Magneto‐Controllable Capture and Release of Cancer Cells by Using a Micropillar Device Decorated with Graphite Oxide‐Coated Magnetic Nanoparticles

Aiming to highly efficient capture and analysis of circulating tumor cells, a micropillar device decorated with graphite oxide‐coated magnetic nanoparticles is developed for magneto‐controllable capture and release of cancer cells. Graphite oxide‐coated, Fe₃O₄ magnetic nanoparticles (MNPs) are synthesized by solution mixing and functionalized with a specific antibody, following by the immobilization of such modified MNPs on our designed micropillar device. For the proof‐of‐concept study, a HCT116 colorectal cancer cell line is employed to exam the capture efficiency. Under magnetic field manipulation, the high density packing of antibody‐modified MNPs on the micropillars increases the local concentration of antibody, as well as the topographic interactions between cancer cells and micropillar surfaces. The flow rate and the micropillar geometry are optimized by studying their effects on capture efficiency. Then, a different number of HCT116 cells spiked in two kinds of cell suspension are investigated, yielding capture efficiency >70% in culture medium and >40% in blood sample, respectively. Moreover, the captured HCT116 cells are able to be released from the micropillars with a saturated efficiency of 92.9% upon the removal of applied magnetic field and it is found that 78% of the released cancer cells are viable, making them suitable for subsequent biological analysis.     

Magnetically Responsive Elastomer–Silicon Hybrid Surfaces for Fluid and Light Manipulation

Stimuli‐responsive surfaces with tunable fluidic and optical properties utilizing switchable surface topography are of significant interest for both scientific and engineering research. This work presents a surface involving silicon scales on a magnetically responsive elastomer micropillar array, which enables fluid and light manipulation. To integrate microfabricated silicon scales with ferromagnetic elastomer micropillars, transfer printing‐based deterministic assembly is adopted. The functional properties of the surface are completely dictated by the scales with optimized lithographic patterns while the micropillar array is magnetically actuated with large‐range, instantaneous, and reversible deformation. Multiple functions, such as tunable wetting, droplet manipulation, tunable optical transmission, and structural coloration, are designed, characterized, and analyzed by incorporating a wide range of scales (e.g., bare silicon, black silicon, photonic crystal scales) in both in‐plane and out‐of‐plane configurations.     

Excellent Anti‐Icing Abilities of Optimal Micropillar Arrays with Nanohairs

Anti‐icing abilities are achieved on surfaces of micropillar arrays with nanohairs that are fabricated by methods of soft replication and crystal growth, i.e., different micropillar arrays with the similar nanohairs, different nanohairs with the same micropillar arrays. It is demonstrated that an optimal micropillar array with nanohairs contributes an excellent anti‐icing or antifogging property at low temperature below zero. As a result, the longest icing delay time is achieved effectively up to ≈9839 s at −10 °C on the optimal surface. As for the optimal surface in humidity, the condensed droplets merge into each other, and meanwhile jump off easily. Accordingly, a largest dry area is up to ≈90.5% at −5 °C in ≈1020 s after breeze action. It is attributed to the stability of less liquid–solid fraction on an optimal surface under low temperature, in addition to cooperation between micropillar arrays and nanohairs in sizes. This finding provides an insight into the design of structure size on micro–nanostructured surface for anti‐icing/antifogging ability effectively, which can be extended into the applications in some surfaces of systems, e.g., microdevices worked in cold or humid environment.     

Quantitative Magnetic Separation of Particles and Cells Using Gradient Magnetic Ratcheting

Extraction of rare target cells from biosamples is enabling for life science research. Traditional rare cell separation techniques, such as magnetic activated cell sorting, are robust but perform coarse, qualitative separations based on surface antigen expression. A quantitative magnetic separation technology is reported using high‐force magnetic ratcheting over arrays of magnetically soft micropillars with gradient spacing, and the system is used to separate and concentrate magnetic beads based on iron oxide content (IOC) and cells based on surface expression. The system consists of a microchip of permalloy micropillar arrays with increasing lateral pitch and a mechatronic device to generate a cycling magnetic field. Particles with higher IOC separate and equilibrate along the miropillar array at larger pitches. A semi‐analytical model is developed that predicts behavior for particles and cells. Using the system, LNCaP cells are separated based on the bound quantity of 1 μm anti‐epithelial cell adhesion molecule (EpCAM) particles as a metric for expression. The ratcheting cytometry system is able to resolve a ±13 bound particle differential, successfully distinguishing LNCaP from PC3 populations based on EpCAM expression, correlating with flow cytometry analysis. As a proof‐of‐concept, EpCAM‐labeled cells from patient blood are isolated with 74% purity, demonstrating potential toward a quantitative magnetic separation instrument.     

