Search

Cirrhosis of the liver is often accompanied by refractory ascites, a condition characterized by fluid buildup in the peritoneal cavity that does not respond to diuretics or recurs shortly after therapeutic paracentesis. There are several management strategies in practice including large-volume paracentesis, transjugular intrahepatic portosystemic shunts, peritoneovenous shunting, and liver transplant. However, each of these options come with limitations such as high cost, poor efficacy, and increased risk of complications. This article explores a new management strategy with a novel biopowered shunt, the PeriLeve device, that moves fluid from the peritoneal cavity to the urinary bladder using natural changes in intra-abdominal pressure (IAP). By doing so, PeriLeve shifts the current paradigm of care from the hospital to the home which reduces costs to healthcare providers and patients while improving quality of life. The basic design of the pump consists of two check (i.e., one-way) valves on either side of a fluid filled cavity which is separated from an air cavity by an elastomeric membrane. This article presents benchtop testing results of a prototype PeriLeve pump. The performance of the pump was analyzed along six parameters: change in intra-abdominal pressure, valve opening pressure, membrane thickness, membrane stiffness, membrane surface area, and air cavity volume. Initial results indicate that, with future optimizations, the pump can ultimately move a clinically significant volume of fluid.

This paper reviews and discusses the rated strength of insulators as defined in ANSI, CSA, and IEC product standards. The definition of rated strength, which for a particular insulator, requires reference to the tests and acceptance requirements given in the appropriate standard for that insulator is discussed. An insulator may not exhibit noticeable changes at loads sufficient for the initiation of irreversible damage, referred to as its damage limit. The significance of this is discussed in the paper as insulator application loads which must be below the damage limit of the insulator and be made in accordance with the relevant standard, including consideration of the allowable variation in strength implied by strict conformance to the standard. Damage limits for ceramic and composite insulators based on the minimum allowable strength according to the current standards are given.

This paper reviews and discusses the mechanical loads that overhead lines experience as defined by applicable codes and the strength requirements for insulators that support the lines as described in ANSI, CSA, and IEC standards. Consideration of the probable occurrence of extreme weather conditions requires the insulator strength to be such that irreversible damage to the insulator does not occur at the maximum load considered. The inherent variation of the mechanical strength of the insulator dielectric material is discussed as a factor for its ultimate strength, and as a basis for limiting line loads below the damage limit for the insulator. Time-load tests suggest that reasonable damage limits for ceramic and composite insulator dielectric materials can be established. As discussed in a companion paper, the assignment of damage limits for specific insulators depends not only on the inherent mechanical strength of the dielectric, but also on the standard upon which the mechanical strength of the insulator is rated. Common line design requirements, coupled with insulator manufacturers' recommendations that the maximum load not exceed the routine proof test load each insulator is subjected to has, in general, given good field experience.

Calcified deposits exist in almost all abdominal aortic aneurysms (AAAs). The significant difference in stiffness between these hard deposits and the compliant arterial wall may result in local stress concentrations and increase the risk of aneurysm rupture. Calcium deposits may also complicate AAA repair by hindering the attachment of a graft or stent-graft to the arterial wall or cause vessel wall injury at the site of balloon dilation or vascular clamp placement. Knowledge of the composition and properties of calcified deposits helps in understanding the risks associated with their presence. This work presents results of elemental composition, microscopic morphology, and mechanical property measurements of human calcified deposits obtained from within AAAs. The elemental analyses indicate the deposits are composed primarily of calcium phosphate with other assorted constituents. Microscopy investigations show a variety of microstructures within the deposits. The mechanical property measurements indicate an average elastic modulus in the range of cortical bone and an average hardness similar to nickel and iron.

Knowledge of impending abdominal aortic aneurysm (AAA) rupture can help in surgical planning. Typically, aneurysm diameter is used as the indicator of rupture, but recent studies have hypothesized that pressure-induced biomechanical stress may be a better predictor. Verification of this hypothesis on a large study population with ruptured and unruptured AAA is vital if stress is to be reliably used as a clinical prognosticator for AAA rupture risk. We have developed an automated algorithm to calculate the peak stress in patient-specific AAA models. The algorithm contains a mesh refinement module, finite element analysis module, and a postprocessing visualization module. Several aspects of the methodology used are an improvement over past reported approaches. The entire analysis may be run from a single command and is completed in less than 1h with the peak wall stress recorded for statistical analysis. We have used our algorithm for stress analysis of numerous ruptured and unruptured AAA models and report some of our results here. By current estimates, peak stress in the aortic wall appears to be a better predictor of rupture than AAA diameter. Further use of our algorithm is ongoing on larger study populations to convincingly verify these findings.

