Joseph N Grima

University of Malta · Department of Chemistry

Olympic-style sliding-seat rowing is a sport that has been extensively researched, with studies investigating aspects related to the physiology, biomechanics, kinematics, and the performance of rowers. In contrast, studies on the more classic form of fixed-seat rowing are sparse. The aim of this study is to address this lacuna by analysing for the...

In this work, an experimental investigation of an accordion-like system with embedded magnetic inclusion is carried out. It is shown that the way that this structure deforms when subjected to an external magnetic field depends on the magnetic moment of the embedded magnets and the length of the arms. Stacking of the accordion-like system leads to t...

The implementation of periodic boundary conditions (PBCs) is one of the most important and difficult steps in the computational analysis of structures and materials. This is especially true in cases such as mechanical metamaterials which typically possess intricate geometries and designs which makes finding and implementing the correct PBCs a diffi...

In this work, through the use of a theoretical model, we analyse the potential of a specific three-dimensional mechanical metamaterial composed of arrowhead-like structural units to exhibit a negative Poisson’s ratio for an arbitrary loading direction. Said analysis allows us to assess its suitability for use in applications where materials must be...

A novel class of chiral structures having intersecting ligaments acting as the node is presented in this study. Depending on the geometric parameters, this arrangement may allow both the ligaments connecting the nodes together as well as the ligaments that compose the node itself to bend upon the application of a uniaxial force. Hence, such structu...

In this work, through the use of a theoretical model and experimental testing, we investigate a novel three-dimensional mechanical metamaterial composed of structural units consisting of two arrowhead-like elements with magnetic inclusions. We show that such system has a capability to exhibit negative Poisson’s ratio and negative incremental stiffn...

Auxetics (negative Poisson's ratio) and materials with negative linear compressibility (NLC) exhibit the anomalous mechanical properties of getting wider rather than thinner when stretched and expanding in at least one direction under hydrostatic pressure, respectively. A novel mechanism-termed the 'triangular elongation mechanism'-leading to such...

Abstract This letter re-assesses a publication in the Journal of Cardiothoracic Surgery entitled ‘Vibration transmittance measures sternotomy stability – a preliminary study in human cadavers.’ The roles of ultrasound in testing for sternotomy stability and that of stress vibration transmittance in cases of fracture of the posterior table of the st...

In this work, through the use of Molecular Dynamics simulations, we show the capability of different mechanical metamaterials to induce their own global rotational motion as a result of an internal deformation. We also show that one of the considered structures, i.e. the hexachiral system, may manifest a superior extent of the global rotation in co...

Through the use of the Monte Carlo simulations utilising the mean-field approach, we show that a dense assembly of separated ultra-small magnetic nanoparticles embedded into a non-magnetic deformable matrix can be characterized by a large isothermal magnetic entropy change even upon applying a weak magnetic field with values much smaller than one T...

In this work, through numerical studies, we show the possibility of designing composites in a form of magneto-mechanical metamaterials which are capable of exhibiting an enhanced impact resistance in comparison to their non-magnetic counterparts. We also show that it is possible to control the impact resistance of the system solely by means of the...

Perforated systems constitute one of the most important classes of mechanical metamaterials. In this work, two types of “I”‐shaped slit perforation patterns are proposed which may be used to design perforated systems that mimic the deformation mechanisms of a variety of re‐entrant and anti‐tetrachiral honeycomb systems. Using finite element analysi...

An extensive investigation is carried out on the missing rib square grid structure using finite element simulations. This showed that, in the cases considered in this study, the type of deformation is primarily dependent on the ratio of the thickness of different ribs with the structure behaving like an anti‐tetrachiral at particular ratios. The an...

Various clamp designs have been proposed over the years with different degrees of complexity and actual performance vis‐à‐vis correct grip and reduced impact on the surface strains of the tendon sample. The previous studies all focused on impact on ultimate tensile strain and Young's modulus of the tendons. Here an innovative clamp design is propos...

