Two-photon Polymerization Technique Research Articles

Perspective on Water-Soluble Two-Photon Initiator for Two-Photon Polymerization.

Two-photon polymerization (TPP) as an unparalleled technology empowers the rapid prototyping of customized three-dimensional (3D) micro/nanostructures, garnering noticeable interest in tissue engineering, drug delivery, and regenerative medicine. These applications have a high requirement on the biocompatibility and integrity of 3D structures. Therefore, it is important to develop two-photon initiator with good water-solubility, initiation efficiency, and biocompatibility. Here, we share our insights into the development of a water-soluble two-photon initiator (WTPI) and applications from the material and manufacturing perspective. We highlight the nonlinear optical properties and the synthesis of WTPI through three pathways. Then we further demonstrate the applications of the TPP technique in the aqueous phase in the fields of tissue engineering, 4D printing, and ceramic manufacturing. Finally, a general conclusion and outlook are provided for the future development and application of WTPI.

Superconducting single-photon detector integrated in DBR with optical microconnector for MM or SM fiber

This paper presents the development of a superconducting nanowire single-photon detector (SNSPD) integrated into a distributed Bragg reflector (DBR) with a design center wavelength of 830 nm and a bandwidth of 200 nm. This SNSPD is made of a superconducting niobium nitride (NbN) thin film that is produced using plasma-enhanced atomic layer deposition. The DBR is made of 15 alternating layers of silicon nitride and silicon oxide that are produced through plasma-enhanced chemical vapor deposition. The reflection efficiency of the mirror is 90% at a wavelength of 830 nm. For sufficient optical coupling, an optical micro-connector optimized for multimode or single-mode optical fibers with a diameter of 128 μm was formed using two-photon polymerization techniques. The niobium nitride film was deposited onto the DBR surface in-situ in two separate reactors connected by a vacuum transfer. The in-situ technique of deposition of a superconducting niobium nitride film and a DBR has allowed achieving a system detection efficiency of 90% at a wavelength of 830 nm and a dark count rate of 10 s−1 at a temperature of 2.5 K. Additionally, the detector jitter was 50 ps.

Direct printing of conductive hydrogels using two-photon polymerization

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) based conductive hydrogels have been extensively used for bioelectronics and tissue engineering applications owing to their high electrical conductivity and ability to interface with soft biological tissues. However, the fabrication of PEDOT:PSS-based hydrogels has mostly relied on conventional fabrication techniques such as casting and transfer printing, which limits the feature size to micro- or mesoscale. To address these limitations, this study offers a novel PEDOT:PSS-based hydrogel material that can be printed directly to nano/microscale structures using the Two-Photon Polymerization (TPP) technique. The TPP-printability of the newly developed hydrogel was analyzed, and its printing window was characterized. The prepared hydrogel exhibits enhanced radial swelling, electrical properties, and biocompatibility compared to the pristine PEDOT:PSS. To validate the micro-manufacturing feasibility of hydrogel, several microstructures were fabricated using the TPP process. The fabricated microstructures demonstrate many properties, including electrical conductivity, biocompatibility, high resolution, and structural stability. The direct printing of highly conductive PEDOT:PSS hydrogel micro-devices overcomes the limitations of traditional conductive hydrogel manufacturing methods. It opens up new possibilities for applications in micro-energy storage devices, flexible micro/nanoelectromechanical systems (MEMS), and flexible biomedical micro-devices. The combination of high-resolution direct printing and unique hydrogel properties offers promising opportunities in these fields.

High-Throughput Two-Photon 3D Printing Enabled by Holographic Multi-Foci High-Speed Scanning.