Active materials respond mechanically to changes in environmental conditions. One example of a compliant active material is a polymer gel. Active polymer gels expand and contract in response to certain environmental stimuli, such as the application of an electric field or a change in the pH level of the surroundings. This ability to achieve large, reversible deformations with no external mechanical loading has generated much interest in the use of these gels as actuators and “artificial muscles”. While much work has been done to study the behavior and properties of these gels, little information is available regarding the full constitutive description of the mechanical and actuation properties. This work focuses on developing a means of characterizing the mechanical properties of compliant active materials. A thermodynamically consistent finite-elastic constitutive model was developed to describe the mechanical and actuation behaviors of these kinds of materials. The mechanical properties of compliant active materials are characterized by a free-energy function, and the model utilizes an evolving internal variable to describe the actuation state. A biaxial testing system has been developed which can measure stresses and deformations of polymer gel films in a variety of liquid environments. This testing system is used to determine the form and parameters of the free-energy function for a specific active polymer gel, poly(vinyl alcohol)–poly(acrylic acid) gel.

Active polymer gels expand and contract in response to certain environmental stimuli, such as the application of an electric field or a change in the pH level of the surroundings. This ability to achieve large, reversible deformations with no external mechanical loading has generated much interest in the use of these gels as biomimetic actuators and ‘artificial muscles’. In previous work, a thermodynamically consistent finite–elastic constitutive model has been developed to describe the mechanical and actuation behaviours of active polymer gels. The mechanical properties were characterized by a free–energy function, and the model uses an evolving internal variable to describe the actuation state. In this work, an evolution law for the internal variable is determined from free actuation experiments on a poly(vinyl alcohol)–poly(acrylic acid) (PVA–PAA) gel. The complete finite–elastic/evolution law constitutive model is then used to predict the response of the PVA–PAA gel to isotonic and isometric loading and actuation. The model is shown to give relatively good agreement with experimental results.

Robotic needle steering is a promising technique to improve the effectiveness of needle-based clinical procedures, such as biopsies and ablation, by computer-controlled, curved insertions of needles within solid organs. In this paper, we explore the capabilities, challenges, and clinical relevance of asymmetric-tip needle steering through experiments in ex vivo and in vivo tissue. We evaluate the repeatability of needle insertion in inhomogeneous biological tissue and compare ex vivo and in vivo needle curvature and insertion forces. Steerable needles curved more in kidney than in liver and prostate, likely due to differences in tissue properties. Pre-bent needles produced higher insertion forces in liver and more curvature in vivo than ex vivo. When compared to straight stainless steel needles, steerable needles did not cause a measurable increase in tissue damage and did not exert more force during insertion. The minimum radius of curvature achieved by prebent needles was 5.23cm in ex vivo tissue, and 10.4cm in in vivo tissue. The curvatures achieved by bevel tip needles were negligible for in vivo tissue. The minimum radius of curvature for bevel tip needles in ex vivo tissue was 16.4cm; however, about half of the bevel tip needles had negligible curvatures. We also demonstrate a potential clinical application of needle steering by targeting and ablating overlapping regions of cadaveric canine liver.

Contains fulltext : 80349.pdf (Publisher’s version ) (Open Access) PURPOSE: To quantify dynamic changes in aortoiliac dimensions using dynamic electrocardiographically (ECG)-gated computed tomographic angiography (CTA) and to investigate any potential impact on preoperative endograft sizing in relation to observer variability. METHODS: Dynamic ECG-gated CTA was performed in 18 patients with abdominal aortic aneurysms. Postprocessing resulted in 11 datasets per patient: 1 static CTA and 10 dynamic CTA series. Vessel diameter, length, and angulation were measured for all phases of the cardiac cycle. The differences between diastolic and systolic aneurysm dimensions were analyzed for significance using paired t tests. To assess intraobserver variability, 20 randomly selected datasets were analyzed twice. Intraobserver repeatability coefficients (RC) were calculated using Bland-Altman analysis. RESULTS: Mean aortic diameter at the proximal neck was 21.4+/-3.0 mm at diastole and 23.2+/-2.9 mm at systole, a mean increase of 1.8+/-0.4 mm (8.5%, p

