Tubular Micro/Nanomotors: Propulsion Mechanisms, Fabrication Techniques and Applications

Abstract

Micro/nanomotors are self-propelled machines that can convert various energy sources into autonomous movement. With the great advances of nanotechnology, Micro/Nanomotors of various geometries have been designed and fabricated over the past few decades. Among them, the tubular Micro/Nanomotors have a unique morphology of hollow structures, which enable them to possess a strong driving force and easy surface functionalization. They are promising for environmental and biomedical applications, ranging from water remediation, sensing to active drug delivery and precise surgery. This article gives a comprehensive and clear review of tubular Micro/Nanomotors, including propulsion mechanisms, fabrication techniques and applications. In the end, we also put forward some realistic problems and speculate about corresponding methods to improve existing tubular Micro/Nanomotors.

Keywords: applications; fabrication techniques; propulsion mechanisms; tubular Micro/Nanomotors.

Figure 1

Representative examples of chemically/biochemically propelled tubular Micro/Nanomotors. (A) Schematic illustration of tubular catalytic micromotor propelled by O₂-bubble ejection in an aqueous H₂O₂ solution (adapted from Reference [46]); (B) Schematic diagram of photocatalytic reaction propelled motion of TiO₂ micromotor in an aqueous H₂O₂ solution under the irradiation of UV light (adapted from [50]); (C) Schematic diagram of motion for tubular polyaniline (PANI)/Zn micromotor in an acidic environment (adopted from [36]); (D) Schematic illustration of the motion of urease-conjugated SiO₂ tubular micromotor (adapted from [56]).

Figure 2

Representative examples of external field or motile microorganism-propelled tubular Micro/Nanomotors. (A) Schematic diagram of Au nanoshells powered by self-thermophoresis upon exposure to a NIR laser (adapted from [57]); (B) Schematic illustration of acoustic droplet vaporization and propulsion of PFC-loaded micromotors triggered by an ultrasound pulse (adapted from [58]); (C) A bio-hybrid tubular micromotor combining a single motile sperm cell with a rolled-up microtube (adapted from [61]).

Figure 2

Representative examples of external field or motile microorganism-propelled tubular Micro/Nanomotors. (A) Schematic diagram of Au nanoshells powered by self-thermophoresis upon exposure to a NIR laser (adapted from [57]); (B) Schematic illustration of acoustic droplet vaporization and propulsion of PFC-loaded micromotors triggered by an ultrasound pulse (adapted from [58]); (C) A bio-hybrid tubular micromotor combining a single motile sperm cell with a rolled-up microtube (adapted from [61]).

Figure 3

Representative examples of the fabrication techniques of tubular Micro/Nanomotors. (A) Schematic diagram of a rolled-up tubular micro/nanomotor consisting of Pt/Au/Fe/Ti multilayers on a photoresist sacrificial layer (adapted from [30]); (B) Three-dimensional schematic illustration of the fabrication method for the nanoporous tubular micromotors used anodic aluminum oxide (AAO) as a sacrificial template (adapted from [68]); (C) Preparation of bilayer PANI/Pt tubular micromotors using polycarbonate membranes (adapted from [31]); (D) Fabrication of polyelectrolyte multilayer tubular nanomotors. Black dots and vertical stripes represent Pt nanoparticles and polyelectrolyte multilayers, respectively (adapted from [75]); (E) Silver wire template-assisted layering approach for preparation of tubular micromotors (adapted from [76]); (F) Schematics to demonstrate the preparation of the TiO₂ tubular micromotor by a dry spinning method and subsequent calcination (adapted from [50]).

Figure 4

Representative examples of tubular Micro/Nanomotors for water remediation. (A) Schematic process for the degradation of polluted water (rhodamine 6G as a model contaminant) into inorganic products by multifunctional micromotors (adapted from [8]); (B) Illustration of a micromotor-based accelerated oxidative decontamination of organophospate nerve agents (adapted from [9]); (C) C6-SAM-modified micromotors with different head functional groups that can (left) or cannot (right) pick up small olive oil droplets (adapted from [34]); (D) Schematic images of GO-micromotors based approach for lead decontamination and recovery (adapted from [82]).

Figure 5

Representative examples of tubular Micro/Nanomotors for environmental sensing. (A) Schematic illustration of the pollutant effect on the micromotor locomotion speed through inhibition of the catalase biocatalytic layer (adapted from [39]); (B) Poisoning of the Pt-based micromotors with small molecules containing sulphur (adapted from [85]); (C) In vitro “off-on” fluorescent detection of ricin-B toxin by FAM-Ricin B aptamer-modified rGO/Pt micromotors (adapted from [91]).

Figure 6

Representative examples of tubular Micro/Nanomotors for biosensing. (A) The principle for DNA detection by introducing Pt nanoparticle–DNA conjugate to the microtube via specific DNA hybridization (adapted from [94]); (B) Use for in situ immunoassay of protein biomarker via motion readout and tag counting (adapted from [98]); (C) A ConA-modified micromotor for selective isolation, transport and release of the target bacteria (adapted from [101]); (D) Schematic illustration of the PAPBA/Ni/Pt micromotor and its “on-the-fly” interaction with glucose (top) and yeast cell (bottom), along with triggered (fructose-induced) release of the cell (adapted from [33]).

Figure 7

Representative examples of tubular Micro/Nanomotors for active drug delivery. (A) Fabrication and light-triggered drug release process of (PLL/BSA)₁₀-DOX-CAT-AuNPs-Gelatin micromotors (adapted from [41]); (B) Schematic representation of the in vivo propulsion and tissue penetration of zinc-based micromotors (adapted from [112]); (C) Schematic illustration of in vivo operation of the enteric Mg micromotors for propulsion and fluorescent images of localized delivery to the gastrointestinal tract (adapted from [113]); (D) Schematic illustration of the firing of nanobullets from the microcannon-structured motor by the spontaneous PFC vaporization upon application of US pulse (adapted from [114]); (E) modified Ni nanotube motors for active drug delivery (adapted from [60]).

Figure 8

Representative examples of tubular Micro/Nanomotors for precise surgery. (A) SEM images of the guided catalytic InGaAs/GaAs/(Cr)Pt micromotors before and after drilling a single cell (adapted from [118]); (B) Schematic representation of imparting magnetic and drug delivery properties to the biotube, as well as fluorescent images of live cells (green) and dead cells (red) before and after microdrilling, respectively (adapted from [43]); (C) Schematic image showing the fuel-free motion of the micromotors towards the center of magnetic field and the drilling operation on pig liver tissue (adapted from [44]); (D) A living New Zealand rabbit eye with a micromotor and rotation of the micromotor around three axes at a rotating magnetic field in the vitreous humor (adapted from [119]).