The von Karman equations, the stress function, and elastic ridges in high dimensions

The elastic energy functional of a thin elastic rod or sheet is generalized to the case of an M-dimensional manifold in N-dimensional space. We derive potentials for the stress field and curvatures and find the generalized von Karman equations for a manifold in elastic equilibrium. We perform a scaling analysis of an M-1 dimensional ridge in an M = N-1 dimensional manifold. A ridge of linear size X in a manifold with thickness h << X has a width w ~ h^{1/3}X^{2/3} and a total energy E ~ h^{M} (X/h)^{M-5/3}. We also prove that the total bending energy of the ridge is exactly five times the total stretching energy. These results match those of A. Lobkovsky [Phys. Rev. E 53, 3750 (1996)] for the case of a bent plate in three dimensions.Comment: corrected references, 27 pages, RevTeX + epsf, 2 figures, Submitted to J. Math. Phy

The Advantages of a Tapered Whisker

The role of facial vibrissae (whiskers) in the behavior of terrestrial mammals is principally as a supplement or substitute for short-distance vision. Each whisker in the array functions as a mechanical transducer, conveying forces applied along the shaft to mechanoreceptors in the follicle at the whisker base. Subsequent processing of mechanoreceptor output in the trigeminal nucleus and somatosensory cortex allows high accuracy discriminations of object distance, direction, and surface texture. The whiskers of terrestrial mammals are tapered and approximately circular in cross section. We characterize the taper of whiskers in nine mammal species, measure the mechanical deflection of isolated felid whiskers, and discuss the mechanics of a single whisker under static and oscillatory deflections. We argue that a tapered whisker provides some advantages for tactile perception (as compared to a hypothetical untapered whisker), and that this may explain why the taper has been preserved during the evolution of terrestrial mammals

Functionalization of Bacterial Microcompartment Shell Proteins With Covalently Attached Heme.

Heme is a versatile redox cofactor that has considerable potential for synthetic biology and bioelectronic applications. The capacity to functionalize non-heme-binding proteins with covalently bound heme moieties in vivo could expand the variety of bioelectronic materials, particularly if hemes could be attached at defined locations so as to facilitate position-sensitive processes like electron transfer. In this study, we utilized the cytochrome maturation system I to develop a simple approach that enables incorporation of hemes into the backbone of target proteins in vivo. We tested our methodology by targeting the self-assembling bacterial microcompartment shell proteins, and inserting functional hemes at multiple locations in the protein backbone. We found substitution of three amino acids on the target proteins promoted heme attachment with high occupancy. Spectroscopic measurements suggested these modified proteins covalently bind low-spin hemes, with relative low redox midpoint potentials (about -210 mV vs. SHE). Heme-modified shell proteins partially retained their self-assembly properties, including the capacity to hexamerize, and form inter-hexamer attachments. Heme-bound shell proteins demonstrated the capacity to integrate into higher-order shell assemblies, however, the structural features of these macromolecular complexes was sometimes altered. Altogether, we report a versatile strategy for generating electron-conductive cytochromes from structurally-defined proteins, and provide design considerations on how heme incorporation may interface with native assembly properties in engineered proteins

Stress condensation in crushed elastic manifolds

We discuss an M-dimensional phantom elastic manifold of linear size L crushed into a small sphere of radius R << L in N-dimensional space. We investigate the low elastic energy states of 2-sheets (M=2) and 3-sheets (M=3) using analytic methods and lattice simulations. When N \geq 2M the curvature energy is uniformly distributed in the sheet and the strain energy is negligible. But when N=M+1 and M>1, both energies appear to be condensed into a network of narrow M-1 dimensional ridges. The ridges appear straight over distances comparable to the confining radius R.Comment: 4 pages, RevTeX + epsf, 4 figures, Submitted to Phys. Rev. Let

Scaling Laws for Mitotic Chromosomes

During mitosis in higher eukaryotes, each chromosome condenses into a pair of rod-shaped chromatids. This process is co-regulated by the activity of several gene families, and the underlying biophysics remains poorly understood. To better understand the factors regulating chromosome condensation, we compiled a database of mitotic chromosome size and DNA content from the tables and figures of &gt;200 published papers. A comparison across vertebrate species shows that chromosome width, length and volume scale with DNA content to the powers ∼1/4, ∼1/2, and ∼1, respectively. Angiosperms (flowering plants) show a similar length scaling, so this result is not specific to vertebrates. Chromosome shape and size thus satisfy two conditions: (1) DNA content per unit volume is approximately constant and (2) the cross-sectional area increases proportionately with chromosome length. Since viscous drag forces during chromosome movement are expected to scale with length, we hypothesize that the cross-section increase is necessary to limit the occurrence of large chromosome elongations that could slow or stall mitosis. Lastly, we note that individual vertebrate karyotypes typically exhibit a wider range of chromosome lengths as compared with angiosperms

The association between care co-ordination and emergency department use in older managed care enrollees

