Two-photon Polymerization Process Research Articles

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.

3D micro/nano hydrogel structures fabricated by two-photon polymerization for biomedical applications.

Hydrogels are three-dimensional natural or synthetic cross-linked networks composed of polymer chains formed by hydrophilic monomers. Due to the ability to simulate many properties of natural extracellular matrix, hydrogels have been widely used in the biomedical field. Hydrogels can be obtained through a variety of polymerization strategies such as heating and redox. However, photochemistry is one of the most interesting methods for researchers in this field. Gelatin-methacryloyl (GelMA) inherits the biological activity of gelatin and has become one of the gold standards in the field of biomaterials. GelMA, as a photopolymerizable hydrogel precursor, can be used to fabricate 3D porous structures for biomedical applications through two-photon polymerization. We report a new formulation of GelMA-based photoresist and used it to manufacture a series of two-photon polymerization structures, with a maximum resolution less than 120nm. The influence of process parameters on 3D structures manufacturing is studied by adjusting the scanning speed, laser power, and layer spacing values in two-photon polymerization processing. In vitro biological tests show that the 3D hydrogel produced by two-photon polymerization in this paper is biocompatible and suitable for MC3T3-E1 cell.

Water repellence of biomimetic structures fabricated via femtosecond laser direct writing

Femtosecond laser direct writing based on two-photon polymerization (TPP), which is a sub-classification of vat photopolymerization, has the advantages of unlimited geometrical freedom and high resolution, and has attracted significant interests from scientists and engineers in recent years. In the present study, TPP fabrication was performed using a laser equipment with wavelength of 514 nm. The collimated beam diameter before the focusing lens was measured as 5 mm and the focused laser spot diameter was calculated as 280 nm. Key TPP processing parameters were investigated for the fabrication of nanowires, and micro/ nanoscale biomimetic structures, including cone and pinecone structures. The finest nanowire has a diameter of 105 nm and is achieved at the laser power of 0.8 μW (near the laser polymerization threshold). For the three-dimensional (3D) structures, laser power, layer thickness and hatch spacing would influence the fabrication accuracy. Generation of a stiff 3D cone structure would require a minimum laser power of 1.4 μW when the hatch spacing and layer thickness are 200 nm. The optimum processing parameters (laser power 2.2 μW, hatch space and layer thickness 100 nm) can generate structures with dimensions close to the design values. For pinecones with nano features, the scanning strategies play a significant role in the nano fabrication accuracy. A row of pinecones and cones were fabricated and tested for their water repellence performance. The relationship between the performance and the nano cone density was investigated. The pinecone structures exhibit superior performance to the ones with no nano-scale structures, indicating that improved hydrophobicity can be achieved by designing the micro/nano hierarchical structures. The increased number density of the nano cones in a micro/nano hierarchical structure was found to profoundly enhance the water repellence of the structures. The mechanisms were discussed with Wenzel and Cassie models.

High-efficiency two-photon polymerization initiators based on coumarin derivatives for additive manufacturing with sub-wavelength resolution

Efficient photoinitiators are very important for two-photon polymerization additive manufacturing. In this paper, a series of new initiators based on coumarin groups have been designed and synthesized, and their optical properties and two-photon polymerization properties have been studied and discussed in detail, and the relationship between their structures and properties has been analyzed. The synthesized initiators can realize a two-photon polymerization process with a speed of 100,000 μm/s under a laser power of 35–50 mW, and successfully obtain a series of clear three-dimensional structures with sub-wavelength resolution. Starting from the principle of two-photon polymerization, the relationship between laser power and writing speed and resolution was discussed, two simple mathematical models were established, and a fitting formula with physical meaning was obtained, which provides a way for the prediction and control of resolution. Finally, under the conditions of 100,000 μm/s and 50 mW laser power, a low-density ring with an outer diameter of 2.4 mm and an inner diameter of 2.0 mm with sub-wavelength resolution was manufactured in just over 10 min for the fusion target experiment. This proves the possibility of using the resulting initiator to manufacture precision micro-nano devices.

