Dynamic Color-Switching of Hydrogel Micropillar Array under Ethanol Vapor for Optical Encryption.

Responsive structural colors from artificially engineered micro/nanostructures are critical to the development of anti-counterfeiting, optical encryption, and intelligent display. Herein, the responsive structural color of hydrogel micropillar array is demonstrated under the external stimulus of ethanol vapor. Micropillar arrays with full color are fabricated via femtosecond laser direct writing by controlling the height and diameter of the micropillars according to the FDTD simulation. Color-switching of the micropillar arrays is achieved in <1 s due to the formation of liquid film among micropillars. More importantly, the structural color blueshift of the micropillar arrays is sensitive to the micropillar diameter, instead of the micropillar height. The micropillar array with a diameter of 772 nm takes 400 ms to complete blueshift under ethanol vapor, while that with a diameter of 522 nm blueshifts at 2400 ms. Microscale patterns are realized by employing the size-dependent color-switching of designed micropillar arrays under ethanol vapor. Moreover, Morse code and directional blueshift of structural colors are realized in the micropillar arrays. The advantages of controllable color-switching of the hydrogel micropillar array would be prospective in the areas of optical encryption, dynamic display, and anti-counterfeiting.

Dual-Stimuli Cooperative Responsive Hydrogel Microactuators Via Two-Photon Lithography.

With the development of bionics as well as materials science, intelligent soft actuators have shown promising applications in many fields such as soft robotics, sensing, and remote manipulation. Microfabrication technologies have enabled the reduction of the size of responsive soft actuators to the micron level. However, it is still challenging to construct microscale actuators capable of responding to different external stimuli in complex and diverse conditions. Here, this work demonstrates a dual-stimuli cooperative responsive hydrogel microactuator by asymmetric fabrication via femtosecond laser direct writing. The dual response of the hydrogel microstructure is achieved by employing responsive hydrogel with functional monomer 2-(dimethylamino)ethyl methacrylate. Raman spectra of the hydrogel microstructures suggest that the pH and temperature response of the hydrogel is generated by the changes in tertiary amine groups and hydrogen bonds, respectively. The asymmetric hydrogel microstructures show opposite bending direction when being heated to high temperature or exposed to acid solution, and can independently accomplish the grasp of polystyrene microspheres. Moreover, this work depicts the cooperative response of the hydrogel microactuator to pH and temperature at the same time. The dual-stimuli cooperative responsive hydrogel microactuators will provide a strategy for designing and fabricating controllable microscale actuators with promising applications in microrobotics and microfluidics.

Cross-Scale Topography Achieved by MOPL with Positive Photoresist to Regulate the Cell Behavior.

Cross-scale micro-nano structures play an important role in semiconductors, MEMS, chemistry, and cell biology. Positive photoresist is widely used in lithography due to the advantages of high resolution and environmental friendliness. However, cross-scale micro-nano structures of positive photoresist are difficult to flexibly pattern, and the feature resolution is limited by the optical diffraction. Here, cross-scale patterned micro-nano structures are achieved using the positive photoresist based on the femtosecond laser maskless optical projection lithography (MOPL) technique. The dependence between exposure dose and groove width is comprehensively analyzed, and a feature size of 112 nm is obtained at 110 µW. Furthermore, large-area topography considering cell size is efficiently fabricated by the MOPL technique, which enables the regulation of cell behavior. The proposed protocol of achieving cross-scale structures with the exact size by MOPL of positive photoresist would provide new avenues for potential applications in nanoelectronics and tissue engineering.

Wetting of Cell Aggregates on Microdisk Topography Structures Achieved by Maskless Optical Projection Lithography.

