03/13/2003
Spines and Other Biominerals
I’m looking out at the five-foot mound of snow that covers a fair portion of our backyard. It makes me want to be back walking the beach at Marco Island in spite of the possibility of an encounter with an alligator (see last week’s column). Sea urchins, with their sharp spines, were another hazard that kept me from testing the waters of the Gulf. In the February 21 2003 issue of Science, I found a scanning electron microscope (SEM) photo of a sea urchin spine. The photo was in an article titled “A Bright Bio-Inspired Future” by Trevor Douglas, a chemist at Montana State University in Bozeman.
I have a fondness for Bozeman dating back to an overnight stay there which may have saved my marriage after three horrendous, thunderstormy nights with my family in a tent in Yellowstone. I may have mentioned before that, on the morning of our departure, my wife threatened to take a plane home after finding me unable to fold the wet tent into an appropriate shape to stuff into the carrier on top of our car. (I’ve never been good at wrapping and folding things.) The clean, dry, pleasant motel in Bozeman that night put us all in a better mood and we’ve never been in a tent again!
But I digress. Back to Trevor Douglas. He and virologist Mark Young made a splash in the media several years ago when they claimed to have carried out experiments using the world’s smallest test tubes. This was accomplished using the “husks” of viruses as the test tubes. A virus is essentially a strand of DNA or RNA in a “box” made of protein. Take out the DNA or RNA and you have left a tiny box or husk. The Bozeman workers found that the pores in the viral husks would open or close, depending on the pH of certain solutions. With the pores open, they could trap the salts or other reactants inside the cage and grow crystals that conform to the shape of the cage. Viruses are very uniform and there are all kinds and shapes. Their work demonstrated the opportunity to make large numbers of identical tiny objects/crystals of different shapes. A visit to the Montana State Web site shows they’re now investigating these viral husks as switches. They are either open or closed and conceivably could have some application in a biocomputer, should such a concept come to pass.
But I digress again. The purpose of Douglas’ article in Science was to provide a perspective on the work reported in another article in that issue by workers at my old stomping grounds, Bell Labs. I was actually led to this article by an item given to me by Brian Trumbore from the March 10 Business Week titled “Bell Labs: Catching up to the Clam”. The item credits the Bell Labs workers with having succeeded in mimicking the process by which mollusks build up their shells. Their paper is titled “Direct Fabrication of Large Micropatterned Single Crystals”. Having spent half my career at Bell Labs growing or studying single crystals, I was used to fairly large uniformly solid crystals or thin single crystal layers for light emitting diodes.
However, Joanna Aizenberg and her colleagues at Bell Labs are growing single crystals that resemble nothing like what I’ve been accustomed to. I was shocked to look at Douglas’ SEM photo of the sea urchin spine, loaded with pores and empty spaces in the structure, and find that it was a single crystal of calcite, a crystalline form of the compound calcium carbonate. The Bell Labs paper includes an SEM photo of part of the skeleton of another sea creature, the brittlestar. The photo shows an array of little microlenses, each about 2 thousandths of an inch in diameter. Each lens is connected to its neighboring lenses by bridges of calcite. As with the sea urchin spine, this leaves lots of pores and open space in the structure and again, the whole array is one single crystal of calcite.
How does nature accomplish this feat of “biomineralization” in which a single orderly arrangement of atoms is maintained throughout a highly porous structure of inorganic calcite? It turns out that these inorganic biominerals contain small but significant amounts of organic molecules in the structure. Offhand, it should be even harder to maintain an orderly arrangement of atoms with interloping organic molecules to disrupt the structure in a brittlestar or sea urchin spine. However, these organic compounds are actually the key to forming the single crystal in the first place.
My understanding of the process is that a mix of certain organic compounds (porphyrins are an example) forms a patterned array. However, the mix of different molecules presents a disorderly surface to the solution (seawater, I presume) from which the calcium carbonate is deposited. This disorderly surface discourages crystallization and induces the formation of layers of amorphous calcium carbonate (ACC). Remember that an amorphous material has no long-range crystal structure (compare amorphous glass with crystalline quartz).
On the other hand, other organic molecules (alkanethiols for example) can form what’s known as a SAM, a self-assembled monolayer. A SAM contains a bunch of lined up molecules that present a very orderly surface to the solution and promotes the growth of crystalline calcium carbonate (calcite). An amorphous material is thermodynamically unstable and wants to become a more stable crystalline material. A SAM gooses this process along.
Aizenberg and her coworkers cheated a bit by using a process known as photolithography, employed in silicon chip manufacture, to form patterned arrays of tiny posts on a glass substrate, regularly spaced. After coating the array of posts with a thin layer of gold or silver, they used an atomic force microscope tip to lay down a nano-size SAM. The rest of the array was covered with a “disorderly” mix of organic compounds. They then simply placed the array in a calcium chloride solution, put it in a desiccator containing powdered ammonium carbonate and waited.
Within a half hour a layer of amorphous calcium carbonate had formed. Within an hour the nano-SAM had begun its job of initiating the crystallization of the amorphous material. The crystallization proceeded to spread throughout the layer until a region of about a millimeter in size had converted to single crystal calcite. The result was a patterned array much like the brittlestar.
In other experiments, they found that the porous microstructure also served as “microsumps” that removed water during the crystallization. Also, by adding dyes, they showed that impurities may also be removed along with the water. The patterning and the resulting porous structure are also thought to help in relieving stresses that arise during the formation of the crystalline calcite. When you have a transformation from amorphous to crystalline, or even from one crystalline form to another, there can be a substantial change in volume. In a normal single crystal, this can lead to cracking of the crystal. In a micro- or nano-size porous structure, the stresses don’t get a chance to build up to the point of cracking. Such cracking wouldn’t make for a happy clam or an effective sea urchin spine!
I’m certainly no expert on the growth of biominerals like clam shells, but it seems to me that this paper makes a strong case for Nature’s preference for patterned porous structures. For me, it’s nice to see that Bell Labs is still engaged in high quality fundamental studies, especially after its episode of scientific fraud last year and the horrendous financial troubles of Lucent Technologies. An item in the paper this week celebrated the issuance of Bell Labs’ 30,000th patent. I hope there are many more to come.
Allen F. Bortrum
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