Hybrid Magnetic Micropillar Arrays for Programmable Actuation

Stimuli-responsive micro/nanostructures that can dynamically and reversibly adapt their configurations according to external stimuli have stimulated a wide scope of engineering applications, ranging from material surface engineering to micromanipulations. However, it remains a challenge to achieve a precise local control of the actuation to realize applications that require heterogeneous and on-demand responses. Here, a new experimental technique is developed for large arrays of hybrid magnetic micropillars and achieve precise local control of actuation using a simple magnetic field. By manipulating the spatial distribution of magnetic nanoparticles within individual elastomer micropillars, a wide range of the magnetomechanical responses from less than 5% to ≈50% for the ratio of the bending deflection to the original length of the pillars is realized. It is demonstrated that the micropillars with different degrees of bending deformation can be configured in any spatial pattern using a photomask-assisted template-casting technique to achieve heterogeneous, site-specific, and programmed bending actuations. This unprecedented local control of the micropillars offers exciting novel applications, as demonstrated here in encryptable surface printing and stamping, direction- and track-programmable microparticle/droplet transport, and smart magnetic micro-tweezers. The hybrid magnetic micropillars reported here provide a versatile prototype for heterogeneous and on-demand actuation using programmable stimuli-responsive micro/nanostructures.     

Unidirectional Wetting in the Hydrophobic Wenzel Regime

Unidirectional wetting surfaces can cause liquid droplets to flow/move in one direction while pinning them in the other directions, a feature that is useful for biosensing, adhesives, thermal management, and microfluidics. Such surfaces can be fabricated by employing structurally or chemically asymmetric nanostructures. While unidirectional wetting in the hydrophobic Wenzel regime had previously been observed on surfaces decorated with chemically asymmetric nanostructures, it has yet to be demonstrated on structurally asymmetric nanostructures. Based on the current understanding of the phenomenon, this can only be achieved using highly bent nanowires. Here, evidence to the contrary is provided by showing that mildly bent nanowires can also bring about unidirectional wetting in the hydrophobic Wenzel regime, even for contact angles beyond the superhydrophobic limit. Using NaCl precipitation, the unidirectional wetting mechanism is analyzed on a nanoscale level and it is found that the criteria for unidirectional wetting to take place in the hydrophobic Wenzel regime are different from that in the hydrophilic Wenzel regime. Moreover, it is revealed that slight wetting in the pinned direction can be caused by large scale deformation of high aspect ratio nanostructures during droplet spreading, which may be part of the reason behind previous observations of near‐unidirectional wetting on bent nanowires with high aspect ratios.     

Regular Aligned 1D Single‐Crystalline Supramolecular Arrays for Photodetectors

Solution‐processed semiconductor single‐crystal patterns possess unique advantages of large scale and low cost, leading to potential applications toward high‐performance optoelectronic devices. To integrate organic semiconductor micro/nanostructures into devices, various patterning techniques have been developed. However, previous patterning techniques suffer from trade‐offs between precision, scalability, crystallinity, and orientation. Herein, a patterning method is reported based on an asymmetric‐wettability micropillar‐structured template. Large‐scale 1D single‐crystalline supramolecular arrays with strict alignment, pure crystallographic orientation, and precise position can be obtained. The wettability difference between tops and sidewalls of micropillars gives rise to the confinement of organic solutions in discrete capillary tubes followed by dewetting and formation of capillary trailing. The capillary trailing enables unidirectional dewetting, regulated mass transport, and confined crystal growth. Owing to the high crystallinity and pure crystallographic orientation with Pt atomic chains parallel to the substrate, the photodetectors based on the 1D arrays exhibit improved responsivity. The work not only provides fundamental understanding on the patterning and crystallization of supramolecular structures but also develops a large‐scale assembly technique for patterning single‐crystalline micro/nanostructures.     