Contains fulltext : 80262.pdf (Publisher’s version ) (Open Access) PURPOSE: To evaluate in-vivo thrombus compressibility in abdominal aortic aneurysms (AAAs) to hopefully shed light on the biomechanical importance of intraluminal thrombus. METHODS: Dynamic electrocardiographically-gated computed tomographic angiography was performed in 17 AAA patients (15 men; mean age 73 years, range 69-76): 11 scheduled for surgical repair and 6 under routine surveillance. The volumes of intraluminal thrombus, the lumen, and the total aneurysm were quantified for each phase of the cardiac cycle. Thrombus compressibility was defined as the percent change in thrombus volume between diastole and peak systole. Continuous data are presented as medians and interquartile ranges (IQR). RESULTS: A substantial interpatient variability was observed in thrombus compressibility, ranging from 0.4% to 43.6% (0.2 to 13.5 mL, respectively). Both thrombus and lumen volumes varied substantially during the cardiac cycle. As lumen volume increased (5.2%, IQR 2.8%-8.8%), thrombus volume decreased (3.0%, IQR 1.0%-4.6%). Total aneurysm volume remained relatively constant (1.3%, IQR 0.4-1.9%). Changes in lumen volume were inversely correlated with changes in thrombus volume (r = -0.73; p = 0.001). CONCLUSION: In-vivo thrombus compressibility varied from patient to patient, and this variation was irrespective of aneurysm size, pulse pressure, and thrombus volume. This suggests that thrombus might act as a biomechanical buffer in some, while it has virtually no effect in others. Whether differences in thrombus compressibility alter the risk of rupture will be the focus of future research.

Object The aim of this study was to explore the possibility that a hybrid aneurysm clip with polymeric jaws bonded to a metal spring could provide mechanical properties comparable to those of an all-metal clip as well as diminished artifacts on computed tomography (CT) scanning. Methods Three clips were created, and Clips 1 and 2 were tested for mechanical properties. Clip 1 consisted of an Elgiloy spring (a cobalt-chromium-nickel alloy) bonded to carbon fiber limbs; Clip 2 consisted of an Elgiloy spring with polymethylmethacrylate (PMMA) jaws; and Clip 3 consisted of PMMA limbs identical to those in Clip 2 but bonded to a titanium spring. Custom testing equipment was set up to measure the aneurysm clip clamping forces and slippage. Clips 2 and 3 were visualized in vivo using a 64-slice CT unit, and the slices were reformatted into 3D images. Results According to the testing apparatus, Clip 2 had a similar closing force but less slippage than three similar commercial aneurysm clips. The artifact from the cobalt alloy spring on CT scanning largely offset the advantage of the nonmetal PMMA limbs, which created no artifact. The hybrid titanium/PMMA clip (Clip 3) created very little artifact on CT and allowed visualization of the phantom through the limbs. Conclusions It is feasible to build a potentially biocompatible hybrid cerebral aneurysm clip with mechanical properties that closely resemble those of conventional metallic clips. Further testing should be directed toward establishing the reliability and biocompatibility of such a clip and optimizing the contour and surface treatments of the polymer limbs.

Magnetic Resonance Elastography (MRE) has emerged as a noninvasive, quantitative physical means of examining the elastic properties of biological tissues. While it is common to assume simplified elasticity models for purposes of MRE image reconstruction, it is well-accepted that many soft tissues display complex time-dependent behavior not described by linear elasticity. Understanding how the mechanical properties of biological materials change with the frequency of the applied stresses and strains is paramount to the reconstructive imaging techniques used in steady-state MRE. Alternative continuum models, such as consolidation theory, offer the ability to model tissue and other materials comprised of two distinct phases, generally consisting of an elastic solid phase and an infiltrating fluid. For these materials, the time-dependent response under a given load is a function not only of the elastic properties of the solid matrix, but also of the rate at which fluid can flow through the matrix under a pressure gradient. To better study the behavior of the dynamic poroelasticity equations, a three-dimensional finite element model was constructed. Confined, time-harmonic excitation of simulated soil and tissue-like columns was performed to determined material deformation and pore pressure distributions, as well as to identify the influence of the key model parameters under loading conditions and frequencies relevant in steady-state MRE. The results show that the finite element implementation is able to represent the analytical behavior with errors on the order of 1% over a broad range of frequencies. Further, differences between poroelastic and elastic responses in the column can be significant over the frequency range relevant to MRE depending on the value of hydraulic conductivity assumed for the medium.

