Recursive Least Squares Filtering Algorithms for On-Line Viscoelastic Characterization of Biosamples

The mechanical characterization of biological samples is a fundamental issue in biology and related fields, such as tissue and cell mechanics, regenerative medicine and diagnosis of diseases. In this paper, a novel approach for the identification of the stiffness and damping coefficients of biosamples is introduced. According to the proposed method, a MEMS-based microgripper in operational condition is used as a measurement tool. The mechanical model describing the dynamics of the gripper-sample system considers the pseudo-rigid body model for the microgripper, and the Kelvin–Voigt constitutive law of viscoelasticity for the sample. Then, two algorithms based on recursive least square (RLS) methods are implemented for the estimation of the mechanical coefficients, that are the forgetting factor based RLS and the normalised gradient based RLS algorithms. Numerical simulations are performed to verify the effectiveness of the proposed approach. Results confirm the feasibility of the method that enables the ability to perform simultaneously two tasks: sample manipulation and parameters identification

Rapid assessment of paint coatings by micro and nano indentation methods

Paint coatings/films having pigments/filler particles, in general, are of technological importance because of their wide usage. Traditional quality control testing methods used in industry, such as tensile tests on bulk paint samples, and quick tests such as pencil scratch test, often could not predict coating performance during usage. Coating performance includes strength of adherence, scratch/mar resistance, erosion resistance, formability, colour fastness and gloss retention. Of particular interest in this thesis, is the determination of the elastic modulus of the coatings, since it can be linked to the cross-linking density and hence to the performance of the paint. The best way to evaluate performance are laboratory simulation tests and field exposure tests, but these tests often take weeks or years (in the latter case) to generate meaningful results, thus are not suitable for quality control (QC) purposes. It is therefore imperative to develop an improved quality control tool for quick assessment of pigmented paint coatings suitable for use in the industrial environment. Unlike unpigmented paint coatings, such as automotive top coats, pigmented coats have an inherent roughness imparted by the colour pigments and filler particles which makes determination of the paint matrix by indentation methods difficult. Three-body abrasion, caused by the dislodgment of these hard particles, also adds to the difficulties of interpreting scratch test results. The relationship between crosslink density in paint matrix and mechanical properties, such as ductility, is known [1], and can be correlated to scratch resistance. However, such correlation was difficult to establish in pigmented paint coatings as dislodgement of pigments during scratch tests sometimes led to accelerated wear. Indentation testing would yield information on paint properties, such as elastic modulus and hardness, without causing the dislodgement of pigments. Each indentation test typically takes a few minutes, making it an ideal candidate as a rapid quality control tool. The major drawback in using indentation techniques on soft, compliant materials such as polymers which make up the paint matrix are the timedependent response (creep at fixed load or stress relaxation on fixed displacement), leading to steeper and even negative unloading slopes and hence inaccurate modulus values. The literature review briefly covers some of the commonly used testing methods employed in industry for polymer coatings. Micro- and nano-indentation methods for the determination of elastic modulus are covered in detail. The effect of creep pertaining to indentation testing, and the treatment thereof, is also reviewed. The experimental work firstly examined the applicability of commercial microindentation equipment as QC tools. The results showed that these instruments could qualitatively differentiate the elastic modulus between paint coatings having different degrees of curing (hence differing crosslink densities and resultant mechanical properties), as well as different pigment/filler types and contents. However, creep affected the calculated values of the elastic modulus. Mechanical models using springs and dashpots to estimate elastic modulus values from the creep response were investigated, as were analytical methods to nullify the effect of creep in the unloading response. From this work it was proposed that the Boltzmann superposition principle (where strain response to a complex stress history for a linear viscoelastic material resulting from a complex loading history, is the algebraic sum of the strains due to each individual step in load) be used to extrapolate and then ‘subtract’ the creep displacement response during unloading to yield a more accurate value for the elastic modulus. This contribution provides the groundwork for possible future development of a rapid QC tool for industrial use

