In recent years, two-photon polymerization (2PP) has emerged as a promising technology to structure customized biomaterials in regenerative medicine. Based on nonlinear absorption phenomena, 2PP allows rapid and flexible fabrication of fully three dimensional (3D) objects with sub-100-nm resolution.
The ever-growing need for biocompatible photoinitiators (PI) necessitates knowledge of the spectral two-photon absorption (2PA) characteristics. Matching the laser wavelength to the peak of the 2PA spectrum of a particular compound can result in a significant increase of the PI’s performance. With the advent of tunable femtosecond laser systems the application window of 2PP has vastly expanded due to the broad spectral range available for structuring.
To reveal the potential of a certain PI design the z-scan technique has become a standard method to measure the non-linear properties. We have developed a completely automated z-scan setup, which requires negligible user input for the characterization. It is based on the same system used for 2PP, which allows direct comparison of the PI absorption and the polymerisation performance.
To ensure reproducibility and accuracy of measurements, our group developed an automated algorithm, which collects the required laser parameters before the scanning process. These are stored in a comprehensive library for every single measurement. Therefore, even large amounts of data are easily handled and correctly evaluated without the need to manually check each measurement.
Our setup allowed us to reliably determine the absorption properties of newly synthesized PIs and adjust the structuring wavelength. The change in wavelength resulted in significant improvement of the structuring process.
Hydrogels are polymeric materials with water contents similar to that of soft tissues. Due to their biomimetic properties, they have been extensively used in various biomedical applications including cell encapsulation for tissue engineering. The utilization of photopolymers provides a possibility for the temporal and spatial controlling of hydrogel cross-links. We produced three-dimensional (3-D) hydrogel scaffolds by means of the two-photon polymerization (2PP) technique. Using a highly efficient water-soluble initiator, photopolymers with up to 80 wt.% water were processed with high precision and reproducibility at a writing speed of 10 mm/s . The biocompatibility of the applied materials was verified using Caenorhabditis elegans as living test organisms. Furthermore, these living organisms were successfully embedded within a 200×200×35 μm 3 hydrogel scaffold. As most biologic tissues exhibit a window of transparency at the wavelength of the applied femtosecond laser, it is suggested that 2PP is promising for an in situ approach. Our results demonstrate the feasibility of and potential for bio-fabricating 3-D tissue constructs in the micrometre-range via near-infrared lasers in direct contact with a living organism.
Recently Fourier-Scatterometry has become of increasing interest for quantitative wafer metrology. But also in other
fields the fast and precise optical characterization of periodical gratings of sub 100 nm size is of great interest. We
present the application of Fourier-Scatterometry, extended by the use of the coherent properties of white light for the
characterization of sub-wavelength periodic gratings of photosensitive material structured by two-photon polymerization.
First a simulation-based sensitivity comparison of Fourier-Scatterometry at one fixed wavelength, Fourier-Scatterometry
using a white light light source and also additionally using a reference-branch for white-light-interference has been
carried out. The investigated structures include gratings produced by two-photon polymerization of photosensitive
material and typical semiconductor test gratings. The simulations were performed using the rigorous-coupled-waveanalysis
included in our software package MicroSim. A sensitivity comparison between both methods is presented for
the mentioned structure types. We also show our experimental implementation of the measurement setup using a whitelight-
laser and a modified microscope with a high-NA (NA: 0.95) objective as well as a Linnik-type reference branch
for the phase sensitive measurements. First measurements for the investigation of the performance of this measurement
setup are presented for comparison with the simulation results.
In this work, we prepare and optically characterize novel, titanium-containing hybrid materials that can be
structured three-dimensionally using two-photon polymerization. We investigate the effect on the structurability
of the increase of titanium isopropoxide and methacrylic acid content in this photosensitive composite. We
show that while it is possible to make transparent thin films with titanium isopropoxide molar content as high as
90%, three-dimensional structures can be made only when the titanium isopropoxide molar content is less than
50%. We measure the refractive index of different titanium isopropoxide: methacrylic acid concentrations in the
composite. We show a linear increase of the composite refractive index with titanium isopropoxide
concentration, while the increase of the methacrylic acid content does not it.
