Detecting nonlinearity in structural systems using the transfer entropy.

The transfer entropy was recently proposed as a means of exploring coupling in dynamical systems. Transfer entropy is an information theoretic that quantifies the degree to which one dynamical process affects the transition probabilities (dynamics) of another. Here we demonstrate how this metric may be utilized to detect the presence of nonlinearity in a system. Using the method of surrogate data, the transfer entropy computed at various lag times are compared to values computed from linearized surrogates. The transfer entropy is shown to be a more sensitive indicator of nonlinearity than is the mutual information for both simulated and experimental data. This technique is particularly applicable to the field of structural health monitoring, where damage is often equated with the presence of a nonlinearity in an otherwise linear system.

Using chaotic interrogation and attractor nonlinear cross-prediction error to detect fastener preload loss in an aluminum frame.

Structural health monitoring is an important field concerned with assessing the current state (or "health") of a structural system or component with regard to its ability to perform its intended function appropriately. One approach to this problem is identifying appropriate features obtained from time series vibration responses of the structure that change as structural degradation occurs. In this work, we present a novel technique adapted from the nonlinear time series prediction community whereby the structure is excited by an applied chaotic waveform, and predictive maps built between structural response attractors are used as the feature space. The structural response is measured at several points on the structure, and pairs of attractors are used to predict each other. As the dynamics of the structure change due to damage, the prediction error rises. This approach is applied to detecting the preload loss in a bolted joint in an aluminum frame structure.

Controlling system dimension: a class of real systems that obey the Kaplan-Yorke conjecture.

The Kaplan-Yorke conjecture suggests a simple relationship between the fractal dimension of a system and its Lyapunov spectrum. This relationship has important consequences in the broad field of nonlinear dynamics where dimension and Lyapunov exponents are frequently used descriptors of system dynamics. We develop an experimental system with controllable dimension by making use of the Kaplan-Yorke conjecture. A rectangular steel plate is driven with the output of a chaotic oscillator. We controlled the Lyapunov exponents of the driving and then computed the fractal dimension of the plate's response. The Kaplan-Yorke relationship predicted the system's dimension extremely well. This finding strongly suggests that other driven linear systems will behave similarly. The ability to control the dimension of a structure's vibrational response is important in the field of vibration-based structural health monitoring for the robust extraction of damage-sensitive features.

Use of chaotic excitation and attractor property analysis in structural health monitoring.

This work explores the utility of attractor-based approaches in the field of vibration-based structural health monitoring. The technique utilizes the unique properties of chaotic signals by driving the structure directly with the output of a chaotic oscillator. Using the Kaplan-Yorke conjecture, the Lyapunov exponents of the driving signal may be tuned to the dominant eigenvalues of the structure, thus controlling the dimension of the structural response. Data are collected at various stages of structural degradation and a simple nonlinear model, constructed from the undamaged data, is used to make predictions for the damaged response data. Prediction error is then introduced as a "feature" for classifying the magnitude of the damage. Results are presented for an experimental cantilevered beam instrumented with fiber-optic strain sensors.

Spatiotemporal nonlinear dynamics of a magnetoelastic ribbon.

Magnetoelastic materials have a strong coupling between strain and magnetization, so applying a magnetic field to a magnetoelastic material can change its shape. This coupling leads to interesting dynamics. We have studied the dynamics of a wide ribbon of Metglass 2605sc which was driven by a magnetic field. The ribbon was suspended as a pendulum in a set of Helmholtz coils, which provided both dc and ac magnetic fields. Laser light was reflected off the ribbon to measure its angular displacement. Two points on the ribbon could be simultaneously illuminated, and one of the laser beams could be scanned over the ribbon. We observed quasiperiodic bifurcations in the motion of the ribbon, and characterized the spatial aspect of the motion with some recently developed statistics.

Emerging technologies for accelerated toxicity evaluation of potential drug candidates.

The current trend in the pharmaceutical industry is to shorten the timelines for all aspects of drug discovery. Although several well-established methods are used for routine toxicological evaluation, new, innovative tools are being designed, developed and implemented that promise to deliver significant advantages. These include technologies that take advantage of large-scale mining of the genome and proteome, such as microarrays and two-dimensional electrophoresis, and methods that enhance the molecular detection of specific proteins and mRNAs, such as mass spectroscopy and branched DNA. These tools, combined with better in vitro model systems, will become instrumental in answering questions about the molecular mechanisms of toxicity, as well as offering productive and effective means for higher-throughput toxicity assessment.

Comparison of responses of base-specific Salmonella tester strains with the traditional strains for identifying mutagens: the results of a validation study.