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

Nonlinear effects in finite elements analysis of colorectal surgical clamping

Minimal Invasive Surgery (MIS) is a procedure that has increased its applications in past few years in different types of surgeries. As number of application fields are increasing day by day, new issues have been arising. In particular, instruments must be inserted through a trocar to access the abdominal cavity without capability of direct manipulation of tissues, so a loss of sensitivity occurs. Generally speaking, the student of medicine or junior surgeons need a lot of practice hours before starting any surgical procedure, since they have to difficulty in acquiring specific skills (hand–eye coordination among others) for this type of surgery. Here is what the surgical simulator present a promising training method using an approach based on Finite Element Method (FEM). The use of continuum mechanics, especially Finite Element Analysis (FEA) has gained an extensive application in medical field in order to simulate soft tissues. In particular, colorectal simulations can be used to understand the interaction between colon and the surrounding tissues and also between colon and instruments. Although several works have been introduced considering small displacements, FEA applied to colorectal surgical procedures with large displacements is a topic that asks for more investigations. This work aims to investigate how FEA can describe non-linear effects induced by material properties and different approximating geometries, focusing as test-case application colorectal surgery. More in detail, it shows a comparison between simulations that are performed using both linear and hyperelastic models. These different mechanical behaviours are applied on different geometrical models (planar, cylindrical, 3D-SS and a real model from digital acquisitions 3D-S) with the aim of evaluating the effects of geometric non-linearity. Final aim of the research is to provide a preliminary contribution to the simulation of the interaction between surgical instrument and colon tissues with multi-purpose FEA in order to help the preliminary set-up of different bioengineering tasks like force-contact evaluation or approximated modelling for virtual reality (surgical simulations). In particular, the contribution of this work is focused on the sensitivity analysis of the nonlinearities by FEA in the tissue-tool interaction through an explicit FEA solver. By doing in this way, we aim to demonstrate that the set-up of FEA computational surgical tools may be simplified in order to provide assistance to non-expert FEA engineers or medicians in more precise way of using FEA tools

Experimental Characterization of Vascular Tissue Viscoelasticity with Emphasis on Elastin's Role

Elucidating how cardiovascular biomechanics is regulated during health and disease is critical for developing diagnostic and therapeutic methods. The extracellular matrix of cardiovascular tissue is composed of multiple fibrillar networks embedded in an amorphous ground substance and has been found to reveal time-dependent mechanical behavior. Given the multiscale nature of tissue biomechanics, an accurate description of cardiovascular biomechanics can be obtained only when microstructural morphology is characterized and put together in correlation with tissue-scale mechanics. This study constitutes the initial steps toward a full description of cardiovascular tissue biomechanics by examining two fundamental questions: How does the elastin microstructure change with tissue-level deformations? And how does the extracellular matrix composition affect tissue biomechanics? The outcome of this dissertation is believed to contribute to the field of cardiovascular tissue biomechanics by addressing some of the fundamental existing questions therein. Assessing alterations in microstructural morphology requires quantified measures which can be challenging given the complex, local and interconnected conformations of tissue structural components embedded in the extracellular matrix. In this study, new image-based methods for quantification of tissue microstructure were developed and examined on aortic tissue under different deformation states. Although in their infancy stages of development, the methods yielded encouraging results consistent with existing perceptions of tissue deformation. Changes in microstructure were investigated by examining histological images of deformed and undeformed tissues. The observations shed light on roles of elastin network in regulating tissue deformation. The viscoelastic behavior of specimens was studied using native, collagen-denatured, and elastin-isolated aortic tissues. The stress-relaxation responses of specimens provide insight into the significance of extracellular matrix composition on tissue biomechanics and how the tissue hydration affects the relaxation behavior. The responses were approximated by traditional spring-dashpot models and the results were interpreted in regards to microstructural composition