3D Two-Photon Microprinting of Nanoporous Architectures

Toward Multiscale, Multimaterial 3D Printing

Biological materials and organisms possess the fundamental ability to self-organize, through which different components are assembled from the molecular level up to hierarchical structures with superior mechanical properties and multifunctionalities. These complex composites inspire material scientists to design new engineered materials by integrating multiple ingredients and structures over a wide range. Additive manufacturing, also known as 3D printing, has advantages with respect to fabricating multiscale and multi-material structures. The need for multifunctional materials is driving 3D printing techniques toward arbitrary 3D architectures with the next level of complexity. In this paper, the aim is to highlight key features of those 3D printing techniques that can produce either multiscale or multimaterial structures, including innovations in printing methods, materials processing approaches, and hardware improvements. Several issues and challenges related to current methods are discussed. Ultimately, the authors also provide their perspective on how to realize the combination of multiscale and multimaterial capabilities in 3D printing processes and future directions based on emerging research.

3D printing of reactive macroporous polymers via thiol-ene chemistry and polymerization-induced phase separation

Using thiol-ene chemistry, polymerization-induced phase separation, and DLP 3D printing, we present a method for manufacturing reactive macroporous 3D structures. This approach enables the fabrication of structures with tunable physicochemical properties and compressibility. Moreover, it facilitates post-functionalization through thiol-Michael addition reactions, thereby expanding performance and application potential.

3D printed porous membrane integrated devices to study the chemoattractant induced behavioural response of aquatic organisms

Biological models with genetic similarities to humans are used for exploratory research to develop behavioral screening tools and understand sensory-motor interactions. Their small, often mm-sized appearance raises challenges in the straightforward quantification of their subtle behavioral responses and calls for new, customisable research tools. 3D printing provides an attractive approach for the manufacture of custom designs at low cost; however, challenges remain in the integration of functional materials like porous membranes. Nanoporous membranes have been integrated with resin exchange using purpose-designed resins by digital light projection 3D printing to yield functionally integrated devices using a simple, economical and semi-automated process. Here, the impact of the layer thickness and layer number on the porous properties - parameters unique for 3D printing - are investigated, showing decreases in mean pore diameter and porosity with increasing layer height and layer number. From the same resin formulation, materials with average pore size between 200 and 600 nm and porosity between 45% and 61% were printed. Membrane-integrated devices were used to study the chemoattractant induced behavioural response of zebrafish embryos and planarians, both demonstrating a predominant behavioral response towards the chemoattractant, spending >85% of experiment time in the attractant side of the observation chamber. The presented 3D printing method can be used for printing custom designed membrane-integrated devices using affordable 3D printers and enable fine-tuning of porous properties through adjustment of layer height and number. This accessible approach is expected to be adopted for applications including behavioural studies, early-stage pre-clinical drug discovery and (environmental) toxicology.

Photochemically Activated 3D Printing Inks: Current Status, Challenges, and Opportunities

3D printing with light is enabled by the photochemistry underpinning it. Without fine control over the ability to photochemically gate covalent bond formation by the light at a certain wavelength and intensity, advanced photoresists with functions spanning from on-demand degradability, adaptability, rapid printing speeds, and tailored functionality are impossible to design. Herein, recent advances in photoresist design for light-driven 3D printing applications are critically assessed, and an outlook of the outstanding challenges and opportunities is provided. This is achieved by classing the discussed photoresists in chemistries that function photoinitiator-free and those that require a photoinitiator to proceed. Such a taxonomy is based on the efficiency with which photons are able to generate covalent bonds, with each concept featuring distinct advantages and drawbacks.

Robust Superhydrophobicity through Surface Defects from Laser Powder Bed Fusion Additive Manufacturing

The robustness of superhydrophobic objects conflicts with both the inevitable introduction of fragile micro/nanoscale surfaces and three-dimensional (3D) complex structures. The popular metal 3D printing technology can manufacture robust metal 3D complex components, but the hydrophily and mass surface defects restrict its diverse application. Herein, we proposed a strategy that takes the inherent ridges and grooves' surface defects from laser powder bed fusion additive manufacturing (LPBF-AM), a metal 3D printing process, as storage spaces for hydrophobic silica (HS) nanoparticles to obtain superhydrophobic capacity and superior robustness. The HS nanoparticles stored in the grooves among the laser-melted tracks serve as the hydrophobic guests, while the ridges' metal network provides the mechanical strength, leading to robust superhydrophobic objects with desired 3D structures. Moreover, HS nanoparticles coated on the LPBF-AM-printed surface can inhibit corrosion behavior caused by surface defects. It was found that LPBF-AM-printed objects with HS nanoparticles retained superior hydrophobicity after 150 abrasion cycles (~12.5 KPa) or 50 cycles (~37.5 KPa). Furthermore, LPBF-AM-printed ships with superhydrophobic coating maintained great water repellency even after 10,000 cycles of seawater swashing, preventing dynamic corrosion upon surfaces. Our proposed strategy, therefore, provides a low-cost, highly efficient, and robust superhydrophobic coating, which is applicable to metal 3D architectures toward corrosion-resistant requirements.

