The Engineering of Biology and Medicine

In celebration of the 100th Anniversary of PNAS, this Special Feature summarizes the enormous progress that has been made in the engineering of biology and medicine

Ginzburg-Landau theory of crystalline anisotropy for bcc-liquid interfaces

The weak anisotropy of the interfacial free-energy $\gamma$ is a crucial parameter influencing dendritic crystal growth morphologies in systems with atomically rough solid-liquid interfaces. The physical origin and quantitative prediction of this anisotropy are investigated for body-centered-cubic (bcc) forming systems using a Ginzburg-Landau theory where the order parameters are the amplitudes of density waves corresponding to principal reciprocal lattice vectors. We find that this theory predicts the correct sign, $\gamma_{100}>\gamma_{110}$, and magnitude, $(\gamma_{100}-\gamma_{110}) / (\gamma_{100}+\gamma_{110})\approx 1$, of this anisotropy in good agreement with the results of MD simulations for Fe. The results show that the directional dependence of the rate of spatial decay of solid density waves into the liquid, imposed by the crystal structure, is a main determinant of anisotropy. This directional dependence is validated by MD computations of density wave profiles for different reciprocal lattice vectors for $\{110\}$ crystal faces. Our results are contrasted with the prediction of the reverse ordering $\gamma_{100}<\gamma_{110}$ from an earlier formulation of Ginzburg-Landau theory [Shih \emph{et al.}, Phys. Rev. A {\bf 35}, 2611 (1987)].Comment: 9 pages, 5 figure

Analysis of the multidimensionality of hallucination-like experiences in clinical and nonclinical Spanish samples and their relation to clinical symptoms: Implications for the model of continuity

Numerous studies have found that hallucinatory experiences occur in the general population. But to date, few studies have been conducted to compare clinical and nonclinical groups across a broad array of clinical symptoms that may co-occur with hallucinations. Likewise, hallucination-like experiences are measured as a multidimensional construct, with clinical and subclinical components related to vivid daydreams, intrusive thoughts, perceptual disturbance, and clinical hallucinatory experiences. Nevertheless, these individual subcomponents have not been examined across a broad spectrum of clinically disordered and nonclinical groups. The goal of the present study was to analyze the differences and similarities in the distribution of responses to hallucination-like experience in clinical and nonclinical populations and to determine the relation of these hallucination-like experiences with various clinical symptoms. These groups included patients with schizophrenia, non-psychotic clinically disordered patients, and a group of individuals with no psychiatric diagnoses. The results revealed that hallucination-like experiences are related to various clinical symptoms across diverse groups of individuals. Regression analysis found that the Psychoticism dimension of the Symptom Check List (SCL-90-R) was the most important predictor of hallucination-like experiences. Additionally, increased auditory and visual hallucination was the only subcomponent that differentiated schizophrenic patients from other groups. This distribution of responses in the dimensions of hallucination-like experiences suggests that not all the dimensions are characteristic of people hearing voices. Vivid daydreams, intrusive thoughts, and auditory distortions and visual perceptual distortions may represent a state of general vulnerability that does not denote a specific risk for clinical hallucinations. Overall, these results support the notion that hallucination-like experiences are closer to a quasi-continuum approach and that total scores on these scales explain a state of vulnerability to general perceptual disturbance

The lower boundary of the accretion column in magnetic cataclysmic variables

Using a parameterised function for the mass loss at the base of the post-shock region, we have constructed a formulation for magnetically confined accretion flows which avoids singularities, such as the infinity in density, at the base associated with all previous formulations. With the further inclusion of a term allowing for the heat input into the base from the accreting white dwarf we are able also to obtain the hydrodynamic variables to match the conditions in the stellar atmosphere. (We do not, however, carry out a mutually consistent analysis for the match). Changes to the emitted X-ray spectra are negligible unless the thickness of mass leakage region at the base approaches or exceeds one percent of the height of the post-shock region. In this case the predicted spectra from higher-mass white dwarfs will be harder, and fits to X-ray data will predict lower white-dwarf masses than previous formulations.Comment: 13 pages, 6 figures, accepted for publication in MNRA

Radiatively-Induced First-Order Phase Transitions: The Necessity of the Renormalization Group