A new inflation test device was developed to study the mechanical properties of aortic tissue. The device was used to measure failure (rupture) strength and to determine the nonlinear, anisotropic elastic properties of porcine thoracic aorta. The tester was designed to stretch initially flat, circular tissue specimens to rupture under uniform biaxial loading. Water was chosen as the pressurizing fluid. Mechanical stretch and radius of curvature during inflation were measured optically in two orthogonal directions, and the Cauchy stress components were calculated from the deformation and the applied pressure. All porcine samples that ruptured successfully did so via a tear in the circumferential direction. Thus, the failure strength was taken to be the stress in the axial direction immediately prior to rupture. The mean failure strength was 1.75 MPa and mean axial stretch at failure was 1.52. These values agree well with published data for other arterial tissues. The nonlinearly elastic deformation behavior was modeled using a hyperelastic constitutive law of the type proposed by Holzapfel et al. [Holzapfel GA, Gasser TC, Ogden RW. J Elasticity 2000;61:1-48]. The results showed that the dominant directions of anisotropy in the porcine aortas were approximately 45 degrees to the axial and circumferential directions, and that the isotropic contribution to the constitutive model was insignificant.

The assessment of patient-specific abdominal aortic aneurysm (AAA) rupture risk using finite element (FE) modeling has been shown to be an improvement over current diameter-based prediction techniques (Fillinger, 2002, 2003). However, certain modeling assumptions are made which may have significant effects on the computed AAA wall stresses. For example, the FE meshes are based on composite computed tomography (CT) images obtained during multiple cardiac cycles. This results in meshes that are averaged representations of pressurized AAAs, yet are assumed to represent AAAs at the zero-stress state. Also, the proximal and distal ends of the AAA FE models are assumed to be fixed in place, which may be satisfactory boundary conditions, but have yet to be validated. Recent developments in dynamic magnetic resonance (dMR) imaging allow AAA cross-sectional images to be obtained at multiple locations and time points throughout a cardiac cycle. These images are used to determine the geometries and positions of AAAs at various times throughout the cardiac cycle.Copyright © 2004 by ASME

Objectives: We previously showed that peak abdominal aortic aneurysm (AAA) wall stress calculated for aneurysms in vivo is higher at rupture than at elective repair. The purpose of this study was to analyze rupture risk over time in patients under observation. Methods: Computed tomography (CT) scans were analyzed for patients with AAA when observation was planned for at least 6 months. AAA wall stress distribution was computationally determined in vivo with CT data, three-dimensional computer modeling, finite element analysis (nonlinear hyperelastic model depicting aneurysm wall behavior), and blood pressure during observation. Results: Analysis included 103 patients and 159 CT scans (mean follow-up, 14 ± 2 months per CT). Forty-two patients were observed with no intervention for at least 1 year (mean follow-up, 28 ± 3 months). Elective repair was performed within 1 year in 39 patients, and emergent repair was performed in 22 patients (mean, 6 ± 1 month after CT) for rupture (n = 14) or acute severe pain. Significant differences were found for initial diameter (observation, 4.9 ±.1 cm; elective repair, 5.9 ±.1 cm; emergent repair, 6.1 ±.2 cm; P

Abdominal Aortic Aneurysms (AAAs) are localized enlargements of the aorta. If untreated, AAAs will grow irreversibly until rupture occurs. Ruptured AAAs are usually fatal and are a leading cause of death in the United States, killing 15,000 per year (National Center for Health Statistics, 2001). Surgery to repair AAAs also carries mortality risks, so surgeons desire a reliable tool to evaluate the risk of rupture versus the risk of surgery.Copyright © 2003 by ASME

