Core-Shell Magnetic Micropillars for Reprogrammable Actuation

Collective and Rapid High Amplitude Magnetic Oscillation of Anisotropic Micropillar Arrays

Magnetic soft actuators allow high-frequency shape reconfiguration of the micropillar array by rapid rotation of an external magnetic field; however, viscoelastic soft actuators cannot instantaneously reach an equilibrium deformation state to minimize the magnetic moment at a given short time scale, resulting in a significant reduction of the strain amplitude. Herein, we report high-frequency magnetic oscillation of a micropillar array without significant reduction in frequency or strain amplitude by programming the magnetization direction of hard magnetic microparticles embedded in a soft elastomer. Various oscillatory motions, including bending, twisting, and torsion under time-varying external magnetic fields, are demonstrated via programming the magnetization of anisotropic micropillars. Hybridization of microparticles and nanorods in magnetic composites improves the magnetic amplitude of micropillars through a synergistic effect. The translation of microscopic oscillatory motion into a macroscopic function is achieved by the rapid and large-amplitude magnetically programmable collective deformation of the micropillar array. Collective oscillatory torsion of the micropillar array functions as the legs in a walking robot as well as micropaddles that can program the chirality of the liquid flow. Point- or line-symmetric torsion enables the flow direction (counterclockwise or clockwise) to be programmed according to the direction of applied magnetic field to the micropillar array.

Discontinuous Directional Wetting Transitions in Polymeric Droplets on the Heterogeneous Microcavity Surface

The wetting transition behaviors of polymeric droplets on microcavity surfaces are familiar and play a vital role in micromanufacturing, microfluidics, and printing industries. Despite previous research indicating that microcavity surfaces can precisely control the droplet wetting state, the understanding of the complex effects of droplet spreading, surface morphology, and property of polymeric droplet on wetting transitions remains incomplete. The air-liquid interfaces (ALIs) typically arise from the entrapped air beneath the droplet on microcavity surfaces, adopting a metastable wetting state caused by either bubble escape or dissolution. Here, we discovered a previously unobserved phenomenon: the time-dependent evolution of regularly arranged ALIs with discontinuous wetting states, along with the pronounced directional wetting transitions from the Cassie-Baxter state to the Wenzel state upon deposition of polymeric droplets on heterogeneous microcavity surfaces. The durability of ALIs in microcavities was quantified, illustrating that the wetting transitions associated with droplet spreading processes obeyed power laws. By integrating the wetting theory and the viscoelastic effect of polymeric droplet, we have proposed a phenomenological coevolution model for wetting transitions that emphasizes the synergistic interaction between adjacent microcavities, resulting in the observed cluster evolution behavior of ALIs within droplets. Our study holds great significance in guiding soft manufacturing techniques utilizing internal ALIs as templates. The established mechanism opens up avenues for investigating the intricate wetting phenomena of polymeric droplets on microtextured substrates.

Multimodal Splitting and Reciprocating Transport of Droplets on a Reprogrammable Functional Surface

Droplet manipulations have important applications in many fields, especially droplet splitting and transport in aseptic operations or biochemical reagent analysis. However, droplet splitting or transport on existing functional surfaces is limited to predesigned microstructures or fixed patterns. It remains a challenge to realize reprogrammable surface microstructures for freely controllable droplet splitting and transport. In this study, a flexible technique for both the multimodal splitting and reciprocating transport of droplets on one surface is proposed. Such a surface is prepared with a facile fabrication method by premixing magnetic particles and softener into the polymer solvent matrix and immersing the solidified matrix in a lubricant. The movable wettability gradient is generated on the surface by an external magnetic field, which can act as an invisible "air knife" to split the droplet in multiple modes. The mechanism and critical conditions of droplet splitting are analyzed and revealed theoretically. Furthermore, the microstructural configurations and surface wettability can be reprogrammed by modulating the magnetic field strength and gradient. Accordingly, the splitting behavior of the droplet is transformed into the reciprocating transport behavior. The influencing factors of such behavior have also been analyzed. The reported reprogrammable manipulation of the droplet on one surface provides a versatile prototype for the actuation of droplets in microfluidic and biological analysis devices.

