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1. 3-D swimming microdrone powered by acoustic bubbles

   https://doi.org/10.1039/d0lc00976h  

   Liu, Fang-Wei ; Cho, Sung Kwon ( January 2021 , Lab on a Chip)

   null (Ed.)

   Mobile microrobots that maneuver in liquid environments and navigate inside the human body have drawn a great interest due to their possibility for medical uses serving as an in vivo cargo. For this system, the effective self-propelling method, which should be powered wirelessly and controllable in 3-D space, is of paramount importance. This article describes a bubble-powered swimming microdrone that can navigate in 3-D space in a controlled manner. To enable 3-D propulsion with steering capability, air bubbles of three lengths are trapped in microtubes that are embedded and three-dimensionally aligned inside the drone body using two-photon polymerization. These bubbles can generate on-demand 3-D propulsion through microstreaming when they are selectively excited at their individual resonance frequencies that depend on the bubble sizes. In order to equip the drone with highly stable maneuverability, a non-uniform mass distribution of the drone body is carefully designed to spontaneously restore the drone to the upright position from disturbances. A mathematical model of the restoration mechanism is developed to predict the restoration behavior showing a good agreement with the experimental data. The present swimming microdrone potentially lends itself to a robust 3-D maneuverable microscale mobile cargo navigating in vitro and in vivo for biomedical applications. 

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2. Reinforcement learning of a multi-link swimmer at low Reynolds numbers

   https://doi.org/10.1063/5.0140662  

   Qin, Ke ; Zou, Zonghao ; Zhu, Lailai ; Pak, On Shun ( March 2023 , Physics of Fluids)

   The use of machine learning techniques in the development of microscopic swimmers has drawn considerable attention in recent years. In particular, reinforcement learning has been shown useful in enabling swimmers to learn effective propulsion strategies through its interactions with the surroundings. In this work, we apply a reinforcement learning approach to identify swimming gaits of a multi-link model swimmer. The swimmer consists of multiple rigid links connected serially with hinges, which can rotate freely to change the relative angles between neighboring links. Purcell [“Life at low Reynolds number,” Am. J. Phys. 45, 3 (1977)] demonstrated how the particular case of a three-link swimmer (now known as Purcell's swimmer) can perform a prescribed sequence of hinge rotation to generate self-propulsion in the absence of inertia. Here, without relying on any prior knowledge of low-Reynolds-number locomotion, we first demonstrate the use of reinforcement learning in identifying the classical swimming gaits of Purcell's swimmer for case of three links. We next examine the new swimming gaits acquired by the learning process as the number of links increases. We also consider the scenarios when only a single hinge is allowed to rotate at a time and when simultaneous rotation of multiple hinges is allowed. We contrast the difference in the locomotory gaits learned by the swimmers in these scenarios and discuss their propulsion performance. Taken together, our results demonstrate how a simple reinforcement learning technique can be applied to identify both classical and new swimming gaits at low Reynolds numbers. 

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3. Buoyant magnetic milliswimmers reveal design rules for optimizing microswimmer performance

   https://doi.org/10.1039/d3nr02846a  

   Benjaminson, Emma ; Imamura, Taryn ; Lorenz, Aria ; Bergbreiter, Sarah ; Travers, Matthew ; Taylor, Rebecca E. ( August 2023 , Nanoscale)

   Magnetically-actuated swimming microrobots are an emerging tool for navigating and manipulating materials in confined spaces. Recent work has demonstrated that it is possible to build such systems at the micro and nanoscales using polymer microspheres, magnetic particles and DNA nanotechnology. However, while these materials enable an unprecedented ability to build at small scales, such systems often demonstrate significant polydispersity resulting from both the material variations and the assembly process itself. This variability makes it difficult to predict, let alone optimize, the direction or magnitude of microswimmer velocity from design parameters such as link shape or aspect ratio. To isolate questions of a swimmer's design from variations in its physical dimensions, we present a novel experimental platform using two-photon polymerization to build a two-link, buoyant milliswimmer with a fully customizable shape and integrated flexible linker (the swimmer is underactuated, enabling asymmetric cyclic motion and net translation). Our approach enables us to control both swimming direction and repeatability of swimmer performance. These studies provide ground truth data revealing that neither the first order nor second order models currently capture the key features of milliswimmer performance. We therefore use our experimental platform to develop design guidelines for tuning the swimming speeds, and we identify the following three approaches for increasing speed: (1) tuning the actuation frequency for a fixed aspect ratio, (2) adjusting the aspect ratio given a desired range of operating frequencies, and (3) using the weaker value of linker stiffness from among the values that we tested, while still maintaining a robust connection between the links. We also find experimentally that spherical two-link swimmers with dissimilar link diameters achieve net velocities comparable to swimmers with cylindrical links, but that two-link spherical swimmers of equal diameter do not. 

