by Michio Kaku
(To visualize this, consider the simple problem of why the Northeast has so many potholes in its highways. Every winter, water seeps into tiny cracks in the asphalt; the water expands as it freezes, causing the asphalt to crumble and gouging out a pothole. But it violates common sense to think that water expands when it freezes. Water does expand because of hydrogen bonding. The water molecule is shaped like a V, with the oxygen atom at the base. The water molecule has a slight negative charge at the bottom and a positive charge at the top. Hence, when you freeze water and stack water molecules, they expand, forming a regular lattice of ice with plenty of spaces between the molecules. The water molecules are arranged like hexagons. So water expands as it freezes since there is more space between the atoms in a hexagon. This is also the reason snowflakes have six sides, and explains why ice floats on water, when by rights it should sink.)
WALKING THROUGH WALLS
In addition to surface tension, hydrogen bonding, and van der Waals forces, there are also bizarre quantum effects at the atomic scale. Normally, we don’t see quantum forces at work in everyday life. But quantum forces are everywhere. For example, by rights, since atoms are largely empty, we should be able to walk through walls. Between the nucleus at the center of the atom and the electron shells, there is only a vacuum. If the atom were the size of a football stadium, then the stadium would be empty, since the nucleus would be roughly the size of a grain of sand.
(We sometimes amaze our students with a simple demonstration. We take a Geiger counter, place it in front of a student, and put a harmless radioactive pellet in back. The student is startled that some particles pass right through his body and trigger the Geiger counter, as if he is largely empty, which he is.)
But if we are largely empty, then why can’t we walk through walls? In the movie Ghost, Patrick Swayze’s character is killed by a rival and turns into a ghost. He is frustrated every time he tries to touch his former fiancée, played by Demi Moore. His hands pass through ordinary matter; he finds that he has no material substance and simply floats through solid objects. In one scene, he sticks his head into a moving subway car. The train races by with his head sticking inside, yet he doesn’t feel a thing. (The movie does not explain why gravity does not pull him through the floor so he falls to the center of the earth. Ghosts, apparently, can pass through anything except floors.)
So why can’t we pass through solid objects like ghosts? The answer resides in a curious quantum phenomenon. The Pauli exclusion principle states that no two electrons can exist in the same quantum state. Hence when two nearly identical electrons get too close, they repel each other. This is the reason objects appear to be solid, which is an illusion. The reality is that matter is basically empty.
When we sit in a chair, we think we are touching it. Actually, we are hovering above the chair, floating less than a nanometer above it, repelled by the chair’s electrical and quantum forces. This means that whenever we “touch” something, we are not making direct contact at all but are separated by these tiny atomic forces. (This also means that if we could somehow neutralize the exclusion principle, then we might be able to pass through walls. However, no one knows how to do this.)
Not only does the quantum theory keep atoms from crashing through one another, it also binds them together into molecules. Imagine for the moment that an atom is like a tiny solar system, with planets revolving around a sun. Now, if two such solar systems collided, then the planets would either crash into one another or fly out in all directions, causing the solar system to collapse. Solar systems are never stable when they collide with another solar system, so by rights, atoms should collapse when they bump into one another.
In reality, when two atoms get very close, they either bounce off each other or they combine to form a stable molecule. The reason atoms can form stable molecules is because electrons can be shared between two atoms. Normally, the idea of an electron being shared between two atoms is preposterous. It is impossible if the electron obeyed the commonsense laws of Newton. But because of the Heisenberg uncertainty principle, you don’t know precisely where the electron is. Instead, it’s smeared out between two atoms, which holds them together.
In other words, if you turn off the quantum theory, then your molecules fall apart when they bump into one another and you would dissolve into a gas of particles. So the quantum theory explains why atoms can bind to form solid matter, rather than disintegrate.
(This is also the reason you cannot have worlds within worlds. Some people imagine that our solar system or galaxy might be an atom in someone else’s gigantic universe. This was, in fact, the final scene in the movie Men in Black, where the entire known universe was in fact just an atom in some alien’s ball game. But according to physics, this is impossible, since the laws of physics change as we go from scale to scale. The rules governing atoms are quite different from the rules governing galaxies.)
Some of the mind-bending principles of the quantum theory are:
• you cannot know the exact velocity and location of any particle—there is always uncertainty
• particles can in some sense be in two places at the same time
• all particles exist as mixtures of different states simultaneously; for example, spinning particles can be mixtures of particles whose axes spin both up and down simultaneously
• you can disappear and reappear somewhere else
All these statements sound ridiculous. In fact, Einstein once said, “the more successful the quantum theory is, the sillier it looks.” No one knows where these bizarre laws come from. They are simply postulates, with no explanation. The quantum theory has only one thing going for it: it is correct. Its accuracy has been measured to one part in ten billion, making it the most successful physical theory of all time.
The reason we don’t see these incredible phenomena in daily life is because we are composed of trillions upon trillions of atoms, and these effects, in some sense, average out.
MOVING INDIVIDUAL ATOMS
Richard Feynman dreamed of the day when a physicist could manufacture any molecule, atom for atom. That seemed impossible back in 1959, but part of that dream is now a reality.
