4D Printing at the Microscale

3D printing of adaptive and dynamic structures, also known as 4D printing, is one of the key challenges in contemporary materials science. The additional dimension refers to the ability of 3D printed structures to change their properties—for example, shape—over time in a controlled fashion as the result of external stimulation. Within the last years, significant efforts have been undertaken in the development of new responsive materials for printing at the macroscale. However, 4D printing at the microscale is still in its early stages. Thus, this progress report will focus on emerging materials for 4D printing at the microscale as well as their challenges and potential applications. Hydrogels and liquid crystalline and composite materials have been identified as the main classes of materials representing the state of the art of the growing field. For each type of material, the challenges and critical barriers in the material design and their performance in 4D microprinting are discussed. Importantly, further necessary strategies are proposed to overcome the limitations of the current approaches and move toward their application in fields such as biomedicine, microrobotics, or optics

Electrochemistry: A basic and powerful tool for micro- and nanomotor fabrication and characterization

Electrochemistry, although an ancient field of knowledge, has become of paramount importance in the synthesis of materials at the nanoscale, with great interest not only for fundamental research but also for practical applications. One of the promising fields in which electrochemistry meets nanoscience and nanotechnology is micro/nanoscale motors. Micro/nano motors, which are devices able to perform complex tasks at the nanoscale, are commonly multifunctional nanostructures of different materials - metals, polymers, oxides- and shapes -spheres, wires, helices- with the ability to be propelled in fluids. Here, we first introduce the topic of micro/nanomotors and make a concise review of the field up to day. We have analyzed the field from different points of view (e.g. materials science and nanotechnology, physics, chemistry, engineering, biology or environmental science) to have a broader view of how the different disciplines have contributed to such exciting and impactful topic. After that, we focus our attention on describing what electrochemical technology is and how it can be successfully used to fabricate and characterize micro/nanostructures composed of different materials and showing complex shapes. Finally, we will review the micro and nanomotors fabricated using electrochemical techniques with applications in biomedicine and environmental remediation, the two main applications investigated so far in this field. Thus, different strategies have thus been shown capable of producing core-shell nanomaterials combining the properties of different materials, multisegmented nanostructures made of, for example, alternating metal and polymer segments to confer them with flexibility or helicoidal systems to favor propulsion. Moreover, further functionalization and interaction with other materials to form hybrid and more complex objects is also shown

Light‐Powered Microrobots: Challenges and Opportunities for Hard and Soft Responsive Microswimmers

Worldwide research in microrobotics has exploded in the past two decades, leading to the development of microrobots propelled in various manners. Despite significant advances in the field and successful demonstration of a wide range of applications, microrobots have yet to become the preferred choice outside a laboratory environment. After introducing available microrobotic propulsion and control mechanisms, microrobots that are manufactured and powered by light are focused herein. Referring to pioneering works and recent interesting examples, light is presented not only as a fabrication tool, by means of twophoton polymerization direct laser writing, but also as an actuator for microrobots in both hard and soft stimuli–responsive polymers. In this scenario, a number of challenges that yet prevent polymeric light-powered microrobots from reaching their full potential are identified, whereas potential solutions to overcome said challenges are suggested. As an outlook, a number of real-world applications that light-powered microrobots should be particularly suited for are mentioned, together with the advances needed for them to achieve such purposes. An interdisciplinary approach combining materials science, microfabrication, photonics, and data science should be conducive to the next generation of microrobots and will ultimately foster the translation of microrobotic applications into the real world

A Review on Active 3D Microstructures via Direct Laser Lithography

Direct laser lithography (DLL) is a key enabling technology for 3D constructs at the microscale and its potential is rapidly growing toward the development of active microstructures. The rationale of this work is based on the different involved methodology, which is referred as indirect, when passive microstructures become active through postprocessing steps, and direct, when active structures are directly obtained by fabricating microstructures with active materials or by introducing heterogeneous mechanical properties and specific design. An in‐depth analysis of both indirect and direct methods is provided. In particular, the wide range of materials and strategies involved in each method is reported, including advantages and disadvantages, as well as examples of fabricated structures and their applications. Finally, the different techniques are briefly summarized, and critically discussed by highlighting how the new synergies between DLL and active materials are opening completely new scenarios, in particular for sensing (e.g., mechanical) and actuation at the microscale

Micro/nanoscale magnetic robots for biomedical applications

Magnetic small-scale robots are devices of great potential for the biomedical field because of the several benefits of this method of actuation. Recent work on the development of these devices has seen tremendous innovation and refinement toward ​improved performance for potential clinical applications. This review briefly details recent advancements in small-scale robots used for biomedical applications, covering their design, fabrication, applications, and demonstration of ability, and identifies the gap in studies and the difficulties that have persisted in the optimization of the use of these devices. In addition, alternative biomedical applications are also suggested for some of the technologies that show potential for other functions. This study concludes that although the field of small-scale robot research is highly innovative ​there is need for more concerted efforts to improve functionality and reliability of these devices particularly in clinical applications. Finally, further suggestions are made toward ​the achievement of commercialization for these devices