Biobased Resin for Sustainable Stereolithography: 3D Printed Vegetable Oil Acrylate Reinforced with Ultra-Low Content of Nanocellulose for Fossil Resin Substitution

The use of biobased materials in additive manufacturing is a promising long-term strategy for advancing the polymer industry toward a circular economy and reducing the environmental impact. In commercial 3D printing formulations, there is still a scarcity of efficient biobased polymer resins. This research proposes vegetable oils as biobased components to formulate the stereolithography (SLA) resin. Application of nanocellulose filler, prepared from agricultural waste, remarkably improves the printed material's performance properties. The strong bonding of nanofibrillated celluloses' (NFCs') matrix helps develop a strong interface and produce a polymer nanocomposite with enhanced thermal properties and dynamical mechanical characteristics. The ultra-low NFC content of 0.1-1.0 wt% (0.07-0.71 vol%) was examined in printed samples, with the lowest concentration yielding some of the most promising results. The developed SLA resins showed good printability, and the printing accuracy was not decreased by adding NFC. At the same time, an increase in the resin viscosity with higher filler loading was observed. Resins maintained high transparency in the 500-700 nm spectral region. The glass transition temperature for the 0.71 vol% composition increased by 28°C when compared to the nonreinforced composition. The nanocomposite's stiffness has increased fivefold for the 0.71 vol% composition. The thermal stability of printed compositions was retained after cellulose incorporation, and thermal conductivity was increased by 11%. Strong interfacial interactions were observed between the cellulose and the polymer in the form of hydrogen bonding between hydroxyl and ester groups, which were confirmed by Fourier-transform infrared spectroscopy. This research demonstrates a great potential to use acrylated vegetable oils and nanocellulose fillers as a feedstock to produce high-performance resins for sustainable SLA 3D printing.

Direct Visualization of Homogeneous Chemical Distribution in Functional Polyradical Microspheres

It is demonstrated that the postfunctionalization of solid polymeric microspheres can generate fully and throughout functionalized materials, contrary to the expectation that core-shell structures are generated. The full functionalization is illustrated on the example of photochemically generated microspheres, which are subsequently transformed into polyradical systems. Given the all-organic nature of the functionalized microspheres, characterization methods with high analytical sensitivity and spatial resolution are pioneered by directly visualizing the inner chemical distribution of the postfunctionalized microspheres based on characteristic electron energy loss signals in transmission electron microscopy (TEM). Specifically, ultrasonic ultramicrotomy is combined successfully with electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) during TEM. These findings open a key avenue for analyzing all-organic low-contrast soft-matter material structures, while the specifically investigated system concomitantly holds promise as an all-radical solid-state functional material.

Superrepellent Porous Polymer Surfaces by Replication from Wrinkled Polydimethylsiloxane/Parylene F

Superrepellent surfaces, such as micro/nanostructured surfaces, are of key importance in both academia and industry for emerging applications in areas such as self-cleaning, drag reduction, and oil repellence. Engineering these surfaces is achieved through the combination of the required surface topography, such as porosity, with low-surface-energy materials. The surface topography is crucial for achieving high liquid repellence and low roll-off angles. In general, the combination of micro- and nanostructures is most promising in achieving high repellence. In this work, we report the enhancement of wetting properties of porous polymers by replication from wrinkled Parylene F (PF)-coated polydimethylsiloxane (PDMS). Fluorinated polymer foam “Fluoropor” serves as the low-surface-energy polymer. The wrinkled molds are achieved via the deposition of a thin PF layer onto the soft PDMS substrates. Through consecutive supercritical drying, superrepellent surfaces with a high surface porosity and a high water contact angle (CA) of >165° are achieved. The replicated surfaces show low roll-off angles (ROA) <10° for water and <21° for ethylene glycol. Moreover, the introduction of the micro-wrinkles to Fluoropor not only enhances its liquid repellence for water and ethylene glycol but also for liquids with low surface tension, such as n-hexadecane.