I had a chance to witness this up close, when I visited the IBM Almaden Research Center in San Jose, California. I came to observe a remarkable instrument, the scanning tunneling microscope, which allows scientists to view and manipulate individual atoms. This device was invented by Gerd Binnig and Heinrich Rohrer of IBM, for which they won the Nobel Prize in 1986. (I remember, as a child, my teacher telling us that we would never be able to see atoms. They are just too small, he said. By then, I had already decided to become an atomic scientist. I realized that I would spend the rest of my life studying something I would never be able to observe directly. But today, not only can we see atoms, but we can play with them, with atomic tweezers.)
The scanning tunneling microscope is actually not a microscope at all. It resembles an old phonograph. A fine needle (with a tip that is only a single atom across) passes slowly over the material being analyzed. A small electrical current travels from the needle, through the material, to the base of the instrument. As the needle passes over the object, the electrical current changes slightly every time it passes over an atom. After multiple passes, the machine prints out the stunning outline of the atom itself. Using an identical needle, the microscope is then capable not just of recording these atoms but also of moving them around. In this way, one can spell out the letters, such as the initials IBM, and in fact even design primitive machines built out of atoms.
(Another recent invention is the atomic force microscope, which can give us stunning 3-D pictures of arrays of atoms. The atomic force microscope also uses the needle with a very small point, but it shines a laser onto it. As the needle passes over the material being studied, the needle jiggles, and this motion is recorded by the laser beam image.)
I found that moving individual atoms around was quite simple. I sat in front of a computer screen, looki
ng at a series of white spheres, each resembling a Ping-Pong ball about an inch across. Actually, each ball was an individual atom. I placed the cursor over an atom and then moved the cursor to another position. I pushed a button that then activated the needle to move the atom. The microscope rescanned the substance. The screen changed, showing that the ball had moved to precisely where I wanted it.
The whole process took only a minute to move each atom to any position I wanted. In fact, in about thirty minutes, I found that I could actually spell out some letters on the screen, made of individual atoms. In an hour, I could make rather complex patterns involving ten or so atoms.
I had to recover from the shock that I had actually moved individual atoms, something that was once thought to be impossible.
MEMS AND NANOPARTICLES
Although nanotechnology is still in its infancy, it has already generated a booming commercial industry in chemical coatings. By spraying thin layers of chemicals only a few molecules thick onto a commercial product, one can make it more resistant to rust or change its optical properties. Other commercial applications today are stain-resistant clothing, enhanced computer screens, stronger metal-cutting tools, and scratch-resistant coatings. In the coming years, more and more novel commercial products will be marketed that have microcoatings to improve their performance.
For the most part, nanotechnology is still a very young science. But one aspect of nanotechnology is now beginning to affect the lives of everyone and has already blossomed into a lucrative $40 billion worldwide industry—microelectromechanical systems (MEMS)—that includes everything from ink-jet cartridges, air bag sensors, and displays to gyroscopes for cars and airplanes. MEMS are tiny machines so small they can easily fit on the tip of a needle. They are created using the same etching technology used in the computer business. Instead of etching transistors, engineers etch tiny mechanical components, creating machine parts so small you need a microscope to see them.
Scientists have made an atomic version of the abacus, the venerable Asian calculating device, that consists of several vertical columns of wires containing wooden beads. In 2000, scientists at the IBM Zurich Research Laboratory made an atomic version of the abacus by manipulating individual atoms with a scanning microscope. Instead of wooden beads that move up and down the vertical wires, the atomic abacus used buckyballs, which are carbon atoms arranged to form a molecule shaped like a soccer ball, 5,000 times smaller than the width of a human hair.
At Cornell, scientists have even created an atomic guitar. It has six strings, each string just 100 atoms wide. Laid end to end, twenty of these guitars would fit inside a human hair. The guitar is real, with real strings that can be plucked (although the frequency of this atomic guitar is much too high to be heard by the human ear).
But the most widespread practical application of this technology is in air bags, which contain tiny MEM accelerometers that can detect the sudden braking of your car. The MEM accelerometer consists of a microscopic ball attached to a spring or lever. When you slam on the brakes, the sudden deceleration jolts the ball, whose movement creates a tiny electrical charge. This charge then triggers a chemical explosion that releases large amounts of nitrogen gas within 1/25 of a second. Already, this technology has saved thousands of lives.
NANOMACHINES IN OUR BODIES
In the near future, we should expect a new variety of nanodevices that may revolutionize medicine, such as nanomachines coursing throughout the bloodstream. In the movie Fantastic Voyage, a crew of scientists and their ship are miniaturized to the size of a red blood cell. They then embark on a voyage through the bloodstream and brain of a patient, encountering a series of harrowing dangers within the body. One goal of nanotechnology is to create molecular hunters that will zoom in on cancer cells and destroy them cleanly, leaving normal cells intact. Science fiction writers have long dreamed about molecular search-and-destroy craft floating in the blood, constantly on the lookout for cancer cells. But critics once considered this to be impossible, an idle dream of fiction writers.
