Tsai, Chi-Tay

Palliation therapy for dysphagia using esophageal stents is the current treatment of choice for those patients with inoperable esophageal malignancies. However, the stents currently used in the clinical setting, regardless of the type of metal mesh or plastic mesh stents (covered/uncovered), may cause complications, such as tumor ingrowth and stent migration into the stomach. Furthermore, metal mesh stents have limited capacities for loading anti-cancer drugs. To effectively reduce/overcome those complications and enhance the efficacy of drug release, we designed and 3D-printed a tubular, flexible polymer stent with spirals, and then load anti-cancer drug, paclitaxel, on the stent for drug release. Non- spiral 3D-printed tubular and mesh polymer stents served as controls. The self-expansion and anti migration properties, cytotoxicity, drug release profile, and cancer cell inhibition of the 3D-printed stent were fully characterized. Results showed the self-expansion force of the 3D-printed polymer stent with spirals was slightly higher than the stent without spirals. The anti-migration force of the 3D-printed stent with spirals was significantly higher than the anti-migration force of a non-spiral stent. Furthermore, the stent with spirals significantly decreased the migration distance compared to the migration distance of the non-spiral 3D-printed polymer stent. The in vitro cytotoxicity of the new stent was examined through the viability test of human esophagus epithelial cells, and results indicated that the polymer stent does not have any cytotoxicity. The results of in vitro cell viability of esophageal cancer cells further indicated that the paclitaxel in the spiral stent treated esophageal cancer cells much more efficiently than that in the mesh stent. Furthermore, the results of the in vitro drug release profile and drug permeation showed that the dense tubular drug-loaded stent could efficiently be delivered more paclitaxel through the esophageal mucosa/submucosa layers in a unidirectional way than mesh stent that delivered less paclitaxel to the esophageal mucosa/submucosa but more to the lumen. In summary, these results showed that the 3D-printed dense polymer stent with spirals has promising potential to treat esophageal malignancies.

Silicon carbide as a representative wide band-gap semiconductor has recently received wide attention due to its excellent physical, thermal and especially electrical properties. It becomes a promising material for electronic and optoelectronic device under high-temperature, high-power and high-frequency and intense radiation conditions. During the Silicon Carbide crystal grown by the physical vapor transport process, the temperature gradients induce thermal stresses which is a major cause of the dislocations multiplication. Although large dimension crystal with low dislocation density is required for satisfying the fast development of electronic and optoelectronic device, high dislocation densities always appear in large dimension crystal. Therefore, reducing dislocation density is one of the primary tasks of process optimization. This dissertation aims at developing a transient finite element model based on the Alexander-Haasen model for computing the dislocation densities in a crystal during its growing process. Different key growth parameters such as temperature gradient, crystal size will be used to investigate their influence on dislocation multiplications. The acceptable and optimal crystal diameter and temperature gradient to produce the lowest dislocation density in SiC crystal can be obtained through a thorough numerical investigation using this developed finite element model. The results reveal that the dislocation density multiplication in SiC crystal are easily affected by the crystal diameter and the temperature gradient. Generally, during the iterative calculation for SiC growth, the dislocation density multiples very rapidly in the early growth phase and then turns to a relatively slow multiplication or no multiplication at all. The results also show that larger size and higher temperature gradient causes the dislocation density enters rapid multiplication phase sooner and the final dislocation density in the crystal is higher.

Micromixer is one of the most significant components of microfluidic systems,
which manifest essential applications in the field of chemistry and biochemistry. Achieving
complete mixing performance at the shortest micro channel length is essential for a
successful micromixer design. We have developed five novel micromixers which have
advantages of high efficiency, simple fabrication, easy integration and ease for mass
production. The design principle is based on the concept of splitting-recombination and
chaotic advection. Numerical models of these micromixers are developed to characterize
the mixing performance. Experiments are also carried out to fabricate the micromixers
and evaluate the mixing performance. Numerical simulation for different parameters such
as fluids properties, inlet velocities and microchannel cross sectional sizes are also
conducted to investigate their effects on the mixing performance. The results show that
critical inlet velocities can be predicted for normal fluid flow in the micromixers. When the inlet velocity is smaller than the critical value, the fluids mixing is dominated by
mechanism of splitting-recombination, otherwise, it is dominated by chaotic advection. If
the micromixer can tolerate higher inlet velocity, the complete mixing length can be further
reduced. Our simulation results will provide valuable information for engineers to design
a micromixer by choosing appropriate geometry to boost mixing performance and broaden
implicational range to fit their specific needs. Accurate and complicated fluidic control,
such as flow mixing or reaction, solution preparation, large scale combination of different
reagents is also important for bio-application of microfluidics. A proposal microfluidic
system is capable of creating 1024 kinds of combination mixtures. The system is composed
of a high density integrated microfluidic chip and control system. The high density
microfluidic chip, which is simply fabricated through soft lithography technique, contains
a pair of 32 flow channels that can be specifically addressed by each 10 actuation channels
based on principle of multiplexor in electronic circuits. The corresponding hardware and
software compose the control system, which can be easy fabricated and modified,
especially for prototype machine developing. Moreover, the control system has general
application. Experiments are conducted to verify the feasibility of this microfluidic system
for multi-optional solution combination.

