Special Issue "Microswimmer"

A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "B:Biology".

Deadline for manuscript submissions: closed (10 January 2019)

Special Issue Editor

Guest Editor
Prof. Takuji Ishikawa

Department of Finemechanics, Graduate School of Engineering Tohoku University 6-6-01, Aoba, Aoba-ku, Sendai 980-8579, Japan
Website | E-Mail
Interests: biomechanics; fluid mechanics; biological flow; swimming

Special Issue Information

Dear Colleagues,

An artificial microswimmer is a cutting-edge technology with engineering and medical applications. A natural microswimmer, such as bacteria and sperm cells, also play important roles in wide varieties of engineering, medical and biological phenomena. Due to the small size of the microswimmer, the inertial effect of the surrounding flow field may be negligible. In such a case, reciprocal body deformation cannot induce migration of a swimmer, which is known as the scallop theorem. To overcome the implications of the scallop theorem, the microswimmer needs to undergo a nonreciprocal body deformation to achieve migration. The swimming strategy is thus completely different from macro-scale swimmers, which should be clarified much further. This Special Issue seeks to showcase research papers, short communications, and review articles that focus on various microswimmers in nature, laboratories and industries. Potential topics will include artificial swimmers, natural swimmers, flowing cells, active matter and soft matter.

Prof. Takuji Ishikawa
Guest Editor

Manuscript Submission Information

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Keywords

  • Microrobot
  • Microorganism
  • Cell
  • Active matter
  • Soft matter
  • Locomotion
  • Microfluidics

Published Papers (6 papers)

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Research

Open AccessArticle
Hydrophobicity Influence on Swimming Performance of Magnetically Driven Miniature Helical Swimmers
Micromachines 2019, 10(3), 175; https://doi.org/10.3390/mi10030175
Received: 15 January 2019 / Revised: 22 February 2019 / Accepted: 28 February 2019 / Published: 6 March 2019
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Abstract
Helical microswimmers have been involved in a wide variety of applications, ranging from in vivo tasks such as targeted drug delivery to in vitro tasks such as transporting micro objects. Over the past decades, a number of studies have been established on the [...] Read more.
Helical microswimmers have been involved in a wide variety of applications, ranging from in vivo tasks such as targeted drug delivery to in vitro tasks such as transporting micro objects. Over the past decades, a number of studies have been established on the swimming performance of helical microswimmers and geometrical factors influencing their swimming performance. However, limited studies have focused on the influence of the hydrophobicity of swimmers’ surface on their swimming performance. In this paper, we first demonstrated through theoretical analysis that the hydrophobicity of swimmer’s surface material of the swimmer does affect its swimming performance: the swimmer with more hydrophobic surface is exerted less friction drag torque, and should therefore exhibit a higher step-out frequency, indicating that the swimmer with more hydrophobic surface should have better swimming performance. Then a series of experiments were conducted to verify the theoretical analysis. As a result, the main contribution of this paper is to demonstrate that one potential approach to improve the helical microswimmers’ swimming performance could be making its surface more hydrophobic. Full article
(This article belongs to the Special Issue Microswimmer)
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Open AccessArticle
Swimming of Spermatozoa in a Maxwell Fluid
Micromachines 2019, 10(2), 78; https://doi.org/10.3390/mi10020078
Received: 11 December 2018 / Revised: 18 January 2019 / Accepted: 21 January 2019 / Published: 24 January 2019
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Abstract
It has been suggested that the swimming mechanism used by spermatozoa could be adopted for self-propelled micro-robots in small environments and potentially applied to biomedical engineering. Mammalian sperm cells must swim through a viscoelastic mucus layer to find the egg cell. Thus, understanding [...] Read more.
It has been suggested that the swimming mechanism used by spermatozoa could be adopted for self-propelled micro-robots in small environments and potentially applied to biomedical engineering. Mammalian sperm cells must swim through a viscoelastic mucus layer to find the egg cell. Thus, understanding how sperm cells swim through viscoelastic liquids is significant not only for physiology, but also for the design of micro-robots. In this paper, we developed a numerical model of a sperm cell in a linear Maxwell fluid based on the boundary element slender-body theory coupling method. The viscoelastic properties were characterized by the Deborah number (De), and we found that, under the prescribed waveform, the swimming speed decayed with the Deborah number in the small-De regime (De < 1.0). The swimming efficiency was independent of the Deborah number, and the decrease in the swimming speed was not significantly affected by the wave pattern. Full article
(This article belongs to the Special Issue Microswimmer)
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Open AccessArticle
Microswimmer Propulsion by Two Steadily Rotating Helical Flagella
Micromachines 2019, 10(1), 65; https://doi.org/10.3390/mi10010065
Received: 31 December 2018 / Revised: 15 January 2019 / Accepted: 15 January 2019 / Published: 18 January 2019
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Abstract
Many theoretical studies of bacterial locomotion adopt a simple model for the organism consisting of a spheroidal cell body and a single corkscrew-shaped flagellum that rotates to propel the body forward. Motivated by experimental observations of a group of magnetotactic bacterial strains, we [...] Read more.
Many theoretical studies of bacterial locomotion adopt a simple model for the organism consisting of a spheroidal cell body and a single corkscrew-shaped flagellum that rotates to propel the body forward. Motivated by experimental observations of a group of magnetotactic bacterial strains, we extended the model by considering two flagella attached to the cell body and rotating about their respective axes. Using numerical simulations, we analyzed the motion of such a microswimmer in bulk fluid and close to a solid surface. We show that positioning the two flagella far apart on the cell body reduces the rate of rotation of the body and increases the swimming speed. Near surfaces, we found that swimmers with two flagella can swim in relatively straight trajectories or circular orbits in either direction. It is also possible for the swimmer to escape from surfaces, unlike a model swimmer of similar shape but with only a single flagellum. Thus, we conclude that there are important implications of swimming with two flagella or flagellar bundles rather than one. These considerations are relevant not only for understanding differences in bacterial morphology but also for designing microrobotic swimmers. Full article
(This article belongs to the Special Issue Microswimmer)
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Open AccessArticle
Stability of a Dumbbell Micro-Swimmer
Micromachines 2019, 10(1), 33; https://doi.org/10.3390/mi10010033
Received: 6 November 2018 / Accepted: 20 November 2018 / Published: 7 January 2019
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Abstract
A squirmer model achieves propulsion by generating surface squirming velocities. This model has been used to analyze the movement of micro-swimmers, such as microorganisms and Janus particles. Although squirmer motion has been widely investigated, motions of two connected squirmers, i.e., a dumbbell squirmer, [...] Read more.
A squirmer model achieves propulsion by generating surface squirming velocities. This model has been used to analyze the movement of micro-swimmers, such as microorganisms and Janus particles. Although squirmer motion has been widely investigated, motions of two connected squirmers, i.e., a dumbbell squirmer, remain to be clarified. The stable assembly of multiple micro-swimmers could be a key technology for future micromachine applications. Therefore, in this study, we investigated the swimming behavior and stability of a dumbbell squirmer. We first examined far-field stability through linear stability analysis, and found that stable forward swimming could not be achieved by a dumbbell squirmer in the far field without the addition of external torque. We then investigated the swimming speed of a dumbbell squirmer connected by a short rigid rod using a boundary element method. Finally, we investigated the swimming stability of a dumbbell squirmer connected by a spring. Our results demonstrated that stable side-by-side swimming can be achieved by pullers. When the aft squirmer was a strong pusher, fore and aft swimming were stable and swimming speed increased significantly. The findings of this study will be useful for the future design of assembled micro-swimmers. Full article
(This article belongs to the Special Issue Microswimmer)
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Graphical abstract

