METHODS FOR ESTIMATING ENERGY IN SOFT TISSUES AND THEIR SIMULANTS – A COMPARATIVE ANALYSIS

Behaviour of 7.62x39mm tracer and API bullets in soft tissue.

Ballistic impacts on human thorax without penetration can produce severe injuries or even death of the carrier. Soft tissue finite element models must capture the non-linear elasticity and strain-rate dependence to accurately estimate the dynamic human mechanical response. The objective of this work is the calibration of a visco-hyperelastic model for soft tissue simulants. Material model parameters have been calculated by fitting experimental stress–strain relations obtained from the literature using genetic algorithms. Several parametric analyses have been carried out during the definition of the optimization algorithm. In this way, we were able to study different optimization strategies to improve the convergence and accuracy of the final result. Finally, the genetic algorithm has been applied to calibrate two different soft tissue simulants: ballistic gelatin and styrene–ethylene–butylene–styrene. The algorithm is able to calculate the constants for visco-hyperelastic constitutive equations with high accuracy. Regarding synthetic stress–strain curves, a short computational time has been shown when using the semi-free strategy, leading to high precision results in stress–strain curves. The algorithm developed in this work, whose code is included as supplementary material for the reader use, can be applied to calibrate visco-hyperelastic parameters from stress–strain relations under different strain rates. The semi-free relaxation time strategy has shown to obtain more accurate results and shorter convergence times than the other strategies studied. It has been also shown that the understanding of the constitutive models and the complexity of the stress–strain objective curves is crucial for the accuracy of the method.

Soft tissue simulant materials in X-ray-based imaging in dentomaxillofacial radiology: a scoping review.

A constitutive model for ballistic gelatin at surgical strain rates

Experimental and numerical investigation of dynamic cavitation in agarose gel as a soft tissue simulant

Characterization of 10% Ballistic Gelatin to Evaluate Temperature, Aging and Strain Rate Effects

The use of waterjet technology is now prevalent in medical applications including surgery, soft tissue resection, bone cutting, waterjet steerable needles, and wound debridement. The depth of the cut (DOC) of a waterjet in soft tissue is an important parameter that should be predicted in these applications. For instance, for waterjet-assisted surgery, selective cutting of tissue layers is a must to avoid damage to deeper tissue layers. For our proposed fracture-directed waterjet steerable needles, predicting the cut depth of the waterjet in soft tissue is important to develop an accurate motion model, as well as control algorithms for this class of steerable needles. To date, most of the proposed models are only valid in the conditions of the experiments and if the soft tissue or the system properties change, the models will become invalid. The model proposed in this paper is formulated to allow for variation in parameters related to both the waterjet geometry and the tissue. In this paper, first the cut depths of waterjet in soft tissue simulants are measured experimentally, and the effect of tissue stiffness, waterjet velocity, and nozzle diameter are studied on DOC. Then, a model based on the properties of the tissue and the waterjet is proposed to predict the DOC of waterjet in soft tissue. In order to verify the model, soft tissue properties (constitutive response and fracture toughness) are measured using low strain rate compression tests, Split-Hopkinson-Pressure-Bar (SHPB) tests, and fracture toughness tests. The results show that the proposed model can predict the DOC of waterjet in soft tissue with acceptable accuracy if the tissue and waterjet properties are known. Graphical Abstract (Left) An overview of the problems of traditional steerable needles and the solutions provided by waterjet steerable needles. (A) Traditional tip-steerable needles and tip-bent needles suffer from poor curvature, especially in soft tissues. (B) Traditional steerable needles are unable to accomplish many bends because the cutting force only results from drastic tissue deformation. (C) The first step for realization of waterjet steerable needles is to understand and model the interaction between waterjet and soft tissues at the tip (predictive model for depth of cut). (D) Then, the equilibrium between shapes cut in the tissue and the straight elastic needle should be understood. (Right) Waterjet steerable needles in which the direction of the tissue fracture is contr olled by waterjet and then the flexible needle follows. The first step for waterjet steerable needle realization is to predict the depth of waterjet cut.

Strain rate sensitive compressive response of gelatine: Experimental and constitutive analysis

Mechanical Properties of Ballistic Gelatin at High Deformation Rates

To evaluate different materials in simulating soft tissues and to analyze the influence of these materials on the mean (MPIV) and standard deviation of pixel intensity values comparing them to a gold-standard in CBCT images. Images of three piglet heads with their soft tissues intact (gold-standard) and different simulant materials were acquired: ice, modelling wax, and ballistic gelatin, with the same thickness of the original soft tissues. The pixel intensities were measured in dental, bone and soft tissues regions, in the mandible and maxilla, for all the groups. Analysis of variance, Dunnet's, Pearson's and linear regression tests were performed. The simulators did not significantly change the MPIV of teeth in comparison with the gold-standard (p = 0.1017). Only ice (p = 0.0156) affected the MPIV of bone. Wax (p= 0.001) and ice (p = 0.0076), but not ballistic gelatin (p = 0.5814), altered the MPIV of soft tissue regions. When assessing the influence of the location (mandible or maxilla) among the simulants, the differences were significant only for the soft tissue regions. Standard deviation was not influenced by simulants (p > 0.05), but ballistic gelatin presented the lower variability. The ballistic gelatin was the best soft tissue simulant since it had the lowest influence on the pixel intensity values for all regions.