A novel three‐dimensional lattice based on a generalization of the anti‐tetrachiral geometry is presented and investigated using experimental, numerical, and analytical methods. The lattice is built through the use of a novel connectivity that is not only able to transmit the force imposed to other orthogonal directions but also induce a contractio...

Poly(phenylacetylene) sheets that mimic the geometry of a wine‐rack‐like structure have been predicted to exhibit negative Poisson's ratios off‐axis. However, their potential to exhibit negative linear compressibility (NLC) has remained largely unexplored. In this work, the compressibility and other mechanical properties of wine‐rack‐like poly(phen...

This work discusses the concept of allowing the control of the stiffness of a particular class of re-entrant auxetic magneto-mechanical metamaterials through the introduction of magnets to the system. It is shown, through experimental testing backed up by a theoretical model, that the appropriate insertion of magnets in such a system will alter its...

The highly symmetric hexachiral honeycomb with an isotropic Poisson's ratio of -1 is one of the earliest and most well-known examples of auxetic mechanical metamaterials and has been proposed for numerous applications ranging from satellite antennas to stents. In this work, we propose a novel, less symmetric generic form of the hexachiral honeycomb...

Background: Air leak post-lobectomy continues to remain a significant clinical problem, with upper lobectomy associated with higher air leak rates. This paper investigated the pathophysiological role of pleural stress in the development of post-lobectomy air leak. Methods: Preoperative characteristics and postoperative data from 367 consecutive vid...

Auxetic Graphene Graphene sheets with a corrugated conformation are made to exhibit giant auxetic behavior (negative Poisson's ratio) in specific directions. This effect, described by Joseph N. Grima and co‐workers in article number 1700330, imparted though the use of patterned defects, can be controlled by varying the distance between defects.

Auxetic structures and materials expand laterally when stretched. It has been argued that this property could be applied in the design of smart filters with tunable sieving properties. This work analyses the filtration properties of a class of auxetic structures which achieve their auxeticity through a rotating rigid unit mechanism, an archetypal m...

The potential of several classes of mechanical metamaterials to induce their own overall rotational motion through the individual rotation of their subunits is examined. Using a theoretical approach, we confirm that for various rotating rigid unit systems, if by design the sum of angular momentum of subunits rotating in different directions is made...

Graphene monolayers engineered to adopt a corrugated conformation are found to exhibit very pronounced lateral expansion characteristics upon uniaxial stretching in specific directions, i.e. giant negative Poisson's ratio (auxeticity). Such anomalous properties are manifested in‐plane and may be controlled through the shape and amplitude of the wav...

‘Wine-rack’ motifs are formally shown to exhibit the unexpected property of a negative Poisson’s ratio (auxetic behaviour) for loading in particular directions. This property is confirmed through the analysis of analytical expressions for the in-plane off-axis mechanical properties derived for an idealised hinging wine-rack model as well as through...

We introduce a mathematical model that describes the allometry of physical characteristics of hollow organs behaving as pressure vessels based on the physics of ideal pressure vessels. The model was validated by studying parameters such as body and organ mass, systolic and diastolic pressures, internal and external dimensions, pressurization energy...

Foams are commonly used for cushioning in protective sporting equipment. Volumetrically compressing open-cell polyurethane foam buckles cell ribs creating a re-entrant structure—set by heating then cooling—which can impart auxetic behaviour. Theoretically, auxetic materials improve impact protection by increasing indentation resistance and energy a...

The term “mechanical metamaterials” encompasses a wide range of systems whose anomalous mechanical properties arise primarily from their structure rather than composition. This unique characteristic gives them an edge over many conventional natural or readily available materials and makes them well-suited for a variety of applications where tailor-...

Auxetics expand laterally when uniaxially stretched. In many cases, this behaviour has been attributed to the interplay between the geometry of the system and its deformation mechanism. This work revisits a class of 2D motifs, the so-called chiral honeycombs, which are closely associated with auxetic behaviour. More specifically, we show that syste...

