Understanding cell morphogenesis during metazoan development requires knowledge of how cells and the extracellular matrix produce and respond to forces. We investigated how apoptosis, which remodels tissue by eliminating supernumerary cells, also contributes forces to a tissue (the amnioserosa) that promotes cell-sheet fusion (dorsal closure) in the Drosophila embryo. We showed that expression in the amnioserosa of proteins that suppress or enhance apoptosis slows or speeds dorsal closure, respectively. These changes correlate with the forces produced by the amnioserosa and the rate of seam formation between the cell sheets (zipping), key processes that contribute to closure. This apoptotic force is used by the embryo to drive cell-sheet movements during development, a role not classically attributed to apoptosis.

Tissue dynamics during dorsal closure, a stage of Drosophila development, provide a model system for cell sheet morphogenesis and wound healing. Dorsal closure is characterized by complex cell sheet movements, driven by multiple tissue specific forces, which are coordinated in space, synchronized in time, and resilient to UV-laser perturbations. The mechanisms responsible for these attributes are not fully understood. We measured spatial, kinematic, and dynamic antero-posterior asymmetries to biophysically characterize both resiliency to laser perturbations and failure of closure in mutant embryos and compared them to natural asymmetries in unperturbed, wild-type closure. We quantified and mathematically modeled two processes that are upregulated to provide resiliency--contractility of the amnioserosa and formation of a seam between advancing epidermal sheets, i.e., zipping. Both processes are spatially removed from the laser-targeted site, indicating they are not a local response to laser-induced wounding and suggesting mechanosensitive and/or chemosensitive mechanisms for upregulation. In mutant embryos, tissue junctions initially fail at the anterior end indicating inhomogeneous mechanical stresses attributable to head involution, another developmental process that occurs concomitant with the end stages of closure. Asymmetries in these mutants are reversed compared to wild-type, and inhomogeneous stresses may cause asymmetries in wild-type closure.

Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity. The voltage tuned resonance is determined by the plasma oscillations of the composite structure. The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood. The phenomenon is potentially important for fast, tunable terahertz detectors. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1497433͔ Double-quantum-well ͑DQW͒ heterostructures are important in the scientific exploration of correlated electron states in two-dimensional electron systems 1 and potentially important for novel field-effect transistors that add functionality by controlling electron transfer between the quantum wells. 2 Interwell transfer can also be promoted by terahertz photon assisted tunneling, opening the possibility of fast, voltage tunable terahertz ͑THz͒ detectors. 3 This motivated our research on THz response of DQW field-effect transistors ͑FETs͒. We report the THz photoconductivity of DQW FETs in which the gate is a periodic metallic grating. Strong photoresponse occurs at the plasma resonance of the composite structure. Other detector proposals make use of plasmon modes in single two-dimensional electron gas ͑2DEG͒ systems. 4 But, the relatively strong resonant response that we report here appears to require the presence of a DQW. We model the resonant response with a transmission line model of the collective modes of the 2DEGs and correlate the observed resonances with standing plasmon resonances under the metallic part of the grating gate. While the work was motivated by the concept of interwell transfer, the actual mechanism that gives rise to this response is not understood.The FETs are fabricated from modulation doped GaAs/ AlGaAs DQW heterostructures grown by molecular beam epitaxy. Both wells are 200 Å wide and are separated by a 70 Å barrier. The nominal electron densities are n upper ϭ1.7 ϫ10 11 cm Ϫ2 and n lower ϭ2.57ϫ10 11 cm Ϫ2 : the 4.2 K mobility is ϳ1.7ϫ10 6 cm 2 /V s. A 2ϫ2 mm mesa is defined and ohmic contacts to both quantum wells form source and drain. A 700-Å-thick TiAu grating gate ͑with no metallization between the grating fingers͒ is evaporated with the lines of the grating perpendicular to the current flow. We explored 4 and 8 m periods; half the period is metal. The grating modulates the electron density when a voltage is applied, selects wave vectors of the excited plasmon and, coincidentally, produces both normal and transverse THz electric fields. See inset in Fig. 1 for a cross section of the sample.We apply a constant source-drain current of 100 A, focus the radiation onto the sample, and study the photoconductive response of the DQW as a function of gate voltage, THz frequency, and temperature. The radiation sources are the free-electron lasers at the University of California, Santa Barbara, which cover a frequency range between 120 GHz and 4.8 THz. The respons...

