The thoracolumbar transversospinal muscles are vital in normal function and are implicated in the pathogenesis of different forms of spinal pathology and pain. They are the target for specific forms of clinical intervention such as exercise regimens and the release of �trigger points�, and are often analysed through imaging studies and electromyographic recordings. Despite the importance of these muscles, there is a paucity of knowledge in regard to aspects of their functional morphology. The aim of this thesis was to examine the thoracolumbar transversospinal muscles between the mid-thoracic spine and sacrum, examining their gross morphology, fibre arrangement, fibre types, and an animal model in order to provide a better understanding of their functional morphology. The gross morphology of these muscles was studied by micro-dissection of cadaveric material. Their form was found to differ from that published in either text book or peer reviewed articles, clearly indicating the existence of a �semispinalis� muscle in the lumbar spine. In addition, the arrangement of these muscles was found to be homologous between the thoracic and lumbar regions, which is contrary to published descriptions. Arrangement of the muscle fibres was examined by identifying motor endplates with acetylcholinesterase histochemistry in all muscles throughout the area of interest. Only one endplate per fibre was observed, and no in-series fibres were found. All muscles showed a complex multipinnate form with large areas of muscle tendon intruding into each muscle. Fibre type proportions in each muscle were investigated by immunohistochemistry. Results indicate the percentage of total muscle area occupied by type I fibres decreased the more caudad the vertebral level of origin, for all muscles. There were significant differences in the area percentage of type I fibres between many different vertebral levels. These differences were mostly found between the most cranial and most caudal levels examined. The percentage of type I fibres recorded suggest all muscles are likely postural in function, and the gradual decrease in type I fibres and the lack of a distinct thoracic / lumbar boundary in the data suggests the thoracic and lumbar transversospinal muscles are homologous. The thoracolumbar transversospinal muscles of the MLC3F nlacZ transgenic mouse were micro-dissected to determine their morphology, and their fibre arrangement subsequently determined using acetylcholinesterase histochemistry. These muscles showed a homogeneous form throughout the thoracolumbar spine, and no in-series muscle fibres were observed with all muscles having one motor endplate per fibre. Results indicated similarities between the morphology of mouse and human transversospinal muscles, perhaps indicative of an adaptation to an upright posture. This thesis provides information that facilitates a more complete understanding of the morphology and function of the thoracolumbar transversospinal muscles. In addition, results indicate that these muscles are homologous through the thoracic and lumbar spine, and therefore the classification and nomenclature used to describe these muscles should be re-examined. Furthermore, the morphological evidence, combined with recent embryological studies, supports the use of the term �spinotransverse� to more accurately describe this muscle group.

During heel-toe running, the vertical ground reaction force (VGRF) profile has both impact and active peaks. Although the mass-spring model (a single mass and a linear spring) is simple and useful to predict running characteristics, its simulation of VGRF profiles produces only a single peak rather than the double peak typically observed in running. In contrast, the mass-spring-damper model (two masses, two springs and a damper) produces a simulated force profile with two separate peak values. Running barefoot versus with shoes of varying stiffness produces VGRF profiles with quite different characteristics. The purpose of this study was to use the mass-spring and mass-spring-damper models to investigate the stiffness characteristics of human running in barefoot, hard-shoe and soft-shoe conditions. Ten recreational runners ran overground at 3.83 m/s and completed five trials of each footwear condition. Force data and two-dimensional kinematic data were recorded simultaneously at 1000 and 250 Hz respectively. Using the mass-spring model, vertical stiffnesses with the barefoot, hard-shoe and soft-shoe conditions were 27.6, 25.3 and 24.6 kN/m, respectively. Hard-shoe and soft-shoe material stiffnesses were about 150 and 100 kNm�����. Considering the leg and shoe as two springs in series, the leg's actual vertical stiffness could be estimated as 30 and 33 kNm����� for hard and soft-shoe conditions. The result suggested that runners increased their actual vertical stiffness with the sequence of barefoot, hard-shoe, and soft-shoe conditions. Using the mass-spring-damper model, the upper spring stiffness was relatively constant while the lower spring stiffness changed with footwear condition: 274, 136 and 126 kN/m, respectively. While it is mathematically convenient to model the leg and body with constant spring characteristics over time, physiologically it is likely that muscle-tendon stiffness does change during stance as muscle activity changes. This suggests that mass-spring models of running would be improved by time varying spring characteristics. Variable stiffness of the simple mass-spring model was tested using a smoothly varying stiffness function. This provided a significantly better force profile simulation for each of the footwear conditions than did the constant stiffness model. Further mass-spring-damper modeling may also be improved through incorporation of such time varying characteristics. / Graduation date: 2002

