Multiphoton Polymerization Research Articles

Initiator‐Free, Multiphoton Polymerization of Gelatin Methacrylamide

AbstractMultiphoton lithography (MPL), based on multiphoton polymerization (MPP), is a powerful technique for the fabrication of 3D structures used in tissue engineering and other biomedical applications, microoptics, nanophotonics, micro‐/nanomechanics, and micro‐/nanofluidics. MPL is based on the localized polymerization/cross‐linking of photosensitive materials induced by femtosecond laser pulses. The use of an appropriate photoinitiator (PI) is required to initiate the polymerization/cross‐linking process. However, several studies have demonstrated the cytotoxicity of the PIs and their derived free radicals. The photo‐cross‐linking of a pre‐synthesized photosensitive gelatin methacrylamide (GelMA) biopolymer using an initiator‐free MPP process is reported for the first time. This is achieved by employing a femtosecond laser operating in the visible range of the spectrum (520 nm). It is demonstrated that multiphoton absorption can cause the cleavage of the polymer double bonds and induce the cross‐linking process. GelMA‐based 3D structures accurately replicating the original computer‐aided design are fabricated, and their excellent biocompatibility is demonstrated in vitro using 3D cell culture experiments with NIH‐3T3 fibroblast cells.

Femtosecond laser microprinting of a polymer fiber Bragg grating for high-sensitivity temperature measurements

We demonstrate the microprinting of a novel suspended polymer fiber Bragg grating for high-sensitivity temperature measurements. The proposed sensor was developed using a femtosecond laser-induced multiphoton polymerization technique. The grating was cured in a single-groove silica tube spliced between two single-mode fibers. Its transmission spectrum, mode field, and temperature response were thoroughly investigated. A sensitivity of -220 pm/°C was achieved over a temperature range of 24°C to 40°C, which is meaningful in biosensing applications. This all-in-fiber polymer Bragg grating exhibits high temperature sensitivity, excellent mechanical strength, and ultrahigh integration. As such, a temperature sensing element of this type would be a beneficial tool for biological measurements.

Multiphoton-Polymerized 3D Protein Assay.

Multiphoton polymerization (MPP) enables 3D fabrication of micro- and nanoscale devices with complex geometries. Using MPP, we create a 3D platform for protein assays. Elevating the protein-binding sites above the substrate surface allows an optically sectioned readout, minimizing the inevitable background signal from nonspecific protein adsorption at the substrate surface. Two fluorescence-linked immunosorbent assays are demonstrated, the first one relying on streptavidin–biotin recognition and the second one on antibody recognition of apolipoprotein A1, a major constituent of high-density lipoprotein particles. Signal-to-noise ratios exceeding 1000 were achieved. The platform has high potential for 3D multiplexed recognition assays with an increased binding surface for on-chip flow cells.

Multiphoton Polymerization Using Femtosecond Bessel Beam for Layerless Three-Dimensional Printing

Photopolymerization enables the printing of three-dimensional (3D) objects through successively solidifying liquid photopolymer on two-dimensional (2D) planes. However, such layer-by-layer process significantly limits printing speed, because a large number of layers need to be processed in sequence. In this paper, we propose a novel 3D printing method based on multiphoton polymerization using femtosecond Bessel beam. This method eliminates the need for layer-by-layer processing, and therefore dramatically increases printing speed for structures with high aspect ratios, such as wires and tubes. By using unmodulated Bessel beam, a stationary laser exposure creates a wire with average diameter of 100 μm and length exceeding 10 mm, resulting in an aspect ratio > 100:1. Scanning this beam on the lateral plane fabricates a hollow tube within a few seconds, more than ten times faster than using the layer-by-layer method. Next, we modulate the Bessel beam with a spatial light modulator (SLM) and generate multiple beam segments along the laser propagation direction. Experimentally observed beam pattern agrees with optics diffraction calculation. This 3D printing method can be further explored for fabricating complex structures and has the potential to dramatically increase 3D printing speed while maintaining high resolution.

