554 research outputs found
Culturing Pancreatic Islets in Microfluidic Flow Enhances Morphology of the Associated Endothelial Cells
Pancreatic islets are heavily vascularized in vivo with each insulin secreting beta-cell associated with at least one endothelial cell (EC). This structure is maintained immediately post-isolation; however, in culture the ECs slowly deteriorate, losing density and branched morphology. We postulate that this deterioration occurs in the absence of blood flow due to limited diffusion of media inside the tissue. To improve exchange of media inside the tissue, we created a microfluidic device to culture islets in a range of flow-rates. Culturing the islets from C57BL6 mice in this device with media flowing between 1 and 7 ml/24 hr resulted in twice the EC-density and -connected length compared to classically cultured islets. Media containing fluorescent dextran reached the center of islets in the device in a flow-rate-dependant manner consistent with improved penetration. We also observed deterioration of EC morphology using serum free media that was rescued by addition of bovine serum albumin, a known anti-apoptotic signal with limited diffusion in tissue. We further examined the effect of flow on beta-cells showing dampened glucose-stimulated Ca2+-response from cells at the periphery of the islet where fluid shear-stress is greatest. However, we observed normal two-photon NAD(P)H response and insulin secretion from the remainder of the islet. These data reveal the deterioration of islet EC-morphology is in part due to restricted diffusion of serum albumin within the tissue. These data further reveal microfluidic devices as unique platforms to optimize islet culture by introducing intercellular flow to overcome the restricted diffusion of media components
Pulsed quantum optomechanics
Studying mechanical resonators via radiation pressure offers a rich avenue
for the exploration of quantum mechanical behavior in a macroscopic regime.
However, quantum state preparation and especially quantum state reconstruction
of mechanical oscillators remains a significant challenge. Here we propose a
scheme to realize quantum state tomography, squeezing and state purification of
a mechanical resonator using short optical pulses. The scheme presented allows
observation of mechanical quantum features despite preparation from a thermal
state and is shown to be experimentally feasible using optical microcavities.
Our framework thus provides a promising means to explore the quantum nature of
massive mechanical oscillators and can be applied to other systems such as
trapped ions.Comment: 9 pages, 4 figure
On RAF Sets and Autocatalytic Cycles in Random Reaction Networks
The emergence of autocatalytic sets of molecules seems to have played an
important role in the origin of life context. Although the possibility to
reproduce this emergence in laboratory has received considerable attention,
this is still far from being achieved. In order to unravel some key properties
enabling the emergence of structures potentially able to sustain their own
existence and growth, in this work we investigate the probability to observe
them in ensembles of random catalytic reaction networks characterized by
different structural properties. From the point of view of network topology, an
autocatalytic set have been defined either in term of strongly connected
components (SCCs) or as reflexively autocatalytic and food-generated sets
(RAFs). We observe that the average level of catalysis differently affects the
probability to observe a SCC or a RAF, highlighting the existence of a region
where the former can be observed, whereas the latter cannot. This parameter
also affects the composition of the RAF, which can be further characterized
into linear structures, autocatalysis or SCCs. Interestingly, we show that the
different network topology (uniform as opposed to power-law catalysis systems)
does not have a significantly divergent impact on SCCs and RAFs appearance,
whereas the proportion between cleavages and condensations seems instead to
play a role. A major factor that limits the probability of RAF appearance and
that may explain some of the difficulties encountered in laboratory seems to be
