Moshe Brand

A myosignal-based powered exoskeleton system

This paper studies the integration of a human arm with a powered exoskeleton (orthotic device) and its experimental implementation in an elbow joint, naturally controlled by the human. The human-machine interface was set at the neuromuscular level, by using the neuromuscular signal (EMG) as the primary command signal for the exoskeleton system. The EMG signal along with the joint kinematics were fed into a myoprocessor which in turn predicted the muscle moments on the elbow joint. The moment-based control system integrated myoprocessor moment prediction with feedback moments measured at the human arm/exoskeleton and external load/exoskeleton interfaces. The exoskeleton structure under study was a two-link, two-joint mechanism, corresponding to the arm limbs and joints, which was mechanically linked by the human operator. Four indices of performance were used to define the quality of the human/machine integration and to evaluate the operational envelope of the system. Experimental results indicate the feasibility of an EMG-based power exoskeleton system as an integrated human-machine system using high-level neurological signals.

Redundancy resolution of a human arm for controlling a seven DOF wearable robotic system

The human arm including the shoulder, elbow, wrist joints and exclusion scapular motion has 7 Degrees of Freedom (DOF) while positioning of the wrist in space and orientating the palm is a task that requires 6 DOF. As such it includes one more DOF than is needed to complete the task. Given the redundant nature of the arm, multiple arm configurations can be used to complete a task, which is expressed mathematically by none unique solution for the inverse kinematics. Despite this mathematical difficulty, the motor control provides a unique solution for the arm redundancy as the arm is moved in space. Resolving this redundancy is becoming critical as the human interacts with a wearable robotic system(exoskeleton) which includes the same redundancy as the human arm. Therefore, the inverse kinematics solution resolving the redundancy of these two coupled systems must be identical in order to guarantee a seamless integration. The redundancy of the arm can be formulated kinematically by defining the swivel angle — the rotation angle of the plane including the upper and lower arm around a virtual axis connecting the shoulder and wrist joints which are fixed in space. Analyzing reaching tasks recorded with a motion capture lab indicates that the swivel angle is selected such that when the elbow joint is flexed, the palm points the head. Based on these experimental results, selecting the point around the center of the head as a stationary target allows to calculate the swivel angle and in that way to resolve the human arm redundancy. Experimental results indicated that by using the proposed redundancy resolution criteria the error between the predicted swivel angle and the actual swivel angle adopted by the motor control system is less then 5 Deg. This criterion or a synthesis of several additional criteria may improve the synergistic relationships between an operator and a wearable robotic system.

The Cardiocoil stent-artery interaction.

An analytical approach for the mechanical interaction of the self-expanding Cardiocoil stent with the stenosed artery is presented. The damage factor as the contact stress at the stent-artery interface is determined. The stent is considered as an elastic helical rod having a nonlinear pressure-displacement dependence, while the artery is modeled by an elastic cylindrical shell. An influence of a moderate relative thickness of the shell is estimated. The equations for both the stent and the artery are presented in the stent-associated helical coordinates. The computational efficiency of the model enabled to carry out a parametric study of the damage factor. Comparative examinations are conducted for the stents made of the helical rods with circular and rectangular cross sections. It was found, in particular, that, under same other conditions, the damage factor for the stent with a circular cross section may be two times larger than that for a rectangular one.

Numerical investigation of a novel aortic cannula aimed at reducing cerebral embolism during cardiovascular bypass surgery.