Ultrafast 3D high precision print of micro structures for optical instrument calibration procedures

Multi-photon polymerization, like the so-called direct laser writing (DLW) technique allows for flexible additive manufacturing of three-dimensional ultra-precise structures on the micro- and nanoscale. A possible application for DLW is the manufacturing of measurement standards for calibration procedures of optical measurement instruments. This requires flexible and high precision manufacturing of individualized geometries with high quality surfaces. However, many of the process parameters in DLW have to be selected based on experience and previous knowledge.In this article, the influence of DLW process parameters on the micro-geometry and surface roughness produced are systematically studied, and optimized in terms of printing speed and manufacturing accuracy. Resulting microstructures are being evaluated with different measurement techniques, i.e., a stylus instrument, SEM and AFM. Based on optimized process parameters, a new measurement standard for the novel interferometric measurement instrument Ellipso-Height-Topometer is manufactured and examined as a case study. As a result, it can be shown, that DLW is able to manufacture ultra-precise micro geometries in a very flexible and very fast way and satisfies the tolerances for manufacturing of the designed measurement standard.

The Characterization of Absorptive Nonlinearities

AbstractThe measurement of the order and magnitude of absorptive nonlinearities is essential to the development and improvement of a wide range of optical materials. This review presents and contrasts some of the most commonly used methods for making such measurements. As background, we introduce the history of, and the essential physics behind, multiphoton absorption (MPA) and phenomena that can interfere with its measurement. We review methods for the direct and indirect measurement of MPA. The direct methods covered include nonlinear transmission, absorption modulation, Z‐scan, and spectrally‐resolved, two‐beam coupling. The indirect methods reviewed, all of which rely on the measurement of multiphoton action cross sections, include nonlinear fluorescence excitation, thermal lensing, MPA‐induced photocurrent, and various techniques for determining the order of effective nonlinear absorption in multiphoton absorption polymerization.

Nearly Aberration-Free Multiphoton Polymerization into Thick Photoresist Layers.

In the era of lab-on-chip (LOC) devices, two-photon polymerization (TPP) is gaining more and more interest due to its capability of producing micrometer-sized 3D structures. With TPP, one may integrate functional structures into microfluidic systems by polymerizing them directly inside microchannels. When the feature of sub-micrometer size is a requirement, it is necessary to use high numerical aperture (NA) oil-immersion objectives that are optimized to work close to the glass substrate-photoresist interface. Further away from the substrate, that is, a few tens of micrometers into the photoresist, the focused beam undergoes focal spot elongation and focal position shift. These effects may eventually reduce the quality of the polymerized structures; therefore, it is desirable to eliminate them. We introduce a method that can highly improve the quality of structures polymerized tens of micrometers away from the substrate-photoresist interface by an oil-immersion, high NA objective. A spatial light-modulator is used to pre-compensate the phase-front distortion introduced by the interfacial refractive index jump on the strongly converging beam.

Functional photoresists for sub-diffraction stimulated emission depletion lithography

Two novel photoresists were developed for the fabrication of sub-diffractionally sized polymeric nanostructures with chemically reactive surfaces. Using multiphoton polymerization as well as stimulated emission depletion (STED) lithography, chemically functional monomers were copolymerized with highly crosslinking triacrylate monomers to yield stable nanostructures. The polymer structure was thereby supplemented with chemical functionalities for further covalent modification reactions. The reactivity of mercapto- and carboxylate groups on the surface of the nanostructures was proved by orthogonally labeling them with reactive fluorophores. The photoresists can be used for stimulated emission depletion lithography and lateral line widths down to 55 nm were achieved. A three-dimensional structure is shown that is made up with three compounds: a frame out of a un-functionalized acrylate photoresist, and two intermediate networks made up with thiol- and carboxyl functional photoresists.

