Bioimpedance of soft tissue under compression

In this paper compression-dependent bioimpedance measurements of porcine spleen tissue are presented. Using a Cole–Cole model, nonlinear compositional changes in extracellular and intracellular makeup; related to a loss of fluid from the tissue, are identified during compression. Bioimpedance measurements were made using a custom tetrapolar probe and bioimpedance circuitry. As the tissue is increasingly compressed up to 50%, both intracellular and extracellular resistances increase while bulk membrane capacitance decreases. Increasing compression to 80% results in an increase in intracellular resistance and bulk membrane capacitance while extracellular resistance decreases. Tissues compressed incrementally to 80% show a decreased extracellular resistance of 32%, an increased intracellular resistance of 107%, and an increased bulk membrane capacitance of 64% compared to their uncompressed values. Intracellular resistance exhibits double asymptotic curves when plotted against the peak tissue pressure during compression, possibly indicating two distinct phases of mechanical change in the tissue during compression. Based on these findings, differing theories as to what is happening at a cellular level during high tissue compression are discussed, including the possibility of cell rupture and mass exudation of cellular material.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98622/1/0967-3334_33_6_1095.pd

Wireless Tissue Palpation for Intraoperative Detection of Lumps in the Soft Tissue

In an open surgery, identification of precise margins for curative tissue resection is performed by manual palpation. This is not the case for minimally invasive and robotic procedures, where tactile feedback is either distorted or not available. In this paper, we introduce the concept of intraoperative wireless tissue palpation. The wireless palpation probe (WPP) is a cylindrical device (15 mm in diameter, 60 mm in length) that can be deployed through a trocar incision and directly controlled by the surgeon to create a volumetric stiffness distribution map of the region of interest. This map can then be used to guide the tissue resection to minimize healthy tissue loss. The wireless operation prevents the need for a dedicated port and reduces the chance of instrument clashing in the operating field. The WPP is able to measure in real time the indentation pressure with a sensitivity of 34 Pa, the indentation depth with an accuracy of 0.68 mm, and the probe position with a maximum error of 11.3 mm in a tridimensional workspace. The WPP was assessed on the benchtop in detecting the local stiffness of two different silicone tissue simulators (elastic modulus ranging from 45 to 220 kPa), showing a maximum relative error below 5%. Then, in vivo trials were aimed to identify an agar-gel lump injected into a porcine liver and to assess the device usability within the frame of a laparoscopic procedure. The stiffness map created intraoperatively by the WPP was compared with a map generated ex vivo by a standard uniaxial material tester, showing less than 8% local stiffness error at the site of the lump

Next generation of atraumatic laparoscopic instruments through analysis of the instrument-tissue interface

Mechanically induced (or iatrogenic) bowel injury from the use of laparoscopic instruments can result in devastating effects on patient outcomes both during and after surgery. The aim of this work was to investigate exactly how colonic tissue behaves both mechanically and structurally when it is subjected to a mechanical load. Analysis of force application in laparoscopic surgery is critical to understanding the nature of the instrument-tissue interaction. The development of a novel method of both histological analysis and mechanical analysis (by which the tool-tissue interaction can be characterised) has evolved through this thesis. Mechanical and histological analysis was undertaken to quantify the instrument-tissue interaction in laparoscopic surgery. This was done in both ex vivo and in vivo experiments, using an indentation method and laparoscopic instrument respectively, in porcine tissue. Mechanical stress was applied to the colon by indentation applied using the Modular Universal Surface Tester (MUST) (FalexTM Tribology USA) in ex vivo experiments to mechanically characterise the response of tissue to mechanical loading. Histological analysis was performed to examine the architecture of the tissue after loading. In vivo analysis of colon grasping was then performed to characterise grasper damage both mechanically and histologically. A mechanical measure of energy input into the tissue has been linked to consistent histological damage, above a 50 N grasping force and a loading input of 300 N.s This work has successfully identified specific loading conditions that result in tissue injury and is the first to make a link between the mechanical analyses of tissue manipulation with change to the architecture of the tissue

