Ocular Impression-Taking-Which Material Is Best?

OBJECTIVES: To assess the efficacy and effect on clinical signs of a polyvinylsiloxane (Tresident; Shütz Dental Group GmbH, Germany) compared with an irreversible hydrocolloid (Orthoprint; Zhermack SpA, Badia Polesine, Italy) for ocular impression-taking. METHODS: Twenty subjects were recruited (13 female and 7 male), with mean age 31.1±4.6 years (SD) (range 25.8-39.7). Subjects attended for 2 sessions, each of 1-hr duration, on 2 separate days. Each session was scheduled at the same time on each day. At each visit, the subject underwent an ocular impression procedure, using either Tresident or Orthoprint, in random order and to one eye only. Investigator 2 was blind to this assignment. Two experienced practitioners conducted the study, investigator 1 performed the ocular impression procedures and investigator 2 observed and assessed the clinical signs: logMAR visual acuity, ocular surface staining, tear break-up time (TBUT), and ocular hyperemia. RESULTS: Visual acuity was unaffected by either material; TBUT was marginally disrupted by both materials, but was not clinically significant according to published criteria; ocular redness increased with both materials; and corneal staining was significantly greater after Orthoprint impression. Less redness and clinically insignificant staining after impression-taking, with fewer clinical complications, was found after use of Tresident. CONCLUSIONS: Tresident offers a quicker, more effective, and clinically viable method of obtaining ocular impression topography compared with the traditional Orthoprint, and Orthoprint causes significantly more superficial punctuate staining of the corneal epithelium than Tresident.

Research on the relationship between cephalosporin structure, solution clarity, and rubber closure compatibility using volatile components profile of butyl rubber closures.

OBJECTIVE: Establish an effective experimental strategy to determine the compatibility of rubber closures for drugs. SIGNIFICANCE: Various types of rubber closures with different compositions are available for drug packaging. Many additives of rubber closures can be released from rubber closures and may affect the quality of drugs and pose a risk to human health. In this study, we aimed to determine the relationship between cephalosporin structure, solution clarity, and rubber closure compatibility using volatile components profile of butyl rubber closures. METHODS: Two opposite polarity gas chromatography (GC) systems and GC-mass spectrometry (MS) were used to achieve rapid qualitative determination of the main volatile components in rubber closures. Simulated adsorption experiment was performed to investigate the adsorption of main volatile components in rubber closures by cephalosporins with different side chain structures, and to determine the effects of adsorption on solution clarity. RESULTS: A volatile components screening library of rubber closures was established and the structures of some volatile component were confirmed. The specific adsorption of the structure of cephalosporins on volatile components from rubber closures was studied. CONCLUSION: Based on the results of this study, rubber closures with good compatibility for cephalosporins with different side chain structures can be selected rapidly. This experimental strategy not only facilitates the screening of suitable rubber closures more effectively, but also enables the quick determination of volatile components adsorbed by drugs.

Elastic Properties of Magnetorheological Elastomers in a Heterogeneous Uniaxial Magnetic Field.

Magnetorheological elastomers (MREs) are stimulus-responsive soft materials that consist of polymeric matrices and magnetic particles. In this study, large-strain response of MREs with 5 vol % of carbonyl iron (CI) particles is experimentally characterized for two different conditions: (1) shear deformation in a uniform magnetic field; and (2), compression in a heterogeneous uniaxial magnetic field. For condition (1), dynamic viscoelastic measurements were performed using a rheometer with a rotor disc and an electric magnet that generated a uniform magnetic field on disc-like material samples. For condition (2), on the other hand, three permanent magnets with different surface flux densities were used to generate a heterogeneous uniaxial magnetic field under cylindrical material samples. The experimental results were mathematically modeled, and the relationship between them was investigated. We also used finite-element method (FEM) software to estimate the uniaxial distributions of the magnetic field in the analyzed MREs for condition (2), and developed mathematical models to describe these phenomena. By using these practicable techniques, we established a simple macroscale model of the elastic properties of MREs under simple compression. We estimated the elastic properties of MREs in the small-strain regime (neo⁻Hookean model) and in the large-strain regime (Mooney⁻Rivlin model). The small-strain model explains the experimental results for strains under 5%. On the other hand, the large-strain model explains the experimental results for strains above 10%.