The mechanisms and research progress of laser fabrication technologies beyond diffraction limit

Laser is recognized as one of the top technological achievements of 20th century and plays an important role in many fields, such as medicine, industry, entertainment and so on. Laser processing technology is one of the earliest and most developed applications of laser. With the rapid development of nanoscience and nanotechnology and micro/nano electronic devices, the micro/nanofabrication technologies become increasingly demanding in manufacturing industries. In order to realize low-cost, large-area and especially high-precision micro-nanofabrication, it has great scientific significance and application value to study and develop the laser fabrication technologies that can break the diffraction limit. In this article, the super resolution laser fabrication technologies are classified into two groups, far-filed laser direct writing technologies and near-field laser fabrication technologies. Firstly, the mechanisms and progress of several far-field laser direct writing technologies beyond the diffraction limit are summarized, which are attributed to the lasermatter nonlinear interaction. The super-diffraction laser ablation was achieved for the temperature-dependent reaction of materials with the Gaussian distribution laser, and the super-diffraction laser-induced oxidation in Metal-Transparent Metallic Oxide grayscale photomasks was realized by the laser-induced Cabrera-Mott oxidation process. Besides, the multi-photon polymerization techniques including degenerate/non-degenerate two-photon polymerization are introduced and the resolution beyond the diffraction limit was achieved based on the third-order nonlinear optical process. Moreover, the latest stimulated emission depletion technique used in the laser super-resolution fabrication is also introduced. Secondly, the mechanisms and recent advances of novel super diffraction near-field laser fabrication technologies based on the evanescent waves or surface plasmon polaritons are recommended. Scanning near-field lithography used a near-field scanning optical microscope coupled with a laser to create nanoscale structures with a resolution beyond 100 nm. Besides, near-field optical lithography beyond the diffraction limit could also be achieved through super resolution near-field structures, such as a bow-tie nanostructure. The interference by the surface plasmon polariton waves could lead to the fabrication of super diffraction interference fringe structures with a period smaller than 100 nm. Moreover, a femtosecond laser beam could also excite and interfere with surface plasmon polaritons to form laser-induced periodic surface structures. Furthermore, the super-resolution superlens and hyperlens imaging lithography are introduced. Evanescent waves could be amplified by using the superlens of metal film to improve the optical lithography resolution beyond the diffraction resolution. The unique anisotropic dispersion of hyperlens could provide the high wave vector component without the resonance relationship, which could also realize the super resolution imaging. Finally, prospective research and development tend of super diffraction laser fabrication technologies are presented. It is necessary to expand the range of materials which can be fabricated by laser beyond the diffraction limit, especially 2D materials.

In situ measurement of the effective nonlinear absorption order in multiphoton photoresists

AbstractKnowledge of the order of the effective nonlinear absorption in multiphoton photoresists is a key element in the development of improved materials for multiphoton absorption polymerization (MAP). The direct measurement of this nonlinearity has proven challenging. A new technique called 2‐beam initiation threshold (2‐BIT) is presented that allows for the unambiguous, in situ measurement of the order of the effective nonlinear absorption using a simple optical arrangement that can be employed with virtually any MAP setup. This technique is benchmarked using three common commercial photoinitiators that have been used previously in MAP and one common dye that acts as a photoinitiator. The linear absorption spectrum is demonstrated to be a poor predictor of the effective order of nonlinear absorption at a given wavelength. Surprisingly, for two of these initiators the effective nonlinear absorption process is dominated by 3‐photon absorption in the 800 nm wavelength range, suggesting that 2‐BIT is a valuable means of identifying initiators that can improve the resolution of MAP. image

Abruptly autofocusing beams enable advanced multiscale photo-polymerization

Tunable-ring Airy beams are experimentally generated and employed for the fabrication of large three-dimensional structures with high resolution using multi-photon polymerization. We demonstrate that these beams can be adjusted to abruptly autofocus over an extended range of working distances while keeping their voxel shape and dimensions almost invariant. This striking property together with the real-time electronically controlled focus tuning makes these beams ideal candidates for long-range multi-photon polymerization. Moreover, the well-controlled remote localized deposition of energy can also impact many other fields of linear and nonlinear optics, like filamentation and remote high-power terahertz generation.

