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Biomedical subjects

A Levenson

Publications and source records attributed to A Levenson.

9 recordsLinked to original sources
PubMed2005

Tuning a two-dimensional photonic crystal resonance via optical carrier injection.

We report on wide wavelength tuning through optical injection of carriers of a photonic resonance observed in reflectivity at 1543 nm in an InP-based two-dimensional photonic crystal slab. An 8-nm blueshift, which represents 20 times the resonance linewidth, is observed when a 4-kW/cm2 intense optical pump is incident on the sample. An analytical model that we developed, based on a coupled-mode nonlinear approach, allows us to describe this phenomenon in detail.

Journal Article↗
PubMed2004

Preparation of F-18 labeled annexin V: a potential PET radiopharmaceutical for imaging cell death.

The clinical response to antitumor therapy is measured using imaging, such as CT or MRI, 6-12 weeks following chemotherapy treatment. The images at that time reflect both tumor cell death and new growth. Therefore, the amount of tumor cell death caused by chemotherapy cannot be efficiently quantified with current imaging modalities. A quantitative measurement of tumor cell death immediately following chemotherapy is needed to help validate both new agents and to optimize administration of existing therapies. Annexin V is a 36kD protein that binds to exposed phosphatidylserine (PS) on dying cells. In order to synthesize a probe that can detect cell death in vivo, the positron emitter F-18 was conjugated to annexin V via the compound N- succinimidyl-4-[18F]fluorobenzoate, [18F]SFB. The decay corrected radiochemical yield of F-18 labeled annexin V from 18F fluoride was 17.6 +/- 5.6% (n = 4) in three hours. The stepwise radiochemical yield of the conjugation step with annexin V was as high as 70% when a protein concentration of 5 mg/ml was used. Cancer cells treated with the chemotherapeutic agent, etoposide, showed an 88% increase in the binding of F-18 labeled annexin V compared to untreated cells. We conclude that [18F] labeled annexin V can be readily prepared by the conjugation of annexin V with [18F]SFB and that the positron-emitting compound is biologically active in detecting apoptosis.

Annexin A5↗
PubMed2003

Second-harmonic generation in one-dimensional photonic edge waveguides.

Diffraction losses in one-dimensional photonic crystal (PC) waveguides are the primary limitation on second-harmonic (SH) conversion efficiency. By using a finite difference time domain (FDTD) code taking into account second-order nonlinear polarization, we investigated these losses numerically, particularly at the SH wavelength. We propose an efficient SH conversion scheme in Al(x)Ga(1-x)As/air-etched waveguides. An analytical model is used to extrapolate the conversion efficiency to a number of periods for which time consumption makes the FDTD codes unsuitable.

Journal Article↗
PubMed2002

Phase-matched frequency doubling at photonic band edges: efficiency scaling as the fifth power of the length.

By exploiting the unique properties of periodic stratified media we demonstrate simultaneously phase matching and enhancement of the optical field under second order nonlinear interaction. This leads to a second harmonic efficiency growth faster than the fifth power of the structure length, far better than the usual quadratic behavior associated with second order nonlinear effects.

Journal Article↗
PubMed1999

Stimulation of type I collagen transcription in human skin fibroblasts by TGF-beta: involvement of Smad 3.

Transforming growth factor-beta (TGF-beta) stimulates the transcription of the alpha2(I) procollagen gene (COL1A2). The intracellular mediators involved in this response remain poorly understood. In this study, we demonstrate that primary human skin fibroblasts express Smads, a novel family of signaling molecules, in vitro in the absence of TGF-beta. The levels of Smad 7 mRNA was rapidly and transiently increased by TGF-beta. Transient overexpression of Smad 3 and Smad 4, but not Smad 1 or Smad 2, caused trans-activation of a CAT reporter gene driven by a 772 bp segment of the human COL1A2 promoter containing putative TGF-beta response elements. Smad stimulation of promoter activity was ligand independent, but was further enhanced by TGF-beta. Overexpression of a phosphorylation-deficient Smad 3 mutant or wild-type Smad 7, which lacks the carboxy-terminal phosphorylation motif, specifically inhibited TGF-beta-induced activation of COL1A2 promoter. A CAGACA sequence shown to be a functional Smad-binding element in the plasminogen activator inhibitor-1 gene promoter was found within the TGF-beta-response region of the proximal COL1A2 promoter. Gel mobility shift assays showed protein phosphorylation-dependent binding activity in fibroblast nuclear extracts specific for this sequence; TGF-beta treatment strongly stimulated the formation of this DNA-protein complex. Smad was identified as a component of the CAGACA-binding transcription complex in TGF-beta-treated fibroblasts by antibody supershifting. These results demonstrate that (i) Smad 3 transmits TGF-beta signals from the receptor to the COL1A2 promoter in human fibroblasts, and is likely to play an important role in stimulation of COL1A2 promoter activity elicited by TGF-beta; (ii) in fibroblasts, Smads appear to function as inducible DNA-binding transcription factors; and (iii) Smad 7 may be involved in autocrine negative feedback in the regulation of COL1A2 promoter activity by TGF-beta.

Cells, Cultured↗
PubMed1993

Coping with recent life events: the interplay of personal and collective resources.

The importance of personal and collective resources in coping with recent life events was studied among 230 kibbutz members. The sense of coherence, a global life orientation that detects the ability to avoid stressors and to choose appropriate coping strategies and resources, represented personal resources. Collective resources, embedded in the social system to which one belongs, were measured by membership in a religious kibbutz, the kibbutz being viewed as a powerful, collective-coping resource by itself. Physical well-being, psychological distress, and functional limitations were used as outcome measures. Both types of resources have a salutogenic effect, but sense of coherence appears to be a better resource for avoiding the effect of recent life events and for moderating psychological distress and functional limitation after experiencing such events. The two types of resources have no additive effect, nor do they compensate for each other. Only one significant interaction was found, suggesting that the combination of the two resources is useful in avoiding functional limitation. It is also suggested that collective resources have a slight positive effect on personal resources, which, in turn, take over and become most valuable in coping with recent life events. When stress affects social functioning, these same personal resources facilitate the mobilization of whatever collective resources are available.

Adaptation, Psychological↗
PubMed1991

Life events, sense of coherence and health: gender differences on the kibbutz.

The effects of recently experienced life events (RLE) and of a personal coping resource--the sense of coherence (SOC)--on the health of men and women were investigated among members of two small and cohesive communities--two kibbutzim in Israel (n = 230). Results of analyses lend support to previous findings about the negative effects of life events on health, and to Antonovsky's theory about the positive influence of SOC on health. Separate analyses for men and women, however, show that while RLEs negatively affect women's health, SOC has no significant counterbalancing effect on their health. Among men, an opposite pattern is found; their health is not affected by RLE, but is significantly affected by their SOC. Our findings lead to the conclusion that men and women are differentially affected by stressors and make different use of their coping resources. These findings should be taken into consideration in further research on stress, coping and health.

Adaptation, Psychological↗
PubMedDate not supplied

Focus on: ORBIS International Flying Eye Hospital, Department of Technical Services.

The ORBIS International Flying Eye Hospital is dedicated to restoring sight to the blind through medical education programs in developing countries. The modification of a DC-10 aircraft to house a teaching hospital for ophthalmic surgery involved a variety of engineering challenges to satisfy standards for both hospital and aircraft safety. The Technical Services Department maintains all medical equipment on the aircraft, encountering situations not found in traditional clinical engineering departments. Technical education is also an important part of the ORBIS biomedical engineering program. Future plans include expansion of existing technical training efforts, as well as using technology to improve the medical education program as a whole.

Aircraft↗