Surface wave propagation control with locally resonant metasurfaces using topology-optimized resonatorsa).

Locally resonant elastodynamic metasurfaces for suppressing surface waves have gained popularity in recent years, especially because of their potential in low-frequency applications such as seismic barriers. Their design strategy typically involves tailoring geometrical features of local resonators to attain a desired frequency bandgap through extensive dispersion analyses. In this paper, a systematic design methodology is presented to conceive these local resonators using topology optimization, where frequency bandgaps develop by matching multiple antiresonances with predefined target frequencies. The design approach modifies an individual resonator's response to unidirectional harmonic excitations in the in-plane and out-of-plane directions, mimicking the elliptical motion of surface waves. Once an arrangement of optimized resonators composes a locally resonant metasurface, frequency bandgaps appear around the designed antiresonance frequencies. Numerical investigations analyze three case studies, showing that longitudinal-like and flexural-like antiresonances lead to nonoverlapping bandgaps unless both antiresonance modes are combined to generate a single and wider bandgap. Experimental data demonstrate good agreement with the numerical results, validating the proposed design methodology as an effective tool to realize locally resonant metasurfaces by matching multiple antiresonances such that bandgaps generated as a result of in-plane and out-of-plane surface wave motion combine into wider bandgaps.

Design of resonant elastodynamic metasurfaces to control S0 Lamb waves using topology optimization.

Control of guided waves has applications across length scales ranging from surface acoustic wave devices to seismic barriers. Resonant elastodynamic metasurfaces present attractive means of guided wave control by generating frequency stop-bandgaps using local resonators. This work addresses the systematic design of these resonators using a density-based topology optimization formulated as an eigenfrequency matching problem that tailors antiresonance eigenfrequencies. The effectiveness of our systematic design methodology is presented in a case study, where topologically optimized resonators are shown to prevent the propagation of the S0 wave mode in an aluminum plate.

When high viscosity of pancreatic cysts precludes effective EUS-FNA: a benchtop comparison of negative pressure devices.

Background and study aims Endoscopic ultrasound-guided fine needle aspiration (EUS-FNA) of pancreatic cystic lesions (PCLs) is an important diagnostic tool; however, it is often unsuccessful due to high viscosity of cystic fluid. In an effort to improve FNA, we objectively compared eight vacuum device configurations to determine the most effective method for aspirating viscous fluid collections. We also tested a high-frequency oscillation (HFO) technique that could be employed in FNA. Materials and methods Maximum gauge pressures of four vacuum devices were measured: two standard EUS-FNA syringes, a 50-cc Alliance II device, and a nonmedical hand vacuum pump. To aspirate a viscous stock solution, 19-gauge and 22-gauge needles were used and flow rates were calculated. HFO was also applied to the needle during aspiration to determine effect on aspiration rate. Results Aspiration devices generated maximum gauge pressures ranging from -21.5 to -27.5 inHg. The 19-gauge FNA needle aspirated viscous fluid 11.3 × faster on average than a 22-gauge needle. HFO increased average flow rates by 29.7 % in 19G and 124.6 % in 22G configurations. Conclusion EUS-FNA of viscous fluid can be optimized by using the lowest possible gauge needle and connecting a vacuum device capable of generating and sustaining near perfect vacuum. This can be accomplished by maximizing syringe volume. In addition, connector-tubing length between the syringe and needle should be minimized, and tubing wall should be sufficiently strong to resist collapse under vacuum. Other novel techniques to increase fluid yield include a hand vacuum pump and application of HFO to FNA.

Analytical model and stability analysis of the leading edge spar of a passively morphing ornithopter wing.

This paper presents the stability analysis of the leading edge spar of a flapping wing unmanned air vehicle with a compliant spine inserted in it. The compliant spine is a mechanism that was designed to be flexible during the upstroke and stiff during the downstroke. Inserting a variable stiffness mechanism into the leading edge spar affects its structural stability. The model for the spar-spine system was formulated in terms of the well-known Mathieu's equation, in which the compliant spine was modeled as a torsional spring with a sinusoidal stiffness function. Experimental data was used to validate the model and results show agreement within 11%. The structural stability of the leading edge spar-spine system was determined analytically and graphically using a phase plane plot and Strutt diagrams. Lastly, a torsional viscous damper was added to the leading edge spar-spine model to investigate the effect of damping on stability. Results show that for the un-damped case, the leading edge spar-spine response was stable and bounded; however, there were areas of instability that appear for a range of spine upstroke and downstroke stiffnesses. Results also show that there exist a damping ratio between 0.2 and 0.5, for which the leading edge spar-spine system was stable for all values of spine upstroke and downstroke stiffnesses.

Development of tasks and evaluation of a prototype forceps for NOTES.

