PMCC PMCC

Search tips
Search criteria

Advanced
Results 1-25 (1136384)

Clipboard (0)
None

Related Articles

1.  Channeling in helium ion microscopy: Mapping of crystal orientation 
Summary
Background: The unique surface sensitivity and the high resolution that can be achieved with helium ion microscopy make it a competitive technique for modern materials characterization. As in other techniques that make use of a charged particle beam, channeling through the crystal structure of the bulk of the material can occur.
Results: Here, we demonstrate how this bulk phenomenon affects secondary electron images that predominantly contain surface information. In addition, we will show how it can be used to obtain crystallographic information. We will discuss the origin of channeling contrast in secondary electron images, illustrate this with experiments, and develop a simple geometric model to predict channeling maxima.
Conclusion: Channeling plays an important role in helium ion microscopy and has to be taken into account when trying to achieve maximum image quality in backscattered helium images as well as secondary electron images. Secondary electron images can be used to extract crystallographic information from bulk samples as well as from thin surface layers, in a straightforward manner.
doi:10.3762/bjnano.3.57
PMCID: PMC3458594  PMID: 23019544
channeling; crystallography; helium ion microscopy; ion scattering
2.  Fabrication of carbon nanomembranes by helium ion beam lithography 
Summary
The irradiation-induced cross-linking of aromatic self-assembled monolayers (SAMs) is a universal method for the fabrication of ultrathin carbon nanomembranes (CNMs). Here we demonstrate the cross-linking of aromatic SAMs due to exposure to helium ions. The distinction of cross-linked from non-cross-linked regions in the SAM was facilitated by transferring the irradiated SAM to a new substrate, which allowed for an ex situ observation of the cross-linking process by helium ion microscopy (HIM). In this way, three growth regimes of cross-linked areas were identified: formation of nuclei, one-dimensional (1D) and two-dimensional (2D) growth. The evaluation of the corresponding HIM images revealed the dose-dependent coverage, i.e., the relative monolayer area, whose density of cross-links surpassed a certain threshold value, as a function of the exposure dose. A complete cross-linking of aromatic SAMs by He+ ion irradiation requires an exposure dose of about 850 µC/cm2, which is roughly 60 times smaller than the corresponding electron irradiation dose. Most likely, this is due to the energy distribution of secondary electrons shifted to lower energies, which results in a more efficient dissociative electron attachment (DEA) process.
doi:10.3762/bjnano.5.20
PMCID: PMC3943867  PMID: 24605285
carbon nanomembranes; dissociative electron attachment; helium ion microscopy; ion beam-organic molecules interactions; self-assembled monolayers
3.  Helium ion microscopy of enamel crystallites and extracellular tooth enamel matrix 
An unresolved problem in tooth enamel studies has been to analyze simultaneously and with sufficient spatial resolution both mineral and organic phases in their three dimensional (3D) organization in a given specimen. This study aims to address this need using high-resolution imaging to analyze the 3D structural organization of the enamel matrix, especially amelogenin, in relation to forming enamel crystals. Chemically fixed hemi-mandibles from wild type mice were embedded in LR White acrylic resin, polished and briefly etched to expose the organic matrix in developing tooth enamel. Full-length amelogenin was labeled with specific antibodies and 10 nm immuno-gold. This allowed us to use and compare two different high-resolution imaging techniques for the analysis of uncoated samples. Helium ion microscopy (HIM) was applied to study the spatial organization of organic and mineral structures, while field emission scanning electron microscopy (FE-SEM) in various modes, including backscattered electron detection, allowed us to discern the gold-labeled proteins. Wild type enamel in late secretory to early maturation stage reveals adjacent to ameloblasts a lengthwise parallel alignment of the enamel matrix proteins, including full-length amelogenin proteins, which then transitions into a more heterogeneous appearance with increasing distance from the mineralization front. The matrix adjacent to crystal bundles forms a smooth and lacey sheath, whereas between enamel prisms it is organized into spherical components that are interspersed with rod-shaped protein. These findings highlight first, that the heterogeneous organization of the enamel matrix can be visualized in mineralized en bloc samples. Second, our results illustrate that the combination of these techniques is a powerful approach to elucidate the 3D structural organization of organic matrix molecules in mineralizing tissue in nanometer resolution.
doi:10.3389/fphys.2014.00395
PMCID: PMC4193210  PMID: 25346697
tooth enamel; matrix organization; amelogenin; immuno-gold labeling; helium ion microscopy; high-resolution microscopy
4.  Cell surface and cell outline imaging in plant tissues using the backscattered electron detector in a variable pressure scanning electron microscope 
Plant Methods  2013;9:40.
Background
Scanning electron microscopy (SEM) has been used for high-resolution imaging of plant cell surfaces for many decades. Most SEM imaging employs the secondary electron detector under high vacuum to provide pseudo-3D images of plant organs and especially of surface structures such as trichomes and stomatal guard cells; these samples generally have to be metal-coated to avoid charging artefacts. Variable pressure-SEM allows examination of uncoated tissues, and provides a flexible range of options for imaging, either with a secondary electron detector or backscattered electron detector. In one application, we used the backscattered electron detector under low vacuum conditions to collect images of uncoated barley leaf tissue followed by simple quantification of cell areas.
Results
Here, we outline methods for backscattered electron imaging of a variety of plant tissues with particular focus on collecting images for quantification of cell size and shape. We demonstrate the advantages of this technique over other methods to obtain high contrast cell outlines, and define a set of parameters for imaging Arabidopsis thaliana leaf epidermal cells together with a simple image analysis protocol. We also show how to vary parameters such as accelerating voltage and chamber pressure to optimise imaging in a range of other plant tissues.
Conclusions
Backscattered electron imaging of uncoated plant tissue allows acquisition of images showing details of plant morphology together with images of high contrast cell outlines suitable for semi-automated image analysis. The method is easily adaptable to many types of tissue and suitable for any laboratory with standard SEM preparation equipment and a variable-pressure-SEM or tabletop SEM.
doi:10.1186/1746-4811-9-40
PMCID: PMC3853341  PMID: 24135233
Backscattered-electron imaging; Plant cell outlines; Image analysis; Arabidopsis thaliana
5.  Contrast and Raman spectroscopy study of single- and few-layered charge density wave material: 2H-TaSe2 
Scientific Reports  2013;3:2593.
In this article, we report the first successful preparation of single- and few-layers of tantalum diselenide (2H-TaSe2) by mechanical exfoliation technique. Number of layers is confirmed by white light contrast spectroscopy and atomic force microscopy (AFM). Vibrational properties of the atomically thin layers of 2H-TaSe2 are characterized by micro-Raman spectroscopy. Room temperature Raman measurements demonstrate MoS2-like spectral features, which are reliable for thickness determination. E1g mode, usually forbidden in backscattering Raman configuration is observed in the supported TaSe2 layers while disappears in the suspended layers, suggesting that this mode may be enabled because of the symmetry breaking induced by the interaction with the substrate. A systematic in-situ low temperature Raman study, for the first time, reveals the existence of incommensurate charge density wave phase transition in single and double-layered 2H-TaSe2 as reflected by a sudden softening of the second-order broad Raman mode resulted from the strong electron-phonon coupling (Kohn anomaly).
doi:10.1038/srep02593
PMCID: PMC3763362  PMID: 24005335
6.  A Conspicuous Clay Ovoid in Nakhla: Evidence for Subsurface Hydrothermal Alteration on Mars with Implications for Astrobiology 
Astrobiology  2014;14(8):651-693.
