Fabrication and multifunction integration of microfluidic chips by femtosecond laser direct writing

One-step fabrication of a self-driven point-of-care chip by femtosecond laser direct writing and its application in cancer cell H2O2 detection via semiconductor-based SERS

Self-driven microfluidic systems have attracted significant attention and demonstrated great potential in the field of point-of-care (POC) testing due to their device simplicity, low power consumption, increased portability, and reduced sample consumption. To develop POC detection chips with diverse characteristics that meet different requirements, there is a strong demand for feasible strategies that enable easy operation and reduce processing time. Here, a one-step processing approach using femtosecond laser direct writing technology was proposed to fabricate a capillary-actuated POC microfluidic chip. The driving force of the chip is highly dependent on its surface wettability, which can be easily adjusted by changing the laser processing parameters. This POC microfluidic chip allowed for the detection of intracellular H2O2 through a catalytic reaction system that incorporated 5-aminosalicylic acid -sensitized colloidal TiO2 nanoparticles and horse radish peroxidase, with integrating semiconductor-based surface-enhanced Raman scattering (SERS) quantitative technique. The concentration of H2O2 was determined by the SERS signal of the catalytic products in the microfluidic chip, resulting in rapid detection with minimal sample consumption. Our method provides a simple, feasible, and alternative strategy for POC testing of H2O2, with a linear range of 10-2∼10-6 M and a limit of detection of 0.55 μM. This approach was successfully applied to rapid detection of intracellular H2O2 in MCF-7 breast cancer cells with high sensitivity and minimal sample consumption. Additionally, this study not only demonstrates the exceptional advantages of femtosecond laser processing technology in fabricating diverse microfluidic chips for various applications, but also presents an efficient POC testing strategy for detecting cell signaling molecules.

Printing 3D Metallic Structures in Porous Matrix

The fabrication of metallic micro/nanostructures has great potential for advancing optoelectronic microdevices. Over the past decade, femtosecond laser direct writing (FsLDW) technology has played a crucial role in driving progress in this field. In this study, silica gel glass is used as a supporting medium, and FsLDW is employed to reduce gold and palladium ions using 7-Diethylamino-3-thenoylcoumarin (DETC) as a two-photon sensitizer, enabling the printing of conductive multilayered and 3D metallic structures. How the pore size of the silica gel glass affects the electrical conductivity of printed metal wires is systematically examined. This 3D printing method is versatile and offers expanded opportunities for applying metallic micro/nanostructures in optoelectronic devices.

Rapid Fabrication of High-Performance Flexible Pressure Sensors Using Laser Pyrolysis Direct Writing

The fabrication of flexible pressure sensors with low cost, high scalability, and easy fabrication is an essential driving force in developing flexible electronics, especially for high-performance sensors that require precise surface microstructures. However, optimizing complex fabrication processes and expensive microfabrication methods remains a significant challenge. In this study, we introduce a laser pyrolysis direct writing technology that enables rapid and efficient fabrication of high-performance flexible pressure sensors with a micro-truncated pyramid array. The pressure sensor demonstrates exceptional sensitivities, with the values of 3132.0, 322.5, and 27.8 kPa-1 in the pressure ranges of 0-0.5, 0.5-3.5, and 3.5-10 kPa, respectively. Furthermore, the sensor exhibits rapid response times (loading: 22 ms, unloading: 18 ms) and exceptional reliability, enduring over 3000 pressure loading and unloading cycles. Moreover, the pressure sensor can be easily integrated into a sensor array for spatial pressure distribution detection. The laser pyrolysis direct writing technology introduced in this study presents a highly efficient and promising approach to designing and fabricating high-performance flexible pressure sensors utilizing micro-structured polymer substrates.

Magnetically actuated glaucoma drainage device for regulating intraocular pressure after implantation

The key risk factor for glaucoma is increased intraocular pressure (IOP). Glaucoma drainage devices implanted in the eye can reduce IOP and thus stop disease progression. However, most devices currently used in clinical practice are passive and do not allow for postsurgical IOP control, which may result in serious complications such as hypotony (i.e., excessively low IOP). To enable noninvasive IOP control, we demonstrate a novel, miniature glaucoma implant that will enable the repeated adjustment of the hydrodynamic resistance after implantation. This is achieved by integrating a magnetic microvalve containing a micropencil-shaped plug that is moved using an external magnet, thereby opening or closing fluidic channels. The microplug is made from biocompatible poly(styrene-block-isobutylene-block-styrene) (SIBS) containing iron microparticles. The complete implant consists of an SIBS drainage tube and a housing element containing the microvalve and fabricated with hot embossing using femtosecond laser-machined glass molds. Using in vitro and ex vivo microfluidic experiments, we demonstrate that when the microvalve is closed, it can provide sufficient hydrodynamic resistance to overcome hypotony. Valve function is repeatable and stable over time. Due to its small size, our implant is a promising, safe, easy-to-implant, minimally invasive glaucoma surgery device.

Two Functions from a Single Photoresist: Tuning Microstructure Degradability from Light-Stabilized Dynamic Materials

A photoresist-based on a light-stabilized dynamic material driven by an out-of-equilibrium photo-Diels-Alder reaction of triazolinediones with naphthalenes-whose ability to intrinsically degrade postprinting can be tuned by a simple adjustment of laser intensity during 3D laser lithography is introduced. The resist's ability to form stable networks under green light irradiation that degrade in the dark is transformed into a tunable degradable 3D printing material platform. In-depth characterization of the printed microstructures via atomic force microscopy before and during degradation reveals the high dependency of the final structures' properties on the writing parameters. Upon identifying the ideal writing parameters and their effect on the network structure, it is possible to selectively toggle between stable and fully degradable structures. This simplifies the direct laser writing manufacturing process of multifunctional materials significantly, which typically requires the use of separate resists and consecutive writing efforts to achieve degradable and nondegradable material sections.

