Hydrogel-based diffractive optical elements (hDOEs) using rapid digital photopatterning

A Sterile, Injectable, and Robust Sericin Hydrogel Prepared by Degraded Sericin

The application of sericin hydrogels is limited mainly due to their poor mechanical strength, tendency to be brittle and inconvenient sterilization. To address these challenges, a sericin hydrogel exhibiting outstanding physical and chemical properties along with cytocompatibility was prepared through crosslinking genipin with degraded sericin extracted from fibroin deficient silkworm cocoons by the high temperature and pressure method. Our reported sericin hydrogels possess good elasticity, injectability, and robust behaviors. The 8% sericin hydrogel can smoothly pass through a 16 G needle. While the 12% sericin hydrogel remains intact until its compression ratio reaches 70%, accompanied by a compression strength of 674 kPa. 12% sericin hydrogel produce a maximum stretch of 740%, with breaking strength and tensile modulus of 375 kPa and 477 kPa respectively. Besides that, the hydrogel system demonstrated remarkable cell-adhesive capabilities, effectively promoting cell attachment and, proliferation. Moreover, the swelling and degradation behaviors of the hydrogels are pH responsiveness. Sericin hydrogel releases drugs in a sustained manner. Furthermore, this study addresses the challenge of sterilizing sericin hydrogels (sterilization will inevitably lead to the destruction of their structures). In addition, it challenges the prior notion that sericin extracted under high temperature and pressure is difficult to directly cross-linked into a stable hydrogel. This developed hydrogel system in this study holds promise to be a new multifunctional platform expanding the application area scope of sericin.

Droplet bioprinting of acellular and cell-laden structures at high-resolutions

Advances in Digital Light Processing (DLP) based (bio) printers have made printing of intricate structures at high resolution possible using a wide range of photosensitive bioinks. A typical setup of a DLP bioprinter includes a vat or reservoir filled with liquid bioink, which presents challenges in terms of cost associated with bioink synthesis, high waste, and gravity-induced cell settling, contaminations, or variation in bioink viscosity during the printing process. Here, we report a vat-free, low-volume, waste-free droplet bioprinting method capable of rapidly printing 3D soft structures at high resolution using model bioinks. A multiphase many-body dissipative particle dynamics (mDPD) model was developed to simulate the dynamic process of droplet-based DLP printing and elucidate the roles of surface wettability and bioink viscosity. Process variables such as light intensity, photo-initiator concentration, and bioink formulations were optimized to print 3D soft structures (∼0.4 to 3 kPa) with an XY resolution of 38 ± 1.5 μm and Z resolution of 237±5.4 μm. To demonstrate its versatility, droplet bioprinting was used to print a range of acellular 3D structures such as a lattice cube, a Mayan pyramid, a heart-shaped structure, and a microfluidic chip with endothelialized channels. Droplet bioprinting, performed using model C3H/10T1/2 cells, exhibited high viability (90%) and cell spreading. Additionally, microfluidic devices with internal channel network lined with endothelial cells showed robust monolayer formation while osteoblast-laden constructs showed mineral deposition upon osteogenic induction. Overall, droplet bioprinting could be a low-cost, no-waste, easy-to-use, method to make customized bioprinted constructs for a range of biomedical applications.

Adaptive focal lengths in white light focusing Fresnel lenses enabled by reflective-type and phase-only spatial light modulator

Fresnel zone plates (FZPs) are widely used in integrated optical systems to meet new cutting-edge demands for photonic integration and device miniaturizing. However, their use in applications of cross-scale fabrication still faces several obstacles, such as low efficiency, fixed focal length, single wavelength, large size, and complicated fabrication. Here, we first examine a novel adaptive focal length in white light focusing by using reflective-type and phase-only spatial light modulator (RLC-SLM) based on a liquid crystal on silicon. The device achieves a maximum diffraction efficiency of approximately 38% at primary focal points of binary phase-type FZPs throughout the visible range (red, green, and blue wavelengths). The RLC-SLM focuses the light of the desired wavelength while other sources are defocused. White light focusing and color separation are demonstrated by sequentially and additively switching different FZPs. These recent advances show that optically tunable FRZs are promising potential candidates to enhance adaptive camera systems, microscopes, holograms, and portable and wearable devices, thereby opening up novel possibilities in optical communications and sensing.