Switchable Adhesion of Micropillar Adhesive on Rough Surfaces

Switchable structured adhesion on rough surfaces is highly desired for a wide range of applications. Combing the advantages of gecko seta and creeper root, a switchable fibrillar adhesive composed of polyurethane (PU) as the backing layer and graphene/shape memory polymer (GSMP) as the pillar array is developed. The photothermal effect of graphene (under UV irradiation) changes GSMP micropillars into the viscoelastic state, allowing easy and intimate contact on surfaces with a wide range of roughness. By controlling the phase state of GSMP via UV irradiation during detachment, the GSMP micropillar array can be switched between the robust‐adhesion state (UV off) and low‐adhesion state (UV on). The state of GSMP micropillars determines the adhesion force capacity and the stress distribution at the detaching interface, and therefore the adhesion performance. The PU‐GSMP adhesive achieves large adhesion strength (278 kPa), high switching ratio (29), and fast switching (10 s) at the same time. The results suggest a design principle for bioinspired structured adhesives, especially for reversible adhesion on surfaces with a wide range of roughness.     

Transport of a soft cargo on a nanoscale ratchet

Surface ratchets can guide droplet transport for microfluidic systems. Here, we demonstrated the actuation of microgels encapsulated in droplets using a unidirectional nanotextured surface, which moves droplets with low vibration amplitudes by a ratcheting mechanism. The nanofilm carries droplets along the ratchets with minimal drop shape deformation to move the encapsulated soft cargo, i.e., microscale hydrogels. The tilted nanorods of the nanofilm produce unidirectional wetting, thereby enabling droplet motion in a single direction. Maximum droplet translation speed on the nanofilm was determined to be 3.5 mm∕s, which offers a pathway towards high throughput microgel assembly applications to build complex constructs.     

Anisotropic Wetting Surfaces with One‐Dimesional and Directional Structures: Fabrication Approaches,Wetting Properties and Potential Applications

This review article provides a brief summary of recent research progress on anisotropic wetting on one‐dimensional (1D) and directionally patterned surfaces, as well as the technical importance in various applications. Inspiration from natural structures exhibiting anisotropic wetting behavior is first discussed. Development of fabrication techniques for topographically and chemically 1D patterned surfaces and directional nanomaterials are then reviewed, with emphasis on anisotropic behavior with topographically (structurally) patterned surfaces. The basic investigation of anisotropic wetting behavior and theoretical simulations for anisotropic wetting are also further reviewed. Perspectives concerning future direction of anisotropic wetting research and its potential applications in microfluidic devices, lab‐on‐a‐chip, sensor, microreactor and self‐cleaning are presented.     

Programmable Liquid Adhesion on Bio‐Inspired Re‐Entrant Structures

Surfaces combining antispreading and high adhesion can find wide applications in the manipulation of liquid droplets, generation of micropatterns and liquid enrichment. To fabricate such surfaces, almost all the traditional methods demand multi‐step processes and chemical modification. And even so, most of them cannot be applied for some liquids with extremely low surface energy. In the past decade, multiply re‐entrant structures have aroused much attention because of their universal and modification‐independent antiadhesion or antipenetration ability. Unfortunately, theories and applications about their liquid adhesion behavior are still rare. In this work, inspired by the springtail skin and gecko feet in the adhered state, it is demonstrated that programmable liquid adhesion is realized on the 3D‐printed micro doubly re‐entrant arrays. By arranging the arrays reasonably, three different Cassie adhesion behaviors can be obtained: I) no residue adhesion, II) tunable adhesion, and III) absolute adhesion. Furthermore, various arrays are designed to tune macro/micro liquid droplet manipulation, which can find applications in the transportation of liquid droplets, liquid enrichment, generation of tiny droplets, and micropatterns.     