Towards More Accurate Medical Simulation via Procedural Instrumentation

University of Minnesota M.S. thesis. July 2019. Major: Biomedical Engineering. Advisor: Timothy Kowalewski. 1 computer file (PDF); vii, 93 pages.By 2030, the United States will find itself short of almost 100,000 physicians to care for its population. This has significant implications not only for patients, hospitals, and health care providers around the country -- but also for educators. The objective of this thesis, in a broad sense, is to explore techniques and science needed for more accurate surgical simulation. More specifically, the particular niche that has been carved out by my work as a Masters student is quantifying the change in tissue properties between the in-vivo tissues that physicians work with, and the ex-vivo tissues that biomechanicians commonly study due to convenience. The implications of this work will hopefully help drive the development of more accurate next generation medical simulators. A prototype device design, experimental protocol, and data analysis strategy is proposed to quantify the change in in-vivo to ex-vivo tissue response. This is validated on n = 4 porcine carcasses and lays the groundwork for an imminent n = 5 in-vivo porcine study. The last chapter of this thesis presents another avenue of improving medical simulators, by providing a case study on the kinematic assessment of urinary catheterization. The unifying theme consists of introducing technologies for procedural instrumentation to bring more quantitative rigor to surgical science

Detrimental Thoracoabdominal Interaction With Lateral Airbag Restraints

Side impact motor vehicle crashes pose unique challenges for occupant protection, particularly with regard to torso injury mitigation. The minimal crush distance between the vehicle exterior and the occupant torso has necessitated advanced passive safety technologies in response to tightened regulatory requirements and increased public awareness of safety issues. In particular, lateral airbag restraints (side airbags) have undergone a rapid and unregulated introduction in recent years, with US availability increasing to over 90% of new vehicles in 2010. As with frontal airbag restraints, the prdissertationsity for injury to occupants in close proximity to side airbag deployment remains a concern. Test protocols have been proposed to evaluate occupant injury risk from airbag deployment with mechanical occupant surrogates. Yet few studies have attempted to characterize thoracoabdominal responses to close-proximity airbag contact in actual crashes, leaving unaddressed the relevance of test protocols and occupant surrogates currently employed. To address this issue, the present study sought to identify and characterize injury and biomechanical responses of the thoracoabdominal region to torso-interacting side airbag restraints. A novel biological experimental approach was developed from a multi-body analysis and from an evaluation of documented restraint performance. Biomechanical responses of deflection, deflection rate, the Viscous Criterion, and deformation obliquity with respect to subject anatomy were quantified. Further, tissue-level material response was examined through a comparative finite element analysis of subject-specific loading. Results indicated that traumatic visceral injury specific to the posterolateral region was associated with close-proximity airbag interaction. Deformation response was uniquely oblique with respect to anatomy, necessitating the refinement of existing injury metrics. Biomechanical tolerances were also determined for risk of trauma to posterolateral viscera. These results are useful for the development of mechanical occupant surrogates and reductions to injury risks from close-proximity side airbag loading

Realistic tool-tissue interaction models for surgical simulation and planning

Surgical simulators present a safe and potentially effective method for surgical training, and can also be used in pre- and intra-operative surgical planning. Realistic modeling of medical interventions involving tool-tissue interactions has been considered to be a key requirement in the development of high-fidelity simulators and planners. The soft-tissue constitutive laws, organ geometry and boundary conditions imposed by the connective tissues surrounding the organ, and the shape of the surgical tool interacting with the organ are some of the factors that govern the accuracy of medical intervention planning.\ud \ud This thesis is divided into three parts. First, we compare the accuracy of linear and nonlinear constitutive laws for tissue. An important consequence of nonlinear models is the Poynting effect, in which shearing of tissue results in normal force; this effect is not seen in a linear elastic model. The magnitude of the normal force for myocardial tissue is shown to be larger than the human contact force discrimination threshold. Further, in order to investigate and quantify the role of the Poynting effect on material discrimination, we perform a multidimensional scaling study. Second, we consider the effects of organ geometry and boundary constraints in needle path planning. Using medical images and tissue mechanical properties, we develop a model of the prostate and surrounding organs. We show that, for needle procedures such as biopsy or brachytherapy, organ geometry and boundary constraints have more impact on target motion than tissue material parameters. Finally, we investigate the effects surgical tool shape on the accuracy of medical intervention planning. We consider the specific case of robotic needle steering, in which asymmetry of a bevel-tip needle results in the needle naturally bending when it is inserted into soft tissue. We present an analytical and finite element (FE) model for the loads developed at the bevel tip during needle-tissue interaction. The analytical model explains trends observed in the experiments. We incorporated physical parameters (rupture toughness and nonlinear material elasticity) into the FE model that included both contact and cohesive zone models to simulate tissue cleavage. The model shows that the tip forces are sensitive to the rupture toughness. In order to model the mechanics of deflection of the needle, we use an energy-based formulation that incorporates tissue-specific parameters such as rupture toughness, nonlinear material elasticity, and interaction stiffness, and needle geometric and material properties. Simulation results follow similar trends (deflection and radius of curvature) to those observed in macroscopic experimental studies of a robot-driven needle interacting with gels