The emerging two-photon polymerization (TPP) technique enables high-resolution printing of complex 3D structures, revolutionizing micro/nano additive manufacturing. Various fast scanning and parallel processing strategies have been proposed to promote its efficiency. However, obtaining large numbers of uniform focal spots for parallel high-speed scanning remains challenging, which hampers the realization of higher throughput. We report a TPP printing platform that combines galvanometric mirrors and liquid crystal on silicon spatial light modulator (LCoS-SLM). By setting the target light field at LCoS-SLM's diffraction center, sufficient energy is acquired to support simultaneous polymerization of over 400 foci. With fast scanning, the maximum printing speed achieves 1.49 × 108 voxels s-1, surpassing the existing scanning-based TPP methods while maintaining high printing resolution and flexibility. To demonstrate the processing capability, functional 3D microstructure arrays are rapidly fabricated and applied in micro-optics and micro-object manipulation. Our method may expand the prospects of TPP in large-scale micro/nanomanufacturing.

Whispering-Gallery Mode Micro-Ring Resonator Integrated with a Single-Core Fiber Tip for Refractive Index Sensing

A micro-ring resonator structure was fabricated via the two-photon polymerization technique directly on a single-mode fiber tip and tested for refractive index sensing application. The micro-ring structure was used to excite whispering-gallery modes, and observations of the changes in the resonance spectrum introduced by changes in the refractive index of the environment served as the sensing principle. The proposed structure has the advantages of a very simple design, allowing for measurements in reflection mode, relatively easy and fast fabrication and integration with a single tip of a standard single-mode fiber, which allowed for quick and convenient measurements in the optical setup. The performance of the structure was characterized, and the resonant spectrum giving high potential for refractive index sensing was measured. Future perspectives of the research are addressed.

Two-photon polymerization of silica glass diffractive micro-optics with minimal lateral shrinkage.

Three-dimensional printing enables the fabrication of silica glass optics with complex structures. However, shrinkage remains a significant obstacle to high-precision 3D printing of glass optics. Here we 3D-printed Dammann gratings (DGs) with low lateral shrinkage (<4%) using a two-photon polymerization (2PP) technique. The process consists of two steps: patterning two-photon polymerizable glass slurry with a 515 nm femtosecond laser to form desired structures and debinding/sintering the structures into transparent and dense silica glass. The sintered structures exhibited distinct shrinkage rates in the lateral against longitudinal directions. As the aspect ratio of the structures increased, the lateral shrinkage decreased, while the longitudinal shrinkage increased. Specifically, the structure with an aspect ratio of approximately 60 achieved a minimal lateral shrinkage of 1.1%, the corresponding longitudinal shrinkage was 61.7%. The printed DGs with a surface roughness below 20 nm demonstrated good beam-shaping performance. The presented technique opens up possibilities for rapid prototyping of silica diffractive optical elements.

High-Precision 3D Printing of Microporous Cochlear Implants for Personalized Local Drug Delivery.

Hearing loss is a highly prevalent multifactorial disorder affecting 20% of the global population. Current treatments using the systemic administration of drugs are therapeutically ineffective due to the anatomy of the cochlea and the existing blood-labyrinth barrier. Local drug delivery systems can ensure therapeutic drug concentrations locally while preventing adverse effects caused by high dosages of systemically administered drugs. Here, we aimed to design, fabricate, and characterize a local drug delivery system for the human cochlea. The design was relevant to the size of the human ear, included two different shapes, and incorporated two different microporous structures acting as reservoirs for drug loading and release. The four cochlear implant designs were printed using the two-photon polymerization (2PP) technique and the IP-Q photoresist. The optimized 2PP process enabled the fabrication of the cochlear implants with great reproducibility and shape fidelity. Rectangular and cylindrical implants featuring cylindrical and tapered tips, respectively, were successfully printed. Their outer dimensions were 0.6 × 0.6 × 2.4 mm3 (L × W × H). They incorporated internal porous networks that were printed with high accuracy, yielding pore sizes of 17.88 ± 0.95 μm and 58.15 ± 1.62 μm for the designed values of 20 μm and 60 μm, respectively. The average surface roughness was 1.67 ± 0.24 μm, and the water contact angle was 72.3 ± 3.0°. A high degree of polymerization (~90%) of the IP-Q was identified after printing, and the printed material was cytocompatible with murine macrophages. The cochlear implants designed and 3D printed in this study, featuring relevant sizes for the human ear and tunable internal microporosity, represent a novel approach for personalized treatment of hearing loss through local drug delivery.

