by Michio Kaku
So in the future, nanotechnology will detect cancer colonies years to decades before they can form a tumor, and nanoparticles circulating in our blood might be used to destroy these cells. The basic science is being done today.
NANOCARS IN OUR BLOOD
One step beyond the nanoparticle is the nanocar, a device that can actually be guided in its travels inside the body. While the nanoparticle is allowed to circulate freely in the bloodstream, these nanocars are like remote-controlled drones that can be steered and piloted.
James Tour and his colleagues at Rice University have made such a nanocar. Instead of wheels, it has four buckyballs. One future goal of this research is to design a molecular car that can push a tiny robot around the bloodstream, zapping cancer cells along the way or delivering lifesaving drugs to precise locations in the body.
But one problem with the molecular car is that it has no engine. Scientists have created more and more sophisticated molecular machines, but creating a molecular power source has been one of the main roadblocks. Mother Nature has solved this problem by using the molecule adenosine triphosphate (ATP) as her energy source. The energy of ATP makes life possible; it energizes every second of our muscles’ motions. This energy of ATP is stored within an atomic bond between its atoms. But creating a synthetic alternative has proven difficult.
Thomas Mallouk and Ayusman Sen of Pennsylvania State University have found a potential solution to this problem. They have created a nanocar that can actually move tens of microns per second, which is the speed of most bacteria. (They first created a nanorod, made of gold and platinum, the size of a bacterium. The nanorod was placed into a mixture of water and hydrogen peroxide. This created a chemical reaction at either end of the nanorod that caused protons to move from one end of the rod to the other. Since the protons push against the electrical charges of the water molecule, this propels the nanorod forward. The rod continues to move forward as long as there is hydrogen peroxide in the water.)
Steering these nanorods is also possible using magnetism. Scientists have embedded nickel disks inside these nanorods, so they act like compass needles. By moving an ordinary refrigerator magnet next to these nanorods, you can steer them in any direction you want.
Yet another way to steer a molecular machine is to use a flashlight. Light can break up the molecules into positive and negative ions. These two types of ions diffuse through the medium at different speeds, which sets up an electric field. The molecular machines are then attracted by these electric fields. So by pointing the flashlight one can steer the molecular machines in that direction.
I had a demonstration of this when I visited the laboratory of Sylvain Martel of the Polytechnic Montréal in Canada. His idea was to use the tails of ordinary bacteria to propel a tiny chip forward in the bloodstream. So far, scientists have been unable to manufacture an atomic motor, like the one found in the tails of bacteria. Martel asked himself: If nanotechnology could not make these tiny tails, why not use the tails of living bacteria?
He first created a computer chip smaller than the period at the end of this sentence. Then he grew a batch of bacteria. He was able to place about eighty of these bacteria behind the chip, so that they acted like a propeller that pushed the chip forward. Since these bacteria were slightly magnetic, Martel could use external magnets to steer them anywhere he wanted.
I had a chance to steer these bacteria-driven chips myself. I looked in a microscope, and I could see a tiny computer chip that was being pushed by several bacteria. When I pressed a button, a magnet turned on, and the chip moved to the right. When I released the button, the chip stopped and then moved randomly. In this way, I could actually steer the chip. While doing this, I realized that one day, a doctor may be pushing a similar button, but this time directing a nanorobot in the veins of a patient.
Molecular robots will be patrolling our bloodstreams, identifying and zapping cancer cells and pathogens. They could revolutionize medicine. (photo credit 4.1)
One can imagine a future where surgery is completely replaced by molecular machines moving through the bloodstream, guided by magnets, homing in on a diseased organ, and then releasing medicines or performing surgery. This could make cutting the skin totally obsolete. Or, magnets could guide these nanomachines to the heart in order to remove a blockage of the arteries.
DNA CHIPS
As we mentioned in Chapter 3, in the future we will have tiny sensors in our clothes, body, and bathroom, constantly monitoring our health and detecting diseases like cancer years before they become a danger. The key to this is the DNA chip, which promises a “laboratory on a chip.” Like the tricorder of Star Trek, these tiny sensors will give us a medical analysis within minutes.
Today, screening for cancer is a long, costly, and laborious process, often taking weeks. This severely limits the number of cancer analyses that can be performed. However, computer technology is changing all this. Already, scientists are creating devices that can rapidly and cheaply detect cancer, by looking for certain biomarkers produced by cancer cells.
Using the very same etching technology used in computer chips, it is possible to etch a chip on which there are microscopic sites that can detect specific DNA sequences or cancer cells.
Using transistor etching technology, DNA fragments are embedded into the chip. When fluids are passed over the chip, these DNA fragments can bind to specific gene sequences. Then, using a laser beam, one can rapidly scan the entire site and identify the genes. In this way, genes do not have to be read one by one as before, but can be scanned by the thousands all at once.
