The purpose of this research is to explore the potential of using carbon-infiltrated carbon nanotubes (CI-CNT) as a material for coronary artery stents. Stents are commonly fabricated from metal, which may not perform as well as many polymers and ceramics in biomedical applications. Pyrolytic carbon, a ceramic, is currently used in medical implant devices due to its preferrable biocompatibility properties. Micro-patterned pyrolytic carbon devices can be created by growing carbon nanotubes, and then filling the space between with amorphous carbon via chemical vapor deposition. We prepared multiple samples of two different planar stent-like flexible geometries and smaller cubic structures out of carbon infiltrated carbon nanotubes. These samples were tested in tension to failure. The cubic structures were used for separate compression tests. We also examined existing auxetic patterns for possible application in the stent designs and a second iteration of design and fabrication was performed using data and understanding obtained from the work in the first iteration. Slight changes were made to the mask design and fabrication processes based on the new geometries and testing considerations. The auxetic planar designs were tested in compression to demonstrate flexibility and collect material data. The testing results show that CI-CNTs can be designed and fabricated into flexible geometries capable of stent-like compression. The samples in this work were found to have moduli ranging from 5 to 27 GPa, with the majority being between 10 and 20 GPa. We also found fracture strength greater than 100 MPa, with it sometimes getting as high as 200 MPa. Lastly, fracture strain values were measured, with the maximum reaching 1.4% and the average between 0.75-1%. We also found that the CI-CNTs material lends itself to fracture at weak locations (if present) before the anticipated fracture strength has been reached and concluded that a tightly controlled process (including fabrication machines) environment is necessary to ensure consistent results and a CI-CNT material whose imperfections have been minimized.

The purpose of this research was to design, develop, and test coronary stent designs composed of carbon-infiltrated carbon nanotubes (CI-CNTs). Coronary stents currently have two major complications: restenosis and thrombosis. CI-CNT stents have potential to address both of these issues, and therefore may provide improved clinical outcomes. CI-CNT stent geometry is patterned using high-resolution photolithography that provide advantages in design possibilities.To develop a coronary stent, a standard design process was followed including: background, design specifications, concept generation, development, analysis, and testing. Background research was first completed and general design specifications for coronary stent performance were compiled. Multiple design concepts were generated, evaluated, and finally a design was selected. This stent design was further developed and optimized using analytical tools along with finite element analysis. This stent design used tapered struts in repeating segments to reduce stress and improve radial force. The design was modeled and analyzed as both a flat geometry as well as in a cylindrical configuration. Mechanics of materials equations and geometry specific finite element analysis were used to guide the final coronary stent design. The stent design was tested mechanically, and additional tests were performed to verify the blood compatibility of the CI-CNT material. The flat version of the stent design was manufactured and mechanically tested to verify performance. The performance of the cylindrical stent configuration was analyzed using an FE model of an atherosclerotic artery. This arterial FE model was created and validated by analyzing balloon angioplasty of a common stainless steel stent. The biocompatibility of CI-CNTs was explored and studied. Blood compatibility testing of CI-CNT samples was performed with results comparable in performance to stainless steel. A method of stent deployment was planned, and several other stent design concepts were analyzed. This research demonstrates that a functioning coronary stent can be manufactured from CI-CNTs. The optimized design has potential to address problems currently associated with stents. However, a major challenge for CI-CNT stent designs is meeting the design requirement of sufficient radial force. CI-CNT stents also need to have excellent blood compatibility to justify being used in stent applications.

Elastic absorption of kinetic energy and distribution of impact forces are required in many applications. This may be achieved through the use of compliant corrugations. An innovative padding concept is investigated for such applications. Also, recent attention given to the potential for using origami in engineering applications may provide new corrugation configurations that are advantageous for energy absorption and force distribution. This work explores three areas related to these concepts.First, the parameters of a compliant, corrugated padding concept are investigated using Finite Element Analyses (FEA) and physical testing. The shape of the corrugation cross section is explored as well as the wavelength and amplitude by employing a full factorial design of experiments. FEA results are used to choose designs for prototyping and physical testing. The results of the physical testing were consistent with the FEA predictions although the FEA tended to underestimate the peak pressure compared to the physical tests. A performance metric is proposed to compare different padding configurations. The concept shows promise for sports padding applications. It may allow for designs which are smaller, more lightweight, and move better with an athlete than current technologies yet still provide the necessary protective functions.Second, the elastic energy absorbing properties of a particular origami folding pattern, the Miura-ori, is investigated. Analytical models for the kinematics and force-deflection of a unit cell based on two different modes of elastic energy absorption are derived. The models are used to explore the effects of the key geometrical parameters of the tessellation. Physical prototypes are compared to the analytical models.Third, a three-stage strategy is presented for selecting materials for origami-inspired corrugations that can deform to achieve a desired motion without yielding, absorb elastic strain energy, and be light weight or cost effective. Two material indices are derived to meet these requirements based on compliant mechanism theory. Using Finite element analysis, it is shown that the properties of Miura-ori pattern has advantages for energy absorption and force distribution when compared to a triangular wave corrugation. While the focus of these studies is the Miura-ori tessellation, the methods developed can be applied to other tessellated patterns used in energy absorbing or force distribution applications.