The generation of emboli during cardiopulmonary bypass (CPB) is profoundly affected by the hemodynamic properties of the aortic cannula used in the current study. The aim of the current work was to numerically investigate the hemodynamic efficiency and feasibility of a novel, backward suction cannula (BSC), designed to drastically reduce the potential risk for cerebral emboli (CEP). In line with the standard cannulae, the BSC provides oxygenated blood from the CPB machine through its primary lumen. However, the unique feature of the BSC lies in its secondary lumen, which is used to suck blood and embolic matter back from the surgical field to the CPB machine for filtration. Analysis included a numerical investigation of the hemodynamic characteristics of 44 different models, encompassing various anatomic orientations, cannula types, cannula orientations and flow conditions. Hemodynamic efficacy and CEP were assessed via trajectories of particle released from the surgical region, while the cannula feasibility was evaluated through potential for atheroembolism (AP) and index for hemolysis (IH). Differences between the investigated cannulae in terms of these measures were tested using analyses of variance tests (ANOVAs). Results indicate that the BSC exhibited a significant improvement of the cannula performance in terms of CEP with no significant change in the risk for other hemodynamic complications, such as hemolysis or atheroembolism (AP and IH). These findings suggest the advantageous use of the BSC in the clinical setting for its potential to diminish the risk for cerebral emboli, which presents the most pertinent cause of noncardiac complications following open heart surgery.

Investigation of Risks for Cerebral Embolism Associated With the Hemodynamics of Cardiopulmonary Bypass Cannula: A Numerical Model

Cerebral emboli originating in the ascending aorta are a major cause of noncardiac complications following cardiac surgery. The hemodynamics of the aortic cannula has been proven to play a significant role in emboli generation and distribution. The aim of the current study was to perform a thorough numerical investigation in order to examine the effect of the design and orientation of the cannula used during cardiopulmonary bypass on the risk to develop cerebral embolism. Hemodynamic analyses compared numerical models of 27 cases consisting of six different cannula orientations, four aortic anatomies, and three cannula designs. The cannula designs included a straight-tip (ST) cannula, a moderately curved tip cannula (TIP1 ), and a sharp-angle curved cannula (TIP2 ). Outcome measures included hemodynamic parameters such as emanating jet velocity, jet velocity drop, maximal shear stress, aortic wall reaction, emboli pathlines and distribution between upper and lower vessels, and stagnation regions. Based on these parameters, the risks for hemolysis, atheroembolism, and cerebral embolism were evaluated and compared. On one hand, the jet emerging from the ST cannula generated large wall-shear stress at the aortic wall; this may have triggered the erosion and distribution of embolic atheromatous debris from the aortic arch. On the other hand, it diverted more emboli from the clamp region to the descending aorta and thus reduced the risk for cerebral embolism. The TIP1 cannula demonstrated less shear stress on the aortic wall and diverted more emboli from the clamp region toward the upper vessels. The TIP2 cannula exhibited a stronger emanating jet, higher shear stress inside the cannula, and highly disturbed flow, which was more stagnant near the clamp region. Current findings support the significant impact of the cannula design and orientation on emboli generation and distribution. Specifically, the straight tip cannula demonstrated a reduced risk of cerebral embolism, which may be pivotal in the clinical setting.

Numerical models of net-structure stents inserted into arteries

INTRODUCTION Restenosis is strongly attributed to stresses caused by stent-artery interactions generated in the artery after balloon angioplasty. Numerical methods are often used to examine the stent-artery mechanical interactions. To overcome the extensive computational requirements demanded by these simulations, simplifications are needed. OBJECTIVE We introduce simplified models to calculate the mechanical interactions between net-structured stents and arteries, and discuss their validity and implications. METHODS 2D simplified numerical models are suggested, which allow cost effective assessment of arterial stresses and the potential damage factor (DF). In these models, several contact problems were solved for arteries with hyper elastic mechanical properties. Stresses were calculated for a large range of cases and for different numerical model types. The effects of model simplifications, oversizing mismatch and stenosis rate and length and symmetry on the resulting stresses were analyzed. RESULTS & CONCLUSIONS Results obtained from planar 2D models were found in good agreement with results obtained from complex 3D models for cases with axisymmetric constant or varying stenosis. This high correlation between the results of 3D cases with varying stenosis and the more simple 2D cases can be used as a simplified and convenient tool for calculating the arterial wall stresses in complex cases. Maximal stresses obtained by the 2D model with an asymmetric stenosis are lower than the maximal stresses obtained in the axisymmetric case with the same stenosis percentage. Therefore, axisymmetric models may provide the worst-case estimation values for a stent of interest.

