Bimaterial lattices as thermal adapters and actuators

The goal of this paper is to demonstrate how anisotropic biomaterial lattices can be used in thermal actuation. Compared to other lattices with tailored thermal expansion, the anisotropy of these bimaterial lattices makes them uniquely suitable for use as thermal actuators. Each individual cell, and hence lattices consisting of such cells, can be designed with widely different predetermined coefficients of thermal expansion (CTE) in different directions, enabling complex shape changes appropriate for actuation with either passive or active control. The lattices are composed of planar non-identical cells that each consist of a skewed hexagon surrounding an irregular triangle. The cells and all members of any cell are connected to each other by pins so that they have no rotational constraints and are able to expand or contract freely. In this case, the skew angles of the hexagon and the ratio of the CTEs of the two component materials determine the overall performance of the lattice. At its boundaries, the lattice is connected to substrates by pins and configured such that the CTE between two neighboring lattice vertices coincides with the CTE of the adjacent substrate. Provided the boundary behavior of the lattice is matched to the thermal properties of the substrates, temperature changes in the structure produce thermal strains without producing any corresponding stresses. Such lattices can be used in three different ways: as adaptive elements for stress-free connection of components with different CTEs; for fine tuning of structures; and as thermally driven actuators. In this paper, we demonstrate some concepts for lattice configurations that produce thermally-driven displacements that enable several actuators: a switch, a valve and tweezers.