Delayed fracture healing is the improper healing of fractures within a reasonable amount of time and is estimated to impact a sixth of all fractures that occur annually in the United States1. While blood- and imaging-based bone turnover biomarkers have been thoroughly investigated throughout the healing process of bone, there is still a lack of understanding on how well these biomarkers can predict union in individual patients. Although conventional radiography is the most common clinical practice for assessing bone healing progression, this imaging technique—as well as the other imaging methods used—fails to discern the in vivo mechanical environment of bone, and therefore the likeliness of union or nonunion. There is a need to identify mechanical biomarkers that could better differentiate between patients who undergo typical healing progression versus delayed fracture healing. In order to identify these mechanical biomarkers, a 3D in vitro cell culture platform that recapitulates the micromechanical environment must be developed and tested. Success of this in vitro platform relies on the generation of rigorous testing protocols for assessing stiffness and fluid flow within this organoid system. This study aims to develop an experimental and computational pipeline for mechanically characterizing 3D printed (3DP) scaffolds—Voronoi, IsoTruss, and Truncated Octahedron (TO) geometries—that will be the foundation for future studies to explore patient-specific mechanical biomarkers in these bone tissue engineered scaffolds A dynamic mechanical analysis (DMA) strain sweep was performed on the scaffolds (n=6 for 4- and 7-day 3T3 fibroblast seeded Voronoi and TO scaffolds, n=4 for 4- and 7-day seeded IsoTruss scaffolds, n=3 for 4- and 7-day soaked controls for each geometry) to measure storage modulus, loss modulus, and the damping coefficient. The Voronoi geometry increased significantly in storage modulus when seeded for seven days compared to four days (p=0.0293). There was also an overall significant decrease in stiffness when the scaffolds were seeded versus non-seeded (p<<0.001). Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was performed to produce fluid flow experimental validation data, and this provided insights on the micromechanical environment of the IsoTruss scaffold that were consistent with the computational fluid dynamics (CFD) simulation model. The CFD model was used to calculate wall shear stresses (WSS) for various inlet velocities (0.05, 0.10, 0.15, 0.20, and 0.25 mm/s), with 0.15 mm/s producing WSS best within the range of extracellular matrix formation. DMA, DCE-MRI, and CFD all confirmed mechanical characteristics of the IsoTruss geometry that were unique to its specific micromechanical architecture. Out of all scaffolds tested, the IsoTruss geometry achieved the maximum (3.47 MPa) and minimum (0.0631 MPa) storage modulus. The computational analysis pipeline revealed that the patterns observed in the DMA experiments could be caused by buckling due to the fourteen-strut intersections and printing infidelity issue related to the IsoTruss geometry. The protocol developed herein for the experimental and computational analyses done on the scaffolds in this thesis will be used in the future on bone organoids to study individualized fracture healing. / Master of Science / Delayed fracture healing and nonunion are prevalent clinical complications with devastating impacts on patient quality of life. The current clinical methods for evaluating bone healing fail to discern the in vivo mechanical environment of bone, and therefore the likeliness of union or nonunion. There is a need to identify mechanical biomarkers that could better differentiate between patients who undergo typical healing progression versus delayed fracture healing. In order to identify these mechanical biomarkers, a 3D in vitro cell culture platform that recapitulates the micromechanical environment must be developed and tested. This study aims to develop an experimental and computational pipeline for mechanically characterizing 3D printed (3DP) scaffolds—Voronoi, IsoTruss, and Truncated Octahedron (TO) geometries— that will be the foundation for future studies to explore fracture healing on an individual, patient-specific level. For experimental characterization, a dynamic mechanical analysis was performed on the scaffolds to measure stiffness and the rate of energy storage and dissipation. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and a computational fluid dynamics (CFD) simulation were conducted to characterize the internal stresses on the scaffolds and optimize them for bone material generation. DMA testing revealed that the Voronoi geometry increased significantly in storage modulus when seeded for seven days compared to four days. DMA, DCE-MRI, and CFD all confirmed mechanical characteristics of the IsoTruss geometry that were unique to its specific micromechanical architecture.