The ability of a TA7000 series of Salmonella his- mutant tester strains to detect mutagens as classified by the traditional tester strains (TA100, TA98, TA1535, TA1537, TA97, TA102 and TA104) was evaluated using 30 coded chemicals, 5 of which were duplicates with different code numbers. The TA7000 series of tester strains were TA7001, TA7002, TA7003, TA7004, TA7005 and TA7006, each of which reverts by a specific base substitution. In addition, each chemical was tested in a mixture of the base-specific strains (the Mix), plus the traditional strains, TA98 and TA1537. A liquid version of the Salmonella mutagenicity assay was performed in microtiter plates to allow partial automation for increased throughput. The results were compared to those in the National Toxicology Program (NTP) database, which were obtained from the traditional strains in the preincubation assay. In the two strains common to both protocols, TA98 and TA1537, the agreement was 80% and 85%, respectively. When compared to the NTP results for TA100, the Mix gave a 72% concordance, while the addition of the frameshift tester strain, TA98, increased the agreement to 76%. The overall agreement on positive or negative classifications of mutagenicity was 88% for the 25 chemicals tested. There were three notable exceptions to the overall agreement. Benzaldehyde was detected as a mutagen in TA7005 in contrast to its classification as a non-mutagen in the NTP database. This does not necessarily contradict the NTP results because the base-specific strains may respond to different mutagens. Two weak mutagens in the NTP database, 1-chloro-2-propanol and isobutyl nitrite, were not detected as mutagens in the base-specific new strains in the liquid protocol. While there are a number of major differences in the two assays, it was concluded that the results from each procedure are comparable.

The CAT-Tox (L) assay: a sensitive and specific measure of stress-induced transcription in transformed human liver cells.

Identifying and measuring the molecular mechanisms of toxicity is an important goal in hazard assessment. We have developed an assay in transformed human liver cells to simultaneously measure the transcriptional responses of 14 stress promoter- or response element-chloramphenicol acetyl transferase (CAT) fusion constructs that are stably integrated into the HepG2 cell line. This assay can measure a wide spectrum of stresses, both toxic and nontoxic, such as protein and protein biosynthesis perturbations, DNA damage, heavy metals, and planar aromatic hydrocarbons. We found that each promoter or response element can be induced by one or more of four chemicals that were tested in the assay. These results have been interpreted in light of the current models of action for each compound. The responses of this assay system can distinguish among compounds that are closely related in their structure and have been shown previously to elicit similar biological activities in simple assay systems. We have designated this technique the CAT-Tox (L)iver assay. It measures a broad range of cellular stresses and toxicants at levels that were comparable to or below those of established methods. The induction profiles generated using the CAT-Tox (L) assay can help to elucidate the molecular mechanisms by which chemicals exert their actions on human cells. These profiles can be indicative of both toxic and nontoxic processes that are occurring in the cell. We propose that this cellular stress assay can serve as a screen for a variety of substances at the molecular level.

Regulation of cytokine gene expression in T helper cell subsets.

We have examined the transcriptional regulation of the Th1-specific IL-2 gene and the Th2-specific IL-4 gene by transient transfection of promoter-chloramphenicol acetyl transferase (CAT) constructs into T cell clones and by electrophoretic mobility shift assays. Transfection of the Th2 clone D10.G4 with IL-4 promoter-CAT constructs demonstrated anti-CD3 inducible IL-4 promoter activity. In contrast, CAT constructs containing murine IL-2 promoter sequences were not inducible in D10.G4 cells. Transfection analyses in the Th1 clone D1.1 demonstrated inducible IL-2 promoter activity but no IL-4 promoter activity. Electrophoretic mobility shift assays using well-defined regulatory elements in the murine IL-2 gene promoter showed that anti-CD3 stimulation of Th2 clones failed to induce a characteristic increase in the ratio of p65-p50:p50-p50 NF-kappa B binding complexes in the nucleus that did occur in IL-2-producing clones. In addition, other protein complexes with NF-kappa B binding sequences were seen using lysates from Th1 and Th0 cells but not Th2 cells. Thus, expression of the promoter-CAT constructs directly correlates with endogenous IL-2 and IL-4 gene expression in Th1 and Th2 clones, confirming that the differential expression of IL-2 and IL-4 genes in these T cells is transcriptionally controlled. Furthermore, the lack of IL-2 transcription in activated Th2 cells is associated with the failure to generate required IL-2 gene promoter binding proteins, particularly NF-kappa B in the nucleus.

Transcription of the interleukin 4 gene is regulated by multiple promoter elements.

Activation of T helper cell 1 (Th1) and Th2 results in transcription of the interleukin 2 (IL-2) and IL-4 cytokine genes, respectively. Whereas many of the regulatory elements and factors responsible for IL-2 transcription in T cells are well defined, little is known about parallel mechanisms that drive transcription of the IL-4 gene. Here we have analyzed the murine IL-4 promoter, both in vivo and in a Th2 clone. 3 kb of IL-4 upstream sequence is shown to be sufficient to achieve tissue-specific and inducible expression of a thymidine kinase reporter gene in vivo in a manner that mirrors the expression of endogenous IL-4. Tissue-specific and inducible expression is also demonstrated in a Th2 clone, but not in a B cell line. Deletional and mutational analysis of the IL-4 promoter demonstrated that sequences from -100 to -28 were necessary for a transcriptional response to Concanavalin A or anti-CD3 monoclonal antibody. An overlapping, yet smaller region, spanning the sequences from -60 to -28 bp was shown to be required for the response to ionomycin. Mutation of an 8-bp region from -43 to -35 of the IL-4 promoter completely abrogated IL-4 gene transcription in response to all stimuli tested. In addition, our results show that the effects of the immunosuppressive agent Cyclosporin A map to the same DNA sequences as the positive control elements. These results identify DNA sequences that are functionally important for the control of IL-4 gene transcription both in vivo and in vitro. Although these sequences are highly conserved in the human and murine IL-4 genes, they are largely not present in the IL-2 enhancer complex. Thus, cytokine-specific cis-acting elements may be one mechanism by which these two cytokine genes are differentially regulated.