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MIT Open Access Articles

Mostrando recursos 41 - 60 de 13,889

41.
2DEG electrodes for piezoelectric transduction of AlGaN/GaN MEMS resonators - Weinstein, Dana; Popa, Laura Cornelia
A 2D electron gas (2DEG) interdigitated transducer (IDT) in Gallium Nitride (GaN) resonators is introduced and demonstrated. This metal-free transduction does not suffer from the loss mechanisms associated with more commonly used metal electrodes. As a result, this transducer can be used for both the direct interrogation of GaN electromechanical properties and the realization of high Q resonators. A 1.2 GHz bulk acoustic resonator with mechanical Q of 1885 is demonstrated, with frequency quality factor product (f·Q) of 2.3×10[superscript 12], the highest measured in GaN to date.

42.
Switchable piezoelectric transduction in AlGaN/GaN MEMS resonators - Weinstein, Dana; Popa, Laura Cornelia
This work presents a new switching mechanism in piezoelectric transduction of AlGaN/GaN bulk acoustic resonators. A piezoelectric transducer is formed in the AlGaN, between a top Schottky electrode and a 2D electron gas (2DEG) as a second electrode. In the off state, this 2DEG can be depleted by applying a negative bias to the top electrode, suppressing transduction and reducing capacitive loading. Switchable AlGaN/GaN resonators are demonstrated from 240MHz to 3.5GHz with frequency-quality factor products up to 1.7×10[superscript 12] in air. Switching is shown in both devices with passive piezoelectric and HEMT sensing, with >19dB suppression in the off state.

43.
An ultralow power athermal silicon modulator - Timurdogan, Erman; Sorace-Agaskar, Cheryl M.; Sun, Jie; Shah Hosseini, Ehsan; Biberman, Aleksandr; Watts, Michael
Silicon photonics has emerged as the leading candidate for implementing ultralow power wavelength–division–multiplexed communication networks in high-performance computers, yet current components (lasers, modulators, filters and detectors) consume too much power for the high-speed femtojoule-class links that ultimately will be required. Here we demonstrate and characterize the first modulator to achieve simultaneous high-speed (25 Gb s[superscript −1]), low-voltage (0.5 V[subscript PP]) and efficient 0.9 fJ per bit error-free operation. This low-energy high-speed operation is enabled by a record electro-optic response, obtained in a vertical p–n junction device that at 250 pm V[superscript −1] (30 GHz V[superscript −1]) is up to 10 times larger than prior demonstrations. In addition, this...

44.
Matrix Remodeling Maintains ESC Self-Renewal by Activating Stat3 - Przybyla, Laralynne M.; Theunissen, Thorold W.; Jaenisch, Rudolf; Voldman, Joel
While a variety of natural and synthetic matrices have been used to influence embryonic stem cell (ESC) self-renewal or differentiation, and ESCs also deposit a rich matrix of their own, the mechanisms behind how extracellular matrix affects cell fate are largely unexplored. The ESC matrix is continuously remodeled by matrix metalloproteinases (MMPs), a process that we find is enhanced by the presence of mouse embryonic fibroblast feeders in a paracrine manner. Matrix remodeling by MMPs aids in the self-renewal of ESCs, as inhibition of MMPs inhibits the ability of ESCs to self-renew. We also find that addition of the interstitial...

45.
Electrokinetic confinement of axonal growth for dynamically configurable neural networks - Honegger, Thibault; Scott, Mark A.; Voldman, Joel; Yanik, Mehmet Fatih
Axons in the developing nervous system are directed via guidance cues, whose expression varies both spatially and temporally, to create functional neural circuits. Existing methods to create patterns of neural connectivity in vitro use only static geometries, and are unable to dynamically alter the guidance cues imparted on the cells. We introduce the use of AC electrokinetics to dynamically control axonal growth in cultured rat hippocampal neurons. We find that the application of modest voltages at frequencies on the order of 10[superscript 5] Hz can cause developing axons to be stopped adjacent to the electrodes while axons away from the...

46.
Image-Predicated Sorting of Adherent Cells Using Photopatterned Hydrogels - Kovac, Joseph; Gerardin, Ylaine; Voldman, Joel
Using photopatterned hydrogels and selective cell encapsulation, populations of adherent cells are examined using microscopy and sorted into viable sub-populations predicated on their imaged phenotypes. The inexpensive method utilizes commercial reagents and equipment available in many labs, making image-predicated cell sorting an accessible technique for a large number of individual labs.

47.
Probing Embryonic Stem Cell Autocrine and Paracrine Signaling Using Microfluidics - Przybyla, Laralynne; Voldman, Joel
Although stem cell fate is traditionally manipulated by exogenously altering the cells' extracellular signaling environment, the endogenous autocrine and paracrine signals produced by the cells also contribute to their two essential processes: self-renewal and differentiation. Autocrine and/or paracrine signals are fundamental to both embryonic stem cell self-renewal and early embryonic development, but the nature and contributions of these signals are often difficult to fully define using conventional methods. Microfluidic techniques have been used to explore the effects of cell-secreted signals by controlling cell organization or by providing precise control over the spatial and temporal cellular microenvironment. Here we review how...