Abstract
A conspicuous biomorphic ovoid structure has been discovered in the Nakhla martian meteorite, made of nanocrystalline iron-rich saponitic clay and amorphous material. The ovoid is indigenous to Nakhla and occurs within a late-formed amorphous mesostasis region of rhyolitic composition that is interstitial to two clinopyroxene grains with Al-rich rims, and contains acicular apatite crystals, olivine, sulfides, Ti-rich magnetite, and a new mineral of the rhoenite group. To infer the origin of the ovoid, a large set of analytical tools was employed, including scanning electron microscopy and backscattered electron imaging, wavelength-dispersive X-ray analysis, X-ray mapping, Raman spectroscopy, time-of-flight secondary ion mass spectrometry analysis, high-resolution transmission electron microscope imaging, and atomic force microscope topographic mapping. The concentric wall of the ovoid surrounds an originally hollow volume and exhibits internal layering of contrasting nanotextures but uniform chemical composition, and likely inherited its overall shape from a preexisting vesicle in the mesostasis glass. A final fibrous layer of Fe-rich phases blankets the interior surfaces of the ovoid wall structure. There is evidence that the parent rock of Nakhla has undergone a shock event from a nearby bolide impact that melted the rims of pyroxene and the interstitial matter and initiated an igneous hydrothermal system of rapidly cooling fluids, which were progressively mixed with fluids from the melted permafrost. Sharp temperature gradients were responsible for the crystallization of Al-rich clinopyroxene rims, rhoenite, acicular apatites, and the quenching of the mesostasis glass and the vesicle. During the formation of the ovoid structure, episodic fluid infiltration events resulted in the precipitation of saponite rinds around the vesicle walls, altered pyrrhotite to marcasite, and then isolated the ovoid wall structure from the rest of the system by depositing a layer of iron oxides/hydroxides. Carbonates, halite, and sulfates were deposited last within interstitial spaces and along fractures. Among three plausible competing hypotheses here, this particular abiotic scenario is considered to be the most reasonable explanation for the formation of the ovoid structure in Nakhla, and although compelling evidence for a biotic origin is lacking, it is evident that the martian subsurface contains niche environments where life could develop. Key Words: Biomorph—Clays—Search for life (biosignatures)—Martian meteorites—Hydrothermal systems. Astrobiology 14, 651–693.
doi:10.1089/ast.2013.1069
PMCID: PMC4126275  PMID: 25046549
7.  High Resolution Helium Ion Scanning Microscopy of the Rat Kidney 
PLoS ONE  2013;8(3):e57051.
Helium ion scanning microscopy is a novel imaging technology with the potential to provide sub-nanometer resolution images of uncoated biological tissues. So far, however, it has been used mainly in materials science applications. Here, we took advantage of helium ion microscopy to explore the epithelium of the rat kidney with unsurpassed image quality and detail. In addition, we evaluated different tissue preparation methods for their ability to preserve tissue architecture. We found that high contrast, high resolution imaging of the renal tubule surface is possible with a relatively simple processing procedure that consists of transcardial perfusion with aldehyde fixatives, vibratome tissue sectioning, tissue dehydration with graded methanol solutions and careful critical point drying. Coupled with the helium ion system, fine details such as membrane texture and membranous nanoprojections on the glomerular podocytes were visualized, and pores within the filtration slit diaphragm could be seen in much greater detail than in previous scanning EM studies. In the collecting duct, the extensive and striking apical microplicae of the intercalated cells were imaged without the shrunken or distorted appearance that is typical with conventional sample processing and scanning electron microscopy. Membrane depressions visible on principal cells suggest possible endo- or exocytotic events, and central cilia on these cells were imaged with remarkable preservation and clarity. We also demonstrate the use of colloidal gold probes for highlighting specific cell-surface proteins and find that 15 nm gold labels are practical and easily distinguishable, indicating that external labels of various sizes can be used to detect multiple targets in the same tissue. We conclude that this technology represents a technical breakthrough in imaging the topographical ultrastructure of animal tissues. Its use in future studies should allow the study of fine cellular details and provide significant advances in our understanding of cell surface structures and membrane organization.
doi:10.1371/journal.pone.0057051
PMCID: PMC3591388  PMID: 23505418
8.  The role of helium gas in medicine 
Medical Gas Research  2013;3:18.
The noble gas helium has many applications owing to its distinct physical and chemical characteristics, namely: its low density, low solubility, and high thermal conductivity. Chiefly, the abundance of studies in medicine relating to helium are concentrated in its possibility of being used as an adjunct therapy in a number of respiratory ailments such as asthma exacerbation, COPD, ARDS, croup, and bronchiolitis. Helium gas, once believed to be biologically inert, has been recently shown to be beneficial in protecting the myocardium from ischemia by various mechanisms. Though neuroprotection of brain tissue has been documented, the mechanism by which it does so has yet to be made clear. Surgeons are exploring using helium instead of carbon dioxide to insufflate the abdomen of patients undergoing laparoscopic abdominal procedures due to its superiority in preventing respiratory acidosis in patients with comorbid conditions that cause carbon dioxide retention. Newly discovered applications in Pulmonary MRI radiology and imaging of organs in very fine detail using Helium Ion Microscopy has opened exciting new possibilities for the use of helium gas in technologically advanced fields of medicine.
doi:10.1186/2045-9912-3-18
PMCID: PMC3751721  PMID: 23916029
Helium; Heliox; Inhalation therapy; Cardioprotection; Neuroprotection; Insufflation
9.  SIMULTANEOUS BACKSCATTER AND ATTENUATION ESTIMATION USING A LEAST SQUARES METHOD WITH CONSTRAINTS 
Ultrasound in medicine & biology  2011;37(12):2096-2104.
Backscatter and attenuation variations are essential contrast mechanisms in ultrasound B-mode imaging. Emerging Quantitative Ultrasound methods extract and display absolute values of these tissue properties. However, in clinical applications, backscatter and attenuation parameters sometimes are not easily measured because of tissues inhomogeneities above the region of interest. We describe a least squares method (LSM) that fits the echo signal power spectra from a region of interest (ROI) to a 3-parameter tissue model that simultaneously yields estimates of attenuation losses and backscatter coefficients.
To test the method, tissue-mimicking phantoms with backscatter and attenuation contrast as well as uniform phantoms were scanned with linear array transducers on a Siemens S2000. Attenuation and backscatter coefficients estimated by the LSM were compared with those derived using a reference phantom method (Yao et al. 1990). Results show that the LSM yields effective attenuation coefficients for uniform phantoms comparable to values derived using the reference phantom method. For layered phantoms exhibiting non-uniform backscatter, the LSM resulted in smaller attenuation estimation errors than the reference phantom method. Backscatter coefficients derived using the LSM were in excellent agreement with values obtained from laboratory measurements on test samples and with theory. The LSM is more immune to depth-dependent backscatter changes than commonly used reference phantom methods.
doi:10.1016/j.ultrasmedbio.2011.08.008
PMCID: PMC3223333  PMID: 21963038
attenuation; backscatter; quantitative ultrasound; least squares method
10.  Oxidation and metal-insertion in molybdenite surfaces: evaluation of charge-transfer mechanisms and dynamics 
Molybdenum disulfide (MoS2), a layered transition-metal dichalcogenide, has been of special importance to the research community of geochemistry, materials and environmental chemistry, and geotechnical engineering. Understanding the oxidation behavior and charge-transfer mechanisms in MoS2 is important to gain better insight into the degradation of this mineral in the environment. In addition, understanding the insertion of metals into molybdenite and evaluation of charge-transfer mechanism and dynamics is important to utilize these minerals in technological applications. Furthermore, a detailed investigation of thermal oxidation behavior and metal-insertion will provide a basis to further explore and model the mechanism of adsorption of metal ions onto geomedia.
The present work was performed to understand thermal oxidation and metal-insertion processes of molybdenite surfaces. The analysis was performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), and nuclear reaction analysis (NRA).
Structural studies using SEM and TEM indicate the local-disordering of the structure as a result of charge-transfer process between the inserted lithium and the molybdenite layer. Selected area electron diffraction measurements indicate the large variations in the diffusivity of lithium confirming that the charge-transfer is different along and perpendicular to the layers in molybdenite. Thermal heating of molybenite surface in air at 400°C induces surface oxidation, which is slow during the first hour of heating and then increases significantly. The SEM results indicate that the crystals formed on the molybdenite surface as a result of thermal oxidation exhibit regular thin-elongated shape. The average size and density of the crystals on the surface is dependent on the time of annealing; smaller size and high density during the first one-hour and significant increase in size associated with a decrease in density with further annealing.
doi:10.1186/1467-4866-9-8
PMCID: PMC2442061  PMID: 18534025
11.  Free-energy Landscapes of Ion-channel Gating Are Malleable: changes in the number of bound ligands are accompanied by changes in the location of the transition state in acetylcholine-receptor channels† 
Biochemistry  2003;42(50):14977-14987.
Acetylcholine-receptor channels (AChRs) are allosteric membrane proteins that mediate synaptic transmission by alternatively opening and closing (‘gating’) a cation-selective transmembrane pore. Although ligand binding is not required for the channel to open, the binding of agonists (for example, acetylcholine) increases the closed ⇌ open equilibrium constant because the ion-impermeable → ion-permeable transition of the ion pathway is accompanied by a low → high affinity change at the agonist-binding sites. The fact that the gating conformational change of muscle AChRs can be kinetically modeled as a two-state reaction has paved the way to the experimental characterization of the corresponding transition state, which represents a snapshot of the continuous sequence of molecular events separating the closed and open states. Previous studies of fully (di-) liganded AChRs, combining single-channel kinetic measurements, site-directed mutagenesis, and data analysis in the framework of the linear free-energy relationships of physical organic chemistry, have suggested a transition-state structure that is consistent with channel opening being an asynchronous conformational change that starts at the extracellular agonist-binding sites and propagates towards the intracellular end of the pore. In this paper, I characterize the gating transition state of unliganded AChRs, and report a remarkable difference: unlike that of diliganded gating, the unliganded transition state is not a hybrid of the closed- and open-state structures but, rather, is almost indistinguishable from the open state itself. This displacement of the transition state along the reaction coordinate obscures the mechanism underlying the unliganded closed ⇌ open reaction but brings to light the malleable nature of free-energy landscapes of ion-channel gating.