An overview is provided of materials and processes research currently being conducted in support of MEMS device design at MIT. Underpinning research is being conducted in five areas: room temperature strength characterization, elevated temperature strength characterization, processing of Si/SiC hybrid structures, modeling of wafer bonding processes and development of high temperature fluid interconnections. Emphasis is placed on the key areas of materials science and engineering. / Singapore-MIT Alliance (SMA)

The goal of this project was to develop techniques and instrumentation to measure the elastic properties of the lens and lens capsule in situ and their changes with age using Atomic Force Microscopy (AFM). The studies include the construction, characterization, and calibration of laboratory-based Atomic Force Microscope (AFM) to measure mechanical properties of ophthalmic tissues. Atomic Force Microscopy is a nanoscale imaging technique that has been applied to mechanical property measurement through nanoindentation. Young's modulus of elasticity is determined by monitoring the cantilever deflections when it contacts the sample. The studies also include the development of tissue preparation techniques to enable measurement of the lens elasticity using AFM. This study found that lens capsule elasticity decreases with age, outer lens cortex elasticity remains constant with age, and the inner lens cortex is stiffer than the outer lens cortex. The effect of the changing biometry and mechanical properties with age was investigated by developing a mathematical model of accommodation. These changes will be the limiting factor to accommodative amplitude. Changes in lens capsule mechanical properties will affect the maximal accommodative amplitude in older eyes.

Carbon nanotubes (CNT) are a new form of carbon with exceptional electrical and mechanical properties. This makes them attractive as inclusions in nanocomposite materials with the potential to provide improvements in electrical and mechanical properties and allows for the creation of a new range of multifunctional materials. In this study single wall carbon nanotubes (SWCNT) were dispersed in polystyrene using a solution mixing method, with the aid of a surfactant. A good dispersion was achieved and the resulting nanocomposites were characterized for electrical conductivity and mechanical properties by 3 point flexural and fracture toughness tests. Results show a significant improvement in electrical properties with electrical percolation occurring between 0.1 and 0.2 wt%. A minor improvement was observed in the flexural modulus but the strength and fracture toughness values in the nanocomposites decreased relative to the neat material. Scanning electron microscopy (SEM) was performed to characterize the morphology and fracture surface of the specimens. The results of testing and microscopy show that the presence of the nanotubes has an adverse effect on the crazing mechanism in Polystyrene (PS) resulting in a deterioration of the mechanical properties that depend on this mechanism.

The internal dynamics of powder systems under compression are as of yet not fully understood, and thus there is a necessity for approaches that can help in further clarifying and enhancing the level of understanding on this subject. To this end, the internal dynamics of powder systems under compression were probed by means of force distributions and a novel compaction equation. The determination of force distributions hinged on the use of carbon paper as a force sensor, where the imprints transferred from it onto white paper where converted through calibration into forces. Through analysis of these imprints, it was found that the absence of friction and bonding capacity between the particles composing the powder bed had no effect on how the applied load was transferred through the system. Additionally, it was found that pellet strength had a role to play in the homogeneity of force distributions, where, upon the occurrence of fracture, force distributions became less homogenous. A novel compaction equation was derived and tested on a series of systems composed of pellets with differing mechanical properties. The main value of the equation lay in its ability to predict compression behavior from single particle properties, and the agreement was especially good when a compact of zero porosity was formed. The utility of the equation was tested in two further studies, using a series of pharmaceutically relevant powder materials. It was established that the A parameter of the equation was a measure of the deformability of the powder material, much like the Heckel 1/K parameter, and can be used as a means to rank powders according to deformability, i.e. to establish plasticity scale. The equation also provided insights into the dominating compression mechanisms through an invariance that could be exploited to determine the point, at which the powder system became constrained, i.e. the end of rearrangement. Additionally, the robustness of the equation was demonstrated through fruitful analysis of a set of diverse materials. In summary, this thesis has provided insights and tools that can be translated into more efficient development and manufacturing of medicines in the form of tablets.

