(333b) Rapid Fabrication and Characterization of 3-D Branched Microvascular Flow Networks

Standard photolithography-based micromachining techniques are widely used to construct 2D microchannel networks, but the inherently planar nature of these processes limits their usefulness in the creation of 3D structures. More recent developments have enabled fully 3D flow networks to be produced using processes including solid freeform fabrication, stereolithography, and 3D printing. But many of these methods involve serial ?direct writing' processes that require timescales on the order of hours to days and are not practical for mass production. In addition, no single technique has proven ideal to construct microchannel networks that incorporate a wide range of size scales (µm to mm). Here, we describe a new process that enables 3-D branched microvascular networks to be constructed in a single step. This technique employs an electrostatic discharge phenomenon that occurs when a dielectric medium is energized by a strong electric field and subsequently discharged to form branched ?tree-like? channels in polymer substrates. Suitable space charge distributions are generated by irradiating the sample with an electron beam so that the energy released upon discharge is sufficient to locally vaporize and fracture the material, leaving behind a network of branched channels in a tree-like fractal structure. Beam intensity and spatial irradiation profile (width and penetration depth) can be adjusted to control the location and morphology of the discharge structures.

The embedded patterns exhibit a self-similar fractal network structure, with channel characteristic dimensions ranging from approximately 100 nm to 1 mm in diameter. The formation of interconnected networks in the discharge structure was demonstrated by nucleating discharges at multiple sites along the substrate perimeter. Interconnectivity of the network between the different nucleation sites was demonstrated by injecting an aqueous dye into the network. The diameter of the microchannels was estimated to be ~ 20 microns using fluorescent beads and confocal microscopy. A microfluidic analog of the discharge structure was developed in parallel to investigate the shear stress and pressure drop in narrow channels in order to determine the feasibility of culturing cells inside these networks. Bovine aortic endothelial cells (BAEC) cultured in the microfluidic analog were viable for 72 h at flow rates of ~ 1 μL/min and demonstrate that cell culture within channels of similar dimensions in discharge structures can be carried out. We have also used this process to form microchannel networks in poly(lactic acid) substrates, suggesting that this technique could be used to generate vascular flow structures in substrates relevant for tissue engineering applications.