Steven A. Clarke

Long-term outcomes of isolated liver transplantation for short bowel syndrome and intestinal failure-associated liver disease.

Background and Aim: A select group of children with short bowel syndrome (SBS) and intestinal failure–associated liver disease (IFALD) fulfill the criteria for isolated liver transplantation (iLTx). Long-term results in this group of patients have not been reported. Methods: A retrospective study of the medical records of 8 survivors of 14 children who underwent iLTx for SBS and IFALD from 1998 to 2005, managed by a multidisciplinary intestinal rehabilitation team at our institution. Results: Median follow-up is 107.5 months (range 89–153 months). Five of 8 children were weaned from parenteral nutrition (PN) to enteral nutrition (EN) in a median of 10 months after iLTx (range 3–32 months). Three of 5 children were subsequently weaned from EN to full oral feeding in 13, 24, and 53 months after stopping PN, whereas the remaining 2 are still receiving EN 118 and 74 months after stopping PN. These 5 children maintain their weight median z scores with a median increase of 1.59 (range 1.24–1.79) compared with the pretransplant z score, whereas the height z scores show fluctuations through the years with a median change of 0.12 (range −0.29 to 0.36). The other 3 of 8 children developed progressive intestinal failure; 2 underwent isolated small bowel transplantation 112 and 84 months after iLTx and the third is receiving PN. Conclusions: Children with SBS and IFALD who have the potential for adaptation in the residual bowel can undergo iLTx, but it is a treatment option to be exercised with extreme caution. These children need close follow-up with an experienced multidisciplinary team to monitor nutritional outcomes and may need consideration for transplant or nontransplant surgery in the long term.

Measurements of the DDT Process in Exploding Bridgewire Detonators

The deflagration‐to‐detonation transition (DDT) of low density (0.88 g/cc) PETN during exploding bridgewire (EBW) initiation has been studied using laser interferometry and streak photography. Cutback experiments using VISAR have confirmed a 1.0 mm run‐distance to detonation in this low density PETN powder. In a detonation system using a combination of low and high density powders, an apparent center of initiation (COI) analysis of streak data has yielded a surprisingly similar result. This data suggested that a compaction of low density powder to near theoretical maximum density (TMD) may occur before the onset of detonation, which is consistent with work done previously. These experiments show this is not the case and COI analysis reveals a non‐ideal initial propagation front. Additionally, data show that although function time increases significantly with decreasing firing voltage, the apparent COI changes very little. This indicates that the detonation criterion is not dependent upon the rate of deflagration, but on a volume of material that must be burned in a confined space to create the critical pressure needed at the compaction front.

Simultaneous photonic doppler velocimetry and ultra-high speed imaging techniques to characterize the pressure output of detonators

Detonator output directed into both ambient air and polymethylmethacrylate (PMMA) windows is simultaneously investigated using ultra-high speed, time-resolved imaging and photonic Doppler velocimetry (PDV) measurements. In air, one-dimensional measurements of detonator cup position are made from timeresolved image sequences and compared to time-integrated velocity curves obtained from the PDV data. The results demonstrate good agreement that validates using the two methods concurrently to measure the motion of the detonator free-surface. In PMMA windows, instantaneous shock velocities are calculated from 1-D time-resolved measurements of shock position and known velocity-Hugoniot data are utilized to map the shock velocity calculations to corresponding values of mass velocity and shock pressure. Simultaneous PDV data describing the motion of the detonator cup/PMMA interface are used to determine the mass velocity and pressure at the interface, and to compare to the mass and shock pressures calculated from...

On the Initiation Mechanism in Exploding Bridgewire and Laser Detonators

Since its invention by Los Alamos during the Manhattan Project era the exploding bridgewire detonator (EBW) has seen tremendous use and study. Recent development of a laser‐powered device with detonation properties similar to an EBW is reviving interest in the basic physics of the deflagration‐to‐detonation (DDT) process in both of these devices. Cutback experiments using both laser interferometry and streak camera observations are providing new insight into the initiation mechanism in EBWs. These measurements are being correlated to a DDT model of compaction to detonation and shock to detonation developed previously by Xu and Stewart. The DDT model is incorporated into a high‐resolution, multi‐material model code for simulating the complete process. Model formulation and the modeling issues required to describe the test data will be discussed.