the presence of molecules which can accumulate without being substrate or
catalyst of any reaction.Comment: pp 113-12
Gender and sustainable livelihoods: linking gendered experiences of environment, community and self
In this essay I explore the economic, social,
environmental and cultural changes taking place in Bolsena,
Italy, where agricultural livelihoods have rapidly
diminished in the last two decades. I examine how gender
dynamics have shifted with the changing values and
livelihoods of Bolsena through three women’s narratives
detailing their gendered experiences of environment,
community and self. I reflect on these changes with Sabrina,
who is engaged in a feminist community-based
organization; Anna, who is running an alternative wine bar;
and Isabella, a jeweler, who is engaged in ecofeminist
practices. My analysis is based on concepts developed by
feminist political ecology: specifically, the theory of rooted
networks from Dianne Rocheleau, Donna Haraway’s concept
of naturecultures (and the work of J. K. Gibson-Graham
on new economic imaginaries emerging from the
politics of place. I aim to think with, reflect upon and
provoke from the ‘‘otherwise’’, taking into account the
lived relations entwining nature and gender. My article
looks at the interconnections of gender, environment and
livelihoods, attentive to the daily needs, embodied interactions
and labours of these three women as part of a
reappropriation, reconstruction and reinvention of Bolsena’s
lifeworld. By listening to the stories of their everyday
lives and struggles, I show the dynamic potential of the
politics of place and the efforts to build diverse economies
and more ethical economic and ecological relationships
based on gender-aware subjectivities and values
Sideband Cooling Micromechanical Motion to the Quantum Ground State
The advent of laser cooling techniques revolutionized the study of many
atomic-scale systems. This has fueled progress towards quantum computers by
preparing trapped ions in their motional ground state, and generating new
states of matter by achieving Bose-Einstein condensation of atomic vapors.
Analogous cooling techniques provide a general and flexible method for
preparing macroscopic objects in their motional ground state, bringing the
powerful technology of micromechanics into the quantum regime. Cavity opto- or
electro-mechanical systems achieve sideband cooling through the strong
interaction between light and motion. However, entering the quantum regime,
less than a single quantum of motion, has been elusive because sideband cooling
has not sufficiently overwhelmed the coupling of mechanical systems to their
hot environments. Here, we demonstrate sideband cooling of the motion of a
micromechanical oscillator to the quantum ground state. Entering the quantum
regime requires a large electromechanical interaction, which is achieved by
embedding a micromechanical membrane into a superconducting microwave resonant
circuit. In order to verify the cooling of the membrane motion into the quantum
regime, we perform a near quantum-limited measurement of the microwave field,
resolving this motion a factor of 5.1 from the Heisenberg limit. Furthermore,
our device exhibits strong-coupling allowing coherent exchange of microwave
photons and mechanical phonons. Simultaneously achieving strong coupling,
ground state preparation and efficient measurement sets the stage for rapid
advances in the control and detection of non-classical states of motion,
possibly even testing quantum theory itself in the unexplored region of larger
size and mass.Comment: 13 pages, 7 figure
Coupling carbon nanotube mechanics to a superconducting circuit
The quantum behaviour of mechanical resonators is a new and emerging field
driven by recent experiments reaching the quantum ground state. The high
frequency, small mass, and large quality-factor of carbon nanotube resonators
make them attractive for quantum nanomechanical applications. A common element
in experiments achieving the resonator ground state is a second quantum system,
such as coherent photons or superconducting device, coupled to the resonators
motion. For nanotubes, however, this is a challenge due to their small size.
