The creation of geometrically complex fluidic devices is a subject of broad fundamental and technological interest. Here, we demonstrate the fabrication of three-dimensional (3D) microvascular networks through direct-write assembly of a fugitive organic ink. This approach yields a pervasive network of smooth cylindrical channels (approximately 10-300 microm) with defined connectivity. Square-spiral towers, isolated within this vascular network, promote fluid mixing through chaotic advection. These vertical towers give rise to dramatic improvements in mixing relative to simple straight (1D) and square-wave (2D) channels while significantly reducing the device planar footprint. We envisage that 3D microvascular networks will provide an enabling platform for a wide array of fluidic-based applications.

The integration of nanotechnology into three-dimensional printing (3DP) offers huge potential and opportunities for the manufacturing of 3D engineered materials exhibiting optimized properties and multifunctionality. The literature relating to different 3DP techniques used to fabricate 3D structures at the macro- and microscale made of nanocomposite materials is reviewed here. The current state-of-the-art fabrication methods, their main characteristics (e.g., resolutions, advantages, limitations), the process parameters, and materials requirements are discussed. A comprehensive review is carried out on the use of metal- and carbon-based nanomaterials incorporated into polymers or hydrogels for the manufacturing of 3D structures, mostly at the microscale, using different 3D-printing techniques. Several methods, including but not limited to micro-stereolithography, extrusion-based direct-write technologies, inkjet-printing techniques, and popular powder-bed technology, are discussed. Various examples of 3D nanocomposite macro- and microstructures manufactured using different 3D-printing technologies for a wide range of domains such as microelectromechanical systems (MEMS), lab-on-a-chip, microfluidics, engineered materials and composites, microelectronics, tissue engineering, and biosystems are reviewed. Parallel advances on materials and techniques are still required in order to employ the full potential of 3D printing of multifunctional nanocomposites.

Recent advances in multi-material 3D and 4D printing (time as the fourth dimension) show that the technology has the potential to extend the design space beyond complex geometries. The potential of these additive manufacturing (AM) technologies allows for functional inclusion in a low-cost single-step manufacturing process. Different composite materials and various AM technologies can be used and combined to create customized multi-functional objects to suit many needs. In this work, several types of 3D and 4D printing technologies are compared and the advantages and disadvantages of each technology are discussed. The various features and applications of 3D and 4D printing technologies used in the fabrication of multi-material objects are reviewed. Finally, new avenues for the development of multi-material 3D and 4D printed objects are proposed, which reflect the current deficiencies and future opportunities for inclusion by AM.

Development of a 3D printable material system possessing inherent piezoelectric properties to fabricate integrable sensors in a single-step printing process without poling is of importance to the creation of a wide variety of smart structures. Here, we study the effect of addition of barium titanate nanoparticles in nucleating piezoelectric β-polymorph in 3D printable polyvinylidene fluoride (PVDF) and fabrication of the layer-by-layer and self-supporting piezoelectric structures on a micro- to millimeter scale by solvent evaporation-assisted 3D printing at room temperature. The nanocomposite formulation obtained after a comprehensive investigation of composition and processing techniques possesses a piezoelectric coefficient, d, of 18 pC N, which is comparable to that of typical poled and stretched commercial PVDF film sensors. A 3D contact sensor that generates up to 4 V upon gentle finger taps demonstrates the efficacy of the fabrication technique. Our one-step 3D printing of piezoelectric nanocomposites can form ready-to-use, complex-shaped, flexible, and lightweight piezoelectric devices. When combined with other 3D printable materials, they could serve as stand-alone or embedded sensors in aerospace, biomedicine, and robotic applications.

Microfluidic systems are essential to a broad range of technological applications, including biotechnology, [1] microelectronics, [2] sensors, [3] chemical reactors, [4] and autonomic materials. [5] Several approaches have emerged for fabricating two-dimensional (2D) [6] and three-dimensional (3D) [7] microfluidic devices, including photolithographic or soft-lithographic techniques, [1b,2a,3c,4b,5b] laser micromachining, [7b] and derivative methods based on soft lithography.[7d±f] These techniques, however, have been confined to relatively thin device architectures (of only a few layers) and limited by materials constraints, [7b] poor resolution, [7c] or the need for extensive manual labor. [7d±f] We recently demonstrated the direct-write assembly of 3D microvascular networks, consisting of 16-layer structures with interconnected microchannels (200 lm in diameter) encapsulated in an epoxy matrix.[7a] Our approach involved the robotic deposition of a fugitive organic ink, which yields the desired microchannel network upon its subsequent removal from the matrix. Difficulties such as deformation of this fugitive-ink scaffold were encountered during assembly that prevented fabrication of larger 3D structures. Here, we report the development of a new fugitive ink that enables the direct-write assembly of 3D scaffolds (with more than a hundred layers) that retain their shape during fabrication and subsequent matrix infiltration under ambient conditions. The fabrication procedure of 3D microvascular networks (see Scheme 1) begins with the robotic deposition [8] of the fugitive ink onto a moving x±y platform, yielding a 2D pattern. After the initial layer is generated, the deposition nozzle, which is mounted on the z-stage, is raised a finite height and another layer is deposited. This process is repeated until the desired 3D scaffold is created. The interstitial pore space between patterned features is then infiltrated with a low-viscosity epoxy. Upon curing, the ink-based scaffold is removed, yielding an interconnected 3D microvascular network. The fugitive inks used in this directed assembly technique must satisfy several criteria. First, the ink must flow through a fine Scheme 1. Schematic representation of the fabrication procedure for 3D microvascular networks by direct-write assembly: a) deposition of fugitive ink (in blue) through cylindrical nozzle; b) multilayer scaffold after ink deposition; c) resin infiltration into scaffold; d) resin solidification to form structural matrix; and e) 3D microvascular network created after removal of fugitive ink.