Cellulose whiskers (CWs) obtained from naturally occuring cellulose are nano-inclusions which show a lot of promise as mechanical reinforcements in polymers. Typically, a relatively high content is added to realize improvement in effective mechanical behavior. This enhancement in modulus is usually followed by a modest increase in strength but generally the ductility and toughness decrease. Our approach is to use small concentrations of CWs so as not to detrimentally affect processability, toughness and ductility. By aligning the small concentrations, we target the same kind of improvement in modulus and strength as reported in the literature, but at much smaller volume contents. In this work, we investigate the effect of AC electric field on the alignment of dispersed nanoscale CW in a polymer. Polyvinyl acetate (PVAc) is used as the model polymer because of the good interaction between CWs and PVAc. A low concentration of 0.4wt% was used for the study. Two dispersion methods, namely basic and modified, were developed. The basic method led to micron scale dispersion. Using the modified method, CWs were individually dispersed in PVAc with average lengths and diameters of 260 nm and 8 nm respectively yielding an aspect ratio of approximately 30. The behavior of CWs (alignment and chain formation) under an applied electric field was found to be a function of applied electric field magnitude, frequency and duration. Following alignment, the CW/PVAc nanocomposites are thermally dried in the presence of electric field to maintain the aligned microstructure. Improvements in dielectric constant and mechanical properties were observed for the aligned cases as compared to random case and pure PVAc. The optimal electric field magnitude, frequency and duration for the alignment and chain formation were found to be 200Vpp/mm, 50 KHz for duration of 20 minutes for the microcomposite and 250Vpp/mm, 10KHz for a duration of 1hr for the nanocomposite. At 0.4wt% concentration, 21% increase in dielectric constant for the optimal nanocomposite case. Above Tg, a 680% improvement in elastic modulus at 0.4wt% concentration for the optimal nanocomposite case. The reason for the significant reinforcement is attributed to alignment (rotation and chain formation) and chain-chain interaction (3D network formation and hydrogen bonding).

In 1995, the Swedish government recommended the discontinuation of amalgam as restorative in paediatric dentistry. Because the mercury content in amalgam constitutes an environmental hazard, its use has declined. The use of resin composites is increasing, but the polymerisation shrinkage of the material is still undesirably high, and the handling of uncured resin can cause contact dermatitis. A new restorative material has recently been developed in Sweden as an alternative to amalgam and resin composite: a calcium aluminate cement (CAC). CAC has been marketed as a ceramic direct restorative for posterior restorations (class I, II) and for class V restorations. This thesis evaluates mechanical properties and clinical durability of the calcium aluminate cement when used for class II restorations. Hardness, in vitro wear, flexural strength, flexural modulus, and surface roughness were evaluated. A scanning electron replica method was used for evaluation of the interfacial adaptation to tooth structures in vivo. The durability was studied in a 2-year intra-individually clinical follow-up of class II restorations. Major results and conclusions from the studies are as follows: • The CAC was a relatively hard material, harder than resin-modified glass ionomer cement but within the range of resin composites. The CAC wore less than resin-modified glass ionomer cement but more than resin composite. • Flexural strength of CAC was in the same range as that of zinc phosphate cement and far below that of both resin composite and resin-modified glass ionomer cement. Flexural modulus of CAC was higher than both resin composite and resin-modified glass ionomer cement. The low flexural strength of CAC precludes its use in stress-bearing areas. • Surface roughness of CAC could be decreased by several polishing techniques. • For CAC restorations, interfacial adaptation was higher to dentin but lower to enamel compared with resin composite restorations. Fractures were found perpendicular to the boarders of all CAC restorations and may indicate expansion of the material. • After 2 years of clinical service, the class II CAC restorations showed an unacceptably high failure rate. Material fractures and tooth fractures were the main reasons for failure.

