Self-Sealed Bionic Long Microchannels with Thin Walls and Designable Nanoholes Prepared by Line-Contact Capillary-Force Assembly

Capillary-Force-Driven Self-Assembly of 4D-Printed Microstructures

Capillary-force-driven self-assembly is emerging as a significant approach for the massive manufacture of advanced materials with novel wetting, adhesion, optical, mechanical, or electrical properties. However, academic value and practical applications of the self-assembly are greatly restricted because traditional micropillar self-assembly is always unidirectional. In this work, two-photon-lithography-based 4D microprinting is introduced to realize the reversible and bidirectional self-assembly of microstructures. With asymmetric crosslinking densities, the printed vertical microstructures can switch to a curved state with controlled thickness, curvature, and smooth morphology that are impossible to replicate by traditional 3D-printing technology. In different evaporating solvents, the 4D-printed microstructures can experience three states: (I) coalesce into clusters from original vertical states via traditional self-assembly, (II) remain curved, or (III) arbitrarily self-assemble (4D self-assembly) toward the curving directions. Compared to conventional approaches, this 4D self-assembly is distance-independent, which can generate varieties of assemblies with a yield as high as 100%. More importantly, the three states can be reversibly switched, allowing the development of many promising applications such as reversible micropatterns, switchable wetting, and dynamic actuation of microrobots, origami, and encapsulation.

Tethered and Untethered 3D Microactuators Fabricated by Two-Photon Polymerization: A Review

Microactuators, which can transform external stimuli into mechanical motion at microscale, have attracted extensive attention because they can be used to construct microelectromechanical systems (MEMS) and/or microrobots, resulting in extensive applications in a large number of fields such as noninvasive surgery, targeted delivery, and biomedical machines. In contrast to classical 2D MEMS devices, 3D microactuators provide a new platform for the research of stimuli-responsive functional devices. However, traditional planar processing techniques based on photolithography are inadequate in the construction of 3D microstructures. To solve this issue, researchers have proposed many strategies, among which 3D laser printing is becoming a prospective technique to create smart devices at the microscale because of its versatility, adjustability, and flexibility. Here, we review the recent progress in stimulus-responsive 3D microactuators fabricated with 3D laser printing depending on different stimuli. Then, an outlook of the design, fabrication, control, and applications of 3D laser-printed microactuators is propounded with the goal of providing a reference for related research.

Self-assembled microcage fabrication for manipulating and selectively capturing microparticles and cells

Single-cell-scale selective manipulation and targeted capture play a vital role in cell behavior analysis. However, selective microcapture has primarily been performed in specific circumstances to maintain the trapping state, making the subsequent in situ characterization and analysis of specific particles or cells difficult and imprecise. Herein, we propose a novel method that combines femtosecond laser two-photon polymerization (TPP) micromachining technology with the operation of optical tweezers (OTs) to achieve selective and targeted capture of single particles and cells. Diverse ordered microcages with different shapes and dimensions were self-assembled by micropillars fabricated via TPP. The micropillars with high aspect ratios were processed by single exposure, and the parameters of the micropillar arrays were investigated to optimize the capillary-force-driven self-assembly process of the anisotropic microcages. Finally, single microparticles and cells were selectively transported to the desired microcages by manipulating the flexibly of the OTs in a few minutes. The captured microparticles and cells were kept trapped without additional forces.

Controllable double-helical microstructures by photonic orbital angular momentum for chiroptical response

Three-dimensional helical microstructures are abundant in nature and can be applied as chiral metamaterials for advanced nanophotonics. Here we report a flexible method to fabricate double-helical microstructures with single exposure by recording the chirality of incident optical vortices. Two coaxial optical vortices can interfere to generate a helical optical field, confirmed by the numerical simulation. The diameters of double-helical microstructures can be tailored by the magnitude of topological charges. This fast manufacturing strategy provides the opportunity to efficiently yield helical microstructures. Finally, the chirality of double-helical microstructures can be reversibly read by optical vortices, demonstrating a strong chiroptical response.

