3D-printed microrobots for biomedical applications

Microrobots, which can perform tasks in difficult-to-reach parts of the human body under their own or external power supply, are potential tools for biomedical applications, such as drug delivery, microsurgery, imaging and monitoring, tissue engineering, and sensors and actuators. Compared with traditional fabrication methods for microrobots, recent improvements in 3D printers enable them to print high-precision microrobots, breaking through the limitations of traditional micromanufacturing technologies that require high skills for operators and greatly shortening the design-to-production cycle. Here, this review first introduces typical 3D printing technologies used in microrobot manufacturing. Then, the structures of microrobots with different functions and application scenarios are discussed. Next, we summarize the materials (body materials, propulsion materials and intelligent materials) used in 3D microrobot manufacturing to complete body construction and realize biomedical applications (e.g., drug delivery, imaging and monitoring). Finally, the challenges and future prospects of 3D printed microrobots in biomedical applications are discussed in terms of materials, manufacturing and advancement.

Interplay of Luminophores and Photoinitiators during Synthesis of Bulk and Patterned Luminescent Photopolymer Blends

Four-dimensional printing with embedded photoluminescence is emerging as an exciting area in additive manufacturing. Slim polymer films patterned with three-dimensional lattices of multimode cylindrical waveguides (waveguide-encoded lattices, WELs) with enhanced fields of view can be fabricated by localizing light as self-trapped beams within a photopolymerizable formulation. Luminescent WELs have potential applications as solar cell coatings and smart planar optical components. However, as luminophore-photoinitiator interactions are expected to change the photopolymerization kinetics, the design of robust luminescent photopolymer sols is nontrivial. Here, we use model photopolymer systems based on methacrylate-siloxane and epoxide homopolymers and their blends to investigate the influence of the luminophore Lumogen Violet (LV) on the photolysis kinetics of the Omnirad 784 photoinitiator through UV-vis absorbance spectroscopy. Initial rate analysis with different bulk polymers reveals differences in the pseudo-first-order rate constants in the absence and presence of LV, with a notable increase (∼40%) in the photolysis rate for the 1:1 blend. Fluorescence quenching studies, coupled with density functional theory calculations, establish that these differences arise due to electron transfer from the photoexcited LV to the ground-state photoinitiator molecules. We also demonstrate an in situ UV-vis absorbance technique that enables real-time monitoring of both waveguide formation and photoinitiator consumption during the fabrication of WELs. The in situ photolysis kinetics confirm that LV-photoinitiator interactions also influence the photopolymerization process during WEL formation. Our findings show that luminophores play a noninnocent role in photopolymerization and highlight the necessity for both careful consideration of the photopolymer formulation and a real-time monitoring approach to enable the fabrication of high-quality micropatterned luminescent polymeric films.

3D-Printed Eosin Y-Based Heterogeneous Photocatalyst for Organic Reactions

Heterogenization of Eosin Y by 3D-printing and its application in photocatalysis are reported. The approach allows a fine tuning of the photocatalyst morphology and its rapid preparation. Photocatalytic activity was evaluated through model organic reactions involving oxidation, reduction, and photosensitization pathways. The efficiency, recyclability and stability of 3D printed EY is remarkable paving the way to new generation of heterogeneous photocatalysts with a perfect control of their shape and adaptable to any photoreactors.

Dansylated Nitrile N-Oxide as the Fluorescent Dye Clickable to Unsaturated Bonds without Catalyst

Fluorescent polymeric materials have been exploited in the fields of aesthetical purposes, biomedical engineering, and three-dimensional printing applications. While the fluorescent materials are prepared by the polymerization of fluorescent monomer or the blending a fluorescent dye with common polymer, the covalent immobilization of fluorescent dye onto common polymers is not the practical technique. In this paper, dansylated nitrile N-oxide (Dansyl-NO) has been designed and synthesized to be a stable nitrile N-oxide as the derivative of 2-hydroxy-1-naphthaldehyde. While Dansyl-NO shows good reactivity to an alkene and an alkyne to give fluorescent Dansyl-Ene and Dansyl-Yne, respectively, it hardly reacts to a nitrile. The results indicate that Dansyl-NO serves as a fluorescent dye clickable to alkenes and alkynes. To know the effects of solvent on the fluorescent properties, the UV-vis and fluorescence spectra of Dansyl-Ene are measured in three solvents. Dansyl-Ene shows fluorescent solvatochromism, which appears to be red-shifted along with the increase in solvent polarity. Poly(styrene-co-butadiene) directly reacts with Dansyl-NO to give fluorescent modified SB. The emission spectrum of modified SB is blue-shifted compared with that of Dansyl-Ene. The blue-shift could be possibly attributed to the presence of less polar polymer skeleton around the dansyl moieties of modified SB.