We advocate a (Wilson) renormalization-group (RG) treatment of finite-temperature first-order phase transitions, in particular those driven by radiative corrections such as occur in the standard model, and other spontaneously-broken gauge theories. We introduce the scale-dependent coarse-grained free energy S_\La[\phi] which we explicitly calculate, using the Wilson RG and a (4-\ep)-expansion, for a scalar toy model that shares many features of the gauged case. As argued by Langer and others, the dynamics of the phase transition are described by S_\La[\phi] with \La of order the bubble wall thickness, and {\it not} by the usual (RG-improved) finite-temperature effective action which is reproduced by S_\La[\phi] for \La\to 0. We argue that for weakly first-order transitions (such as that in the standard model) the (4-\ep)-expansion is necessary to control an inevitable growth of the effective scale-dependent coupling towards the strong-coupling regime, and that diagrammatic resummation techniques are unlikely to be appropriate.Comment: 26 pages, two figures, LaTex macropackage. References added and appendix A revised. LBL preprint LBL-3457

Metastability in Two Dimensions and the Effective Potential

We study analytically and numerically the decay of a metastable phase in (2+1)-dimensional classical scalar field theory coupled to a heat bath, which is equivalent to two-dimensional Euclidean quantum field theory at zero temperature. By a numerical simulation we obtain the nucleation barrier as a function of the parameters of the potential, and compare it to the theoretical prediction from the bounce (critical bubble) calculation. We find the nucleation barrier to be accurately predicted by theory using the bounce configuration obtained from the tree-level (``classical'') effective action. Within the range of parameters probed, we found that using the bounce derived from the one-loop effective action requires an unnaturally large prefactor to match the lattice results. Deviations from the tree-level prediction are seen in the regime where loop corrections would be expected to become important.Comment: 13pp, LaTex with Postscript figs, CLNS 93/1202, DART-HEP-93/0

New fields on super Riemann surfaces

A new $(1,1)$-dimensional super vector bundle which exists on any super Riemann surface is described. Cross-sections of this bundle provide a new class of fields on a super Riemann surface which closely resemble holomorphic functions on a super Riemann surface, but which (in contrast to the case with holomorphic functions) form spaces which have a well defined dimension which does not change as odd moduli become non-zero.Comment: 12pp, kcl-th-94-

A new model for in vitro testing of vitreous substitute candidates

Purpose: To describe a new model for in vitro assessment of novel vitreous substitute candidates. Methods: The biological impact of three vitreous substitute candidates was explored in a retinal explant culture model; a polyalkylimide hydrogel (Bio-Alcamid®), a two component hydrogel of 20 wt.% poly (ethylene glycol) in phosphate buffered saline (PEG) and a cross-linked sodium hyaluronic acid hydrogel (Healaflow®). The gels where applied to explanted adult rat retinas and then kept in culture for 2, 5 and 10 days. Gel-exposed explants were compared with explants incubated under standard tissue culture conditions. Cryosections of the specimens were stained with hematoxylin and eosin, immunohistochemical markers (GFAP, Vimentin, Neurofilament 160, PKC, Rhodopsin) and TUNEL. Results: Explants kept under standard conditions as well as PEG-exposed explants displayed disruption of retinal layers with moderate pyknosis of all neurons. They also displayed moderate labeling of apoptotic cells. Bio-Alcamid®-exposed explants displayed severe thinning and disruption of retinal layers with massive cell death. Healaflow®-treated explants displayed normal retinal lamination with significantly better preservation of retinal neurons compared with control specimens, and almost no signs of apoptosis. Retinas exposed to Healaflow® and retinas kept under standard conditions showed variable labeling of GFAP with generally low expression and some areas of upregulation. PEG-exposed retinas showed increased GFAP labeling and Bio-Alcamid®-exposed retinas showed sparse labeling of GFAP. Conclusions Research into novel vitreous substitutes has important implications for both medical and surgical vitreoretinal disease. The in vitro model presented here provides a method of biocompatibility testing prior to more costly and cumbersome in vivo experiments. The explant culture system imposes reactions within the retina including disruption of layers, cell death and gliosis, and the progression of these reactions can be used for comparison of vitreous substitute candidates. Bio-Alcamid® had strong adverse effects on the retina which is consistent with results of prior in vivo trials. PEG gel elicits reactions similar to the control retinas whereas Healaflow® shows protection from culture-induced trauma indicating favorable biocompatibility.Swedish Research CouncilUniversity of Lund. Medical FacultyPrincess Margaret's Foundation for Blind ChildrenKnut and Alice Wallenberg FoundationGeneral Sir John Monash Foundation (Scholarship)In Vivo Therapeutics Corporatio