Giant Helical Dichroism of Single Chiral Nanostructures with Photonic Orbital Angular Momentum

Optical activity, demonstrating the chiral light-matter interaction, has attracted tremendous attention in both fundamental theoretical research and advanced applications of high-efficiency enantioselective sensing and next-generation chiroptical spectroscopic techniques. However, conventional chiroptical responses are normally limited in large assemblies of chiral materials by circularly polarized light, exhibiting extremely weak chiroptical signals in a single chiral nanostructure. Here, we demonstrate that an alternative chiral freedom of light-orbital angular momentum-can be utilized for generating strong helical dichroism in single chiral nanostructures. The helical dichroism by monochromatic vortex beams can unambiguously distinguish the intrinsic chirality of nanostructures, in an excellent agreement with theoretical predictions. The single planar-chiral nanostructure can exhibit giant helical dichroism of ∼20% at the visible wavelength. The vortex-dependent helical dichroism, expanding to single nanostructures and two-dimensional space, has implications for high-efficiency chiroptical detection of planar-chiral nanostructures in chiral optics and nanophotonic systems.

3D Bioinspired Microstructures for Switchable Repellency in both Air and Liquid

In addition to superhydrophobicity/superoleophobicity, surfaces with switchable water/oil repellency have also aroused considerable attention because of their potential values in microreactors, sensors, and microfluidics. Nevertheless, almost all those as-prepared surfaces are only applicable for liquids with higher surface tension (γ > 25.0 mN m-1) in air. In this work, inspired by some natural models, such as lotus leaf, springtail skin, and filefish skin, switchable repellency for liquids (γ = 12.0-72.8 mN m-1) in both air and liquid is realized via employing 3D deformable multiply re-entrant microstructures. Herein, the microstructures are fabricated by a two-photon polymerization based 3D printing technique and the reversible deformation is elaborately tuned by evaporation-induced bending and immersion-induced fast recovery (within 30 s). Based on 3D controlled microstructural architectures, this work offers an insightful explanation of repellency/penetration behavior at any three-phase interface and starts some novel ideas for manipulating opposite repellency by designing/fabricating stimuli-responsive microstructures.

Capillary flow in microchannel circuitry of scleral lenses

Continuous monitoring of biomarkers in a quantitative manner at point-of-care settings can advance early diagnosis in medicine. Contact lenses offer a minimally-invasive platform to continuously detect biomarkers in tear fluid. Microfluidic components as lab-on-a-chip technology have the potential to transform contact lenses into fully-integrated multiplexed sensing devices. Here, simple and complex microchannels are created in scleral lenses that perform microfluidic operations via capillary action. The engraving of microchannels in scleral lenses were performed by laser micromilling, where a predictive computational model was developed to simulate the effect of laser power and exposure time on polymer behavior. Experimentally varying the CO2 laser power (1.2-3.6 W) and speed (38-100 mm s-1) allowed the micromilling of concave microchannels with groove depths of 10-240 μm and widths of 35-245 μm on polymetric substrates. The demonstrated laser micromilled circuitry in scleral lenses included linear channels, T/Y junctions, multiplexed arrays, mixers, and spiral channels, as well as serially organized multicomponent channels. Capillary forces acting in the microchannels allowed flowing rhodamine dye within the microfluidic components, which was visualized by optical microscopy in reflection and transmission modes simultaneously. The developed microfluidic components in scleral lenses may enable tear sampling, storage, analysis, and multiplexed detection capabilities for continuous monitoring applications.

Two-Photon Nanolithography of Tailored Hollow three-dimensional Microdevices for Biosystems

Functional three-dimensional (3D) microstructures incorporating accessible interiors have emerged as a versatile platform for biosystem applications. By configuring their 3D geometric features, these biosystem microdevices can accurately evaluate and control targeted bioenvironments. However, classical fabrication techniques based on photolithography-etching processes cannot precisely and programmably control the geometric of the entire hollow 3D microstructures. Here, we proposed the use of a two-photon polymerization (TPP)-based technique for the precise, straightforward, and customizable preparation of hollow 3D microstructure devices with small opening(s). Factors governing the formation of hollow 3D biosystem microdevices, including material composition, laser input, and (post-) development treatment, have been systematically investigated and a set of optimized conditions are presented as a starting point for the development of novel hollow biosystem microdevices. To evaluate the broad applicability of this approach, a series of tailored hollow 3D microdevices with small opening(s), including a micropore, microneedle, microelectrode, microvalve, and micromachine, were successfully prepared using our direct laser writing-TPP technique. To further validate the feasibility of these biosystem microdevices in practical implementations, we demonstrated the use of hollow 3D micropore devices for the robust resistive-pulse analysis of nanoparticles.

