3-Dimensional Submicron Polymerization of Acrylamide by Multiphoton Excitation of Xanthene Dyes

Multiphoton Laser Fabrication of Hybrid Photo-Activable Biomaterials

The possibility to shape stimulus-responsive optical polymers, especially hydrogels, by means of laser 3D printing and ablation is fostering a new concept of “smart” micro-devices that can be used for imaging, thermal stimulation, energy transducing and sensing. The composition of these polymeric blends is an essential parameter to tune their properties as actuators and/or sensing platforms and to determine the elasto-mechanical characteristics of the printed hydrogel. In light of the increasing demand for micro-devices for nanomedicine and personalized medicine, interest is growing in the combination of composite and hybrid photo-responsive materials and digital micro-/nano-manufacturing. Existing works have exploited multiphoton laser photo-polymerization to obtain fine 3D microstructures in hydrogels in an additive manufacturing approach or exploited laser ablation of preformed hydrogels to carve 3D cavities. Less often, the two approaches have been combined and active nanomaterials have been embedded in the microstructures. The aim of this review is to give a short overview of the most recent and prominent results in the field of multiphoton laser direct writing of biocompatible hydrogels that embed active nanomaterials not interfering with the writing process and endowing the biocompatible microstructures with physically or chemically activable features such as photothermal activity, chemical swelling and chemical sensing.

Multiphoton microfabrication and micropatterning (MMM) - An all-in-one platform for engineering biomimetic soluble cell niches

Engineered biomimetic cell niches represent a valuable in vitro tool for investigating physiological and pathological cellular activities, while developing an all-in-one technology to engineer cell niches, particularly soluble cell niche factors, with retained bioactivities, remains challenging. Here, we report a mask-free, non-contact and biocompatible multiphoton microfabrication and micropatterning (MMM) technology in engineering a spatially and quantitatively controllable bone morphogenetic protein-2 (BMP-2) soluble niche, by immobilizing optimally biotinylated BMP-2 (bBMP-2) on micro-printed neutravidin (NA) micropatterns. Notably, the micropatterned NA bound-bBMP-2 niche elicited a more sustained and a higher level of the downstream Smad signaling than that by free BMP-2, in C2C12 cells, suggesting the advantages of immobilizing soluble niche factors on engineered micropatterns or scaffold materials. This work reports a universal all-in-one cell niche engineering platform and contributes to reconstituting heterogeneous native soluble cell niches for signal transduction modeling and drug screening studies.

Hydrophilic dyes as photosensitizers for photopolymerization of dental adhesives

OBJECTIVES

This study explored hydrophilic dyes as photosensitizers for application in dental adhesives. The goal was to identify dyes that enhance the degree of conversion (DC) of the hydrophilic-rich phase without impairing polymerization of the hydrophobic-rich phase.

METHODS

Properties that were investigated included the molar extinction coefficient at 480 nm, relative normalized photon absorption efficiency (PAE), rate of polymerization and degree of conversion (DC). The following hydrophilic dyes: Bromophenol blue sodium salt, Rosebengal sodium salt, Erythrosin B, New Fuchsin and Victoria blue B were identified as suitable photosensitizers.

RESULTS

In this study it was observed that dyes such as Bromophenol blue sodium salt, New Fuchsin, Victoria blue B and Rosebengal sodium salt were suitable candidates for dental adhesive photopolymerization, leading to substantial degree of conversion to both the hydrophilic-rich phase and the hydrophobic-rich phase.

CONCLUSIONS

In addition to the ability of the photosensitizer to absorb light in the visible range and transition to an excited state as a result of the absorbed energy, other factors such as the efficiency of the photosensitizer/light curing unit (LCU) combination, stability/efficiency of the excited state of the photosensitizer and/or initiating reactive species play an important role in the photopolymerization of the dental adhesive.

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.

3D-Printing of Structure-Controlled Antigen Nanoparticles for Vaccine Delivery

Targeted delivery of antigens to immune cells using micro/nanocarriers may serve as a therapeutic application for vaccination. However, synthetic carriers have potential drawbacks including cytotoxicity, low encapsulation efficiency of antigen, and lack of a morphological design, which limit the translation of the delivery system to clinical use. Here, we report a carrier-free and three-dimensional (3D)-shape-designed antigen nanoparticle by multiphoton lithography-based 3D-printing. This simple, versatile 3D-printing approach provides freedom for the precise design of particle shapes with a nanoscale resolution. Importantly, shape-designed antigen nanoparticles with distinct aspect ratios show shape-dependent immune responses. The 3D-printing approach for the rational design of nanomaterials with increasing safety, complexity, and efficacy offers an emerging platform to develop vaccine delivery systems and mechanistic understanding.

