Mechanically guided assembly of 3D microfluidic architectures
The platforms described in the following, which we refer to as 3D microvascular systems due to their micro/millimeter scale geometries and soft mechanical properties, transform from lithographically fabricated 2D precursor structures through compressive buckling, following concepts previously demonstrated in various thin-film materials and devices in areas other than microfluidics (41, 42, 44, 50, 52, 53). This assembly approach applies across a wide range of length scales, from nanometers to meters, and it is compatible with nearly any class of material, hard or soft, organic or inorganic. Recent publications on these schemes highlight various capabilities and applications in areas ranging from microelectromechanical systems, to electronic and optoelectronic devices, to energy harvesters, and to cell scaffolds (41–53).
Figure 1 presents a schematic illustration of the process as implemented here, beginning with the soft lithographic preparation of 2D microfluidic precursors (geometries in fig. S1) and the subsequent controlled buckling mechanisms that convert them into 3D systems (see Materials and Methods for details; movie S1). This structure is a double-layered 3D architecture that has geometrical features comparable to those of basic biological vascular networks (Fig. 1, A and B). Specifically, the layout involves a stepwise change in the widths of the arrays of microchannels (from 100 μm to 30 μm and 10 μm, and then back to 30 μm and 100 μm) and a three-level branching configuration (Fig. 1C and figs. S2 and S3) to mimic a collection of arteries, arterioles, capillaries, venules, and veins. The narrowest channels have widths of 10 μm, comparable to the sizes of capillaries in human body. The overall 3D shape approximates a spheroid, with an enclosed internal cavity between the top and bottom layers, in resemblance to biological constructs like glomeruli and alveoli. This spheroidal configuration follows from a computationally guided design approach that includes contributions from constituent structural components bonded at a collection of sites (Fig. 1D) to an underlying elastomer substrate with a biaxial prestrain of 50% (figs. S1, S2, and S4). Note S1 (and fig. S4) summarizes key mechanical considerations in design choices.
As described in detail in Materials and Methods, fabrication of this construct begins with formation of a 2D microfluidic system in PDMS (Fig. 1E) by casting and curing a layer of this material against a mold that consists of photodefined patterns of microchannel geometries on a silicon wafer. A laser cutting process defines inlets and outlets for fluid introduction and removal, respectively. Peeling this PDMS structure from the mold and bonding it to a flat layer of PDMS cast and cured against an unpatterned silicon wafer form a sealed 2D microchannel network. Another laser cutting process yields an open 2D architecture that follows the geometry of the microfluidic channels. Peeling this system (i.e., 2D precursor) from the wafer, bonding it at selected locations on a prestretched elastomer substrate, and then releasing this prestretch creates compressive forces that act at the bonding locations to trigger buckling processes and associated geometrical transformation into a 3D layout. The example highlighted here uses two such precursors transferred and selectively bonded to the prestretched substrate as an aligned multilayer assembly, at locations denoted as orange dots in Fig. 1D. The same processes can be applied to broad classes of materials, including not only materials for microfluidic systems but also those for electronic and photonic capabilities. On the basis of these straightforward procedures, 3D microchannel geometries with complexity and multifunctional integration are readily accessible, as described in the following sections.