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4. Dynamics of rigid achiral magnetic microswimmers in shear-thinning fluids

   https://doi.org/10.1063/5.0167307  

   Quashie, David ; Wang, Qi ; Jermyn, Sophie ; Katuri, Jaideep ; Ali, Jamel ( September 2023 , Physics of Fluids)

   Here, we use magnetically driven self-assembled achiral swimmers made of two to four superparamagnetic micro-particles to provide insight into how swimming kinematics develop in complex, shear-thinning fluids. Two model shear-thinning polymer fluids are explored, where measurements of swimming dynamics reveal contrasting propulsion kinematics in shear-thinning fluids vs a Newtonian fluid. When comparing the velocity of achiral swimmers in polymer fluids to their dynamics in water, we observe kinematics dependent on (1) no shear-thinning, (2) shear-thinning with negligible elasticity, and (3) shear-thinning with elasticity. At the step-out frequency, the fluidic environment's viscoelastic properties allow swimmers to propel faster than their Newtonian swimming speed, although their swimming gait remains similar. Micro-particle image velocimetry is also implemented to provide insight into how shear-thinning viscosity fluids with elasticity can modify the flow fields of the self-assembled magnetic swimmers. Our findings reveal that flow asymmetry can be created for symmetric swimmers through either the confinement effect or the Weissenberg effect. For pseudo-chiral swimmers in shear-thinning fluids, only three bead swimmers show swimming enhancement, while four bead swimmers always have a decreased step-out frequency velocity compared to their dynamics in water.

    

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5. Co-designing Ethical Supports for Technology Practitioners

   https://doi.org/10.1109/ETHICS57328.2023.10155098  

   Li, Ziqing ; Obi, Ike ; Chivukula, Shruthi Sai ; Will, Matthew ; Johns, Janna ; Pivonka, Anne C. ; Carlock, Thomas ; Menon, Ambika R. ; Bharadwaj, Aayushi ; Gray, Colin M. ( May 2023 , 2023 IEEE International Symposium on Ethics in Engineering, Science, and Technology (ETHICS))

   In an era of ubiquitous digital interfaces and systems, technology and design practitioners must address a range of ethical dilemmas surrounding the use of persuasive design techniques and how to balance shareholder and end-user needs [2], [5]. Similarly, the increasing user concerns about unethical products and services [1] is paralleling a rise in regulatory interests in enforcing ethical design and engineering practices among technology practitioners, surfacing a need for further support. Although various scholars have developed frameworks and methods to support practitioners in navigating these challenging contexts [3], [4], often, there is a lack of resonance between these generic methods and the situated ethical complexities facing the practitioner in their everyday work. In this project, we designed and implemented a three-hour cocreation workshop with designers, engineers, and technologists to support them to develop bespoke ethics-focused action plans that are resonant with the ethical challenges they face in their everyday practice. In developing the co-creation session, we sought to answer the following questions to empower practitioners: • How can we support practitioners in developing action plans to address ethical dilemmas in their everyday work? and • How can we empower designers to design more responsibly? Building on these questions as a guide, we employed Miro, a digital whiteboard platform, to develop the co-creation experience. The final c o-creation e xperience w as d esigned w ith the visual metaphor of a “house” with four floors and multiple rooms that allowed participants to complete different tasks per room, all aimed towards the overall goal of developing participants' own personalized action plan in an interactive and collaborative way. We invited participants to share their stories and ethical dilemmas to support their creation and iteration of a personal action plan that they could later use in their everyday work context. Across the six co-creation sessions we conducted, participants (n=26) gained a better understanding of the drivers for ethical action in the context of their everyday work and developed an action plan through the co-creation workshop that enabled them to constructively engage with ethical challenges in their professional context. At the end of the session, participants were provided the action plans they created to allow them to use it in their practice. Furthermore, the co-design workshops were designed such that practitioners could take them away (the house and session guide) and run them independently at their organization or another context to support their objectives. We describe the building and the activities conducted in each floor below and will provide a pictorial representation of the house with the different floors, rooms, and activities on the poster presentation. a) First floor-Welcome, Introduction, Reflection: The first floor of the virtual house was designed to allow participants to introduce themselves and to reflect on and discuss the ethical concerns they wished to resolve during the session. b) Second floor-Shopping for ethics-focused methods: The second floor of the virtual house was designed as a “shopping” space where participants selected from range of ethicsfocused building blocks that they wish to potentially adapt or incorporate into their own action plan. They were also allowed to introduce their own methods or tools. c) Third floor-DIY Workspace: The third floor was designed as a DIY workspace to allow the participants to work in small groups to develop their own bespoke action plan based on building blocks they have gathered from their shopping trip and by using any other components they wish. The goal here was to support participants in developing methods and action plans that were resonant with their situated ethical complexities. d) Fourth floor-Gallery Space: The fourth floor was designed as a gallery to allow participants to share and discuss their action plans with other participants and to identify how their action plans could impact their future practice or educational experiences. Participants were also provided an opportunity at this stage to reflect on their experience participating in the session and provide feedback on opportunities for future improvement. 

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