Part of this dream is being realized today. In 1992, Jerome Schentag of the University at Buffalo invented the smart pill, which we mentioned earlier, a tiny instrument the size of a pill that you swallow and that can be tracked electronically. It can then be instructed to deliver medicines to the proper location. Smart pills have been built that contain TV cameras to photograph your insides as they go down your stomach and intestines. Magnets can be used to guide them. In this way, the device can search for tumors and polyps. In the future, it may be possible to perform minor surgery via these smart pills, removing any abnormalities and doing biopsies from the inside, without cutting the skin.
A much smaller device is the nanoparticle, a molecule that can deliver cancer-fighting drugs to a specific target, which might revolutionize the treatment of cancer. These nanoparticles can be compared to a molecular smart bomb, designed to hit a specific target with a chemical payload, vastly reducing collateral damage in the process. While a dumb bomb hits everything, including healthy cells, smart bombs are selective and home in on just the cancer cells.
Anyone who has experienced the horrific side effects of chemotherapy will understand the vast potential of these nanoparticles to reduce human suffering. Chemotherapy works by bathing the entire body with deadly toxins, killing cancer cells slightly more efficiently than ordinary cells. The collateral damage from chemotherapy is widespread. The side effects—including nausea, loss of hair, loss of strength, etc.—are so severe that some cancer patients would rather die of cancer than subject themselves to this torture.
Nanoparticles may change all this. Medicines, such as chemotherapy drugs, will be placed inside a molecule shaped like a capsule. The nanoparticle is then allowed to circulate in the bloodstream, until it finds a particular destination, where it releases its medicine.
The key to these nanoparticles is their size: between 10 to 100 nanometers, too big to penetrate a blood cell. So nanoparticles harmlessly bounce off normal blood cells. But cancer cells are different; their cell walls are riddled with large, irregular pores. The nanoparticles can enter freely into the cancer cells and deliver their medicine but leave healthy tissue untouched. So doctors do not need complicated guidance systems to steer these nanoparticles to their target. They will naturally accumulate in certain types of cancerous tumors.
The beauty of this is that it does not require complicated and dangerous methods, which might have serious side effects. These nanoparticles are simply the right size: too big to attack normal cells but just right to penetrate a cancer cell.
Another example is the nanoparticles created by the scientists at BIND Biosciences in Cambridge, Massachusetts. Its nanoparticles are made of polylactic acid and copolylactic acid/glycolic acid, which can hold drugs inside a molecular mesh. This creates the payload of the nanoparticle. The guidance system of the nanoparticle is the peptides that coat the particle and specifically bind to the target cell.
What is especially appealing about this work is that these nanoparticles form by themselves, without complicated factories and chemical plants. The various chemicals are mixed together slowly, in proper sequence, under very controlled conditions, and the nanoparticles self-assemble.
“Because the self-assembly doesn’t require multiple complicated chemical steps, the particles are very easy to manufacture …. And we can make them on a kilogram scale, which no one else has done,” says BIND’s Omid Farokhzad, a physician at the Harvard Medical School. Already, these nanoparticles have proven their worth against prostate, breast, and lung cancer tumors in rats. By using colored dyes, one can show that these nanoparticles are accumulating in the organ in question, releasing their payload in the desired way. Clinical trials on human patients start in a few years.
ZAPPING CANCER CELLS
Not only can these nanoparticles seek out cancer cells and deliver chemicals to kill them, they might actually be able to kill them on the spot. The principle behind this is simple. These nanopart
icles can absorb light of a certain frequency. By focusing laser light on them, they heat up, or vibrate, destroying any cancer cells in the vicinity by rupturing their cell walls. The key, therefore, is to get these nanoparticles close enough to cancer cells.
Several groups have already developed prototypes. Scientists at the Argonne National Laboratory and the University of Chicago have created titanium dioxide nanoparticles (titanium dioxide is a common chemical found in sunscreen). This group found that they could bind these nanoparticles to an antibody that naturally seeks out certain cancer cells called glioblastoma multiforme (GBM). So these nanoparticles, by hitching a ride on this antibody, are carried to the cancer cells. Then a white light is illuminated for five minutes, heating and eventually killing the cancer cells. Studies have shown that 80 percent of the cancer cells can be destroyed in this way.
These scientists have also devised a second way to kill cancer cells. They created tiny magnetic disks that can vibrate violently. Once these disks are led to the cancer cells, a small external magnetic field can be passed over them, causing them to shake and tear apart the cell walls of the cancer. In tests, 90 percent of the cancer cells were killed after just 10 minutes of shaking.
This result is not a fluke. Scientists at the University of California at Santa Cruz have devised a similar system using gold nanoparticles. These particles are only 20 to 70 nanometers across and only a few atoms thick, arranged in the shape of a sphere. Scientists used a certain peptide that is known to be attracted to skin cancer cells. This peptide was made to connect with the gold nanoparticles, which then were carried to the skin cancer cells in mice. By shining an infrared laser, these gold particles could destroy the tumor cells by heating them up. “It’s basically like putting a cancer cell in hot water and boiling it to death. The more heat the metal nanospheres generate, the better,” says Jin Zhang, one of the researchers.
