Constitutive Model of Radiation Aging Effects in Filled Silicone Elastomers under Strain.

Filled silicone elastomers, an essential component in many technological applications, are often subjected to controlled or unintended radiation for a variety of reasons. Radiation exposure can lead to permanent mechanical and structural changes in the material, which is manifested as altered mechanical response, and in some cases, a permanent set. For unfilled elastomers, network theories developed and refined over decades can explain these effects in terms of chain-scission and cross-link formation and a hypothesis involving independent networks formed at different strain levels of the material. Here, we expose a filled silicone rubber to gamma radiation while being under finite elongational strain and show that the observed mechanical and structural changes can be quantitatively modeled within the same theoretical framework developed for unfilled elastomers as long as nuances associated with the Mullins effect are accounted for in a consistent manner. In this work, we employ Ogden's incompressible hyperelastic model within the framework of Tobolsky's two-network scheme to describe the observed permanent set and mechanical modulus changes as a function of radiation dosage. In the process, we conclude that gamma radiation induces both direct cross-linking at chain crossings (H-links) and main-chain-scission followed by cross-linking (Y-links). We provide an estimate of the ratio of chain-scission to cross-linking rates, which is in reasonable agreement with previous experimental estimate from Charlesby-Pinner analysis. We use density functional theory (DFT)-based quantum mechanical calculations to explore the stability of -Si and -SiO radicals that form upon a radiation-induced chain-scission event, which sheds light on the relative rates of Y-linking and H-linking processes.

Age-aware constitutive materials model for a 3D printed polymeric foam.

Traditional open or closed-cell stochastic elastomeric foams have wide-ranging applications in numerous industries: from thermal insulation, shock absorbing/gap-filling support cushions, packaging, to light-weight structural and positional components. Recent developments in 3D printing technologies by direct ink-write have opened the possibility of replacing stochastic foam parts by more controlled printed micro-structures with superior stress-distribution and longer functional life. For successful deployment as mechanical support or structural components, it is crucial to characterize the response of such printed materials to long-term external loads in terms of stress-strain behavior evolution and in terms of irreversible structural and load-bearing capacity changes over time. To this end, here we report a thermal-age-aware constitutive model for a 3D printed close-packed foam structure under compression. The model is based on the Ogden hyperfoam strain-energy functional within the framework of Tobolsky two-network scheme. It accurately describes experimentally measured stress-strain response, compression set, and load retention for various aging times and temperatures. Through the technique of time-temperature-superposition the model enables the prediction of long-term changes along with the quantification of uncertainty stemming from sample-to-sample variation and measurement noise. All aging parameters appear to possess the same Arrhenius activation barrier, which suggests a single dominant aging mechanism at the molecular/network level.

Age and factors associated with self-clean intermittent catheterization in patients with spina bifida.

PURPOSE: The purpose of this study is twofold: 1) to determine the age when a child with spina bifida (SB) will most likely transition from caregiver clean intermittent catheterization (CIC) to self-CIC, and 2) to identify factors associated with self-CIC in children older than that age. METHODS: This is a retrospective, single-institution cohort study of individuals with SB. Data were collected prospectively as part of the National Spina Bifida Patient Registry. For Aim 1, we identified all individuals who perform self-CIC and who had a documented transition from caregiver-CIC. We then determined the age of transition to self-CIC. For Aim 2, we compared individuals over age 10 years (age cutoff determined by Aim 1) who use self-CIC to those who use caregiver-CIC to determine what variables were associated with self-CIC. RESULTS: From our SB population, 206 individuals used self-CIC. Of these, 64 patients had documented ages of transition from caregiver- to self-CIC. 46 (71.9%) and 56 (87.5%) patients had transitioned to self-CIC by 10 and 14 years, respectively. For Aim 2, we used age 10 as a cutoff, based on the findings from Aim 1, and found that 287/696 patients were ⩾ 10 years and using CIC. Factors independently associated with lower likelihood of self-CIC were thoracic spinal lesions (odds ratio (OR) 0.45) and Medicaid insurance (OR 0.24). CONCLUSIONS: The ages at self-CIC transition vary, although most patients transition by age 10. Thoracic-level spinal lesions and Medicaid insurance are associated with lower odds of self-CIC.