In 1997, the Affymetrix company released the first commercial DNA chip that could rapidly analyze 50,000 DNA sequences. By 2000, 400,000 DNA probes were available for a few thousand dollars. By 2002, prices had dropped to $200 for even more powerful chips. Prices continue to plunge due to Moore’s law, down to a few dollars.
Shana Kelley, a professor at the University of Toronto’s medical school, said, “Today, it takes a room filled with computers to evaluate a clinically relevant sample of cancer biomarkers and the results aren’t quickly available. Our team was able to measure biomolecules on an electronic chip the size of your fingertip.” She also envisions the day when all the equipment to analyze this chip will be shrunk to the size of a cell phone. This lab on a chip will mean that we can shrink a chemical laboratory found in a hospital or university down to a single chip that we can use in our own bathrooms.
Doctors at Massachusetts General Hospital have created their own custom-made biochip that is 100 times more powerful than anything on the market today. Normally, circulating tumor cells (CTCs) make up fewer than one in a million cells in our blood, but these CTCs eventually kill us if they proliferate. The new biochip is sensitive enough to find one in a billion CTCs circulating in our blood. As a result, this chip has been proven to detect lung, prostate, pancreatic, breast, and colorectal cancer cells by analyzing as little as a teaspoon of blood.
Standard etching technology carves out chips containing 78,000 microscopic pegs (each 100 microns tall). Under an electron microscope, they resemble a forest of round pegs. Each peg is coated with an antibody for the epithelial cell adhesion molecule (EpCAM), which is found in many types of cancer cells but is absent in ordinary cells. EpCAM is vital for cancer cells to communicate with one another as they form a tumor. If blood is passed through the chip, the CTC cells stick to the round pegs. In clinical trials, the chip successfully detected cancers in 115 out of 116 patients.
The proliferation of these labs on a chip will also radically affect the cost of diagnosing disease. At present, it may cost several hundred dollars to have a biopsy or chemical analysis, which might take a few weeks. In the future, it may cost a few pennies and take a few minutes. This could revolutionize the speed and accessibility of cancer diagnoses. Every time we brush our teeth, we will have a thorough checkup for a variety of diseases, including cancer.
Leroy Hood and his colleagues at the University of Washingto
n created a chip, about 4 centimeters wide, that can test for specific proteins from a single drop of blood. Proteins are the building blocks of life. Our muscles, skin, hair, hormones, and enzymes are all made of proteins. Detecting proteins from diseases like cancer could lead to an early warning system for the body. At present, the chip costs only ten cents and can identify a specific protein within ten minutes, so it is several million times more efficient than the previous system. Hood envisions a day when a chip will be able to rapidly analyze hundreds of thousands of proteins, alerting us to a wide variety of diseases years before they become serious.
CARBON NANOTUBES
One preview of the power of nanotechnology is carbon nanotubes. In principle, carbon nanotubes are stronger than steel and can also conduct electricity, so carbon-based computers are a possibility. Although they are enormously strong, one problem is that they must be in pure form, and the longest pure carbon fiber is only a few centimeters long. But one day, entire computers may be made of carbon nanotubes and other molecular structures.
Carbon nanotubes are made of individual carbon atoms bonded to form a tube. Imagine chicken wire, where every joint is a carbon atom. Now roll up the chicken wire into a tube, and you have the geometry of a carbon nanotube. Carbon nanotubes are formed every time ordinary soot is created, but scientists never realized that carbon atoms could bond in such a novel way.
The near-miraculous properties of carbon nanotubes owe their power to their atomic structure. Usually, when you analyze a solid piece of matter, like a rock or wood, you are actually analyzing a huge composite of many overlapping structures. It is easy to create tiny fractures within this composite, which cause it to break. So the strength of a material depends on imperfections in its molecular structure. For example, graphite is made of pure carbon, but it is extremely soft because it is made of layers that can slide past each other. Each layer consists of carbon atoms, each of which is bonded with three other carbon atoms.
Diamonds are also made of pure carbon, but they are the strongest naturally occurring mineral. The carbon atoms in diamonds are arranged in a tight, interlocking crystal structure, giving them their phenomenal strength. Similarly, carbon nanotubes owe their amazing properties to their regular atomic structure.
Already, carbon nanotubes are finding their way into industry. Because of their conductivity, they can be used to create cables to carry large amounts of electrical power. Because of their strength, they can be used to create substances tougher than Kevlar.
But perhaps the most important application of carbon will be in the computer business. Carbon is one of several candidates that may eventually succeed silicon as the basis of computer technology. The future of the world economy may eventually depend on this question: What will replace silicon?
POST-SILICON ERA
As we mentioned earlier, Moore’s law, one of the foundations of the information revolution, cannot last forever. The future of the world economy and the destiny of nations may ultimately hinge on which nation develops a suitable replacement for silicon.
The question—When will Moore’s law collapse?—sends shudders throughout the world economy. Gordon Moore himself was asked in 2007 if he thought the celebrated law named after him could last forever. Of course not, he said, and predicted that it would end in ten to fifteen years.
