A biological system can be exceedingly small. Many of the cells are very tiny, but they are very active; they manufacture various substances; they walk around; they wiggle; and they do all kinds of marvelous things – all on a very small scale. Also, they store information. Consider the possibility that we too can make a thing very small which does what we want – that we can manufacture an object that maneuvers at that level!
– Richard Feynman, “There's Plenty of Room at the Bottom”
In his classic 1959 presentation to the American Physical Society at Caltech, Richard Feynman mentioned both nanocomputing and maneuverable nanorobots. This is a great example of what a visionary pioneer can do with an educated mind, an innovative spirit, and a sense of what's possible. Some of Dr. Feynman's challenges were accepted during his lifetime. Some are just beginning to be realized now.
By turning his vision into a challenge, Dr. Feynman extended his personal reach more than half a century into the future. Today we are accepting his original challenge, crediting him with jump-starting the field of nanotechnology, and fulfilling a dream that started with “thinking small” and continues even farther into the future.
One of the most intriguing concepts originally raised by Dr. Feynman involves engineering and designing functions that are common in biological systems. This is known today as “biomimicry” or “biomimetics.”
While many of the innovations mentioned in this classic 1959 talk were not possible to achieve at the time and wouldn't be possible for decades to come, they were very real in Dr. Feynman's imagination. Dr. Feynman offered some intriguing targets for the bionano research community that would actually be achieved – although most people didn't know it at the time. If we could have traveled forward to the year 2015 from December 29, 1959 when Dr. Feynman gave his speech, we would be amazed to learn that most of his predictions have come true, and progress has been made on all fronts.
Of course, when Dr. Feynman presented his challenge, he probably wasn't thinking that one day we would be creating new functions based on engineering human DNA, borrowing ideas from the feet of a lizard, or creating structures that replicate the properties of a butterfly wing.
Two of the innovations that were mentioned in Dr. Feynman's 1959 talk were nanocomputers that “store information” and nanorobots that “maneuver.” This prediction relates to biological as well as electronic computing.
This prediction is being brought to life by researchers from the Weizmann Institute of Science in Reghovot, Israel, who in 2002 demonstrated a programmable molecular computing machine that used DNA molecules and enzymes. In the April 2004 issue of Nature, the team suggested that their DNA computer could theoretically diagnose cancerous activity within a cell, and release an anticancer drug. These functions could be performed autonomously. The team members included Ehud Shapiro, Yaakov Benenson, Binyamin Gil, Uri Ben-Dor, and Rivka Adar.
In 2009, Shapiro and his colleague Tom Ran reported the implementation of simple logic programs that enabled a DNA computer to answer “yes” or “no” and correctly answer a question such as “Is Socrates mortal?” Their research included development of a program that linked a computer programming language to DNA computing code, which further extended the utility of their DNA computing technology . The ultimate applications are in programmable autonomous computing devices that can operate in a biological environment, Dr. Shapiro suggested.
In June 2011, Caltech researchers Lulu Qian and Erik Winfree built the largest DNA computational circuit to date, using an approach that is both simple and scalable. The researchers formed 130 synthetic DNA strands, which they used to create a 74 molecule, 4-bit circuit that was able to compute the square root of any number up to 15, and round the answer to the nearest integer. Their DNA circuit used biochemical logic gates to produce binary (on–off) signals, similar to silicon-based integrated circuits. They had previously tested a DNA computer comprised of 12 DNA molecules.
In the eloquent introduction to their June 2011 article in Science, they wrote: “The power and mystery of life is entangled within the information processing at the heart of all cellular machinery. Engineering molecular information processing systems may allow us to tap into that power and elucidate principles that will help us to understand and appreciate the mystery” .
In a commentary that accompanied the researchers' Science article , Duke University science professor John Reif observed that the execution of a single gate could take 30 min–1 h, and computing a 4-bit square root could take as long as 6–10 h, although the researchers indicated that they can increase the speed of their circuit up to 100 times or more using higher concentrations and other strategies. They also recognized that implementation challenges remain such as “increased spurious binding” at larger scales that could slow the reaction rates and affect the processes. A possible solution might be to utilize DNA origami techniques invented by Caltech colleague Paul Rothemund, they suggested.