Piezoelectric sensors are one of the primary devices used in smart structures because of their capability to act as both, sensors and actuators. A finite element model has been developed to predict elastic behavior and electrical response of laminate composites with embedded piezoelectric sensors. Correlations with experimental results indicate that the model is capable of forecasting the elastic and electrical response of the structure with good accuracy. The important issue of debonding of any of the faces of the sensors is also studied in the current work. Finite element results indicate significant changes in the elastic response caused by debonding, as well as unreliable electrical outputs.

The objective of the work is to verify the feasibility of converting a large FEA code into a massively parallel FEA code in terms of computational speed and cost. Sequential subroutines in the Research EPIC hydro code, a Lagrangian finite element analysis code for high velocity elastic-plastic impact problems, are individually converted into parallel code using Cray Adaptive Fortran (CRAFT). The performance of massively parallel subroutines running on 32 PEs on Cray-T3D is faster than their sequential counterparts on Cray-YMP. At next stage of the research, Parallel Virtual Machine (PVM) directives is used to develop a PVM version of the EPIC hydro code by connecting the converted parallel subroutines running on multiple PEs of T3D to the sequential part of the code running on single PE. With an incremental increase in the massively parallel subroutines into the PVM EPIC hydro code, the performance with respect to speedup of the code increased accordingly. The results indicate that significant speedup can be achieved in the EPIC hydro code when most or all of the subroutines are massively parallelized.

The finite element method is a very powerful tool used to analyze a variety of problems in engineering. This thesis looks at the finite element method as a tool and several important modeling features of concern. A well known finite element software package, ANSYS, will be used to demonstrate a diverse number of its capabilities, and several procedures followed in solving a specific engineering problem. The subject matter involves a nonlinear contact analysis of a pressure vessel having a threaded closure. The choice of this application is prompted by an interest in better understanding how the finite element method is implemented in the design and analysis of different pressure vessel parameters. A parametric finite element analysis was performed. Load and stress distributions along the threaded region of the vessel were examined for parameters including number of threads, thread pitch, diameter ratio, closure plug length, and thread profile.

The Heterojunction Bipolar Transistor (HBT) is capable of delivering high current density at microwave frequencies and are now being implemented in microwave circuitry as high power amplifiers. The heat generated during device operation is dissipated through the Gallium Arsenide substrate. Because of its poor thermal conductivity the junction temperature rise can be large enough to degrade and thermally limit the performance of the device. The power HBT with multiple emitter fingers are susceptible to the thermal effect due to non-uniform temperature distribution. This results in a thermal effect called thermal runaway causing thermal-induced current instability and hot spot formation thus destroying the device. Thermal shunt technique which has been developed to suppress this non-uniform temperature involves fabrication of a thick metal thermal shunt connecting all the fingers thus forming a strong thermal coupling between the emitter fingers. In this thesis 2 and 3-dimensional thermal simulations were carried out using Finite Element techniques to study the thermal behavior of the HBT's as a function of thermal shunt and other device design configurations such as the number of emitter fingers, thickness of thermal shunt, emitter spacing, Silicon as a substrate material, power variation etc. The results are projected as a design guideline for HBT device.

Piezo-Transducer-Vibrators are miniature devices that emit both audio and silent signals and are currently targeted for use as an integral part of wristwatch technology. Utilizing nonlinear finite element analysis is essential for obtaining a greater understanding of the system response under varying conditions. Dyna3D nonlinear finite element code is applied in this analysis with the focus on the mechanical aspects of the vibrator. Four impact variables, the velocity, the plate gap, the weight and the velocity angle are studied to determine the effects on the system response. Each impact variable is assigned three separate values, creating twelve programs for analysis. For each program, responses to impact conditions are studied demonstrating the deformed mode shapes, maximum principal stresses and maximum displacements using state database plots and time-history plots.

The dislocation density in the Gallium Arsenide (GaAs) crystal is generated by the excessive thermal stresses during Czochralski (CZ) growth process. A constitutive equation which couples the dislocation density with the plastic deformation is employed to simulate the dislocation density in the crystal. The temperature distribution in the crystal during growth process is obtained by solving the quasi-steady-state (QSS) heat transfer equation. The thermal stresses induced by the temperature distribution are calculated by finite element method. The resolved shear stress (RSS) in each slip system is obtained by stress transformation. The dislocation motion and multiplication in each slip system are simulated using the constitutive equation and the total dislocation density in the crystal is obtained. The dislocation density is also found to be affected by the growth orientation, growth speed, ambient temperature and the radius of the crystal. The dislocation density in GaAs crystals grown from different growth orientation and crystal radius at various ambient temperatures will be calculated so that the influence of these growth parameters on the dislocation density can be understood. Consequently, one can control the growth parameters to reduce the dislocation density generated in the crystal during the CZ growth process.

Thermal stresses are induced by temperature variations in gallium arsenide(GaAs) crystal growth. The thermal stresses cause plastic deformations by dislocation and dynamic interaction in the crystal. In this study, firstly the temperature distribution in the Czochralski technique (CZ) growth of GaAs crystal is obtained according to the Jordan model. Secondly a visco-plastic response function for the GaAs crystal is developed from the Haasen model. Finally a nonlinear finite element model is employed to simulate the dislocation generation during CZ growth of GaAs crystal.