Mode-field matching design, 3D fabrication and characterization of down-tapers on single-mode optical fiber tips for coupling to photonic integrated circuits

Photonic Integrated Circuits have made it possible to decrease the footprint of traditionally bulky optical systems and they create opportunities for various new and fascinating applications. One of the limiting factors for the widespread adaption of PICs is their connection to the outside world. As the mode field diameter of optical modes in waveguides tends to be an order of magnitude smaller than in their fiber counterparts, creating an efficient, robust and alignmenttolerant fiber-to-chip interface remains a challenge. In this work, we investigate the optimization of the fiber-side of the optical interface, whereas the chip itself remains untouched and makes use of spot-size convertors. Optical fiber tips can be functionalized using two-photon polymerization-based 3D nanoprinting technology, which offers full 3D design freedom and sub-micrometer resolution. We present a down-taper design strategy to match the mode-field diameter of single-mode optical fibers to the modefield diameter of waveguides with spot-size converters on PICs. The 3D printed down-tapers are characterized towards their geometry and mode shape, and we experimentally demonstrate their use for coupling towards a Silicon-On-Insulator chip with spot-size convertors. Furthermore, the performance of these down-tapered fibers is compared to conventional lensed fibers in terms of optical coupling efficiency

SERS using two-photon polymerized nanostructures for mycotoxin detection

Improved chemical- and bio-sensing with Surface Enhanced Raman Spectroscopy (SERS) requires nanostuctures that can be flexibly designed and fabricated with different physical and optical properties. Here, we present nano-pillar arrays ranging from 200 nm to 600 nm as SERS substrates for mycotoxin detection that are fabricated by means of two-photon polymerization. We built a nominal shape and a voxel-based model for simulating the enhancement of the electric field of the nano-pillar arrays using the Finite-Difference Time-Domain (FDTD) method. A new model was built based on the Atomic Force Microscopy (AFM) data obtained from the fabricated nanostructures and introduced into a FDTD model. We demonstrated the enhancement behavior by measuring the Raman spectrum of Rhodamine B solutions. Both the simulations and experimental results suggest that the 200 nm nano-pillar array has the highest Enhancement Factor (EF). Besides, we determined the limit of detection of the 200 nm pillar array by performing Raman measurements on Rhodamine B solutions with different concentrations. The detection limit of our 200 nm nano-pillar array is 0.55 μM. Finally we discriminated 1 ppm deoxynivalenol and 1.25 ppm fumonisin b1 in acetonitrile solutions by our SERS substrate in combination with principal component analysis. This versatile approach for SERS substrates fabrication gives new opportunities for material characterization in chemical and biological applications

Technological advancements for the development of stem cell-based models for hepatotoxicity testing

Stem cells are characterized by their self-renewal capacity and their ability to differentiate into multiple cell types of the human body. Using directed differentiation strategies, stem cells can now be converted into hepatocyte-like cells (HLCs) and therefore, represent a unique cell source for toxicological applications in vitro. However, the acquired hepatic functionality of stem cell-derived HLCs is still significantly inferior to primary human hepatocytes. One of the main reasons for this is that most in vitro models use traditional two-dimensional (2D) setups where the flat substrata cannot properly mimic the physiology of the human liver. Therefore, 2D-setups are progressively being replaced by more advanced culture systems, which attempt to replicate the natural liver microenvironment, in which stem cells can better differentiate towards HLCs. This review highlights the most recent cell culture systems, including scaffold-free and scaffold-based three-dimensional (3D) technologies and microfluidics that can be employed for culture and hepatic differentiation of stem cells intended for hepatotoxicity testing. These methodologies have shown to improve in vitro liver cell functionality according to the in vivo liver physiology and allow to establish stem cell-based hepatic in vitro platforms for the accurate evaluation of xenobiotics

Increasing the microfabrication performance of synthetic hydrogel precursors through molecular design

Implementation of hydrogel precursors in two-photon polymerization (2PP) technology provides promising opportunities in the tissue engineering field thanks to their soft characteristics and similarity to extracellular matrix. Most of the hydrogels, however, are prone to post-fabrication deformations, leading to a mismatch between the computer-aided design and the printed structure. In the present work, we have developed novel synthetic hydrogel precursors to overcome the limitations associated with 2PP processing of conventional hydrogel precursors such as post-processing deformations and a narrow processing window. The precursors are based on a poly(ethylene glycol) backbone containing urethane linkers and are, on average, functionalized with six acrylate terminal groups (three on each terminal group). As a benchmark material, we exploited a precursor with an identical backbone and urethane linkers, albeit functionalized with two acrylate groups, that were reported as state-of-the-art. An in-depth characterization of the hexafunctional precursors revealed a reduced swelling ratio (36 MPa Young’s modulus) compared to their difunctional analogs. The superior physical properties of the newly developed hydrogels lead to 2PP-based fabrication of stable microstructures with excellent shape fidelity at laser scanning speeds up to at least 90 mm s–1, in contrast with the distorted structures of conventional difunctional precursors. The hydrogel films and microscaffolds revealed a good cell interactivity after functionalization of their surface with a gelatin methacrylamide-based coating. The proposed synthesis strategy provides a one-pot and scalable synthesis of hydrogel building blocks that can overcome the current limitations associated with 2PP fabrication of hydrogel microstructures