MIT engineers and colleagues from the University of California are reporting a unique design of a "smart surface" that can reversibly switch properties in response to an external stimulus. The work paves the way for systems that could, for example, release or absorb cells and chemicals from surfaces on demand.
In the Jan. 17 issue of Science, the researchers describe an example of their new approach in which they engineered a surface that can change from water-attracting to water-repelling with the application of a weak electric field. Switch the electrical potential of that field from positive to negative and the surface reverts to its initial affinity for water.
The general technique, which is patent pending, could also be applied to the dynamic control of other surface characteristics such as adhesion, friction and biocompatibility. "We started with a fundamental system to prove that the overall concept of reversibly modifying a surface via conformational transitions works," said Thanh-Nga Tran, a graduate student in the Harvard-MIT Division of Health Sciences and Technology (HST) and the Department of Chemical Engineering.
"This opens the door to a variety of applications, including novel drug-delivery systems and smart templates for the bioseparation of one molecule from another," said Robert Langer, MIT's Germeshausen Professor of Chemical and Biomedical Engineering and leader of the work. Langer has appointments in chemical engineering, HST and MIT's Biological Engineering Division.
"This is the first time to our knowledge that anyone has created a truly reversible switch of a surface's property exploiting monomolecular layers," said Joerg Lahann, a postdoctoral associate in chemical engineering. As a result, "we believe this study lays the fundamental groundwork for a new paradigm in surface engineering that may be of considerable significance in materials science, biology and medicine."
In addition to Langer, Lahann and Tran, other authors of the Science paper are former undergraduate Hiroki Kaido and former postdoctoral associate Insung S. Choi (both in chemical engineering); Samir Mitragotri and Jagannathan Sundaram of the University of California at Santa Barbara (Mitragotri is a former MIT chemical engineering Ph.D. and postdoctoral associate); and Saskia Hoffer and Gabor A. Somorjai of the University of California at Berkeley.
The new switchable surface essentially consists of a forest of molecules only a billionth of a meter tall, engineered to stand at a precise distance from each other. In this particular case, the team makes the top of each molecule negatively charged and hydrophilic (water-loving), and the trunk positively charged and hydrophobic (water-repelling).
When a positive electrical potential is applied, the induced attractive force causes the top to bend down. The resulting loop that is now exposed is hydrophobic. Reverse the electrical potential and the molecules will straighten to their full height, the hydrophilic tops once more attracting water.
"Look at your hand," Langer explained. "Imagine that your fingertips have property A and your knuckles have property B. We've created a reversible way to move those 'fingers' up and down," exposing either the fingertips or the knuckles with their different properties."
One important challenge for the team was finding a way to create a molecular forest, or self-assembled monolayer (SAM), with enough space between molecules to allow each to bend down. Conventional SAMs are characterized by very dense assemblies of molecules so tightly packed together that they have no room to move.
The MIT engineers solved the problem by adding bulky "hats" to each molecule during the assembly of the SAM, creating the equivalent of a field of molecular mushrooms. By then removing the hats, "we ended up with a low-density monolayer," Tran explained. The molecules now stood at the perfect distance from each other (a distance determined by the team's earlier calculations).
From there, the team had to prove the molecules could switch from one property to another. To do so, they assessed the surface's interactions with water by analyzing the contact angle of a water droplet on it. They also looked at the microscopic properties of the new surface.
The latter "was actually a very difficult task since the monolayer was only about one nanometer thick, or one-millionth the thickness of a dime," Lahann said. Somorjai and colleagues at Berkeley, however, had established a novel method called sum-frequency generation (SFG) spectroscopy--a very sensitive surface analysis technique. "SFG allows you to study a single layer of molecules on a surface, and that's exactly what was required in our case," said Lahann.
Future work will include developing surfaces that have different switchable properties as well as tailoring the proof-of-concept system for different applications.
This work was funded by the National Science Foundation through its Materials Research Science and Engineering Centers program; the U.S. Army Research Office through MIT's Institute for Soldier Nanotechnologies; the National Institutes of Health; the Fonds der Chemischen Industrie in Germany; and the Whitaker Foundation.