Thermosetting polymers can cure at a gradient of cure temperatures due to a variety of factors, including heat transfer in the thermoset during heating and the exotherm due to the chemical reaction occurring during the cure. A new method for assessing the effect of cure conditions on mechanical behavior of toughened thermosets has been developed. Modeling of the phase separation process of a model thermoset system provided detailed understanding of the mechanism of property variation with cure temperature for this material. Subsequent characterization of gradient temperature cured samples has shown important variations, illustrating not only the importance of cure conditions, but the possibility of producing materials with new and useful properties. A special mold was developed to cure samples in a controlled gradient of temperature. Example systems known to show pronounced variations in microstructure cured in this gradient mold showed large variations of microstructure as a function of position within the sample, corresponding to the cure temperature at that point. A model toughened thermoset system was developed to demonstrate gradients of properties following cure in the gradient temperature mold. Cyanate ester materials were modified with hydroxyl-terminated butadiene-acrylonitrile copolymers as well as low Tg amorphous polyesters. The polyesters showed very desirable properties for a toughener, including relatively good thermo-oxidative stability in comparison with the butadiene-acrylonitrile toughener. However, the variation of properties of the cured materials with temperature was small, and to better understand the property variation possible using a gradient cure temperature technique, the butadiene-acrylonitrile toughened cyanate ester system was chosen for further study. This system showed a significant variation of glass transition temperature of the cyanate-rich phase as a function of cure temperature. Modeling of the phase separation process of this material was varied out employing a modeling procedure developed for epoxy materials. Various characteristics of the system were determined in order to apply the model to the chosen toughened thermoset. These included viscosity, surface, and thermodynamic parameters in addition to a careful characterization of the morphological parameters developed during cure at the chosen temperatures. Results show excellent predictive capability of the model for microstructure. Prediction of phase composition as a function of cure temperature is also possible, again with good agreement with experiment results. Higher cure temperatures result in a non-equilibrium phase composition, depressing the glass transition temperature of the continuous cyanate ester rich phase. This provides a mechanism by which properties of the system change as a function of position within a gradient temperature cured sample. Dynamic mechanical analysis was employed to characterize the relaxation properties of gradient and isothermally cured samples. The Havriliak Negami equation was chosen to describe the relaxation behavior of these samples. Comparison of the fitting of isotherms over the small, experimentally accessible range of frequencies showed that the use of time-temperature superpositioning could more reliably discern relatively small differences. The breadth of the relaxation corresponding to the glass transition of the polycyanurate phase was increased with a gradient cure temperature relative to isothermally cured samples. This increased broadness was expressed in an alternative way through the use of an autocorrelation function, which allows direct comparison of the time-dependent transition from a fully unrelaxed condition to a fully relaxed one. / Ph. D.

Two commercial carboxymethylcellulose acetate butyrate (CMCAB) polymers, high and low molecular weight (MW) forms, were analyzed in this study. High-solids water-borne dispersions of these polymers were studied as renewable wood adhesives. Neat polymer analyses revealed that the apart from MW, the CMCAB systems had different acid values, and that the high MW system was compromised with gel particle contaminants. Formulation of the polymer into water-dispersions was optimized for this study, and proved the "direct method", in which all formulation components were mixed at once in a sealed vessel, was the most efficient preparation technique. Applying this method, 4 high-solids water dispersions were prepared and evaluated with viscometry, differential scanning calorimetry, dynamic mechanical analysis, light and fluorescence microscopy, and mode I fracture testing. Thermal analyses showed that the polymer glass transition temperature significantly increased when bonded to wood. CMCAB dispersions produced fairly brittle adhesive-joints; however, it is believed toughness can likely be improved with further formulation optimization. Lastly, dispersion viscosity, film formation, adhesive penetration and joint-performance were all dependent on the formulation solvents, and moreover, these properties appeared to correlate with each other. / Master of Science

