Steven P. Marra

Behavior of Tip-Steerable Needles in Ex Vivo and In Vivo Tissue

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 5.23 cm in ex vivo tissue, and 10.4 cm 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 16.4 cm; 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.

Automated Methodology for Determination of Stress Distribution in Human Abdominal Aortic Aneurysm

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 1 h 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.

The mechanical properties of lead-titanate/polymer 0–3 composites

Abstract 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.

In-vivo imaging of changes in abdominal aortic aneurysm thrombus volume during the cardiac cycle.

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.

Mechanical characterization of active poly(vinyl alcohol)–poly(acrylic acid) gel

Abstract 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.

Impact of dynamic computed tomographic angiography on endograft sizing for endovascular aneurysm repair.

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<0.01). The RC for the aortic diameter at the level of the proximal aneurysm neck was 1.9 mm (8.9%). At the distal sealing zones, the mean increase in diameter was 1.7±0.3 mm (14.1%, p<0.01) for the right and 1.8±0.5 mm (14.2%, p<0.01) for the left common iliac artery (CIA). At both distal sealing zones, the mean increase in CIA diameter exceeded the RC (10.0% for the right CIA and 12.6% for the left CIA). Conclusion: The observed changes in aneurysm dimension during the cardiac cycle are small and in the range of intraobserver variability, so dynamic changes in proximal aneurysm neck diameter and aneurysm length likely have little impact on preoperative endograft selection. However, changes in diameter at the distal sealing zones may be relevant to sizing, so distal oversizing of up to 20% should be considered to prevent distal type I endoleak.

The actuation of a biomimetic 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.

The mechanical and electromechanical properties of calcium-modified lead titanate/poly(vinylidene fluoride-trifluoroethylene) 0-3 composites

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.

Prototypical metal/polymer hybrid cerebral aneurysm clip: in vitro testing for closing force, slippage, and computed tomography artifact : Laboratory investigation

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 I 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

Characterization and modeling of compliant active materials

Abstract 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.