Engineered functional neural interfaces (fNIs) serve as essential abiotic–biotic transducers between an engineered system and the nervous system. They convert external physical stimuli to cellular signals in stimulation mode or read out biological processes in recording mode. Information can be exchanged using electricity, light, magnetic fields, mechanical forces, heat, or chemical signals. fNIs have found applications for studying processes in neural circuits from cell cultures to organs to whole organisms. fNI-facilitated signal transduction schemes, coupled with easily manipulable and observable external physical signals, have attracted considerable attention in recent years. This enticing field is rapidly evolving toward miniaturization and biomimicry to achieve long-term interface stability with great signal transduction efficiency. Not only has a new generation of neuroelectrodes been invented, but the use of advanced fNIs that explore other physical modalities of neuromodulation and recording has begun to increase. This review covers these exciting developments and applications of fNIs that rely on nanoelectrodes, nanotransducers, or bionanotransducers to establish an interface with the nervous system. These nano fNIs are promising in offering a high spatial resolution, high target specificity, and high communication bandwidth by allowing for a high density and count of signal channels with minimum material volume and area to dramatically improve the chronic integration of the fNI to the target neural tissue. Such demanding advances in nano fNIs will greatly facilitate new opportunities not only for studying basic neuroscience but also for diagnosing and treating various neurological diseases.
Background Despite significant advancements of optical imaging techniques for mapping hemodynamics in small animal models, it remains challenging to combine imaging with spatially resolved electrical recording of individual neurons especially for longitudinal studies. This is largely due to the strong invasiveness to the living brain from the penetrating electrodes and their limited compatibility with longitudinal imaging.
Understanding brain functions at the circuit level requires time-resolved simultaneous measurement of a large number of densely distributed neurons, which remains a great challenge for current neural technologies. In particular, penetrating neural electrodes allow for recording from individual neurons at high temporal resolution, but often have larger dimensions than the biological matrix, which induces significant damage to brain tissues and therefore precludes the high implant density that is necessary for mapping large neuronal populations with full coverage. Here, it is demonstrated that nanofabricated ultraflexible electrode arrays with cross-sectional areas as small as sub-10 µm2 can overcome this physical limitation. In a mouse model, it is shown that these electrodes record action potentials with high signal-to-noise ratio; their dense arrays allow spatial oversampling; and their multiprobe implantation allows for interprobe spacing at 60 µm without eliciting chronic neuronal degeneration. These results present the possibility of minimizing tissue displacement by implanted ultraflexible electrodes for scalable, high-density electrophysiological recording that is capable of complete neuronal circuitry mapping over chronic time scales.
The design, construction and preliminary measurements of a new test stand for accurately assessing the shear stress acting at the fluid surface interface of wall bounded flows is discussed. This stand is based on control volume analysis whereby a fully developed turbulent velocity profile produces shear forces which equate to the pressure drop measured between fixed points in a constant area pipe. The calibration stand is designed to facilitate both subsonic and supersonic flow. Subsonic flow conditions are achieved by placing different diameter nozzles at the exhaust of the test section thereby permitting different free stream velocities and mass flow rates for a given ratio of the total pressure to static pressure in the pipe. The advantages of this facility is in its ability to produce a broadrange of Reynolds numbers (based on centerline velocity and pipe diameter) and elevatedpressures that are required to gauge the sensitivity of modern shear stress sensors.
Reduced-order models of the airwake produced by the flow over a simple frigate shipare developed using POD based methods. The focus is to understand the trade spacebetween cost and accuracy, where different forms of the POD technique are concerned.Of particular importance is the upfront expense of employing ‘classical’ or snapshot formsof the POD technique in both scalar and vector forms using either time suppressed data(conventional-POD), or kernels constructed from cross-spectral densities of the fluctuatingvelocity. The latter approach is referred to as harmonic-POD so as not to exclude harmonictransforms in space. The flow over a simple frigate ship is an ideal test bed given that it isunsteady, three-dimensional, inhomogenous in all spatial directions, and stationary in time. The spatial modes from all three techniques are shown to correspond to unique time-scales, thereby demonstrating how the preservation of the temporal behavior associated with a particular spatial scale is not unique to the harmonic-POD approach.
Conventional theory predicts that ultrahigh lattice thermal conductivity can only occur in crystals composed of strongly bonded light elements, and that it is limited by anharmonic three-phonon processes. We report experimental evidence that departs from these long-held criteria. We measured a local room-temperature thermal conductivity exceeding 1000 watts per meter-kelvin and an average bulk value reaching 900 watts per meter-kelvin in bulk boron arsenide (BAs) crystals, where boron and arsenic are light and heavy elements, respectively. The high values are consistent with a proposal for phonon-band engineering and can only be explained by higher-order phonon processes. These findings yield insight into the physics of heat conduction in solids and show BAs to be the only known semiconductor with ultrahigh thermal conductivity.