Resilin is an almost perfect elastic protein found in many insects. It can be stretched up to 300% of its resting length and is not affected by creep or stress relaxation. While much is known about the static mechanical properties of resilin, it is most often used dynamically by insects. Unfortunately, the dynamic mechanical properties of resilin over the biologically relevant frequency range are unknown. Here, nearly pure samples of resilin were obtained from the dragonfly, Libellua luctuosa, and dynamic mechanical analysis was performed with a combination of time-temperature and time-concentration superposition to push resilin through its glass transition. The tensile properties for resilin were found over five different ethanol concentrations (65, 70, 82, 86 and 90% by volume in water) between temperatures of -5°C and 60°C, allowing for the quantification of resilin's dynamic mechanical properties over the entire master curve. The glass transition frequency of resilin in water at 22°C was found to be 106.3 Hz. The rubber storage modulus was 1.6 MPa, increasing to 30 MPa in the glassy state. At 50 Hz and 35% strain over 98% of the elastic strain energy can returned each cycle, decreasing to 81% at the highest frequencies used by insects (13 kHz). However, despite its remarkable ability to store and return energy, the resilin tendon in dragonflies does not act to improve the energetic efficiency of flight or as a power amplifying spring. Rather, it likely functions to passively control and stabilize the trailing edge of each wing during flight. / Master of Science

The purpose of this research was to determine the effects of cellulose nanocrystals (CNCs), as potential additives, on the properties and performance of phenol–formaldehyde (PF) adhesive resin. The steady-state viscosity of a commercial PF resol resin and three CNC–resin mixtures, containing 1–3 wt % CNCs, based on solids content, was measured with a rheometer as a function of shear rate. The viscosity of the PF resin itself was independent of shear rate. The viscosity–shear rate curves of the CNC–resin mixtures showed two regions, a shear thinning region at lower shear rates and a Newtonian region at higher shear rates. The low-shear-rate viscosity of the resin was greatly increased by the CNCs. The structure of the CNC–resin mixtures under quiescent conditions was analyzed by polarized light microscopy. The mixtures contained CNC aggregates, which could be disrupted by ultrasound treatment. The curing progressions of the resin and CNC–resin mixtures were analyzed by non-isothermal differential scanning calorimetry (DSC). The DSC curves showed two exotherms followed by an endotherm. The energy of activation for the first exotherm was reduced by the CNCs whereas the energy of activation for the second exotherm was not affected by the CNCs. Increasing CNC contents caused higher degrees of reaction conversion during the first curing stage and a greater loss of sample mass, attributed to formaldehyde release during resin cure. For analysis of the mechanical properties during and after cure, sandwich-type test specimens were prepared from southern yellow pine strips and the resin and CNC–resin mixtures. The mechanical properties of the test specimens were measured as a function of time and temperature by dynamic mechanical analysis (DMA). The time to incipient storage modulus increase decreased and the rate of relative storage modulus increase increased with increasing CNC content. The ultimate sample stiffness increased with increasing CNC content for CNC contents between 0 and 2 wt %, which was attributed to mechanical reinforcement of the resin by the CNCs. At a CNC content of 3 wt %, the ultimate sample stiffness was lower than at a CNC content of 2 wt % and the second tan δ maximum occurred earlier in the experiment, indicating an earlier onset of vitrification. The lower ultimate sample stiffness was attributed to premature quenching of the curing reactions through CNC-induced depression of the vitrification point. For analysis of the fracture performance, double cantilever beam test specimens were prepared from southern yellow pine beams and the resin and CNC–resin mixtures, using different hot-pressing times. Fracture energies were measured by mode I cleavage tests. Bondline characteristics were analyzed by light microscopy. At a hot-pressing time of 10 min, the fracture energy decreased with increasing CNC content, whereas it stayed constant for CNC contents between 1 and 3 wt % at a hot-pressing time of 8 min. The bondlines of resin mixtures containing CNCs exhibited voids, whereas those of the pure resin did not. CNCs had both benefitial and detrimental effects on the properties and performace of PF resin. / Master of Science