Nano-Confined Polymer Structures for Protein Binding

Stimulated emission depletion lithography (STED-lithography), a further development of multiphoton polymerization (MPP) lithography, belongs to the most promising methods for 2D and 3D structuring of polymer scaffolds with structure sizes in the nanometer range. Already in the early developmental stages of STED-microscopy, it was pointed out that a nano-confined effective point-spread function (PSF) can be applied to spatially confine photo-chemical reactions to sub-diffraction volumes. In optical STED-lithography, one laser pulse excites photo-initiators for radical polymerization and a second donut-shaped laser beam locally inhibits these starter molecules in the outer rim of the PSF. This confinement of the excitation volume leads to a restriction of the size of the polymerized structures. Such a STED-based approach facilitates the creation of structures with sizes below the limits of conventional MPP. Currently, feature sizes as small as 55 nm and a resolution of 120 nm of adjacent lines can be achieved.We use stimulated emission depletion (STED) lithography for the assembly of polymeric structures down to several nanometers in any desired geometry. Protein adhesive photoresists allow manufacturing of nano-confined, protein functionalized structures. The structures show good biocompatibility and allow an easy biofunctionalization with proteins down to a single protein level. Furthermore, we use STED-lithography to create 3D compound structures of different acrylate polymers with distinct properties. A µm-sized protein repellent scaffold consists of multi-photon lithography (MPP) fabricated features, carrying STED-lithography written sub-100 nm protein-binding sites.Combining STED lithography with fluorescence microscopy allows us to produce well characterized, biocompatible structures, applicable to biological assays.

Electrostatically tunable plasmonic devices fabricated on multi-photon polymerized three-dimensional microsprings.

Electrostatically tunable plasmonic devices on three-dimensional (3D) microsprings were fabricated using multi-photon polymerization followed by metal deposition. These plasmonic devices comprised a nanostructured Au microplate and two 3D microsprings. The maximum plasmon excitation efficiency was 35%, a value achieved with incident light of wavelength 632.8 nm. The efficiency could be continuously changed from almost zero to maximum by inclining the microplates with the application of DC voltage up to 50 V. Such dynamic functionality is useful for the realization of highly integrated optoelectronic devices and tunable metamaterials.

3D-printed polymers for biomedical applications

cant technology leaps take place simultaneously in several complementary areas. This is the case in polymeric biomaterials’ technology. Amazing advances have been made in the additive manufacturing technologies, i.e. 3D-printing. It is a family of different layer by layer manufacturing methods like fused deposition modeling, powder laser sintering, ink-jet printing, stereo lithography, two or multi photon polymerization and hydrogel 3D-printing. All of them applicable for processing of polymeric materials. Essential developments have taken place also in biomedical imaging and 3D-design to create the necessary digital models needed for the 3D-printing. At the same time great progress has been achieved in biopolymers’ development. They can be tailored to be biostable or bioresorbable in a controlled way, or in some cases even bioactive. Bioactivity has been achieved through addition of bioactive compounds like bioactive glass or through functionalization of the polymeric structure (www.kunststoffe-international.com/1332070, DOI: 10.1016/j.actbio.2011.06. 039, DOI: 10.1016/j.compscitech.2012.10.014, DOI: 10.1002/jbm.b.32863, DOI: 10.1002/pola.27400, DOI: 10.1039/C5TB01468A). The parallel advance which has taken place in the biomedical sciences has significantly improved the options in regenerative medicine and may even enable tissue engineering. For polymer research this important and fast developing area offers great opportunities of high societal impact. However, the demands for polymer properties are strict for the novel biomaterials. They should meet the various physical criteria set by the needs of the application, but in addition the demand of non-toxicity is a self-evident must in biomedical applications. Successful 3D-additive manufacturing sets requirements to master the viscosities and liquid to solid transitions in the printing process. In stereo lithography the oligomeric low viscosity prepolymers need to be fast photo curing; with photo curing chemistry that is non-toxic in the biological environment and can get approved by the authorities. Especially for resorbable polymers there is a need to find alternative curing chemistries in addition to acrylate or methacrylate based systems. (www.kunststoffe-international.com/1325854) Complementary research interplay is essential in this field of science. An excellent example of such an effort has been the ArtiVasc 3D-project funded by the 7 Framework Program of the European Union to prepare polymeric scaffolds for vascularized, cell tissue cultivated skin like tissue. Outcome of the project is presented in the video: ArtiVasc 3D on the Way to Bio-Artificial Tissue. Novel technologies, breakthroughs in biomedical sciences together with innovative novel polymers enable jointly significant enhancements for life!