Two-Photon Polymerization: Fundamentals, Materials, and Chemical Modification Strategies

Two-photon polymerization (TPP) has become a premier state-of-the-art method for microscale fabrication of bespoke polymeric devices and surfaces. With applications ranging from the production of optical, drug delivery, tissue engineering, and microfluidic devices, TPP has grown immensely in the past two decades. Significantly, the field has expanded from standard acrylate- and epoxy-based photoresists to custom formulated monomers designed to change the hydrophilicity, surface chemistry, mechanical properties, and more of the resulting structures. This review explains the essentials of TPP, from its initial conception through to standard operating principles and advanced chemical modification strategies for TPP materials. At the outset, the fundamental chemistries of radical and cationic polymerization are described, along with strategies used to tailor mechanical and functional properties. This review then describes TPP systems and introduces an array of commonly used photoresists including hard polyacrylic resins, soft hydrogel acrylic esters, epoxides, and organic/inorganic hybrid materials. Specific examples of each class-including chemically modified photoresists-are described to inform the understanding of their applications to the fields of tissue-engineering scaffolds, micromedical, optical, and drug delivery devices.

Femtosecond Laser Micromachining of the Mask for Acoustofluidic Device Preparation

Surface acoustic wave (SAW)-based acoustofluidic devices have shown broad applications in microfluidic actuation and particle/cell manipulation. Conventional SAW acoustofluidic device fabrication generally includes photolithography and lift-off processes and thus requires accessing cleanroom facilities and expensive lithography equipment. In this paper, we report a femtosecond laser direct writing mask method for acoustofluidic device preparation. By micromachining of steel foil to form the mask and direct evaporation of metal on the piezoelectric substrate using the mask, the interdigital transducer (IDT) electrodes of the SAW device are generated. The minimum spatial periodicity of the IDT finger is about 200 μm, and the preparation for LiNbO₃ and ZnO thin films and flexible PVDF SAW devices is verified. Meanwhile, we have demonstrated various microfluidic functions, including streaming, concentration, pumping, jumping, jetting, nebulization, and particle alignment using the fabricated acoustofluidic (ZnO/Al plate, LiNbO₃) devices. Compared to the traditional manufacturing process, the proposed method omits spin coating, drying, lithography, developing, and lift-off processes and thus has advantages of simple, convenient, low cost, and environment friendliness.

3D printing of hollow geometries using blocking liquid substitution stereolithography

Micrometer scale arbitrary hollow geometries within a solid are needed for a variety of applications including microfluidics, thermal management and metamaterials. A major challenge to 3D printing hollow geometries using stereolithography is the ability to retain empty spaces in between the solidified regions. In order to prevent unwanted polymerization of the trapped resin in the hollow spaces-known as print-through-significant constraints are generally imposed on the primary process parameters such as resin formulation, exposure conditions and layer thickness. Here, we report on a stereolithography process which substitutes the trapped resin with a UV blocking liquid to mitigate print-through. We investigate the mechanism of the developed process and determine guidelines for the formulation of the blocking liquid. The reported method decouples the relationship between the primary process parameters and their effect on print-through. Without having to optimize the primary process parameters to reduce print-through, hollow heights that exceed the limits of conventional stereolithography can be realized. We demonstrate fabrication of a variety of complex hollow geometries with cross-sectional features ranging from tens of micrometer to hundreds of micrometers in size. With the framework presented, this method may be employed for 3D printing functional hollow geometries for a variety of applications, and with improved freedom over the printing process (e.g. material choices, speed and resulting properties of the printed parts).

Visible-Light-Degradable 3D Microstructures in Aqueous Environments

The additive manufacturing technique direct laser writing (DLW), also known as two-photon laser lithography, is becoming increasingly established as a technique capable of fabricating functional 3D microstructures. Recently, there has been an increasing effort to impart microstructures fabricated using DLW with advanced functionalities by introducing responsive chemical entities into the underpinning photoresists. Herein, a novel photoresist based on the photochemistry of the bimane group is introduced that can be degraded upon exposure to very mild conditions, requiring only water and visible light (λ_max  = 415-435 nm) irradiation. The degradation of the microstructures is tracked and quantified using AFM measurements of their height. The influence of the writing parameters as well as the degradation conditions is investigated, unambiguously evidencing effective visible light degradation in aqueous environments. Finally, the utility of the photodegradable resist system is demonstrated by incorporating it into multimaterial 3D microstructures, serving as a model for future applications.

Femtosecond Laser-Fabricated Photonic Chips for Optical Communications: A Review

Integrated optics, having the unique properties of small size, low loss, high integration, and high scalability, is attracting considerable attention and has found many applications in optical communications, fulfilling the requirements for the ever-growing information rate and complexity in modern optical communication systems. Femtosecond laser fabrication is an acknowledged technique for producing integrated photonic devices with unique features, such as three-dimensional fabrication geometry, rapid prototyping, and single-step fabrication. Thus, plenty of femtosecond laser-fabricated on-chip devices have been manufactured to realize various optical communication functions, such as laser generation, laser amplification, laser modulation, frequency conversion, multi-dimensional multiplexing, and photonic wire bonding. In this paper, we review some of the most relevant research progress in femtosecond laser-fabricated photonic chips for optical communications, which may break new ground in this area. First, the basic principle of femtosecond laser fabrication and different types of laser-inscribed waveguides are briefly introduced. The devices are organized into two categories: active devices and passive devices. In the former category, waveguide lasers, amplifiers, electric-optic modulators, and frequency converters are reviewed, while in the latter, polarization multiplexers, mode multiplexers, and fan-in/fan-out devices are discussed. Later, photonic wire bonding is also introduced. Finally, conclusions and prospects in this field are also discussed.

Fabrication of a thin PDMS film with complex liquid metal electrodes embedded and its application as skin sensors

Ultra-thin PDMS films with complex liquid metal patterns embedded. The thickness is as thin as 119 μm.