It is understandable that the first DNA computers can only perform a few calculations, and very slowly – but the fact that they can make these calculations at all is a step in the right direction.
While Dr. Feynman did not use the term “nanorobot” or “nanobot,” it was clear that he was referring to the capabilities of what we now call nanobots when he talked about nanoscale objects that can “maneuver.” A great deal has been written about whether nanobots would be mechanical or biological, with an edge going to biological systems since most self-replicating systems in Nature are biological. One of the amazing innovations to come out of the convergence of biology and nanotechnology has been the use of DNA to create very early versions of nanobots.
Some structures that have been called nanobots are actually nanoscale containers that can hold, transport, and release toxins and other agents to kill fast-growing cancer cells. These take the form of nanoshells made of gold or carbon, boxes made of DNA – even boxes with “locks,” and clamshell-shaped containers with “latches” that release when they come into contact with a specific type of cell. These nanoshells are more like packaging than maneuverable nanoscale robots. However, there are several streams of research that are creating maneuverable nanoscale devices, using the principles of DNA nanotechnology and DNA origami.
In the past half decade, DNA has been used to develop molecular robots <4 nm in diameter that have the ability to “walk” toward a target. In 2010, Milan Stojanovic at Columbia University led a team that developed a molecular robot made from DNA. The team included Erik Winfree (Caltech), Han Yan (Arizona State University), and Nils Walter (University of Michigan, Ann Arbor).
Their nanoscale walking structures have four “legs” made from biotin-labeled DNA strands that attach and detach to the surface of a pathway or track until the strands reach a patch of DNA that they can bind to but cannot cut, that stops them. The researchers call their DNA robot a “spider” and refer to the stopping surface as “flypaper.”
Their first molecular robots could take only three steps. In subsequent tests, the DNA spider took ∼50 “steps” to travel 100 nm . The number of legs and their length can be varied, and their motion, gait, and “stickiness” can be adjusted to help them locate targets on a two-dimensional lattice. This concept was elaborated on by Dr. Stojanovic in a paper coauthored with Oleg Semenov and Darko Stefanovic from the University of Mexico .
These examples are very early-stage indications that illustrate the potential to design biological computers and nanoscale biorobots.
Biomimetics – also known as biomimicry – is the use of models, systems, and elements found in Nature, to design and engineer materials and machines. While scientists have been learning from Nature for centuries, the term “biomimetics” was coined during the 1950s by American biophysicist Otto Schmitt at the University of Minnesota. The concept received renewed attention in 1997 with the publication of Janine Benyus' book, Biomimicry: Innovation Inspired by Nature. Janine is the founder of the Biomimicry Guild and the Biomimicry Institute and the author of Nature's 100 Best.
“Biomimicry is innovation inspired by nature,” Janine declared in her classic book. “In a society accustomed to dominating or “improving” nature, this respectful imitation is a radically new approach, a revolution really. Unlike the Industrial Revolution, the Biomimicry Revolution introduces an era based not on what we can extract from nature, but on what we can learn from her….”
In a radio interview with TreeHugger, Janine said: “Nature is nano. It's one of life's principles, really, that life builds from the bottom up. What that means is that nanotechnology basically has to do with scale. It's a term that refers to scale – how small something is. Your body builds out of a complete nano construction” .
While scientists and engineers have been drawing lessons from Nature for decades, it was only in the past decade that nanotechnology has enabled scientists to mimic Nature at the scale of atoms and molecules. In this sense, we might extend the term “biomimicry” to create a new term, “nanomimicry” – emulating and replicating structures and processes that occur in Nature at the nanoscale. Two of the most intriguing examples of nanomimicry are a dry adhesive based on the footpads of a lizard, and a colorfast waterproof material inspired by a butterfly.
Most people are familiar with a small friendly looking lizard called a gecko. In many countries of the world, this cute-looking insect-eating lizard is a common sight in homes and offices. When I was in my 20 s, as a young Army officer serving in Southeast Asia, there always seemed to be one or two geckos on the wall, the ceiling, or the window – how they managed to stick to the ceiling or the smooth surface of a glass mirror was a fascinating mystery.