An abdominal aortic aneurysm (AAA) is an abnormal, localized enlargement of the aorta. If untreated, a AAA will continue to enlarge in size and eventually rupture. Currently, AAA diameter is used as the principal indicator of impending rupture. However, this method it is not totally reliable. In an effort to improve the estimation of rupture risk, some researchers are currently studying the mechanical wall stresses of AAAs using patient-specific medical imaging techniques and finite element modeling [1,2]. The accuracy of these models depends significantly on the constitutive law used to describe the mechanical properties of the AAA tissue. To date, only isotropic constitutive laws have been used in these models.Copyright © 2002 by ASME

Clinical experience with regard to predicting abdominal aortic aneurysm (AAA) rupture has shown that although AAA diameter is a good indicator, there are likely other risk factors. Some researchers have explored a biomechanical approach to predicting aneurysm rupture risk [1,2] based on the hypothesis that aneurysm rupture occurs when the mechanical stresses in the aortic wall exceed the wall failure strength. Therefore, knowledge of wall stresses in a particular AAA may help identify impending rupture. Recently, researchers have used patients’ abdominal CT scan data and blood pressure to estimate in-vivo AAA wall stresses [3]. In the present project, an improved automated methodology is used to predict AAA wall stress. The underlying correlation between mechanical stress and aneurysm wall rupture is also investigated.

The ability of some polymeric gels to shrink and swell with changes in their environment makes them of interest in many applications such as artificial muscles and drug delivery systems. While much work has been done to study the behavior and properties of these gels, little information is available regarding the full constitutive description of the mechanical and actuation properties. This work is focused on developing constitutive descriptions of the mechanical properties of such gels, and to determine how these properties change due to changes in the environment. Since these gels can undergo finite elastic deformations, uniaxial tests do not provide sufficient property information and a combination of loading conditions must be used. A biaxial testing system has been developed to test thin sheets of these films, and includes the ability to monitor and change the environmental conditions around the specimen. Initial tests were performed on latex to determine the quality of the testing apparatus. Preliminary results on a polyacrylonitrile gel are presented.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Active polymer gels can achieve large, reversible deformations in response to environmental stimuli, such as the application of an electric field or a change in pH level. Consequently, great interest exists in using these gels as actuators and artificial muscles. The goal of this work is to characterize the mechanical properties of ionic polymer gels and to describe how these properties evolve as the gel actuates. Experimental results of uniaxial tests on poly(vinyl alcohol)-poly(acrylic acid) gels are presented for both acidic and basic environments. These materials are shown to be to be slightly viscoelastic and compressible and capable of large recoverable deformations. The gels also exhibit similar stress in response to mechanical deformation in both the acid and the base.

Thin film composite sensors have been fabricated which incorporate piezoelectric ceramic particles in apolymer matrix. These composites are more compliant than pure piezoelectric ceramics and can beembedded in thick structures to monitor internal mechanical conditions, such as the evolution of damage. Composite films consisting of Ca-modified lead titanate particles in a poly(vinylidene fluoride- trifluoroethylene) matrix are examined in this paper. The viscoelastic properties of these composites (withvarious volume fractions, up to 60% ceramic) have been measured over a range of frequencies (0.01 to 100rad/sec). The complex h1 piezoelectric coefficient has also been measured for these composites for various volume fractions and over a frequency range from 5 to 100 rad/sec. The magnitudes and phase angles of thepiezoelectric coefficients are shown to be highly frequency dependent.Keywords: piezoelectric, composite, sensor, lead titanate, P(VDF-TrFE) 1. INTRODUCTION Much information regarding the mechanical status of a structure can be obtained through surface monitoringsensors such as strain gages. However, as thick polymeric composite structures become increasingly utilized,many mechanical issues (such as residual stresses and the progressive evolution of internal damage) would bebetter sensed from within the structure. Currently, embedded sensors are being used to detect stress waves in

The potential applications of pure piezoelectric ceramics as embedded sensors are limited by the stiffness and brittleness of these materials. To achieve a more compliant sensor, composites have been developed which incorporate calcium-modified lead titanate particles in an polymer matrix. Such compliant sensors, in thin film form, may be useful within thick composite structures such as tank hulls and helicopter blades. The mechanical response of 0 - 3 composite films of Ca-modified lead titanate in polyvinylidene fluoride-trifluoroethylene and Epon828 epoxy matrices is investigated in this work. The electrical response of these composites is currently being studied by Wenger et al. The viscoelastic properties of these thin film composites with various volume fractions have been measured over a wide range of frequencies and temperatures. The observed mechanical response of these heterogeneous materials is compared with the predictions of several simple models for such composites. Preliminary piezoelectric results are also presented.

