3D‐Printing of Functionally Graded Porous Materials Using On‐Demand Reconfigurable Microfluidics

Abstract

Tailoring the morphology of macroporous structures remains one of the biggest challenges in material synthesis. Herein, we present an innovative approach for the fabrication of custom macroporous materials in which pore size varies throughout the structure by up to an order of magnitude. We employed a valve‐based flow‐focusing junction (vFF) in which the size of the orifice can be adjusted in real‐time (within tens of milliseconds) to generate foams with on‐line controlled bubble size. We used the junction to fabricate layered and smoothly graded porous structures with pore size varying in the range of 80–800 μm. Additionally, we mounted the vFF on top of an extrusion printer and 3D‐printed constructs characterized by a predefined 3D geometry and a controlled, spatially varying internal porous architecture, such as a model of a bone. The presented technology opens new possibilities in macroporous material synthesis with potential applications ranging from tissue engineering to aerospace industry and construction.

Understanding the multiscale self-assembly of metal–organic polyhedra towards functionally graded porous gels

“…4 Inspired by biological systems, synthetic materials with such gradients have been designated as functionally graded materials (FGM), 5 and porous materials containing gradients can give unique mechanical properties and permeability, with applications in various elds from tissue engineering 6 to the aerospace industry. 7 In most cases, the fabrication procedures to engineer porosity have been limited to top-down approaches including microuidics 8 or 3D printing technologies; 9 indeed, inducing gradients of porosity over the mesoporous and microporous regimes remains a challenge.…”

Architected Polymer Foams via Direct Bubble Writing

“…We therefore selected a flow rate Q = 10 mL min −1 for all experiments, because it allows for controlled bubble ejection over a wide range of gas pressures. Note, this flow rate is more than two orders of magnitude higher than those used for generating polymer foams within microfluidic devices . At this flow rate, we created a representative 3D polymer foam printed in the monodisperse bubble regime (Figure m–o), which demonstrates that the cell (bubble) size is well controlled.…”

“…In addition, bubble drainage and Ostwald ripening (gas transport from small to large cells) within the liquid foam prior to polymerization further broaden the bubble size distribution. To overcome this inhomogeneity, microfluidic techniques have recently been used to generate polymer foams composed of locally monodisperse bubbles of controlled connectivity or gradients . However, it is difficult to scale up these techniques to create large production volumes and freestanding or spanning features are still out of reach.…”

Microfluidics Mediated Production of Foams for Biomedical Applications

“…Although a number of conventional methods exist for producing solid foams (e.g., melt molding [65,66] and gas foaming [61,101]), liquid foam templating is one example that can be mediated with microfluidics. This is because its scaffold structure is built on the formation of liquid foams [36,38,59], whose connectivity and bubble size can be carefully controlled with microfluidics up to 800 µm [103], as has been widely reported in the literature [38,100,[103][104][105][106][107][108][109].…”

“…Solidfication of the liquid foam template involves heating [111], freeze-drying [113,114], and/or cross-linking [103] through the presence of photoinitiators, as in the case of gelatin methacryloyl (GM) foams [115], where generated liquid foams from microfluidic bubbling is exposed to UV light. Recent developments have also employed valve-based microfluidic flow-focusing (vFF), where the orifice size is controlled in real time during the passage of two immiscible fluids, thereby allowing immediate variation of bubble sizes [106]. When vFF techniques are incorporated with an extrusion printer, 3D structures can be fabricated with varying yet controlled internal porous architecture as a result of bubble size variation.…”