Hydroxymethyl resorcinol (HMR) is an effective adhesion promoter (primer) for wood bonding; it dramatically improves adhesion and enhances bond durability against moisture exposure. In an effort to improve understanding of the HMR mechanism of action, this work compared HMR with two other chemical treatments investigated as wood primers: alkyl-HMR (a-HMR), an HMR variant having reduced crosslink density, and a 5% solution of polymeric methylenebis(phenylisocyanate) in N-methylpyrrolidone (solution referred to as "pMDI"). The experimental system was red oak (Quercus rubra) bonded with a moisture-cure polyurethane adhesive (PUR). The objective was to document wood rheological changes induced by the three primers, and determine if these changes correlated to primer efficacy in PUR-bonded red oak. Adhesion was tested in mode-I (opening) fracture using dual cantilever beam specimens. HMR and a-HMR proved to be highly effective primers for PUR-bonded red oak; both primers dramatically improved bondline toughness and durability. Relative to HMR, the reduced crosslink density in a-HMR did not impair primer efficacy. In contrast, the pMDI primer was ineffective; it reduced bondline toughness and durability. Solvent-submersion, torsional dynamic mechanical analysis (DMA) was conducted on primer-treated red oak (with specimens immersed in dimethylformamide). Using all three grain orientations, the lignin glass/rubber transition was carefully studied with attention directed towards primer-induced changes in stiffness (storage modulus), the glass transition temperature (Tg), the associated damping (tan ° maximum intensity), and the breadth of tan ° transition. It was found that primer effectiveness correlated with a reduction in damping intensity, and also with a Tg increase greater than 5°C. Determination of these correlations was complicated by grain dependency, and also by rheological changes caused by solvent treatments that were used as primer control treatments. / Master of Science

Resilin is a cuticular protein found in a variety of insects. It can stretch up to 300% of its natural length without any creep or relaxation. Further, it operates across a wide frequency range from 5 Hz in locomotion to 13 kHz in sound production. Both the protein sequence and composition of natural resilin as well as the dynamic mechanical properties vary substantially across species. This suggests that mechanical properties may be evolutionarily tuned for specific functions within an insect. Here, samples of resilin obtained from the tibia-tarsal joint of the cockroach, Periplaneta americana, were tested using a custom built dynamic mechanical analyzer. The material properties in compression are obtained from the rubbery to glassy domain with time-temperature superposition (-2C to 55C) and time-concentration superposition (0 % to 93% ethanol by volume in water). At low frequency the storage modulus was found to be 1.5 MPa increasing to about 5 MPa in the transition zone. The glass transition frequency at 23C in complete hydration was found to be 200 kHz. The data shows that cockroach resilin is less resilient than dragonfly resilin at low frequencies, returning about 79% of the elastic strain energy at 25 Hz compared to 97% for dragonfly resilin. However, at the glass transition (200 kHz) the material returns about 47% of the elastic strain energy compared to 30% in dragonfly (2MHz ). The resilin pad in cockroach is a composite structure, acting as a compressive spring to passively extend the tibia-tarsal joint during cockroach locomotion. Its mechanical properties are more similar to the composite locust pre-alar arm than to the pure resilin dragonfly tendon, suggesting that macroscopic structural influences may be as important as molecular sequence differences in setting properties. / Master of Science

No / Low frequency (0.1¿2 Hz) dynamic mechanical analysis on individual type I collagen fibrils has been carried out using atomic force microscopy (AFM). Both the elastic (static) and viscous (dynamic) responses are correlated to the characteristic axial banding, gap and overlap regions. The elastic modulus (¿5 GPa) on the overlap region, where the density of tropocollagen is highest, is 160% that of the gap region. The amount of dissipation on each region is frequency dependent, with the gap region dissipating most energy at the lowest frequencies (0.1 Hz) and crossing over with the overlap region at ¿0.75 Hz. This may reflect an ability of collagen fibrils to absorb energy over a range of frequencies using more than one mechanism, which is suggested as an evolutionary driver for the mechanical role of type I collagen in connective tissues and organs. / BBSRC