We simulate, analyse and compare the mechanical properties of a number of molecular sheet-like systems based on fully substituted, penta-substituted, tetra-substituted and tri-substituted poly(phenylacetylene) using static force-field based methods. The networks are modeled in a 3D environment with and without inter-layer interactions in analogy to...

Although high heels are known to affect the biomechanics of human movement, most notably by increasing forefoot plantar pressure, it is commonly worn due to the influence of fashion. To deal with the resulting clinical issues, some prescriptions include the use of cushioning pads to redistribute the plantar loads and increase comfort. This prelimin...

Background: The adherence of the heart to physical laws, such as Laplace's Law, may act as a measure of the organ's relative efficiency. Allometric relationships were investigated to assess the heart's efficiency concerning end-diastolic and end-systolic volumes, cardiac pressurization energy, cardiac output and mass. Methods: Data to generate a...

In this work, we propose the concept that a device based on mechanical metamaterials can be used to induce and control its own rotational motion as a result of internal deformations due to the conversion of translational degrees of freedom into rotational ones. The application of a linear force on the structural units of the system may be fine-tune...

A novel class of hexagonal nanoscale honeycombs made from penta and tetra substituted polyphenlyacetylenes is proposed and modeled as crystalline systems using force-field based simulations. It is shown that, in-plane, these systems behave rather similarly to crystalline forms of graphyne, graphdiyne, and other fully substituted equivalents but ben...

In this work, we investigate the evolution of magnetic domains with time in magnetic auxetic systems using the Ising model. We show that the mechanical deformation of the structure affects the way how domains evolve and that their rate of growth does not follow a well-known power law associated with the Ising model for systems corresponding to a sp...

In this work, we investigate the deformation mechanism of auxetic hierarchical rotating square systems through a dynamics approach. We show how their deformation behaviour, hence their mechanical properties and final configuration for a given applied load, can be manipulated solely by altering the resistance to rotational motion of the hinges withi...

Different methods may be used to produce nanoparticles, for instance in 1951 Turkevich and co-workers proposed that gold nanoparticles can be produced from the reaction of trisodum citrate, which acts as a stabilizing and reducing agent, with chloroauric acid, the source of gold nanoparticles. By changing chloroauric acid to silver nitrate, silver...

Introduction: Fresh-frozen cadaveric samples are often utilized for research and the study of anatomy. Thiel embalming offers the possibility of preserving human tissue and bone without requiring special freezing equipment. Thus mechanically testing Thiel embalmed bone to compare with fresh frozen samples becomes increasingly important. Method: The...

We describe a new class of negative Poisson’s ratio (NPR) open cell PU-PE foams produced by blocking the shape memory effect in the polymer. Contrary to classical NPR open cell thermoset and thermoplastic foams that return to their auxetic phase after re-heating (and therefore limit their use in technological applications), this new class of cellul...

The possibility of achieving the anomalous property of negative linear compressibility through a mechanism involving the rotations of rigid units is presented and discussed through analytical modelling. It is shown that some rotating rigid units with particular geometric features and connectivities will expand rather than contract in certain direct...

Molecular mechanics simulations were used to analyse the on-axis mechanical properties of a series of nanonetworks. These consisted of naphthacene-like units linked together through acetylene chains, and designed to mimic the geometry of a winerack-like structure in one of the major planes, a geometry which is closely associated with negative linea...

Composites with elliptic inclusions of long semi-axis a and short semi-axis b are studied by the Finite Element method. The centres of ellipses form a square lattice of the unit lattice constant. The neighbouring ellipses are perpendicular to each other and their axes are parallel to the lattice axes. The influence of geometry and material characte...

Objective: A hypothesis is presented suggesting that the pathogenesis of apical lung disease is due to progression of subclinical congenital apical bullae in people with low Body Mass Index (BMI), a combination present in 15% of the population, due to high pleural stress levels present in the antero-posteriorly flattened chests of these individual...

Unimode metamaterials made from rotating rigid triangles are analysed mathematically for their mechanical and thermal expansion properties. It is shown that these unimode systems exhibit positive Poisson's ratios irrespective of size, shape and angle of aperture, with the Poisson's ratio exhibiting giant values for certain conformations. When the P...