The SciMED RADIUS™ Stent-Artery Interaction

Main cause of restenosis after balloon angioplasty is due to the stresses generated in the artery as well as from the stent artery interaction. Understanding the factors that are involved in this interaction and the ability to evaluate the stresses that are formed in the artery, could help to lessen the number of failures. The goal of the present study is to develop computationally efficient analytical model for estimating the potential damage factor as the contact stresses, and to investigate their influence upon stent design, artery and plaque parameters. The artery was stipulated to be thick walled cylinder and its stress-strain state was determined from analytic solution of Lame problem. An analytic model based on the analysis of the beams deformation in the framework Euler-Bernoulli assumptions was formulated for the stent. The radial pressure, which is exerted on the inner surface of the artery, is assumed to be an average of contact stresses applied by the stent and the blood pressure. The variation in the Potential Damage Factor value as a function of the mismatch between stent’s and artery’s diameters is linear, and as much as the diameter of the artery increases, the Potential Damage Factor for the same mismatch decreases. For arteries with 75% blocking and mismatch of 1mm, the potential damage factor is 4.5.Copyright

Numerical Models of an Artery with a Net Structured Stent

Main cause of restenosis after balloon angioplasty is due to the stresses generated in the artery as well as from the stent artery interaction. Understanding the factors that are involved in this interaction, and the ability to evaluate the stresses that are formed in the artery, could help to lessen the number of failures. The goal of the present study is to develop computationally efficient numerical models for estimating the Damage Factor (DF) as the contact stresses, and to investigate their influence upon stent design, artery and plaque parameters. At first the artery was taken as a hollow cylinder with homogenous, linear elastic properties of the material. Later, the artery was taken as a two dissimilar layers model, with non-linear hyper-elastic properties. The variation in the Damage Factor value as a function of the mismatch between stent’s and artery’s diameters is nearly linear, and as much as the diameter of the artery increases, the Damage Factor for the same mismatch decreases. For arteries with 75% blocking and mismatch of 1mm, the Damage Factor is 3.8.

Synthesizing two criteria for redundancy resolution of human arm in point tasks

The human arm may be considered to be a redundant mechanism given a pointing task. As a result, multiple arm configurations can be used to complete a pointing task in which the tip of the index finger is brought to a preselected point in space. A kinematic model of the human arm with four degrees of freedom (DOF) and the synthesis of two criteria were developed as an analytical tool for studying position tasks. The two criteria were: (1) minimizing angular joint displacement (Minimal Angular Displacement (MAD)) and (2) averaging limits of the shoulder joint range (Joint Range Availability (JRA)). Joint angles predicted by a weighted model synthesizing the MAD and JRA models was linearly correlated (slope=0.97; r2=0.81) with experimental data compared to individual criteria (MAD slope=0.76; r2=0.67 or JRA slope=1; r2=0.56). The partial contributions to the synthesized criterion were 70% MAD and 30% JRA. Solving the inverse kinematics problem of articulated redundant serials mechanism such as the human or robotic arm has applications in fields of human-robot interaction and wearable robotics, ergonomics, and computer graphics animation.

THE INFLUENCE OF POST ANGIOPLASTY STENT IMPLANT PROFILE ON ARTERIAL WALL STRESS

One of the main causes for post angioplasty arterial restenosis is the excessive stress induced in the arterial wall during and after the medical intervention. The closed stent is introduced in the artery wrapped around the deflated balloon catheter and is expanded in the final position by inflating the balloon. Unfortunately, this process also stretches the arterial wall. Additionally, for the stent to be successful, its diameter must be slightly larger than the diameter of the inflated artery. The stent is usually a dense mesh of interconnected beams. Therefore, it is often considered that it applies a constant pressure to the artery / stent interface. However, in reality each beam individually presses against the innermost layer of the artery (intima). The current study proposes a model, which predicts the arterial wall subsurface stress field due to individual stent beams. It was found that the local shape of the contact (beam cross section) plays an important role close to the stent / intima contact. Sharper edged cross sections (e.g. square) promote higher stresses. It was observed that during restenosis a new inner layer (neointima) is formed, significantly reducing the stent efficiency. This could be related to local stress concentrations due to the choice of stent beam profile.Copyright