The muscle acetylcholine receptor channel (AChR)1 is the neurotransmitter-gated ion channel that mediates neuromuscular synaptic transmission in vertebrates (1). Although the structure of this large pentameric transmembrane protein (∼470 residues per subunit) is not known with atomic resolution, a wealth of structural information exists, mainly from mutational studies, affinity labeling, chemical modification of specific residues, electron microscopy, and crystallography (reviewed in ref. 2). As is the case of any other allosteric protein, the dynamic behavior of this receptor-channel can be understood in the framework of thermodynamic cycles, with conformational changes and ligand-binding events as the elementary steps (3-5). Thus, the AChR can adopt a variety of different conformations that can interconvert (closed, open, and desensitized ‘states’), and each conformation has a distinct ligand-binding affinity (low affinity in the closed state and high affinity in the open and desensitized states) and a particular ‘catalytic efficiency’ (ion-impermeable in the closed and desensitized states, and ion-permeable in the open state). To meet the physiological requirement of a small closed ⇌ open (‘gating’) equilibrium constant for the unliganded receptor, and a large gating equilibrium constant for the ACh-diliganded receptor, the affinity of the AChR for ACh must be higher in the open than in the closed conformation (4-6). This follows from the notion that the equilibrium constants governing the different reaction steps (ligand binding and gating) of these cyclic reaction schemes are constrained by the principle of detailed balance.
Hence, irrespective of whether the receptor is diliganded, monoliganded or unliganded, two changes must take place in going from the closed state (low ligand affinity and ion-impermeable) to the open state (high ligand affinity and ion-permeable): a) the pore becomes permeable to ions, and b) the transmitter-binding sites, some 50 Å away from the pore domain (7), increase their affinity for the ligand (with the reverse changes taking place during closing). The apparent lack of stable intermediates between the closed and open conformations, inferred from kinetic modeling of the diliganded-gating reaction (8), suggests that these two changes occur as a result of a one-step, global conformational change. The question, then, arises as to whether this concerted conformational change proceeds synchronously (i.e., every residue of the protein moves ‘in unison’) or asynchronously (i.e., following a sequence of events; ref. 9) and, if the latter were the case, whether multiple, few, or just one sequence of events is actually traversed by the channel to ‘connect’ the end states.
Analysis of the correlation between rate and equilibrium constants of gating in diliganded AChRs has allowed us to address some of these issues by probing the structure of the transition state (8, 10-12), that is, the intermediate species between the end states of a one-step reaction that can be most easily studied. Interpretation of these results in the framework of the classical rate-equilibrium free-energy relationships of physical organic chemistry (13, 14), revealed that AChR diliganded gating is a highly asynchronous reaction, and suggested that the transition-state ensemble is quite homogeneous, as if the crossing of the energy barrier were confined to a narrow pass at the top of the energy landscape. In the opening direction, the conformational rearrangement that leads to the low-to-high affinity change at the extracellular binding sites precedes the conformational rearrangement of the pore that renders the channel ion-permeable. This propagated global conformational change, which we have referred to as a ‘conformational wave’ (11), must reverse during channel closing so that closing starts at the pore and propagates all the way to the binding sites.
It is not at all obvious why the diliganded-gating conformational change starts at the binding sites when the channel opens, nor even why the conformational change propagates at all through the receptor, instead of taking place synchronously throughout the protein. Is there any correlation between the location of the domain that binds agonist and the location of the initiation site for the opening conformational change? Could the latter have started from the intracellular end of the pore, for example, and have propagated to the (extracellular) transmitter-binding sites? What difference does it make to be liganded or unliganded as far as the mechanism of the gating conformational change is concerned? To address these issues, I set out to explore the mechanism of gating in unliganded AChRs by probing the structure of the corresponding transition state using kinetic measurements, site-directed mutagenesis, and the concepts of rate-equilibrium free-energy relationships and Φ-value analysis.
Briefly, a Φ-value can be assigned to any position in the protein by estimating the slope of a ‘Brönsted plot’2 [log (gating rate constant) versus log (gating equilibrium constant)] where each point corresponds to a different amino-acid substitution at that given position. More coarsegrained Φ-values can also be obtained by using different agonists or different transmembrane potentials, for example, as a means of altering the rate and equilibrium constants of gating. Very often, rate-equilibrium plots are linear, and 0 < Φ < 1. A value of Φ = 0 suggests that the position in question (in the case of a mutation series) experiences a closed-state-like environment at the transition state whereas a value of Φ = 1 suggests an open-state-like environment. A fractional Φ-value suggests an environment that is intermediate between those experienced in the closed and open states (16).
Earlier results indicated that the Φ-values obtained by varying the transmembrane potential are different in diliganded and unliganded AChRs. These Φ-values, which are a measure of the closed-state-like versus open-state-like character of the channel’s voltage-sensing elements at the transition state, are 0.070 ± 0.060 in diliganded receptors (17), and 1.025 ± 0.053 in unliganded AChRs (11, 18). The present study reveals that residues at the transmitter-binding sites (Figure 1), the extracellular loop that links the second (M2) and third (M3) transmembrane segments (M2-M3 linker), and the upper and lower half of M2, which during diliganded gating have Φ-values of ∼1 (ref. 11), ∼0.7 (ref. 10), ∼0.35 (refs 8, 11, 12), and ∼0 (ref. 12), respectively, have also Φ-values very close to 1 during unliganded gating. This generalized shift in Φ-values suggests that the diliganded → unliganded perturbation deforms the energy landscape of gating in such a way that the ‘new’ transition state occurs very close to the open state, to such an extent that all tested positions experience an open-state-like environment at the transition state of unliganded gating. Thus, the transition state occurs so ‘late’ (i.e., so close to the open state) that its inferred structure does not provide any clues as to the intermediate stages of this reaction.
Hence, the mechanism of unliganded gating remains obscure. The change in the position of the transition state along a reaction coordinate, as a result of perturbations to the energy landscape, is a very well known phenomenon in organic chemistry (e.g., refs 20-26), and protein folding (e.g., refs 27-34). In this paper, I show that this phenomenon can also take place in the case of allosteric transitions and, therefore, that the structure of the transition state of a global conformational change need not be fixed; rather, it can change depending on the experimental conditions.
doi:10.1021/bi0354334
PMCID: PMC1463891  PMID: 14674774
12.  Intrastromal Corneal Ring Implants for Corneal Thinning Disorders 
Executive Summary
Objective
The purpose of this project was to determine the role of corneal implants in the management of corneal thinning disease conditions. An evidence-based review was conducted to determine the safety, effectiveness and durability of corneal implants for the management of corneal thinning disorders. The evolving directions of research in this area were also reviewed.
Subject of the Evidence-Based Analysis
The primary treatment objectives for corneal implants are to normalize corneal surface topography, improve contact lens tolerability, and restore visual acuity in order to delay or defer the need for corneal transplant. Implant placement is a minimally invasive procedure that is purported to be safe and effective. The procedure is also claimed to be adjustable, reversible, and both eyes can be treated at the same time. Further, implants do not limit the performance of subsequent surgical approaches or interfere with corneal transplant. The evidence for these claims is the focus of this review.
The specific research questions for the evidence review were as follows:
Safety
Corneal Surface Topographic Effects:
Effects on corneal surface remodelling
Impact of these changes on subsequent interventions, particularly corneal transplantation (penetrating keratoplasty [PKP])
Visual Acuity
Refractive Outcomes
Visual Quality (Symptoms): such as contrast vision or decreased visual symptoms (halos, fluctuating vision)
Contact lens tolerance
Functional visual rehabilitation and quality of life
Patient satisfaction:
Disease Process:
Impact on corneal thinning process
Effect on delaying or deferring the need for corneal transplantation
Clinical Need: Target Population and Condition
Corneal ectasia (thinning) comprises a range of disorders involving either primary disease conditions such as keratoconus and pellucid marginal corneal degeneration or secondary iatrogenic conditions such as corneal thinning occurring after LASIK refractive surgery. The condition occurs when the normally round dome-shaped cornea progressively thins causing a cone-like bulge or forward protrusion in response to the normal pressure of the eye. Thinning occurs primarily in the stoma layers and is believed to be a breakdown in the collagen network. This bulging can lead to an irregular shape or astigmatism of the cornea and, because the anterior part of the cornea is largely responsible for the focusing of light on the retina, results in loss of visual acuity. This can make even simple daily tasks, such as driving, watching television or reading, difficult to perform.
