Design Optimization of Compliant Structures for Radiofrequency Ablation and Additive Manufacturing
Open Access
- Author:
- Hanks, Bradley Wright
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 21, 2020
- Committee Members:
- Mary I Frecker, Dissertation Advisor/Co-Advisor
Mary I Frecker, Committee Chair/Co-Chair
Reuben H Kraft, Committee Member
Jason Zachary Moore, Committee Member
Edward William Reutzel, Outside Member
Timothy W. Simpson, Committee Member
Karen Ann Thole, Program Head/Chair - Keywords:
- design optimization
compliant mechanism
additive manufacturing
lattice structures
radiofrequency ablation - Abstract:
- Recent advances in the capabilities of model simulation and manufacturing technology are paving the way for systematic design tools. These tools enable designers to explore organic and nonconventional designs that were previously not possible to analyze or manufacture. Through design optimization, these nonconventional designs may be used to maximize part performance in a variety of applications such as medical, aerospace, and defense industries. One sector of nonconventional designs that have been relatively unexplored is that of compliant mechanisms or tailored mechanical properties for metamaterial design. Compliant mechanisms are structures which gain motion through deflection or deformation of a material rather than traditional hinges or joints. Metamaterials gain their properties based their structure as opposed to material composition. Using material deformation to gain motion allows for generation of unique structures that reduce the wear commonly seen in hinges, remove the need for lubrication in harsh environments, reduce part count, and improve dexterity. While these advantages present unique opportunities in a variety of situations, it remains challenging to design and optimize these nonconventional structures. In this dissertation two systematic design optimization approaches are described for compliant structures: a shape matching approach for a deployable surgical tool for radiofrequency ablation (RFA) of tumors and generation of lattice structures for additive manufacturing (AM). Radiofrequency ablation is an increasing common minimally invasive treatment option for abdominal tumors. During the procedure, an electrode is inserted into the tumor and the surrounding tissue is heated. To effectively destroy a tumor with RFA, it is critical that the shape of the treatment match the shape of the tumor with a small margin around the periphery. The shape of the treatment, or ablation zone, is largely dependent on the shape of the electrode. To improve treatment for endoscopic RFA of tumors, a novel compliant deployable RFA electrode is presented along with a systematic design optimization approach for shape matching the treatment zone and tumor. The optimization approach includes a finite element model of RFA coupled with a genetic algorithm, an approach that may be applied to tumors throughout the abdomen and mediastinum. A specific example of the approach is demonstrated through pancreatic tumor ablation. The results of the approach show treatment efficiency is increased from 25% to 87% for a 2.5cm spherical tumor. In addition, experimental validation of the finite element RFA model is reported. With the recent advances in AM, systematic design optimization approaches are needed for generating structures which take advantage of the design freedom available. With the added design freedom, lattice structures, which consist of a repeating unit cell topology patterned throughout a structure, may be used to reduce weight or increase functionality while maintaining part performance. For AM, nearly infinite possibilities are available for the unit cell topology that will populate the lattice structure. However, there is a lack of fundamental understanding of unit cell topology selection. As a result, lattice structures are used without full consideration for how the lattice structure changes the properties of the part. For development of tailored compliance, this dissertation presents a design for AM (DfAM) systematic design optimization approach for generating manufacturable and customized unit cell topologies. Using a ground structure topology optimization (GSTO) approach, novel unit cell topologies may be generated to meet application specific needs. By incorporating a library of unique optimization objectives, constraints, and penalties, Additive Lattice Topology Optimization (ALTO) is an application-agnostic approach to unit cell design, demonstrated through various case studies for unique lattice structure applications. Two case studies are presented for multi-functional lattices that combine minimization of strain energy and maximization of thermal conductance. The results show how DfAM considerations can be included in the optimization to improve manufacturability and are demonstrated with printed examples of the optimized solution. In addition, these multi-functional lattices highlight the importance of multi-objective optimization and its ability to generate a Pareto-optimal set of solutions. Three other case studies are presented for optimizing structures that match a target constitutive matrix. The first of these three demonstrates a novel powder removability factor to generate a unit cell topology that has the same orthogonal effective elastic moduli as a baseline unit cell topology. The optimized solution shows improved powder removal compared to the baseline unit cell through mass measurement, CT scan analysis, and tortuosity quantification. Experimental compression testing of both the optimized and baseline unit cell topologies validate the predicted results within 6%. The second of these case studies uses optimization to design three novel unit cell topologies that utilize mechanism-like behavior to reduce stiffness without reducing weight. This enables lattice structures with a non-uniform stiffness while maintaining a uniform weight distribution. The third case study demonstrates generation of an orthotropic unit cell topology, with three times the effective modulus in one direction as compared another. As an additional part of this work, a compilation of data from analytical models, experimental characterization, and finite-element analysis of lattice structures has resulted in the largest public database of mechanical property information for metal lattice structures fabricated with AM. The Lattice Unit-cell Characterization Interface for Engineers (LUCIE) offers Ashby-style plots for unit cell topology trends in mechanical properties. It represents a compilation of 69 sources, resulting in 1400 experimental data points, 200+ finite element points, and 45+ analytical models for a total of 18 different unit cell topologies. The culmination of this work presents research contributions for novel design optimization approaches for RFA of tumors and DfAM of metal lattice structures through numerous case studies and experimental validation.