The visceral response to underbody blast

Blast is the most common cause of injury and death in contemporary warfare. Blast injuries may be categorised based upon their mechanism with underbody blast describing the effect of an explosive device detonating underneath a vehicle. Torso injuries are highly lethal within this environment and yet their mechanism in response to underbody blast is poorly understood. This work seeks to understand the pattern and mechanism of these injuries and to link them to physical underbody blast loading parameters in order to enable mitigation and prevention of serious injury and death. An analysis of the United Kingdom Joint Theatre Trauma Registry for underbody blast events demonstrates that torso injury is a major cause of morbidity and mortality from such incidents. Mediastinal injury, including those trauma to the heart and thoracic great vessels is shown confer the greatest lethality within this complex environment. This work explores the need for a novel in vivo model of underbody loading in order to explore the mechanisms of severe torso injury and to define the relationship between the “dose” of underbody loading and resultant injury. The work includes the development of a new rig which causes underbody blast analogous vertical accelerations upon a seated rat model. Injuries causes by this loading to both the chest and abdomen can be best predicted by the examining the kinematic response of the torso to the loading. Axial compression of the torso, a previously undescribed injury metric is shown to be the best predictor of injury. The ability of these results to translate to a human model is explored in detail, with focus upon the biomechanical rationale; that torso organ injuries occur through both direct compression and shearing of tethering attachments. Survivability of underbody blast could be improved by applying these principles to the design and modification of seats, vehicles and posture.Open Acces

The Design and Development of an Intelligent Atraumatic Laparoscopic Grasper

A key tool in laparoscopic surgery is the grasper, which is the surgeon’s main means of manipulating tissue within the body. However inappropriate use may lead to tissue damage and poor surgical outcomes. This thesis presents a novel approach to the assessment and prevention of tissue damage caused by laparoscopic graspers. The research focusses on establishing typical grasping characteristics used in surgery and thus developing a model of mechanically induced tissue trauma. A review explored the state-of-the-art in devices for measuring surgical grasping, tissue mechanics, and damage quantification to inform the research. An instrumented grasper was developed to characterise typical surgical tasks, enabling the grasping force and jaw displacement to be measured. This device was then used to quantitatively characterise grasper use in an in-vivo porcine model where the device was used to perform organ retraction and manipulation tasks. From this work, the range of forces and the grasping times used in certain tasks were determined and this information was used to guide the rest of the study. The in-vivo investigation highlighted a need for grasping in a controlled environment where the tissue’s mechanical properties could be studied. A grasper test rig was designed and developed to provide automated controlled grasping of ex-vivo tissue. This allowed the mechanical properties of tissue to be determined and analysed for indications of tissue damage. A series of experimental studies were conducted with this system which showed how the mechanical response of tissue varies depending on the applied grasping force characteristics, and how this is indicative of tissue damage through comparison to histological analysis. These data were then used to develop a model which predicts the likelihood and severity of tissue damage during grasping, based on the input conditions of grasping force and time. The model was integrated into the instrumented grasper system to provide a tool which could enable real-time grading and feedback of grasping during surgery, or be used to inform best practice in training scenarios

Development of a Reality-Based, Haptics-Enabled Simulator for Tool-Tissue Interactions