Biomaterial-based microstructures fabricated by two-photon polymerization microfabrication technology

Two-photon polymerization (TPP) microfabrication technology can freely prepare micro/nano structures with different morphologies and high accuracy for micro/nanophotonics, micro-electromechanical systems, microfluidics, tissue engineering and drug delivery.

Microtubes with Complex Cross Section Fabricated by C-Shaped Bessel Laser Beam for Mimicking Stomata That Opens and Closes Rapidly

This article presents a new method for fabricating complex cross-sectional microtubes with a high aspect ratio at micro/nanoscale. The microtubes are directly written in a photoresist using a femtosecond pulsed laser combined with a spatial light modulator (SLM). A new method for generating a C-shaped Bessel beam by modifying the Bessel beams with a SLM is reported for the first time. Using this gap-ring-shaped light field, microtubes with special cross section (trefoil-shaped, clover-shaped, spiral, etc.) have been first achieved through two-photo polymerization rapidly. The microtube wall can reach about 800 nm and the diameter of the gap-ring structure is only a few micrometers. As a demonstration, artificial stomata were manufactured with the same size as actual plants stomata consisting of gap-ring microtubes. This artificial stomata can mimic the function of the real stomata with rapid opening and closing, demonstrating its ability to trap and release microparticles regulated by rinse solvent.

Mechanical-Tunable Capillary-Force-Driven Self-Assembled Hierarchical Structures on Soft Substrate

Capillary-force-driven self-assembly (CFSA) has been combined with many top-down fabrication methods to be alternatives to conventional single micro/nano manufacturing techniques for constructing complicated micro/nanostructures. However, most CFSA structures are fabricated on a rigid substrate, and little attention is paid to the tuning of CFSA, which means that the pattern of structures cannot be regulated once they are manufactured. Here, by combining femtosecond laser direct writing with CFSA, a flexible method is proposed to fabricate self-assembled hierarchical structures on a soft substrate. Then, the tuning of the self-assembly process is realized with a mechanical-stretching strategy. With this method, different patterns of tunable self-assembled structures are obtained before tuning and after release, which is difficult to achieve with other techniques. In addition, as a proof-of-concept application, this mechanical tunable self-assembly of microstructures on a soft substrate is used for smart displays and versatile micro-object trapping.

Microfluidic chip with movable layers for the manipulation of biochemicals

A simple and effective platform that can conglomerate various microfluidic functions in a single chip is essential for many bioassays, especially for point-of-care testing applications. Here, a chip that exploits surface tension in solutions with movable top and bottom layers is presented, for use in fluid transport, mixing, maintaining metered volumes, and biomolecule capture and release. The chip has open chambers in vertically mobile top layers and rotationally mobile bottom layers to exploit surface tension in biochemical solutions, and implements control over fluid motion. To manipulate biomolecules, a vertically mobile tip with a permanent magnet at the top layer performs collection, transport, release, and dispersion of magnetic beads. Thus, the chip orchestrates various fluidic control functions without using on-chip valves and pumps that increase operational and structural complexity. To demonstrate its utility, the chip performs automated DNA extraction by obtaining genomic DNA from a sample containing cells. Our approach provides a useful and effective alternative to numerous platforms that use active and passive on-chip components for bioassays.

Femtosecond laser direct writing of ion exchangeable multifunctional microstructures

We report on the fabrication of ion exchangeable microstructures by femtosecond laser direct writing of an ion exchange photopolymer, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS). The resultant microstructures are negatively charged in aqueous solution, and can adsorb positively charged species, such as metal ions, nanoparticles, and proteins by electrostatic interaction, forming functional components for chip functionalization. In addition, it is possible to modify the microstructures with positively charged species that make the microstructures sensitive to negatively charged species. As a typical example, a crossed 3D microvessel functionalized with antibodies was fabricated, which reveals great potential for organ-on-a-chip systems. The fabrication of ion exchangeable microstructures holds great promise for flexible chip functionalization.