Nanotechnology and Human Health

Nanotechnology Background

I believe Richard Jones' assertion, in the book Soft Machines, that nature and evolution-over billions of years-have created soft machines that are nearly inconceivably well constructed and adapted for their purposes. Although we can attempt to design nanostructures, we would be well served to model our endeavors after the effective biological structures that already exist in nature. We can look to living organisms to see all sorts of sophisticated machines which have evolved over billions of years. "At the sub cellular level, nature's designs are in fact highly optimized. Only our experience of designing things in a realm in which physics is very different prevents us from recognizing this". ⁴ So we must embrace the physics at the nanoscale and the example of nature to successfully design nanomachines.

The fundamental concept that I would like to get across to my physics students is that the physics that is relevant is scale dependent! This means that classical mechanical physics that we utilize to design and construct in the macro world, which is heavily reliant on momentum and inertia, may not be true or useful in the nanoworld, which is dominated by Brownian motion, stickiness, fluid dynamics, viscosity, heat, friction and quantum mechanics. Brownian motion is the result of microscopic collisions that are temperature dependent and which result in a "random walk" of larger particles. It turns out that Brownian motion, which translates to the bulk property of diffusion, is an effective means of moving molecules over short distances but a terrible means for long distances, because the distance covered varies only with the square root of the number of "steps". In the nanoworld, a bacterium is small enough to rely on diffusion to mix molecules within it. ⁵ The only way to eliminate Brownian motion, which depends on the temperature, or the average kinetic energy of the surrounding molecules, is to eliminate all kinetic energy which means to achieve absolute zero and to isolate the object from its surroundings. This is impossible, but even to reduce the Brownian motion, which varies as the square root of the absolute temperature, by a factor of ten, you have to lower the temperature to around 3K (-270 C). Additionally, since the desire is to have the nanotechnology interact with the surrounding environment, this is impossibile. Brownian motion at the nanoscale must be accepted.

Stickiness is another fundamental aspect of the nanoworld. Materials, especially those made of the same material or oppositely charged (but even neutral ones), tend to stick together. Of the four forces (gravity, electromagnetism, the strong nuclear force and the weak nuclear force), electromagnetism dominates over the other forces even more at the nanoscale because although EM forces are much weaker than the strong nuclear force their range of action is much greater. Quantum mechanical effects become unavoidable!

The optical properties of very small pieces of matter, known as quantum dots, are determined by their size because photons are only absorbed at allowable energies and only certain wavelengths can achieve a "standing wave". According to the Uncertainty principle, quantum dots restrict electrons to spaces that limit their position, and consequently must have higher momentum, which is known as confinement energy, and results in higher frequencies for smaller nanodots. This means that the color of a quantum dot is determined by its size! Similarly, gold particles of different sizes in the nanometer range, when in solution are able to produce the entire spectrum of visible light. ⁶ As bizarre as all of this physics seems at the nanoscale, these are inevitable consequences of quantum mechanics, which has been demonstrated to be true at all scales and supplants Newtonian mechanics as a better explanation of reality.

Advances in quantum mechanics have allowed for incredible technological advances. One major advance is in our comprehension of electricity which has led to tremendous technological advances, primarily in information technology. A great success of quantum physics has been in understanding the way in which different types of solids conduct electricity. In solids it is the flow of electrons that results in electrical currents. "It is a great triumph of quantum mechanics that it can explain what makes different substances metals, insulators or semiconductors. It is no exaggeration to say that it is this quantum mechanical understanding of materials that has led directly to the present technological revolution, with its accompanying avalanche of new and cheap consumer goods ranging from stereo systems and color TVs to computers and mobile phones." ⁷ Quantum mechanics has enabled the information explosion and technological advances of the last half a century.