Model of diffusion-assisted direct laser writing by means of nanopolymerization in the presence of radical quencher

Diffusion-assisted direct laser writing (DA-DLW) by multiphoton polymerization has been recently shown to be one of the most promising methods for the high-resolution 3D nanofabrication [I. Sakellari, et al., ACS Nano 6, 2302 (2012)]. The improvement of the writing spatial resolution has been observed under certain conditions when the mobile radical quencher (polymerization inhibitor) is added to the photosensitive composition. In this work, we present a theoretical study of this method, focusing on the resolution capabilities and optimal writing parameters. The laser beam absorption in the polymerizable composition causes the localized depletion of the quencher molecules. If the quencher depletion is balanced by its diffusion from the outside of the focal volume, the quasi-stationary non-equillibrium concentration spatial profile with zero minimum can be obtained. The polymer is then effectively formed only in the domain where the quencher is depleted. The spatially-distributed quencher, in this case, has the effect similar to that of the vortex beam in STimulated Emission Microscopy (STED).

Femtosecond laser internal manufacturing of three-dimensional microstructure devices

Potential applications for three-dimensional microstructure devices developed rapidly across numerous fields including microoptics, microfluidics, microelectromechanical systems, and biomedical devices. Benefiting from many unique fabricating advantages, internal manufacturing methods have become the dominant process for three-dimensional microstructure device manufacturing. This paper provides a brief review of the most common techniques of femtosecond laser three-dimensional internal manufacturing (3DIM). The physical mechanisms and representative experimental results of 3D manufacturing technologies based on multiphoton polymerization, laser modification, microexplosion and continuous hollow structure internal manufacturing are provided in details. The important progress in emerging applications based on the 3DIM technologies is introduced as well.

A Fiber-Endface, Fabry–Perot Vapor Microsensor Fabricated by Multiphoton Polymerization

A Fabry-Perot micro-optical sensing resonator fabricated by direct laser writing on the endface of a standard telecom fiber is demonstrated. This endface optical fiber sensing probe has been tested for tracing the vapors of common organic solvents. The device operates in the spectral region lying between 1440 and 1660 nm, while the spectra recorded in reflection mode correlate to the refractive index changes due to the different vapors trapped inside the microcavity. The performance of the device operation with respect to the reflecting surface porosity of the fabricated microstructure is presented, while the sensing of ethanol traces down to 4 ppm concentrations was achieved.

Burr-like, laser-made 3D microscaffolds for tissue spheroid encagement.

The modeling, fabrication, cell loading, and mechanical and in vitro biological testing of biomimetic, interlockable, laser-made, concentric 3D scaffolds are presented. The scaffolds are made by multiphoton polymerization of an organic-inorganic zirconium silicate. Their mechanical properties are theoretically modeled using finite elements analysis and experimentally measured using a Microsquisher(®). They are subsequently loaded with preosteoblastic cells, which remain live after 24 and 72 h. The interlockable scaffolds have maintained their ability to fuse with tissue spheroids. This work represents a novel technological platform, enabling the rapid, laser-based, in situ 3D tissue biofabrication.

3D direct laser writing using a 405 nm diode laser.

Three-dimensional (3D) direct laser writing commonly uses near-infrared femtosecond laser pulses. Here, we use a quasi-cw blue diode laser at a 405nm wavelength. As prerequisite, we identify photoresist systems that unambiguously show nonlinear multiphoton polymerization at this excitation wavelength. Next, we obtain a diffraction-limited focus with a high-numerical-aperture objective lens (NA=1.4), which is crucial to actually benefit from the wavelength advantage. To evaluate the anticipated reduced linewidths and improved resolution, we fabricate and characterize 2D and 3D benchmark structures. Finally, we also demonstrate dip-in direct laser writing with our setup.

Design, Fabrication and Computational Characterization of a 3D Micro-Valve Built by Multi-Photon Polymerization

We report on the design, modeling and fabrication by multi-photon polymerization of a complex medical fluidic device. The physical dimensions of the built micro-valve prototype are compared to those of its computer-designed model. Important fabrication issues such as achieving high dimensional resolution and ability to control distortion due to shrinkage are presented and discussed. The operational performance of both multi-photon and CAD-created models under steady blood flow conditions was evaluated and compared through computational fluid dynamics analysis.