Fabrication and mechanical characterization of hydrogel-based 3D cell-like structures.

In this article, we demonstrate the fabrication of 3D cell-like structures using a femtosecond laser-based two-photon polymerization technique. By employing poly(ethylene glycol) diacrylate monomers as a precursor solution, we fabricate 3D hemispheres that resemble morphological and biomechanical characteristics of natural cells. We employ an optical tweezers-based microrheology technique to measure the viscoelastic properties of the precursor solutions inside and outside the structures. In addition, we demonstrate the interchangeability of the precursor solution within fabricated structures without impairing the microstructures. The combination of two-photon polymerization and microrheological measurements by optical tweezers demonstrated here represents a powerful toolbox for future investigations into cell mimic and artificial cell studies.

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.

Two-photon polymerization based reusable master template to fabricate polymer microneedles for drug delivery

Microneedle patches have been widely used in a minimally invasive manner for various drug delivery applications. However, for developing these microneedle patches, master molds are required, which are generally made of metal and are very expensive. Two-photon polymerization (2PP) technique can be used for fabricating microneedles more precisely and at a much lower cost. This study reports a novel strategy for developing microneedle master templates using the 2PP method. The main advantage of this technique is that there is no requirement for post-processing after laser writing, and that for the fabrication of polydimethylsiloxane (PDMS) molds, harsh chemical treatments such as silanization are not required. This is a one-step process for manufacturing of microneedle templates which allows easy replication of negative PDMS molds. This is done by adding resin to the master-template and annealing at a specific temperature, thereby making the PDMS peel-off easy and allowing re-use of the master template multiple times. Using this PDMS mold, two types of polyvinyl alcohol (PVA)-rhodamine (RD) microneedle patches were developed, namely, dissolving (D-PVA) and hydrogel (H-PVA) patches and were characterized using suitable techniques. This technique is affordable, efficient and does not require post-processing for development of microneedle templates required for drug delivery applications.•Two photon polymerization can be used for cost-effective fabrication of polymer microneedles for transdermal drug delivery.•Post-processing or surface-modification procedures are not required for these master templates.•Using a simple annealing step, the master template becomes reusable and robust for peeling off polymers like PDMS.

Generation of Bessel beams via femtosecond direct laser writing 3D phase plates.

Featuring diffracting-free propagation and self-reconstruc-tion, Bessel beams (BBs) have sparked great interest in the scientific community. These properties give the potential for application in optical communications, laser machining, and optical tweezers. However, generating such beams with high quality is still challenging. Here, by using the femtosecond direct laser writing (DLW) based on two-photon polymerization (TPP) technique, we convert phase distributions of ideal BBs with different topological charges into polymer phase plates. The experimentally generated zeroth- and higher-order BBs are propagation-invariant up to 800 mm. Our work may facilitate the applications of non-diffracting beams in integrated optics.

Advanced Lab-on-Tip ultrasound detectors: A numerical analysis

The Lab-on-Fiber technology is giving an important contribution to the development of compact and minimal invasive devices for biomedical applications. By exploiting the high degree of miniaturization combined with the biocompatibility of the materials used, these devices can be integrated inside medical needles and taken in the human body for in vivo diagnosis and treatments, with important advantages in terms of performances, effectiveness and invasiveness. In this framework, with a look toward high-resolution ultrasound based imaging, here we analyze a novel Lab-on-Fiber 3D micro-structure for ultrasound detection, consisting in a polymeric membrane sustained by six pillars, directly integrated above the tip of a standard single mode fiber. Such a structure essentially works as a Fabry-Perot cavity, providing interference fringes in the fiber reflection spectrum, which shift accordingly with the structure vibrations caused by the incident acoustic waves. Our numerical studies, based on finite element method, demonstrated that by opportunely dimensioning the geometrical parameters involved it is possible to tune the working frequency range up to tens of MHz, reaching sensitivities higher than the standard configurations proposed so far, given by a polymeric slab directly attached above the fiber tip. Interestingly, the investigated structure can be effectively realized by exploiting the Two-Photon polymerization technique directly applied to the optical fiber tip.

