temperature which is proportional to the hole density, and appear to reach a constant value at lower temperatures.

The regulative role of neurite mechanical tension in network development.

A bewildering series of dynamical processes take part in the development of the nervous system. Neuron branching dynamics, the continuous formation and elimination of neural interconnections, are instrumental in constructing distinct neuronal networks, which are the functional building blocks of the nervous system. In this study, we investigate and validate the important regulative role of mechanical tension in determining the final morphology of neuronal networks. To single out the mechanical effect, we cultured relatively large invertebrate neurons on clean quartz surfaces. Applied to these surfaces were isolated anchoring sites consisting of carbon nanotube islands to which the cells and the neurites could mechanically attach. Inspection of branching dynamics and network wiring upon development revealed an innate selection mechanism in which one axon branch wins over another. The apparent mechanism entails the build-up of mechanical tension in developing axons. The tension is maintained by the attachment of the growth cone to the substrate or, alternatively, to the neurites of a target neuron. The induced tension promotes the stabilization of one set of axon branches while causing retraction or elimination of axon collaterals. We suggest that these findings represent a crucial, early step that precedes the formation of synapses and regulates neuronal interconnections. Mechanical tension serves as a signal for survival of the axonal branch and perhaps for the subsequent formation of synapses.

Compact self-wiring in cultured neural networks.

We present a novel approach for patterning cultured neural networks in which a particular geometry is achieved via anchoring of cell clusters (tens of cells/each) at specific positions. In addition, compact connections among pairs of clusters occur spontaneously through a single non-adherent straight bundle composed of axons and dendrites. The anchors that stabilize the cell clusters are either poly-D-lysine, a strong adhesive substrate, or carbon nanotubes. Square, triangular and circular structures of connectivity were successfully realized. Monitoring the dynamics of the forming networks in real time revealed that the self-assembly process is mainly driven by the ability of the neuronal cell clusters to move away from each other while continuously stretching a neurite bundle in between. Using the presented technique, we achieved networks with wiring regions which are made exclusively of neuronal processes unbound to the surface. The resulted network patterns are very stable and can be maintained for as long as 11 weeks. The approach can be used to build advanced neuro-chips for bio-sensing applications (e.g. drug and toxin detection) where the structure, stability and reproducibility of the networks are of great relevance.

Carbon nanotube electrodes for effective interfacing with retinal tissue.

We have investigated the use of carbon nanotube coated microelectrodes as an interface material for retinal recording and stimulation applications. Test devices were micro-fabricated and consisted of 60, 30 μm diameter electrodes at spacing of 200 μm. These electrodes were coated via chemical vapor deposition of carbon nanotubes, resulting in conducting, three dimensional surfaces with a high interfacial area. These attributes are important both for the quality of the cell-surface coupling as well as for electro-chemical interfacing efficiency. The entire chip was packaged to fit a commercial multielectrode recording and stimulation system. Electrical recordings of spontaneous spikes from whole-mount neonatal mouse retinas were consistently obtained minutes after retinas were placed over the electrodes, exhibiting typical bursting and propagating waves. Most importantly, the signals obtained with carbon nanotube electrodes have exceptionally high signal to noise ratio, reaching values as high as 75. Moreover, spikes are marked by a conspicuous gradual increase in amplitude recorded over a period of minutes to hours, suggesting improvement in cell-electrode coupling. This phenomenon is not observed in conventional commercial electrodes. Electrical stimulation using carbon nanotube electrodes was also achieved. We attribute the superior performances of the carbon nanotube electrodes to their three dimensional nature and the strong neuro-carbon nanotube affinity. The results presented here show the great potential of carbon nanotube electrodes for retinal interfacing applications. Specifically, our results demonstrate a route to achieve a reduction of the electrode down to few micrometers in order to achieve high efficacy local stimulation needed in retinal prosthetic devices.

Engineered neuronal circuits shaped and interfaced with carbon nanotube microelectrode arrays

Standard micro-fabrication techniques which were originally developed to fabricate semi-conducting electronic devices were inadvertently found to be adequate for bio-chip fabrication suited for applications such as stimulation and recording from neurons in-vitro as well as in-vivo. However, cell adhesion to conventional micro-chips is poor and chemical treatments are needed to facilitate the interaction between the device surface and the cells. Here we present novel carbon nanotube-based electrode arrays composed of cell-alluring carbon nanotube (CNT) islands. These play a double role of anchoring neurons directly and only onto the electrode sites (with no need for chemical treatments) and facilitating high fidelity electrical interfacing–recording and stimulation. This method presents an important step towards building nano-based neurochips of precisely engineered networks. These neurochips can provide unique platform for studying the activity patterns of ordered networks as well as for testing the effects of network damage and methods of network repair.