Progress in Photocurable 3D Printing of Photosensitive Polyurethane: A Review

In recent years, as a class of advanced additive manufacturing (AM) technology, photocurable 3D printing has gained increasing attention. Based on its outstanding printing efficiency and molding accuracy, it is employed in various fields, such as industrial manufacturing, biomedical, soft robotics, electronic sensors. Photocurable 3D printing is a molding technology based on the principle of area-selective curing of photopolymerization reaction. At present, the main printing material suitable for this technology is the photosensitive resin, a composite mixture consisting of a photosensitive prepolymer, reactive monomer, photoinitiator, and other additives. As the technique research deepens and its application gets more developed, the design of printing materials suitable for different applications is becoming the hotspot. Specifically, these materials not only can be photocured but also have excellent properties, such as elasticity, tear resistance, fatigue resistance. Photosensitive polyurethanes can endow photocured resin with desirable performance due to their unique molecular structure including the inherent alternating soft and hard segments, and microphase separation. For this reason, this review summarizes and comments on the research and application progress of photocurable 3D printing of photosensitive polyurethanes, analyzing the advantages and shortcomings of this technology, also offering an outlook on this rapid development direction.

Improving Printability of Digital-Light-Processing 3D Bioprinting via Photoabsorber Pigment Adjustment

Digital-light-processing (DLP) three-dimensional (3D) bioprinting, which has a rapid printing speed and high precision, requires optimized biomaterial ink to ensure photocrosslinking for successful printing. However, optimization studies on DLP bioprinting have yet to sufficiently explore the measurement of light exposure energy and biomaterial ink absorbance controls to improve the printability. In this study, we synchronized the light wavelength of the projection base printer with the absorption wavelength of the biomaterial ink. In this paper, we provide a stepwise explanation of the challenges associated with unsynchronized absorption wavelengths and provide appropriate examples. In addition to biomaterial ink wavelength synchronization, we introduce photorheological measurements, which can provide optimized light exposure conditions. The photorheological measurements provide precise numerical data on light exposure time and, therefore, are an effective alternative to the expendable and inaccurate conventional measurement methods for light exposure energy. Using both photorheological measurements and bioink wavelength synchronization, we identified essential printability optimization conditions for DLP bioprinting that can be applied to various fields of biological sciences.

Carbon Dots Embedded in Cellulose Film: Programmable, Performance-Tunable, and Large-Scale Subtle Fluorescent Patterning by in Situ Laser Writing

Fluorescent patterns with multiple functions enable high-security anti-counterfeiting labels. Complex material synthesis and patterning processes limit the application of multifunctional fluorescent patterns, so the technology of in situ fluorescent patterning with tunable multimodal capabilities is becoming more necessary. In this work, an in situ fluorescent patterning technology was developed using laser direct writing on solid cellulose film at ambient conditions without masks. The fluorescent intensity and surface microstructure of the patterns could be adjusted by programmable varying of the laser parameters simultaneously. During laser direct writing, carbon dots are generated in situ in a cellulose ester polymer matrix, which significantly simplifies the fluorescent patterning process and reduces the manufacturing cost. Interestingly, the tunable fluorescent intensity empowers the fabrication of visual stereoscopic fluorescent patterns with excitation dependence, further improving its anti-counterfeiting performance. The obtained fluorescent patterns still show ultrahigh optical properties after being immersed in an acid/base solution (pH 5-12) over one month. In addition, the anti-UV performance of the obtained laser-patterned film with transmittance around 90% is comparable to that of commercial UV-resistant films. This work provided an advanced and feasible approach to fabricating programmable, performance-tunable, subtle fluorescent patterns in large-scale for industrial application.

Tailoring a hybrid three-component photoinitiating system for 3D printing

In the field of additive manufacturing DLP vat technologies are promising 3D printing techniques. The need of highly efficient photoiniating systems drives us to the development of photocyclic 3-component initiators. In order to improve the 3D printing sensitivity, we present in this paper the use of synthesized clay to tune up the photochemistry underlying the initiating radical production. Therefore, a three-component initiating system, based on a cationic dye, two coinitiators and with a clay filler suitable for DLP 3D printing of acrylate resins leading to high quality of parts and low printing time, is developed.

Materials Testing for the Development of Biocompatible Devices through Vat-Polymerization 3D Printing

Light-based 3D printing techniques could be a valuable instrument in the development of customized and affordable biomedical devices, basically for high precision and high flexibility in terms of materials of these technologies. However, more studies related to the biocompatibility of the printed objects are required to expand the use of these techniques in the health sector. In this work, 3D printed polymeric parts are produced in lab conditions using a commercial Digital Light Processing (DLP) 3D printer and then successfully tested to fabricate components suitable for biological studies. For this purpose, different 3D printable formulations based on commercially available resins are compared. The biocompatibility of the 3D printed objects toward A549 cell line is investigated by adjusting the composition of the resins and optimizing post-printing protocols; those include washing in common solvents and UV post-curing treatments for removing unreacted and cytotoxic products. It is noteworthy that not only the selection of suitable materials but also the development of an adequate post-printing protocol is necessary for the development of biocompatible devices.