Acoustic streaming-assisted two-photon polymerization process for the production of multimaterial microstructures

Acoustic streaming-assisted two-photon polymerization process for the production of multimaterial microstructures

3D fiber-probe surface plasmon resonance microsensor towards small volume sensing

Integrating surface plasmon resonance (SPR) structures on fiber end facets enables the sensing system to be miniature, label-free, real-time, and remote detection. Yet, the wave vector matching condition for SPR excitation is hard to be satisfied by regulating the wave vector of light emitted from fiber ends. Here, we demonstrate a novel SPR microsensor realized by integrating a 3D micro waveguide onto the fiber tip. The mechanism of SPR excitation is illustrated by wave vector matching between the in-plane component of waveguide mode and surface plasmon polaritons. Using two-photon polymerization (2PP) and metallization process, the designed 3D waveguide probe with a length of 10 µm is fabricated on the fiber tip. Then, the microsensor was applied for the immune-sensing of anti-human IgG and human IgG interactions, and a LOD of 1.11 nM was achieved. The ultracompact structure endows with the capability for high-performance detection in the micrometre scale through a convenient “dip-and-read” manner. As a proof-of-concept for this “dip-and-read” small volume sensing, immunoassay detection in a small sample volume of 5 μL was also successfully demonstrated. The obtained results indicated that our 3D fiber-probe SPR microsensor may provide a powerful tool for small-volume biosensing applications.

Bio-inspired Compact, High-resolution Snapshot Hyperspectral Imaging System with 3D Printed Glass Lightguide Array.

To address the major challenges to obtain high spatial resolution in snapshot hyperspectral imaging, 3D printed glass lightguide array has been developed to sample the intermediate image in high spatial resolution and redistribute the pixels in the output end to achieve high spectral resolution. Curved 3D printed lightguide array can significantly simplify the snapshot hyperspectral imaging system, achieve better imaging performance, and reduce the system complexity and cost. We have developed two-photon polymerization process to print glass lightguide array, and demonstrated the system performance with biological samples. This new snapshot technology will catalyze new hyperspectral imaging system development and open doors for new applications from UV to IR.

A Calibration Method for the Resolution of 2D TPP Laser Direct Writing.

To improve the fabrication efficiency of the two-photon polymerization (TPP) laser direct writing, the TPP exposure process was set to complete by a single-line scan, which was named 2D TPP. The voxel of the 2D TPP should be large enough to cross the photoresist and the underlayer. To explore the resolution limit of the 2D TPP considering the thickness of the photoresist, a new method named the 45° scanning method was proposed. Meanwhile, a two-photon micro-nano fabrication platform was developed. A group of experiments based on the orthogonal decomposition method was carried out to analyze the width and length of the voxel on the S1805 photoresist under different laser power and processing speed. To confirm whether the resolution of the micro-nano structures fabricated by 2D TPP is consistent with the width of the voxel, aluminum wire grids were fabricated through the 2D TPP and the metal lift-off process. A second-order regression equation of the machining resolution and input parameters of the 2D TPP is deduced. The correlation coefficient between the width of the voxel and the aluminum wire grids is 0.961, which means a significant positive correlation between them. Finally, the second-order regression model derived from the fabrication resolution of the 2D TPP was validated by experiments. Full 2D grids were fabricated using 2D TPP and mental lift-off process. This paper provides a convenient, low-cost, and high-efficiency method for calibrating the fabrication resolution of 2D TPP on various photoresists.