Stimulated-responsive refractive-diffractive biological hydrogel micro-optical element enabling achromatism via femtosecond laser lithography

Herein, we report a novel biological hydrogel-based achromatic refractive-diffractive micro-optical element with single-material apochromatism. Benefiting from the stimulated responsive property of the hydrogel, pH modulation yielded swelling and affected the refractive index of the element, enabling multi-wavelength focusing performance tuning and chromatic aberration adjustment. Using femtosecond laser lithography, we fabricated a separate hydrogel microlens and Fresnel zone plate and measured the tunable focusing performance while varying pH; the results were consistent with our simulation results. Furthermore, we designed and fabricated a hydrogel-based achromatic refractive-diffractive micro-optical element and demonstrated achromatism with respect to three wavelengths using only one material consisting of a microlens and a Fresnel zone plate. We characterized the optical focusing properties and observed smaller chromatic aberration. The potential applications of such hybrid microoptical elements include biomedical imaging and optical biology sensing.

Printing Double-Network Tough Hydrogels Using Temperature-Controlled Projection Stereolithography (TOPS)

We report a new method to shape double-network (DN) hydrogels into customized 3D structures that exhibit superior mechanical properties in both tension and compression. A one-pot prepolymer formulation containing photo-cross-linkable acrylamide and thermoreversible sol-gel κ-carrageenan with a suitable cross-linker and photoinitiators/absorbers is optimized. A new TOPS system is utilized to photopolymerize the primary acrylamide network into a 3D structure above the sol-gel transition of κ-carrageenan (80 °C), while cooling down generates the secondary physical κ-carrageenan network to realize tough DN hydrogel structures. 3D structures, printed with high lateral (37 μm) and vertical (180 μm) resolutions and superior 3D design freedoms (internal voids), exhibit ultimate stress and strain of 200 kPa and 2400%, respectively, under tension and simultaneously exhibit a high compression stress of 15 MPa with a strain of 95%, both with high recovery rates. The roles of swelling, necking, self-healing, cyclic loading, dehydration, and rehydration on the mechanical properties of printed structures are also investigated. To demonstrate the potential of this technology to make mechanically reconfigurable flexible devices, we print an axicon lens and show that a Bessel beam can be dynamically tuned via user-defined tensile stretching of the device. This technique can be broadly applied to other hydrogels to make novel smart multifunctional devices for a range of applications.

Laser-Induced Fabrication of Micro-Optics on Bioresorbable Calcium Phosphate Glass for Implantable Devices

In this study, a single-step nanosecond laser-induced generation of micro-optical features is demonstrated on an antibacterial bioresorbable Cu-doped calcium phosphate glass. The inverse Marangoni flow of the laser-generated melt is exploited for the fabrication of microlens arrays and diffraction gratings. The process is realized in a matter of few seconds and, by optimizing the laser parameters, micro-optical features with a smooth surface are obtained showing a good optical quality. The tunability of the microlens' dimensions is achieved by varying the laser power, allowing the obtaining of multi-focal microlenses that are of great interest for three-dimensional (3D) imaging. Furthermore, the microlens' shape can be tuned between hyperboloid and spherical. The fabricated microlenses exhibited good focusing and imaging performance and the variable focal lengths were measured experimentally, showing good agreement with the calculated values. The diffraction gratings obtained by this method showed the typical periodic pattern with a first-order efficiency of about 5.1%. Finally, the dissolution characteristics of the fabricated micropatterns were studied in a phosphate-buffered saline solution (PBS, pH = 7.4) demonstrating the bioresorbability of the micro-optical components. This study offers a new approach for the fabrication of micro-optics on bioresorbable glass, which could enable the manufacturing of new implantable optical sensing components for biomedical applications.

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.

