Polymer translocation through a nanopore under an applied external field

We investigate the dynamics of polymer translocation through a nanopore under an externally applied field using the 2D fluctuating bond model with single-segment Monte Carlo moves. We concentrate on the influence of the field strength $E$, length of the chain $N$, and length of the pore $L$ on forced translocation. As our main result, we find a crossover scaling for the translocation time $\tau$ with the chain length from $\tau \sim N^{2\nu}$ for relatively short polymers to $\tau \sim N^{1 + \nu}$ for longer chains, where $\nu$ is the Flory exponent. We demonstrate that this crossover is due to the change in the dependence of the translocation velocity v on the chain length. For relatively short chains $v \sim N^{- \nu}$, which crosses over to $v \sim N^{- 1}$ for long polymers. The reason for this is that with increasing $N$ there is a high density of segments near the exit of the pore, which slows down the translocation process due to slow relaxation of the chain. For the case of a long nanopore for which $R_\parallel$, the radius of gyration $R_{g}$ along the pore, is smaller than the pore length, we find no clear scaling of the translocation time with the chain length. For large $N$, however, the asymptotic scaling $\tau \sim N^{1 + \nu}$ is recovered. In this regime, $\tau$ is almost independent of $L$. We have previously found that for a polymer, which is initially placed in the middle of the pore, there is a minimum in the escape time for $R_\parallel \approx L$. We show here that this minimum persists for a weak fields $E$ such that $EL$ is less than some critical value, but vanishes for large values of $EL$.Comment: 25 Pages, 10 figures. Submitted to J. Chem. Phys. J. Chem. Phys. 124, in press (2006

Polymer translocation out of confined environments

We consider the dynamics of polymer translocation out of confined environments. Analytic scaling arguments lead to the prediction that the translocation time scales like $\tau\sim N^{\beta+\nu_{2D}}R^{1+(1-\nu_{2D})/\nu}$ for translocation out of a planar confinement between two walls with separation $R$ into a 3D environment, and $\tau \sim N^{\beta+1}R$ for translocation out of two strips with separation $R$ into a 2D environment. Here, $N$ is the chain length, $\nu$ and $\nu_{2D}$ are the Flory exponents in 3D and 2D, and $\beta$ is the scaling exponent of translocation velocity with $N$, whose value for the present choice of parameters is $\beta \approx 0.8$ based on Langevin dynamics simulations. These scaling exponents improve on earlier predictions.Comment: 5 pages, 5 figures. To appear in Phys. Rev.

Polymer translocation through a nanopore: a two-dimensional Monte Carlo simulation

We investigate the problem of polymer translocation through a nanopore in the absence of an external driving force. To this end, we use the two-dimensional (2D) fluctuating bond model with single-segment Monte Carlo moves. To overcome the entropic barrier without artificial restrictions, we consider a polymer which is initially placed in the middle of the pore, and study the escape time required for the polymer to completely exit the pore on either end. In particular, we examined the effect of the pore length on the escape time.Comment: 16Pages, 6 figure

Evaluation of prognostic risk models for postoperative pulmonary complications in adult patients undergoing major abdominal surgery: a systematic review and international external validation cohort study

Background Stratifying risk of postoperative pulmonary complications after major abdominal surgery allows clinicians to modify risk through targeted interventions and enhanced monitoring. In this study, we aimed to identify and validate prognostic models against a new consensus definition of postoperative pulmonary complications. Methods We did a systematic review and international external validation cohort study. The systematic review was done in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched MEDLINE and Embase on March 1, 2020, for articles published in English that reported on risk prediction models for postoperative pulmonary complications following abdominal surgery. External validation of existing models was done within a prospective international cohort study of adult patients (≥18 years) undergoing major abdominal surgery. Data were collected between Jan 1, 2019, and April 30, 2019, in the UK, Ireland, and Australia. Discriminative ability and prognostic accuracy summary statistics were compared between models for the 30-day postoperative pulmonary complication rate as defined by the Standardised Endpoints in Perioperative Medicine Core Outcome Measures in Perioperative and Anaesthetic Care (StEP-COMPAC). Model performance was compared using the area under the receiver operating characteristic curve (AUROCC). Findings In total, we identified 2903 records from our literature search; of which, 2514 (86·6%) unique records were screened, 121 (4·8%) of 2514 full texts were assessed for eligibility, and 29 unique prognostic models were identified. Nine (31·0%) of 29 models had score development reported only, 19 (65·5%) had undergone internal validation, and only four (13·8%) had been externally validated. Data to validate six eligible models were collected in the international external validation cohort study. Data from 11 591 patients were available, with an overall postoperative pulmonary complication rate of 7·8% (n=903). None of the six models showed good discrimination (defined as AUROCC ≥0·70) for identifying postoperative pulmonary complications, with the Assess Respiratory Risk in Surgical Patients in Catalonia score showing the best discrimination (AUROCC 0·700 [95% CI 0·683–0·717]). Interpretation In the pre-COVID-19 pandemic data, variability in the risk of pulmonary complications (StEP-COMPAC definition) following major abdominal surgery was poorly described by existing prognostication tools. To improve surgical safety during the COVID-19 pandemic recovery and beyond, novel risk stratification tools are required. Funding British Journal of Surgery Society

Crypto-Securities Regulation: ICOs, Token Sales and Cryptocurrencies under EU Financial Law

Cryptocurrencies, such as bitcoin and ethereum, have not only risen to public attention as novel means of payments, but also as facilitators of initial coin offerings (ICOs, also called token sales). In these entirely online-mediated offerings, entrepreneurs sell tokens registered on a blockchain in exchange for cryptocoins. Buyers receive tokens that can be understood as cryptographically-secured coupons which embody a bundle of rights and obligations. In July 2017, the SEC released an investigative report that highlighted that such tokens can be subject to the full scope of US securities regulation. It is unclear, however, to what extent EU securities regulation is applicable to ICOs and, particularly, whether issuers have to publish and register a prospectus in order to avoid criminal and civil prospectus liability in the EU. In conceptual terms, this depends on whether tokens are considered “securities” under the EU prospectus regulation regime. Against this background, this paper develops a nuanced approach that distinguishes between three archetypes of tokens: currency, investment, and utility tokens. It analyzes the differential implications of each of these types, and their hybrid forms, for EU securities regulation, and develops policy proposals for their regulation.Cryptocurrencies, such as bitcoin and ethereum, have not only risen to public attention as novel means of payments, but also as facilitators of initial coin offerings (ICOs, also called token sales). In these entirely online-mediated offerings, entrepreneurs sell tokens registered on a blockchain in exchange for cryptocoins. Buyers receive tokens that can be understood as cryptographically-secured coupons which embody a bundle of rights and obligations. In July 2017, the SEC released an investigative report that highlighted that such tokens can be subject to the full scope of US securities regulation. It is unclear, however, to what extent EU securities regulation is applicable to ICOs and, particularly, whether issuers have to publish and register a prospectus in order to avoid criminal and civil prospectus liability in the EU. In conceptual terms, this depends on whether tokens are considered “securities” under the EU prospectus regulation regime. Against this background, this paper develops a nuanced approach that distinguishes between three archetypes of tokens: currency, investment, and utility tokens. It analyzes the differential implications of each of these types, and their hybrid forms, for EU securities regulation, and develops policy proposals for their regulation