Open AccessArticle
Performance Evaluation of a Magnetically Actuated Capsule Microrobotic System for Medical Applications
Micromachines 2018, 9(12), 641; https://doi.org/10.3390/mi9120641
Received: 23 October 2018 / Revised: 8 November 2018 / Accepted: 14 November 2018 / Published: 4 December 2018
Cited by 1 | PDF Full-text (13568 KB) | HTML Full-text | XML Full-text
Abstract
The paper aims to propose a magnetic actuated capsule microrobotic system, which is composed of a magnetically actuated microrobot with a screw jet mechanism, a driving system, and a positioning system. The magnetically actuated microrobot embedded an O-ring magnet as an actuator has [...] Read more.
The paper aims to propose a magnetic actuated capsule microrobotic system, which is composed of a magnetically actuated microrobot with a screw jet mechanism, a driving system, and a positioning system. The magnetically actuated microrobot embedded an O-ring magnet as an actuator has potential for achieving a particular task, such as medical diagnose or drug delivery. The driving system composes of a three axes Helmholtz coils to generate a rotational magnetic field for controlling the magnetically actuated microrobot to realize the basic motion in pipe, e.g., forward/backward motion and upward/downward motion. The positioning system is used to detect the pose of the magnetically actuated microrobot in pipe. We will discuss the shape of the Helmholtz coils and the magnetic field around the O-ring magnet to obtain an optimal performance of the magnetically actuated microrobot. The experimental result indicated that the microrobot with screw jet motion has a flexible movement in pipe by adjusting the rotational magnetic field plane and the magnetic field changing frequency. Full article
(This article belongs to the Special Issue Microswimmer)
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Open AccessArticle
Automatic Manipulation of Magnetically Actuated Helical Microswimmers in Static Environments
Micromachines 2018, 9(10), 524; https://doi.org/10.3390/mi9100524
Received: 18 September 2018 / Revised: 9 October 2018 / Accepted: 12 October 2018 / Published: 16 October 2018
Cited by 1 | PDF Full-text (4130 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Electromagnetically actuated microswimmers have been widely used in various biomedical applications due to their minor invasive traits and their easy access to confined environments. In order to guide the microswimmers autonomously towards a target, an obstacle-free path must be computed using path planning [...] Read more.
Electromagnetically actuated microswimmers have been widely used in various biomedical applications due to their minor invasive traits and their easy access to confined environments. In order to guide the microswimmers autonomously towards a target, an obstacle-free path must be computed using path planning algorithms, meanwhile a motion controller must be formulated. However, automatic manipulations of magnetically actuated microswimmers are underdeveloped and still are challenging topics. In this paper, we develop an automatic manipulation system for magnetically actuated helical microswimmers in static environments, which mainly consists of a mapper, a path planner, and a motion controller. First, the mapper processes the captured image by morphological transformations and then labels the free space and the obstacle space. Second, the path planner explores the obstacle-free space to find a feasible path from the start to the goal by a global planning algorithm. Last, the motion controller guides the helical microswimmers along the desired path by a closed-loop algorithm. Experiments are conducted to verify the effectiveness of the proposed automatic manipulation. Furthermore, our proposed approach presents the first step towards applications of microswimmers for targeted medical treatments, such as micromanipulation, targeted therapy, and targeted drug delivery. Full article
(This article belongs to the Special Issue Microswimmer)
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