Stimuli-Responsive Polymers for Tubal Actuators

Stimuli-responsive polymers for tubal actuators have garnered significant attention due to their potential applications in soft robotics, artificial blood vessels, controlled liquid transportation, and microchemical reactors. This perspective emphasizes the advantages, response mechanisms, and fundamental design principles of stimuli-responsive polymers for tubal actuators. It also addresses the biological and engineering applications, current challenges, and future prospects of stimuli-responsive polymers for tubal actuators. The discussion categorizes stimuli-responsive polymers for tubal actuators based on various properties, including liquid crystal elastomer actuators, hydrogel actuators, and shape memory polymer actuators. The subsequent sections focuses on the structural features, design principles, and biological applications of stimuli-responsive polymers for tubal actuator, elucidating their potential interrelationships. The molecular architectures and design principles are intricately linked to the stimuli-responsive mechanisms. Finally, this perspective outlines the challenges faced by stimuli-responsive polymers for tubal actuators. This article aims to facilitate broader applications of stimuli-responsive polymers for tubal actuators, thereby promoting progress across multiple fields.

Development of High-Aspect-Ratio Soft Magnetic Microarrays for Magneto-Mechanical Actuation via Field-Induced Injection Molding

Magnetorheological elastomers (MREs) are in demand in the field of high-tech microindustries and nanoindustries such as biomedical applications and soft robotics due to their exquisite magneto-sensitive response. Among various MRE applications, programmable actuators are emerging as promising soft robots because of their combined advantages of excellent flexibility and precise controllability in a magnetic system. Here, we present the development of magnetically programmable soft magnetic microarray actuators through field-induced injection molding using MREs, which consist of styrene-ethylene/butylene styrene (SEBS) elastomer and carbonyl iron powder (CIP). The ratio of the CIP/SEBS matrix was designed to maximize the CIP fraction based on a critical solids loading. Further, as part of the design of the magnetization distribution in micropillar arrays, the magnetorheological response of the molten composites was analyzed using the static and dynamic viscosity results for both the on and off magnetic states, which reflected the particle dipole interaction and subsequent particle alignment during the field-induced injection molding process. To develop a high-aspect-ratio soft magnetic microarray, X-ray lithography was applied to prepare the sacrificial molds with a height-to-width ratio of 10. The alignment of the CIP was designed to achieve a parallel magnetic direction along the micropillar columns, and consequently, the micropillar arrays successfully achieved the uniform and large bending actuation of up to approximately 81° with an applied magnetic field. This study suggests that the injection molding process offers a promising manufacturing approach to build a programmable soft magnetic microarray actuator.

Cutting-Edge Research in Nanoscience and Nanotechnology: Celebrating the 130th Anniversary of Wuhan University

Thanks to the fast-paced progress of microscopic theories and nanotechnologies, a tremendous world of fundamental science and applications has opened up at the nanoscale. Ranging from quantum physics to chemical and biological mechanisms and from device functionality to materials engineering, nanoresearch has become an essential part of various fields. As one of the top universities in China, Wuhan University (WHU) aims to promote cutting-edge nanoresearch in multiple disciplines by leveraging comprehensive academic programs established throughout 130 years of history. As visible in prestigious scientific journals such as ACS Nano, WHU has made impactful advancements in various frontiers, including nanophotonics, functional nanomaterials and devices, biomedical nanomaterials, nanochemistry, and environmental science. In light of these contributions, WHU will be committed to serving talents and scientists wholeheartedly, fully supporting international collaborations and continuously driving innovative research.

Coupling of static ultramicromagnetic field with elastic micropillar-structured substrate for cell response

Micropillars have emerged as promising tools for a wide range of biological applications, while the influence of magnetic fields on cell behavior regulation has been increasingly recognized. However, the combined effect of micropillars and magnetic fields on cell behaviors remains poorly understood. In this study, we investigated the responses of H9c2 cells to ultramicromagnetic micropillar arrays using NdFeB as the tuned magnetic particles. We conducted a comparative analysis between PDMS micropillars and NdFeB/PDMS micropillars to assess their impact on cell function. Our results revealed that H9c2 cells exhibited significantly enhanced proliferation and notable cytoskeletal rearrangements on the ultramicromagnetic micropillars, surpassing the effects observed with pure PDMS micropillars. Immunostaining further indicated that cells cultured on ultramicromagnetic micropillars displayed heightened contractility compared to those on PDMS micropillars. Remarkably, the ultramicromagnetic micropillars also demonstrated the ability to decrease reactive oxygen species (ROS) levels, thereby preventing F-actin degeneration. Consequently, this study introduces ultramicromagnetic micropillars as a novel tool for the regulation and detection of cell behaviors, thus paving the way for advanced investigations in tissue engineering, single-cell analysis, and the development of flexible sensors for cellular-level studies.

