Femtosecond Laser Direct Write Integration of Multi-Protein Patterns and 3D Microstructures into 3D Glass Microfluidic Devices

Mechanism and performance evaluation of transient and selective laser processing of glass based on optical monitoring

Femtosecond laser processing has been widely applied in glass processing owing to its ability to fabricate microscale components. To improve processing efficiency, a transient and selective laser (TSL) processing technique was previously developed, in which electron excitation was induced inside a transparent medium by a single pulse of femtosecond (fs) laser, and a single pulse of microsecond (µs) laser can be selectively absorbed in this excited region to heat and remove the material. However, because of its high speed removal process, the unclear mechanism and inefficient evaluation of its processing performance limit its further application. This study analyzes the transient spatiotemporal evolution of the induced plasma and the related material removal mechanism of the TSL processing using a side high-speed monitoring method. To achieve a rapid performance evaluation, a quantitative analysis of the optical plasma signals (on a microsecond timescale) generated in TSL processing was performed by employing a developed coaxial high-speed monitoring method using a photodetector. The variations in the shapes, intensity distribution, and dimensions of the plasma were quantitatively investigated. In addition, the relation between the plasma signal and drilling performance under different laser parameters, including hole depth, hole types, and cracks, was explored and quantitatively analyzed. The revealed mechanism is expected to contribute to the broadening of the application of TSL processing in microfabrication. Furthermore, the developed high-speed and precision monitoring technology can be utilized for high-speed evaluation and precision control of machining quality in real time during ultrahigh-speed laser machining, without time-consuming camera observations.

Water-Soluble Photoinitiators in Biomedical Applications

Light-initiated polymerization processes are currently an important tool in various industrial fields. The advancement of technology has resulted in the use of photopolymerization in various biomedical applications, such as the production of 3D hydrogel structures, the encapsulation of cells, and in drug delivery systems. The use of photopolymerization processes requires an appropriate initiating system that, in biomedical applications, must meet additional criteria such as high water solubility, non-toxicity to cells, and compatibility with visible low-power light sources. This article is a literature review on those compounds that act as photoinitiators of photopolymerization processes in biomedical applications. The division of initiators according to the method of photoinitiation was described and the related mechanisms were discussed. Examples from each group of photoinitiators are presented, and their benefits, limitations, and applications are outlined.

Three-Dimensional Printing of Pure Proteinaceous Microstructures by Femtosecond Laser Multiphoton Cross-Linking

Laser direct write (LDW) is a promising three-dimensional (3D) printing technology for creating proteinaceous microstructures in which the proteins retain their original function, enabling the manufacture of complex biomimetic 3D microenvironments and versatile enhancement of medical microdevices. A photoactivator has commonly been used to date in the laser direct write of proteins to enhance the cross-linking process. However, incomplete conversion results in photoactivator molecules remaining trapped inside the protein microstructure, causing their gradual leaching and subsequent undesirable effect on biological applications. Here, we demonstrate the 3D fabrication of microstructures made of pure serum albumin protein using photoactivator-free fabrication, confirmed by Raman data. For the first time, acid-catalyzed hydrolysis of the created structures provides evidence that chemical cross-links are induced by exposure to femtosecond laser irradiation. The diversity of the biomaterial protein available for the precursors for LDW offers capability of the fabrication of complex biomimetic 3D microenvironments and biochip applications.

From Simple to Architecturally Complex Hydrogel Scaffolds for Cell and Tissue Engineering Applications: Opportunities Presented by Two-Photon Polymerization

Direct laser writing via two-photon polymerization (2PP) is an emerging micro- and nanofabrication technique to prepare predetermined and architecturally precise hydrogel scaffolds with high resolution and spatial complexity. As such, these scaffolds are increasingly being evaluated for cell and tissue engineering applications. This article first discusses the basic principles and photoresists employed in 2PP fabrication of hydrogels, followed by an in-depth introduction of various mechanical and biological characterization techniques used to assess the fabricated structures. The design requirements for cell and tissue related applications are then described to guide the engineering, physicochemical, and biological efforts. Three case studies in bone, cancer, and cardiac tissues are presented that illustrate the need for structured materials in the next generation of clinical applications. This paper concludes by summarizing the progress to date, identifying additional opportunities for 2PP hydrogel scaffolds, and discussing future directions for 2PP research.