In-depth characterization of polyolefin plastomers/elastomers (ethylene/1-octene copolymers) through hyphenated chromatographic techniques.

The chemical composition distribution (CCD) of three single site made ethylene/1-octene copolymers was investigated through offline-hyphenation of solvent gradient interaction chromatography (SGIC) with 1H NMR. Thus, a clear, non-linear correlation between SGIC elution time and chemical composition was found under the specific measurement conditions applied here. The application of 1H NMR as detection allowed to determine the CCD with unprecedented accuracy. 2D-LC of the copolymers revealed the correlation between CCD and molar mass distribution (MMD) in a quantitative manner. Furthermore, this approach allowed a comparison between the response behavior of an evaporative light scattering detector (ELSD, semi-quantitative, commonly applied in SGIC) and that of an infrared (IR) detector (quantitative, commonly applied in SEC). As a result, it could be shown that ELSD results are close to IR results for the system investigated here, in other words, the often-criticized semi-quantitative response behavior of the ELSD is affecting results in an acceptable manner.

Evaluation of Container Closure System Integrity for Storage of Frozen Drug Products: Impact of Capping Force and Transportation.

Frozen-state storage and cold-chain transport are key operations in the development and commercialization of biopharmaceuticals. Today, several marketed drug products are stored (and/or shipped) under frozen conditions to ensure sufficient stability, particularly for live viral vaccines. When these products are stored in glass vials with stoppers, the elastomer of the stopper needs to be flexible enough to seal the vial at the target's lowest temperature to ensure container closure integrity and thus both sterility and safety of the drug product. The container closure integrity assessment in the frozen state (e.g., -20°C, -80°C) should include container closure integrity (CCI) of the container closure system (CCS) itself, impact of processing (e.g., capping process on CCI), and impact of shipment and movement on CCI in the frozen state. The objective of this work was to evaluate the impact of processing and shipment on CCI of a CCS in the frozen state. The impact on other quality attributes was not investigated. In this light, the ThermCCI method was applied to evaluate the impact of shipping stress and variable capping force on CCI of frozen vials and to evaluate the temperature limits of rubber stoppers. In conclusion, retaining CCI during cold storage is mostly a function of vial-stopper combination, and temperatures below -40°C may pose a risk to the CCI of a frozen drug product. Variable capping force may have an influence on the CCI of a frozen drug product if not appropriately assessed. Regarding the impact of shipment on the CCI of glass vials, no indication was given at room temperature, -20°C, or -75°C when compared with static storage at such temperatures.LAY ABSTRACT: Today, several marketed products are stored (and/or shipped) under frozen conditions to ensure sufficient stability. When these products are stored in glass vials with stoppers, the elastomer of the stopper needs to be flexible enough to seal the vial and ensure container closure integrity and thus both sterility and safety of the drug product. The impact of processing and shipment on the container closure integrity (CCI) of a container closure system (vial, stopper, and flip-off cap) in the frozen state is assessed. A helium-leakage test at low temperature (ThermCCI) was used to evaluate the impact of shipping stress and variable capping force on CCI of frozen vials as well as the temperature limits of rubber stoppers. In conclusion, it was found that retaining CCI during cold storage is mostly a function of vial-stopper combination and that temperatures below -40°C may pose a risk to the CCI of a frozen drug product. Variable capping force may have an influence on the CCI of a frozen drug product if not appropriately assessed. Additionally, it was observed that the shipment of the frozen glass vials did not affect the CCI.

Chemically defined, ultrasoft PDMS elastomers with selectable elasticity for mechanobiology.