Human dermal fibroblasts exhibit comparable cellular and molecular effects when exposed to THz radiation and hyperthermic stress. These findings suggest that radiation at 2.52 THz generates primarily thermal effects in mammalian cells. Therefore, we conclude that THz-induced bioeffects may be accurately predicted with conventional thermal damage models.

Epithelial movements are key morphogenetic events in animal development. They are driven by multiple mechanisms, including signal-dependent changes in cytoskeletal organization and in cell adhesion. Such processes must be controlled precisely and coordinated to accurately sculpt the three-dimensional form of the developing organism. By observing the Drosophila epidermis during embryonic development using confocal time-lapse microscopy, we have investigated how signaling through the Jun-N-terminal kinase (JNK) pathway governs the tissue sheet movements that result in dorsal closure (DC). We find that JNK controls the polymerization of actin into a cable at the epidermal leading edge as previously suggested, as well as the joining (zipping) of the contralateral epithelial cell sheets. Here, we show that zipping is mediated by regulation of the integrins myospheroid and scab. Our data demonstrate that JNK signaling regulates a set of target genes that cooperate to facilitate epithelial movement and closure. Developmental Dynamics 235:427-434, 2006.

Terahertz spectrometers and imaging systems are currently being evaluated as biomedical tools for skin burn assessment. These systems show promise, but due to their size and weight, they have restricted portability, and are impractical for military and battlefield settings where space is limited. In this study, we developed and tested the performance of a compact, light, and portable THz time-domain spectroscopy (THz-TDS) device. Optical properties were collected with this system from 0.1 to 1.6 THz for water, ethanol, and several ex vivo porcine tissues (muscle, adipose, skin). For all samples tested, we found that the index of refraction (n) decreases with frequency, while the absorption coefficient (μ(a)) increases with frequency. Muscle, adipose, and frozen/thawed skin samples exhibited comparable n values ranging between 2.5 and 2.0, whereas the n values for freshly harvested skin were roughly 40% lower. Additionally, we found that the freshly harvested samples exhibited higher μ(a) values than the frozen/thawed skin samples. Overall, for all liquids and tissues tested, we found that our system measured optical property values that were consistent with those reported in the literature. These results suggest that our compact THz spectrometer performed comparable to its larger counterparts, and therefore may be a useful and practical tool for skin health assessment.

Selenoproteins play an important role in the human body by accomplishing essential biological functions like oxido-reductions, antioxidant defense, thyroid hormone metabolism and immune response; therefore, the possibility to synthesize selenium nanoparticles free of any contaminants is exciting for future nano-medical applications. This paper reports the first synthesis of selenium nanoparticles by femtosecond pulsed laser ablation in de-ionized water. Those pure nanoparticles have been successfully used to inhibit the formation of Candida albicans biofilms. Advanced electron microscopy images showed that selenium nanoparticles easily adhere on the biofilm, then penetrate into the pathogen, and consequently damage the cell structure by substituting with sulfur. 50% inhibition of Candida albicans biofilm was obtained at only 25 ppm. Finally, the two physical parameters proved to affect strongly the viability of Candida albicans are the crystallinity and particle size.

Graphene based field effect transistor for the detection of ammonia J. Appl. Phys. 112, 064304 (2012) Unipolar behavior of asymmetrically doped strained Si0.5Ge0.5 tunneling field-effect transistors Appl. Phys. Lett. 101, 123501 (2012) Efficient physical-thermal model for thermal effects in AlGaN/GaN high electron mobility transistors Appl. Phys. Lett. 101, 122101 (2012) Light/negative bias stress instabilities in indium gallium zinc oxide thin film transistors explained by creation of a double donorThe terahertz absorption spectrum of plasmon modes in a grid-gated double-quantum-well ͑DQW͒ field-effect transistor structure is analyzed theoretically and numerically using a first principles electromagnetic approach and is shown to faithfully reproduce important physical features of recent experimental observations. We find that the essential character of the response-multiple resonances corresponding to spatial harmonics of standing plasmons under the metal grating-is caused by the static spatial modulation of electron density in the channel. Higher order plasmon modes become more optically active as the depth of the electron density modulation in the DQW tends towards unity. The maximum absorbance, at plasma resonance, is shown to be 1/2. Furthermore, the strongest absorption also occurs when the standing plasmon resonance coincides with the fundamental dipole mode of the ungated portion of the channel.