Embolectomy in a rabbit acute arterial occlusion model using a novel electromechanical extraction device.

A prototype endovascular electromechanical clot-extraction device was fabricated using a combination of shape memory polymer and shape memory nickel-titanium alloy (nitinol). Five embolic vascular occlusions were created in 4 rabbits by injecting thermally coagulated blood through a 4F catheter in the common carotid artery. Angiography immediately after clot injection showed complete or partial occlusion of the common carotid artery. Posttreatment angiography showed complete (2/5), partial (2/5), or no (1/5) restoration of blood flow.

3D printed cellular solid outperforms traditional stochastic foam in long-term mechanical response.

3D printing of polymeric foams by direct-ink-write is a recent technological breakthrough that enables the creation of versatile compressible solids with programmable microstructure, customizable shapes, and tunable mechanical response including negative elastic modulus. However, in many applications the success of these 3D printed materials as a viable replacement for traditional stochastic foams critically depends on their mechanical performance and micro-architectural stability while deployed under long-term mechanical strain. To predict the long-term performance of the two types of foams we employed multi-year-long accelerated aging studies under compressive strain followed by a time-temperature-superposition analysis using a minimum-arc-length-based algorithm. The resulting master curves predict superior long-term performance of the 3D printed foam in terms of two different metrics, i.e., compression set and load retention. To gain deeper understanding, we imaged the microstructure of both foams using X-ray computed tomography, and performed finite-element analysis of the mechanical response within these microstructures. This indicates a wider stress variation in the stochastic foam with points of more extreme local stress as compared to the 3D printed material, which might explain the latter's improved long-term stability and mechanical performance.

Age-related changes in levels of tyrosine kinase B receptor and fibroblast growth factor receptor 2 in the rat inferior colliculus: implications for neural senescence.

Brain-derived neurotrophic factor and fibroblast growth factor 2, and their respective binding sites, tyrosine kinase B receptor and fibroblast growth factor receptor 2, are known to regulate neurite outgrowth and antioxidant enzyme activity. Several studies suggest that brain-derived neurotrophic factor and fibroblast growth factor are contained in the inferior colliculus. Previous work in our laboratories revealed dendritic and synaptic losses in the inferior colliculus of aged Fischer-344 rats, along with coincident increases in lipid peroxidation possibly linked to a decrease in activity of antioxidant enzymes. In an effort to identify potential causal mechanisms underlying age-related synaptic and dendritic losses that occur in the inferior colliculus, the present study attempted to determine if inferior colliculus levels of tyrosine kinase B receptor and fibroblast growth factor receptor 2 expression are altered with age. Immunocytochemistry was performed in the inferior colliculus, hippocampus and cerebellum of 3-month-old F344 rats to study distributions of the full-length and truncated isoforms of tyrosine kinase B receptor, and fibroblast growth factor receptor 2. The latter two brain regions served as positive controls. For all three antigens, immunolabeling was localized primarily in somata and proximal dendrites in all subdivisions of the inferior colliculus, and in the dentate gyrus and Ammon's horn of the hippocampus. In the cerebellum, the somata and dendrites of the Purkinje cells were also immunolabeled.A significant reduction in levels of the full-length form of tyrosine kinase B receptor in 18- and 25-month-old rats (respectively, approximately 20% and 30% relative to 3-month-olds) was revealed using western blot analyses. Inferior colliculus and hippocampal levels of the truncated form were modestly decreased ( approximately 7%) as well in the two older age groups. In contrast, levels of fibroblast growth factor receptor 2 in the inferior colliculus and hippocampus were elevated by approximately 35% in the two older age groups when compared to 3-month-olds. Changes in cerebellar levels of tyrosine kinase B receptor and fibroblast growth factor receptor 2, while similar to those in the inferior colliculus and hippocampus among the age groups, did not achieve statistical significance in this study. These findings give rise to the possibility that age-related reductions in tyrosine kinase B receptor levels could be a causal factor in the degenerative changes observed in the inferior colliculus of aged animals, including mitochondrial damage and dendritic regression. The observed increases in fibroblast growth factor receptor 2 levels may be compensatory to the increased oxidative stress. The effectiveness of the fibroblast growth factor receptor 2 response is questionable given the damage that occurs in the inferior colliculus and hippocampus of aged animals. However, the deficits could worsen in the absence of an increase in fibroblast growth factor receptor 2.