Soft metamaterial with programmable ferromagnetism

Magnetopolymers are of interest in smart material applications; however, changing their magnetic properties post synthesis is complicated. In this study, we introduce easily programmable polymer magnetic composites comprising 2D lattices of droplets of solid-liquid phase change material, with each droplet containing a single magnetic dipole particle. These composites are ferromagnetic with a Curie temperature defined by the rotational freedom of the particles above the droplet melting point. We demonstrate magnetopolymers combining high remanence characteristics with Curie temperatures below the composite degradation temperature. We easily reprogram the material between four states: (1) a superparamagnetic state above the melting point which, in the absence of an external magnetic field, spontaneously collapses to; (2) an artificial spin ice state, which after cooling forms either; (3) a spin glass state with low bulk remanence, or; (4) a ferromagnetic state with high bulk remanence when cooled in the presence of an external magnetic field. We observe the spontaneous emergence of 2D magnetic vortices in the spin ice and elucidate the correlation of these vortex structures with the external bulk remanence. We also demonstrate the easy programming of magnetically latching structures.

Aligned Magnetic Nanocomposites for Modularized and Recyclable Soft Microrobots

Creating reconfigurable and recyclable soft microrobots that can execute multimodal locomotion has been a challenge due to the difficulties in material processing and structure engineering at a small scale. Here, we propose a facile technique to manufacture diverse soft microrobots (∼100 μm in all dimensions) by mechanically assembling modular magnetic microactuators into different three-dimensional (3D) configurations. The module is composed of a cubic micropillar supported on a square substrate, both made of elastomer matrix embedded with prealigned magnetic nanoparticle chains. By directionally bonding the sides or backs of identical modules together, we demonstrate that assemblies from only two and four modules can execute a wide range of locomotion, including gripping microscale objects, crawling and crossing solid obstacles, swimming within narrow and tortuous microchannels, and rolling along flat and inclined surfaces, upon applying proper magnetic fields. The assembled microrobots can additionally perform pick-transfer-place and cargo-release tasks at the microscale. More importantly, like the game of block-building, the microrobots can be disassembled back to separate modules and then reassembled to other configurations as demanded. The present study not only provides a versatile and economic manufacturing technique for reconfigurable and recyclable soft microrobots, enabling unlimited design space for diverse robotic locomotion from limited materials and module structures, but also extends the functionality and dexterity of existing soft robots to microscale that should facilitate practical applications at such small scale.

Precise Controlling of Friction and Adhesion on Reprogrammable Shape Memory Micropillars

Microstructured surfaces with stimuli-responsive performances have aroused great attention in recent years, but it still remains a significant challenge to endow surfaces with precisely controlled morphological changes in microstructures, so as to get the precise control of regional properties (e.g., friction, adhesion). Herein, a kind of carbonyl iron particle-doped shape memory polyurethane micropillar with precisely controllable morphological changes is realized, upon remote near-infrared light (NIR) irradiation. Owing to the reversible transition of micropillars between bent and upright states, the micro-structured surface exhibits precisely controllable low-to-high friction transitions, together with the changes of friction coefficient ranging from ∼0.8 to ∼1.2. Hence, the changes of the surface friction even within an extremely small area can be precisely targeted, under local NIR laser irradiation. Moreover, the water droplet adhesion force of the surface can be reversibly switched between ∼160 and ∼760 μN, demonstrating the application potential in precisely controllable wettability. These features indicate that the smart stimuli-responsive micropillar arrays would be amenable to a variety of applications that require remote, selective, and on-demand responses, such as a refreshable Braille display system, micro-particle motion control, lab-on-a-chip, and microfluidics.

Creating three-dimensional magnetic functional microdevices via molding-integrated direct laser writing

Magnetically driven wireless miniature devices have become promising recently in healthcare, information technology, and many other fields. However, they lack advanced fabrication methods to go down to micrometer length scales with heterogeneous functional materials, complex three-dimensional (3D) geometries, and 3D programmable magnetization profiles. To fill this gap, we propose a molding-integrated direct laser writing-based microfabrication approach in this study and showcase its advanced enabling capabilities with various proof-of-concept functional microdevice prototypes. Unique motions and functionalities, such as metachronal coordinated motion, fluid mixing, function reprogramming, geometrical reconfiguring, multiple degrees-of-freedom rotation, and wireless stiffness tuning are exemplary demonstrations of the versatility of this fabrication method. Such facile fabrication strategy can be applied toward building next-generation smart microsystems in healthcare, robotics, metamaterials, microfluidics, and programmable matter.