Background: Storage of fruit, vegetables and other products is used worldwide with a wide variety of foods. In many cases, though environmental conditions such as air humidity and temperature are taken into consideration, air filters are not employed even though such filters could potentially reduce the perishing of product during storage. Scope an...

Perforated systems with quasi-disordered arrays of slits are found to exhibit auxetic characteristics almost as much as their traditional ordered "rotating-squares" counterparts. This provides a highly robust methodology for constructing auxetics that may be used for various practical applications such as skin grafting, where a high degree of preci...

Introduction The human rib cage resembles a masonry dome in shape. Masonry domes have a particular construction that mimics stress distribution. Rib cortical thickness and bone density were analyzed to determine whether the morphology of the rib cage is sufficiently similar to a shell dome for internal rib structure to be predicted mathematically....

We show that a mechanically driven magnetocaloric effect (MCE) in magneto-auxetic systems (MASs) in the vicinity of room temperature is possible and the effect can be colossal. Even at zero external magnetic field, the magnetic entropy change in this reversible process can be a few times larger in magnitude than in the case of the giant MCE discove...

Existent rigid unit mode (RUM) models based on rotating squares, which may explain the phenomenon of negative thermal expansion (NTE), are generalized so as to assess the NTE potential for novel systems made from rectangular or rhombic rigid units. Analytical models for the area coefficients of thermal expansion (CTE) of these innovative networks a...

α‐cristobalite is a metastable polymorph of silica which has the remarkable property of exhibiting a negative Poisson's ratio (auxetic). In this study, the mechanical properties and deformation mechanisms of this system were investigated through a force‐field based approach. Besides simulations on the actual structure of α‐cristobalite, a number of...

Over the years, great advances have been made in the field of auxetic metamaterials where one of the main focuses was the production of systems which can be produced through simple and relatively inexpensive means. In this work, auxetic systems created through the introduction of elliptical grooves, meant to mimic the rotating units mechanism, are...

Negative linear compressibility, i.e., the phenomenon of expansion rather than shrinkage in at least one direction upon the application of a hydrostatic compressive pressure is an unusual mechanical property which is attracting more interest in the recent years. Here, through analysis of published data by Hu et al. [J. Superhard Mater. 36, 257–269...

Auxetic metamaterials have generated a great deal of interest in recent years. In this letter, a novel approach which can be used to produce such auxetic metamaterials is presented. This method involves the introduction of patterned slit perforations within a sheet/block of material in order to create systems which resemble a large variety of auxet...

No abstract is available for this article.

Tendons are visco-elastic structures that connect bones to muscles and perform the basic function of force transfer to and from the skeleton. They are essential for positioning as well as energy storing when involved in more abrupt movements such as jumping. Unfortunately, they are also prone to damage, and when injuries occur, they may have dilapi...

Stishovite, a mineral which is estimated to be abundant in the Earth's mantle, exhibits negative Poisson's ratios for a range of loading directions at specific ambient pressure ranges. Here, we investigate the deformation mechanisms which lead to auxetic behaviour in the (001) plane in further detail, and show that the two-dimensional projection of...

A negative thermal expansion effect is created by two dissimilar materials A and B , where A has a high Young's modulus , CTE and a needle like structure embedded in a matrix of material B which is soft and has low CTE and a high positive Poisson's ratio. When the structure is heated , the needle-like inclusion elongates in the axial direction at a...

Chiral honeycombs are one of the most important and oft studied classes of auxetic systems due to their vast number of potential applications which range from stent geometries to composites, sensors and satellite components. Despite numerous works on these systems, however, relatively few studies have investigated the effect of structural disorder...

This study was aimed at determining the cause for the high incidence of tuberculosis (TB) reactivation occurring in males with a low body mass index (BMI). Current thinking about pulmonary TB describes infection in the lung apex resulting in cavitation after reactivation. A different hypothesis is put forward for TB infection, suggesting that this...