OBJECTIVE: To investigate the association between care co-ordination and use of the Emergency Department (ED) in older managed care enrollees. DESIGN: Nested case-control with 103 cases (used the ED) and 194 controls (did not use the ED). PATIENTS AND METHODS: Older patients with multiple chronic illnesses enrolled in a care management programme of a large group-model health maintenance organisation with more than 50,000 members over the age of 64. Better care co-ordination was defined as timely follow-up after a change in treatment; fewer decision-makers involved with the care plan; and a higher patient-perceived rating of overall care co-ordination. Logistic regression was used to assess the relationship between ED use (the outcome variable) and measures of care co-ordination (the predictor variables). RESULTS: Self-reported care co-ordination was not significantly different between cases and controls for any of the four classifications of inappropriate ED use. Similarly, no differences were found in the number of different physicians or medication prescribers involved in the patients' care. Four-week follow-up after potentially high-risk events for subsequent ED use, including changes in chronic disease medications, missed encounters, and same day encounters, did not differ between subjects with inappropriate ED use and controls. CONCLUSION: Existing measures of care co-ordination were not associated with inappropriate ED use in this study of older adults with complex care needs. The absence of an association may, in part, be attributable to the paucity of validated measures to assess care co-ordination, as well as the methodological complexity inherent in studying this topic. Future research should focus on the development of new measures and on approaches that better isolate the role of care co-ordination from other potential variables that influence utilisation

Development and testing of a measure designed to assess the quality of care transitions

BACKGROUND: To improve the quality of care delivered to older persons receiving care across multiple settings, interventions are needed. However, the absence of a patient-centred measure specifically designed to assess this care has constrained innovation. OBJECTIVE: To develop a rigorously designed and tested measure, the Care Transition Measure (CTM). SETTING: A large, integrated managed care organisation in Colorado with approximately 55,000 members over the age of 65 years. PARTICIPANTS: Patients 65 years and older who were recently discharged from hospital and received subsequent skilled nursing care in a facility or in the home. METHODS: Six focus groups of older persons and their caregivers (n=49) were established. Standard qualitative analytic techniques were applied to written transcripts and four key domains were identified: (1) information transfer; (2) patient and caregiver preparation; (3) self-management support; and (4) empowerment to assert preferences. Specific CTM items were developed, pilot tested, and refined. Psychometric testing, conducted in a different population but selected using the same entry criteria (n=60), included content and construct validity, intra-item variation, and floor/ceiling properties. RESULTS: Older patients and clinicians found the measure to be highly relevant and comprehensive (i.e. content validity). Construct validity was assessed by comparing items from the CTM to selected items from a measure developed by Hendriks and colleagues (Medical Care 2001; 39(3): 270–283). Inter-item Spearman correlations ranged 0.388–0.594. No significant floor or ceiling effects were detected. CONCLUSIONS: The CTM was developed with substantial input from older patients and their caregivers. Psychometric testing suggested that the measure was valid. The CTM may serve to fill an important gap in health system performance evaluation by measuring the quality of care delivered across settings

Exponentiation of the Drell-Yan cross section near partonic threshold in the DIS and MSbar schemes

It has been observed that in the DIS scheme the refactorization of the Drell-Yan cross section leading to exponentiation of threshold logarithms can also be used to organize a class of constant terms, most of which arise from the ratio of the timelike Sudakov form factor to its spacelike counterpart. We extend this exponentiation to include all constant terms, and demonstrate how a similar organization may be achieved in the MSbar scheme. We study the relevance of these exponentiations in a two-loop analysis.Comment: 20 pages, JHEP style, no figure

Universal Power Law in the Noise from a Crumpled Elastic Sheet

Using high-resolution digital recordings, we study the crackling sound emitted from crumpled sheets of mylar as they are strained. These sheets possess many of the qualitative features of traditional disordered systems including frustration and discrete memory. The sound can be resolved into discrete clicks, emitted during rapid changes in the rough conformation of the sheet. Observed click energies range over six orders of magnitude. The measured energy autocorrelation function for the sound is consistent with a stretched exponential C(t) ~ exp(-(t/T)^{b}) with b = .35. The probability distribution of click energies has a power law regime p(E) ~ E^{-a} where a = 1. We find the same power law for a variety of sheet sizes and materials, suggesting that this p(E) is universal.Comment: 5 pages (revtex), 10 uuencoded postscript figures appended, html version at http://rainbow.uchicago.edu/~krame

A 2.1 Solar Mass Pulsar Measured by Relativistic Orbital Decay

PSR J0751+1807 is a millisecond pulsar in a circular 6 hr binary system with a helium white dwarf secondary. Through high precision pulse timing measurements with the Arecibo and Effelsberg radio telescopes, we have detected the decay of its orbit due to emission of gravitational radiation. This is the first detection of the relativistic orbital decay of a low-mass, circular binary pulsar system. The measured rate of change in orbital period, corrected for acceleration biases, is dP_b/dt=(-6.4+-0.9)x10^-14. Interpreted in the context of general relativity, and combined with measurement of Shapiro delay, it implies a pulsar mass of 2.1+-0.2 solar masses, the most massive pulsar measured. This adds to the emerging trend toward relatively high neutron star masses in neutron star--white dwarf binaries. Additionally, there is some evidence for an inverse correlation between pulsar mass and orbital period in these systems. We consider alternatives to the general relativistic analysis of the data, and we use the pulsar timing data to place limits on violations of the strong equivalence principle.Comment: 9 pages, Submitted to Ap