Here, we couple a carbon nanoelectromechanical (NEMS) device to a
superconducting circuit. Suspended carbon nanotubes act as both superconducting
junctions and moving elements in a Superconducting Quantum Interference Device
(SQUID). We observe a strong modulation of the flux through the SQUID from
displacements of the nanotube. Incorporating this SQUID into superconducting
resonators and qubits should enable the detection and manipulation of nanotube
mechanical quantum states at the single-phonon level
Microwave amplification with nanomechanical resonators
Sensitive measurement of electrical signals is at the heart of modern science
and technology. According to quantum mechanics, any detector or amplifier is
required to add a certain amount of noise to the signal, equaling at best the
energy of quantum fluctuations. The quantum limit of added noise has nearly
been reached with superconducting devices which take advantage of
nonlinearities in Josephson junctions. Here, we introduce a new paradigm of
amplification of microwave signals with the help of a mechanical oscillator. By
relying on the radiation pressure force on a nanomechanical resonator, we
provide an experimental demonstration and an analytical description of how the
injection of microwaves induces coherent stimulated emission and signal
amplification. This scheme, based on two linear oscillators, has the advantage
of being conceptually and practically simpler than the Josephson junction
devices, and, at the same time, has a high potential to reach quantum limited
operation. With a measured signal amplification of 25 decibels and the addition
of 20 quanta of noise, we anticipate near quantum-limited mechanical microwave
amplification is feasible in various applications involving integrated
electrical circuits.Comment: Main text + supplementary information. 14 pages, 3 figures (main
text), 18 pages, 6 figures (supplementary information
Fate specification and tissue-specific cell cycle control of the <i>Caenorhabditis elegans</i> intestine
Coordination between cell fate specification and cell cycle control in multicellular organisms is essential to regulate cell numbers in tissues and organs during development, and its failure may lead to oncogenesis. In mammalian cells, as part of a general cell cycle checkpoint mechanism, the F-box protein β-transducin repeat-containing protein (β-TrCP) and the Skp1/Cul1/F-box complex control the periodic cell cycle fluctuations in abundance of the CDC25A and B phosphatases. Here, we find that the Caenorhabditis elegans β-TrCP orthologue LIN-23 regulates a progressive decline of CDC-25.1 abundance over several embryonic cell cycles and specifies cell number of one tissue, the embryonic intestine. The negative regulation of CDC-25.1 abundance by LIN-23 may be developmentally controlled because CDC-25.1 accumulates over time within the developing germline, where LIN-23 is also present. Concurrent with the destabilization of CDC-25.1, LIN-23 displays a spatially dynamic behavior in the embryo, periodically entering a nuclear compartment where CDC-25.1 is abundant
Quantum nondemolition measurement of mechanical motion quanta
The fields of opto- and electromechanics have facilitated numerous advances
in the areas of precision measurement and sensing, ultimately driving the
studies of mechanical systems into the quantum regime. To date, however, the
quantization of the mechanical motion and the associated quantum jumps between
phonon states remains elusive. For optomechanical systems, the coupling to the
environment was shown to preclude the detection of the mechanical mode
occupation, unless strong single photon optomechanical coupling is achieved.
Here, we propose and analyse an electromechanical setup, which allows to
overcome this limitation and resolve the energy levels of a mechanical
oscillator. We find that the heating of the membrane, caused by the interaction
with the environment and unwanted couplings, can be suppressed for carefully
designed electromechanical systems. The results suggest that phonon number
measurement is within reach for modern electromechanical setups.Comment: 8 pages, 5 figures plus 24 pages, 11 figures supplemental materia
Critical Role of Gap Junction Coupled K(ATP) Channel Activity for Regulated Insulin Secretion
Pancreatic β-cells secrete insulin in response to closure of ATP-sensitive K(+) (K(ATP)) channels, which causes membrane depolarization and a concomitant rise in intracellular Ca(2+) (Ca(i)). In intact islets, β-cells are coupled by gap junctions, which are proposed to synchronize electrical activity and Ca(i) oscillations after exposure to stimulatory glucose (>7 mM). To determine the significance of this coupling in regulating insulin secretion, we examined islets and β-cells from transgenic mice that express zero functional K(ATP) channels in approximately 70% of their β-cells, but normal K(ATP) channel density in the remainder. We found that K(ATP) channel activity from approximately 30% of the β-cells is sufficient to maintain strong glucose dependence of metabolism, Ca(i), membrane potential, and insulin secretion from intact islets, but that glucose dependence is lost in isolated transgenic cells. Further, inhibition of gap junctions caused loss of glucose sensitivity specifically in transgenic islets. These data demonstrate a critical role of gap junctional coupling of K(ATP) channel activity in control of membrane potential across the islet. Control via coupling lessens the effects of cell–cell variation and provides resistance to defects in excitability that would otherwise lead to a profound diabetic state, such as occurs in persistent neonatal diabetes mellitus
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