Microflow multi-layer diffraction optical element processed by hybrid manufacturing technology

Traditional planar diffractive optical elements (DOEs) are challenged in imaging systems due to diffraction efficiency and chromatic dispersion. In this paper, we have designed a microfluidic diffractive optical element (MFDOE), which is processed by digital micromirror device (DMD) maskless lithography (DMDML) assisted femtosecond laser direct writing (FsLDW). MFDOE is a combination of photoresist-based multi-layer harmonic diffraction surface and liquid, realizing diffraction efficiency of more than 90% in the visible band. And it shows achromatic characteristics in the two bands of 469 nm (±20 nm) and 625 nm (±20 nm). These results show that MFDOE has good imaging performance.

Femtosecond Laser Densification of Hydrogels to Generate Customized Volume Diffractive Gratings

Inspired by nature's ability to shape soft biological materials to exhibit a range of optical functionalities, we report femtosecond (fs) laser-induced densification as a new method to generate volume or subsurface diffractive gratings within ordinary hydrogel materials. We characterize the processing range in terms of fs laser power, speed, and penetration depths for achieving densification within poly(ethylene glycol) diacrylate (PEGDA) hydrogel and characterize the associated change in local refractive index (RI). The RI change facilitates the fabrication of custom volume gratings (parallel line, grid, square, and ring gratings) within PEGDA. To demonstrate this method's broad applicability, fs laser densification was used to generate line gratings within the phenylboronic acid (PBA) hydrogel, which is known to be responsive to changes in pH. In the future, this technique can be used to convert ordinary hydrogels into multicomponent biophotonic systems.

Rapid trapping and tagging of microparticles in controlled flow by in situ digital projection lithography

Real-time and fast trapping and tagging of microfeatures, such as microparticles and cells, are of great significance for biomedical research. In this work, we propose a novel in situ digital projection lithography technology that integrates real-time, in situ generation of digital masks for particle processing and fluid control into conventional DMD-based projection lithography. With the help of image recognition technology, we rapidly resolve the information of the microparticle profile or channel location, combining the selection of existing masks of different shapes, thus enabling in situ generation of user-customized micro-trap arrays and microfilter arrays for particle trapping and tagging. The success in trapping and filtering single particles, particle arrays, and cells has indicated the promising prospects of this novel technology for broad applications in microfluidics, single-cell analysis, and early-stage disease diagnostics.

Correction of a digital micromirror device lithography system for fabrication of a pixelated liquid crystal micropolarizer array

The combination of a digital micromirror device (DMD) lithography system and a rotatable polarizer provides a simple and convenient method to achieve the pixelated liquid crystal micropolarizer (LCMP) array for polarization imaging. In this paper, two crucial problems restricting the high-precision fabrication of LCMP array are pointed out and settled: the dislocation of LCMP pixels caused by parallelism error of the rotating polarizer and the grid defect caused by the gap between micromirrors. After correction, the maximum deviation of the fabricated LCMP pixels was reduced from 3.23 µm to 0.11 µm and the grid defect is eliminated. The correction method reported here lays a good foundation for the fine processing of liquid crystal devices with arbitrary photoalignment structure by using the DMD system.

Polarization-modulated grating interferometer by conical diffraction

The grating interferometer in the Littrow configuration uses quarter wave plates (QWPs) to modulate the polarization in the measurement system to determine the autocollimation optical path. Fabrication errors and mounting errors of the QWPs lead to phase changes in the grating interferometer that generate measurement errors. As an alternative, we propose a grating interferometer that produces conical diffraction. Using the grating instead of QWPs to modulate the beam's polarization bypasses this source of error. A 45 mm range experiment was performed that yielded a repeated measurement error of 40 nm. Experiments show that the system has a simple structure and good repeatability and is capable of high-precision displacement measurements.