Two-photon absorption cross-section investigation of visible-light photoinitiators under Q-switched Nd:YAG nanosecond pulse laser at 1064 nm

Two-photon polymerization (TPP) technologies commonly rely on femtosecond lasers such as Ti:sapphire which limits their accessibility due to high costs and complexities. Recently, multiple reports showed TPP under near-infrared irradiation which enables the use of alternative light sources such as Neodymium-doped lasers known to be affordable and efficient for a nanosecond and picosecond pulsed generation. 4,4′-Bis(dimethyl-amino) benzophenone or Michler’s ketone (MK), one of the photoinitiators commonly used for photopolymerization under UV irradiation, also shows an absorption band in the visible region which allows for two-photon polymerization at the fundamental wavelength of Neodymium-doped lasers at 1064 nm. In this report, we investigated the two-photon absorption (TPA) of MK in contrast with Irgacure-784 and Indane-1,3-dione, reported to also be promising photoinitiators for the same TPP process. Among them, MK showed a large TPA cross-section measured via the nonlinear transmission method and Z-scan technique with Q-switched Nd:YAG nanosecond pulse laser at 1064 nm, demonstrating MK as a promising photoinitiator for the low-cost two-photon polymerization.

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 <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.

Structuring light using solgel hybrid 3D-printed optics prepared by two-photon polymerization.

Three-dimensional direct laser writing based on a two-photon polymerization process of hybrid organic-inorganic material was used to print micrometer-scale refractive phase elements that were designed to manipulate incoming Gaussian beams into line and square intensity-flattened profiles. Here we present new results of shaping light beams, enabled by tailoring a two-photon absorption process for printing hybrid material structures based on a fast solgel process. The optical design and calculations of the optical elements are described, along with characterization of their performance in manipulating incoming light beams. The novelty described in this work, to the best of our knowledge, is the implementation of 3D solgel materials as better and improved micro-optics. This new ability provides upgraded 3D high resolution and smooth, printed optical phase structures using tailored hybrids with improved optical and mechanical properties compared to standard common photoresists. This opens new and exciting opportunities for compact and robust beam shaping by reaching glassy material properties and overcoming limitations of organic polymers.

Two-photon polymerization of anisotropic composites using acoustic streaming

In this work, an acoustic streaming-assisted Two-Photon Polymerization process (TPP) was developed for manufacturing anisotropic particle-polymer composites. Acoustic field (AF)-assisted constant microcirculation of nanoparticles within a droplet, also known as acoustic streaming (AS), results in nanoparticle trapping within TPP printed grooved surfaces. The effect of the acoustic voltage on the flow velocity and particle trapping efficiency is modeled and characterized. The identified optimum input voltage was used to generate the proper acoustic streaming to trap particles within polymer grooves during TPP process to produce a three-dimensional (3D) anisotropic particle-polymer composite in a layer-by-layer fashion. Experimental results validated the feasibility of the proposed manufacturing approach.

Acoustic Field-Assisted Two-Photon Polymerization Process

Abstract This study successfully integrates acoustic patterning with the Two-Photon Polymerization (TPP) process for printing nanoparticle–polymer composite microstructures with spatially varied nanoparticle compositions. Currently, the TPP process is gaining increasing attention within the engineering community for the direct manufacturing of complex three-dimensional (3D) microstructures. Yet the full potential of TPP manufactured microstructures is limited by the materials used. This study aims to create and demonstrate a novel acoustic field-assisted TPP (A-TPP) process, which can instantaneously pattern and assemble nanoparticles in a liquid droplet, and fabricate anisotropic nanoparticle–polymer composites with spatially controlled particle–polymer material compositions. It was found that the biggest challenge in integrating acoustic particle patterning with the TPP process is that nanoparticles move upon laser irradiation due to the photothermal effect, and hence, the acoustic assembly is distorted during the photopolymerization process. To cure acoustic assembly of nanoparticles in the resin through TPP with the desired nanoparticle patterns, the laser power needs to be carefully tuned so that it is adequate for curing while low enough to prevent the photothermal effect. To address this challenge, this study investigated the threshold laser power for polymerization of TPP resin (Pthr) and photothermal instability of the nanoparticle (Pthp). Patterned nanoparticle–polymer composite microstructures were fabricated using the novel A-TPP process. Experimental results validated the feasibility of the developed acoustic field-assisted TPP process on printing anisotropic composites with spatially controlled material compositions.