Keratoconus (KC) is the most common form of corneal thinning disorder and is a noninflammatory chronic disease process. Although the specific causes of the biomechanical alterations that occur in KC are unknown, there is a growing body of evidence to suggest that genetic factors may play an important role. KC is a rare condition (<0.05% of the population) and is unique among chronic eye diseases as it has an early age of onset (median age of 25 years). Disease management for this condition follows a step-wise approach depending on disease severity. Contact lenses are the primary treatment of choice when there is irregular astigmatism associated with the disease. When patients can no longer tolerate contact lenses or when lenses no longer provide adequate vision, patients are referred for corneal transplant.
Keratoconus is one of the leading indications for corneal transplants and has been so for the last three decades. Yet, despite high graft survival rates of up to 20 years, there are reasons to defer receiving transplants for as long as possible. Patients with keratoconus are generally young and life-long term graft survival would be an important consideration. The surgery itself involves lengthy time off work and there are potential complications from long term steroid use following surgery, as well as the risk of developing secondary cataracts, glaucoma etc. After transplant, recurrent KC is possible with need for subsequent intervention. Residual refractive errors and astigmatism can remain challenging after transplantation and high refractive surgery rates and re-graft rates in KC patients have been reported. Visual rehabilitation or recovery of visual acuity after transplant may be slow and/or unsatisfactory to patients.
Description of Technology/Therapy
INTACS® (Addition Technology Inc. Sunnyvale, CA, formerly KeraVision, Inc.) are the only currently licensed corneal implants in Canada. The implants are micro-thin poly methyl methacrylate crescent shaped ring segments with a circumference arc length of 150 degrees, an external diameter of 8.10 mm, an inner diameter of 6.77 mm, and a range of different thicknesses. Implants act as passive spacers and, when placed in the cornea, cause local separation of the corneal lamellae resulting in a shortening of the arc length of the anterior corneal curvature and flattening the central cornea. Increasing segment thickness results in greater lamellar separation with increased flattening of the cornea correcting for myopia by decreasing the optical power of the eye. Corneal implants also improve corneal astigmatism but the mechanism of action for this is less well understood.
Treatment with corneal implants is considered for patients who are contact lens intolerant, having adequate corneal thickness particularly around the area of the implant incision site and without central corneal scarring. Those with central corneal scarring would not benefit from implants and those without an adequate corneal thickness, particularly in the region that the implants are being inserted, would be at increased risk for corneal perforation. Patients desiring to have visual rehabilitation that does not include glasses or contact lenses would not be candidates for corneal ring implants.
Placement of the implants is an outpatient procedure with topical anesthesia generally performed by either corneal specialists or refractive surgeons. It involves creating tunnels in the corneal stroma to secure the implants either by a diamond knife or laser calibrated to an approximate depth of 70% of the cornea. Variable approaches have been employed by surgeons in selecting ring segment size, number and position. Generally, two segments of equal thickness are placed superiorly and inferiorly to manage symmetrical patterns of corneal thinning whereas one segment may be placed to manage asymmetric thinning patterns.
Following implantation, the major safety concerns are for potential adverse events including corneal perforation, infection, corneal infiltrates, corneal neovascularization, ring migration and extrusion and corneal thinning. Technical results can be unsatisfactory for several reasons. Treatment may result in an over or under-correction of refraction and may induce astigmatism or asymmetry of the cornea.
Progression of the corneal cone with corneal opacities is also invariably an indication for progression to corneal transplant. Other reasons for treatment failure or patient dissatisfaction include foreign body sensation, unsatisfactory visual quality with symptoms such as double vision, fluctuating vision, poor night vision or visual side effects related to ring edge or induced or unresolved astigmatism.
Evidence-Based Analysis Methods
The literature search strategy employed keywords and subject headings to capture the concepts of 1) intrastromal corneal rings and 2) corneal diseases, with a focus on keratoconus, astigmatism, and corneal ectasia. The initial search was run on April 17, 2008, and a final search was run on March 6, 2009 in the following databases: Ovid MEDLINE (1996 to February Week 4 2009), OVID MEDLINE In-Process and Other Non-Indexed Citations, EMBASE (1980 to 2009 Week 10), OVID Cochrane Library, and the Centre for Reviews and Dissemination/International Agency for Health Technology Assessment. Parallel search strategies were developed for the remaining databases. Search results were limited to human and English-language published between January 2000 and April 17, 2008. The resulting citations were downloaded into Reference Manager, v.11 (ISI Researchsoft, Thomson Scientific, U.S.A), and duplicates were removed. The Web sites of several other health technology agencies were also reviewed including the Canadian Agency for Drugs and Technologies in Health (CADTH), ECRI, and the United Kingdom National Institute for Clinical Excellence (NICE). The bibliographies of relevant articles were scanned.
Inclusion Criteria
English language reports and human studies
Any corneal thinning disorder
Reports with corneal implants used alone or in conjunction with other interventions
Original reports with defined study methodology
Reports including standardized measurements on outcome events such as technical success, safety, effectiveness, durability, vision quality of life or patient satisfaction
Case reports or case series for complications and adverse events
Exclusion Criteria
Non-systematic reviews, letters, comments and editorials
Reports not involving outcome events such as safety, effectiveness, durability, vision quality or patient satisfaction following an intervention with corneal implants
Reports not involving corneal thinning disorders and an intervention with corneal implants
Summary of Findings
In the MAS evidence review on intrastromal corneal ring implants, 66 reports were identified on the use of implants for management of corneal thinning disorders. Reports varied according to their primary clinical indication, type of corneal implant, and whether or not secondary procedures were used in conjunction with the implants. Implants were reported to manage post LASIK thinning and/or uncorrected refractive error and were also reported as an adjunctive intervention both during and after corneal transplant to manage recurrent thinning and/or uncorrected refractive error.
Ten pre-post cohort longitudinal follow-up studies were identified examining the safety and effectiveness of INTAC corneal implants in patients with keratoconus. Five additional cohort studies were identified using the Ferrara implant for keratoconus management but because this corneal implant is not licensed in Canada these studies were not reviewed.
The cohorts implanted with INTACS involved 608 keratoconus patients (754 eyes) followed for 1, 2 or 3 years. Three of the reports involved ≥ 2 years of follow-up with the longest having 5-year follow-up data for a small number of patients. Four of the INTAC cohort studies involved 50 or more patients; the largest involved 255 patients. Inclusion criteria for the studies were consistent and included patients who were contact lens intolerant, had adequate corneal thickness, particularly around the area of the implant incision site, and without central corneal scarring. Disease severity, thinning pattern, and corneal cone protrusions all varied and generally required different treatment approaches involving defined segment sizes and locations.
A wide range of outcome measures were reported in the cohort studies. High levels of technical success or ability to place INTAC segments were reported. Technically related complications were often delayed and generally reported as segment migration attributable to early experience. Overall, complications were infrequently reported and largely involved minor reversible events without clinical sequelae.
The outcomes reported across studies involved statistically significant and clinically relevant improvements in corneal topography, refraction and visual acuity, for both uncorrected and best-corrected visual acuity. Patients’ vision was usually restored to within normal functioning levels and for those not achieving satisfactory correction, insertion of intraocular lenses was reported in case studies to result in additional gains in visual acuity. Vision loss (infrequently reported) was usually reversed by implant exchange or removal. The primary effects of INTACS on corneal surface remodelling were consistent with secondary improvements in refractive error and visual acuity. The improvements in visual acuity and refractive error noted at 6 months were maintained at 1 and 2-year follow-up
Improvements in visual acuity and refractive error following insertion of INTACS, however, were not noted for all patients. Although improvements were not found to vary across age groups there were differences across stages of disease. Several reports suggested that improvements in visual acuity and refractive outcomes may not be as large or predictable in more advanced stages of KC. Some studies have suggested that the effects of INTACs were much greater in flattening the corneal surface than in correcting astigmatism. However, these studies involved small numbers of high risk patients in advanced stages of KC and conclusions made from this group are limited.
INTACS were used for other indications other than primary KC. The results of implant insertion on corneal topography, refraction, and visual acuity in post-LASIK thinning cases were similar to those reported for KC. The evidence for this indication, however, only involved case reports and small case series. INTACS were also successfully used to treat recurrent KC after corneal transplant but this was based on only a single case report. Corneal implants were compared to corneal transplantation but these studies were not randomized and based on small numbers of selected patients.