3D Printing Mechanically Robust and Transparent Polyurethane Elastomers for Stretchable Electronic Sensors

Advanced stretchable electronic sensors with a complex structure place higher requirements on the mechanical properties and manufacturing process of the stretchable substrate materials. Herein, three kinds of polyurethane acrylate oligomers were synthesized successfully and mixed with a commercial acrylate monomer (isobornyl acrylate) to prepare photocurable resins with a low viscosity for a digital light processing three-dimensional (3D) printer without custom equipment. Results showed that the resin containing poly(tetrahydrofuran) units (PPTMGA-40) exhibited optimal mechanical properties and shape recoverability. The tensile strength and elongation at break of PPTMGA-40 were 15.7 MPa and 414.3%, respectively. The unprecedented fatigue resistance of PPTMGA-40 allowed it to withstand 100 compression cycles at 80% strain without fracture. The transmittance of PPTMGA-40 reached 89.4% at 550 nm, showing high transparency. An ionic hydrogel was coated on the surface of 3D-printed structures to fabricate stretchable sensors, and their conductivity, transparency, and mechanical performance were characterized. A robust piezoresistive strain sensor with a high strength (∼6 MPa) and a wearable finger guard sensor were fabricated, demonstrating that this hydrogel-elastomer system can meet the requirements of applications for advanced stretchable electronic sensors and expand the usage scope.

Photocurable modification of inorganic fillers and their application in photopolymers for 3D printing

The reinforcement of photo-crosslinkable calcium sulfate whiskers and their reaction mechanism in photopolymers for 3D printing technology.

Three-Dimensional Printed Photoluminescent Polymeric Waveguides

In this work, we propose an innovative strategy for obtaining functional objects employing a light-activated three-dimensional (3D) printing process without affecting the materials' printability. In particular, a dye is a necessary ingredient in a formulation for a digital light processing 3D printing method to obtain precise and complex structures. Here, we use a photoluminescent dye specifically synthesized for this purpose that enables the production of 3D printed waveguides and splitters able to guide the luminescence. Moreover, copolymerizing the dye with the polymeric network during the printing process, we are able to maintain the solvatochromic properties of the dye toward different solvents in the printed structures, enabling the development of solvents' polarity sensors.

Novel Materials for 3D Printing by Photopolymerization

The field of 3D printing, also known as additive manufacturing (AM), is developing rapidly in both academic and industrial research environments. New materials and printing technologies, which enable rapid and multimaterial printing, have given rise to new applications and utilizations. However, the main bottleneck for achieving many more applications is the lack of materials with new physical properties. Here, some of the recent reports on novel materials in this field, such as ceramics, glass, shape-memory polymers, and electronics, are reviewed. Although new materials have been reported for all three main printing approaches-fused deposition modeling, binder jetting or laser sintering/melting, and photopolymerization-based approaches, apparently, most of the novel physicochemical properties are associated with materials printed by photopolymerization approaches. Furthermore, the high resolution that can be achieved using this type of 3D printing, together with the new properties, has resulted in new implementations such as microfluidic, biomedical devices, and soft robotics. Therefore, the focus here is on photopolymerization-based additive manufacturing including the recent development of new methods, novel monomers, and photoinitiators, which result in previously inaccessible applications such as complex ceramic structures, embedded electronics, and responsive 3D objects.

3D-Printable Photochromic Molecular Materials for Reversible Information Storage

The formulation of advanced molecular materials with bespoke polymeric ionic-liquid matrices that stabilize and solubilize hybrid organic-inorganic polyoxometalates and allow their processing by additive manufacturing, is effectively demonstrated. The unique photo and redox properties of nanostructured polyoxometalates are translated across the scales (from molecular design to functional materials) to yield macroscopic functional devices with reversible photochromism. These properties open a range of potential applications including reversible information storage based on controlled topological and temporal reduction/oxidation of pre-formed printed devices. This approach pushes the boundaries of 3D printing to the molecular limits, allowing the freedom of design enabled by 3D printing to be coupled with the molecular tuneability of polymerizable ionic liquids and the photoactivity and orbital engineering possible with hybrid polyoxometalates.

High-Performance Nano-Photoinitiators with Improved Safety for 3D Printing

In this work, we report the first hybrid nanosized photoinitiators with low cytotoxicity and migration by coupling of polyhedral oligomeric silsesquioxanes (POSS) to benzophenone derivatives. This new series of photoinitiators were fully characterized and showed many favorable properties such as uniform sizes, extremely low tendency to migrate, less effect on resin viscosity, enhanced thermal stability and mechanical strength, increased photoactivity, and significantly lower cell toxicity compared to their corresponding benzophenone molecules. The utility of these hybrid nanosized photoinitiators in 3D printing was demonstrated in printing of various 3D structures with high resolution and accuracy.