Anyone who has taken a swim while wearing an adhesive bandage knows that however tightly such a dressing may cling when dry, it performs poorly when wet. Coating it with a waterproof adhesive might solve the problem, but it would make it excruciating, if not impossible, to remove. Imagine, then, the advantages of an adhesive material that is easy to remove and reapply and that stays put, even underwater.
Phil Messersmith, professor of biomedical engineering, imagined just such a material — with potential applications extending far beyond the everyday adhesive patch. His idea was to mimic and unite the adhesive properties of two different species: geckos and mussels. To transform his idea into reality Messersmith sought help from a few members of the diverse crew of postdoctoral fellows and graduate students that forms his research group. The result, achieved in a little less than two years, was “geckel,” a revolutionary biomimetic material that caught the attention of the international press and won a spot on the cover of the July 19 issue of the journal Nature.
Not every brilliant idea enjoys such relatively quick success. Messersmith, who joined the Northwestern faculty in 1997, says, “In my career, for every 10 ideas, maybe one works well.” The success of geckel might not be the norm, but the discovery process that Messersmith’s lab undertook to create geckel is replicated every day in every lab around the world: Researchers dream up ideas, test them, learn from mistakes, and arrive at solutions.
How do researchers come up with ideas? “I like to move in the fringes of established fields, where totally different fields interact,” says Messersmith. “That’s where the opportunities are.”
Messersmith’s background — an undergraduate degree in life sciences, a master’s degree in bioengineering, and a PhD in materials science — allows him to travel comfortably from one discipline to another and seize those opportunities. “I tell my students that I’m not particularly interested in being mainstream. I’d like to do something different from everyone else.
The reality of research, however, is that you have to have funding, says Messersmith, whose geckel work was funded by the National Institutes of Health (NIH) and NASA. “To write a grant, you usually have an application in mind. But a grant is not a contract. It allows us to pursue things we don’t necessarily propose. If all we do in my lab is accomplish what we say in the proposal, I consider it a failure. You find new things, you recognize opportunities — perhaps even in a failed experiment —and generate a new idea from that.” For example, says Messersmith, “Geckel wasn’t even on the radar when I wrote the proposal [to the NIH for mimicking mussel adhesive proteins]. The word ‘gecko’ didn’t appear once.”
What happened was that in the midst of his lab’s research on mussel adhesive proteins, Messersmith read a scientific paper that quantitatively compared gecko adhesion in air and water. “Always survey the literature,” Messersmith tells his students. “You can’t know your place in the research world without the knowledge of what everybody else is doing.”
On land, geckos can cling to almost any surface, even skittering upside down across a ceiling. This remarkable ability to adhere to a surface and, just as easily, detach from it derives from a mechanical principle known as contact splitting. The pad of a gecko’s foot is densely packed with very fine hairs split at the ends into spatulae — filaments as small as 200 nanometers in diameter. Each of those clings to a surface by means of weak attractive forces such as capillary and van der Waals forces. Multiplied across millions of hairs, those forces increase significantly. Flies, bees, and other insects use this strategy; geckos are the largest creatures to do so.
The paper Messersmith read noted a dramatic drop in adhesion for geckos in water, something that had been long observed and was now measured. That, he says, really got him thinking: “Geckos are the classic model for scientists studying dry adhesion in nature, whereas mussels are the classic model for wet adhesion” — a phenomenon apparent to anyone who has tried to pry a mussel from a wet rock. Messersmith’s lab was already studying mussel adhesives, focusing on the proteins excreted by the common blue mussel. Other labs had tried to replicate the holding power of geckos’ spatulae by fabricating polymers and multiwalled carbon nanotubes, although their efforts had not worked for more than a few contact cycles and had not been tested underwater. Messersmith wondered if there might be a way to combine the adhesive properties of both the gecko and the mussel.
Making it happen
To test his idea Messersmith approached one of his graduate students, Haeshin Lee, whom Messersmith describes as an exceptional student capable of managing several projects simultaneously. Messersmith says he made a couple of sketches and asked Lee, “Wouldn’t it be neat if we could do this?”
“It took me only a couple minutes to realize that Phil was talking about an excellent idea,” says Lee. “I was also happy to study a totally different field during my PhD work,” he adds, sounding a little like his mentor. Since completing his undergraduate degree in biological sciences in his native South Korea in 1997, Lee had studied drug delivery systems and wanted “to study something different but related to proteins.” He had also learned how to measure the binding force of proteins at the single-molecule level, a skill that would prove essential to his work on geckel.
Lee, who will complete his PhD work by early spring, says that when he joined Messersmith’s lab in 2003 he was pleasantly surprised by the amount of freedom his research adviser gave him. “Instead of telling me what to do, he guides me in the right direction,” says Lee, who found plenty of help available. “Phil’s input is everywhere, and I am surrounded by very talented postdocs and graduate students who have answers for my questions almost all the time.”