4D Printing of a Light-Driven Soft Actuator with Programmed Printing Density

There is a growing interest in the concept of four-dimensional (4D) printing that combines a three-dimensional (3D) manufacturing process with dynamic modulation for bioinspired soft materials exhibiting more complex functionality. However, conventional approaches have drawbacks of low resolution, control of internal micro/nanostructure, and creation of fast, complex actuation due to a lack of high-resolution fabrication technology and suitable photoresist for soft materials. Here, we report an approach of 4D printing that develops a bioinspired soft actuator with a defined 3D geometry and programmed printing density. Multiphoton lithography (MPL) allows for controlling printing density in gels at pixel-by-pixel with a resolution of a few hundreds of nanometers, which tune swelling behaviors of gels in response to external stimuli. We printed a 3D soft actuator composed of thermoresponsive poly(N-isopropylacrylamide) (PNIPAm) and gold nanorods (AuNRs). To improve the resolution of printing, we synthesized a functional, thermoresponsive macrocrosslinker. Through plasmonic heating by AuNRs, nanocomposite-based soft actuators undergo nonequilibrium, programmed, and fast actuation. Light-mediated manufacture and manipulation (MPL and photothermal effect) offer the feasibility of 4D printing toward adaptive bioinspired soft materials.

Increase in efficacy of near-infrared femtosecond micromachining in ophthalmic hydrogels with the addition of sodium fluorescein, rose bengal, and riboflavin

We report on the effect of exogenous doping agents with large two-photon absorption cross sections on the efficacy of near-infrared femtosecond micromachining in ophthalmic hydrogels. Contaflex GM Advance 58 hydrogels were solution doped in low concentrations of sodium fluorescein, rose bengal, and riboflavin dissolved in balanced salt solution prior to femtosecond micromachining with three near-infrared wavelengths: 720, 800, and 1035 nm. Using any of the three doping agents in the concentrations studied produced an increase in the amount of optical phase change induced in the material at all writing wavelengths and reduced the amount of power needed to induce a desired amount of phase change.

Biomaterial-based microstructures fabricated by two-photon polymerization microfabrication technology

Two-photon polymerization (TPP) microfabrication technology can freely prepare micro/nano structures with different morphologies and high accuracy for micro/nanophotonics, micro-electromechanical systems, microfluidics, tissue engineering and drug delivery.

3D and 4D Bioprinting of the Myocardium: Current Approaches, Challenges, and Future Prospects

3D and 4D bioprinting of the heart are exciting notions in the modern era. However, myocardial bioprinting has proven to be challenging. This review outlines the methods, materials, cell types, issues, challenges, and future prospects in myocardial bioprinting. Advances in 3D bioprinting technology have significantly improved the manufacturing process. While scaffolds have traditionally been utilized, 3D bioprinters, which do not require scaffolds, are increasingly being employed. Improved understanding of the cardiac cellular composition and multiple strategies to tackle the issues of vascularization and viability had led to progress in this field. In vivo studies utilizing small animal models have been promising. 4D bioprinting is a new concept that has potential to advance the field of 3D bioprinting further by incorporating the fourth dimension of time. Clinical translation will require multidisciplinary collaboration to tackle the pertinent issues facing this field.

From Microscale Devices to 3D Printing: Advances in Fabrication of 3D Cardiovascular Tissues

Current strategies for engineering cardiovascular cells and tissues have yielded a variety of sophisticated tools for studying disease mechanisms, for development of drug therapies, and for fabrication of tissue equivalents that may have application in future clinical use. These efforts are motivated by the need to extend traditional 2-dimensional (2D) cell culture systems into 3D to more accurately replicate in vivo cell and tissue function of cardiovascular structures. Developments in microscale devices and bioprinted 3D tissues are beginning to supplant traditional 2D cell cultures and preclinical animal studies that have historically been the standard for drug and tissue development. These new approaches lend themselves to patient-specific diagnostics, therapeutics, and tissue regeneration. The emergence of these technologies also carries technical challenges to be met before traditional cell culture and animal testing become obsolete. Successful development and validation of 3D human tissue constructs will provide powerful new paradigms for more cost effective and timely translation of cardiovascular tissue equivalents.