Living animal cells are strongly influenced by the mechanical properties of their environment. To model physiological conditions ultrasoft cell culture substrates, in some instances with elasticity (Young's modulus) of only 1 kPa, are mandatory. Due to their long shelf life PDMS-based elastomers are a popular choice. However, uncertainty about additives in commercial formulations and difficulties to reach very soft materials limit their use. Here, we produced silicone elastomers from few, chemically defined and commercially available substances. Elastomers exhibited elasticities in the range from 1 kPa to 55 kPa. In detail, a high molecular weight (155 kg/mol), vinyl-terminated linear silicone was crosslinked with a multifunctional (f = 51) crosslinker (a copolymer of dimethyl siloxane and hydrosilane) by a platinum catalyst. The following different strategies towards ultrasoft materials were explored: sparse crosslinking, swelling with inert silicone polymers, and, finally, deliberate introduction of dangling ends into the network (inhibition). Rheological experiments with very low frequencies led to precise viscoelastic characterizations. All strategies enabled tuning of stiffness with the lowest stiffness of ~1 kPa reached by inhibition. This system was also most practical to use. Biocompatibility of materials was tested using primary cortical neurons from rats. Even after several days of cultivation no adverse effects were found.

Deformation and fatigue of tough 3D printed elastomer scaffolds processed by fused deposition modeling and continuous liquid interface production.

Polyurethane (PU) based elastomers continue to gain popularity in a variety of biomedical applications as compliant implant materials. In parallel, advancements in additive manufacturing continue to provide new opportunities for biomedical applications by enabling the creation of more complex architectures for tissue scaffolding and patient specific implants. The purpose of this study was to examine the effects of printed architecture on the monotonic and cyclic mechanical behavior of elastomeric PUs and to compare the structure-property relationship across two different printing approaches. We examined the tensile fatigue of notched specimens, 3D crosshatch scaffolds, and two 3D spherical pore architectures in a physically crosslinked polycarbonate urethane (PCU) printed via fused deposition modeling (FDM) as well as a photo-cured, chemically-crosslinked, elastomeric PU printed via continuous liquid interface production (CLIP). Both elastomers were relatively tolerant of 3D geometrical features as compared to stiffer synthetic implant materials such as PEEK and titanium. PCU and crosslinked PU samples with 3D porous structures demonstrated a reduced tensile failure stress as expected without a significant effect on tensile failure strain. PCU crosshatch samples demonstrated similar performance in strain-based tensile fatigue as solid controls; however, when plotted against stress amplitude and adjusted by porosity, it was clear that the architecture had an impact on performance. Square shaped notches or pores in crosslinked PU appeared to have a modest effect on strain-based tensile fatigue while circular shaped notches and pores had little impact relative to smooth samples. When plotted against stress amplitude, any differences in fatigue performance were small or not statistically significant for crosslinked PU samples. Despite the slight difference in local architecture and tolerances, crosslinked PU solid samples were found to perform on par with PCU solid samples in tensile fatigue, when appropriately adjusted for material hardness. Finally, tests of samples with printed architecture localized to the gage section revealed an effect in which fatigue performance appeared to drastically improve despite the localization of strain.

Viscoelastic and mechanical behavior of recombinant protein elastomers.

Recombinant DNA synthesis was employed to produce elastin-mimetic protein triblock copolymers containing chemically distinct midblocks. These materials displayed a broad range of mechanical and viscoelastic responses ranging from plastic to elastic when examined as hydrated gels and films. These properties could be related in a predictable fashion to polymer block size and structure. While these materials could be easily processed into films and gels, electrospinning proved a feasible strategy for creating protein fibers. All told, the range of properties exhibited by this new class of protein triblock copolymer in combination with their easy processability suggests potential utility in a variety of soft prosthetic and tissue engineering applications.

Mechanical properties and remodeling of hybrid cardiac constructs made from heart cells, fibrin, and biodegradable, elastomeric knitted fabric.

Hybrid cardiac constructs with mechanical properties suitable for in vitro loading studies and in vivo implantation were constructed from neonatal rat heart cells, fibrin (Fn), and biodegradable knitted fabric (Knit). Initial (2-h) constructs were compared with native heart tissue, studied in vitro with respect to mechanical function (stiffness, ultimate tensile strength [UTS], failure strain epsilon(f), strain energy density E) and compositional remodeling (collagen, DNA), and implanted in vivo. For 2-h constructs, stiffness was determined mainly by the Fn and was half as high as that of native heart, whereas UTS, epsilon(f), and E were determined by the Knit and were, respectively, 8-, 7-, and 30-fold higher than native heart. Over 1 week of static in vitro culture, cell-mediated, serum-dependent remodeling was demonstrated by a 5-fold increase in construct collagen content and maintenance of stiffness not observed in cell-free constructs. Cyclic stretch further increased construct collagen content in a manner dependent on loading regimen. The presence of cardiac cells in cultured constructs was demonstrated by immunohistochemistry (troponin I) and Western blot (connexin 43). However, in vitro culture reduced Knit mechanical properties, decreasing UTS, epsilon(f), and E of both constructs and cell-free constructs and motivating in vivo study of the 2-h constructs. Constructs implanted subcutaneously in nude rats for 3 weeks exhibited the continued presence of cardiomyocytes and blood vessel ingrowth by immunostaining for troponin I, connexin 43, and CD-31. Together, the data showed that hybrid cardiac constructs initially exhibited supraphysiologic UTS, epsilon(f), and E, and remodeled in response to serum and stretch in vitro and in an ectopic in vivo model.