Minimally invasive surgery (MIS) has several advantages over traditional methods. Scaling MIS instruments to smaller sizes and increasing their performance will enable surgeons to offer new procedures to a wider range of patients. In this work, two compliant mechanism-based minimally invasive surgical instrument wrist or gripper mechanisms are designed and analyzed.The cylindrical cross-axis flexural pivot (CCAFP) is a single-degree-of-freedom wrist mechanism that could be combined with existing gripper mechanisms to create a multi-degree-of freedom instrument. The simplicity of the CCAFP mechanism facilitates analysis and implementation. The flexures of the CCAFP are integral with the instrument shaft, enabling accessories to be passed through the lumen. The CCAFP is analyzed and determined to be a viable wrist mechanism for MIS instruments based on research results. A finite element (FE) model of the mechanism is created to analyze the force-deflection and strain-deflection relationships. Experimental results are used to verify the FE model. A 3 mm design is created that could undergo an angular deflection of +/- 90 degrees. The addition of cam surfaces to help guide the flexures and limit the maximum stress during deflection is explored. These cam surfaces can be integral to the instrument shaft along with the flexures. A 2 degree-of-freedom (DoF) CCAFP with intersecting axes of rotation is also introduced. The inverted L-Arm gripper compliant mechanism has 2 DoF, one wrist and one gripping. Three challenges associated with using compliant mechanisms in MIS instruments are considered: inadequate performance in compression, large flexure deformations, and a highly variable mechanical advantage. These challenges were resolved in the L-Arm design by inverting the flexures, tailoring flexure geometry and employing nitinol, and integrating pulleys into each jaw of the mechanism. The L-Arm was prototyped at several sizes to demonstrate functionality and scalability. A finite element model of the L-Arm flexure was created to determine the strain-deflection relationship. A fatigue test was completed to characterize nitinol for use in compliant mechanism MIS instruments.These concepts demonstrate the ability of compliant mechanisms to overcome the design and manufacturing challenges associated with MIS instruments at the 3 mm scale. The models and principles included in this work could be used in the application of compliant mechanisms to design new MIS instruments as well as in other areas that employ compliant mechanisms in a cylindrical form factor.

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The objectives of the research presented in this thesis are the development of a seven degree-of-freedom morphing airplane and the design and integration of a variable camber compliant tail. The morphing airplane was designed and manufactured to study the benefits of large planform changes and flight control morphing. Morphing capabilities of each wing consist of 8 in. wing extension and contraction, 40° of wing sweep and ±20.25° of outboard wing twist in addition to 6 in. of tail extension and contraction. Initial wind-tunnel tests proved that for a large range of lift coefficients, the optimal airplane configuration changes to minimize the drag. Another portion of this research deals with the development of a structural optimization program to design a variable camber compliant tail. The program integrates ANSYS, aerodynamic thin airfoil theory and the Method of Moving Asymptotes to optimize the shape of an airfoil tail for maximum trailing edge deflection. An objective function is formulated to maximize the trailing edge tip deflection subject to stress constraints. The optimal structure needs to be flexible to maximize the tip deflection, but stiff enough to minimize the deflection of the tip due to aerodynamic loading. The results of the structural optimization program created a compliant tail mechanism that can deflect the trailing edge tip with a single actuator ±4.27°. / Master of Science

