Photodegradation actuated shape-changing hydrogels

Hydrogels are attractive materials for generating 4D shapes due to their ability to undergo pronounced volume changes in response to several stimuli, including light. We previously reported shape-changing hydrogels actuated by long-wave UV and visible light in the presence of live cells using poly(ethylene glycol) macromers incorporating different photodegradable ortho-nitrobenzyl (o-NB) groups. In this comprehensive study, we determine the effect of chemical structure of different o-NB macromers (which influences molar absorptivity and rate constant of degradation), composition (macromer weight percent), fabrication design (initial gel thickness) and environment (ionic strength of solution) on light-induced hydrogel folding. We demonstrate successful photopolymerization and subsequent photodegradation of hydrogels, multistep folding, and live-cell encapsulation. This hydrogel system may be useful as new tool in stem cell differentiation and developmental biology research, facilitating the in vitro investigation of processes that are sensitive to both physical and temporal stimuli.     

Nonreciprocal and directional wave propagation in a two-dimensional lattice with bilinear properties

Nonlinear Dynamics - A passive method of realizing nonreciprocal wave propagation in a two-dimensional (2D) lattice is proposed, using bilinear springs combined with the necessary spatial asymmetry...     

Pseudorapidity distributions of charged particles from Au + Au collisions at the maximum RHIC energy,square root[s(NN)] = 200 GeV

We present charged-particle multiplicities as a function of pseudorapidity and collision centrality for the 197Au+197Au reaction at square root[s(NN)] = 200 GeV. For the 5% most central events we obtain dN(ch)/deta/(eta = 0) = 625+/-55 and N(ch)/(-4.7< or =eta < or =4.7) = 4630 +/- 370, i.e., 14% and 21% increases, respectively, relative to square root[s(NN)] = 130 GeV collisions. Charged-particle production per pair of participant nucleons is found to increase from peripheral to central collisions around midrapidity. These results constrain current models of particle production at the highest RHIC energy.     

Optimal orientation of anisotropic solids

Results are presented for finding the optimal orientation ofan anisotropic elastic material. The problem is formulated asminimizing the strain energy subject to rotation of the materialaxes, under a state of uniform stress. It is shown that a stationaryvalue of the strain energy requires the stress and strain tensorsto have a common set of principal axes. The new derivation ofthis well-known coaxiality condition uses the six-dimensionalexpression of the rotation tensor for the elastic moduli. Usingthis representation it is shown that the stationary conditionis a minimum or a maximum if an explicit set of conditions issatisfied. Specific results are given for materials of cubic,transversely isotropic (TI) and tetragonal symmetries. In eachcase the existence of a minimum or maximum depends on the signof a single elastic constant. The stationary (minimum or maximum)value of energy can always be achieved for cubic materials.Typically, the optimal orientation of a solid with cubic materialsymmetry is not aligned with the symmetry directions. Expressionsare given for the optimal orientation of TI and tetragonal materials,and are in agreement with results of Rovati and Taliercio obtainedby a different procedure. A new concept is introduced: the straindeviation angle, which defines the degree to which a state ofstress or strain is not optimal. The strain deviation angleis zero for coaxial stress and strain. An approximate formulais given for the strain deviation angle which is valid for materialsthat are weakly anisotropic.     

Incidence of apparent restricted diffusion in three different models of cerebral infarction

High speed magnetic resonance imaging (MRI) and short diffusion times are used to investigate the appearance of restricted diffusion in three different models of cerebral infarction. The models are: the middle cerebral artery occlusion (MCAO) model in the rat, the carotid occlusion model in the gerbil, and the Rose Bengal microvascular occlusion model in the rat. All three were investigated for 16 b-values equally spaced between 10 and 1510 s/mm² using two distinct experiments. In the ct (constant time) experiment, the diffusion time was held constant at 11.7 ms while the b-value was varied with the gradient strength. In the cg (constant gradient) experiment, the gradient strength was held constant and the b-value increased by varying the diffusion time from 4.4 to 11.7 ms. A monoexponential decay of the signal intensity with b-value in the ct experiment accompanied by nonmonoexponential (NME) decay in the cg experiment is indicative of restricted diffusion. As this phenomenon is detectable only at short diffusion times, it cannot be due to restriction by impermeable membranes, and we have thus termed this apparent restriction. For the MCAO model and the carotid occlusion model, apparent restriction was found both inside the infarct territory and in some regions outside it. No definite evidence for restriction was found for the Rose Bengal model, which was, however, only studied from 24 h post-insult.     

A Flexible, Fast, and Optimal Modeling Approach Applied to Crew Rostering at London Underground

We present a general modeling approach to crew rostering and its application to computer-assisted generation of rotation-based rosters (or rotas) at the London Underground. Our goals were flexibility, speed, and optimality, and our approach is unique in that it achieves all three. Flexibility was important because requirements at the Underground are evolving and because specialized approaches in the literature did not meet our flexibility-implied need to use standard solvers. We decompose crew rostering into stages that can each be solved with a standard commercial MILP solver. Using a 167 MHz Sun UltraSparc 1 and CPLEX 4.0 MILP solver, we obtained high-quality rosters in runtimes ranging from a few seconds to a few minutes within 2% of optimality. Input data were takes from different depots with crew sizes ranging from 30–150 drivers, i.e., with number of duties ranging from about 200–1000. Using an argument based on decomposition and aggregation, we prove the optimality of our approach for the overall crew rostering problem.     

The energy of a growing elastic surface

The potential energy of the elastic surface of an elastic body which is growing by the coherent addition of material is derived. Several equivalent expressions are presented for the energy required to add a single atom, also known as the chemical potential. The simplest involves the Eshelby stress tensors for the bulk medium and for the surface. Dual Lagrangian/Eulerian expressions are obtained which are formally similar to each other. The analysis employs two distinct types of variations to derive the governing bulk and surface equations for an accreting elastic solid. The total energy of the system is assumed to comprise bulk and surface energies, while the presence of an external medium can be taken into account through an applied surface forcing. A detailed account is given of the various formulations possible in material and current coordinates, using four types of bulk and surface stresses: the Piola-Kirchhoff stress, the Cauchy stress, the Eshelby stress and a fourth, called the nominal energy-momentum stress. It is shown that inhomogeneity surface forces arise naturally if the surface energy density is allowed to be position dependent.     

Reactions of several monosaccharide-derived alcohols with p-acetamidobenzenesulfonyl azide and DBU

Attempted diazo transfer to 1-O-(2-phenylacetyl)-2,3;5,6-di-O-isopropylidene-α-d-mannofuranose using p-acetamidobenzenesulfonyl azide (p-ABSA) and DBU as base affords 1-O-(2-diazo-2-phenylacetyl)-2,3;5,6-di-O-isopropylidene-α-d-mannofuranose in low yield along with 2,3;5,6-di-O-isopropylidene-α-d-mannofuranose, 1-azido-2,3;5,6-di-O-isopropylidene-β-d-mannofuranose, as well as the unreacted starting material. The azido sugar likely arises from α-mannofuranosyl sulfonate ester formation, through displacement of azide from p-ABSA by the sugar lactol, followed by stereospecific displacement by azide anion on the furanosyl sulfonate ester. This outcome has been studied further with the conditions being applied to several common monosaccharide derivatives. Accessible substrates afford the azido sugar in an overall one-pot alcohol-to-azide conversion, while hindered substrates yield the sulfonate esters.