Hierarchical Nano/Micro-Structured Surfaces With High Surface Area/Volume Ratios

Abstract Recently, many studies have investigated additive manufacturing (AM) of hierarchical surfaces with high surface area/volume (SA/V) ratios, and their performance has been characterized for applications in next-generation functional devices. Despite recent advances, it remains challenging to design and manufacture high SA/V ratio structures with desired functionalities. In this study, we established the complex correlations among the SA/V ratio, surface structure geometry, functionality, and manufacturability in the two-photon polymerization (TPP) process. Inspired by numerous natural structures, we proposed a 3-level hierarchical structure design along with the mathematical modeling of the SA/V ratio. Geometric and manufacturing constraints were modeled to create well-defined three-dimensional hierarchically structured surfaces with a high accuracy. A process flowchart was developed to design the proposed surface structures to achieve the target functionality, SA/V ratio, and geometric accuracy. Surfaces with varied SA/V ratios and hierarchy levels were designed and printed. The wettability and antireflection properties of the fabricated surfaces were characterized. It was observed that the wetting and antireflection properties of the 3-level design could be easily tailored by adjusting the design parameter settings and hierarchy levels. Furthermore, the proposed surface structure could change a naturally hydrophilic surface to near-superhydrophobic. Geometrical light trapping effects were enabled and the antireflection property could be significantly enhanced (> 80% less reflection) by the proposed hierarchical surface structures. Experimental results implied the great potential of the proposed surface structures for various applications such as microfluidics, optics, energy, and interfaces.

Three-dimensional Fabry–Pérot cavities sculpted on fiber tips using a multiphoton polymerization process

This paper presents 3D Fabry–Pérot (FP) cavities fabricated directly onto cleaved ends of low-loss optical fibers by a two-photon polymerization (2PP) process. This fabrication technique is quick, simple, and inexpensive compared to planar microfabrication processes, which enables rapid prototyping and the ability to adapt to new requirements. These devices also utilize true 3D design freedom, facilitating the realization of microscale optical elements with challenging geometries. Three different device types were fabricated and evaluated: an unreleased single-cavity device, a released dual-cavity device, and a released hemispherical mirror dual-cavity device. Each iteration improved the quality of the FP cavity’s reflection spectrum. The unreleased device demonstrated an extinction ratio around 1.90, the released device achieved 61, and the hemispherical device achieved 253, providing a strong signal to observe changes in the free spectral range of the device’s reflection response. The reflectance of the photopolymer was also estimated to be between 0.2 and 0.3 over the spectrum of interest. The dual-cavity devices include both an open cavity, which can interact with an interstitial medium, and a second solid cavity, which provides a static reference reflection. The hemispherical dual-cavity device further improves the quality of the reflection signal with a more consistent resonance, and reduced sensitivity to misalignment. These advanced features, which are very challenging to realize with traditional planar microfabrication techniques, are fabricated in a single patterning step. The usability of these FP cavities as thermal radiation sensors with excellent linear response and sensitivity over a broad range of temperature is reported. The 3D structuring capability the 2PP process has enabled the creation of a suspended FP heat sensor that exhibited linear response over the temperature range of 20 ºC –120 ºC; temperature sensitivity of ∼50 pm ºC−1 at around 1550 nm wavelength; and sensitivity improvement of better than 9x of the solidly-mounted sensors.

Design and fabrication of an electrothermal MEMS micro-actuator with 3D printing technology

This study presents the design and fabrication results of an electrothermal micro-electro-mechanical system (MEMS) actuator. Unlike traditional one-directional U-shaped actuators, this bi-directional electrothermal (BET) micro-actuator can produce displacements in two directions as a single device. The BET micro-actuator was fabricated using two-photon polymerization (2PP) and digital light processing (DLP) methods, which are 3D printing techniques. These methods have been compared to see the success of BET micro-actuator fabrication. The compound of these methods and the essential coefficients through the 3D printing operation were applied. Evaluation experiments have demonstrated that in both methods, the 3D printer can print materials smaller than 95.7 μm size features. Though the same design was used for the 2PP and DLP methods, the supporting structures were not produced with the 2PP. The BET micro-actuator was manufactured by removing the supports from the original design in the 2PP. The number of supports, the diameter, and height on the arms of the micro-actuator is 18, 4 μm, and 6 μm, respectively. Although 4 μm diameter supports could be produced with the DLP, it was not possible to produce them with 3D printing device based on 2PP. Besides, the DLP was found to be better than the 2PP for the manufacturing of asymmetrical support structures. The fabrication process has been carried out successfully by two methods. When the fabrication success is compared, the surface quality and fabrication speed of the micro-actuator fabricated with DLP is better than the 2PP. Presented results show the efficiency of the 3D printing technology and the simplicity of fabrication of the micro-actuator via 2PP and DLP. An experimental study was carried out to characterize the relationship between displacement and input voltage for the micro-actuator. Experimental results show that the displacement range of the micro-actuator is 8 μm with DLP, while 6 μm with 2PP.