Assessment of skeletal muscle viability by PET.

UNLABELLED: We investigated the use of [18F]fluoro-2-deoxyglucose (FDG) PET scanning for assessment of skeletal muscle viability in patients with peripheral vascular disease and in patients following free-flap skeletal muscle transfer for closure of open wounds. METHODS: We obtained 32 FDG-PET scans from 30 patients, either at the time of admission for peripheral vascular disease (n = 16) or between 1 and 15 days after surgery for skeletal muscle transfer (n = 16). Ratios between injured and contralateral limb FDG tracer activity uptake were correlated with clinical outcome at 1 mo to 3 yr follow-up. RESULTS: Viable muscle uptake ratios ranged from 0.47 to 7.88 (mean: 2.26 +/- 1.81; n = 26), while nonviable muscle uptake ratios ranged from 0.12 to 0.46 (mean: 0.27 +/- 0.12; n = 6; p < 0.02). After skeletal muscle transfer, two patients with viable tissue, as documented by PET, required amputation due to osteomyelitis, and one patient with peripheral vascular disease who showed viable tissue by PET required amputation 3 mo after the PET scan because of recurrent ulcers. CONCLUSION: FDG-PET scanning can determine skeletal muscle viability in patients with peripheral vascular disease and in patients following free-flap transfer.

Recent observations on leptospirosis in Northern Ireland and their bearing on current diagnostic methods.

An investigation was undertaken to assess the present importance of leptospiral infections in Northern Ireland, and in particular to look for evidence of infection by leptospiral serotypes other than L. icterohaemorrhagiae and L. canicola. Blood samples from 335 patients, sent to the laboratory for a variety of tests, were examined. After initial screening with five groups of pooled antigens, tests for leptospiral agglutinins were completed with formolized antigens prepared from 13 different serotypes. In seven patients a diagnosis of acute leptospirosis was made while nine others showed serological evidence of previous leptospiral infection. Attempts to isolate leptospirae by culture from 29 blood samples were unsuccessful. The serological results indicate that two additional leptospiral serotypes, namely L. ballum and L. bratislava, are causing human infection in Northern Ireland, and presumably also in other parts of the British Isles. Some clinical and epidemiological features associated with different types of leptospiral infection are described. It is stressed that leptospirosis is essentially a febrile illness, that meningeal symptoms are common, and that (contrary to popular belief) jaundice is by no means a constant occurrence. The implications of these findings are discussed, with special reference to the diagnosis of leptospiral infections. Laboratory diagnostic procedures are briefly reviewed, and the possible deficiencies of the agglutination test commonly used in Britain are pointed out. Some suggestions are made concerning both clinical and laboratory aspects of diagnosis, and the need for a reliable screening test for all forms of leptospiral infection is emphasized.

Immunoglobulin G (IgG) serological response to Branhamella catarrhalis in patients with acute bronchopulmonary infections.

Over 12 months serum was collected from 45 inpatients, with acute bronchopulmonary infection, in whose sputum Branhamella catarrhalis predominated, or was the sole pathogen. Serum was examined for IgG against B catarrhalis using an immunofluorescence antibody test. Acute and convalescent sera were compared with sera of age and sex matched controls. The convalescent sera had significantly higher titres than the acute sera which in turn had higher titres than the controls. The findings confirm the role of B catarrhalis as a respiratory pathogen and suggest that patients with chronic pulmonary disease are more prone to infection with this organism than the general population.

Cryptococcal meningitis associated with steroid therapy.

Two patients on prolonged steroid therapy developed meningitis due to Cryptococcus neoformans. The first responded satisfactorily to treatment with amphotericin B, both initially and again following relapse. The second died shortly after treatment was begun. Pathogenicity studies suggest that the strain isolated from the fatal case was the more virulent. Cryptococcal meningitis probably occurs more often in Britain than is generally appreciated, and this possibility should be remembered when investigating patients with obscure forms of meningitis; if not, then the correct diagnosis may not be made. Attention is drawn to the increasing number of recently reported cases of this disease which have been associated with long-term steroid therapy.