The foremost limitation of the evidence base is the basic study design in the reports that involved longitudinal follow-up only for the treated group; there were no randomized trials. Follow-up in the trials (although at prescribed intervals) often had incomplete accounts of losses at follow-up and estimates of change were often not reported or based on group differences. Second, although standardized outcome measures were reported, contact lens tolerance (a key treatment objective) was infrequently specified. A third general limitation was the lack of reporting of patients’ satisfaction with their vision quality or functional vision. Outcome measures for vision quality and impact on patient quality of life were available but rarely reported and have been noted to be a limitation in ophthalmological literature in general. Fourth, the longitudinal cohort studies have not followed patients long enough to evaluate the impact of implants on the underlying disease process (follow-up beyond 3 years is limited). Additionally, only a few of these studies directly examined corneal thinning in follow-up. The overall quality of evidence determined using the GRADE hierarchy of evidence was moderate.
There is some evidence in these studies to support the claim that corneal implants do not interfere with, or increase the difficultly of, subsequent corneal transplant, at least for those performed shortly after INTAC placement. Although it’s uncertain for how long implants can delay the need for a corneal transplant, given that patients with KC are often young (in their twenties and thirties), delaying transplant for any number of years may still be a valuable consideration.
Conclusion
The clinical indications for corneal implants have evolved from management of myopia in normal eyes to the management of corneal thinning disorders such as KC and thinning occurring after refractive surgery. Despite the limited evidence base for corneal implants, which consists solely of longitudinal follow-up studies, they appear to be a valuable clinical tool for improving vision in patients with corneal thinning. For patients unable to achieve functional vision, corneal implants achieved statistically significant and clinically relevant improvements in corneal topography, refraction, and visual acuity, providing a useful alternative to corneal transplant. Implants may also have a rescue function, treating corneal thinning occurring after refractive surgery in normal eyes, or managing refractive errors following corneal transplant. The treatment offers several advantages in that it’s an outpatient based procedure, is associated with minimal risk, and has high technical success rates. Both eyes can be treated at once and the treatment is adjustable and reversible. The implants can be removed or exchanged to improve vision without limiting subsequent interventions, particularly corneal transplant.
Better reporting on vision quality, functional vision and patient satisfaction, however, would improve evaluation of the impact of these devices. Information on the durability of the implants’ treatment effects and their affects on underlying disease processes is limited. This information is becoming more important as alternative treatment strategies, such as collagen cross-linking aimed at strengthening the underlying corneal tissue, are emerging and which might prove to be more effective or increase the effectiveness of the implants, particularly in advances stages of corneal thinning.
Ontario Health System Considerations
At present there are approximately 70 ophthalmologists in Canada who’ve had training with corneal implants; 30 of these practice in Ontario. Industry currently sponsors the training, proctoring and support for the procedure. The cost of the implant device ranges from $950 to $1200 (CAD) and costs for instrumentation range from $20,000 to $30,000 (CAD) (a one time capital expenditure). There is no physician services fee code for corneal implants in Ontario but assuming that they are no higher than those for a corneal transplant, the estimated surgical costs would be $914.32(CAD) An estimated average cost per patient, based on device costs and surgical fees, for treatment is $1,964 (CAD) (range $1,814 to $2,114) per eye. There have also been no out of province treatment requests. In Ontario the treatment is currently being offered in private clinics and an increasing number of ophthalmologists are being certified in the technique by the manufacturer.
KC is a rare disease and not all of these patients would be eligible candidates for treatment with corneal implants. Based on published population rates of KC occurrence, it can be expected that there is a prevalent population of approximately 6,545 patients and an incident population of 240 newly diagnosed cases per year. Given this small number of potential cases, the use of corneal implants would not be expected to have much impact on the Ontario healthcare system. The potential impact on the provincial budget for managing the incident population, assuming the most conservative scenario (i.e., all are eligible and all receive bilateral implants) ranges from $923 thousand to $1.1 million (CAD). This estimate would vary based on a variety of criteria including eligibility, unilateral or bilateral interventions, re-interventions, capacity and uptake
Keywords
Keratoconus, corneal implants, corneal topography, corneal transplant, visual acuity, refractive error
PMCID: PMC3385416  PMID: 23074513
13.  Molecular Remodeling of Tip Links Underlies Mechanosensory Regeneration in Auditory Hair Cells 
PLoS Biology  2013;11(6):e1001583.
Backscatter scanning electron microscopy and conventional whole cell patch-clamp experiments reveal a two-step mechanism for the regeneration of tip links, the crucial element of mechanotransduction machinery in the hair cells of the inner ear.
Sound detection by inner ear hair cells requires tip links that interconnect mechanosensory stereocilia and convey force to yet unidentified transduction channels. Current models postulate a static composition of the tip link, with protocadherin 15 (PCDH15) at the lower and cadherin 23 (CDH23) at the upper end of the link. In terminally differentiated mammalian auditory hair cells, tip links are subjected to sound-induced forces throughout an organism's life. Although hair cells can regenerate disrupted tip links and restore hearing, the molecular details of this process are unknown. We developed a novel implementation of backscatter electron scanning microscopy to visualize simultaneously immuno-gold particles and stereocilia links, both of only a few nanometers in diameter. We show that functional, mechanotransduction-mediating tip links have at least two molecular compositions, containing either PCDH15/CDH23 or PCDH15/PCDH15. During regeneration, shorter tip links containing nearly equal amounts of PCDH15 at both ends appear first. Whole-cell patch-clamp recordings demonstrate that these transient PCDH15/PCDH15 links mediate mechanotransduction currents of normal amplitude but abnormal Ca2+-dependent decay (adaptation). The mature PCDH15/CDH23 tip link composition is re-established later, concomitant with complete recovery of adaptation. Thus, our findings provide a molecular mechanism for regeneration and maintenance of mechanosensory function in postmitotic auditory hair cells and could help identify elusive components of the mechanotransduction machinery.
Author Summary
The inner ear detects sound when stereocilia, the mechanosensory projections on the apical surface of the hair cells, are deflected and tug on tiny extracellular tip links. These links interconnect stereocilia and convey forces to the mechanosensitive transduction channels. Current models postulate a static composition of the tip link with protocadherin 15 (PCDH15) at the link's bottom end and cadherin 23 (CDH23) at the upper end. Tip links are subjected to substantial sound-induced forces. Although hair cells can renew (regenerate) disrupted tip links and restore hearing, the molecular details of this process are unknown. Our study provides mechanistic insight into tip link regeneration. We used backscatter scanning electron microscopy to monitor the distribution of immuno-gold labeled molecular components of the tip links during their re-formation and a conventional whole-cell patch-clamp technique to follow the concomitant recovery of mechano-electrical transduction. According to our data, the mechanotransduction machinery is initially re-established by the formation of functional (mechanotransduction-mediating) links of a previously unknown composition, PCDH15–PCDH15. Transition to the PCDH15–CDH23 composition underlies final maturation of mechanotransduction. This two-step mechanism of tip link regeneration was unexpected. As tip links are continuously stressed by loud sounds and regenerated throughout an organism's life, we provide a plausible molecular mechanism for the life-long maintenance of mechanosensory function in nonregenerating cochlear hair cells.
doi:10.1371/journal.pbio.1001583
PMCID: PMC3679001  PMID: 23776407
14.  Direct observation of crystal defects in an organic molecular crystals of copper hexachlorophthalocyanine by STEM-EELS 
Scientific Reports  2012;2:252.
The structural analysis of crystal defects in organic thin films provides fundamental insights into their electronic properties for applications such as field effect transistors. Observation of crystal defects in organic thin films has previously been performed at rather low resolution by conventional transmission electron microscopy based on phase-contrast imaging. Herein, we apply for the first time annular dark-field imaging to the direct observation of grain boundaries in copper hexachlorophthalocyanine thin films at the atomic resolution level by using an aberration-corrected scanning transmission electron microscope combined with electron energy-loss spectroscopy. By using a low-dose technique and an optimized detection angle, we were able to visualize the contrast of light element (C and N) together with the heavier elements (Cl and Cu) within the molecular column. We were also able to identify unexpected molecular orientations in the grain boundaries along the {110} crystallographic planes giving rise to stacking faults.
doi:10.1038/srep00252
PMCID: PMC3273783  PMID: 22355764
15.  Synthesis of Pt nanoparticles and their burrowing into Si due to synergistic effects of ion beam energy losses 
Summary
We report the synthesis of Pt nanoparticles and their burrowing into silicon upon irradiation of a Pt–Si thin film with medium-energy neon ions at constant fluence (1.0 × 1017 ions/cm2). Several values of medium-energy neon ions were chosen in order to vary the ratio of the electronic energy loss to the nuclear energy loss (S e/S n) from 1 to 10. The irradiated films were characterized using Rutherford backscattering spectroscopy (RBS), atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). A TEM image of a cross section of the film irradiated with S e/S n = 1 shows ≈5 nm Pt NPs were buried up to ≈240 nm into the silicon. No silicide phase was detected in the XRD pattern of the film irradiated at the highest value of S e/S n. The synergistic effect of the energy losses of the ion beam (molten zones are produced by S e, and sputtering and local defects are produced by S n) leading to the synthesis and burrowing of Pt NPs is evidenced. The Pt NP synthesis mechanism and their burrowing into the silicon is discussed in detail.
doi:10.3762/bjnano.5.197
PMCID: PMC4222290  PMID: 25383298
atomic force microscopy; burrowing of nanoparticles; medium-energy ion irradiation; nuclear and electronic energy loss; Rutherford backscattering spectroscopy; scanning electron microscopy; thin films; transmission electron microscopy
16.  The p7 Protein of Hepatitis C Virus Forms Structurally Plastic, Minimalist Ion Channels 
PLoS Computational Biology  2012;8(9):e1002702.