Enhanced sensitivity to residual dipolar couplings of elastomers by higher-order multiple-quantum NMR.

The homonuclear and heteronuclear residual dipolar couplings in elastomers reflect changes in the cross-link density, temperature, the uniaxial and biaxial extension or compression as well as the presence of penetrant molecules. It is shown theoretically that for an isolated methyl group the relative changes in the intensity of the homonuclear double-quantum buildup curves in the initial time regime due to variation of the residual dipolar coupling strength is less sensitive than the changes in the triple-quantum filtered NMR signal when considering the same excitation/reconversion time. For a quadrupolar nucleus with spin I=2 the sensitivity enhancement was simulated for four-quantum, triple-quantum, and double-quantum buildup curves. In this case the four-quantum build-up curve shows the highest sensitivity to changes of spin couplings. This enhanced sensitivity to the residual dipolar couplings was tested experimentally by measuring 1H double-quantum, triple-quantum, and four-quantum buildup curves of differently cross-linked natural rubber samples. In the initial excitation/reconversion time regime, where the residual dipolar couplings can be measured model free, the relative changes in the intensity of the four-quantum buildup curves are about five times higher than those of the double-quantum coherences. For the first time proton four-quantum coherences were recorded for cross-linked elastomers.

Evaluation of current compendial physiochemical test procedures for pharmaceutical elastomeric closures and development of an improved HPLC procedure.

The interaction between elastomeric container closures and the solutions they confine presents a potential hazard to the consumer due to extraction of closure ingredients into the dosage form. Each of the major Pharmacopeias, the United States, the European, and the Japanese, prescribe testing procedures for elastomeric closures. These consist of a series of non-specific wet chemical analyses performed on samples extracted into water or, in some cases, isopropanol (IPA) or the drug product vehicle. No consideration is given to the extracting potential of the drug product. Results from our testing on ten randomly selected closure samples indicated that these tests are not sensitive or specific enough to accurately measure the levels of extractables. Therefore, an HPLC gradient method was developed which had the required sensitivity and specificity. The prescribed compendial extractions, when performed on the various stopper types, proved inefficient and experiments were conducted in an attempt to improve them. These included increasing the time of the extractions, increasing the closure surface area, and increasing the strength of the extracting solvent (methylene chloride). The HPLC gradient method and the compendial wet chemical tests were then used to evaluate the stopper extractables. Results of the compendial analyses on the prescribed aqueous extractions were inconclusive as the number and relative amount of extractables in the closure could not be measured. The results of the compendial testing were only marginally improved using the stronger extraction conditions. Testing was dramatically improved, however, using the HPLC gradient method. As many as twenty extractables were detected in some of the samples and, unlike the compendial analysis, low level extractables were detected in the water samples. Identification of some of the extractables was accomplished via GC/MS.

Environmental-sanitary risk analysis procedure applied to artificial turf sports fields.

