Demetrios T. Venetsanos

DIESYS?dynamically non-linear dielectric elastomer energy generating synergetic structures: perspectives and challenges

Dielectric elastomer based generators (DEGs) offer some unique properties over energy generators based on other materials. These properties include high energy density, high efficiency over a broad range of frequencies, low compliance, the ability to produce high strain, large area, low cost films with no toxic materials and wide range environmental tolerance. As further shown in this paper, DEG materials can also exhibit a non-linear dynamic behavior, enhancing broad-band energy transfer. More specifically, dielectric elastomer (DE) energy generating synergetic structures (DIESYS) are considered as dynamic energy absorbers. Two elementary characteristic DIESYS design concepts are examined, leading to a typical antagonistic configuration for in-plane oscillations and a typical synagonistic configuration for out-of-plane oscillations. Originally, all the DE elements of the structure are assumed to be always in tension during all the phases of the harvesting cycle, conforming to the traditional concept of operation of DE structures. As shown in this paper, the traditional always-in-tension concept results in a linear dynamic system response, despite the fact that the implemented (DE) parts are considered to have been made of a non-linear (hyperelastic) material. In contrast, the proposed loose-part concept ensures the appearance of a non-linear broad-band system response, enhancing energy transfer from the environmental source.

Investigation on the distal screw of a trochanteric intramedullary implant (Fi-nail) using a simplified finite element model

Numerous studies have been published concerning the characteristics and the behaviour of the intramedullary devices in the treatment of the intertrochanteric hip fractures. However, there is still room for further exploration and exploitation concerning the implant behaviour with respect to the parts of the implant assembly (nail, lag screw and distal screw). Towards this direction, the present paper aimed at revealing the effect of the position of the distal screw on the mechanical behaviour of the fixation device. For this purpose, a simplified model was developed and analysed with the finite element method. In total, five different locations for the distal screw were examined. In all cases, the bone was fixed at its distal end while the external load was applied at the tip of the lag screw towards the hip and in the form of orthonormal force components applied individually. The results of the FE analyses were illustrated in appropriately formed plots revealing the sensitivity of the behaviour of the implant with respect to the location of the distal screw. The main conclusion derived from the present investigation was that moving the distal screw apically decreases the stresses on the distal screw but increases the stresses on the lag screw. In turn, this indicates the existence of a location for the distal screw that compromises these two effects in an optimum way.

Optimization Aspects on the Hand of the Fabrics

Fabric hand is an essential property of textile fabrics. It affects the sensorial perception of humans when they touch a fabric. The feeling of hand depends on many factors; mainly on the low stress mechanical behavior of the fabric as well as on the subjective aspects of the person assessing the fabric such as sensitivity, culture, etc. The hand of the fabrics can be approached either by subjective estimations or by objective measurements. Both evaluation methods provide the necessary information for the calculation of the total hand value (THV) of the fabric. The THV is a function of multiple variables: the primary hand values. The complex expression of the THV introduces difficulties in predicting the conditions for the maximization of its final value. The present work provides a mathematical tool for a parametric study and definition of the quantities, which have to change, in order to maximize the THV of the fabric.

A new evolutionary optimization method for osteoporotic bone augmentation

Bone augmentation is a preventative osteoporosis intervention, comprising the injection of bone cement into an osteoporotic bone. As injection of excessive amounts of bone cement may result into thermal necrosis of bone tissue or even embolism, the minimum cement volume required to achieve a predefined level of augmentation must be sought. To this end, the present paper introduces a new evolutionary optimization method, applicable to any osteoporotic bone. The method was numerically evaluated through a typical case of femoral augmentation and compared to another powerful optimization method. The results demonstrate the efficiency and low computational cost of the proposed method.

The importance of parameter choice in modelling dynamics of the eye lens

The lens provides refractive power to the eye and is capable of altering ocular focus in response to visual demand. This capacity diminishes with age. Current biomedical technologies, which seek to design an implant lens capable of replicating the function of the biological lens, are unable as yet to provide such an implant with the requisite optical quality or ability to change the focussing power of the eye. This is because the mechanism of altering focus, termed accommodation, is not fully understood and seemingly conflicting theories require experimental support which is difficult to obtain from the living eye. This investigation presents finite element models of the eye lens based on data from human lenses aged 16 and 35 years that consider the influence of various modelling parameters, including material properties, a wide range of angles of force application and capsular thickness. Results from axisymmetric models show that the anterior and posterior zonules may have a greater impact on shape change than the equatorial zonule and that choice of capsular thickness values can influence the results from modelled simulations.