Springtail‐Inspired Superamphiphobic Ordered Nanohoodoo Arrays with Quasi‐Doubly Reentrant Structures

The skin of springtails is well‐known for being able to repel water and organic liquids using their hexagonally arranged protrusions with reentrant structures. Here, a method to prepare 100 nm‐sized nanohoodoo arrays with quasi‐doubly reentrant structures over square centimeters through combining the nanosphere lithography and the template‐protected selective reactive ion etching technique is demonstrated. The top size of the nanohoodoos, the intra‐nanohoodoo distance, and the height of the nanohoodoos can be readily controlled by the plasma‐etching time of the polystyrene (PS) spheres, the size of the PS spheres used, and the reactive ion etching time of silicon. The strong structural control capability allows for the study of the relationship between the nanohoodoo structure and the wetting property. Superamphiphobic nanohoodoo arrays with outstanding water/organic liquid repellent properties are finally obtained. The superamphiphobic and liquid repellent properties endow the nanohoodoo arrays with remarkable self‐cleaning performance even using hot water droplets, anti‐fogging performance, and the surface‐enhanced Raman scattering sensitivity improvement by enriching the analyte molecules on the nanohoodoo arrays. Overall, the simple and massive production of the superamphiphobic nanohoodoo structures will push their practical application processes in diverse fields where wettability and liquid repellency need to be carefully engineered.     

Large‐Scale,Long‐Range‐Ordered Patterning of Nanocrystals via Capillary‐Bridge Manipulation

Deterministic assembly of nanoparticles with programmable patterns is a core opportunity for property‐by‐design fabrication and large‐scale integration of functional materials and devices. The wet‐chemical‐synthesized colloidal nanocrystals are compatible with solution assembly techniques, thus possessing advantages of high efficiency, low cost, and large scale. However, conventional solution process suffers from tradeoffs between spatial precision and long‐range order of nanocrystal assembly arising from the uncontrollable dewetting dynamics and fluid flow. Here, a capillary‐bridge manipulation method is demonstrated for directing the dewetting of nanocrystal inks and deterministically patterning long‐range‐ordered superlattice structures. This is achieved by employing micropillars with programmable size, arrangement, and shape, which permits deterministic manipulation of geometry, position, and dewetting dynamics of capillary bridges. Various superlattice structures, including one‐dimensional (1D), circle, square, pentagon, hexagon, pentagram, cross arrays, are fabricated. Compared to the glassy thin films, long‐range‐ordered superlattice arrays exhibit improved ferroelectric polarization. Coassembly of nanocrystal superlattice and organic functional molecule is further demonstrated. Through introducing azobenzene into superlattice arrays, a switchable ferroelectric polarization is realized, which is triggered by order–disorder transition of nanocrystal stacking in reversible isomerization process of azobenzene. This method offers a platform for patterning nanocrystal superlattices and fabricating microdevices with functionalities for multiferroics, electronics, and photonics.     

25th Anniversary Article: Scalable Multiscale Patterned Structures Inspired by Nature: the Role of Hierarchy

Multiscale, hierarchically patterned surfaces, such as lotus leaves, butterfly wings, adhesion pads of gecko lizards are abundantly found in nature, where microstructures are usually used to strengthen the mechanical stability while nanostructures offer the main functionality, i.e., wettability, structural color, or dry adhesion. To emulate such hierarchical structures in nature, multiscale, multilevel patterning has been extensively utilized for the last few decades towards various applications ranging from wetting control, structural colors, to tissue scaffolds. In this review, we highlight recent advances in scalable multiscale patterning to bring about improved functions that can even surpass those found in nature, with particular focus on the analogy between natural and synthetic architectures in terms of the role of different length scales. This review is organized into four sections. First, the role and importance of multiscale, hierarchical structures is described with four representative examples. Second, recent achievements in multiscale patterning are introduced with their strengths and weaknesses. Third, four application areas of wetting control, dry adhesives, selectively filtrating membranes, and multiscale tissue scaffolds are overviewed by stressing out how and why multiscale structures need to be incorporated to carry out their performances. Finally, we present future directions and challenges for scalable, multiscale patterned surfaces.     

Electromagnetic synergetic actuators based on polypyrrole/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> hybrid nanotube arrays

Conducting polymer actuators that can undergo complex and coordinated motions are generally obtained by using complex microfabrication methods to pattern several conducting polymer components. Herein, we describe a facile approach for fabricating electromagnetic synergetic actuators based on polypyrrole/Fe₃O₄ hybrid nanotube arrays. The actuator can perform biomimetic movements like arm-hand coordination. In this case, a magnetic field is used for primary actuation like an arm, i.e., large-scale angular movement, and an electric potential is used for secondary adjustment like a hand, i.e., small-scale angular movement.     