Optimization of selective laser-induced etching (SLE) for fabrication of 3D glass microfluidic device with multi-layer micro channels

We present the selective laser-induced etching (SLE) process and design guidelines for the fabrication of three-dimensional (3D) microfluidic channels in a glass. The SLE process consisting of laser direct patterning and wet chemical etching uses different etch rates between the laser modified area and the unmodified area. The etch selectivity is an important factor for the processing speed and the fabrication resolution of the 3D structures. In order to obtain the maximum etching selectivity, we investigated the process window of the SLE process: the laser pulse energy, pulse repetition rate, and scan speed. When using potassium hydroxide (KOH) as a wet etchant, the maximum etch rate of the laser-modified glass was obtained to be 166 μm/h, exhibiting the highest selectivity about 333 respect to the pristine glass. Based on the optimized process window, a 3D microfluidic channel branching to three multilayered channels was successfully fabricated in a 4 mm-thick glass. In addition, appropriate design guidelines for preventing cracks in a glass and calibrating the position of the dimension of the hollow channels were studied.

Ablation and Patterning of Carbon Nanotube Film by Femtosecond Laser Irradiation

Carbon nanotube (CNT) film can be used as thin film electrodes and wearable electronic devices due to their excellent mechanical and electrical properties. The femtosecond laser has the characteristics of an ultra-short pulse duration and an ultra-high peak power, and it is one of the most suitable methods for film material processing. The ablation and patterning of CNT film are performed by a femtosecond laser with different parameters. An ablation threshold of 25 mJ/cm2 was obtained by investigating the effects of laser pulse energy and pulse number on ablation holes. Raman spectroscopy and scanning electron microscope (SEM) were used to characterize the performance of the pattern groove. The results show that the oligomer in the CNT film was removed by the laser ablation, resulting in an increase in Raman G band intensity. As the laser increased, the ablation of the CNTs was caused by the energy of photons interacting with laser-induced thermal elasticity when the pulse energy was increased enough to destroy the carbon–carbon bonds between different carbon atoms. Impurities and amorphous carbon were found at and near the cut edge while laser cutting at high energy, and considerable distortion and tensile was produced on the edge of the CNTs’ groove. Furthermore, appropriate cutting parameters were obtained without introducing defects and damage to the substrate, which provides a practical method applied to large-area patterning machining of CNT film.

Fabrication of 3D microchannels for tissue engineering in photosensitive glass using NIR femtosecond laser radiation

The biocompatibility of photosensitive glasses allows various biomedical applications; one is the field of tissue engineering and more precisely microengineered tissue-on-a-chip platforms to study the tissue microenvironment and disease modelling. Three dimensional architectures of adapted components are required for modern materials. A photosensitive lithiumalumosilicate glass FS21 was investigated regarding the interaction with a Ti:Sapphire laser systemto build three dimensional buried channels inside the glass. Femtosecond laser radiation with a wavelength of 800 nm and pulse duration of 140 fs was used to modify the glass structure. Subsurface channel geometries were achieved by a subsequent thermal treatment and were formed into capillaries using wet chemical etching of the exposed and crystallised channels. Contrary to ultraviolet (UV) exposure, spectral optical investigations showed that fs laser exposure caused various radiation induced defects in the base glass coupled with the generation of photoelectrons for the photochemical modification of silver ions. We observed an outgassing of different species coming from raw materials of the original glass batch during the glass crystallisation process. Etch rate ratios differ between 1:25 and 1:45 and are dependent on: stoichiometric deviation between surface and bulk, crystal size and distribution and exchange of the etching agent in narrow capillaries.

3D Biomimetic Chips for Cancer Cell Migration in Nanometer-Sized Spaces Using "Ship-in-a-Bottle" Femtosecond Laser Processing

Cancer cells undergo dramatic morphology changes when migrating in confined spaces narrower than their diameter during metastasis, and thus it is necessary to understand the deformation mechanism and associated molecular events in order to study tumor progression. To this end, we propose a new biochip with three-dimensional (3D) polymer nanostructures in a closed glass microfluidic chip. "Ship-in-a-bottle" femtosecond laser processing is an exclusive technique to flexibly create 3D small details in biochips. The wavefront correction by the spatial light modulator significantly improves the fabrication resolution of this technique. The device could then accommodate defect-free 3D biomimetic nanoconfigurations for the evaluation of prostate cancer cell migration in confined spaces. Specifically, polymeric channels with widths of ∼900 nm, which is more than one order of magnitude smaller than the cell size, are integrated by femtosecond laser inside glass channels. The cells are responsive to an in-channel gradient of epidermal growth factor and can migrate a distance greater than 20 μm. After migration, the cells suffer partial cytokinesis, followed by fusion of the divided parts back into single cell bodies.