3D printed metasurface for generating a Bessel beam with arbitrary focusing directions

In this Letter, a metasurface combined with emerging 3D printing technology is proposed. The proposed metasurface regards the simple cube as the unit cell, and the height of the cube is the only variable. A nearly linear transmission phase range covering 360° operating at 20 GHz is obtained when the height is regulated in [2.26 mm, 11.20 mm]. Therefore, the proposed unit cell can be adopted to any metasurface with various functions. Taking the generation of a non-diffractive Bessel beam as an example, two metasurfaces composed of 30×30 units with different focusing directions are designed based on non-diffractive theory and the generalized law of refraction. Two prototypes are 3D printed and measured by a near-field scanning system. The measured results validate our design with satisfactory focusing and beam deflection performance. Additionally, the 3D printed metasurface has lower cost and a shorter processing cycle, and avoids metal loss. Therefore, a 3D printed metasurface is an excellent candidate that can be applied in millimeter wave or even higher frequency bands.

Tunable multi-wavelength optofluidic Dammann grating with beam splitting property

Dammann grating (DG) is a binary beam splitter. Traditional DG is pure solid and cannot be modulated for different working wavelength. We report a tunable multi-wavelength DG based on a liquid-solid hybrid structure. Two glass plates are bonded by UV adhesive strips, one has a periodic grooves structure made by photoresist, the other has two drilled holes as inlet and outlet, respectively. A microfluidic mixer connected the inlet mixes of two miscible liquids with different flow rates to adjust the refractive index of the mixed liquid entering DG from 1.351 to 1.473. In the experiment, the real-time tunability has shown the DG achieves well beam splitting effect when parameter N is integer, 7 × 7 light spots are arranged in order with good uniformity. For λ = 632.8 nm, spot size uniformity is about 78.38% and power uniformity is ∼71.01%. For λ = 532 nm, the spot size and power uniformity are about 77.17% and 64.32%, respectively. The experiment also demonstrates this DG's suitability for near-infrared light. This work is the first study of tunable DG based on liquid-solid hybrid structure and possesses special merits as compared to its solid counterpart, such as simple fabrication, tunability and multi-wavelength applicability, which make it have an extensive prospect in optofluidic networks and optical devices.

Measurement and compensation of a stitching error in a DMD-based step-stitching photolithography system

The step-stitching issue occurring in digital micromirror device (DMD)-based step lithography, which refers to overlapping and misalignment, has dramatically influenced the overall accuracy of the exposed patterns. To address this technical challenge, this paper proposes a testing method to resolve the system tolerance parameters, inclination angle with 0.060^∘±0.003^∘, and magnification with 3.60399±0.00020, which induce the stitching problem. With these two parameters, a compensation strategy on motion is implemented to precisely control the step distance of the stage so that the edge-to-edge stitching error is reduced to about 0.150 µm and the corner-to-corner stitching error is less than 0.500 µm. The changes of the linewidth induced by the displacement error due to the stage control accuracy and illumination nonuniformity caused by the light source are simulated and analyzed, and the image preprocessing method based on a gradual grayscale mask is employed to improve the quality of stitching. Using this method, the linewidth difference is controlled to be within 0.150 µm. After finishing all the corrections and imaging preprocessing, the transverse error has become almost invisible, and the longitudinal error has been reduced by 97.72%. Experimental results demonstrate that the improved stitching accuracy could achieve high-fidelity devices.

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.

Lithographic pattern quality enhancement of DMD lithography with spatiotemporal modulated technology

In this paper, we propose spatiotemporal modulation projection lithography (STPL) technology, which is a spatiotemporal modulation technology applied to the conventional digital micromirror device (DMD) projection lithography system. Through coordinating the micro-movement of the piezoelectric stage, the flexible pattern generation of DMD, and the exposure time, the proposed STPL enables us to fabricate a microstructure with smooth edges, accurate linewidth, and accurate line position. Further application on fabricating a diffraction lens has been implemented. The edge sawtooth of the Fresnel zone plate fabricated by using the STPL is reduced to 0.3 µm, the error between the actual measured linewidth and the ideal linewidth is only within ±0.1µm, and the focal length is 15 mm, which is basically consistent with the designed focal length. These results indicated that STPL can serve a significant role in the micromanufacturing field for achieving high-fidelity microdevices.