Fabrication of three-dimensional multi-protein microstructures for cell migration and adhesion enhancement

In this study, three-dimensional (3D) multi-component microstructures were precisely fabricated via multiphoton excited photochemistry using a femtosecond laser direct-writing system with proposed repetition positioning and vector scanning techniques. Extracellular matrix (ECM) proteins, such as fibronectin (FN), are difficult to stack and form 3D structures larger than several-hundred microns in height due to the nature of their protein structure. Herein, to fabricate complex 3D microstructures with FN, a 3D scaffold was designed and formed from bovine serum albumin (BSA), after which human FN was inserted at specific locations on the BSA scaffold; in this manner, the fabricated ECM microstructure can guide cells in a 3D environment. A human breast cancer cell line, MDA-MB-231, was used to investigate the behavior of cell migration and adhesion on the fabricated human FN and BSA protein structures. Experimental results indicate that many cells are not able to attach or climb on a 3D structure's inclined plane without FN support; hence, the influence of cell growth in a 3D context with FN should being taken into consideration. This 3D multi-protein fabrication technique holds potential for cell studies in designed complex 3D ECM scaffolds.

Spatial and temporal analysis of extracellular matrix proteins in the developing murine heart: a blueprint for regeneration

The extracellular matrix (ECM) of the embryonic heart guides assembly and maturation of cardiac cell types and, thus, may serve as a useful template, or blueprint, for fabrication of scaffolds for cardiac tissue engineering. Surprisingly, characterization of the ECM with cardiac development is scattered and fails to comprehensively reflect the spatiotemporal dynamics making it difficult to apply to tissue engineering efforts. The objective of this work was to define a blueprint of the spatiotemporal organization, localization, and relative amount of the four essential ECM proteins, collagen types I and IV (COLI, COLIV), elastin (ELN), and fibronectin (FN) in the left ventricle of the murine heart at embryonic stages E12.5, E14.5, and E16.5 and 2 days postnatal (P2). Second harmonic generation (SHG) imaging identified fibrillar collagens at E14.5, with an increasing density over time. Subsequently, immunohistochemistry (IHC) was used to compare the spatial distribution, organization, and relative amounts of each ECM protein. COLIV was found throughout the developing heart, progressing in amount and organization from E12.5 to P2. The amount of COLI was greatest at E12.5 particularly within the epicardium. For all stages, FN was present in the epicardium, with highest levels at E12.5 and present in the myocardium and the endocardium at relatively constant levels at all time points. ELN remained relatively constant in appearance and amount throughout the developmental stages except for a transient increase at E16.5. Expression of ECM mRNA was determined using quantitative polymerase chain reaction and allowed for comparison of amounts of ECM molecules at each time point. Generally, COLI and COLIII mRNA expression levels were comparatively high, while COLIV, laminin, and FN were expressed at intermediate levels throughout the time period studied. Interestingly, levels of ELN mRNA were relatively low at early time points (E12.5), but increased significantly by P2. Thus, we identified changes in the spatial and temporal localization of the primary ECM of the developing ventricle. This characterization can serve as a blueprint for fabrication techniques, which we illustrate by using multiphoton excitation photochemistry to create a synthetic scaffold based on COLIV organization at P2. Similarly, fabricated scaffolds generated using ECM components, could be utilized for ventricular repair.

Cell Adhesion on Micro-Structured Fibronectin Gradients Fabricated by Multiphoton Excited Photochemistry

Concentration gradients of ECM proteins play active roles in many areas of cell biology including wound healing and metastasis. They may also form the basis of tissue engineering scaffolds, as these can direct cell adhesion and migration and promote new matrix synthesis. To better understand cell-matrix interactions on attractive gradients, we have used multiphoton excited (MPE) photochemistry to fabricate covalently linked micro-structured gradients from fibronectin (FN). The gradient design is comprised of a parallel series of individual linear gradients with overall dimensions of approximately 800 × 800 μm, where a linear dynamic range of nearly 10-fold in concentration was achieved. The adhesion dynamics of 3T3 fibroblasts were investigated, where the cell morphology and actin cytoskeleton became increasingly elongated and aligned with the direction of the gradient at increasing protein concentration. Moreover, the cell morphologies are distinct when adhered to regions of differing FN concentration but with similar topography. These results show that the fabrication approach allows investigating the roles of contact guidance and ECM cues on the cell-matrix interactions. We suggest this design overcomes some of the limitations with other fabrication methods, especially in terms of 3D patterning capabilities, and will serve as a new tool to study cell-matrix interactions.