This study describes the mechanical response of thermoset polymers under high compressive loads. The study is divided into two parts. The first part focuses on the behaviour of a powder coating when used in a clamping force joint and how the properties vary when the chemical and physical structure of the coating is changed. The second part discusses the fundamental understanding of the behaviour of thermoset polymers with small thickness-to-width ratio subjected to compressive stresses, the aim being to develop mathematical material models for viscoelastic materials under high compressive loads. In the first part polyester powder coatings were used with variations in molecular weight, number of functional groups of the resin, amount and type of filler and thickness of the coating. The coatings were subjected to conventional tests for coatings and polymers and also to specially designed tests developed to study the behaviour of powder coatings in clamping force joints. The high compressive loads in a clamping force joint put high demands on the relaxation and creep resistance of the coating and the study shows the importance of crosslink density, filler content, and also coating thickness in order to achieve the desired mechanical properties of a coating. A high reactivity of the resin, facilitating a high crosslink density and hence a high Tg, is the most important property of the coating. A film with high crosslink density shows increase in relaxation time and in apparent yield strength under compression, and also an increase in relaxation modulus and storage modulus in tension at temperatures above Tg. Addition of fillers reduces the deformation during compression and tension, but also induces a lower strain at break and hence a more brittle coating. The reinforcing effect of the fillers is pronounced when increasing the crosslink density of the coating, especially in the compression tests. The effect is evident in compression even at low amounts of fillers, where the relaxation time and resistance to deformation are strongly increased. The combination of high crosslink density and addition of fillers is therefore desirable since fillers then can be used moderately in order to achieve a reinforcing effect in compression while minimising embrittlement. The study also showed that increased coating thickness will give rise to defects in the coating, especially voids and blisters due to evaporation of water formed during the curing of the polyester powder coating. These defects will give rise to stress concentrations and increased plastic deformations in the coating, impairing the properties of the clamping force joint. The results from relaxation tests in tension were used to create a micromechanical model. This model was used in finite element modelling to estimate the loss of clamping force in a screw joint and to correlate with the experimental results of the powder coatings. In the second part of the study a well-defined free radically cured vinyl ester resin was used and studied in six different geometries in order to determine the dependence of apparent mechanical properties on the particular size and shape of a sample when it is subjected to high compressive loads. Variation of the specimen thickness, boundary conditions and loading conditions reveals that the geometry of the sample has a significant effect on the mechanical performance of the polymer. The apparent modulus and the yield strength increase dramatically when the thickness-to-width ratio of the sample is reduced, whereas they decrease when the friction between the sample and the compression plate is reduced. The creep strain rate decreases when the thickness of the material is reduced and it decreases even more when the amount of material surrounding the compressed part of the sample is increased. Creep and strain recovery tests on large specimens were used to develop a mathematical model including non-linear viscoelastic and viscoplastic response of a thermoset vinyl ester. The model was used in FEM calculations where the experimental results were compared with the calculated results in order to model the trends of the material response when varying the sample geometry. / QC 20100921

In this Thesis Ti-Al-C thin films deposited by cathodic arc at 700, 800 and 900 °C were investigated with respect to composition, structure and mechanical properties. The highest growth temperature resulted in close to single crystalline Ti2AlC MAX phase.   A high oxygen incorporation of 7-12 at.% was detected in all the films, likely originating from residual gas and the Al2O3 substrate. It was evident that the characteristic nanolaminated MAX phase structure was retained upon deflection from the ideal MAX phase stoichiometry.   Hardness and elastic modulus of the sample grown at 900 °C were 16 and 259 GPa, respectively, as determined by nanoindentation using a Berkovich tip. Nanoindentation measurements with a cube corner tip were also performed on all three samples in order to extract elastic moduli.   Analysis of loading-unloading curves and SPM images revealed no relation between pop-in events and pile-ups around the residual imprints, indicating that other mechanisms than formation of kink bands may be responsible for formation of pile-ups. This was also confirmed by cross-sectional TEM investigation of an indent: Ti2AlC MAX phase deformed without kinking and delamination, as opposed to the observations in single crystalline Ti3SiC2 films. Several possible reasons for the different deformation mechanism observed are discussed.    These results are of importance for the fundamental understanding of the origin of material characteristics, and serve as an initial study initiating further investigations of the influence of defects on MAX phase properties.