Hand-Powered Inertial Microfluidic Syringe-Tip Centrifuge

Conventional sample preparation techniques require bulky and expensive instruments and are not compatible with next-generation point-of-care diagnostic testing. Here, we report a manually operated syringe-tip inertial microfluidic centrifuge (named i-centrifuge) for high-flow-rate (up to 16 mL/min) cell concentration and experimentally demonstrate its working mechanism and performance. Low-cost polymer films and double-sided tape were used through a rapid nonclean-room process of laser cutting and lamination bonding to construct the key components of the i-centrifuge, which consists of a syringe-tip flow stabilizer and a four-channel paralleled inertial microfluidic concentrator. The unstable liquid flow generated by the manual syringe was regulated and stabilized with the flow stabilizer to power inertial focusing in a four-channel paralleled concentrator. Finally, we successfully used our i-centrifuge for manually operated cell concentration. This i-centrifuge offers the advantages of low device cost, simple hand-powered operation, high-flow-rate processing, and portable device volume. Therefore, it holds potential as a low-cost, portable sample preparation tool for point-of-care diagnostic testing.

Complex 3D microfluidic architectures formed by mechanically guided compressive buckling

Microfluidic technologies have wide-ranging applications in chemical analysis systems, drug delivery platforms, and artificial vascular networks. This latter area is particularly relevant to 3D cell cultures, engineered tissues, and artificial organs, where volumetric capabilities in fluid distribution are essential. Existing schemes for fabricating 3D microfluidic structures are constrained in realizing desired layout designs, producing physiologically relevant microvascular structures, and/or integrating active electronic/optoelectronic/microelectromechanical components for sensing and actuation. This paper presents a guided assembly approach that bypasses these limitations to yield complex 3D microvascular structures from 2D precursors that exploit the full sophistication of 2D fabrication methods. The capabilities extend to feature sizes <5 μm, in extended arrays and with various embedded sensors and actuators, across wide ranges of overall dimensions, in a parallel, high-throughput process. Examples include 3D microvascular networks with sophisticated layouts, deterministically designed and constructed to expand the geometries and operating features of artificial vascular networks.

Dual Channel Microfluidics for Mimicking the Blood-Brain Barrier

High-resolution imaging is essential for analysis of the steps and way stations of cargo transport in in vitro models of the endothelium. In this study, we demonstrate a microfluidic system consisting of two channels horizontally separated by a cell-growth-promoting membrane. Its design allows for high-resolution (down to single-molecule level) imaging using a high numerical aperture objective with a short working distance. To reduce optical aberrations and enable single-molecule-sensitive imaging, an observation window was constructed in the membrane via laser cutting with subsequent structuring using 3D multiphoton lithography for improved cell growth. The upper channel was loaded with endothelial cells under flow conditions, which showed polarization and junction formation. A coculture of human vascular endothelial cells with pericytes was developed that mimics the blood-brain barrier. Finally, this dual channel microfluidics system enabled 3D localization microscopy of the cytoskeleton and 3D single-molecule-sensitive tracing of lipoprotein particles.

Surface Wettability and Electrical Resistance Analysis of Droplets on Indium-Tin-Oxide Glass Fabricated Using an Ultraviolet Laser System

Indium tin oxide (ITO) is widely used as a substrate for fabricating chips because of its optical transparency, favorable chemical stability, and high electrical conductivity. However, the wettability of ITO surface is neutral (the contact angle was approximately 90°) or hydrophilic. For reagent transporting and manipulation in biochip application, the surface wettability of ITO-based chips was modified to the hydrophobic or nearly hydrophobic surface to enable their use with droplets. Due to the above demand, this study used a 355-nm ultraviolet laser to fabricate a comb microstructure on ITO glass to modify the surface wettability characteristics. All of the fabrication patterns with various line width and pitch, depth, and surface roughness were employed. Subsequently, the contact angle (CA) of droplets on the ITO glass was analyzed to examine wettability and electrical performance by using the different voltages applied to the electrode. The proposed approach can succeed in the fabrication of a biochip with suitable comb-microstructure by using the optimal operating voltage and time functions for the catch droplets on ITO glass for precision medicine application. The experiment results indicated that the CA of droplets under a volume of 20 μL on flat ITO substrate was approximately 92° ± 2°; furthermore, due to its lowest surface roughness, the pattern line width and pitch of 110 μm exhibited a smaller CA variation and more favorable spherical droplet morphology, with a side and front view CA of 83° ± 1° and 78.5° ± 2.5°, respectively, while a laser scanning speed of 750 mm/s was employed. Other line width and pitch, as well as scanning speed parameters, increased the surface roughness and resulted in the surface becoming hydrophilic. In addition, to prevent droplet morphology collapse, the droplet’s electric operation voltage and driving time did not exceed 5 V and 20 s, respectively. With this method, the surface modification process can be employed to control the droplet’s CA by adjusting the line width and pitch and the laser scanning speed, especially in the neutral or nearly hydrophobic surface for droplet transporting. This enables the production of a microfluidic chip with a surface that is both light transmittance and has favorable electrical conductivity. In addition, the shape of the microfluidic chip can be directly designed and fabricated using a laser direct writing system on ITO glass, obviating the use of a mask and complicated production processes in biosensing and biomanipulation applications.