Owing to the extensive use of artificial turfs worldwide, over the past 10 years there has been much discussion about the possible health and environmental problems originating from styrene-butadiene recycled rubber. In this paper, the authors performed a Tier 2 environmental-sanitary risk analysis on five artificial turf sports fields located in the city of Turin (Italy) with the aid of RISC4 software. Two receptors (adult player and child player) and three routes of exposure (direct contact with crumb rubber, contact with rainwater soaking the rubber mat, inhalation of dusts and gases from the artificial turf fields) were considered in the conceptual model. For all the fields and for all the routes, the cumulative carcinogenic risk proved to be lower than 10(-6) and the cumulative non-carcinogenic risk lower than 1. The outdoor inhalation of dusts and gases was the main route of exposure for both carcinogenic and non-carcinogenic substances. The results given by the inhalation pathway were compared with those of a risk assessment carried out on citizens breathing gases and dusts from traffic emissions every day in Turin. For both classes of substances and for both receptors, the inhalation of atmospheric dusts and gases from vehicular traffic gave risk values of one order of magnitude higher than those due to playing soccer on an artificial field.

New approach to the ecotoxicological risk assessment of artificial outdoor sporting grounds.

Artificial surfaces for outdoor sporting grounds may pose environmental and health hazards that are difficult to assess due to their complex chemical composition. Ecotoxicity tests can indicate general hazardous impacts. We conducted growth inhibition (Pseudokirchneriella subcapitata) and acute toxicity tests (Daphnia magna) with leachates obtained from batch tests of granular infill material and column tests of complete sporting ground assemblies. Ethylene propylene diene monomer rubber (EPDM) leachate showed the highest effect on Daphnia magna (EC(50) < 0.4% leachate) and the leachate of scrap tires made of styrene butadiene rubber (SBR) had the highest effect on P. subcapitata (EC(10) = 4.2% leachate; EC(50) = 15.6% leachate). We found no correlations between ecotoxicity potential of leachates and zinc and PAH concentrations. Leachates obtained from column tests revealed lower ecotoxicological potential. Leachates of column tests of complete assemblies may be used for a reliable risk assessment of artificial sporting grounds.

Material properties of a maxillofacial chlorinated polyethylene elastomer stored in simulated skin secretions.

STATEMENT OF PROBLEM: Facial prostheses deteriorate in a service environment primary due to exposition to various environmental factors, including sebaceous oils (sebum) and perspiration. PURPOSE: This study investigated the physical properties of an experimental, facial prosthetic after immersion for 6 months in simulated sebum, and perspiration at 37 degrees C. MATERIAL AND METHODS: Chlorinated polyethylene (CPE) specimens were immersed in simulated perspiration as well as in sebum. Compression tests were conducted on a Zwick testing machine. Shore A hardness measurements were carried out in a CV digital Shore A durometer. Melting and glass transition temperatures were evaluated with a differential scanning calorimeter. Weight changes were measured and color changes were determined in the CIE LAB system using a MiniScan XE spectrophotometer. Simple mathematical models were developed to correlate the measured properties with immersion time. The data were also subjected to analyses of variance (ANOVA) and the Tukey multiple range tests at a level of alpha = 0.05. RESULTS: Specimens immersed in perspiration became harder due to facilitation of the propagation of cross-linking reaction that probably occurred during aging of the CPE samples. Some weight increase was observed for the specimens immersed into the aqueous solutions, whereas for those immersed in sebum, weight loss was recorded, probably because of extraction of some compounds. The color change was higher for the specimens immersed in sebum than that corresponding to simulated perspiration. CONCLUSIONS: The chlorinated polyethylene specimens aged for a period, which simulates 1.5 years of clinical service1, showed significant deformations in their physical properties.

Estudo da alteração dimensional linear dos elásticos ortodônticos em diferentes pigmentos e tempos de armazenamento / Study of linear dimensional change of orthodontic elastics in different pigments and storage times