Cell aggregates as a 3D culture model can effectively mimic the physiological processes such as embryonic development, immune response, and tissue renewal in vivo. Researches show that the topography of biomaterials plays an important role in regulating cell proliferation, adhesion, and differentiation. It is of great significance to understand how cell aggregates respond to surface topography. Herein, microdisk array structures with the optimized size are used to investigate the wetting of cell aggregates. Cell aggregates exhibit complete wetting with distinct wetting velocities on the microdisk array structures of different diameters. The wetting velocity of cell aggregates reaches a maximum of 293 µm h-1 on microdisk structures with a diameter of 2 µm and is a minimum of 247 µm h-1 on microdisk structures of 20 µm diameter, which suggests that the cell-substrates adhesion energy on the latter is smaller. Actin stress fibers, focal adhesions (FAs), and cell morphology are analyzed to reveal the mechanisms of variation of wetting velocity. Furthermore, it is demonstrated that cell aggregates adopt climb and detour wetting modes on small and large-sized microdisk structures, respectively. This work reveals the response of cell aggregates to micro-scale topography, providing guidance for better understanding of tissue infiltration.

Micropattern of Silver/Polyaniline Core-Shell Nanocomposite Achieved by Maskless Optical Projection Lithography.

With the development of device miniaturization, a flexible and fast preparation method is in demand for achieving microstructures with desired patterns. We develop a novel photoreduction-polymerization method for preparing conductive metal-polymer patterns. Ag/polyaniline (PANI) nanocomposites have been successfully synthesized by maskless optical projection lithography (MOPL) technology, which is based on multiphoton absorption and the localized surface plasmon resonance (LSPR) effect. The individualized design and synthesis of the nanocomposite patterns at the micro-nano scale are flexibly realized on a variety of substrates. The surface-enhanced Raman scattering (SERS) effect of Rhodamine 6G (R6G) is demonstrated on the microstructure of a square maze-shaped Ag/PANI nanocomposite. The electrical conductivity of the as-prepared nanocomposite is obtained. The preparation protocol proposed in this study opens up new avenues for the fabrication of micro-nano devices such as sensors and detectors.

Consistent pattern printing of the gap structure in femtosecond laser DMD projection lithography.

Maskless lithography technologies have been developed and played an important role in the fabrication of functional micronano devices for microelectronics, biochips and photonics. Optical projection lithography based on digital micromirror device (DMD) is an efficient maskless lithography technology that can rapidly fabricate complex structures. The precise modulation of gap width by DMD maskless optical projection lithography (MOPL) using femtosecond laser becomes important for achieving micronano structures. Herein, we have investigated the relationship between the structure morphology and the light intensity distribution at the image plane by multi-slit diffraction model and Abbe imaging principle, and optimized the gap width more accurately by modulating exposure energy. The aperture diameter of the objective lens has a substantial effect on the pattern consistency. The continuously adjustable structural gap widths of 2144 nm, 2158 nm and 1703 nm corresponding to 6, 12, 24 pixels are obtained by varying the exposure energy in the home-built MOPL system. However, the ideal gap structure cannot be obtained only by adjusting the exposure energy when the gap width is small, such as 1 or 2 pixels. Furthermore, we have proposed an alternative way to achieve fine gap structures through the structural decomposition design and precise control of exposure energy in different regions without changing the MOPL optical system. This study would provide a promising protocol for fabricating gap microstructures with controllable configuration using MOPL technique.

λ/12 Super Resolution Achieved in Maskless Optical Projection Nanolithography for Efficient Cross-Scale Patterning.

The emerging demand for device miniaturization and integration prompts the patterning technique of micronano-cross-scale structures as an urgent desire. Lithography, as a sufficient patterning technique, has been playing an important role in achieving functional micronanoscale structures for decades. As a promising alternative, we have proposed and demonstrated the maskless optical projection nanolithography (MLOP-NL) technique for efficient cross-scale patterning. A minimum feature size of 32 nm, which is λ/12 super resolution breaking the optical diffraction limit, has been achieved by a single exposure. Furthermore, multiscale two-dimensional micronano-hybrid structures with the size over hundreds of micrometers and the precision at tens of nanometers have been fabricated by simply controlling the exposure conditions. The proposed MLOP-NL technique provides a powerful tool for achieving cross-scale patterning with both large-scale and precise configuration with high efficiency, which can be potentially used in the fabrication of multiscale integrated microsystems.