DNA Printing Integrated Multiplexer Driver Microelectronic Mechanical System Head (IDMH) and Microfluidic Flow Estimation

The system designed in this study involves a three-dimensional (3D) microelectronic mechanical system chip structure using DNA printing technology. We employed diverse diameters and cavity thickness for the heater. DNA beads were placed in this rapid array, and the spray flow rate was assessed. Because DNA cannot be obtained easily, rapidly deploying DNA while estimating the total amount of DNA being sprayed is imperative. DNA printings were collected in a multiplexer driver microelectronic mechanical system head, and microflow estimation was conducted. Flow-3D was used to simulate the internal flow field and flow distribution of the 3D spray room. The simulation was used to calculate the time and pressure required to generate heat bubbles as well as the corresponding mean outlet speed of the fluid. The "outlet speed status" function in Flow-3D was used as a power source for simulating the ejection of fluid by the chip nozzle. The actual chip generation process was measured, and the starting voltage curve was analyzed. Finally, experiments on flow rate were conducted, and the results were discussed. The density of the injection nozzle was 50, the size of the heater was 105 μm × 105 μm, and the size of the injection nozzle hole was 80 μm. The maximum flow rate was limited to approximately 3.5 cc. The maximum flow rate per minute required a power between 3.5 W and 4.5 W. The number of injection nozzles was multiplied by 100. On chips with enlarged injection nozzle density, experiments were conducted under a fixed driving voltage of 25 V. The flow curve obtained from various pulse widths and operating frequencies was observed. The operating frequency was 2 KHz, and the pulse width was 4 μs. At a pulse width of 5 μs and within the power range of 4.3-5.7 W, the monomer was injected at a flow rate of 5.5 cc/min. The results of this study may be applied to estimate the flow rate and the total amount of the ejection liquid of a DNA liquid.

Bioinspired Surfaces With Switchable Wettability

The surface wettability of plants exhibits many unique advantages, which enhances the environmental adaptability of plants. In view of the rapid development of responsive materials, smart surfaces have been explored extensively to regulate surface wettability through external stimuli. Herein, we summarized recent advancements in bioinspired surfaces with switchable wettability. Typical bioinspired surfaces with switchable wettability and their emerging applications have been reviewed. In the end, we have discussed the remaining challenges and provided perspective on future development.

Deformation Behavior of Glass Nanostructures in Hot Embossing

In the nanoscale glass formation, the flow and deformation behavior of glass materials are quite different from those in the macroscale because the mold cavity influences the viscous flow behaviors of glass because of the size effect. The knowledge of macroglass molding process no longer applies to the fabrication of glass microparts by hot embossing. To investigate the size effect of the mold cavity on glass flow behavior during squeeze flow, patterned molds with different length scales and shapes were used for glass embossing. The experimental results demonstrated that glass structures with ultrafine and atomic scale surface could be fabricated by using precision embossing. The nanostructures of embossed glass at 100 and 500 nm wide cavity were found to exhibit nanoscale effect during squeeze flow. Molecular confinement accelerates the tectonic deformation of embossed glass at smaller length scales. At the microscale filling, the tectonic deformation of embossed glass is mainly dominated by elastic recovery, surface tension, hydrostatic pressure, and viscous flow. As the length scale reduces to submicron, the dual-peak filling mode gradually transfers to the single-peak filling mode. Additionally, deformation modes have little influence on the shapes of the mold cavity. This work sheds light on the fabrication of glass nano/microstructures.

Microfluidic polyimide gas dynamic virtual nozzles for serial crystallography

Free liquid jets are a common sample delivery method in serial femtosecond x-ray (SFX) crystallography. Gas dynamic virtual nozzles (GDVNs) use an outer gas stream to focus a liquid jet down to a few micrometers in diameter. Such nozzles can be fabricated through various methods (capillary grinding, soft lithography, digital light processing, and two-photon polymerization) and materials, such as glass, polydimethylsiloxane, and photosensitive polyacrylates. Here, we present a broadly accessible, rapid prototyping laser ablation approach to micromachine solvent-resistant and inert Kapton polyimide foils with highly reproducible geometric features that result in 3D flow-focused GDVNs suitable for crystallography experiments at synchrotrons and free-electron laser facilities.

Shining Light on Poly(ethylene glycol): From Polymer Modification to 3D Laser Printing of Water Erasable Microstructures

The implementation of stimuli-responsive bonds into 3D network assemblies is a key concept to design adaptive materials that can reshape and degrade. Here, a straightforward but unique photoresist is introduced for the tailored fabrication of poly(ethylene glycol) (PEG) materials that can be readily erased by water, even without the need for acidic or basic additives. Specifically, a new class of photoresist is developed that operates through the backbone crosslinking of PEG when irradiated in the presence of a bivalent triazolinedione. Hence, macroscopic gels are obtained upon visible light-emitting diode irradiation (λ > 515 nm) that are stable in organic media but rapidly degrade upon the addition of water. Photoinduced curing is also applicable to multiphoton laser lithography (λ > 700 nm), hence providing access to 3D printed microstructures that vanish when immersed in water at 37 °C. Materials with varying crosslinking densities are accessed by adapting the applied laser writing power, thereby allowing for tunable hydrolytic erasing timescales. A new platform technology is thus presented that enables the crosslinking and 3D laser printing of PEG-based materials, which can be cleaved and erased in water, and additionally holds potential for the facile modification and backbone degradation of polyether-containing materials in general.

Digital Manufacturing for Microfluidics

The microfluidics field is at a critical crossroads. The vast majority of microfluidic devices are presently manufactured using micromolding processes that work very well for a reduced set of biocompatible materials, but the time, cost, and design constraints of micromolding hinder the commercialization of many devices. As a result, the dissemination of microfluidic technology-and its impact on society-is in jeopardy. Digital manufacturing (DM) refers to a family of computer-centered processes that integrate digital three-dimensional (3D) designs, automated (additive or subtractive) fabrication, and device testing in order to increase fabrication efficiency. Importantly, DM enables the inexpensive realization of 3D designs that are impossible or very difficult to mold. The adoption of DM by microfluidic engineers has been slow, likely due to concerns over the resolution of the printers and the biocompatibility of the resins. In this article, we review and discuss the various printer types, resolution, biocompatibility issues, DM microfluidic designs, and the bright future ahead for this promising, fertile field.

