Heart attack (myocardial infarction) is the number one killer in the United States. People who are fortunate to survive a heart attack are left with a weakened heart muscle due to death of the tissue that was supplied with blood via the blocked vein. Scar tissue that forms at the blockage site impedes electrical and chemical communication between heart cells, increasing the risk of death from arrhythmias. To improve the outlook for heart attack survivors, a team of Johns Hopkins University researchers led by Andre Levchenko, Associate Professor of Biomedical Engineering, is working on a biologically inspired nanoengineered patch to repair the damaged area in the heart.
The Right Texture
Looking at rat heart muscle under an electron microscope, Levchenko and colleagues noticed that layers of cardiomyocytes were well-aligned with fibers of the underlying extracellular matrix (ECM), a scaffold that supports tissue development. Based on this observation, they supposed that the disorganized growth of heart cells on a flat dish was due to lack of an aligned ECM, specifically its rippled structure.
To test this hypothesis, they measured the size and spacing between fibers in the heart ECM and fabricated similar surfaces using a technique called UV-assisted capillary molding. Scaffolds with different sizes of ridges and grooves (50–800 nm) were designed to mimic the nanoscale texture variations in the natural myocardium. The scaffolds were made of polyethylene glycol (PEG) hydrogel, a biocompatible material that allows exchange of nutrients, oxygen, and waste products.
As anticipated, newborn rat heart cells grown on the nanofabricated scaffolds aligned with the ridges and grooves. Scanning electron microscope analysis of the interface between the cell layer and the nanoscaffold revealed that cells hugged the ridges by penetrating into grooves between them. What was completely unexpected, however, was that size differences of the scaffold ridges affected the cells’ shape, orientation, alignment, communication with neighboring cells, and propagation of electrical impulses in very different ways, suggesting that cells can sense differences in the nano-texture of the surrounding matrix.
The investigators first suspected that the cells had special nanosensors that detected texture but "later discovered that the cells' ability to sense differences in textures was likely because of variations in the degree of penetration between ridges," clarifies Levchenko. When the grooves are too narrow for cells to penetrate, interaction between the cells and the underlying scaffold is diminished. This is one of the first studies showing that nanosized surface features are a major driver of cellular behavior, a potentially important discovery for the field of tissue engineering.
Next Steps – Mending an Injured Heart
Several obstacles must be overcome to make the patch amenable to therapeutic use. "Replacement tissues need to mimic as closely as possible the function of the host tissue on which they are being grafted. The electrical function of the cells should match that of the host myocardium to avoid creating electrical disturbances that could result in arrhythmias," states Levchenko’s collaborator, Leslie Tung, Professor of Biomedical Engineering at Johns Hopkins University. Growing cells on the ECM-like scaffold would prepare them better for integration with the host heart. However, the current versions of the nanoengineered scaffold provide only mechanical cues to the cells. Coating the PEG scaffold with various components of the ECM would improve chemical communication between cells.
In a recent study [unpublished at the time of this writing], Levchenko’s research team implanted a nanofabricated patch containing PEG scaffold with heart stem cells into an animal model of heart attack. Within 3 weeks of implantation, the stem cells started multiplying and transforming into heart cells. "We would like to see whether maturation of stem cells might be affected by the geometry of the underlying matrix,” says Deok-Ho Kim, Assistant Research Professor of Biomedical Engineering at Johns Hopkins University and lead investigator on the study. MRI could show the size and shape of the heart after transplantation of the patch. "Using ultrasound imaging, we might be able to see whether implanted cells beat in synchrony with the native heart tissue," explains Kim. He is also trying to add compounds that would make the scaffold biodegradable in the body.
Tung indicates that nanoengineered scaffolds "may be important components to include in any kind of synthetic cardiac tissue." Different types of cells and multiple layers could be combined with a scaffold to recreate complex tissues. Levchenko has begun preliminary discussions with companies that are interested in heart repair. "We hope that the idea of patching the heart with nanostructure surfaces ultimately becomes a reality, but we’ll need to figure out the best model," he says.
This work is supported in part by the National Institute of Biomedical Imaging and Bioengineering.
Kim DH, Lipke EA, Kim P, Cheong R, Thompson S, Delannoy M, Suh KY, Tung L, Levchenko A. Nanoscale cues regulate the structure and function of macroscopic cardiac tissue constructs. Proc Natl Acad Sci USA. 2010 Jan 12;107(2):565–70.