Hepatitis C virus (HCV) p7 is a membrane-associated oligomeric protein harboring ion channel activity. It is essential for effective assembly and release of infectious HCV particles and an attractive target for antiviral intervention. Yet, the self-assembly and molecular mechanism of p7 ion channelling are currently only partially understood. Using molecular dynamics simulations (aggregate time 1.2 µs), we show that p7 can form stable oligomers of four to seven subunits, with a bias towards six or seven subunits, and suggest that p7 self-assembles in a sequential manner, with tetrameric and pentameric complexes forming as intermediate states leading to the final hexameric or heptameric assembly. We describe a model of a hexameric p7 complex, which forms a transiently-open channel capable of conducting ions in simulation. We investigate the ability of the hexameric model to flexibly rearrange to adapt to the local lipid environment, and demonstrate how this model can be reconciled with low-resolution electron microscopy data. In the light of these results, a view of p7 oligomerization is proposed, wherein hexameric and heptameric complexes may coexist, forming minimalist, yet robust functional ion channels. In the absence of a high-resolution p7 structure, the models presented in this paper can prove valuable as a substitute structure in future studies of p7 function, or in the search for p7-inhibiting drugs.
Author Summary
Hepatitis C remains a serious global health problem affecting more than 2% of the world's population, and current therapies are effective in only a subset of patients, necessitating an ongoing search for new treatments. The p7 viroporin is considered to be an attractive possible drug target, but rational drug design is hampered by the absence of a high-resolution p7 structure. In this paper, we explore possible structures of oligomeric p7 channels, and discuss the strengths and shortcomings of these models with respect to experimentally determined properties, such as pore-lining residues, ion conductance, and compatibility with low-resolution electron microscopy images. Our results present an image of p7 as a rudimentary, minimalistic ion channel, capable of existing in multiple oligomeric states but exhibiting a bias towards hexamers and heptamers. We believe that the work presented here will be valuable for future research by providing plausible 3-dimensional atomic-resolution models for the visualization of the p7 viroporin and serve as a basis for future computational studies.
doi:10.1371/journal.pcbi.1002702
PMCID: PMC3447957  PMID: 23028296
17.  Physical assembly of Ag nanocrystals on enclosed surfaces in monocrystalline Si 
Scientific Reports  2014;4:6744.
Growth of thin crystals on external substrate surfaces by many different methods is a well-known technique, but its extension to inner, enclosed surfaces of large defects in monocrystalline materials has not yet been reported. The literature on thin film growth and defects in materials can be leveraged to fabricate new structures for a variety of applications. Here we show a physical process of nucleation and evolution of nanocrystalline silver inside voids in monocrystalline silicon. We found that the Ag growth is hetero-epitaxial using a coincident site lattice. Alignment of Ag and Si atomic planes is uniformly observed by high resolution transmission electron microscopy and macroscopically by channeling Rutherford backscattering spectrometry.
doi:10.1038/srep06744
PMCID: PMC4223668  PMID: 25376502
18.  Ion-Abrasion Scanning Electron Microscopy Reveals Surface-Connected Tubular Conduits in HIV-Infected Macrophages 
PLoS Pathogens  2009;5(9):e1000591.
HIV-1-containing internal compartments are readily detected in images of thin sections from infected cells using conventional transmission electron microscopy, but the origin, connectivity, and 3D distribution of these compartments has remained controversial. Here, we report the 3D distribution of viruses in HIV-1-infected primary human macrophages using cryo-electron tomography and ion-abrasion scanning electron microscopy (IA-SEM), a recently developed approach for nanoscale 3D imaging of whole cells. Using IA-SEM, we show the presence of an extensive network of HIV-1-containing tubular compartments in infected macrophages, with diameters of ∼150–200 nm, and lengths of up to ∼5 µm that extend to the cell surface from vesicular compartments that contain assembling HIV-1 virions. These types of surface-connected tubular compartments are not observed in T cells infected with the 29/31 KE Gag-matrix mutant where the virus is targeted to multi-vesicular bodies and released into the extracellular medium. IA-SEM imaging also allows visualization of large sheet-like structures that extend outward from the surfaces of macrophages, which may bend and fold back to allow continual creation of viral compartments and virion-lined channels. This potential mechanism for efficient virus trafficking between the cell surface and interior may represent a subversion of pre-existing vesicular machinery for antigen capture, processing, sequestration, and presentation.
Author Summary
Current treatment regimens for HIV-infected individuals are not capable of eradicating HIV infection, even though combinations of highly potent antiviral drugs are used. Indeed, drug regimens must be periodically altered as the virus resurges from a persistent reservoir. Macrophages, which serve as “search-and-destroy” immune surveillance cells of the body, are now thought to be a key component of this reservoir. Evidence suggests that macrophages can harbor infectious HIV virions for long periods, and transmit them to bystander T cells. We have used a new technique called ion abrasion scanning electron microscopy (IA-SEM) to image entire HIV-infected human macrophages at a resolution high enough to see individual HIV virions and their location within the cell. This approach revealed that HIV is present in a system of nanoscale tubes, barely larger than a virus at some places, which connect internal viral reservoirs to the cell surface. These tubes could allow the macrophage to deliver HIV virions to bystander cells from its continually replenished stores of ammunition, held deep within the cell. Our work provides a glimpse of how the structure of these reservoirs allows macrophages to accomplish viral delivery. Discovery of these virion-channeling tubes provides a potential drug target to address the problem of persistent HIV infection.
doi:10.1371/journal.ppat.1000591
PMCID: PMC2743285  PMID: 19779568
19.  High throughput secondary electron imaging of organic residues on a graphene surface 
Scientific Reports  2014;4:7032.
Surface organic residues inhibit the extraordinary electronic properties of graphene, hindering the development of graphene electronics. However, fundamental understanding of the residue morphology is still absent due to a lack of high-throughput and high-resolution surface characterization methods. Here, we demonstrate that secondary electron (SE) imaging in the scanning electron microscope (SEM) and helium ion microscope (HIM) can provide sub-nanometer information of a graphene surface and reveal the morphology of surface contaminants. Nanoscale polymethyl methacrylate (PMMA) residues are visible in the SE imaging, but their contrast, i.e. the apparent lateral dimension, varies with the imaging conditions. We have demonstrated a quantitative approach to readily obtain the physical size of the surface features regardless of the contrast variation. The fidelity of SE imaging is ultimately determined by the probe size of the primary beam. HIM is thus evaluated to be a superior SE imaging technique in terms of surface sensitivity and image fidelity. A highly efficient method to reveal the residues on a graphene surface has therefore been established.
doi:10.1038/srep07032
PMCID: PMC4229663  PMID: 25391356
20.  Iodine Vapor Staining for Atomic Number Contrast in Backscattered Electron and X-ray Imaging 
Microscopy Research and Technique  2014;77(12):1044-1051.