Periodic Microstructures Fabricated by Laser Interference with Subsequent Etching

Periodic nanostructures have wide applications in micro-optics, bionics, and optoelectronics. Here, a laser interference with subsequent etching technology is proposed to fabricate uniform periodic nanostructures with controllable morphologies and smooth surfaces on hard materials. One-dimensional microgratings with controllable periods (1, 2, and 3 μm) and heights, from dozens to hundreds of nanometers, and high surface smoothness are realized on GaAs by the method. The surface roughness of the periodic microstructures is significantly reduced from 120 nm to 40 nm with a subsequent inductively coupled plasma (ICP) etching. By using laser interference with angle-multiplexed exposures, two-dimensional square- and hexagonal-patterned microstructures are realized on the surface of GaAs. Compared with samples without etching, the diffraction efficiency can be significantly enhanced for samples with dry etching, due to the improvement of surface quality.

A self-driven microfluidic surface-enhanced Raman scattering device for Hg2+ detection fabricated by femtosecond laser

In this paper, we proposed a novel approach for rapid and flexible fabrication of self-driven microfluidic surface enhanced Raman scattering (SERS) chips for quantitative analysis of Hg²⁺ by femtosecond laser direct writing. In contrast to traditional microfluidic chips, the microchannels of the device can drive a liquid sample flow without external driving force. The sample flow speed is tunable since the wettability and capillarity properties of the channels, which depend on the roughness and the inner diameter of the microchannels, can be controlled by optimizing the laser processing parameters. The SERS active detection sites, which exhibit high enhancement effects and fine reproducibility, were integrated through the femtosecond laser-induced periodic surface structures (LIPSS), followed by 30 nm Ag deposition. The SERS performance of the as-prepared microfluidic SERS detection chip was studied with R6G as probe molecules. The quantitative analysis of Hg²⁺ was realized by simply injecting the Hg²⁺ sample and the probe molecules R6G from the two inlets, separately, and collecting the SERS signal at the detection site. The lowest detection limit for Hg²⁺ is 10^-9 M. It should be mentioned that this study is not only limited to Hg²⁺ quantitative analysis, but is also mainly aimed to develop a new technique for the design and fabrication of novel self-driven microfluidic devices depending on practical application requirements.

Polymers for Regenerative Medicine Structures Made via Multiphoton 3D Lithography

Multiphoton 3D lithography is becoming a tool of choice in a wide variety of fields. Regenerative medicine is one of them. Its true 3D structuring capabilities beyond diffraction can be exploited to produce structures with diverse functionality. Furthermore, these objects can be produced from unique materials allowing expanded performance. Here, we review current trends in this research area. We pay particular attention to the interplay between the technology and materials used. Thus, we extensively discuss undergoing light-matter interactions and peculiarities of setups needed to induce it. Then, we continue with the most popular resins, photoinitiators, and general material functionalization, with emphasis on their potential usage in regenerative medicine. Furthermore, we provide extensive discussion of current advances in the field as well as prospects showing how the correct choice of the polymer can play a vital role in the structure’s functionality. Overall, this review highlights the interplay between the structure’s architecture and material choice when trying to achieve the maximum result in the field of regenerative medicine.

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.

Tailoring the Mechanical Properties of 3D Microstructures Using Visible Light Post-Manufacturing

The photochemistry of anthracene, a new class of photoresist for direct laser writing, is used to enable visible-light-gated control over the mechanical properties of 3D microstructures post-manufacturing. The mechanical and viscoelastic properties (hardness, complex elastic modulus, and loss factor) of the microstructures are measured over the course of irradiation via dynamic mechanical analysis on the nanoscale. Irradiation of the microstructures leads to a strong hardening and stiffening effect due to the generation of additional crosslinks through the photodimerization of the anthracene functionalities. A relationship between the loss of fluorescence-a consequence of the photodimerization-and changes in the mechanical properties is established. The fluorescence thus serves as a proxy read-out for the mechanical properties. These photoresponsive microstructures can potentially be used as "mechanical blank slates": their mechanical properties can be readily adjusted using visible light to serve the demands of different applications and read out using their fluorescence.

3D printed nozzles on a silicon fluidic chip

Serial femtosecond crystallography is a new method for protein structure determination utilizing intense and destructive X-ray pulses generated by free-electron lasers. The approach requires the means to deliver hydrated protein crystals to a focused X-ray beam and replenish them at the repetition rate of the pulses. A liquid-jet sample delivery system where a gas dynamic virtual nozzle is printed directly on a silicon-glass microfluidic chip using a 2-photon-polymerization 3D printing process is implemented. This allows for rapid prototyping and high-precision production of nozzles to suit the characteristics of a particular sample and opens up the possibility for high-throughput and versatile sample delivery systems that can integrate microfluidic components for sample detection, characterisation, or control. With the hybrid system described here, stable liquid jets with diameters between 1.5 µm at liquid flow rate of 1.5 µl/min and more than 20 µm at liquid flow rate of 100 µl/min under atmospheric and vacuum conditions are generated. The combination of 2D lithography with direct 3D printing may streamline the integration of free-form-features and also facilitate scale-up production of such integrated microfluidic devices that may be useful in many other applications such as flow cytometry and optofluidics.

Multilayered skyscraper microchips fabricated by hybrid "all-in-one" femtosecond laser processing

Multilayered microfluidic channels integrated with functional microcomponents are the general trend of future biochips, which is similar to the history of Si-integrated circuits from the planer to the three-dimensional (3D) configuration, since they offer miniaturization while increasing the integration degree and diversifying the applications in the reaction, catalysis, and cell cultures. In this paper, an optimized hybrid processing technology is proposed to create true multilayered microchips, by which "all-in-one" 3D microchips can be fabricated with a successive procedure of 3D glass micromachining by femtosecond-laser-assisted wet etching (FLAE) and the integration of microcomponents into the fabricated microchannels by two-photon polymerization (TPP). To create the multilayered microchannels at different depths in glass substrates (the top layer was embedded at 200 μm below the surface, and the underlying layers were constructed with a 200-μm spacing) with high uniformity and quality, the laser power density (13~16.9 TW/cm²) was optimized to fabricate different layers. To simultaneously complete the etching of each layer, which is also important to ensure the high uniformity, the control layers (nonlaser exposed regions) were prepared at the upper ends of the longitudinal channels. Solvents with different dyes were used to verify that each layer was isolated from the others. The high-quality integration was ensured by quantitatively investigating the experimental conditions in TPP, including the prebaking time (18~40 h), laser power density (2.52~2.94 TW/cm²) and developing time (0.8~4 h), all of which were optimized for each channel formed at different depths. Finally, the eight-layered microfluidic channels integrated with polymer microstructures were successfully fabricated to demonstrate the unique capability of this hybrid technique.