Investigation of a nosocomial outbreak of Pseudomonas aeruginosa pneumonia in an intensive care unit by random amplification of polymorphic DNA assay.

From July to September 1993 in the intensive care unit of the Royal Victoria Hospital there were 10 cases of pneumonia associated with sputum culture of Pseudomonas aeruginosa. The isolates had an identical biotype and pyocine typing profile. The same strain of P. aeruginosa was recovered from the sink plug-hole in two rooms, and the tap handles and ventilator tubing in a third room. All strains were retrospectively typed by the random amplification of polymorphic DNA (RAPD) method using a 26-mer oligonucleotide primer, and were identical in profile. Recommendations to medical and nursing staff included secretion isolation precautions, terminal disinfection after patient discharge, use of disposable vinyl gloves by hospital staff for all body substance contacts, thorough handwashing with 4% chlorhexidine gluconate before and after dealing with all patient contacts, and prompt, appropriate antibiotic treatment for P. aeruginosa pneumonia. RAPD is a simple and effective method to determine the relatedness of P. aeruginosa isolates, and typing results are available within a single working day; thus dramatically increasing its clinical relevance over existing molecular methods.

Atypical endocrine tumors of the lung.

Seven malignant peripheral lung tumors that were diagnosed using light microscopy as large-cell carcinomas or as epidermoid or adenocarcinomas were studied by light and electron microscopic histochemistry. All tumors contained numerous dense-core granules. The cells were joined by desmosomes and contained well-developed tonofilament bundles. Serotonin was demonstrated in six of seven tumors and argyrophilic granules were demonstrated in five of six tumors tested. Four tumors produced mucus. All tumors extended to the visceral pleura and two invaded the chest wall. The existence of lung tumors that contain serotonin and bear argyrophilic putative endocrine granules, but that do not have a light-microscopic histology characteristic of either carcinoid or oat-cell carcinomas, is confirmed. The presumptive endocrine nature of such tumors usually passes unrecognized because they lack criteria to allow diagnosis by routine methods.

Immunohistochemical studies of keratin in human bronchus and lung tumors.

Lung tumors of various types, fixed in 4% formaldehyde-1% glutaraldehyde, were stained for keratin proteins. The results were compared with previous ultrastructural evidence of intermediate filament bundles (IFBs), presumed to be keratin. Electron and light microscopic methods were largely complimentary and the results were in agreement in 79% of cases. Light microscopy was superior for demonstrating keratinizing foci containing numerous well-developed IFBs, whereas electron microscopy was superior when keratin filaments were sparsely distributed in cells throughout a tumor. Fetal and adult bronchial specimens were also studied. Normal adult bronchus, fixed in aldehydes, was unreactive but keratin was observed in similarly fixed bronchi that showed epidermoid metaplasia and/or intraepithelial carcinoma. Keratin was demonstrated in normal adult bronchi fixed in ethanol. Keratin was not observed in fetal lung until the 14th week of gestation, when it appeared in basal cells and a few columnar cells of the larger bronchi. Thereafter, keratin progressively appeared in the more distal branches. As specimens from gestations of less than 14 weeks were fixed in aldehydes, the apparent lack of immunoreactivity may have been artifactual. Nevertheless, keratin was demonstrable in aldehyde-fixed fetal bronchi at 16 and 23 weeks' gestation.

Elaboration of human chorionic gonadotropin by lung tumors: an immunocytochemical study.

The highly sensitive peroxidase-antiperoxidase immunocytochemical method was used to demonstrate the presence of the beta-subunit of human chorionic gonadotropin (HCG) within paraffin sections of human lung tumors of diverse histologic patterns. Of the 61 tumors studied, 51 (84%) displayed HCG-like immunoreactivity. This is a much higher incidence than was expected considering studies by other investigators in which serum samples from patients with lung cancer were assayed for HCG. Our results are, however, consistent with data from studies by other investigators in which tumor extracts were assayed for HCG. In addition, it was found that HCG production was often linked with glycogen storage within tumor cells. This may explain the association of HCG production with large-cell carcinomas of the lung, because these tumors often contain copious quantities of glycogen.