In recent years, triply periodic minimal surfaces (TPMS) have drawn much attention in research mainly due to their smooth, highly symmetrical surfaces, non-self-intersecting features, and mathematically controllable topologies. TPMS can have pre-defined physical and mechanical properties. The advancement of additive manufacturing technology enables us to fabricate these intricate geometric structures which was not possible by traditional manufacturing methods. In this study, the vat photopolymerization technique was used to manufacture Primitive, Gyroid, and Diamond structures. Samples were cured under ultraviolet (UV) rays after printing. Uniaxial compression experiments were conducted to assess the compressive modulus and strength of these lightweight structures. The compressive behavior of TPMS structures was also predicted using finite element analysis (FEA). Dynamic mechanical analysis (DMA) was used to compare the behavior of these structures at different temperatures. UV-cured samples exhibited improved thermo-mechanical characteristics. The primitive structure had the highest compressive strength among other structures. FEA also revealed the stress concentration areas for each sandwich structure. The DMA findings indicate that TPMS sandwich structures demonstrate significantly reduced storage modulus compared to solid structures. A numerical investigation was performed to understand the heat exchanger application of TPMS structures. The velocity profile, temperature, and pressure distributions were observed for the Primitive heat exchanger. The results of this investigation provide valuable information regarding the enhanced structural and thermal characteristics of these structures manufactured using vat photopolymerization.

In order to reduce over-dependency on fossil fuels and to create an environment that is free of non-degradable plastics, and most importantly to reduce greenhouse gas emission, bio-based products are being developed from renewable resources through intense research to substitute conventional petrochemical-based plastics with renewable alternatives and to replace synthetic fibers with natural fibers. Many authors have done quite a lot of work on synthesizing polymers from renewable origin. Polylactic acid (PLA) has been developed and characterized, and it was found that it has enormous potential and can serve as an alternative to conventional thermoplastics in many applications. Modification of the plant oil triglycerides has been discussed by many authors, and research is still going on in this area. The challenge is how to make these renewable polymers more competitive in the market, and if possible to make them 100% bio-based. There is also a major disadvantage to using a bio-based polymer from plant oils because of the high viscosity, which makes impregnation of fibers difficult. Although natural fibers are hydrophilic in nature, the problem of compatibility with the hydrophobic matrix must be solved; however, the viscosity of the bio-based resin from plant oils will complicate the situation even more. This is why many authors have reported blending of the renewable thermoset resin with styrene. In the process of solving one problem, i.e reducing the viscosity of the renewable thermoset resin by blending with reactive diluents such as styrene, another problem which we intended to solve at the initial stage is invariably being created by using a volatile organic solvent like styrene. The solution to this cycle of problems is to synthesize a thermoset resin from plant oils which will have lower viscosity, and at the same time have higher levels of functionality. This will increase the crosslinking density, and they can be cured at room temperature or relatively low temperature. In view of the above considerations, the work included in this thesis has provided a reasonable solution to the compounded problems highlighted above. Three types of bio-based thermoset resins were synthesized and characterized using NMR, DSC, TGA, and FT-IR, and their processability was studied. The three resins were subsequently reinforced with natural fibers (woven and non-woven), glass fibers, and Lyocell fiber and the resulting natural fiber composites were characterized by mechanical, dynamic mechanical, impact, and SEM analyses. These composites can be used extensively in the automotive industry, particularly for the interior components, and also in the construction and furniture industries. Methacrylated soybean oil (MSO), methacrylic anhydride-modified soybean oil (MMSO), and acetic anhydride-modified soybean oil (AMSO) were found to be suitable for manufacture of composites because of their lower viscosity. The MMSO and MSO resins were found to be promising materials because composites manufactured by using them as a matrix showed very good mechanical properties. The MMSO resin can completely wet a fiber without the addition of styrene. It has the highest number of methacrylates per triglyceride and high crosslink density. / Akademisk avhandling för avläggande av teknologie doktorsexamen vid Chalmers Tekniska högskola försvaras vid offentlig disputation, den 6:e maj, Chalmers, KE-salen, Kemigården 4, Göteborg, kl. 10.00.