The advent of complex surgical procedures has driven the need for finite element based surgical training simulators which provide realistic visual and haptic feedback throughout the surgical task. The foundation of a simulator stems from the use of accurate, reality-based models for the global tissue response as well as the tool-tissue interactions. To that end, ex vivo and in vivo tests were conducted for soft-tissue probing and in vivo tests were conducted for soft-tissue cutting for the purpose of model development. In formulating a surgical training system, there is a desire to replicate the surgical task as accurately as possible for haptic and visual realism. However, for many biological tissues, there is a discrepancy between the mechanical characteristics of ex vivo and in vivo tissue. The efficacy of utilizing an ex vivo model for simulation of in vivo probing tasks on porcine liver was evaluated by comparing the simulated probing task to an identical in vivo probing experiment. The models were then further improved upon to better replicate the in vivo response. During the study of cutting modeling, in vivo cutting experiments were performed on porcine liver to derive the force-displacement response of the tissue to a scalpel blade. Using this information, a fracture mechanics based approach was applied to develop a fully defined cohesive zone model governing the separation properties of the liver directly in front of the scalpel blade. Further, a method of scaling the cohesive zone parameters was presented to minimize the computational expense in an effort to apply the cohesive based cutting approach to real-time simulators. The development of the models for the global tissue response and local tool-tissue interactions for probing and cutting of soft-tissue provided the framework for real-time simulation of basic surgical skills training. Initially, a pre-processing approach was used for the development of reality-based, haptics enabled simulators for probing and cutting of soft tissue. Then a real-time finite element based simulator was developed to simulate the probing task without the need to know the tool path prior to simulation

Assessment of the Non-linear Stress-strain Characteristics of Poly (vinyl alcohol) Cryogel

Creation of tissue-mimicking constructs is of great importance in the field of biomedical engineering. Poly (vinyl Alcohol) (PVA) is a biomaterial capable of simulating a wide range of geometries and mechanical properties of biological tissues. It is nontoxic, biocompatible, and easy to produce. PVA can be physically crosslinked by repeated cycles of freezing and thawing. The final product of this process is called PVA cryogel (PVA-C). The mechanical properties of PVA-C can be accurately controlled by changing PVA molecular weight, PVA concentration, and number and duration of freeze/thaw cycles (FTC). In this study, the stress-strain behavior of PVA cryogel was studied for different strain ranges. Unconfined compression and rigid indentation tests were conducted to study the behavior of the material at large strains. Additionally, viscoelastic and poroelastic behavior of the PVA-C were investigated by conducting indentation force relaxation tests on different samples. Furthermore, mechanical behavior of the PVA-C at different strain rates was investigated by conducting unconfined compression tests. A piezoelectric ring actuator was used to estimate the velocity of shear wave propagation in the samples and therefore, to obtain reference shear moduli of the samples at strains of approximately 10-‑5. Finally, the mechanical behavior of the PVA-C for small strains was investigated by using the resonant column apparatus. Young’s modulus, shear wave velocity and shear modulus at different strains were obtained. Unconfined compression and indentation testing showed shear moduli that ranged between about 0.008 and 0.3 MPa at strains of 3%. It was found that there was some sample size dependency of the shear moduli for certain formulations. The cooperative diffusion coefficient of the PVA-C obtained from force relaxation test was found to decrease from 1.9 10-8 m2/s for 5% PVA, 6FTC to 6.1 10-10 m2/s for 20% PVA, 6FTC. Results of the tests conducted with piezoelectric ring actuator on samples with different PVA concentrations showed that shear wave velocity varies from 45 to 65 m/s in different samples, and has a positive correlation with PVA concentration and number of freeze/thaw cycles. Reference shear moduli of different samples were also found to range between 2.5 and 4 MPa. The results of a damping test on 20% PVA with 6 FTC showed that the damping ratio varies between 6 to 14% over strain magnitudes of 10-4 to 10-2. Finally, comparison between the results of the different mechanical tests shows that the shear moduli of PVA-C samples are generally constant for very low strains (less than 0.001). Shear moduli are found to decrease when the strain ranges between 0.001 and 0.05 (medium strain), and finally increase again for strains beyond 0.05 (large strain). Comparison of the mechanical behavior of PVA-C with tests reported in the literature for a range of biological tissues suggests that a number of formulations of PVA-C investigated in this study would be possible tissue mimics for these materials