Single-mode SU-8 waveguide fabricated using ultrafast direct laser writing

In this work, we demonstrated the application of femtosecond laser direct writing using the two-photon polymerization (TPP) technique in the fabrication of single-mode optical waveguide in SU-8 photoresist. It was found that the polymerization of SU-8 using 780 nm femtosecond laser requires a fluence of 1.8–10 kJ/cm2, where 2.5 kJ/cm2 is sufficient to create a flat-top surface waveguide for a 1.8 µm thick SU-8 film. To reduce the coupling loss of the waveguide, the waveguide width was enlarged with double scan of the femtosecond laser by shifting the sample laterally with a center-to-center spacing of 0.5 µm between scans. The optimized waveguide has an insertion loss of 11 dB, with an approximated propagation loss of 1.27 dB/cm. Based on the information obtained, a directional coupler was fabricated. The resulting coupling ratio of the directional coupler is 47 % and 53 % at each port. To further demonstrate the viability of the proposed technique to fabricate complex waveguide structures, a vertically suspended waveguide has been designed and fabricated.

Customizable and highly sensitive 3D micro-springs produced by two-photon polymerizations with improved post-treatment processes

Springs are ubiquitous in a variety of scientific and engineering fields. However, the comprehensive study on mechanical properties of micro-spring has not been fully conducted yet due to a lack of reliable productions of varied-shaped micro-springs. Here, we report the design and manufacturing of triple-helix-shaped springs employing two-photon polymerization (TPP) technologies and present a systemic study on the mechanical properties of micro-springs particularly involving spring constants of k. To construct high-quality hollow microstructures, we optimize the TPP process by combining violet light post-treatment with a proper selection of cleaning liquid. Consequently, we demonstrate that the sensitives k can be actively tuned over a range of two orders of magnitude, from ∼1.5 to ∼108.2 μN/μm while maintaining a high resolution of ∼1 μN/μm. Furthermore, compression tests showcase an excellent agreement among all force-vs-displacement lineshapes, resulting in a small k fluctuation of &amp;lt;1%. On the whole, we expected that the modified TPP technique along with proposed helical springs opens an alternative avenue toward micro-scale force detection, leading to potential applications in the field of bio-sensing, where typical forces to be measured exist within a broad range from several piconewtons to several micronewtons.

Engineering of a Microscale Niche for Pancreatic Tumor Cells Using Bioactive Film Coatings Combined with 3D-Architectured Scaffolds.

Two-photon polymerization has recently emerged as a promising technique to fabricate scaffolds for three-dimensional (3D) cell culture and tissue engineering. Here, we combined 3D-printed microscale scaffolds fabricated using two-photon polymerization with a bioactive layer-by-layer film coating. This bioactive coating consists of hyaluronic acid and poly(l-lysine) of controlled stiffness, loaded with fibronectin and bone morphogenic proteins 2 and 4 (BMP2 and BMP4) as matrix-bound proteins. Planar films were prepared using a liquid handling robot directly in 96-well plates to perform high-content studies of cellular processes, especially cell adhesion, proliferation, and BMP-induced signaling. The behaviors of two human pancreatic cell lines PANC1 (immortalized) and PAN092 (patient-derived cell line) were systematically compared and revealed important context-specific cell responses, notably in response to film stiffness and matrix-bound BMPs (bBMPs). Fibronectin significantly increased cell adhesion, spreading, and proliferation for both cell types on soft and stiff films; BMP2 increased cell adhesion and inhibited proliferation of PANC1 cells and PAN092 on soft films. BMP4 enhanced cell adhesion and proliferation of PANC1 and showed a bipolar effect on PAN092. Importantly, PANC1 exhibited a strong dose-dependent BMP response, notably for bBMP2, while PAN092 was insensitive to BMPs. Finally, we proved that it is possible to combine a microscale 3D Ormocomp scaffold fabricated using the two-photon polymerization technique with the bioactive film coating to form a microscale tumor tissue and mimic the early stages of metastatic cancer.

3D printed devices and infrastructure for liquid sample delivery at the European XFEL.