Fast multiphoton microfabrication of freeform polymer microstructures by spatiotemporal focusing and patterned excitation

One of the limits of conventional scanning multiphoton microfabrication is its low throughput due to point-by-point processing. In order to surpass this limit, a multiphoton microfabrication system based on spatiotemporal focusing and patterned excitation has been developed to quickly provide three-dimensional (3D) freeform polymer microstructures. 3D freeform polymer microstructures using Rose Bengal as the photoinitiator are created by sequentially stacking two-dimensional fabricating patterns. The size of each fabrication area can be larger than 300 × 170 μm2 (full width at half maximum). Compared to conventional scanning multiphoton excitation and fixed mask pattern generation, this approach offers freeform microstructures and a greater than three-order increase in fabrication speed. Furthermore, the system is capable of optically sectioning the fabricated microstructures for providing 3D inspection.

Enhanced two-photon excited fluorescence in three-dimensionally crosslinked bovine serum albumin microstructures

In this study, the intensity of two-photon excited fluorescence (TPEF) of xanthene dye, Rose Bengal (RB), was significantly enhanced via bovine serum albumin (BSA) microstructures fabricated by the two-photon crosslinking (TPC) technique. The RB was utilized as the photoactivator in the TPC processing and the enhanced TPEF intensity correlates with the concentration of fabricated crosslinked BSA microstructures via the power control and pulse selection of the employed femtosecond laser. As a result, fabrication of three-dimensional BSA microstructures can be simultaneously monitored by the use of TPEF intensity. The crosslinked BSA microstructures synthesized may be used as an ordered biomaterial for fluorescence enhancement.

Multiphoton fabrication of freeform polymer microstructures with gold nanorods

In this study, three-dimensional (3D) polyacrylamide microstructures containing gold nanorods (AuNRs) were fabricated by two-photon polymerization (TPP) using Rose Bengal (RB) as the photoinitiator. To retain AuNRs in the 3D polymer microstructures, the laser wavelength was chosen for two-photon RB absorption for improved TPP efficiency, but not for enhancing the longitudinal plasmon resonance of AuNRs which may result in photothermal damage of AuNRs. After TPP processing, the laser wavelength was tuned for the longitudinal plasmon resonance and the laser power was increased to beyond the damage threshold of the AuNRs for reshaping the AuNRs into gold nanospheres. As a result, AuNRs in designated positions of the fabricated 3D microstructures can be achieved. Two-photon luminescence from the doped AuNRs can also act as contrast agent for the visualization of 3D polymer microstructures.

Two-photon lithography in the future of cell-based therapeutics and regenerative medicine: a review of techniques for hydrogel patterning and controlled release

In recent decades, there has been considerable interest in using photochemistry to produce biomaterials, owing to their ability to be used in the presence of biological material. Two-photon-induced photoreactions have been used to produce materials for optical data storage and microfabrication and, recently, researchers have exploited two-photon-induced chemical processes to create biomaterials. Researchers have used two-photon-induced lithography to fabricate hydrogels with well-defined chemical and physical properties in 3D through network polymerization, functionalization, uncaging and degradation, as described in this article. Fabrication and modification of chemical and physical architecture of biomaterials in 3D with submicron resolution will allow the elucidation of more complex relationships in cell behavior and tissue development and introduce pathways to engineering complex tissues.

Multiphoton Fabrication

Chemical and physical processes driven by multiphoton absorption make possible the fabrication of complex, 3D structures with feature sizes as small as 100 nm. Since its inception less than a decade ago, the field of multiphoton fabrication has progressed rapidly, and multiphoton techniques are now being used to create functional microdevices. In this Review we discuss the techniques and materials used for multiphoton fabrication, the applications that have been demonstrated, as well as those being pursued. We also consider the outlook for this field, both in the laboratory and in industrial settings.

Global Tuning of Local Molecular Phenomena: An Alternative Approach to Bionanoelectronics

We have applied simultaneous horizontal and vertical bias to a single molecule (2 nm(2)) in an ordered and disordered matrix to virtually isolate and tune its property without taking it out physically from its environment. Using a dedicated electrode system, we have locally tuned nanoscale properties vertically by STM, while stabilizing its environment by applying a global electric field horizontally. Using this technique, we report tuning of molecular conformations in room temperature, whose evolution of states has been statistically investigated. We have also shown control on switching of a few selected conformations by applying dual bias simultaneously. As we avoid any direct injection of charge into the system via electrode contact, this technique could be used as a generalized method to tune phenomena evolved in an environment of weak interaction from a large distance without destroying the property.