48.
Design and Fabrication of Ultralight Weight, Adjustable Multi-electrode Probes for Electrophysiological Recordings in Mice - Brunetti, Philip M.; Wimmer, Ralf D.; Liang, Li; Siegle, Joshua H.; Voigts, Jakob; Halassa, Michael M.; Wilson, Matthew A.
The number of physiological investigations in the mouse, mus musculus, has experienced a recent surge, paralleling the growth in methods of genetic targeting for microcircuit dissection and disease modeling. The introduction of optogenetics, for example, has allowed for bidirectional manipulation of genetically-identified neurons, at an unprecedented temporal resolution. To capitalize on these tools and gain insight into dynamic interactions among brain microcircuits, it is essential that one has the ability to record from ensembles of neurons deep within the brain of this small rodent, in both head-fixed and freely behaving preparations. To record from deep structures and distinct cell layers...

49.
Emergent Majorana mass and axion couplings in superfluids - Wilczek, Frank
Axions (in the general sense) may acquire qualitatively new couplings inside superfluids. Their conventional couplings to fermions, in empty space, involve purely imaginary masses; the new couplings involve emergent Majorana masses. A generalized concept of Majorana mass is proposed, which allows continuous interpolation between Majorana and Dirac fermions, and also extension to bosons. The possibility of weak links for axions, recently put forward, is analyzed and replaced with a non-local analogue.

50.
Coherent feedback that beats all measurement-based feedback protocols - Jacobs, Kurt; Wang, Xiaoting; Wiseman, Howard M.
We show that when the speed of control is bounded, there is a widely applicable minimal-time control problem for which a coherent feedback protocol is optimal, and is faster than all measurement-based feedback protocols, where the latter are defined in a strict sense. The superiority of the coherent protocol is due to the fact that it can exploit a geodesic path in Hilbert space, a path that measurement-based protocols cannot follow.

51.
Single particle tracking reveals spatial and dynamic organization of the Escherichia coli biofilm matrix - Birjiniuk, Alona; Nance, Elizabeth; Hanes, Justin; Ribbeck, Katharina; Billings, Amanda Nicole; Doyle, Patrick S.
Biofilms are communities of surface-adherent bacteria surrounded by secreted polymers known as the extracellular polymeric substance. Biofilms are harmful in many industries, and thus it is of great interest to understand their mechanical properties and structure to determine ways to destabilize them. By performing single particle tracking with beads of varying surface functionalization it was found that charge interactions play a key role in mediating mobility within biofilms. With a combination of single particle tracking and microrheological concepts, it was found that Escherichia coli biofilms display height dependent charge density that evolves over time. Statistical analyses of bead trajectories and...

52.
Persistent Cellular Motion Control and Trapping Using Mechanotactic Signaling - Zhu, Xiaoying; Bouffanais, Roland; Yue, Dick K. P.
Chemotactic signaling and the associated directed cell migration have been extensively studied owing to their importance in emergent processes of cellular aggregation. In contrast, mechanotactic signaling has been relatively overlooked despite its potential for unique ways to artificially signal cells with the aim to effectively gain control over their motile behavior. The possibility of mimicking cellular mechanotactic signals offers a fascinating novel strategy to achieve targeted cell delivery for in vitro tissue growth if proven to be effective with mammalian cells. Using (i) optimal level of extracellular calcium ([Ca2[superscript +] ][subscript ext] = 3 mM) we found, (ii) controllable fluid...

53.
CRISPRseek: A Bioconductor Package to Identify Target-Specific Guide RNAs for CRISPR-Cas9 Genome-Editing Systems - Zhu, Lihua J.; Holmes, Benjamin R.; Aronin, Neil; Brodsky, Michael H.
CRISPR-Cas systems are a diverse family of RNA-protein complexes in bacteria that target foreign DNA sequences for cleavage. Derivatives of these complexes have been engineered to cleave specific target sequences depending on the sequence of a CRISPR-derived guide RNA (gRNA) and the source of the Cas9 protein. Important considerations for the design of gRNAs are to maximize aimed activity at the desired target site while minimizing off-target cleavage. Because of the rapid advances in the understanding of existing CRISPR-Cas9-derived RNA-guided nucleases and the development of novel RNA-guided nuclease systems, it is critical to have computational tools that can accommodate a...