Lee gives Messersmith full credit for the idea of an adhesive based on geckos and mussels — “It would never have occurred to me,” he says — while Messersmith credits Lee with implementing it. “It’s one thing for me to tell a student what to do,” says Messersmith. “It’s another for me to just plant a seed and watch him transform it. Until you do the experiment, you can never tell how good an idea is.” This is a lesson Messersmith learned during his own graduate work: “I was handed a project that was loosely defined. I failed for a couple years, and then I got it to work in a new form. It was valuable experience.”
The work on geckel proceeded with relatively few setbacks. The first step was to fabricate an array of nanoscale pillars to mimic the split hairs on the pads of geckos’ feet. To do this, the researchers used electron-beam lithography to create a pattern of holes on a thin film of acrylic supported on a silicon wafer; this would serve as a mold, with a liquid polymer cast into the holes. Once the polymer cured, it was lifted from the mold to reveal something resembling the bristles on a nanoscale brush. After fabricating arrays with pillars ranging from 200 to 600 nanometers in diameter and from 600 to 700 nanometers in height, the team tested the adhesion of pillars 400 nanometers in diameter and 600 nanometers high.
The next step was to coat the pillars — the gecko part of the hybrid — with a synthetic polymer that mimics the proteins excreted by mussels. Here the team was able to build on earlier work in which Messersmith’s research group had created mussel-mimetic polymers. Because the “glue” proteins of mussels have a high concentration of a catecholic amino acid — 3,4-L-dihydroxyphenylalanine (DOPA) — the project required a polymer with a high DOPA content as well as low water solubility. That polymer was synthesized by Bruce P. Lee, a former graduate student of Messersmith’s and senior scientist at Nerites Corporation, a biotechnology established fields, where totally company in Madison, Wisconsin, where Messersmith serves as chief scientific adviser. The arrays were then dip coated to deposit a thin film, less than 20 nanometers, over the pillars.
The result was a one-square-centimeter patch of material that, says Messersmith, “you might mistake for a piece of flat tape.” In their Nature article Haeshin Lee, Bruce Lee, and Messersmith write, “We refer to the resulting flexible organic nanoadhesive as ‘geckel,’ reflecting the inspiration from both gecko and mussel.”
Testing of the fabricated material came at several stages: the adhesion of the uncoated and the coated pillar arrays was measured both in air and underwater. Measurement at the nanoscale level meant using an atomic force microscope to determine the force of adhesion — gauged not by how tightly a material sticks to something but rather by how much force is needed to pull it away from an object. In the case of geckel, researchers used a tipless cantilever to make contact with an array of pillars and then retracted it, measuring the force versus distance needed to separate the cantilever from the array.
The results were dramatic: the pillar arrays coated with the mussel-mimetic polymer improved wet adhesion 15 times over the uncoated pillar arrays. Furthermore, after 1,100 contact cycles the wet-and dry-adhesion power of the geckel patch was only slightly diminished. Previously, other gecko-mimetic adhesives had stayed sticky for only a few contact cycles, and none had been shown to work underwater.
It was in the measurement of the adhesion forces that Haeshin Lee says he experienced a true “Eureka!” moment: “The data showed simultaneous loading and detachment of up to six geckel nanopillars. I made a short video clip because it looks cool — you can download it from the Natureweb site. It is the adhesion force measurement with the highest resolution ever.”
What does geckel feel like on human skin? “As you would expect, it is sticky,” says Lee, who confesses that he tested a sample on his own skin, an experiment he won’t repeat because it would exclude the sample from further experiments. Messersmith and Lee are currently experimenting with changing the shape of the tips of the pillars for maximum contact area. “In a gecko the ends are shaped like a spatula,” says Messersmith. “The closest we can fabricate is a mushroom shape.”
As tightly as geckel adheres, it is also easy to detach. Messersmith and Lee accomplished this by controlling the composition of the polymer. Messersmith compares removal to peeling away a sticky note: “As you pull it away from a surface, the closest points of connection are under the highest stress. You literally pull away the bonds, one by one.”
Taking it to the next level
This detachability makes geckel a promising candidate for medical uses: to replace sutures for wound closure or to enable follow-up surgery; for drug-delivery patches; or for everyday bandages that would allow patients to bathe. Military and industrial uses might include creating an unmanned vehicle capable of crawling on the ocean floor. The researchers have applied for a patent on the new material. How quickly these potential applications are realized may depend on the involvement of industry. “We’ve demonstrated a concept,” says Messersmith, “but it will be necessary to develop a patterning approach that works on a large scale so that geckel can be mass produced in a cost-effective way.”
Meanwhile Messersmith’s lab is engaged in six or seven lines of research. The group’s latest success, published in Science, is another mussel-inspired project: a two-step, room-temperature method that allows a coating to be applied to material of any size, shape, or type, including traditionally difficult-to-clad materials such as Teflon®. For this work Haeshin Lee and Messersmith collaborated with William Miller, professor of chemical and biological engineering, and graduate student Shara Dellatore. “It’s an astonishingly simple and versatile approach to functional surface modification of materials,” says Messersmith. “The ability to form self-assembled monolayers on nonmetal surfaces is the biggest part of this.”
What research project will be next for Messersmith? “Who knows?” he asks. “I wasn’t doing gecko research two years ago.”
(Originally published in the fall 2007 issue of McCormick Magazine)