3D microphotonic probe for high resolution deep tissue imaging.

Ultra-compact miniaturized optical components for microendoscopic tools and miniaturized microscopes are required for minimally invasive imaging. Current microendoscopic technologies used for deep tissue imaging procedures are limited to a large diameter and/or low resolution due to manufacturing restrictions. We demonstrate a platform for miniaturization of an optical imaging system for microendoscopic applications with a resolution of 1 µm. We designed our probe using cascaded micro-lenses and waveguides (lensguide) to achieve a probe as small as 100 µm x 100 µm with a field of view of 60 µm in diameter. We demonstrate wide-field microscopy based on our polymeric probe fabricated using photolithography and a two-photon polymerization process.

Hollow waveguide array with subwavelength dimensions as a space-variant polarization converter

A hollow waveguide array with subwavelength dimensions is demonstrated as a polarization converter. An individual waveguide in the array has a rectangular cross-section, which leads to an anisotropy of propagation constants and therefore to a phase shift between vertical and horizontal field components upon propagation. The hollow waveguide array was fabricated by a two-photon polymerization process followed by electrochemical deposition of gold. The fabricated array consists of about 2000×2500 hollow waveguide cells, each with dimensions of 1150 nm×930 nm and height of 2 μm. With these dimensions, the structure can, for example, be used at a wavelength of 1550 nm. Other wavelengths and phase shifts are accessible by changing the dimensions of the cross-section and the height. Since the widths of individual waveguides are variable, space-variant operation can be implemented.

Exposure-dependent refractive index of Nanoscribe IP-Dip photoresist layers.

The refractive indices of photoresists used for direct laser writing (DLW) have been determined after exposure to ultraviolet (UV) light. However, it was anticipated that the refractive index will differ when applying a two-photon polymerization (TPP) process. In this Letter, we demonstrate that this is indeed the case. Making use of a guided mode coupling approach, we measure the dispersive real part of the refractive index (n) of a commercial photoresist (IP-Dip, Nanoscribe) at very high accuracy. Additionally, the imaginary part of the refractive index (k) is determined from absorption measurements for wavelengths in the range 300 to 1700nm. TPP layers exhibit a significantly lower refractive index than their UV exposed bulk counterparts (Δn up to 0.01). Furthermore, when fabricating a TPP shell and UV exposing the interior, the refractive index of the shell will not change. This is an important consideration for optical component design and opens the possibility for low refractive index difference wave guiding.

Fabrication of hybrid Fabry-Pérot microcavity using two-photon lithography for single-photon sources.

For an efficient single-photon source a high-count rate into a well-defined spectral and spatial mode is desirable. Here we have developed a hybrid planar Fabry-Pérot microcavity by using a two-photon polymerization process (2PP) where coupling between single-photon sources (diamond colour centres) and resonance modes is observed. The first step consists of using the 2PP process to build a polymer table structure around previously characterized nitrogen-vacancy (NV) centres on top of a distributed Bragg reflector (DBR) with a high reflectivity at the NV zero-phonon line (ZPL). Afterwards, the polymer structure is covered with a silver layer to create a weak (low Q) cavity where resonance fluorescence measurements from the NVs are shown to be in good agreement with analytical and Finite Difference Time Domain (FDTD) results.