54.
Quantum-Locked Key Distribution at Nearly the Classical Capacity Rate - Lupo, Cosmo; Lloyd, Seth
Quantum data locking is a protocol that allows for a small secret key to (un)lock an exponentially larger amount of information, hence yielding the strongest violation of the classical one-time pad encryption in the quantum setting. This violation mirrors a large gap existing between two security criteria for quantum cryptography quantified by two entropic quantities: the Holevo information and the accessible information. We show that the latter becomes a sensible security criterion if an upper bound on the coherence time of the eavesdropper’s quantum memory is known. Under this condition, we introduce a protocol for secret key generation through a...

55.
Structural Analysis of Laplacian Spectral Properties of Large-Scale Networks - Preciado, Victor M.; Jadbabaie, Ali; Verghese, George C.
Using methods from algebraic graph theory and convex optimization, we study the relationship between local structural features of a network and the eigenvalues of its Laplacian matrix. In particular, we propose a series of semidefinite programs to find new bounds on the spectral radius and the spectral gap of the Laplacian matrix in terms of a collection of local structural features of the network. Our analysis shows that the Laplacian spectral radius is strongly constrained by local structural features. On the other hand, we illustrate how local structural features are usually insufficient to accurately estimate the Laplacian spectral gap. As...

56.
Model-Based Noninvasive Estimation of Intracranial Pressure from Cerebral Blood Flow Velocity and Arterial Pressure - Kashif, Faisal M.; Verghese, George C.; Heldt, Thomas; Novak, Vera; Czosnyka, Marek
Intracranial pressure (ICP) is affected in many neurological conditions. Clinical measurement of pressure on the brain currently requires placing a probe in the cerebrospinal fluid compartment, the brain tissue, or other intracranial space. This invasiveness limits the measurement to critically ill patients. Because ICP is also clinically important in conditions ranging from brain tumors and hydrocephalus to concussions, noninvasive determination of ICP would be desirable. Our model-based approach to continuous estimation and tracking of ICP uses routinely obtainable time-synchronized, noninvasive (or minimally invasive) measurements of peripheral arterial blood pressure and blood flow velocity in the middle cerebral artery (MCA), both...

57.
Fully Key-Homomorphic Encryption, Arithmetic Circuit ABE and Compact Garbled Circuits - Boneh, Dan; Gentry, Craig; Gorbunov, Sergey; Halevi, Shai; Nikolaenko, Valeria; Segev, Gil; Vaikuntanathan, Vinod; Vinayagamurthy, Dhinakaran
We construct the first (key-policy) attribute-based encryption (ABE) system with short secret keys: the size of keys in our system depends only on the depth of the policy circuit, not its size. Our constructions extend naturally to arithmetic circuits with arbitrary fan-in gates thereby further reducing the circuit depth. Building on this ABE system we obtain the first reusable circuit garbling scheme that produces garbled circuits whose size is the same as the original circuit plus an additive poly(λ,d) bits, where λ is the security parameter and d is the circuit depth. All previous constructions incurred a multiplicative poly(λ) blowup.
We...

58.
Max-Weight Scheduling in Queueing Networks With Heavy-Tailed Traffic - Markakis, Mihalis G.; Tsitsiklis, John N.; Modiano, Eytan H.
We consider the problem of scheduling in a single-hop switched network with a mix of heavy-tailed and light-tailed traffic and analyze the impact of heavy-tailed traffic on the performance of Max-Weight scheduling. As a performance metric, we use the delay stability of traffic flows: A traffic flow is delay-stable if its expected steady-state delay is finite, and delay-unstable otherwise. First, we show that a heavy-tailed traffic flow is delay-unstable under any scheduling policy. Then, we focus on the celebrated Max-Weight scheduling policy and show that a light-tailed flow that conflicts with a heavy-tailed flow is also delay-unstable. This is true...

59.
Queueing system topologies with limited flexibility - Tsitsiklis, John N.; Xu, Kuang
We study a multi-server model with n flexible servers and rn queues, connected through a fixed bipartite graph, where the level of flexibility is captured by the average degree, d(n), of the queues. Applications in content replication in data centers, skill-based routing in call centers, and flexible supply chains are among our main motivations. We focus on the scaling regime where the system size n tends to infinity, while the overall traffic intensity stays fixed. We show that a large capacity region (robustness) and diminishing queueing delay (performance) are jointly achievable even under very limited flexibility (d(n) l n). In...

60.
Convergence of Type-Symmetric and Cut-Balanced Consensus Seeking Systems - Hendrickx, Julien M.; Tsitsiklis, John N.
We consider continuous-time consensus seeking systems whose time-dependent interactions are cut-balanced, in the following sense: if a group of agents influences the remaining ones, the former group is also influenced by the remaining ones by at least a proportional amount. Models involving symmetric interconnections and models in which a weighted average of the agent values is conserved are special cases. We prove that such systems always converge. We give a sufficient condition on the evolving interaction topology for the limit values of two agents to be the same. Conversely, we show that if our condition is not satisfied, then these...