Low threshold photonic crystal laser based on a Rhodamine dye doped high gain polymer.

We demonstrate low threshold lasing oscillation in a photonic crystal (PhC) laser by using tert-butyl Rhodamine B (t-Bu-RhB) doped gain media. Lactonic t-Bu-RhB is synthesized to improve doping concentration in polymethylmethacrylate (PMMA) films, and then isomerized to the zwitterion form to achieve highly fluorescent gain medium. The t-Bu-RhB doped PMMA film is sandwiched by a pair of polystyrene colloidal crystals to construct a PhC resonating cavity. Single-mode laser oscillation at 592 nm is observed when the PhC resonating cavity is pumped by a Nd:YAG laser. The lasing threshold is 0.12 MW cm(-2) utilizing 6.9 wt% t-Bu-RhB doped PMMA films, which is only 1/60 of that with 3 wt% t-Bu-RhB doped PMMA films. The concentration-dependent lasing action is attributed to different gain factors of the t-Bu-RhB doped PMMA films. Furthermore, a spatially and spectrally coherent laser beam from the PhC resonating cavity is verified by exploring the far-field image and angular dependence of the lasing emission. The approach provides a facile and efficient strategy to reduce the lasing threshold for fabricating low threshold PhC lasers.

Transition of lasing modes in polymeric opal photonic crystal resonating cavity.

We demonstrate the transition of lasing modes in the resonating cavity constructed by polystyrene opal photonic crystals and 7 wt. % tert-butyl Rhodamine B doped polymer film. Both single mode and multiple mode lasing emission are observed from the resonating cavity. The lasing threshold is determined to be 0.81 µJ/pulse for single mode lasing emission and 2.25 µJ/pulse for multiple mode lasing emission. The single mode lasing emission is attributed to photonic lasing resulting from the photonic bandgap effect of the opal photonic crystals, while the multiple mode lasing emission is assigned to random lasing due to the defects in the photonic crystals. The result would benefit the development of low threshold polymeric solid state photonic crystal lasers.

Steering electromagnetic beams with conical curvature singularities.

We describe how the transformation-optics technique can be used to design an effective medium mimicking the conical curvature singularity. Anholonomic coordinate transformation gives rise to linear topological defects that break the rotational symmetry. The bending and splitting of the optical beams are found analytically and numerically, depending on the incident direction and the topological charge. Beyond their practical applications to omnidirectional beam steering for photonics, our findings set forth an attractive realm to simulate the relevant physical phenomena in the optical laboratory.

Multipatterned Chondrocytes' Scaffolds by FL-MOPL with a BSA-GMA Hydrogel to Regulate Chondrocytes' Morphology.

Repairing articular cartilage damage is challenging due to its low regenerative capacity. In vitro, cartilage regeneration is a potential strategy for the functional reconstruction of cartilage defects. A hydrogel is an advanced material for mimicking the extracellular matrix (ECM) due to its hydrophilicity and biocompatibility, which is known as an ideal scaffold for cartilage regeneration. However, chondrocyte culture in vitro tends to dedifferentiate, leading to fibrosis and reduced mechanical properties of the newly formed cartilage tissue. Therefore, it is necessary to understand the mechanism of modulating the chondrocytes' morphology. In this study, we synthesize photo-cross-linkable bovine serum albumin-glycidyl methacrylate (BSA-GMA) with 65% methacrylation. The scaffolds are found to be suitable for chondrocyte growth, which are fabricated by homemade femtosecond laser maskless optical projection lithography (FL-MOPL). The large-area chondrocyte scaffolds have holes with interior angles of triangle (T), quadrilateral (Q), pentagon (P), hexagonal (H), and round (R). The FL-MOPL polymerization mechanism, swelling, degradation, and biocompatibility of the BSA-GMA hydrogel have been investigated. Furthermore, cytoskeleton and nucleus staining reveals that the R-scaffold with larger interior angle is more effective in maintaining chondrocyte morphology and preventing dedifferentiation. The scaffold's ability to maintain the chondrocytes' morphology improves as its shape matches that of the chondrocytes. These results suggest that the BSA-GMA scaffold is a suitable candidate for preventing chondrocyte differentiation and supporting cartilage tissue repair and regeneration. The proposed method for chondrocyte in vitro culture by developing biocompatible materials and flexible fabrication techniques would broaden the potential application of chondrocyte transplants as a viable treatment for cartilage-related diseases.