BACKGROUND AND OBJECTIVES: Few standardized testing procedures exist for instruments intended for Natural Orifice Translumenal Endoscopic Surgery. These testing procedures are critical for evaluating surgical skills and surgical instruments to ensure sufficient quality. This need is widely recognized by endoscopic surgeons as a major hurdle for the advancement of Natural Orifice Translumenal Endoscopic Surgery. METHODS: Beginning with tasks currently used to evaluate laparoscopic surgeons and instruments, new tasks were designed to evaluate endoscopic surgical forceps instruments. RESULTS: Six tasks have been developed from existing tasks, adapted and modified for use with endoscopic instruments, or newly designed to test additional features of endoscopic forceps. The new tasks include the Fuzzy Ball Task, Cup Drop Task, Ring Around Task, Material Pull Task, Simulated Biopsy Task, and the Force Gauge Task. These tasks were then used to evaluate the performance of a new forceps instrument designed at Pennsylvania State University. CONCLUSIONS: The need for testing procedures for the advancement of Natural Orifice Translumenal Endoscopic Surgery has been addressed in this work. The developed tasks form a basis for not only testing new forceps instruments, but also for evaluating individual performance of surgical candidates with endoscopic forceps instruments.

Lost Mold-Rapid Infiltration Forming of Mesoscale Ceramics: Part 2, Geometry and Strength Improvements.

Iterative process improvements have been used to eliminate strength-limiting geometric flaws in mesoscale bend bars composed of yttria-tetragonal zirconia polycrystals (Y-TZP). These improvements led to large quantities of high bend strength material. The metrology of Y-TZP mesoscale bend bars produced using a novel lost mold-rapid infiltration-forming process (LM-RIF) is characterized over several process improvements. These improvements eliminate trapezoidal cross sections in the parts, reduce concave upper surfaces in cross section, and minimize warping along the long axis of 332 x 26 x 17 mum mesoscale bend bars. The trapezoidal cross sections of earlier, first-generation parts were due to the absorption of high-energy ultraviolet (UV) light during the photolithographic mold-forming process, which produced nonvertical mold walls that the parts mirrored. The concave upper surfaces in cross section were eliminated by implementing a RIF-buffing process. Warping during sintering was attributed to impurities in the substrate, which creates localized grain growth and warping as the tetragonal phase becomes destabilized. Precision in the part dimensions is demonstrated using optical profilometry on bend bars and a triangular test component. The bend bar dimensions have a 95% confidence interval of < +/-1 mum, and the tip radius of the triangular test component is 3 mum, consistent with the UV-photolithographic process used to form the mold cavities. The average bend strength of the mesoscale Y-TZP bend exceeds 2 GPa with a Weibull modulus equal to 6.3.

Lost Mold Rapid Infiltration Forming of Mesoscale Ceramics: Part 1, Fabrication.

Free-standing mesoscale (340 mum x 30 mum x 20 mum) bend bars with an aspect ratio over 15:1 and an edge resolution as fine as a single grain diameter ( approximately 400 nm) have been fabricated in large numbers on refractory ceramic substrates by combining a novel powder processing approach with photoresist molds and an innovative lost-mold thermal process. The colloid and interfacial chemistry of the nanoscale zirconia particulates has been modeled and used to prepare highly concentrated suspensions. Engineering solutions to challenges in mold fabrication and casting have yielded free-standing, crack-free parts. Molds are fabricated using high-aspect-ratio photoresist on ceramic substrates. Green parts are formed using a rapid infiltration method that exploits the shear thinning behavior of the highly concentrated ceramic suspension in combination with gelcasting. The mold is thermally decomposed and the parts are sintered in place on the ceramic substrate. Chemically aided attrition milling disperses and concentrates the as-received 3Y-TZP powder to produce a dense, fine-grained sintered microstructure. Initial three-point bend strength data are comparable to that of conventional zirconia; however, geometric irregularities (e.g., trapezoidal cross sections) are present in this first generation and are discussed with respect to the distribution of bend strength.

Laparoscopic multifunctional instruments: design and testing of initial prototypes.

BACKGROUND: Advances in minimally invasive surgical techniques will require new types of instrument end-effectors for smaller, longer, and flexible instruments. These include a new class of multifunctional instruments capable of performing more than 1 task with a single set of working jaws. Furthermore, it is desired that multifunctional instruments be designed to provide improved dexterity compared with that in currently commercially available instruments. METHODS: Three prototypes of multifunctional laparoscopic surgical instruments are described: (1) a mechanical scissors-grasper, (2) a mechanical scissors-grasperarticulator, and (3) a compliant mechanism scissors-grasper. Methods of baseline analysis, design methods and considerations, and subjective evaluations of interim prototypes are presented. RESULTS: The 3 prototypes demonstrate promising early results. However, based on subjective evaluation, these prototypes do not perform individual functions as well as basic disposable single-function laparoscopic instruments do. CONCLUSIONS: The concept of multifunctionality and increased end-effector dexterity is achievable as demonstrated by the prototypes presented. Further work is required to refine, simplify, and improve the multifunctional instruments to a point where they may be useful as surgical tools.

Design of a multifunctional compliant instrument for minimally invasive surgery.

A new multifunctional compliant instrument has been designed for use in minimally invasive surgery. The instrument combines scissors and forceps into a single multifunctional device. The main advantage of using multifunctional instruments for minimally invasive surgery is that instrument exchanges can be reduced, thus reducing procedure time and risk of inadvertent tissue injury during instrument exchanges. In this paper, the length, width, and thickness of the multifunctional compliant mechanism tool tip is optimized to maximize the jaw opening and the grasping force. The optimized design is then modeled to simulate the stresses encountered in the scissors mode. A 5.0 mm diameter stainless steel prototype is fabricated using electro-discharge machining and is shown to grasp and cut successfully.