Kinematic Analysis of the Human Thumb with Foldable Palm

There have been numerous attempts to develop anthropomorphic robotic hands with varying levels of dexterous capabilities. However, these robotic hands often suffer from a lack of comprehensive understanding of the musculoskeletal behavior of the human thumb with integrated foldable palm. This paper proposes a novel kinematic model to analyze the importance of thumb-palm embodiment in grasping objects. The model is validated using human demonstrations for five precision grasp types across five human subjects. The model is used to find whether there are any co-activations among the thumb joint angles and muskuloskeletal parameters of the palm. In this paper we show that there are certain pairs of joints that show stronger linear relationships in the torque space than in joint angle space. These observations provide useful design guidelines to reduce control complexity in anthropomorphic robotic thumbs.

Gradient moduli lens models: how material properties and application of forces can affect deformation and distributions of stress

The human lens provides one-third of the ocular focussing power and is responsible for altering focus over a range of distances. This ability, termed accommodation, defines the process by which the lens alters shape to increase or decrease ocular refractive power; this is mediated by the ciliary muscle through the zonule. This ability decreases with age such that around the sixth decade of life it is lost rendering the eye unable to focus on near objects. There are two opponent theories that provide an explanation for the mechanism of accommodation; definitive support for either of these requires investigation. This work aims to elucidate how material properties can affect accommodation using Finite Element models based on interferometric measurements of refractive index. Gradients of moduli are created in three models from representative lenses, aged 16, 35 and 48 years. Different forms of zonular attachments are studied to determine which may most closely mimic the physiological form by comparing stress and displacement fields with simulated shape changes to accommodation in living lenses. The results indicate that for models to mimic accommodation in living eyes, the anterior and posterior parts of the zonule need independent force directions. Choice of material properties affects which theory of accommodation is supported.

Numerical investigation of the effect of bone cement porosity on osteoporotic femoral augmentation

Femoroplasty is the injection of bone cement into the proximal femur, enhances the bone load capacity, and is typically applied to osteoporotic femora. To minimize the required injected volume of bone cement and maximize the load capacity enhancement, an optimization problem must be solved, where the modulus of elasticity of the augmented bone is a key element. This paper, through the numerical investigation of a fall on the greater trochanter of an osteoporotic femur, compares different ways to calculate this modulus and introduces an approach, based on the concept of bone cement porosity, which provides results statistically similar to those obtained with other considerations. Based on this approach, the present paper quantifies the correlation between degree of osteoporosis and optimum volume of bone cement. It concludes with an exhaustive search that reveals the effect of the bone cement porosity on the optimum volume of PMMA, for various combinations of the frontal and transverse angles of the fall on the greater trochanter.

The Role of Morphology of the Thumb in Anthropomorphic Grasping: A Review

The unique musculoskeletal structure of the human hand brings in wider dexterous capabilities to grasp and manipulate a repertoire of objects than the non-human primates. It has been widely accepted that the orientation and the position of the thumb plays an important role in this characteristic behavior. There have been numerous attempts to develop anthropomorphic robotic hands with varying levels of success. Nevertheless, manipulation ability in those hands is to be ameliorated even though they can grasp objects successfully. An appropriate model of the thumb is important to manipulate the objects against the fingers and to maintain the stability. Modeling these complex interactions about the mechanical axes of the joints and how to incorporate these joints in robotic thumbs is a challenging task. This paper presents a review of the biomechanics of the human thumb and the robotic thumb designs to identify opportunities for future anthropomorphic robotic hands.

Strength Analysis Of Buried Curved Pipes Due To Blast Explosions

With the advent of new low-cost and powerful computers and explicit time-integration FEM codes, three-dimensional models are now available for the transient response analysis of buried structures due to blast explosions in their proximity. In the industrial domain, a most interesting issue is to determine the critical distance between the center of the explosion and the buried structure in order to insure its integrity. In the present paper a systematic study is presented on the stresses induced to buried pipelines, focussing on their curved parts such as elbows and tees. In more detail, it was examined whether the curved parts of a pipeline suffer more compared to the straight parts. For the purposes of the present study, the distance from the center of the explosion, the depth from the ground surface and the impulse of the explosion are considered to be fixed. On the contrary, the shape of the pipeline is variable, as well as the relative position of the pipeline with respect to the center of the explosion. The entire analysis was based on a 3D explicit code (LS-DYNA, version 950). The main outcome of the study is that in all cases taken into account, the equivalent von Mises stresses exerted on the curved parts are either smaller or at most equal to those exerted on the straight parts of the structure. The numerical results lead to the conclusion that for fast and safe decision making, the curved parts of a pipeline may be ignored. Nevertheless, a more thorough investigation is required in order to extend this conclusion to other cases where the radius of curvature is different than the one chosen for the needs of the present study. Finally, the influence of other parameters, such as the impulse characteristics, the soil layer characteristics, the depth, the backfill etc, should also be further examined.