Physicists from the Institute for Medicine and Engineering (IME) at the University of Pennsylvania have found a new class of materials that self-assemble into flat, two-dimensional "crystallites" made from tiny plastic beads the size of bacteria. Laurence Ramos, PhD, a postdoctoral fellow in the physics department, and her colleagues at IME and the University of Delaware used membranes similar to soap bubbles as templates to direct the assembly of clusters of the beads into an intriguing new microstructured material. Along the way, the team uncovered a surprise: Under the right conditions, their beads seemed to defy the basic physical principle that oppositely charged objects attract. Their findings appear in this week's issue of Science.
Electrostatic self-assembly of objects onto membranes is a relatively new technique with such potential biological applications as DNA and protein chips, gene-delivery vehicles, and industrial catalysts. The researchers had long-studied the components of their new structures separately, but nothing in their experience prepared them for the highly organized structures Ramos saw when the two were mixed. "The controlled manufacture of these microarrays could mimic and exploit the remarkable organization seen in many natural biomaterials," says coauthor Philip Nelson, PhD, a professor of physics at Penn.
The membrane portion of the material is essentially a thin, positively
charged soaplike bubble. The surface of the bubble serves as a temporary
template on which the raft of negatively charged spheres assembles. In many
cases, the positively charged membrane attracted only a few dozen negatively
charged spheres, then repelled all others. "At this point there was a lot of
head-scratching, to put it mildly," says Nelson. "Every high-school student is
taught that oppositely charged objects attract -- so how could the membrane
switch from attracting to repelling the beads?" The key to the puzzle, say the
Contact: Karen Young Kreeger
University of Pennsylvania School of Medicine