As ribs adapt to stress like all bones, and the chest behaves as a pressure vessel, the effect of stress on the ribs can be determined by measuring rib height and thickness. Rib height and thickness (depth) were measured using CT scans of seven rib cages from anonymized cadavers. A Finite Element Analysis (FEA) model of a rib cage was constructed u...

Auxetic mechanical metamaterials are engineered systems that exhibit the unusual macroscopic property of a negative Poisson's ratio due to sub-unit structure rather than chemical composition. Although their unique behaviour makes them superior to conventional materials in many practical applications, they are limited in availability. Here, we propo...

Auxetics are materials which have a negative Poisson’s ratio, that is, upon uniaxial tensile loading, they also expand in a direction perpendicular to the applied force. Here, we analyze the elastic constants of stishovite, a high pressure silica polymorph which is known to be a significant constituent of the earth’s mantle, and show that it exhibi...

Graphene can be made auxetic through the introduction of vacancy defects. This results in the thinnest negative Poisson's ratio material at ambient conditions known so far, an effect achieved via a nanoscale de-wrinkling mechanism that mimics the behavior at the macroscale exhibited by a crumpled sheet of paper when stretched. © 2014 WILEY-VCH Verl...

Using experimental data, it is shown that the Poisson's ratios of standard (untreated) open cell polyurethane foam vary when loading in different loading directions, since foam is inherently asymmetric and anisotropic. The loading direction which is in plane orthogonal to the rise direction of the foam constantly exhibited the most negative Poisson...

An optimized theatre environment, including personal temperature regulation, can help maintain concentration, extend work times and may improve surgical outcomes. However, devices, such as cooling vests, are bulky and may impair the surgeon's mobility. We describe the use of a low-cost, low-energy ‘bladeless fan’ as a personal cooling device. The s...

Objectives: Biomechanical modelling of the forces acting on a median sternotomy can explain the mechanism of sternotomy dehiscence, leading to improved closure techniques. Methods: Chest wall forces on 40 kPa coughing were measured using a novel finite element analysis (FEA) ellipsoid chest model, based on average measurements of eight adult mal...

Chiral systems may exhibit auxetic behavior, i.e. they may have a negative Poisson’s ratio. This particular property has led to their being studied extensively by several authors. A Finite Elements Study is presented here, investigating the mode of connection between the nodes and ligaments in the anti-tetrachiral structure. The results show that t...

Primary spontaneous pneumothorax has long been explained as being without apparent cause. This paper deals with the effect of chest wall shape and explains how this may lead to the pathogenesis of primary spontaneous pneumothorax. Rib cage measurements were taken from chest radiographs in 12 male pneumothorax patients and 12 age-matched controls. A...

It is shown that the self-consistency condition which is the basic equation for calculating the mean-field order parameter of any mean-field model Hamiltonian can be replaced by the standard Metropolis Monte Carlo scheme. The advantage of this method is its ease of implementation for both the homogeneous mean-field order parameter and the heterogen...

Auxetics, i.e. systems with a negative Poisson’s ratio, exhibit the unexpected property of becoming wider when stretched and narrower when compressed. This property arises from the manner in which the internal geometric units within the system deform when the system is submitted to a stress and may be explained in terms of ‘geometry–deformation mec...

Auxetics exhibit the anomalous property of getting wider rather than thinner when uniaxially stretched, i.e. have a negative Poisson's ratio. This work presents the results of simulations performed on a number of hypothetical poly(phenylacetylene) networks which are meant to mimic the behaviour of rotating scalene triangles. It was shown that such...

It is a well-known fact that the mechanical properties of coronary stents are defined mainly by two components, the constituting material and the design pattern of the stent itself. The latter especially has attracted the interest of entrepreneurs and scientists alike with a plethora of patents being issued for numerous stent designs. Despite this...

Auxetic foams have been widely studied in view of their superior properties and many useful applications and various models have been developed to help explain the auxetic behavior in such foams. One such model involves the description of auxetic foams in terms of rotating units (e.g. the joints where different cell walls meet), a mechanism, which...