Multiphoton excited fabricated nano and micro patterned extracellular matrix proteins direct cellular morphology

We use multiphoton excited (MPE) photochemistry to fabricate patterned extracellular matrices (ECM) and to investigate the morphology of human dermal fibroblasts adhered to the resulting photocrosslinked linear structures of fibronectin (FN), fibrinogen (FG), and bovine serum albumin (BSA). These proteins were chosen to systematically investigate the roles of topography and ECM biochemistry on cell spreading, as fibroblasts bind directly to both FN and FG at RGD sites through known integrins, whereas BSA provides no comparable ECM cues for cell binding. MPE crosslinked patterns are created from parallel linear structures 600 nm in width, 200 microm in length, and spaced by either 10 or 40 microm. Immunofluorescence staining of FN and FG was used to assay the functionality of crosslinked proteins. The metrics of orientation, elongation, and cell perimeter were used to quantitate the resulting cellular behavior on the crosslinked protein patterns. These parameters all reflect statistical differences for cells on BSA, relative to the similar statistical behavior on fibronectin and fibrinogen. Cells on the BSA patterns are constrained by physical guidance and orientation between linear structures. In contrast, cells adhered on both FN and FG had a greater propensity to spread across adjacent structures, indicating the importance of cell matrix interactions. Focal adhesion staining of cells adhered to the protein structures revealed similar trends. These findings are consistent with our hypothesis that these crosslinked matrix protein structures are expected to direct cell adhesion and spreading and that the topography and ECM cues lead to different forms of guidance.

Soft-lithographic replication of 3D microstructures with closed loops

There is growing interest in lithographic technologies for creating 3D microstructures. Such techniques are generally serial in nature, prohibiting the mass production of devices. Soft-lithographic techniques show great promise for simple and rapid replication of arrays of microstructures but have heretofore not been capable of direct replication of structures with closed loops. We demonstrate that 3D microstructures created with multiphoton absorption polymerization can be replicated by using microtransfer molding to afford complex daughter structures containing closed loops. This method relieves many of the topological constraints of soft lithography, paving the way for the large-scale replication of true 3D microstructures.

Toward nanometer-scale optical photolithography: utilizing the near-field of bowtie optical nanoantennas

Optically resonant metallic bowtie nanoantennas are utilized as fabrication tools for the first time, resulting in the production of polymer resist nanostructures <30 nm in diameter at record low incident multiphoton energy densities. The nanofabrication is accomplished via nonlinear photopolymerization, which is initiated by the enhanced, confined optical fields surrounding the nanoantenna. The position, size, and shape of the resist nanostructures directly correlate with rigorous finite-difference time-domain computations of the field distribution, providing a nanometer-scale measurement of the actual field confinement offered by single optical nanoantennas. In addition, the size of the photoresist regions yields strong upper bounds on photoacid diffusion and resist resolution in SU-8, demonstrating a technique that can be generalized to the study of many current and yet-to-be-developed photoresist systems.

Measurement of normal and anomalous diffusion of dyes within protein structures fabricated via multiphoton excited cross-linking

We demonstrate microscale spatial and chemical control of diffusion within protein matrixes created through the use of nonlinear multiphoton excited photochemistry. The mobility of fluorescent dyes of different mass and composition within controlled cross-linked environments has been measured using two-photon excited fluorescence recovery after photobleaching (FRAP). The diffusion times for several rhodamine and sulforhodamine dyes within these fabricated structures were found to be approximately 3-4 orders of magnitude slower than in free solution. The precise diffusion times can be tuned by varying the laser exposure during the fabrication of the matrix, and the diffusion can be correlated with the mesh size determined by TEM and Flory-Rehner analysis. We find that the hydrophobic Texas Red dyes (sulforhodamines) exhibit diffusion that is highly anomalous, indicative of a strong interaction with the hydrophobic cross-linked protein matrix. These results suggests the use of these cross-linked protein matrixes as ideal model systems in which to systematically study anomalous diffusion. Finally, the diffusion can be tuned within a multilayered protein matrix, and this in conjunction with slow diffusion also suggests the use of these structures in controlled release applications.

Multiphoton laser direct writing of two-dimensional silver structures

We report a novel and efficient method for the laser direct writing of two-dimensional silver structures. Multiphoton absorption of a small fraction of the output of a Ti:sapphire oscillator is sufficient to photoreduce silver nitrate in a thin film of polyvinylpyrrolidone that has been spin-coated on a substrate. The polymer can then be washed away, leaving a pattern consisting of highly interconnected silver nanoparticles. We report the characterization of the silver patterns using scanning electron and atomic force microscopies, and demonstrate the application of this technique in the creation of diffraction gratings.