2D Arrays of Soft Actuators Performing Out-of-Plane Local Inflation for Shape Displays

Elementary actuators performing branching or surface swelling are the primary units in the actuator integration system that is leveraged in works requiring a high versatility and complex motion. However, those primary actuator units often lack scalability or compatibility at assembly into a compact form due to the complexity of the structure and the actuation interference between adjacent units. Herein, it is shown that the phase-change actuator in a simple bilayer structure of a top active layer and a bottom constraint layer achieves 1D surface swelling, such that the closely packed 2D array system of this actuator is easily constructed. Upon resistive heating, the active layer inflates based on the phase change of microliquid droplets embedded in an elastomer body. The inflation along the lateral direction of the actuator is suppressed by controlling the thickness ratio between the active and the constraint layers. The actuation of individual units in the array system is performed independently using a switching device with a microcontroller for the parallel application of resistive heating. The application of 2D shape morphing of the actuator arrays in beam steering and shape displays is investigated.     

A Microfabricated Sandwiching Assay for Nanoliter and High‐Throughput Biomarker Screening

Cells secrete substances that are essential to the understanding of numerous immunological phenomena and are extensively used in clinical diagnoses. Countless techniques for screening of biomarker secretion in living cells have generated valuable information on cell function and physiology, but low volume and real‐time analysis is a bottleneck for a range of approaches. Here, a simple, highly sensitive assay using a high‐throughput micropillar and microwell array chip (MIMIC) platform is presented for monitoring of biomarkers secreted by cancer cells. The sensing element is a micropillar array that uses the enzyme‐linked immunosorbent assay (ELISA) mechanism to detect captured biomolecules. When integrated with a microwell array where few cells are localized, interleukin 8 (IL‐8) secretion can be monitored with nanoliter volume using multiple micropillar arrays. The trend of cell secretions measured using MIMICs matches the results from conventional ELISA well while it requires orders of magnitude less cells and volumes. Moreover, the proposed MIMIC is examined to be used as a drug screening platform by delivering drugs using micropillar arrays in combination with a microfluidic system and then detecting biomolecules from cells as exposed to drugs.     

Photodeformable Azobenzene‐Containing Liquid Crystal Polymers and Soft Actuators

Photodeformable liquid crystal polymers (LCPs) that adapt their shapes in response to light have aroused a dramatic growth of interest in the past decades, since light as a stimulus enables the remote control and diverse deformations of materials. This review focuses on the growing research on photodeformable LCPs, including their basic actuation mechanisms, the various deformation modes, the newly designed molecular structures, and the improvement of processing techniques. Special attention is devoted to the novel molecular structures of LCPs, which allow for easy processing and alignment. The soft actuators with various deformation modes such as bending, twisting, and rolling in response to light are also covered with the emphasis on their photo‐induced bionic functions. Potential applications in energy harvesting, self‐cleaning surfaces, sensors, and photo‐controlled microfluidics are further illustrated. The existing challenges and future directions are discussed at the end of this review.     

微结构表面润湿模式转换的实验研究

采用等离子体刻蚀技术及表面硅烷化处理制备了一组硅基方柱阵列样品,测量了其表面与水的表观接触角,简要分析了表面微结构几何参数对润湿模式转换的影响,探讨了方柱阵列微结构表面发生润湿模式转换的原因.结果表明,微结构表面润湿模式转换受微结构几何参数的影响,原因是微结构几何参数的改变会引起表面上的液滴能量的变化,最终导致液滴的润湿状态发生变化.     

On‐Site Formation of Emulsions by Controlled Air Plugs

Air plugs are usually undesirable in microfluidic systems because of their detrimental effect on the system's stability and integrity. By controlling the wetting properties as well as the topographical geometry of the microchannel, it is reported herein that air plugs can be generated in pre‐defined locations to function as a unique valve, allowing for the on‐site formation of various emulsions including single‐component droplets, composite droplets with droplet‐to‐droplet concentration gradient, blood droplets, paired droplets, as well as bubble arrays without the need for precious flow control, a difficult task with conventional droplet microfluidics. Moreover, the self‐generated air valve can be readily deactivated (turned off) by the introduction of an oil phase, allowing for the on‐demand release of as‐formed droplets for downstream applications. It is proposed that the simple, yet versatile nature of this technique can act as an important method for droplet microfluidics and, in particular, is ideal for the development of affordable lab‐on‐a‐chip systems without suffering from scalability and manufacturing challenges that typically confound the conventional droplet microfluidics.