Dip-In Photoresist for Photoinhibited Two-Photon Lithography to Realize High-Precision Direct Laser Writing on Wafer

For decades, photoinhibited two-photon lithography (PI-TPL) has been continually developed and applied into versatile nanofabrication. However, ultrahigh precision fabrication on wafer by PI-TPL remains challenging, due to the lack of a refractive index (n) matched photoresist (Rim-P) with effective photoinhibition capacity for dip-in mode. In this paper, various Rim-P are developed and then screened for their applications in PI-TPL. In addition, different lithography methods (in terms of oil-mode and dip-in mode) are analyzed by use of optical simulations combined with experiments. Remarkably, one type of Rim-P (n = 1.518) shows effective photoinhibition capacity, which represents an outstanding breakthrough in the field of PI-TPL. In contrast to photoresist with an unsuitable refractive index, optical aberrations are almost completely eliminated in the dip-in mode by using the Rim-P. Consequently, features with a minimum critical dimension as small as 39 nm are successfully achieved on wafer by dip-in PI-TPL, which paves the way for subdiffraction silicon-based chip manufacturing by PI-TPL. Moreover, through a combination of the Rim-P and dip-in mode, the ability to achieve tall and high-precision three-dimensional nanostructures is no longer problematic.

Emerging porous organic polymers for biomedical applications

This review summarizes and discusses the recent progress in porous organic polymers for diverse biomedical applications such as drug delivery, biomacromolecule immobilization, phototherapy, biosensing, bioimaging, and antibacterial applications.

3D Laser Nanoprinting of Optically Functionalized Structures with Effective-Refractive-Index Tailorable TiO2 Nanoparticle-Doped Photoresin

The advanced direct laser printing of functional devices with tunable effective index is a key research topic in numerous emerging fields, especially in micro-/nano-optics, nanophotonics, and electronics. Photosensitized nanocomposites, consisting of high-index materials (e.g., titanium dioxide, TiO₂) embedded in polymer matrix, are emerging as attractive platforms for advanced additive manufacturing. Unfortunately, in the currently applied techniques, the preparation of optically functionalized structures based on these photosensitized nanocomposites is still hampered by many issues like hydrolysis reaction, high-temperature calcinations, and, especially, the complexity of experimental procedures. In this study, we demonstrate a feasible strategy for fabricating micro-/nanostructures with a flexibly manipulated effective refractive index by incorporating TiO₂ nanoparticles in the matrix of acrylate resin, i.e., TiO₂-based photosensitized nanocomposites. It was found that the effective refractive index of nanocomposite can be easily tuned by altering the concentration of titanium dioxide nanoparticles in the monomer matrix. For TiO₂ nanoparticle concentrations up to 30 wt%, the refractive index can be increased over 11.3% (i.e., altering from 1.50 of pure monomer to 1.67 at 532 nm). Based on such a photosensitized nanocomposite, the grating structures defined by femtosecond laser nanoprinting can offer vivid colors, ranging from crimson to magenta, as observed in the dark-field images. The minimum printing width and printing resolution are estimated at around 70 nm and 225 nm, indicating that the proposed strategy may pave the way for the production of versatile, scalable, and functionalized opto-devices with controllable refractive indices.

Density Regulation and Localization of Cell Clusters by Self-Assembled Femtosecond-Laser-Fabricated Micropillar Arrays

Tumor cell clusters of varying sizes and densities have different metastatic potentials. Three-dimensional (3D) patterned structures with rational topographical and mechanical properties are capable of guiding the 3D clustering of tumor cells. In this study, single femtosecond laser pulses were used to fabricate individual high-aspect-ratio micropillars via two-photon polymerization (TPP). By combining this approach with capillary-force self-assembly, complex 3D microstructure patterns were constructed with a high efficiency. The microstructures were able to regulate the formation of cell clusters at different cell seeding densities and direct self-guided 3D assembly of cell clusters of various sizes and densities. Localization of cell clusters was achieved using grid-indexed samples to address individual cell clusters, which holds great promise for in situ cell cluster culture and monitoring and for applications such as RNA sequencing of cell clusters.

3D Printing of Superhydrophobic Objects with Bulk Nanostructure

The rapid development of 3D printing (or additive manufacturing) technologies demands new materials with novel properties and functionalities. Superhydrophobic materials, owing to their ultralow water adhesion, self-cleaning, anti-biofouling, or superoleophilic properties are useful for myriad applications involving liquids. However, the majority of the methods for making superhydrophobic surfaces have been based on surface functionalization and coatings, which are challenging to apply to 3D objects. Additionally, these coatings are vulnerable to abrasion due to low mechanical stability and limited thickness. Here, a new materials concept and methodology for 3D printing of superhydrophobic macroscopic objects with bulk nanostructure and almost unlimited geometrical freedom is presented. The method is based on a specific ink composed of hydrophobic (meth)acrylate monomers and porogen solvents, which undergoes phase separation upon photopolymerization to generate inherently nanoporous and superhydrophobic structures. Using a desktop Digital Light Processing printer, superhydrophobic 3D objects with complex shapes are demonstrated, with ultralow and uniform water adhesion measured with scanning droplet adhesion microscopy. It is shown that the 3D-printed objects, owing to their nanoporous structure throughout the entire volume, preserve their superhydrophobicity upon wear damage. Finally, a superhydrophobic 3D-printed gas-permeable and water-repellent microfluidic device and a hierarchically structured 3D-printed super-oil-absorbent are demonstrated.