Mecanismo Flexível é um dispositivo mecânico utilizado para transformar movimento, força ou energia entre as portas de entrada e saída sem a presença de juntas, pinos baseados em uma estrutura em monolítica, em outras palavras, a transformação do movimento é dada pela flexibilidade de sua estrutura. Deste modo a transformação pode ser direcionada em uma direção em específico, amplificando ou reduzindo o deslocamento ou força aplicados. Por este motivo mecanismos flexíveis tem grandes aplicações em micromanipulação e nano posicionamento. A concepção deste tipo de mecanismo é complexa e uma das possibilidades de elaboração deste dispositivo mecânico é através da distribuição de flexibilidade ou rigidez dentro do domínio de projeto utilizando o Método de Otimização Topológica (MOT), que essencialmente combina algoritmos de otimização numéricos como Método de Elementos Finitos (MEF), por exemplo. A grande maioria das classes de mecanismos flexíveis existentes trabalha sob pequenos deslocamentos, na ordem de micro ou nano metros, no entanto, existe uma classe de mecanismos que utiliza o recurso da flambagem não linear para operar com grandes deslocamentos. O procedimento de concepção desta de classe de mecanismo é complexa e ainda se encontra em estagio inicial, necessitando de aprimoramentos que permitam o seu projeto completo via métodos computacionais. Portanto, esta tese foi desenvolvida como objetivo desenvolver uma metodologia computacional para projetar esta classe de mecanismo flexível inovador que emprega a flambagem não linear na sua estrutura como meio para obter sob grandes deslocamentos na porta de saída. A metodologia desenvolvida se baseia no MOT para obter a topologia da estrutura que satisfaça as restrições de projeto. A modelagem do comportamento físico da estrutura utiliza uma formulação variacional não linear do problema elástico, considerando a cinemática não linear com um modelo constitutivo policonvexo. O modelo de material aplicado para obter a topologia da estrutura do mecanismo foi o Solid IsotropicMaterial with Penalization (SIMP) com um algoritmo de otimização numérico baseado no método de ponto interior, onde foi utilizada a implementação do IpOpt em conjunto com a plataforma Python FEniCS de soluções de Equações Diferenciais Parciais (EDPs). São apresentados resultados bidimensionais de mecanismos considerando algumas configurações de geometria, condições de contorno e restrições de flambagem não-linear, como incremento de carga. / The compliant mechanism is a mechanical device used to transform displacement, force or energy between the input and output ports without joints, pins based on a monolithic structure, in other words, the motion transformation is given by the flexibility of its structure. In this way the movement can be defined to a specific axis direction, amplifying or reducing the applied displacement or force. For this reason, the compliant mechanism has significant applications in micromanipulation and nanopositioning system. The design of this type of device is intricate, and one way to achieve such design is trying to distribution flexibility or rigidity within the design domain using the Topology Optimization Method (TOM), which essentially combines numerical optimization algorithms with Finite ElementMethod (FEM), for example. Most models of existing compliant mechanism work under small displacements, in the order of micro or nanometers, nevertheless, there is a class of such mechanisms that uses the nonlinear buckling behavior to operate under large displacements. The design process of this mechanism type is complicated and is still at early stages, requiring improvements that allow a complete design process via computational methods. Therefore, this thesis goal is to develop a computational methodology to create this class of innovative compliant mechanism that employs nonlinear buckling behavior to work under large displacement at the output port. The approach developed is based on TOM to achieve the optimal structure topology that satisfies the design and optimization constraints. The modeling of the elasticity behavior of the structure relies on the nonlinear variational formulation, applying the nonlinear kinematics with a polyconvex constitutive model. The SIMP is employed as a material model to obtain the optimal topology of the mechanismstructure with a numeric optimization algorithm based on the interior point method, where the IpOpt implementation was used with the high-level Python interfaces to FEniCS to solve the partial differential equations (PDEs) problem. Two-dimensional results ofmechanisms are presented considering some geometric, boundary configuration, and including nonlinear buckling as design constraints.

Motivated by the interests to understand bio-structure deformation and exploit their advantages to create bio-inspired systems for engineering applications, a curvature-based model for analyzing compliant mechanisms capable of large deformation in a three dimensional space has been developed. Unlike methods (such as finite element) that formulate problems based on displacements and/or rotational angles, superposition holds for curvatures in the case of finite rotation but not for rotational angles; thus the curvature-based formulation presents an advantage in presenting nonlinear geometries. Along with a generalized constraint that relaxes traditional boundary constraints (such as fixed, pinned or sliding constraint) on compliant mechanisms, the method of deriving the compliant members in the same global referenced frame is presented. The attractive features of the method, which greatly simplifies the models and improves the computation efficiency of multi-body system deformation where compliant beams play an important role, have been experimentally validated. To demonstrate the applicability of this proposed method to a broad spectrum of applications, three practical examples are given; the first example verifies the generalized constraint by analyzing the multi-axis rotation motion within a natural human knee joint and investigates the human-exoskeleton interactions through dynamic analysis. The second example studies a deformable bio-structure by incorporating the generalized joint constraint into the curvature-based model for automated poultry meat processing. The last example designs a bio-inspired robot with a compliant mechanism to serve as a flexonic mobile node for ferromagnetic structure health monitoring. The analytical models have been employed (with experimental validation) to investigate the effects of different joint constraints on the mechanism deformations. It is expected that the proposed method will find a broad range of applications involving compliant mechanisms.

Cellular structures are present from the atomic level all the way up to patterns found in human skeleton. They are prevailing structures in the nature and known for their excellent mechanical, thermal, and acoustic properties. Two typical types of cellular structures, lightweight structures and compliant mechanisms, are investigated. Lightweight structures are rigid and designed to reduce weight, while increasing strength and stiffness. Compliant mechanisms are designed to transform motions and forces. Most available artificial lightweight structures are patterns of primitives. However, the performance of lightweight structures can be enhanced by using adaptive cellular structures with conformal strut orientations and sizes, like the trabeculae in femoral bone. Bending, torsion, and nonlinear behaviors of compliant mechanisms have not been sufficiently studied. In order to design adaptive cellular structures, a new unit cell, the unit truss is proposed. The unit truss approach facilitates the design of adaptive cellular structures for enhanced mechanical properties via geometric modeling, finite element analysis, shape optimization, and additive fabrication. Four research questions, which address representation, structural analysis, design synthesis, and manufacturing respectively, are raised and answered. Unit truss enables representation and mechanics analysis for adaptive cellular structures. A synthesis method using engineering optimization algorithms is developed to systematically design adaptive cellular structure. Two examples, graded cellular structure for prosthesis and compliant mechanism for morphing wings, are studied to test the unit truss approach.