Flesh turned to stone—it has been the stuff of myth and legend since ancient times, a ghastly end for failed heroes or even the merely hapless. But petrifaction isn’t just for people any more. Cells and organelles can be “fixed,” too. And so petrification is emerging as a theme in science. But unlike the gorgons, fanciful beasts, and vengeful spirits of old, scientists have benign intentions, such as tracking the evolution of stem cells and examining the structures of cancers and other growths. They also have in mind the fabrication of biomimetic materials.
Scientists at Sandia National Laboratories have developed a technique to transmute living cells into more permanent materials that defy decay and can endure high-powered probes. The technique has been the subject of several papers, the most recent of which appeared December 8 in Nature Communications.
The Nature Communications paper (“Synthetic fossilization of soft biological tissues and their shape-preserving transformation into silica or electron-conductive replicas”) briefly mentions other “fixing” techniques, and their shortcomings, and introduces the new approach, which is called silica bioreplication.
The work that led to silica bioreplication was initiated by Sandia’s Bryan Kaehr, Ph.D., and then-University of Mexico (UNM) postdoctoral student Jason Townson. These researchers had been using a silica slurry, and they eventually realized that the slurry’s silica molecules had an unexpected property. At a reasonably low pH level, the silica molecules, instead of clotting with each other, bound only to surfaces against which they rested, forming the thinnest of coatings.
Dr. Kaehr wondered if a similar coating on biological cells would strengthen cell structures so they could be examined for longer periods with more powerful tools. So the researchers put cultured tissue cells in a silica solution and let the mix harden overnight. Then they raised the temperature to burn off the biomaterial. What remained, astonishingly, were perfectly replicated cells, like little row houses of glass.
But the replicated cells were so sturdy that Dr. Kaehr surmised that the slurry must have coated the cells inside as well as out. Breaking a row of cells as one would a tiny pane of glass, the team examined their interiors with an electron microscope. They found they had indeed replicated the nanoscopic organelles of the cell as well as its exterior. They had discovered a way to create a near-perfect silica counterfeit of a biological organism, from its overall shape down to its nanostructures.
Silica bioreplication is now being used in various biological investigations. For example, it is being used by biologists in Finland to create three-dimensional models that preserve the different stages of stem cells as they evolve to their final form, said Jeff Brinker, Ph.D., a co-author of the Nature Communications paper. Dr. Brinker is also a Sandia fellow and UNM professor.
Dr. Townson, now on the faculty at UNM, uses the method to research the movements of cancer-fighting nanoparticles inserted into chicken cells prior to their conversion to silica. “With optical microscopy, it is difficult to form an image of the interactions of nanoparticles with cells while preserving a three-dimensional context,” he said. Bioreplication, where the sample can be mechanically dissected and investigated with electron microscopes, offers better 3D resolution at the nanoscale.
In the Nature Communications paper, the authors noted that “[silica bioreplication] imparts structural rigidity enabling the preservation of shape and nano-to-macroscale dimensional features upon drying to form a biocomposite.” Then, with high-temperature oxidative calcination, the authors continued, it is possible to form silica replicas. Also, with reductive pyrolysis, it is possible to form electrically conductive carbon replicas of complete organisms.
The new work took silica bioreplication to a larger scale—that of a whole liver. Doing so, the authors suggested, could produce insights that should enable the development of new classes of biomimetic composite materials. Also, they indicated that introducing carbonization opened new possibilities for electron microscopy. Carbon, because it conducts electricity instead of absorbing it, is not weakened and destroyed like protoplasm. For example, the carbonized method could be used to better examine cancers and other growths without the often tedious and expensive processes normally necessary to “fix,” process, and stabilize the organic material for examination to prevent it from falling apart under electron-beam analysis.