Alleviating Distortion and Improving the Young's Modulus in Two-Photon Polymerization Fabrications

Two-photon polymerization enables the extremely high resolution three-dimensional printing of micro-structures. To know the mechanical properties, and better still, to be able to adjust them is of paramount importance to ensuring the proper structural integrity of the printed products. In this work, the Young’s modulus is measured on two-photon polymerized micro-cantilever bars. Optimizing the scanning trajectory of the laser focus points is important in alleviating distortion of the printed bars. By increasing the laser power and decreasing the inter-voxel distances we can double the Young’s modulus. Post-curing with ultraviolet light can approximately quadruple the Young’s modulus. However, the resulting modulus is still only about 0.3% of that of the bulk polymerized material.

Femtosecond laser induced selective etching in fused silica: optimization of the inscription conditions with a high-repetition-rate laser source

Femtosecond laser induced selective etching (FLISE) of dielectric materials is a promising technique for fabricating various microfluidic devices. Here we experimentally studied the dependence of the selective etching speed in fused silica glass on laser pulse energy, repetition rate, and inscription speed using a 1030 nm femtosecond laser. The evolution of micromorphology of the laser inscribed lines was revealed with optical microscopy, scanning electron microscopy, as well as anisotropic diffraction of the optical gratings formed by these inscribed lines. A single pulse energy threshold is required to initiate the FLISE. Further, a laser repetition rate window between an upper threshold and a lower threshold was observed, which were limited by the thermal-induced disruption of the nanogratings and by the disconnection of successive pulses modified spots respectively. The synergetic influences of the above factors were evaluated by the exposure laser energy density, which shows a common threshold for different inscription conditions and demonstrates itself to be an excellent criterion for choosing appropriate parameters in FLISE. The formation of continuous nanogratings is confirmed to be the major mechanism of FLISE in fused silica. Our observations not only help one to understand the micro mechanism in FLISE of fused silica, but also are of great use for fabricating large-scale microfluidic circuits.

High signal-to-noise ratio ultra-compact lab-on-a-chip microflow cytometer enabled by silicon optical antennas

We experimentally demonstrate an all-silicon nanoantenna-based micro-optofluidic cytometer showing a combination of high signal-to-noise ratio (SNR) > 14 dB and ultra-compact size. Thanks to the ultra-high directivity of the antennas (>150), which enables a state-of-the-art sub-micron resolution, we are able to avoid the use of the bulky devices typically employed to collimate light on chip (such as lenses or fibers). The nm-scale antenna cross section allows a dramatic reduction of the optical system footprint, from the mm-scale of previous approaches to a few µm², yielding a notable reduction in the fabrication costs. This scheme paves the way to ultra-compact lab-on-a-chip devices that may enable new applications with potential impact on all branches of biological and health science.

Microfluidic Contact Lenses

Contact lens is a ubiquitous technology used for vision correction and cosmetics. Sensing in contact lenses has emerged as a potential platform for minimally invasive point-of-care diagnostics. Here, a microlithography method is developed to fabricate microconcavities and microchannels in a hydrogel-based contact lens via a combination of laser patterning and embedded templating. Optical microlithography parameters influencing the formation of microconcavities including ablation power (4.3 W) and beam speed (50 mm s^-1 ) are optimized to control the microconcavity depth (100 µm) and diameter (1.5 mm). The fiber templating method allows the production of microchannels having a diameter range of 100-150 µm. Leak-proof microchannel and microconcavity connections in contact lenses are validated through flow testing of artificial tear containing fluorescent microbeads (Ø = 1-2 µm). The microconcavities of contact lenses are functionalized with multiplexed fluorophores (2 µL) to demonstrate optical excitation and emission capability within the visible spectrum. The fabricated microfluidic contact lenses may have applications in ophthalmic monitoring of metabolic disorders at point-of-care settings and controlled drug release for therapeutics.

Arch-like microsorters with multi-modal and clogging-improved filtering functions by using femtosecond laser multifocal parallel microfabrication

Conventional micropore membranes based size sorting have been widely applied for single-cell analysis. However, only a single filtering size can be achieved and the clogging issue cannot be completely avoided. Here, we propose a novel arch-like microsorter capable of multimodal (high-, band- and low-capture mode) sorting of particles. The target particles can pass through the front filter and are then trapped by the back filter, while the non-target particles can bypass or pass through the microsorter. This 3D arch-like microstructures are fabricated inside a microchannel by femtosecond laser parallel multifocal scanning. The designed architecture allows for particles isolation free of clogging over 20 minutes. Finally, as a proof of concept demonstration, SUM159 breast cancer cells are successfully separated from whole blood.

Bifunctional plasmonic-magnetic particles for an enhanced microfluidic SERS immunoassay

Surface-Enhanced Raman Scattering (SERS) is emerging as a promising strategy for the quantification of immunoglobulin G (IgG) due to its inherent high sensitivity and specificity; however, it remains challenging to integrate SERS detection with a microfluidic system in a simple, efficient and low-cost manner. Here, we report on a novel bifunctional plasmonic-magnetic particle-based immunoassay, in which plasmonic nanoparticles act as soluble SERS immunosubstrates, whereas magnetic particles are for promoting micromixing in a microfluidic chip. With this novel SERS immunosubstrate in conjunction with the unique microfluidic system, we could substantially reduce the assay time from 4 hours to 80 minutes as well as enhance the detection specificity by about 70% in comparison to a non-microfluidic immunoassay. Compared to previous microfluidic SERS systems, our strategy offers a simple microfluidic chip design with only one well for mixing, washing and detection.