Femtosecond Laser Maskless Optical Projection Lithography of Cartilage PCM Inspired 3D Protein Matrix to Chondrocyte Phenotype.

Hydrogels containing chondrocytes have exhibited excellent potential in regenerating hyaline cartilage. However, chondrocytes are vulnerable to dedifferentiation during in vitro culture, leading to fibrosis and mechanical degradation of newly formed cartilage. It is proposed to modulate cartilage formation via the developed chondrocyte pericellular matrix (PCM) -like scaffolds for the first time, in which the S, M, and L-sized scaffolds are fabricated by femtosecond laser maskless optical projection lithography (FL-MOPL) of bovine serum albumin-glyceryl methacrylate hydrogel. Chondrocytes on the M PCM-like scaffold can maintain round morphology and synthesize extracellular matrix (ECM) to induce regeneration of hyaline cartilage microtissues by geometrical restriction. A series of M PCM-like scaffolds is fabricated with different stiffness and those with a high Young's modulus are more effective in maintaining the chondrocyte phenotype. The proposed PCM-like scaffolds are effective in modulating cartilage formation influenced by pore size, depth, and stiffness, which will pave the way for a better understanding of the geometric cues of mechanotransduction interactions in regulating cell fate and open up new avenues for tissue engineering.

Two-photon induced photopolymerization using photoinitiator with an intramolecular radical quenching moiety for nanolithography.

We have investigated the photoinitiating properties of the photoinitiator 3,6-bis[2-(4-nitrophenyl)-ethynyl]-9-(4-methoxy-benzyl)-carbazole (BNMBC), which has an intramolecular radical quenching group "p-methoxybenzyl," in the substrate lines fabrication of two-photon induced photopolymerization (TPIP). Another photoinitiator 3,6-bis[2-(4-nitrophenyl)-ethynyl]-9-benzyl-carbazole (BNBC) with similar chemical structure but not radical quenching group was studied for comparison. Their photopolymerization properties were studied with resins in which BNBC and BNMBC were used as photoinitiators with a molar ratio of 0.02%, respectively. The linewidth of polymer lines fabricated by TPIP of the photoresist that contained BNMBC could be decreased to 65% of those using BNBC. Besides, we introduced a radical quencher, phenyl methyl ether (PhOCH3), to BNBC and further studied the photopolymerization properties by using BNBC, BNBC/PhOCH3 and BNMBC as photoinitiators with a molar ratio of 0.1%. The results further indicated that BNMBC was effective to confine the radical diffusion in polymerization due to the radical quenching effect of moiety. This intramolecular radical quenching moiety exhibited more effective confining effect of radical diffusion compared to the intermolecular radical quencher PhOCH3. Furthermore, arbitrary complex three-dimensional (3D) microstructure was achieved using the photoresist with photoinitiator BNMBC. The approach could open up broad prospect for improving the resolution in TPIP.

22 nm Resolution Achieved by Femtosecond Laser Two-Photon Polymerization of a Hyaluronic Acid Vinyl Ester Hydrogel.

