Hierarchical Bio-Inspired Vascular Networks: Electrical Treeing

Vascular networks provide a method to distribute fluid throughout a system. Artificial vascular materials with enhanced properties are being developed that could ultimately be integrated into systems reliant upon fluid transport while retaining their structural properties. An uninterrupted and controllable supply of liquid is optimal for many applications such as continual self-healing materials, in-situ delivery of optically index matched fluids, thermal management (sweating) and drug delivery systems could benefit from a bio-inspired approach that combines complex network geometries with minimal processing parameters. One such approach to induce vascular networks whilst mimicking nature's design is electrical treeing (ET).

                (a)                              (b)                                         (c)                                       (d)

Figure 1. Optical image of ET (a) in a EPON 828/PACM system, (b) under AC driven electrical current showing “bush-like” features, (c) under DC driven electrical current showing “tree-like” features and filling of an ET grown vascular network with a UV visualization dye.

Electrical treeing (ET) is the result of partial discharges in a dielectric material. In the vicinity of a small diameter electrode, the local electric field is greater than the global dielectric strength, causing a localized, step-wise, breakdown to occur forming a highly branched interconnected structure (Fig. 1a). The growth of these structures is influenced by the configuration of the electrodes, with geometries of a point lead electrode to a point or plane ground electrode being of most interest. ET is a viable method to produce networks in 2D systems and in more robust 3D systems on a smaller, micron, scale than the products of the EHVF method. AC driven electrical current (Fig. 1b), harnessing a sine wave at 100 Hz, grows a “bush-like” structure with many branches and therefore a larger volume within the epoxy samples. DC driven electrical current (Fig. 1c) produces a more “tree-like” structure with fewer branches and bifurcations. The surface of the electrodes were modified with dispersed multi-walled carbon nanotubes (MWCNTs) to aid in increasing the local electric field, and thus enable a higher rate of tree initiation and growth. Inclusion of particles was investigated to determine if the growth direction can be manipulated. The use of self-clearing electrodes (as a grounding material) was investigated with the infiltration of a UV dye through the hollow channels produced by ET resulting in a vascularized network capable of repeated fillings and evacuations. Fluid delivery (Fig. 1d) can be tailored through the applications of different electrode and ground manufacturing techniques for optimized flow rates for a given application.