Iodine imparts strong contrast to objects imaged with electrons and X-rays due to its high atomic number (53), and is widely used in liquid form as a microscopic stain and clinical contrast agent. We have developed a simple technique which exploits elemental iodine's sublimation-deposition state-change equilibrium to vapor stain specimens with iodine gas. Specimens are enclosed in a gas-tight container along with a small mass of solid I2. The bottle is left at ambient laboratory conditions while staining proceeds until empirically determined completion (typically days to weeks). We demonstrate the utility of iodine vapor staining by applying it to resin-embedded tissue blocks and whole locusts and imaging them with backscattered electron scanning electron microscopy (BSE SEM) or X-ray microtomography (XMT). Contrast is comparable to that achieved with liquid staining but without the consequent tissue shrinkage, stain pooling, or uneven coverage artefacts associated with immersing the specimen in iodine solutions. Unmineralized tissue histology can be read in BSE SEM images with good discrimination between tissue components. Organs within the locust head are readily distinguished in XMT images with particularly useful contrast in the chitin exoskeleton, muscle and nerves. Here, we have used iodine vapor staining for two imaging modalities in frequent use in our laboratories and on the specimen types with which we work. It is likely to be equally convenient for a wide range of specimens, and for other modalities which generate contrast from electron- and photon-sample interactions, such as transmission electron microscopy and light microscopy. Microsc. Res. Tech. 77:1044–1051, 2014. © 2014 The Authors. Microscopy Research Technique published by Wiley Periodocals, Inc.
doi:10.1002/jemt.22435
PMCID: PMC4285820  PMID: 25219801
SEM; microCT; desublimation; attenuation; segmentation; soft tissue; histology
21.  Collagen Cross-Linking Using Riboflavin and Ultraviolet-A for Corneal Thinning Disorders 
Executive Summary
Objective
The main objectives for this evidence-based analysis were to determine the safety and effectiveness of photochemical corneal collagen cross-linking with riboflavin (vitamin B2) and ultraviolet-A radiation, referred to as CXL, for the management of corneal thinning disease conditions. The comparative safety and effectiveness of corneal cross-linking with other minimally invasive treatments such as intrastromal corneal rings was also reviewed. The Medical Advisory Secretariat (MAS) evidence-based analysis was performed to support public financing decisions.
Subject of the Evidence-Based Analysis
The primary treatment objective for corneal cross-linking is to increase the strength of the corneal stroma, thereby stabilizing the underlying disease process. At the present time, it is the only procedure that treats the underlying disease condition. The proposed advantages for corneal cross-linking are that the procedure is minimally invasive, safe and effective, and it can potentially delay or defer the need for a corneal transplant. In addition, corneal cross-linking does not adversely affect subsequent surgical approaches, if they are necessary, or interfere with corneal transplants. The evidence for these claims for corneal cross-linking in the management of corneal thinning disorders such as keratoconus will be the focus of this review.
The specific research questions for the evidence review were as follows:
Technical: How technically demanding is corneal cross-linking and what are the operative risks?
Safety: What is known about the broader safety profile of corneal cross-linking?
Effectiveness - Corneal Surface Topographic Affects:
What are the corneal surface remodeling effects of corneal cross-linking?
Do these changes interfere with subsequent interventions, particularly corneal transplant known as penetrating keratoplasty (PKP)?
Effectiveness -Visual Acuity:
What impacts does the remodeling have on visual acuity?
Are these impacts predictable, stable, adjustable and durable?
Effectiveness - Refractive Outcomes: What impact does remodeling have on refractive outcomes?
Effectiveness - Visual Quality (Symptoms): What impact does corneal cross-linking have on vision quality such as contrast vision, and decreased visual symptoms (halos, fluctuating vision)?
Effectiveness - Contact lens tolerance: To what extent does contact lens intolerance improve after corneal cross-linking?
Vision-Related QOL: What is the impact of corneal cross-linking on functional visual rehabilitation and quality of life?
Patient satisfaction: Are patients satisfied with their vision following the procedure?
Disease Process:
What impact does corneal cross-linking have on the underling corneal thinning disease process?
Does corneal cross-linking delay or defer the need for a corneal transplant?
What is the comparative safety and effectiveness of corneal cross-linking compared with other minimally invasive treatments for corneal ectasia such as intrastromal corneal rings?
Clinical Need: Target Population and Condition
Corneal ectasia (thinning) disorders represent a range of disorders involving either primary disease conditions, such as keratoconus (KC) and pellucid marginal corneal degeneration, or secondary iatrogenic conditions, such as corneal thinning occurring after laser in situ keratomileusis (LASIK) refractive surgery.
Corneal thinning is a disease that occurs when the normally round dome-shaped cornea progressively thins causing a cone-like bulge or forward protrusion in response to the normal pressure of the eye. The thinning occurs primarily in the stroma layers and is believed to be a breakdown in the collagen process. This bulging can lead to irregular astigmatism or shape of the cornea. Because the anterior part of the cornea is responsible for most of the focusing of the light on the retina, this can then result in loss of visual acuity. The reduced visual acuity can make even simple daily tasks, such as driving, watching television or reading, difficult to perform.
Keratoconus is the most common form of corneal thinning disorder and involves a noninflammatory chronic disease process of progressive corneal thinning. Although the specific cause for the biomechanical alterations in the corneal stroma is unknown, there is a growing body of evidence suggesting that genetic factors may play an important role. Keratoconus is a rare disease (< 0.05% of the population) and is unique among chronic eye diseases because it has an early onset, with a median age of 25 years. Disease management for this condition follows a step-wise approach depending on disease severity. Contact lenses are the primary treatment of choice when there is irregular astigmatism associated with the disease. Patients are referred for corneal transplants as a last option when they can no longer tolerate contact lenses or when lenses no longer provide adequate vision.
Keratoconus is one of the leading indications for corneal transplants and has been so for the last 3 decades. Despite the high success rate of corneal transplants (up to 20 years) there are reasons to defer it as long as possible. Patients with keratoconus are generally young and a longer-term graft survival of at least 30 or 40 years may be necessary. The surgery itself involves lengthy time off work and postsurgery, while potential complications include long-term steroid use, secondary cataracts, and glaucoma. After a corneal transplant, keratoconus may recur resulting in a need for subsequent interventions. Residual refractive errors and astigmatism can remain challenges after transplantation, and high refractive surgery and regraft rates in KC patients have been reported. Visual rehabilitation or recovery of visual acuity after transplant may be slow and/or unsatisfactory to patients.
Description of Technology/Therapy
Corneal cross-linking involves the use of riboflavin (vitamin B2) and ultraviolet-A (UVA) radiation. A UVA irradiation device known as the CXL® device (license number 77989) by ACCUTECH Medical Technologies Inc. has been licensed by Health Canada as a Class II device since September 19, 2008. An illumination device that emits homogeneous UVA, in combination with any generic form of riboflavin, is licensed by Health Canada for the indication to slow or stop the progression of corneal thinning caused by progressive keratectasia, iatrogenic keratectasia after laser-assisted in situ keratomileusis (LASIK) and pellucid marginal degeneration. The same device is named the UV-X® device by IROCMedical, with approvals in Argentina, the European Union and Australia.
UVA devices all use light emitting diodes to generate UVA at a wavelength of 360-380 microns but vary in the number of diodes (5 to 25), focusing systems, working distance, beam diameter, beam uniformity and extent to which the operator can vary the parameters. In Ontario, CXL is currently offered at over 15 private eye clinics by refractive surgeons and ophthalmologists.
The treatment is an outpatient procedure generally performed with topical anesthesia. The treatment consists of several well defined procedures. The epithelial cell layer is first removed, often using a blunt spatula in a 9.0 mm diameter under sterile conditions. This step is followed by the application of topical 0.1% riboflavin (vitamin B2) solution every 3 to 5 minutes for 25 minutes to ensure that the corneal stroma is fully penetrated. A solid-state UVA light source with a wavelength of 370 nm (maximum absorption of riboflavin) and an irradiance of 3 mW/cm2 is used to irradiate the central cornea. Following treatment, a soft bandage lens is applied and prescriptions are given for oral pain medications, preservative-free tears, anti-inflammatory drops (preferably not nonsteroidal anti-inflammatory drugs, or NSAIDs) and antibiotic eye drops. Patients are recalled 1 week following the procedure to evaluate re-epithelialization and they are followed-up subsequently.
Evidence-Based Analysis Methods
A literature search was conducted on photochemical corneal collagen cross-linking with riboflavin (vitamin B2) and ultraviolet-A for the management of corneal thinning disorders using a search strategy with appropriate keywords and subject headings for CXL for literature published up until April 17, 2011. The literature search for this Health Technology Assessment (HTA) review was performed using the Cochrane Library, the Emergency Care Research Institute (ECRI) and the Centre for Reviews and Dissemination. The websites of several other health technology agencies were also reviewed, including the Canadian Agency for Drugs and Technologies in Health (CADTH) and the United Kingdom’s National Institute for Clinical Excellence (NICE). The databases searched included OVID MEDLINE, MEDLINE IN-Process and other Non-Indexed Citations such as EMBASE.
As the evidence review included an intervention for a rare condition, case series and case reports, particularly for complications and adverse events, were reviewed. A total of 316 citations were identified and all abstracts were reviewed by a single reviewer for eligibility. For those studies meeting the eligibility criteria, full-text articles were obtained. Reference lists were also examined for any additional relevant studies not identified through the search.
