Design and Optimization of Narrow-Gauged Contact-Aided Compliant Mechanisms for Advanced Minimally Invasive Surgery

Open Access
Author:
Aguirre, Milton
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 21, 2011
Committee Members:
  • Mary I Frecker, Dissertation Advisor
  • Mary I Frecker, Committee Chair
  • Eric M Mockensturm, Committee Member
  • Matthew B Parkinson, Committee Member
  • Christopher Muhlstein, Committee Member
  • Harriet Black Nembhard, Committee Member
Keywords:
  • Design
  • Contact-Aided
  • Compliant Mechanism
  • Advanced MIS
  • NOTES
  • Optimization
Abstract:
The goal of this research is to develop design and optimization methods for contact-aided compliant mechanisms intended for Natural Orifice Translumenal Endoscopic Surgery (NOTES). The design methods are used to develop prototypes for the purpose of evaluating tool performance in benchtop and end-user testing procedures. The work described in this dissertation is part of a collaborative effort between mechanical design engineers and material scientists. Recent advancements in materials sciences and a need for mesoscale surgical instruments to advance NOTES, drive the development of a novel design and manufacturing method. NOTES has been referred to as the ‘coming revolution of surgery’ and motivates development of new design and manufacturing methods. New instruments must be smaller in diameter to minimize the size of incisions coupled with the flexibility to approach the surgical site through the body’s natural orifices. Once access to the surgical site is attained, the instruments must be capable of precise and effective manipulation to complete complex procedures. Collaborating with clinicians at the Penn State Hershey Medical Center, a surgical forceps is selected for design, taking into account constraints imposed by manufacturing processes. A significant challenge is designing a surgically useful device under the severe geometric constraints of narrow-gauge instruments. The goal is to design a forceps that can open large enough and provide sufficient grasping force to effectively execute complex surgical maneuvers. This research seeks to attain this goal through compliant mechanism design. The device is a monolithic contact-aided compliant mechanism, which relies on contact stress-relief to obtain large elastic tip deflections for effective grasping. Because of the proposed instrument scale and required feature sizes, Penn State’s lost mold rapid infiltration forming process is selected for part fabrication. Since quantitative surgical instrument requirements are not well defined in the literature, they are established within this dissertation. Design constraints are defined in terms of the current limitations imposed by the manufacturing process. Computational methods are developed to simulate and predict tool performance using nonlinear finite element analysis. Size and shape optimization methods are used to determine optimal tool dimensions for ceramic and metallic parts. Once parts are fabricated they are characterized and tested to compare experimental and theoretical tool performance. Good agreement between experimental and theoretical tests verified that the prototypes merit assessment in end-user surgical simulators. Collaborators at the Penn State Hershey Medical Center volunteered to execute a set of standardized tasks to compare the prototype’s performance side-by-side with a standard commercially available endoscopic forceps. The prototype was rated superior at fine grasping and its ability to control intermediate positions between the open and closed positions of the jaws. The standard instrument had a slightly higher average measured pull-off force; however, the maximum pull-off force recorded for both instruments was identical at 1.46 N. The prototype was also identified as being not suitable for biopsies, which can be accredited to the prototype’s square cross-sectional arms. Although various design modifications were suggested to improve prototype performance, the most common critique was that the jaw opening of the device should be increased. Since the prototype’s grasping force was rated satisfactory, alternative design methods focus on increasing the total jaw opening while preserving the current grasping capabilities. These design methods, which adhere to the current two-dimensional manufacturing constraint, include a hybrid, two material forceps design and a multiple contact-aided compliant mechanism design. For both alternative design concepts intermediate solutions are provided for the consideration of future work. The hybrid design method was developed to investigate the performance benefits of two-material contact-aided compliant mechanisms. A hybrid approach could eliminate the tradeoff between flexibility and stiffness by isolating the two desired properties. Results show that larger jaw openings and blocked forces can be achieved, compared to a homogeneous design. Additionally, a design method was developed to examine whether multiple contact-aided compliant mechanisms can further exploit the benefits of contact stress-relief. Theoretically, designs with additional contact elements result in more stress-relief to maximize the re-distribution of stresses and achieve larger elastic tip deflections. Results prove that the device generates larger elastic tip deflections with each additional contact element. When comparing the performance of a multi-contact forceps to the existing prototypes, the predicted jaw opening improved by a factor of three; however, to obtain comparable grasping forces three-dimensional or hybrid manufacturing capabilities are required. The design and optimization techniques developed in this dissertation were demonstrated through the design of a narrow-gauge forceps for NOTES. End-user tests identified design modifications for the next generation of prototypes. Future work to improve the tool performance should focus on incorporating flexible materials and/or 3D manufacturing into the design and fabrication process.