Multilayers and Water Don't Mix

Water falling on a lotus leaf forms droplets that roll smoothly over the leaf's sloping surface and fall off. The water-repulsion, or lotus effect, is due to a hydrophobic coat on the leaf that is roughened on the nanoscale, resulting in reduced contact area between the water droplets and the leaf's surface.

Chemists at Florida State University (FSU), Tallahassee, have now simulated the lotus effect with ultrathin layers of fluorinated polyelectrolytes containing nanorods of a naturally occurring material. They have also shown that the hydrophobic properties of fluorinated polyelectrolytes can be exploited to control the adhesion and growth of rat muscle cells on patterned surfaces.

To create the lotus effect, the FSU group, led by chemistry professor Joseph B. Schlenoff, prepared ultrahydrophobic nanocomposite films from a novel combination of fluorinated polyanions and polycations and the mineral attapulgite [Angew. Chem. Int. Ed., 44, 782 (2005)]. The mineral is a rare magnesium aluminum silicate clay consisting of needlelike nanorods 1 to 2 µm long and 10 to 50 nm thick.

"In the lotus effect, the surface of the leaf is rough on the micrometer scale and decorated with hydrophobic wax particles on the nanometer scale," Schlenoff explains. "In our system, the micrometer roughness is produced by clusters of attapulgite particles, and the nanometer roughness is mimicked by the clay nanorods themselves. Everything gets coated by a hydrophobic fluorinated polyelectrolyte layer."

The Florida researchers used sequential layer-by-layer assembly of oppositely charged polymers to prepare the films. The technique was developed in the early 1990s by a group led by Gero Decher, who is now a professor of chemistry at Louis Pasteur University, Strasbourg, France.

"Layer-by-layer assembly is a simple, low-cost, environmentally friendly technology that allows one to fabricate 'built-to-order' surfaces," Decher says. "Rather than thinking about which chemistry to use for realizing some functionality on a surface, layer-by-layer assembly makes it possible to design functional surfaces, in this case ultrahydrophobic surfaces, and to engineer them with a set of tools exceeding by far all other existing technologies for surface functionalization."

SCHLENOFF'S GROUP used a tri-strata approach to produce their ultrahydrophobic surfaces. The first stratum consists of several layer pairs of poly(diallyldimethylammonium) cations [PDADMA] and poly(4-styrenesulfonate) anions deposited from a salt solution onto a silicon wafer. The second stratum consists of alternating layers of the clay particles, which bear a negative surface charge, and PDADMA. The final stratum is made of layers of two fluorinated polyelectrolytes: Nafion (DuPont's sulfonated fluorinated anionic polymer), and a polycation synthesized from poly(vinylpyridine) and a fluorinated alkyl iodide.

Finally, the chemists measured the contact angles of drops of water dispensed on the multilayer surfaces. They use the term "ultrahydrophobic" for surfaces exhibiting large water-contact angles.

The use of layer-by-layer sequential adsorption for preparing ultrathin films with ultrahydrophobic surfaces has been previously reported, but the FSU method is the first to use fluorinated polyelectrolytes, Schlenoff points out.

"Although a number of groups have formed such surfaces, the method described by Schlenoff and coauthors is particularly simple," remarks Merlin L. Bruening, an expert on ultrathin films who is an associate professor of chemistry at Michigan State University, East Lansing. "The method involves no annealing or silanization steps and could be applied to many types of surfaces with varying geometries. However, there are still interesting questions regarding the actual film architecture and what is required to achieve ultrahydrophobicity."

Schlenoff notes that fluorinated multilayers have a divergent range of potential applications. "Their antiwetting properties mean they could be used as low-volatility stain-resistant coatings for fabrics and carpets," he says. "They contain much less water than regular multilayers, and we have found them to be much more effective at corrosion control. Fluorinated surfaces have a low coefficient of friction, and because the films have low dielectric constants, they could be good for high-speed electronic applications."