Three-dimensional (3D) bioinspired hydrogels have played an important role in tissue engineering, owing to their advantage of excellent biocompatibility. Here, the two-photon polymerization (TPP) of a 3D hydrogel with high precision has been investigated, using the precursor with hyaluronic acid vinyl ester (HAVE) as the biocompatibility hydrogel monomer, 3,3'-((((1E,1'E)-(2-oxocyclopentane-1,3-diylidene) bis(methanylylidene)) bis(4,1-phenylene)) bis(methylazanediyl))dipropanoate as the water-soluble initiator, and dl-dithiothreitol (DTT) as the click-chemistry cross-linker. The TPP properties of the HAVE precursors have been comprehensively investigated by adjusting the solubility and the formulation of the photoresist. The feature line width of 22 nm has been obtained at a processing laser threshold of 3.67 mW, and the 3D hydrogel scaffold structures have been fabricated. Furthermore, the average value of Young's modulus is 94 kPa for the 3D hydrogel, and cell biocompatibility has been demonstrated. This study would provide high potential for achieving a 3D hydrogel scaffold with highly precise configuration in tissue engineering and biomedicine.

Synthesis of biocompatible BSA-GMA and two-photon polymerization of 3D hydrogels with free radical type I photoinitiator.

Although the development of three-dimensional (3D) printing technology is growing rapidly in the biomedical field, it remains a challenge to achieve arbitrary 3D structures with high resolution and high efficiency. Protein hydrogels fabricated by two- photon polymerization (TPP) have excellent mechanical properties, high precision, and 3D architecture. However, a large number of the amino acid group in bovine serum albumin (BSA) would be consumed when the protein-based hydrogels use dyes of free radical type II photoinitiators. In this study, we use glycidyl methacrylate (GMA) to modify BSA molecules to obtain a series of BSA-GMA materials, allowing the protein material to be two-photon polymerized with a water-soluble free radical type I photoinitiator. The precisely controllable 3D structure of the BSA-GMA hydrogel was fabricated by adjusting the concentration of the precursor solution, the degree of methacrylation, and the processing parameters of the TPP technique. Importantly, BSA-GMA materials are free of acidic hazardous substances. Meanwhile, the water-soluble initiator lithium phenyl (2,4,6-trimethylbenzoyl) phosphite (LAP) allows TPP on the vinyl group of the GMA chain and thus without consuming its amino acid group. The as-prepared BSA-GMA hydrogel structure exhibits excellent autofluorescence imaging, pH responsiveness, and biocompatibility, which would provide new avenues for potential applications in tissue engineering and biomedical fields to meet specific biological requirements.

Asymmetric fishnet metamaterials with strong optical activity.

We investigate the optical properties of mono- and double-layer asymmetric fishnet metamaterials with orientated elliptical holes, which exhibit exotic spectral and polarization rotating characteristics in the visible spectral range. Our results show that nontrivial orientations of the holes with respect to the reciprocal lattice vectors of the periodic lattice in both systems produce strong polarization rotation as well as additional enhanced optical transmission peaks. Analysis of the electromagnetic field distribution shows the unusual effect is produced by the spinning localized surface plasmon resonances due to the asymmetric geometry. High sensitivity of the hybridized mode on the dielectric spacing, the aspect ratio of the holes and the embedding media in double-layer structure is also observed. The dependence of spectral and polarization response on the orientation of the holes and the embedding media is useful for design of chiral metamaterials at optical frequencies and tailoring the polarization behavior of the metallic nano-structures.

C2v symmetrical two-photon polymerization initiators with anthracene core: synthesis, optical and initiating properties.