Histochemical studies of dense-core granulated tumors of the lung. Neuron-specific enolase as a marker for granulated cells.

It is well known that carcinoid tumors and some small-cell carcinomas of the lung contain dense-core granules (DCGs). Moreover, a small number of tumors presenting with epidermoid, large-cell, or adenocarcinoma histologic characteristics (so-called atypical endocrine tumors), also contain DCGs. Herein, we describe certain histochemical features of DCG tumors and compare them with other major lung tumor types that lack DCGs (non-DCG tumors). All DCG tumors contained neuron-specific enolase and many contained serotonin. These markers were not present in any non-DCG tumor. Other histochemical markers (glycogen, mucosubstances, corticotropin, beta-human chorionic gonadotropin, keratin, somatostatin, and calcitonin) were found in a proportion of DCG and non-DCG tumors, but were, in general, more common in non-DCG tumors and atypical endocrine tumors than in carcinoids and small-cell carcinomas. alpha-Fetoprotein was rarely found in non-DCG tumors, and was never observed in DCG tumors. The atypical endocrine group represents a class of tumors with a remarkably mixed and varied phenotype. Their potential significance is discussed and methods to facilitate their diagnosis are suggested.

LASIK surgery.

The laser in situ keratomileusis (i.e., LASIK) procedure reshapes the surface of the cornea to focus visual images directly onto the retina, thereby improving visual acuity. In 1998, approximately 420,000 LASIK procedures were performed in the United States, and approximately two million people throughout the world already are benefiting from the LASIK vision correction procedure.

Mullins effect in a filled elastomer under uniaxial tension.

Modulus softening and permanent set in filled polymeric materials due to cyclic loading and unloading, commonly known as the Mullins effect, can have a significant impact on their use as support cushions. A quantitative analysis of such behavior is essential to ensure the effectiveness of such materials in long-term deployment. In this work we combine existing ideas of filler-induced modulus enhancement, strain amplification, and irreversible deformation within a simple non-Gaussian constitutive model to quantitatively interpret recent measurements on a relevant PDMS-based elastomeric cushion. We find that the experimental stress-strain data is consistent with the picture that during stretching (loading) two effects take place simultaneously: (1) the physical constraints (entanglements) initially present in the polymer network get disentangled, thus leading to a gradual decrease in the effective cross-link density, and (2) the effective filler volume fraction gradually decreases with increasing strain due to the irreversible pulling out of an initially occluded volume of the soft polymer domain.

Post-Polymerization Crosslinked Polyurethane Shape-Memory Polymers.

Novel urethane shape-memory polymers (SMPs) of significant industrial relevance have been synthesized and characterized. Chemically crosslinked SMPs have traditionally been made in a one-step polymerization of monomers and crosslinking agents. However, these new post-polymerization crosslinked SMPs can be processed into complex shapes by thermoplastic manufacturing methods and later crosslinked by heat exposure or by electron beam irradiation. Several series of linear, olefinic urethane polymers were made from 2-butene-1,4-diol, other saturated diols, and various aliphatic diisocyanates. These thermoplastics were melt-processed into desired geometries and thermally crosslinked at 200°C or radiation crosslinked at 50 kGy. The SMPs were characterized by solvent swelling and extraction, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile testing, and qualitative shape-recovery analysis. Swelling and DMA results provided concrete evidence of chemical crosslinking, and further characterization revealed that the urethanes had outstanding mechanical properties. Key properties include tailorable transitions between 25 and 80°C, tailorable rubbery moduli between 0.2 and 4.2 MPa, recoverable strains approaching 100%, failure strains of over 500% at T(g), and qualitative shape-recovery times of less than 12 seconds at body temperature (37°C). Because of its outstanding thermo-mechanical properties, one polyurethane was selected for implementation in the design of a complex medical device. These post-polymerization crosslinked urethane SMPs are an industrially relevant class of highly processable shape-memory materials.