Tree-shaped networks are now being considered as small-scale architectures for high-densities in electronics cooling and fuel cells design. This paper documents the optimization of tree-shaped inserts of high thermal conductivity. The new feature is the presence of loops in the tree canopy. Every feature of the tree-with-loops architecture is optimized numerically. Two classes of trees with loops are considered: loops with one size, and loops with two sizes. The performance of trees with loops is compared with that of trees without loops and designs with purely radial inserts. It is shown that dendrites and loops are features that become attractive as scales decrease and complexity increases. In the same direction, the robustness of tree-with-loops architectures increases. (c) 2006 Elsevier Ltd. All rights reserved.