O objetivo deste trabalho foi avaliar a alteração dimensional linear dos elásticos ortodônticos sendo submetidos ao tempo de armazenagem em saliva artificial a 37ºC durante o período imediato (T0), 24 horas (T24) e 30 dias (T30). Os grupos de elásticos foram divididos em cinco cores com dez (n=10) amostras por grupo: cinza (C); verde (V); preta (P); rosa (R) e transparentes (T), e marcas comerciais: Morelli (a); 3M Unitek (b); GAC (c). Utilizou-se a máquina de análise de dimensão (perfilômetro) da marca Mitutoyo, que avaliou a alteração de dimensão conforme as distâncias de extremidade dos elásticos e os fatores envolvidos como o tempo de armazenamento e a marca comercial. Em seguida, os dados foram analisados estatisticamente, obtendo os resultados e realizadas as tabelas, gráficos e as correlações lineares. Para esta avaliação foram utilizados os testes Anova e de Tukey (p < 0,01). Os resultados apontaram os seguintes valores: grupo V ­ t0:3,09 (± 0,24), t24:3,12 (± 0,36) e t30d:3,15 (± 0,22); grupo T ­ t0:3,07 (± 0,16), t24:3,10 (± 0,15), t30d:3,14 (± 0,10), grupo P ­ t0:3,13 (± 0,24), t24:,16 (± 0,19), t30d:3,18 (± 0,20); grupo R ­ t0:3,08 (± 0,25), t24:3,11 (± 0,14), t30d:3,13 (± 0,18); grupo C ­ t0:3,15 (± 0,31), t24h:3,18 (± 0,15) e t30d:3,19 (± 0,59). O grupo P apresentou diferença estatisticamente significante em relação aos demais. As marcas comerciais apresentaram pequenas alterações dimensionais, aceitáveis clinicamente de acordo com a literatura. Concluiu-se que os pigmentos, que determinam a cor específica dos elásticos, são fatores que interferem na propriedade deste material. O tempo de armazenamento é determinante e fundamental para avaliação do comportamento de estabilidade do material polimérico.

The objective of this study was to evaluate the linear dimensional change of orthodontic elastics submitted to the storage in artificial saliva 37ºC at baseline (T0), 24-hour (T24), and at 30-day (T30) periods. The groups (n=10) were divided into five colors: gray (C), green (V), black (P), pink (R), and clear (T) with the following brands: Morelli (a), 3m Unitek (b), and GAC (c). A profilometer (Mitutoyo) was used to analyze the dimensional changes. Next, means were obtained also with tables, plots, and linear correlations. The Anova and Tukey´s tests (p<0.01) were used to evaluate data. The results showed that group V presented values: t0:3.09 (± 0.24), t24:3.12 (± 0.36) and t30d:3.15 (± 0.22); T group: t0:3.07 (± 0.16), t24:3.10 (± 0.15), t30d:3.14 (± 0.10), P group: t0:3.13 (± 0.24), t 24:16 (± 0.19), t30d:3.18 (± 0.20); R group: t0:3.08 (± 0.25), t24:3.11 (± .14), t30d:3.13 (± 0.18), C group: t0:3.15 (± 0.31), t24h:3.18 (± 0.15) and t30d: 3.19 (± 0.59), being statistically different from the P group compared to other groups. The commercial brands presented small dimensional changes and according the literature. It can be concluded that pigments can interfere with the elastic property.The storage time is fundamental to evaluate the stability of the polymeric material.

Mechanical manipulation of molecular lattice parameters in smectic elastomers.

Smectic liquid crystalline elastomers (SLCE) represent unique materials that combine a 1-D molecular lattice arrangement and orientational order with rubber-elasticity mediated by a polymer network. Such materials may exhibit large thermo-mechanical, opto-mechanical and electro-mechanical effects, due to the coupling of macroscopic sample geometry and microscopic structural features. It is shown that the molecular layer dimensions in the smectic phases can be influenced reversibly by macroscopic strain of the material. We present a microscopic model on the basis of experimental results obtained by mechanical dilatation measurements, optical interferometry, X-ray scattering, (13)C NMR, FTIR and polarizing microscopy data. The model gives an explanation of the controversial results obtained in different types of smectic elastomers.

Avaliação da degradação da força liberada por ligaduras elásticas convencionais e temáticas / Force degradation assessment released by conventional and thematic orthodontic elastic ties