Self-Assembly

One of the fundamental physics principles, which we must comprehend thoroughly, is entropy, embodied in the Second Law of Thermodynamics. The Law of Entropy states that all systems will tend to go from states of low entropy (or high order) to high entropy (or greater disorder). Self-assembly is a process through which a system becomes more structured or ordered. Self-assembly would seem to violate the law of entropy because self assembly seemingly results in greater order; however, this is only an apparent paradox. Decreased entropy in one area of a system can be achieved by the increase of entropy in another area of the system of equal or greater size. Another intriguing consequence of entropy is that a decrease in temperature leads to a decrease in entropy. However, the information also increases because the consequence of Planck's constant is that a decrease in momentum of a system leads to greater knowledge of the system.

In order to achieve self assembly, we need a balance between stickiness and Brownian motion and these structures are dynamic. Self assembly is soft and self-healing. It is soft because the contents are continually in motion and self-healing because although the parts of the structure fall out of place their tendency is to regain their organization.

Soap is a good example of a system in which molecules create complex structures at the nanoscale. Soap molecules have hydrophilic ("water loving") and hydrophobic ("water hating") ends. When placed in water, which is a polar substance, the soap will arrange its molecules with its hydrophilic ends towards the water and its hydrophobic ends away from the water. One way this is achieved by soap molecules is by staying on the surface with the hydrophobic ends sticking up into the air so that they can get as far away from the water, like oil on water. Or if the soap is forced into the water the soap will form structures that make it as "happy" as possible. This can be done by forming pockets or spherical bubbles called micelles, where the hydrophobic ends face each other and the hydrophilic ends face the water!

Lipids, which are even more complex than soap molecules, but also have hydrophilic and hydrophobic ends, are centrally important building blocks of biological membranes. ⁸ Bilayer lipids are more complex structures where two layers, like the one layer on the surface of the water, have their hydrophilic ends pointing out and their hydrophobic ends pointing toward each other. This makes a membrane layer that is flexible and can become any shape, including the self enclosing cell membrane! These bilayer lipids, which can make up membranes, and surprisingly are in an ordered state which have higher entropy than a disordered state! ⁹ Although this seems counter-intuitive, it is so because some entropy is associated with the degree of order of the packing of individual molecules while another part of the entropy is determined by how much room each individual phospholipid has to move around, and bilayering allows for the tightest packing and therefore the most independent motion of each phospholipid! This is maximal spacing and maximal entropy.

Self-assembly is an essential aspect of designing many nano machines. It is the process of assembling molecules in bulk rather than one at a time. "What we need for self-assembly to work is for the surface forces, that stick the units together, and the Brownian motion that shakes them apart, to be roughly in balance. So our self assembling units can be small molecules or large known as polymers or macromolecules. Macromolecules are the building blocks of biology." ¹⁰ So when this balance exists the building blocks of biology can exist. Jones suggests that this is like a wood puzzle with sufficiently strong glue on the appropriate edges placed in a bag and shaken. Eventually, if shaken just hard enough and with the glue only sticking in the appropriate places, the puzzle might self-assemble.

Proteins

Jones suggests that the best way to make nano-machines is to model nature. Proteins are 3D structures and the structure determines the function. Proteins are key examples, because they demonstrate the construction of specific 3D shapes by the process of self-assembly. The fact that linear proteins self-assemble into specific 3D shapes is central to creating complex organisms from simple linear code! ¹¹ Genes encode linear sequences that become 3D proteins. These proteins achieved highly specified shapes through the process of evolution! So proteins self-assemble from the code of genes. Bits of genes evolved because they were useful for catalyzing reactions and gave the organism an advantage.