Fluorinated polyelectrolyte multilayers are also potentially useful for the development of materials with bioadhesive or bioinert surfaces. "Because proteins play an important role in the adhesion, spreading, and growth of cells, considerable effort has been expended in the development of polyelectrolyte thin films with properties that make the surface adhesive or resistant to protein adsorption," note Schlenoff and coworkers in a recent paper [Biomacromolecules, 6, 161 (2005)].

In the paper, Schlenoff and coauthors report investigations into the effects of surface charge and surface hydrophobicity of polyelectrolyte multilayers on the behavior of rat aortic smooth muscle cells cultured on the multilayers.

They show that surface charge and surface hydrophobicity alter the cell shape and attachment of the cultured cells. These surface properties can be used to manipulate the adhesive properties of the cells.

"Substrate properties such as charge and degree of hydrophobicity determine how well smooth muscle cells adhere and spread on the substrate and also affect whether the cells organize their actin filaments in contractile or synthetic smooth muscle cells," Schlenoff explains.

In the contractile state, smooth muscle cells use a system based on actins--filamentous proteins--to produce contractile force, but the cells do not crawl or divide. Synthetic smooth muscle cells rearrange the actin filament system to crawl and divide, but they are no longer able to produce contractile force.

"In vascular smooth muscle, contractile smooth muscle cells produce force for blood pressure regulation," Schlenoff observes. "When blood vessels are injured, a signal is released that prompts smooth muscle cells to change from the contractile state to the synthetic state. The synthetic smooth muscle cells migrate to the site of injury, where they grow and divide, repairing the damaged blood vessel. The newly formed smooth muscle cells eventually convert back to the contractile state and lose their ability to crawl and divide."

HOW SMOOTH MUSCLE cells interact with polyelectrolyte multilayer surfaces has implications for evaluating biocompatibility, according to Schlenoff. For example, the surface properties of stents implanted inside coronary arteries during surgical procedures are thought to play a significant role in the process of restenosis. In this process, vascular smooth muscle cells migrate to cover the stents and in many cases build up layers of tissue. The process can result in narrowing of the arteries and occlusion of blood flow.

The FSU chemists show that cells cultured on the hydrophobic surface of a multilayer film made from Nafion and a positive fluorinated polyelectrolyte spread and adhere. On the other hand, cells cultured on a hydrophilic surface of multilayers of a zwitterionic copolymer adhere poorly, if at all. The copolymer contains acrylic acid and the zwitterion 3-[2-(acrylamido)-ethyldimethyl ammonio] propane sulfonate.

The researchers note that the headgroups of three of the four major cell membrane phospholipids are zwitterions. Polymer surfaces with exposed zwitterion groups may therefore mimic some biological surfaces better than uniformly charged surfaces.

"The use of fluorinated multilayers for cell growth is counterintuitive, as fluorinated surfaces are highly nonnatural," Schlenoff says. "We used two sets of novel polymers. The fluorinated ones impart hydrophobicity, and the zwitterionic copolymer is very hydrophilic and cell resistant. We didn't observe any adsorption of cells or proteins on the zwitterionic copolymer. Our system has unusually high hydrophobic contrast."

Schlenoff and colleagues show that "hydrophobic contrast"--a term that they coined--can be used with polymer-on-polymer stamping to control and direct cell behavior. The researchers use a stamp made of polydimethylsiloxane to apply micropatterns of cell-adhesive Nafion onto cell-nonadhesive zwitterionic copolymer surfaces.

"When presented with this micropatterned surface, smooth muscle cells attach to, and spread on, only the fluorinated areas with very high selectivity," Schlenoff says.

The paper on cell adhesion is important, according to Bruening. "Although there have been many studies of cell adhesion to a variety of surfaces, including polyelectrolyte multilayers, this paper demonstrates clearly the control over cell attachment that is available by utilizing a wide variety of polyelectrolytes with differing charge states and hydrophobicities," he comments.