On the development of a laser detonator

The initiation of explosives by laser illumination has been known for many years. In this paper we will discuss the development of a working detonator design that reduces the energy required for detonation in a low-density secondary explosive by vaporizing a thin metal coating. We present data on the development of the design for a workhorse laser detonator that provides enhanced safety over existing exploding bridgewire detonators (EBWs). Comparison of this laser initiated data to an exploding-bridgewire (EBW) provides insight into the mechanism of initiation of detonation in low-density PETN by the plasma source. A novel diagnostic technique to determine the run-distance to detonation also known as the apparent Center-of-Initiation (COI) will also be discussed.

USING SCHLIEREN VISUALIZATION TO TRACK DETONATOR PERFORMANCE

Several experiments will be presented that are part of a phased plan to understand the evolution of detonation in a detonator from initiation shock through run to detonation, to full detonation, to transition, to booster and booster detonation. High‐speed multiframe schlieren imagery has been used to study several explosive initiation events, such as exploding bridgewires (EBWs), exploding foil initiators (EFIs or “slappers”), direct optical initiation (DOI), and electrostatic discharge. Additionally, a series of tests has been performed on “cut‐back” detonators with varying initial pressing heights. We have also used this diagnostic to visualize a range of EBW, EFI, and DOI full‐up detonators. Future applications to other explosive events, such as boosters and insensitive high explosives booster evaluation, will be discussed. The EPIC finite element code has been used to analyze the shock fronts from the schlieren images to solve iteratively for consistent boundary or initial conditions to determine the ...

Plasmonic enhancement of direct optical initiation of explosives

Current Direct Optical Initiation (DOI) detonators use a laser focused onto a thin metal layer to drive a hot plasma and/or fragments into PETN powder. Previous studies showed a dramatic decrease in laser energies required to initiate the detonation using this approach over direct laser illumination of the PETN powder. Plasmonic metal nanostructures have been shown capable of strongly coupling laser energy into adjacent materials. We have incorporated gold nanospheres into PETN powder and are investigating their plasmonic enhancement of direct optical initiation via measurements of threshold laser energies and streak camera measurements for calculation of run to detonation distances compared to other DOI schemes.

Nanoscale topography of dynamic surfaces with ultrafast time resolution

We describe an optical system for detecting the movement of a surface with subnanosecond temporal and nanometer vertical displacement resolution. The system is fielded on an experiment to determine the distortion of a laser-ablated metal layer and compare the results with hydrodynamic simulations. We also discuss errors that can arise and potential means to mitigate them. The resultant data show one can examine dynamic changes to a reflective surface with accuracy down to tens of nanometers at hundreds of picoseconds time resolution.

Optical initiation spot size effects in low-density PETN

Los Alamos National Laboratory is currently designing a series of direct optically initiated (DOI) detonators. The primary purpose of this series of detonators is to achieve a level of safety in the face of unintentional initiation from an electrical source. The purpose of these experiments is to determine the minimum spotsize that will initiate the low density initial pressing in these laser detonators. With this information it is expected that a more robust optically initiated detonator can be designed and manufactured. Results from a series of experiments will be discussed. First a range of small core diameter fiber optics with varying energy injection levels will be tested to find the minimum energy level necessary to achieve reliable initiation. Second, a range of apertures will be employed to trim the spotsize down to a minimum size that will still maintain reliable initiation. This information will help to understand whether the initiation criteria for the DOI Laser Detonator are dominated by energy density, total energy or a combination of these criteria.

Laser initiation of iron-based photoactive explosives

Photoactive explosives show promise to be relatively insensitive to impact and friction compared to pentaerythritol tetranitrate (PETN) and other detonator materials, but can be more easily initiated with laser light. Metal-ligand charge transfer (MLCT) complexes have been previously shown to have tunable explosive properties and absorption profiles, making them strong candidates for laser detonator materials. Here, we provide a summary of the synthesis and characterization of multiple iron-based MLCT complexes, as well as results from recent efforts to initiate detonation in a high-density PETN output pellet. Paths forward will be discussed.Photoactive explosives show promise to be relatively insensitive to impact and friction compared to pentaerythritol tetranitrate (PETN) and other detonator materials, but can be more easily initiated with laser light. Metal-ligand charge transfer (MLCT) complexes have been previously shown to have tunable explosive properties and absorption profiles, making them strong candidates for laser detonator materials. Here, we provide a summary of the synthesis and characterization of multiple iron-based MLCT complexes, as well as results from recent efforts to initiate detonation in a high-density PETN output pellet. Paths forward will be discussed.