Structure-Processing-Property Relationships of 3D Printed Porous Polymeric Materials

Imparting porosity to 3D printed polymeric materials is an attractive option for producing lightweight, flexible, customizable objects such as sensors and garments. Although methods currently exist to introduce pores into 3D printed objects, little work has explored the structure-processing-property relationships of these materials. In this study, photopolymer/sacrificial paraffin filler composite inks were produced and printed by a direct ink writing (DIW) technique that leveraged paraffin particles as sacrificial viscosity modifiers in a matrix of commercial elastomer photocurable resin. After printing, paraffin was dissolved by immersion of the cured part in an organic solvent at elevated temperature, leaving behind a porous matrix. Rheometry experiments demonstrated that composites with between 40 and 70 wt % paraffin particles were able to be successfully 3D printed; thus, the porosity of printed objects can be varied from 43 to 73 vol %. Scanning electron microscopy images demonstrated that closed-cell porous structures formed at low porosity values, whereas open-cell structures formed at and above approximately 53 vol % porosity. Tensile tests revealed a decrease in elastic modulus as the porosity of the material was increased. These tests were simulated using finite element analysis (FEA), and it was found that the Neo-Hookean model was appropriate to represent the 3D printed porous material at lower and higher void fractions within a 75% strain, and the Ogden model also gave good predictions of porous material performance. The transition between closed- and open-cell behaviors occurred at 52.4 vol % porosity in the cubic representative volume elements used for FEA, which agreed with experimental findings that this transition occurred at approximately 53 vol % porosity. This work demonstrates that the tandem use of rheometry, FEA, and DIW enables the design of complex, tailorable 3D printed porous structures with desired mechanical performance.

3D printing of inherently nanoporous polymers via polymerization-induced phase separation

3D printing offers enormous flexibility in fabrication of polymer objects with complex geometries. However, it is not suitable for fabricating large polymer structures with geometrical features at the sub-micrometer scale. Porous structure at the sub-micrometer scale can render macroscopic objects with unique properties, including similarities with biological interfaces, permeability and extremely large surface area, imperative inter alia for adsorption, separation, sensing or biomedical applications. Here, we introduce a method combining advantages of 3D printing via digital light processing and polymerization-induced phase separation, which enables formation of 3D polymer structures of digitally defined macroscopic geometry with controllable inherent porosity at the sub-micrometer scale. We demonstrate the possibility to create 3D polymer structures of highly complex geometries and spatially controlled pore sizes from 10 nm to 1000 µm. Produced hierarchical polymers combining nanoporosity with micrometer-sized pores demonstrate improved adsorption performance due to better pore accessibility and favored cell adhesion and growth for 3D cell culture due to surface porosity. This method extends the scope of applications of 3D printing to hierarchical inherently porous 3D objects combining structural features ranging from 10 nm up to cm, making them available for a wide variety of applications.

3D printing offers flexibility in fabrication of polymer objects but fabrication of large polymer structures with micrometer-sized geometrical features are challenging. Here, the authors introduce a method combining advantages of 3D printing and polymerization-induced phase separation, which enables formation of 3D polymer structures with controllable inherent porosity.

Shrinkage-Considered Mold Design for Improvement of Micro/Nano-Structured Optical Element Performance

Polymer shrinkage in nano-imprint lithography (NIL) is one of the critical issues that must be considered in order to produce a quality product. Especially, this condition should be considered during the manufacture of optical elements, because micro/nano-structured optical elements should be controlled to fit the desired shape in order to achieve the intended optical performance. In this paper, during NIL, we characterized the shrinkage of polymeric resin on micro lens array (MLA), which is one of the representative micro/nano-structured optical elements. The curvature shape and optical performance of MLA were measured to check the shrinkage tendency during the process. The master mold of MLA was generated by the two-photon polymerization (2PP) additive manufacturing method, and the tested samples were replicated from the master mold with NIL. Several types of resin were adjusted to prepare the specimens, and the shrinkage effects in each case were compared. The shrinkage showed different trends based on the NIL materials and MLA shapes. These characterizations can be applied to compensate for the MLA design, and the desired performance of MLA products can be achieved with a corrected master mold.

Photonic Crystals

The past and present goal of photonic technology stems in the fine and arbitrary control of light propagation within miniaturized devices that can possibly integrate different functionalities [...]