Aliasghar Ajami

Initiation efficiency and cytotoxicity of novel water-soluble two-photon photoinitiators for direct 3D microfabrication of hydrogels

The lack of efficient water-soluble two-photon absorption (TPA) photoinitiators has been a critical obstruction for three dimensional hydrogel microfabrications with high water load by two-photon induced polymerization (TPIP). In this paper, a series of cyclic benzylidene ketone-based two-photon initiators, containing carboxylic acid sodium salts to improve water solubility, were synthesized via classical aldol condensation reactions. The cytotoxicity of cyclopentanone-based photoinitiators is as low as that of the well-known biocompatible photoinitiator Irgacure 2959 as assessed in the dark with MG63 cell line. In z-scan measurement, the TPA cross sections of the investigated initiators are only moderate in water, while the TPA values for hydrophobic analogues measured in chloroform were much higher. All novel initiators exhibited broad processing windows in TPIP tests using hydrophilic photopolymers with up to 50 wt% of water. Impressively, microfabrication of hydrogels with excellent precision was possible at a writing speed as high as 100 mm s−1.

Two-photon absorption cross section measurements of various two-photon initiators for ultrashort laser radiation applying the Z-scan technique

In this paper, we describe the Z-scan measurements of the two-photon absorption (TPA) cross section of various two-photon initiators that are suitable for real three-dimensional structuring of photo-polymerizable formulation. The value of the TPA cross section for the initiator P3K was measured to be 256 GM as the maximum value among the synthesized initiators. Procedures for a precise Z-scan measurement including the measurement of Rayleigh length and beam waist radius are presented. The effect of the pulse width on the Z-scan signal is demonstrated. We also used a flow-cell geometry instead of a static cell in order to refresh the materials to avoid the decomposition of the molecules. This resulted in a more realistic TPA cross section which has been proven by suitable reference compounds.

Measurement of degenerate two-photon absorption spectra of a series of developed two-photon initiators using a dispersive white light continuum Z-scan

To achieve efficient micro- and nanostructuring based on two-photon polymerization (2PP), the development and evaluation of specialized two-photon initiators (2PIs) are essential. Hence, a reliable method to determine the two-photon absorption (2PA) spectra of the synthesized 2PIs used for 2PP structuring is crucial. A technique by which absolute visible-to-near-infrared 2PA spectra of degenerate nature can be determined via performing a single dispersive white-light continuum (WLC) Z-scan has been realized. Using a dispersed white light beam containing 8 fs pulses at wavelengths ranging from 650 nm to 950 nm, the nonlinear transmittance as a function of the sample position can be measured for all spectral components by performing a single scan along the laser beam propagation direction. In this work, the 2PA spectrum of three different 2PIs was determined using this technique. 2PP structuring was also accomplished using the developed 2PIs at different wavelengths. Tuning the wavelength of the laser to ma...

Study on contribution of the asymmetric stress to the birefringence induced by an ultrashort single laser pulse inside fused silica glass

Stress induced birefringence due to asymmetry in axial and radial directions that is generated because of the interaction of ultrashort laser pulses with a transparent material is numerically studied. The coupled equations of nonlinear Schrodinger and plasma density evolution are solved to calculate the deposited energy density and initial temperature profile. Fouriers heat equation and the equations related to the thermo-elasto plastic model are solved to calculate the temperature evolution and distribution of induced displacement inside the material, respectively. Finally, by numerically calculating the distribution of the induced refractive index changes experienced by both axially and radially probe beams, induced stress birefringence is calculated for different characteristics of writing pulses. Furthermore, the induced stress birefringence is experimentally realized, and the effect of the energy of the writing pulse is investigated. To know how the induced refractive index changes and birefringence...

Z-scan measurements of two-photon absorption for ultrashort laser radiation

We have developed a low cost apparatus for open- and closed-aperture Z-scan measurements of multi-photon absorption (MPA) cross-sections of solid and liquid samples. The experimental setup uses simple diodes for light detection. The signals are recorded with a low-cost two-channel PC-scope. We have developed a LabView based software, which analyzes single laser pulses and allows averaging over several shots. First measurements on a CR-39 polymer demonstrated the functioning of the method. Furthermore, we have shown that for 25fsec ultra short pulses three-photon absorption (ThPa) must be considered in addition to two-photon absorption (TPA). The appropriate nonlinear absorption (TPA-, ThPA-) coefficients and the nonlinear refractive index can be obtained via a best fit of the data to theoretical curves, which have been derived and adapted for ThPA from formulas for TPA accessible in the literature.

A biocompatible diazosulfonate initiator for direct encapsulation of human stem cells via two-photon polymerization

Direct cell encapsulation is a powerful tool for fabrication of biomimetic 3D cell culture models in vitro. This method allows more precise recapitulation of the natural environment and physiological functions of cells compared to classical 2D cultures. In contrast to seeding cells on prefabricated scaffolds, cell encapsulation offers benefits regarding high initial cell loading, uniformity of cell distribution and more defined cell–matrix contact. Two-photon polymerization (2PP) based 3D printing enables the precise engineering of cell-containing hydrogel constructs as tissue models. Two-photon initiators (2PIs) specifically developed for this purpose still exhibit considerable cyto- and phototoxicity, impairing the viability of encapsulated cells. This work reports the development of the first cleavable diazosulfonate 2PI DAS, largely overcoming these limitations. The material was characterized by standard spectroscopic methods, white light continuum two-photon absorption cross-section measurements, and its photosensitization of cytotoxic singlet oxygen was compared to the well-established 2PI P2CK. When DAS is used at double concentration to compensate for the lower two-photon cross section, its performance in 2PP-printing of hydrogels is similar to P2CK based on structuring threshold and structure swelling measurements. PrestoBlue metabolic assay showed vastly improved cytocompatibility of DAS in 2D. Cell survival in 3D direct encapsulation via 2PP was up to five times higher versus P2CK, further demonstrating the excellent biocompatibility of DAS and its potential as superior material for laser-based biofabrication.

Laser Analytical Probing of Ultra-Short-Laser-Matter Interaction: Ejected Particles And Surface Topography

Ultra fast laser interaction with surfaces results in fast electronic and thermal ablation processes. The way, how the energy is deposited, determines to a large extent, how efficiently the different processes contribute to laser‐matter interaction. It will be shown, how laser analytical techniques can help to identify the time behavior of the energy deposition, the composition and energy of the emitted particles. Atomic Force Microscopy (AFM) allows determining the surface topography after different stages of ultra‐short laser‐matter interaction. Both techniques combined can yield substantial information for better understanding the physics involved. The appearance of so‐called nano‐hillocks on the surface can be regarded as a typical topographic feature associated with fast electronic processes (correlated with the existence of hot electrons), in particular, demonstrating the efficient localization of energy in small volumes.