Self-Sealed Bionic Long Microchannels with Thin Walls and Designable Nanoholes Prepared by Line-Contact Capillary-Force Assembly

Long microchannels with thin walls, small width, and nanoholes or irregular shaped microgaps, which are similar to capillaries or cancerous vessels, are urgently needed to simulate the physiological activities in human body. However, the fabrication of such channels remains challenging. Here, microchannels with designable holes are manufactured by combining laser printing with line-contact capillary-force assembly. Two microwalls are first printed by femtosecond laser direct-writing, and subsequently driven to collapse into a channel by the capillary force that arises in the evaporation of developer. The channel can remain stable in solvent due to the enhanced Van der Waals' force caused by the line-contact of microwalls. Microchannels with controllable nanoholes and almost arbitrary patterns can be fabricated without any bonding or multistep processes. As-prepared microchannels, with wall thicknesses less than 1 µm, widths less than 3 µm, lengths more than 1 mm, are comparable with human capillaries. In addition, the prepared channels also exhibit the ability to steer the flow of liquid without any external pump.

Underwater Transparent Miniature "Mechanical Hand" Based on Femtosecond Laser-Induced Controllable Oil-Adhesive Patterned Glass for Oil Droplet Manipulation

Development of underwater superoleophobic surfaces has captured the imagination of researchers because of their applications; especially, oil manipulation based on such surfaces has attracted much attention. Here, we show a simple and effective way to fabricate an underwater transparent miniature "mechanical hand" based on controllable oil-adhesive patterned glass using a femtosecond laser. The underwater oil-adhesive force of the patterned glasses that compose the "mechanical hand" device can be controlled from ultralow to ultrahigh by adjusting the ratio of the untreated flat glass area to the laser-ablated rough area. These surfaces also showed favorable transparency in water. Various oils such as chloroform, hexadecane, n-dodecane, decane, liquid paraffin, and petroleum ether were tested, and their repellency against the as-prepared surfaces in water medium was confirmed. Moreover, the "mechanical hand" was used to implement oil transportation, fusion, and rapid capture, which can be applied in the construction of microfluidic devices, in situ detectors, and bioreactors.

Optical screw-wrench for microassembly

For future micro- and nanotechnologies, the manufacturing of miniaturized, functionalized, and integrated devices is indispensable. In this paper, an assembly technique based on a bottom-up strategy that enables the manufacturing of complex microsystems using only optical methods is presented. A screw connection is transferred to the micrometer range and used to assemble screw- and nut-shaped microcomponents. Micro-stereolithography is performed by means of two-photon polymerization, and microstructures are fabricated and subsequently trapped, moved, and screwed together using optical forces in a holographic optical tweezer set-up. The design and construction of interlocking microcomponents and the verification of a stable and releasable joint form the main focus of this paper. The assembly technique is also applied to a microfluidic system to enable the pumping or intermixing of fluids on a microfluidic chip. This strategy not only enables the assembly of microcomponents but also the combination of different materials and features to form complex hybrid microsystems.

Performance Characterization of an xy-Stage Applied to Micrometric Laser Direct Writing Lithography

This article concerns the characterization of the stability and performance of a motorized stage used in laser direct writing lithography. The system was built from commercial components and commanded by G-code. Measurements use a pseudo-periodic-pattern (PPP) observed by a camera and image processing is based on Fourier transform and phase measurement methods. The results report that the built system has a stability against vibrations determined by peak-valley deviations of 65 nm and 26 nm in the x and y directions, respectively, with a standard deviation of 10 nm in both directions. When the xy-stage is in movement, it works with a resolution of 0.36 µm, which is an acceptable value for most of research and development (R and D) microtechnology developments in which the typical feature size used is in the micrometer range.

Slanted channel microfluidic chip for 3D fluorescence imaging of cells in flow

Three-dimensional cellular imaging techniques have become indispensable tools in biological research and medical diagnostics. Conventional 3D imaging approaches employ focal stack collection to image different planes of the cell. In this work, we present the design and fabrication of a slanted channel microfluidic chip for 3D fluorescence imaging of cells in flow. The approach employs slanted microfluidic channels fabricated in glass using ultrafast laser inscription. The slanted nature of the microfluidic channels ensures that samples come into and go out of focus, as they pass through the microscope imaging field of view. This novel approach enables the collection of focal stacks in a straight-forward and automated manner, even with off-the-shelf microscopes that are not equipped with any motorized translation/rotation sample stages. The presented approach not only simplifies conventional focal stack collection, but also enhances the capabilities of a regular widefield fluorescence microscope to match the features of a sophisticated confocal microscope. We demonstrate the retrieval of sectioned slices of microspheres and cells, with the use of computational algorithms to enhance the signal-to-noise ratio (SNR) in the collected raw images. The retrieved sectioned images have been used to visualize fluorescent microspheres and bovine sperm cell nucleus in 3D while using a regular widefield fluorescence microscope. We have been able to achieve sectioning of approximately 200 slices per cell, which corresponds to a spatial translation of ∼ 15 nm per slice along the optical axis of the microscope.

Optimized holographic femtosecond laser patterning method towards rapid integration of high-quality functional devices in microchannels

Rapid integration of high-quality functional devices in microchannels is in highly demand for miniature lab-on-a-chip applications. This paper demonstrates the embellishment of existing microfluidic devices with integrated micropatterns via femtosecond laser MRAF-based holographic patterning (MHP) microfabrication, which proves two-photon polymerization (TPP) based on spatial light modulator (SLM) to be a rapid and powerful technology for chip functionalization. Optimized mixed region amplitude freedom (MRAF) algorithm has been used to generate high-quality shaped focus field. Base on the optimized parameters, a single-exposure approach is developed to fabricate 200 × 200 μm microstructure arrays in less than 240 ms. Moreover, microtraps, QR code and letters are integrated into a microdevice by the advanced method for particles capture and device identification. These results indicate that such a holographic laser embellishment of microfluidic devices is simple, flexible and easy to access, which has great potential in lab-on-a-chip applications of biological culture, chemical analyses and optofluidic devices.