This rough assessment agreed with a previous estimate made by Paolo Gargini, an Intel Fellow, who is responsible for all external research at Intel. Since the Intel Corporation sets the pace for the entire semiconductor industry, his words were carefully analyzed. At the annual Semicon West conference in 2004, he said, “We see that for at least the next fifteen to twenty years, we can continue staying on Moore’s law.”
The current revolution in silicon-based computers has been driven by one overriding fact: the ability of UV light to etch smaller and smaller transistors onto a wafer of silicon. Today, a Pentium chip may have several hundred million transistors on a wafer the size of your thumbnail. Because the wavelength of UV light can be as small as 10 nanometers, it is possible to use etching techniques to carve out components that are only thirty atoms across. But this process cannot continue forever. Sooner or later, it collapses, for several reasons.
First, the heat generated by powerful chips will eventually melt them. One naive solution is to stack the wafers on top of one another, creating a cubical chip. This would increase the processing power of the chip but at the expense of creating more heat. The heat from these cubical chips is so intense you could fry an egg on top of them. The problem is simple: there is not enough surface area on a cubical chip to cool it down. In general, if you pass cool water or air across a hot chip, the cooling effect is greater if you have more surface contact with the chip. But if you have a cubical chip, the surface area is not enough. For example, if you could double the size of a cubical chip, the heat it generates goes up by a factor of eight (since the cube contains eight times more electrical components), but its surface area increases only by a factor of four. This means that the heat generated in a cubical chip rises faster than the ability to cool it down. The larger the cubical chip, the more difficult it is to cool it. So cubical chips will provide only a partial, temporary solution to the problem.
Some have suggested that we simply use X-rays instead of UV light to etch the circuits. In principle, this might work, since X-rays can have a wavelength 100 times smaller than UV light. But there is a trade-off. As you move from UV light to X-rays, you also increase the energy of the beam by a factor of 100 or so. This means that etching with X-rays may destroy the wafer you are trying to etch. X-ray lithography can be compared to an artist trying to use a blowtorch to create a delicate sculpture. X-ray lithography has to be very carefully controlled, so X-ray lithography is only a short-term solution.
Second, there is a fundamental problem posed by the quantum theory: the uncertainty principle, which says that you cannot know for certain the location and velocity of any atom or particle. Today’s Pentium chip may have a layer about thirty atoms thick. By 2020, that layer could be five atoms across, so that the electron’s position is uncertain, and it begins to leak through the layer, causing a short circuit. Thus, there is a quantum limit to how small a silicon transistor can be.
As I mentioned earlier, I once keynoted a major conference of 3,000 of Microsoft’s top engineers in their headquarters in Seattle, where I highlighted the problem of the slowing down of Moore’s law. These top software engineers confided to me that they are now taking this problem very seriously, and parallel processing is one of their top answers to increase computer processing power. The easiest way to solve this problem is to string a series of chips in parallel, so that a computer problem is broken down into pieces and then reassembled at the end.
Parallel processing is one of the keys to how our own brain works. If you do an MRI scan of the brain as it thinks, you find that various regions of the brain light up simultaneously, meaning that the brain breaks up a task into small pieces and processes each piece simultaneously. This explains why neurons (which carry electrical messages at the excruciatingly slow pace of 200 miles per hour) can outperform a supercomputer, in which messages travel at nearly the speed of light. What our brain lacks in speed, it more than makes up for by doing billions of small calculations simultaneously and then adding them all up.
The difficulty with parallel processing is that every problem has to be broken into several pieces. Each piece is then processed by different chips, and the problem is reassembled at the end. The coordination of this breakup can be exceedingly complicated, and it depends specifically on each problem, making a general procedure very difficult to find. The human brain does this effortlessly, but Mother Nature has had millions of years to solve this problem. Software engineers have had only a decade or so.
ATOMIC TRANSISTORS
One possible replacement for silicon chips is transistors made of individual atoms. If silicon transistors fail because wires and layers in a chip are going dow
n in size to the atomic scale, then why not start all over again and compute on atoms?
One way of realizing this is with molecular transistors. A transistor is a switch that allows you to control the flow of electricity down a wire. It’s possible to replace a silicon transistor with a single molecule, made of chemicals like rotaxane and benzenethiol. When you see a molecule of benzenethiol, it looks like a long tube, with a “knob,” or valve, made of atoms in the middle. Normally, electricity is free to flow down the tube, making it conductive. But it is also possible to twist the “knob,” which shuts off the flow of electricity. In this way, the entire molecule acts like a switch that can control the flow of electricity. In one position, the knob allows electricity to flow, which can represent the number “1.” If the knob is turned, then the electric flow is stopped, which represents the number “0.” Thus, digital messages can be sent by using molecules.
Molecular transistors already exist. Several corporations have announced that they have created transistors made of individual molecules. But before they can be commercially viable, one must be able to wire them up correctly and mass-produce them.