Inclusion Criteria
English-language reports and human studies
patients with any corneal thinning disorder
reports with CXL procedures used alone or in conjunction with other interventions
original reports with defined study methodology
reports including standardized measurements on outcome events such as technical success, safety effectiveness, durability, vision quality of life or patient satisfaction
systematic reviews, meta-analyses, randomized controlled trials, observational studies, retrospective analyses, case series, or case reports for complications and adverse events
Exclusion Criteria
nonsystematic reviews, letters, comments and editorials
reports not involving outcome events such as safety, effectiveness, durability, vision quality or patient satisfaction following an intervention with corneal implants
reports not involving corneal thinning disorders and an intervention involving CXL
Summary of Evidence Findings
In the Medical Advisory Secretariat evidence review on corneal cross-linking, 65 reports (16 case reports) involving 1403 patients were identified on the use of CXL for managing corneal thinning disorders. The reports were summarized according to their primary clinical indication, whether or not secondary interventions were used in conjunction with CXL (referred to as CXL-Plus) and whether or not it was a safety-related report.
The safety review was based on information from the cohort studies evaluating effectiveness, clinical studies evaluating safety, treatment response or recovery, and published case reports of complications. Complications, such as infection and noninfectious keratitis (inflammatory response), reported in case reports, generally occurred in the first week and were successfully treated with topical antibiotics and steroids. Other complications, such as the cytotoxic effects on the targeted corneal stroma, occurred as side effects of the photo-oxidative process generated by riboflavin and ultraviolet-A and were usually reversible.
The reports on treatment effectiveness involved 15 pre-post longitudinal cohort follow-up studies ranging from follow-up of patients’ treated eye only, follow-up in both the treated and untreated fellow-eye; and follow-up in the treated eye only and a control group not receiving treatment. One study was a 3-arm randomized control study (RCT) involving 2 comparators: one comparator was a sham treatment in which one eye was treated with riboflavin only; and the other comparator was the untreated fellow-eye. The outcomes reported across the studies involved statistically significant and clinically relevant improvements in corneal topography and refraction after CXL. In addition, improvements in treated eyes were accompanied by worsening outcomes in the untreated fellow-eyes. Improvements in corneal topography reported at 6 months were maintained at 1- and 2-year follow-up. Visual acuity, although not always improved, was infrequently reported as vision loss. Additional procedures such as the use of intrastromal corneal ring segments, intraocular lenses and refractive surgical practices were reported to result in additional improvements in topography and visual acuity after CXL.
Considerations for Ontario Health System
The total costs of providing CXL therapy to keratoconus patients in Ontario was calculated based on estimated physician, clinic, and medication costs. The total cost per patient was approximately $1,036 for the treatment of one eye, and $1,751 for the treatment of both eyes. The prevalence of keratoconus was estimated at 4,047 patients in FY2011, with an anticipated annual incidence (new cases) of about 148 cases. After distributing the costs of CXL therapy for the FY2011 prevalent keratoconus population over the next 3 years, the estimated average annual cost was approximately $2.1 million, of which about $1.3 million would be physician costs specifically.
Conclusion
Corneal cross-linking effectively stabilizes the underlying disease, and in some cases reverses disease progression as measured by key corneal topographic measures. The affects of CXL on visual acuity are less predictable and the use of adjunct interventions with CXL, such as intrastromal corneal ring segments, refractive surgery, and intraocular lens implants are increasingly employed to both stabilize disease and restore visual acuity. Although the use of adjunct interventions have been shown to result in additional clinical benefit, the order, timing, and risks of performing adjunctive interventions have not been well established.
Although there is potential for serious adverse events with corneal UVA irradiation and photochemical reactions, there have been few reported complications. Those that have occurred tended to be related to side effects of the induced photochemical reactions and were generally reversible. However, to ensure that there are minimal complications with the use of CXL and irradiation, strict adherence to defined CXL procedural protocols is essential.
Keywords
Keratoconus, corneal cross-linking, corneal topography, corneal transplant, visual acuity, refractive error.
PMCID: PMC3377552  PMID: 23074417
22.  Helium Ion Microscopy (HIM) for the imaging of biological samples at sub-nanometer resolution 
Scientific Reports  2013;3:3514.
Scanning Electron Microscopy (SEM) has long been the standard in imaging the sub-micrometer surface ultrastructure of both hard and soft materials. In the case of biological samples, it has provided great insights into their physical architecture. However, three of the fundamental challenges in the SEM imaging of soft materials are that of limited imaging resolution at high magnification, charging caused by the insulating properties of most biological samples and the loss of subtle surface features by heavy metal coating. These challenges have recently been overcome with the development of the Helium Ion Microscope (HIM), which boasts advances in charge reduction, minimized sample damage, high surface contrast without the need for metal coating, increased depth of field, and 5 angstrom imaging resolution. We demonstrate the advantages of HIM for imaging biological surfaces as well as compare and contrast the effects of sample preparation techniques and their consequences on sub-nanometer ultrastructure.
doi:10.1038/srep03514
PMCID: PMC3865489  PMID: 24343236
23.  Digging gold: keV He+ ion interaction with Au 
Summary
Helium ion microscopy (HIM) was used to investigate the interaction of a focused He+ ion beam with energies of several tens of kiloelectronvolts with metals. HIM is usually applied for the visualization of materials with extreme surface sensitivity and resolution. However, the use of high ion fluences can lead to significant sample modifications. We have characterized the changes caused by a focused He+ ion beam at normal incidence to the Au{111} surface as a function of ion fluence and energy. Under the influence of the beam a periodic surface nanopattern develops. The periodicity of the pattern shows a power-law dependence on the ion fluence. Simultaneously, helium implantation occurs. Depending on the fluence and primary energy, porous nanostructures or large blisters form on the sample surface. The growth of the helium bubbles responsible for this effect is discussed.
doi:10.3762/bjnano.4.53
PMCID: PMC3740815  PMID: 23946914
formation and healing of defects in crystals; helium ion microscopy; ion beam/solid interactions; vacancies in crystals
24.  Ultra-thin resistive switching oxide layers self-assembled by field-induced oxygen migration (FIOM) technique 
Scientific Reports  2014;4:6871.
High-performance ultra-thin oxide layers are required for various next-generation electronic and optical devices. In particular, ultra-thin resistive switching (RS) oxide layers are expected to become fundamental building blocks of three-dimensional high-density non-volatile memory devices. Until now, special deposition techniques have been introduced for realization of high-quality ultra-thin oxide layers. Here, we report that ultra-thin oxide layers with reliable RS behavior can be self-assembled by field-induced oxygen migration (FIOM) at the interface of an oxide-conductor/oxide-insulator or oxide-conductor/metal. The formation via FIOM of an ultra-thin oxide layer with a thickness of approximately 2–5 nm and 2.5% excess oxygen content is demonstrated using cross-sectional transmission electron microscopy and secondary ion mass spectroscopy depth profile. The observed RS behavior, such as the polarity dependent forming process, can be attributed to the formation of an ultra-thin oxide layer. In general, as oxygen ions are mobile in many oxide-conductors, FIOM can be used for the formation of ultra-thin oxide layers with desired properties at the interfaces or surfaces of oxide-conductors in high-performance oxide-based devices.
doi:10.1038/srep06871
PMCID: PMC4217097  PMID: 25362933
25.  The use of quartz crystal microbalance with dissipation (QCM-D) for studying nanoparticle-induced platelet aggregation 
Interactions between blood platelets and nanoparticles have both pharmacological and toxicological significance and may lead to platelet activation and aggregation. Platelet aggregation is usually studied using light aggregometer that neither mimics the conditions found in human microvasculature nor detects microaggregates. A new method for the measurement of platelet microaggregation under flow conditions using a commercially available quartz crystal microbalance with dissipation (QCM-D) has recently been developed. The aim of the current study was to investigate if QCM-D could be used for the measurement of nanoparticle-platelet interactions. Silica, polystyrene, and gold nanoparticles were tested. The interactions were also studied using light aggregometry and flow cytometry, which measured surface abundance of platelet receptors. Platelet activation was imaged using phase contrast and scanning helium ion microscopy. QCM-D was able to measure nanoparticle-induced platelet microaggregation for all nanoparticles tested at concentrations that were undetectable by light aggregometry and flow cytometry. Microaggregates were measured by changes in frequency and dissipation, and the presence of platelets on the sensor surface was confirmed and imaged by phase contrast and scanning helium ion microscopy.
doi:10.2147/IJN.S26679
PMCID: PMC3263416  PMID: 22275839
platelet aggregation; nanoparticles; light aggregometer; quartz crystal microbalance with dissipation; scanning helium ion microscopy

Results 1-25 (1136384)