Study of Different Sol-Gel Coatings to Enhance the Lifetime of PDMS Devices: Evaluation of Their Biocompatibility

A study of PDMS (polydimethylsiloxane) sol-gel–coated channels fabricated using soft lithography and a laser direct writing technique is presented. PDMS is a biocompatible material that presents a high versatility to reproduce several structures. It is widely employed in the fabrication of preclinical devices due to its advantages but it presents a rapid chemical deterioration to organic solvents. The use of sol-gel layers to cover the PDMS overcomes this problem since it provides the robustness of glass for the structures made with PDMS, decreasing its deterioration and changing the biocompatibility of the surface. In this work, PDMS channels are coated with three different kinds of sol-gel compositions (60MTES/40TEOS, 70MTES/30TISP and 80MTES/20TISP). The endothelial cell adhesion to the different coated devices is evaluated in order to determine the most suitable sol-gel preparation conditions to enhance cellular adhesion.

Fabrication of microchannels by space-selective control of phase separation in glass

Microchannels have important scientific applications in many fields, because they enable precise control, manipulation, and analysis of fluid on a micrometer scale. Herein, we demonstrate an effective strategy for fabrication of microchannels, based on the space-selective phase separation in glass induced by a femtosecond laser. The proposed method shows its abilities in fabrication of three-dimensional microchannels with ∼5  mm length scale and a uniform cross section. Moreover, we also achieve the modulation of the morphology on the inner surface of microchannels by using objective lenses with various numerical-apertures. The physical mechanism of the phase separation and microstructure evolution is discussed. Our method provides new opportunities to fabricate microchannels with complex structures and multifunctional integration.

Making the invisible visible: a microfluidic chip using a low refractive index polymer

Microfluidic frameworks known as micro-total-analysis-systems or lab-on-a-chip have become versatile tools in cell biology research, since functional biochips are able to streamline dynamic observations of various cells. Glass or polymers are generally used as the substrate due to their high transparency, chemical stability and cost-effectiveness. However, these materials are not well suited for the microscopic observation of cell migration at the fluid boundary due to the refractive index mismatch between the medium and the biochip material. For this reason, we have developed a new method of fabricating three-dimensional (3D) microfluidic chips made of the low refractive index fluoric polymer CYTOP. This novel fabrication procedure involves the use of a femtosecond laser for direct writing, followed by wet etching with a dilute fluorinated solvent and annealing, to create high-quality 3D microfluidic chips inside a polymer substrate. A microfluidic chip made in this manner enabled us to more clearly observe the flagellum motion of a Dinoflagellate moving in circles near the fluid surface compared to the observations possible using conventional microfluidic chips. We believe that CYTOP microfluidic chips made using this new method may allow more detailed analysis of various cell migrations near solid boundaries.

Photodynamic assembly of nanoparticles towards designable patterning

Recent advancements in nanotechnology have continued to stimulate the development of functional devices based on nanomaterials. However, the controllable assembly of these tiny nanomaterials into functional structures is still a big challenge for further applications; nowhere is this more obvious than in the field of nanodevices. Currently, despite the fact that self-assembly technologies have revealed great potential to reach this end, serious problems with respect to morphology control, designable assembly and even flexible patterning set huge obstacles to the fabrication of functional devices. Nowadays, in addition to self-assembly technologies that make use of interaction forces between different objects, photodynamic assembly (PDA) technology has emerged as a promising route to architect functional materials with the help of optical driving forces towards device fabrication. In this review, we summarize the recent developments in PDA technology for the designable patterning of nanoparticles (NPs). The basic fundamentals of PDA that resort to optical trapping (OT) and typical examples regarding far-field/near-field OT for the PDA of various NPs have been reviewed. In particular, femtosecond laser induced photodynamic assembly (FsL-PDA), which enables the designable patterning of NPs through a direct writing manner, has been introduced. Finally, the current challenges and future prospects of this dynamic field are discussed based on our own opinions.

3D printed microfluidic devices: enablers and barriers

3D printing has the potential to significantly change the field of microfluidics. The ability to fabricate a complete microfluidic device in a single step from a computer model has obvious attractions, but it is the ability to create truly three dimensional structures that will provide new microfluidic capability that is challenging, if not impossible to make with existing approaches. This critical review covers the current state of 3D printing for microfluidics, focusing on the four most frequently used printing approaches: inkjet (i3DP), stereolithography (SLA), two photon polymerisation (2PP) and extrusion printing (focusing on fused deposition modeling). It discusses current achievements and limitations, and opportunities for advancement to reach 3D printing's full potential.

The upcoming 3D-printing revolution in microfluidics

In the last two decades, the vast majority of microfluidic systems have been built in poly(dimethylsiloxane) (PDMS) by soft lithography, a technique based on PDMS micromolding. A long list of key PDMS properties have contributed to the success of soft lithography: PDMS is biocompatible, elastomeric, transparent, gas-permeable, water-impermeable, fairly inexpensive, copyright-free, and rapidly prototyped with high precision using simple procedures. However, the fabrication process typically involves substantial human labor, which tends to make PDMS devices difficult to disseminate outside of research labs, and the layered molding limits the 3D complexity of the devices that can be produced. 3D-printing has recently attracted attention as a way to fabricate microfluidic systems due to its automated, assembly-free 3D fabrication, rapidly decreasing costs, and fast-improving resolution and throughput. Resins with properties approaching those of PDMS are being developed. Here we review past and recent efforts in 3D-printing of microfluidic systems. We compare the salient features of PDMS molding with those of 3D-printing and we give an overview of the critical barriers that have prevented the adoption of 3D-printing by microfluidic developers, namely resolution, throughput, and resin biocompatibility. We also evaluate the various forces that are persuading researchers to abandon PDMS molding in favor of 3D-printing in growing numbers.