Magnetically Driven Micro and Nanorobots

Manipulation and navigation of micro and nanoswimmers in different fluid environments can be achieved by chemicals, external fields, or even motile cells. Many researchers have selected magnetic fields as the active external actuation source based on the advantageous features of this actuation strategy such as remote and spatiotemporal control, fuel-free, high degree of reconfigurability, programmability, recyclability, and versatility. This review introduces fundamental concepts and advantages of magnetic micro/nanorobots (termed here as "MagRobots") as well as basic knowledge of magnetic fields and magnetic materials, setups for magnetic manipulation, magnetic field configurations, and symmetry-breaking strategies for effective movement. These concepts are discussed to describe the interactions between micro/nanorobots and magnetic fields. Actuation mechanisms of flagella-inspired MagRobots (i.e., corkscrew-like motion and traveling-wave locomotion/ciliary stroke motion) and surface walkers (i.e., surface-assisted motion), applications of magnetic fields in other propulsion approaches, and magnetic stimulation of micro/nanorobots beyond motion are provided followed by fabrication techniques for (quasi)spherical, helical, flexible, wire-like, and biohybrid MagRobots. Applications of MagRobots in targeted drug/gene delivery, cell manipulation, minimally invasive surgery, biopsy, biofilm disruption/eradication, imaging-guided delivery/therapy/surgery, pollution removal for environmental remediation, and (bio)sensing are also reviewed. Finally, current challenges and future perspectives for the development of magnetically powered miniaturized motors are discussed

Innovative designs and applications of Janus micromotors with (photo)-catalytic and magnetic motion

El objetivo principal de esta Tesis Doctoral es el diseño y desarrollo de micromotores Janus biocompatibles y su aplicación en ámbitos relevantes de la salud y de la protección medioambiental. Los micromotores Janus son dispositivos en la microescala autopropulsados que tienen al menos dos regiones en su superficie con diferentes propiedades físicas y químicas, lo que les convierte en una clase distintiva de materiales que pueden combinar características ópticas, magnéticas y eléctricas en una sola entidad. Como la naturaleza del micromotor Janus -el dios romano de las dos caras- los objetivos de esta Tesis Doctoral presentan naturaleza dual y comprenden desarrollos de química fundamental y de química aplicada. En efecto, por una parte, el objetivo central aborda el diseño, síntesis y ensamblaje, así como la caracterización de micromotores Janus poliméricos propulsados por mecanismos (foto)-catalíticos y/o accionados por campos magnéticos. Por otra parte, el objetivo central implica la aplicación de los micromotores desarrollados para resolver desafíos sociales relevantes en los ámbitos químico-analítico, biomédico y ambiental. Partiendo de estas premisas, en la primera parte de la Tesis Doctoral, se sintetizaron micromotores Janus de policaprolactona propulsados químicamente integrando nanomateriales para el diseño de sensores móviles para la detección selectiva de endotoxinas bacterianas. De esta forma, el movimiento autónomo del micromotor mejora la mezcla de fluidos y la eficacia de las reacciones implicadas permitiendo detectar el analito en pocos minutos, incluso en muestras viscosas y medios donde la agitación no es posible. Además, esta autopropulsión es altamente compatible con su empleo en formatos ultra-miniaturizados para el desarrollo de futuros dispositivos portátiles en el marco de la tecnología point of care para aplicaciones clínicas y agroalimentarias. Con el fin de incrementar su biocompatibilidad para aplicaciones in vivo, en una segunda etapa de la Tesis Doctoral, se diseñaron micromotores Janus con propulsión autónoma utilizando luz visible para la eliminación de toxinas relevantes en procesos inflamatorios. El fenómeno autopropulsivo del micromotor y su capacidad de interacción con agentes tóxicos condujo a metodologías más rápidas y eficaces infiriéndose un futuro prometedor de estos micromotores para el tratamiento del shock séptico o intoxicación. En una tercera etapa, se sintetizaron micromotores propulsados por campos magnéticos. Estos micromotores utilizan una aproximación elegante de propulsión, exenta del empleo de combustibles químicos tóxicos como sucede en la propulsión catalítica y, en consecuencia, biocompatible. Asimismo, este mecanismo propulsivo permite controlar e incluso programar su trayectoria para aplicaciones que requieran de un guiado y de un control preciso de esta. De manera específica, estos micromotores han sido aplicados en esta Tesis Doctoral para la liberación controlada de fármacos en el tratamiento de cáncer pancreático y como elementos de remediación ambiental en la eliminación de agentes nerviosos en aguas contaminadas

Actuation and control of microfabricated structures using flagellated bacteria

In this work methods of actuation and control of microfabricated structures are investigated using bacteria as configurable, scalable actuators. Bacteria offer many benefits as microfluidic actuators. They draw chemical energy directly from their environment, they can be operated in a wide range of temperature and pH, and literally billions of bacteria may be cultured within hours. Additionally, the well-documented responses of individual motile bacterial cells may be expected to scale up to arrays of cells. On this population scale, the cellular responses can be employed en masse creating controlled forces that actuate inorganic microfabricated elements. For these investigations the bacterium Serratia marcescens has been chosen. S. marcescens has properties that are particularly appropriate for engineering applications. When cultured on soft agar, the bacteria demonstrate a form of surface motility known as swarming. These investigations start with an experimental analysis of the swarming cell motility using a non-labeled cell tracking technique. The results of these studies reveal that the most energetic bacteria populate the progressing edge of the swarm. A technique of biocompatible microfabrication and chemical release of bacteria-driven microstructures is also presented. This method is used to pattern structure surfaces with the rigorous swarming cells by direct blotting. The self-coordinated motion of the cells is investigated for use as arrays of actuators. Control mechanisms are investigated to adjust rotational and translational motion using optical and electrical stimuli, respectively. The fundamentals of the electrokinetics are also investigated and integrated into a system demonstrating controlled manipulation of target objects and phenotypic chemical sensing.Ph.D., Mechanical Engineering -- Drexel University, 200