In order for organisms to develop, it is essential that bilayer lipids form membranes that create internal, protected spaces from the outside environment. This is fundamental to life! This membrane defines the cellular unit. However, it is not sufficient to simply enclose an interior space because there must be traffic across the membrane and once again it is proteins that make this possible. Membrane proteins create pores that allow specific substances to pass through. In the most elegant example of utilizing a membrane protein across a differential to produce usable energy, the pore for hydrogen evolved in such a way that it produces ATP, the energy source used for virtually all of the processes of life!" ¹² This is the source of energy that allows for lower entropy for the existence of highly ordered organisms and life. The mitochondria use the driving force of osmotic pressure for rotation because the gradient in bacteria outside and inside of hydrogen ions create ATP from ADP. Remarkably this is mechanical. ¹³ This is a truly incredible example of evolution of a self-assembled structure.

Evolution explains the variety of life. However as we look to the cellular level and further to the molecular level, we find "less variety in life, not more! The nanoscale soft machines on which life depends are remarkably similar, and the more fundamental they are to the business of living, the more similar they seem to be. The ribosome, the machine that puts protein together and the ATP—synthase, the fundamental machine of energy conversion— these are recognizably the same in the simplest bacteria as in ourselves." ¹⁴ This is stunning!!! Evolution is crucial, but we need to understand how molecules themselves evolve. There is virtually no trace of how this occurred. Consider the breadth of the 3D design space in which evolution works: a typical protein of 100 amino acids has ~20 ¹⁰⁰ possible linear configurations… making it seem statistically impossible to create or find useful proteins. Our understanding is crude, but Jones suggests that finding small useful proteins that reliably fold at all would be difficult. However, those proteins that did fold reliably would perpetuate themselves because they would be useful. This is the theory. It is very exciting that our understanding of the physics of the nanoworld and our comprehension of chemistry and biology can unify so remarkably into a greater understanding and appreciation for the existence of living organisms.

The advance of science in all of these disciplines indicates that there is a fundamental unity of the physics, chemistry, and biology at the nanoscale. The descriptions are either mechanical or they delve into the bizarre aspects of quantum mechanics and Brownian motion that is essential to appreciating the difference of the nano- and macroscopic world. I am captivated by the relevance of nanotechnology at this level, the accessibility of the physics, the awesome beauty of our evolutionary mechanisms that make life possible, and a desire to explain how all of this is related to physics.

Nanotechnology is our future. In The New Quantum Universe, Paul Davis is quoted as predicting that, "The nineteenth century was known as the machine age, the twentieth century will go down as the information age. I believe the twenty-first century will be the quantum age," and the authors go on to say that, "In the course of the next decades we will see how far this vision will be realized. Certainly, we believe that the influence on our society of this coming nanotechnology revolution, underpinned by quantum mechanics, will be at least as substantial as the fall-out from the present bio-informatics explosion. We hope that this book will assist in stimulating the imagination of a new generation of quantum engineers." ¹⁵ It is my goal as a physics teacher to prepare students to become part of this revolution. In the 1960's Richard Feynman challenged the scientific community to make significant advances in nanotechnology. Hey and Walters believe that we must continue to pursue Feynman's vision and they indicate that as Feynman indicated in his famous lecture "There is Plenty of Room at the Bottom" that there is a whole world at the nanoscale and they recognize that life at the bottom is fundamentally quantum mechanical. ¹⁶

Students, who live to use the newest and best cell phones, iPods, and information technology, may not be aware of Moore's Law, that computing power will double every 18-24 months, but they certainly have grown up with that expectation. Their ingenuity and understanding of quantum mechanics may be necessary for this progress to continue. "At the level of integration envisaged by the semiconductor industry in 2010, there will still be many thousands of electrons participating in the storage of a bit or in the action of a transistor. The techniques described above make it possible to envisage devices that work with very small numbers of electrons. This in turn will enable the number of transistors on a chip to continue to increase without excessive power generation. To avoid problems with fluctuations in the number of electrons participating in such devices it will be necessary to use the principle of Coulomb blockade to control individual electrons. There are still many technological problems to be overcome but quantum engineering of new semiconductor devices may be able to keep Moore's Law true for another 35 years." ¹⁷ The nanotechnology revolution is here and our students must understand quantum mechanics to participate in its development.