One of the rapidly advancing femtosecond laser technologies is three-dimensional micro- and nanostructuring by two-photon
polymerization (2PP) technique. This technique allows the fabrication of any computer-generated 3D structure by
direct laser "recording" into the volume of a photosensitive material. Because of the threshold behavior and nonlinear
nature of the 2PP process, a resolution beyond the diffraction limit can be realized by controlling the laser pulse energy
and number of applied pulses. Many different applications of 2PP technique are discussed.
Two-photon polymerization (2PP) is a novel technology which allows the fabrication of complex three-dimensional (3D)
microstructures and nanostructures. The number of applications of this technology is rapidly increasing; it includes the
fabrication of 3D photonic crystals [1-4], medical devices, and tissue scaffolds [5-6].
In this contribution, we discuss current applications of 2PP for microstructuring of biomedical devices used in drug
delivery. While in general this sector is still dominated by oral administration of drugs, precise dosing, safety, and
convenience are being addressed by transdermal drug delivery systems. Currently, main limitations arise from low
permeability of the skin. As a result, only few types of pharmacological substances can be delivered in this manner .
Application of microneedle arrays, whose function is to help overcome the barrier presented by the epidermis layer of
the skin, provides a very promising solution. Using 2PP we have fabricated arrays of hollow microneedles with different
geometries. The effect of microneedle geometry on skin penetration is examined. Our results indicate that microneedles
created using 2PP technique are suitable for in vivo use, and for integration with the next generation of MEMS- and
NEMS-based drug delivery devices.
The recently developed two-photon polymerisation technique is used for the fabrication of two- and three-dimensional
structures in photosensitive inorganic-organic hybrid material (ORMOCER), in SU8 , and in positive tone resist with
resolutions down to 100nm. In this contribution we present applications of this powerful technology for the realization of
micromechanical systems and microoptical components. We will demonstrate results on the fabrication of complex
movable three-dimensional micromechanical systems and microfluidic components which cannot be realized by other
technologies. This approach of structuring photosensitive materials also provides unique possibilities for the fabrication
of different microoptical components such as arbitrary shaped microlenses, microprisms, and 3D-photonic crystals with
high optical quality.
Conventional microlens arrays consist of a repetitive arrangement of a unit cell on a fixed pitch. In a chirped array, the inflexibility of a regular structure has been overcome. Here, the array consists of individually shaped lenses which are defined by a parametric description of the cells optical function. We propose different fabrication methods for chirped microlens arrays and present experimentally obtained data. Reflow of photoresist is an established technology for the fabrication of microlenses with superior optical performance. For the generation of a chirped microlens array the photolithographic mask for patterning the resist to be melted has to be chirped. We present an algorithm for mask generation with an example of an ultra-thin camera objective. Inherent to the reflow process stringent limitations to viable surfaces apply. For achieving more arbitrary surfaces, laser lithography and also 2-photon polymerization are employed. In both methods the structures are decomposed into pixels. In laser lithography the local height is converted into an intensity value for the exposure. This variable dose writing locally changes the solubility of the resist in the development process leading to the required surface profile. We propose a writing scheme enabling structure heights of several ten microns with sufficient height discretization. 2-photon polymerization is a rapid prototyping method. Here, a small volume of a UV-curing organic-inorganic co-polymer is hardened in the tight focus of the writing beam. The volume pixel to be exposed is addressed by piezoelectric translation stages. Experimentally obtained structures and performed tests of the optical quality are presented.
Rapid progress in ultrafast laser systems opened many exciting possibilities for high-resolution material processing. These laser systems allow to control and deliver optical energy and laser pulses in time and space with unprecedented precision. It is not surprising that these high-quality optical pulses have revolutionized microfabrication technologies. Femtosecond lasers enabled processing of a wide range of materials (including heat sensitive and thermo reactive) with a sub-micrometer resolution. At present, nearly arbitrary shaped 2D and 3D structures can be produced by direct write photofabrication techniques using femtosecond laser pulses. In this paper we present a brief review of our recent progress in femtosecond (maskless, direct-write, nonlinear) laser lithography and 3D photofabrication technique.