Pistol bullet deflection through soft tissue simulants

A strain rate and temperature dependent model for describing nonlinear unloading stress-strain response after warm and hot compression deformation

A finite nonlinear hyper-viscoelastic model for soft biological tissues

The effects of strain rate on bulk tensile stress strain properties of solder have been investigated by many researchers, however the effects of strain rate on stress strain properties of solder joint requires further study. Another aspect which is important for solder joint failure study is the effects of mixed mode loading on solder joint failure behavior and mixed mode fracture toughness measurement and analysis. In this PhD thesis, the strain rate dependent mechanical properties and stress strain behavior of 95.5Sn3.8Ag0.7Cu (SAC387) lead-free solder are investigated for a range of strain rates. The strain rate dependent elastic modulus, yield stress properties and stress strain constitutive model of the solder material are characterized using tensile tests. Tensile tests on dog-bone shaped bulk solder specimens were conducted using a non-contact video extensometer system. Iso-strain rate uni-axial tensile tests were conducted over the strain rates of 0.001, 0.01, 0.1 and 1 (s-1) at 25ºC. For all the tests conducted, higher strain rate is well correlated with higher elastic modulus and yield stress of the solder. Nanoindentation tests were conducted from slow to intermediate strain rates of 0.001, 0.01, 0.1, 1 and 10 (s-1) by using the continuous stiffness measurement (CSM) technique. The strain rate dependent yield stress results from nanoindentation test are expressed in a Cowper-Symonds model where the dynamic flow stress can be estimated over a wide range of strain rates. Strain rate sensitivity effects on the plastic yield stress behavior of SAC387 solder from nanoindentation and tensile test were investigated. The stress strain constitutive model behavior of 95.5Sn3.8Ag0.7Cu solder was investigated further under compression over strain rates ranging from 0.022 s-1 to 9.266 s-1 and under tension over strain rates ranging from 0.001 s-1 to 1.0 s-1. A new Ramberg-Osgood model is developed to describe the stress strain curve model at one particular strain rate. Modifications are done on the original Ramberg-Osgood model in order to form a strain rate dependent model that can describe stress strain curves over all range of strain rates. The model expressions are able to capture the strain rate dependence of the yield and work hardening parameters within the range of the test conditions reported in this thesis. A modified Ramberg-Osgood model consisting of ten constants is derived for strain rate dependent expressions by using linear regression curve fitting. Fracture behavior of solder joints were investigated for soldered tensile test specimens and for a complex combined loading complex mixed mode (CMM) test fixture developed for tensile and shear combined load tests over an intermediate strain rate range from 0.001s-1 to 0.1s-1. The fracture behavior of solder joints subjected to pure tensile, pure shear or varying combination of mixed-mode (tensile and shear) loading combinations were investigated in detail. The observed failure modes vary from brittle intermetallic (IMC) layer failure to ductile bulk solder shear failure. Under mixed-mode loading, a complex combination of IMC and solder failure mechanism was observed. The CMM tests were investigated with cracks fabricated in the solder joint and the fracture behavior of solder joint subject to mode mixity is reported. A fracture mechanics based failure assessment curve (FAC) criteria approach is proposed using interfacial fracture mechanics theory. Finite element analysis results provided mixed mode stress intensity factors for interface cracks used to calculate the fracture toughness parameters.

Needle insertions are common during surgical procedures. Accurately delivering the needle at a specific location in the human body is of importance for the clinical outcome of the procedure. Studies have already shown that robotically inserting traditional needles with a bevel tip can improve targeting accuracy. However, steering of such needles requires spinning the needle, which may lead to additional tissue damage. Therefore, we propose a novel design consisting of a flexible needle with a tendon-driven actuated-tip. Changing the orientation of the actuated-tip allows to control the steering direction of the needle and the amount of deflection. We derive the kinematic model which describes the needle path given the actuated-tip orientation based on nonholonomic kinematics. We present a method for steering the needle towards a target location in soft tissue. This method incorporates online parameter estimation in order to adapt for changes in tissue stiffness. Needle insertion experiments are performed in soft-tissue simulants, made from porcine gelatin. Needle tip pose is measured during insertion using Fiber Bragg Grating (FBG) based shape reconstruction. Results show that the needle can be steered towards targets located at 20 mm from the initial insertion axis, at a depth of 100 mm with a mean targeting error of 2.02 mm.