The Sample Environment and Characterization (SEC) group of the European X-ray Free-Electron Laser (EuXFEL) develops sample delivery systems for the various scientific instruments, including systems for the injection of liquid samples that enable serial femtosecond X-ray crystallography (SFX) and single-particle imaging (SPI) experiments, among others. For rapid prototyping of various device types and materials, sub-micrometre precision 3D printers are used to address the specific experimental conditions of SFX and SPI by providing a large number of devices with reliable performance. This work presents the current pool of 3D printed liquid sample delivery devices, based on the two-photon polymerization (2PP) technique. These devices encompass gas dynamic virtual nozzles (GDVNs), mixing-GDVNs, high-viscosity extruders (HVEs) and electrospray conical capillary tips (CCTs) with highly reproducible geometric features that are suitable for time-resolved SFX and SPI experiments at XFEL facilities. Liquid sample injection setups and infrastructure on the Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography (SPB/SFX) instrument are described, this being the instrument which is designated for biological structure determination at the EuXFEL.

Wide-Gamut Biomimetic Structural Colors from Interference-Assisted Two-Photon Polymerization.

Two-photon polymerization (TPP) is an emerging direct laser writing technique for the fabrication of structural colors. However, its coloration ability is suppressed as the vertical resolution is up to several microns. To solve this issue, an interference-assisted TPP technique was employed. Laser interference at a highly reflective interface produced the periodic energy redistribution along the vertical direction, turning the laser voxel into multilayer structures and confirming this technology as a facile and robust method for precise control of its vertical feature size. Biomimetic structural colors (BSCs) inspired from the ridge-lamella configurations in the Morph butterflies were fabricated using this improved TPP technique. The coloration mechanisms of the multilayer interference from the lamella layers, the thin-film interference from the fusion of multilayers, and the hybrid situations were systematically studied. These BSC colors were grouped as pixel palettes with various TPP parameters corresponding to each other, and they spanned almost the entire standard red-green-blue color space. Moreover, under optimized conditions, it was possible to fabricate a 1 cm2 area within 2.5 h. These features make interference-assisted TPP an ideal coloration method for practical applications, such as display, decoration, sensing, and so on.

Rapid prototyping and customizable microneedle design: Ultra-sharp microneedle fabrication using two-photon polymerization and low-cost micromolding techniques

In recent years, interest in microneedle devices for drug and vaccine delivery and point-of-care diagnostics has grown due to low cost, convenience and minimal invasiveness. A cluster of miniature needles on a small patch could enhance the quality of human health care, revolutionizing test and drug and vaccine delivery systems. However, current fabrication methods are not viable for cost-effective large-scale manufacture. This study reports fabrication of ultra-sharp microneedles with microfluidic channels using two-photon polymerization (2PP) which enables flexible designs with resolution down to 100 nm. The technique is ideally suited to prototyping and the fabrication of master molds from which elastomeric negative-molds have been used in a rapid micromolding technique to make batches of ultra-sharp microneedles. This micromolding process has mass manufacturing potential.

Direct laser writing spiral Sagnac waveguide for ultrahigh magnetic field sensing

A high-birefringence spiral Sagnac waveguide (SSW) device fabricated via direct laser writing (DLW) using a two-photon polymerization (2PP) technique is proposed, designed, and experimentally demonstrated as an ultrahigh magnetic field sensor. The sensor comprises a Y-style tapered waveguide and an SSW containing two microfluidic channels. The SSW has a total length of ∼ 2.4 mm and a spiral radius of ∼ 200 μm . Due to the asymmetric structure, the SSW has a high birefringence of 0.016, which can be designed as a magnetic field sensor, as a magnetic fluid can be filled into the microfluidic channel changing the guiding mode and the birefringence and consequently leading to a change in phase of the interferometer when the applied magnetic field changes. The experimental results show that the proposed photonic device has a sensitivity to magnetic fields as high as 0.48 nm/Oe within a range from 10 to 100 Oe. The proposed device is very stable and easy to fabricate, and it can therefore be used for weak magnetic field detection.

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.