A series of C(2v) symmetrical two-photon absorption compounds with anthracene core, 2,7-bis[2-(4-substituted phenyl)-vinyl]-9,10-dipentyloxyanthracenes designated as I, II and III (the substituted groups at the 4-position of phenyl of I, II and III were dimethylamino, methyl and cyano, respectively) were designed and synthesized as initiators in two-photon induced polymerization (TPIP). The anthracene ring was modified by linking vinylphenyl groups to the 2,7-position to extend conjugation system length and two pentyloxy groups to the 9,10-position to serve as electronic donors. Two-photon absorption cross section of I was around 300 GM, which was much larger than the 10 GM of II and 29 GM of III at 800 nm. I of 0.18% molar ratio in resin composed of methacrylic acid and dipentaerythritol hexaacrylate exhibited a dramatically low threshold of 0.64 mW compared with commercial photoinitiator benzil at a scanning speed 10 µm s(-1). Moreover, the threshold of photoinitiator I was only increased to 2.53 mW at a scanning speed of 1000 µm s(-1). The dependency of threshold on the concentration and exposure time was in accordance with theoretical calculation. Finally, a reasonable mechanism of the two-photon initiating process was proposed. This study provides good prospects for developing low threshold photoinitiator in TPIP.

λ/30 inorganic features achieved by multi-photon 3D lithography.

It's critically important to construct arbitrary inorganic features with high resolution. As an inorganic photoresist, hydrogen silsesquioxane (HSQ) has been patterned by irradiation sources with short wavelength, such as EUV and electron beam. However, the fabrication of three- dimensional nanoscale HSQ features utilizing infrared light sources is still challenging. Here, we demonstrate femtosecond laser direct writing (FsLDW) of HSQ through multi-photon absorption process. 26 nm feature size is achieved by using 780 nm fs laser, indicating super-diffraction limit photolithography of λ/30 for HSQ. HSQ microstructures by FsLDW possess nanoscale resolution, smooth surface, and thermal stability up to 600 °C. Furthermore, we perform FsLDW of HSQ to construct structural colour and Fresnel lens with desirable optical properties, thermal and chemical resistance. This study demonstrates that inorganic features can be flexibly achieved by FsLDW of HSQ, which would be prospective for fabricating micro-nano devices requiring nanoscale resolution, thermal and chemical resistance.

Morphology modification of silver microstructures fabricated by multiphoton photoreduction.

We have investigated the morphology modification of silver microstructures fabricated by the multiphoton photoreduction process. The microstructures have been fabricated by a femtosecond laser under different irradiation time and repeated scanning numbers. Trans-4-[4-(dimethylamino)-N-methylstilbazolium] p-tosylate (DAST) was used as photosensitizer and effectively reduced the laser power to 0.66 mW. The increase of the irradiation time and repeated scanning induced more reduction in the multiphoton photoreduction microfabrication process, resulting in the optimization of the linewidth. The fusion of silver nanoparticles was confirmed, which led to the morphology change of silver microstructures for achieving the compact metallic microstructures. The result would provide an important protocol to fabricate the metallic microstructures for the electronic and photonic applications.

Modulation of Cell Behavior by 3D Biocompatible Hydrogel Microscaffolds with Precise Configuration.

Three-dimensional (3D) micronano structures have attracted much attention in tissue engineering since they can better simulate the microenvironment in vivo. Two-photon polymerization (TPP) technique provides a powerful tool for printing arbitrary 3D structures with high precision. Here, the desired 3D biocompatible hydrogel microscaffolds (3D microscaffold) with structure design referring to fibroblasts L929 have been fabricated by TPP technology, particularly considering the relative size of cell seed (cell suspension), spread cell, strut and strut spacing of scaffold. Modulation of the cell behavior has been studied by adjusting the porosity from 69.7% to 89.3%. The cell culture experiment results reveal that the obvious modulation of F-actin can be achieved by using the 3D microscaffold. Moreover, cells on 3D microscaffolds exhibit more lamellipodia than those on 2D substrates, and thus resulting in a more complicated 3D shape of single cell and increased cell surface. 3D distribution can be also achieved by employing the designed 3D microscaffold, which would effectively improve the efficiency of information exchange and material transfer. The proposed protocol enables us to better understand the cell behavior in vivo, which would provide high prospects for the further application in tissue engineering.