Advanced Materials and Sensors for Microphysiological Systems: Focus on Electronic and Electrooptical Interfaces

Advanced in vitro cell culture systems or microphysiological systems (MPSs), including microfluidic organ-on-a-chip (OoC), are breakthrough technologies in biomedicine. These systems recapitulate features of human tissues outside of the body. They are increasingly being used to study the functionality of different organs for applications such as drug evolutions, disease modeling, and precision medicine. Currently, developers and endpoint users of these in vitro models promote how they can replace animal models or even be a better ethically neutral and humanized alternative to study pathology, physiology, and pharmacology. Although reported models show a remarkable physiological structure and function compared to the conventional 2D cell culture, they are almost exclusively based on standard passive polymers or glass with none or minimal real-time stimuli and readout capacity. The next technology leap in reproducing in vivo-like functionality and real-time monitoring of tissue function could be realized with advanced functional materials and devices. This review describes the currently reported electronic and optical advanced materials for sensing and stimulation of MPS models. In addition, an overview of multi-sensing for Body-on-Chip platforms is given. Finally, one gives the perspective on how advanced functional materials could be integrated into in vitro systems to precisely mimic human physiology.

Robust and Highly Sensitive Cellulose Nanofiber-Based Humidity Actuators

The design of humidity actuators with high response sensitivity (especially actuation time) while maintaining favorable mechanical properties is important for advanced intelligent manufacturing, like soft robotics and smart devices, but still remains a challenge. Here, we fabricate a robust and conductive composite film-based humidity actuator with synergetic benefits from one-dimensional cellulose nanofibers (CNFs) and carbon nanotubes (CNTs) as well as two-dimensional graphene oxide (GO) via an efficient vacuum-assisted self-assembly method. Owing to the excellent moisture sensitivity of CNF and GO, the hydrophobic CNT favoring rapid desorption of water molecules, and the unique porous structure with numerous nanochannels for accelerating the water exchange rate, this CNF/GO/CNT composite film delivers excellent actuation including an ultrafast response/recovery (0.8/2 s), large deformation, and sufficient cycle stability (no detectable degradation after 1000 cycles) in response to ambient gradient humidity. Intriguingly, the actuator could also achieve a superior flexibility, a good mechanical strength (201 MPa), a desirable toughness (6.6 MJ/m³), and stable electrical conductivity. Taking advantage of these benefits, the actuator is conceptually fabricated into various smart devices including mechanical grippers, crawling robotics, and humidity control switches, which is expected to hold great promise toward practical applications.

Control of the asymmetric growth of nanowire arrays with gradient profiles

A novel electrochemical methodology for the growth of arrays of Ni and Co nanowires (NWs) with linear and non-linear varying micro-height gradient profiles (μHGPs), has been developed. The growth mechanism of these microstructures consists of a three-dimensional growth originating from the allowed electrical contact between the electrolyte and the edges of the cathode at the bottom side of porous alumina membranes. It has been shown that the morphology of these microstructures strongly depends on electrodeposition parameters like the cation material and concentration and the reduction potential. At constant reduction potentials, linear Ni μHGPs with trapezoid-like geometry are obtained, whereas deviations from this simple morphology are observed for Co μHGPs. In this regime, the μHGPs average inclination angle decreases for more negative reduction potential values, leading as a result to more laterally extended microstructures. Besides, more complex morphologies have been obtained by varying the reduction potential using a simple power function of time. Using this strategy allows us to accelerate or decelerate the reduction potential in order to change the μHGPs morphology, so to obtain convex- or concave-like profiles. This methodology is a novel and reliable strategy to synthesize μHGPs into porous alumina membranes with controlled and well-defined morphologies. Furthermore, the synthesized low dimensional asymmetrically loaded nanowired substrates with μHGPs are interesting for their application in micro-antennas for localized electromagnetic radiation, magnetic stray field gradients in microfluidic systems, non-reciprocal microwave absorption, and super-capacitive devices for which a very large surface area and controlled morphology are key requirements.