As ligaduras elásticas são consideradas importantes fontes de força na movimentação ortodôntica. No entanto, apresentam a desvantagem de não serem capazes de liberar níveis de forças constantes no meio bucal, por sofrerem alterações em suas propriedades físicas (dimensão, características individuais), prejudicando suas propriedades mecânicas. O objetivo deste trabalho foi avaliar a degradação de força das ligaduras elásticas ortodônticas convencionais e temáticas. Dezoito grupos experimentais foram avaliados (n=10), tendo como fatores em estudo o formato das ligaduras, temática (Orthosource) e convencional (Morelli); o tempo de armazenagem em saliva artificial a 37°C (24 horas e 30 dias) e a cor (vermelho, rosa e azul). Para a determinação da intensidade das forças liberadas, todas as ligaduras elásticas foram distendidas quatro vezes o valor do seu comprimento original (1 mm), em máquina de ensaio universal Instron 5965 à velocidade constante de 5,08 mm/min, sendo a resistência máxima à tração registrada em N. Os dados foram analisados pelos testes Anova e Tukey para comparações entre os grupos (α=0,05). Os resultados demonstraram que a degradação de força após 24 horas foi muito superior nas ligaduras elásticas convencionais (24,09%, 18,90% e 14,45%) do que nas ligaduras temáticas (1,31%, 2,55% e 2,99%) nas cores vermelho, rosa e azul, respectivamente. Após 30 dias, a degradação de força continuou sendo superior nas ligaduras elásticas convencionais (33,20%, 27,23% e 21,87%), embora a degradação nas ligaduras temáticas tenha aumentado drasticamente neste período (11,89%, 15,55% e 18,53%) nas cores vermelho, rosa e azul, respectivamente...

Elastic ties are considered an important source of force for orthodontic movement. However, they present the disadvantage of not being able to release constant force levels overtime in the oral environment due to changes in their physical properties (dimension, individual characteristics) affecting their mechanical properties. The purpose of this study was to evaluate the force degradation of conventional and thematic orthodontic elastic ties. Eighteen experimental groups were evaluated (n=10) having as variables the elastic tie format of two commercial brands, thematic (Orthosource) and conventional (Morelli), the storage time in artificial saliva at 37°C (immediate, 24 hours and 30 days) and color (red, pink and blue). To determine the magnitude of the forces released, all elastic ties were stretched four times their original size (1 mm) in a universal testing machine (Instron 5965) at a constant speed rate of 5.08 mm/min with the tensile strength measured in N. The data were analyzed by Anova and Tukey statistical tests for multiple comparisons among the groups (α=0.05). The results demonstrated that force degradation after 24h was much greater for the conventional (24,09%, 18,90% and 14,45%) than for the thematic elastic ties (1,31%, 2,55% and 2,99%) in red, pink and blue colors, respectively. After 30 days, force degradation continued to be greater with the conventional elastic ties (33,20%, 27,23% and 21,87%) although force degradation with the thematic elastic ties increased dramatically (11,89%, 15,55% and 18,53%) in red, pink and blue colors, respectively...

Hairpin rubber elasticity.

We model the elastic properties of main chain liquid crystalline elastomers, formed by cross linking chains in a strongly nematic state, when they have hairpin defects. We study the response of the elastomer to imposed uniaxial extension along the nematic direction, and employ a microscopic model of how the deformation is distributed non-affinely amongst the hairpin and straight chain populations. The rubber shows a plateau in the stress as a function of the elongation imposed along the director. It is a consequence of the depletion of the actively stretching population of hairpin chains and should not be confused with soft elasticity effects associated with director rotation.

Cavity growth in soft adhesives.

The growth process of cavities nucleated at the interface between a rigid surface and a soft adhesive layer has been investigated with a probe method. A tensile stress was applied to the highly confined layer resulting in a negative hydrostatic pressure in the layer. The statistics of appearance and rate of growth of cavities as a function of applied negative stress were monitored with a CCD camera. If large germs of cavities were initially present, most of the cavities became optically visible above a critical level of stress independent of layer thickness. Cavities grew simultaneously and at the same expansion rate as a function of applied stress. In the absence of large germs, cavities became optically visible one after another, reaching a limiting size controlled by the thickness of the layer independently and very rapidly. Although, for each sample, we observed a statistical distribution of critical stress levels where a cavity expanded, the mean cavitation stress depended both on surface topography and more surprisingly on layer thickness. We believe that this new and somewhat surprising result can be interpreted with a model for the growth of small germs in finite size layers (J. Dollhofer, A. Chiche, V. Muralidharan et al., Int. J. Solids Struct. 41, 6111 (2004)). This model is mainly based on the dual notion of an energy activated transition from an unexpanded metastable state to an expanded stable state and to the proportionality of the activation energy with the elastic energy stored in the adhesive layer.