Thin film composite sensors have been fabricated which incorporate piezoelectric ceramic particles in a polymer matrix. Such composites are more compliant than pure piezoelectric ceramics and can be embedded in thick structures to monitor internal mechanical conditions such as the evolution of damage. The mechanical properties and electromechanical interaction of composite films consisting of Ca-modified lead titanate particles in a poly(vinylidene fluoride-trifluoroethylene) matrix are examined in this paper. The viscoelastic properties of these composites (with various volume fractions, up to 60% ceramic) have been measured over a range of frequencies (0.01 to 100 rad ). These composite properties are primarily controlled by the viscoelastic properties of the polymer matrix, and are shown to depend greatly on frequency and ceramic content. The complex piezoelectric coefficients and have been measured for these composites for various volume fractions and over a frequency range from 5 to 100 rad . The magnitudes and phase angles of the piezoelectric coefficients are also shown to be highly frequency dependent.

Ionic polymer gels shrink and swell in response to certain environmental stimuli, such as the application of an electric field or a change in the pH level of the surroundings. This ability to achieve large, reversible deformations with no external mechanical loading has generated much interest in the use of these gels as actuators and artificial muscles. This work focuses on developing a means of characterizing the mechanical properties of such ionic gels and describing how these properties evolve as the gel actuates. A thermodynamically consistent finite elastic constitutive model of an active polymer gel is developed to describe this behavior. The mechanical properties of the gel are characterized by a strain-energy function and the model utilizes an evolving internal variable to describe the actuation state. Applications of the mode to poly(vinyl alcohol)-poly (acrylic acid) gels are presented.

The stiffness and brittleness of pure piezoelectric ceramics limit the application of these materials as embedded sensors within composites. Thin-film compliant sensors have recently been developed by incorporating piezoelectric ceramic particles in a polymer matrix. When embedded in thicker composite structures, these thin-film sensors may be useful in monitoring internal mechanical conditions such as the evolution of damage. The mechanical properties of 0–3 composite films of calcium-modified lead titanate in Epon 828 epoxy and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) matrices have been investigated in this work. The linear viscoelastic properties of these thin-film composites are measured, and the influences of strain, frequency, and ceramic content on these properties are investigated. Analytical and finite-element modeling are used to predict the effective viscoelastic properties of the composites, and these predictions are compared with the experimental results. The storage moduli predicted by these models are comparable to experimental results, while the predicted loss moduli are significantly less than the measured loss moduli. It is shown that the introduction of a third “interphase” layer at the ceramic/polymer boundary within the finite element model results in a better fit to the experimental data.

Active polymer gels expand and contract in response to certain environmental stimuli, such as the application of an electric field or a change in the pH level of the surroundings. This ability to achieve large, reversible deformations with no external mechanical loading has generated much interest in the use of these gels as actuators and “artificial muscles.” This work focuses on developing a means of characterizing the mechanical properties of active polymer gels and describing how these properties evolve as the gel actuates. Poly(vinyl alcohol)–poly(acrylic acid) (PVA–PAA) gel was chosen as the model material for this work because it is relatively simple and safe to both fabricate and actuate. PVA–PAA gels are fabricated on-site using a solvent-casting technique. These gels expand when moved from acidic to basic solutions, and contract when moved from basic to acidic solutions. The mechanical properties of the gel were characterized by conducting uniaxial tests on thin PVA–PAA gel films. A testing system has been developed which can measure stress and deformations of these films in a variety of liquid environments. The experimental results on PVA–PAA gels show these materials to be relatively compliant, and slightly viscoelastic and compressible. These gels are also capable of large recoverable deformations in both acidic and basic environments.