Portland, Ore A new world of applications for shape-changing materials might become available if man-made rigid structures like an airplane wing, which derives its strength from being "fixed" could adopt the abilities of rigid structures that can change shape, like a flower stem that twists to follow the sun. Airplane wings imbued with such ability might, for example, self-trim as they sensed the plane's speed, or prepare for an imminent landing by cupping the air as a bird's wing does.
The Defense Advanced Research Projects Agency has granted four contracts under its Nastic Materials Program (www.darpa.mil/dso/thrust/matdev/nastic.htm) to explore these possibilities. One includes a 17-month, $2.1 million effort at the Virginia Polytechnic Institute and State University in Blacksburg, Va. "We will use the concept of nastic biological transport to create a synthetic template for making shape-changing materials," said Virginia Tech professor Don Leo, leader of a team that will develop a class of materials that mimic plant protein structures.
Project researchers at other universities, research organizations and corporations have expertise in molecular biology, polymer chemistry, structural modeling and control, and systems integration.
Leo's associates at Virginia Tech's Center for Intelligent Materials Systems and Structures (CIMMS) have successfully created synthetic templates for artificial muscles soft polymers that change shape to work as actuators (see related story, page 1). "Our program manager really wants these to be hard, structural materials probably not metals, but polymers that are hard enough to hold up some structural load," said Leo.
The Nastic Materials program manager, John Main, intends to generate motion in structural materials by controlling the fluid pressure within 10- to 100-micron-diameter "cells" similar to plant cells. Modulating the pressure actively transports fluid through the cell walls, enabling a highly distributed and redundant hydraulic actuation system.
The program seeks to mimic biological "hydraulics" to generate at least 10 megapascals of blocked stress and 20 percent free strain in a reversible process. Demonstrating a power density of this magnitude, according to Main, will indicate that nastic systems could outperform conventional hydraulic systems as well as enable adaptive morphing in military vehicles.
"Biological energy sources have a lot of energy per unit volume," Leo said. "We want to use that energy as directly as possible."
Nastic biological systems work by a variety of mechanisms, such as osmosis, to pump fluid into thousands of tiny, micron-scale capsules that expand because of the pressure of the biological fuels. Instead of using a centralized pump like those engineered into man-made hydraulic systems, a natural system is characterized by fuel and a control mechanism. By supplying the correct fuel, the natural mechanism can be turned on or off without the need for a centralized "power supply."
To emulate such a biological mechanism in a smart material, the researchers plan to harness one of several biological mechanisms that account for nastic plant behaviors. Casting the mechanism into a synthetic template will allow them to test their theories using synthetic materials.
The synthetic template, once crafted to mimic plant biology, will be applied to polymer matrices in hopes of creating rigid, yet shape-changing, materials.
"Within 17 months we want to have 'coupons' of the material that is, little 5 x 5-centimeter plates that exhibit a shape-changing capability," Leo said. "We will consider our demonstration a success if we can supply the coupon with a biological fuel and get a shape change as a result."
CIMMS director Dan Inman, meanwhile, is working to conquer the engineering hurdles to realizing morphing airplane wings under Darpa's Morphing Aircraft Structures program, which aims to demonstrate a seamless wing that can radically change shape for example, from "reconnaissance" mode to "attack" form. Leo predicts "cross pollination" between these separate Darpa-sponsored efforts at CIMMS.