Geckos are lizards that vary in size from a few grams to half a kilogram, with large eyes that many people find endearing. Geckos are common in tropical regions of the world. Some people keep them as pets. In the United States, the gecko is the symbol of a well-known insurance company. Geckos are among the few lizards that vocalize sounds, making clicks and trills and chirping noises to communicate. Geckos have flattened disk-shaped toe pads that help them climb and stick to surfaces from rocks to windows. They are able to stick to a surface using a form of dry adhesion, as opposed to frogs and other creatures that excrete a sticky glue-like substance and leave sticky trails where they walk.
Geckos have an uncanny ability to walk on vertical window glass, on walls, and ceilings. But how does a gecko stick to a smooth glass surface or keep from falling when it's walking upside down? That's what a team of researchers at the University of Massachusetts, Amherst, asked themselves when they decided to try to replicate how a gecko's footpads allow it to cling to smooth surfaces (Figure 11.1). The result is called Geckskin™, which replicates the gecko's secret in an entirely new dry adhesive technology that allows heavy objects to be stuck to smooth surfaces. The objects can be removed without leaving any residue.
Geckos can weigh up to 17 ounces, although most are less than an ounce. When a 5 ounce gecko scrambles across a wall or ceiling, the pads on their feet exert an adhesive force of about 9 lb. They can walk upside down on a smooth surface, suspend themselves by 1′, and cling to surfaces in high winds.1) This is made possible by structures on the pads of their feet called scansors, which are lined with rows of lamellae, which in turn are covered with tiny elastic hair called setae that have branched nanoscale tips called spatulae. Controlling the flow of blood to and from the footpads allows the structures to change shape, attach, and detach. These structures are self-cleaning and water repellant. They are one of Nature's most incredible innovations (Figure 11.2).
So what does this have to do with nanoinnovation?
First, it's important to understand that Geckskin was not an overnight innovation. Biologists had studied the ecology and anatomy of geckos for over 50 years, carefully mapping many aspects of anatomy that enable them to climb. However, trying to replicate gecko-like adhesion was not easy.
In 2011, a team of polymer scientists collaborated with a biologist at the University of Massachusetts, Amherst, to mimic the structure of the gecko's footpads to create a dry adhesive they call “Geckskin.” The Geckskin team demonstrated that a patch of this material only 100 cm2 can support a 700 lb object on a smooth surface. The adhesive patch can be peeled off without leaving a residue. The researchers have shown that a small Geckskin patch can hold a 42″ large-screen television to a living room wall (Figure 11.3).
The research team was led by Al Crosby, a polymer scientist, and biologist Duncan Irschick, a functional morphologist who has been studying the gecko's climbing and clinging abilities for over 20 years. Their team united by happenstance: Al Crosby's laboratory had been working for several years on a simple, yet powerful way to mimic gecko feet in a synthetic adhesive. They derived a new theory for how adhesives work, which relied on the principle of draping, much like a tablecloth drapes over a table. They believed that, contrary to most understanding of how adhesives work, making adhesives increasingly stiff would enable powerful adhesion. They also wanted to know if this theory was the secret to the geckos' adhesive success. To do so, they wanted to team up with a gecko biologist. Not knowing of any on campus, they Googled “geckos UMASS” and – voilà! – to their surprise, one of the world's experts in gecko adhesion – UMass biologist Duncan J. Irschick – popped up in their search. Team members had also been reading his work. Duncan Irschick first began studying gecko adhesion as an undergraduate, and performed many studies on the lizard's remarkable ability to climb surfaces.
Mike Bartlett and Dan King, two graduate students in the Polymer Sciences Department, joined the team and the researchers delved deeper into the mysteries of “gecko adhesion.” The team verified that geckos have stiff tendons that branch into their soft toe pads. These tendons had been previously ignored as being unimportant in geckos, but as it turned out, this feature is vital to the success of both geckos and Geckskin. The stiff tendons enable the soft pad of the gecko foot to achieve high levels of adhesion over a large area, thus enabling even large geckos (half a kilogram) to climb. Similarly, a stiff tendon in Geckskin, made of fabrics such as carbon fiber or Kevlar, enabled the soft pad to adhere across a very wide area and with great effectiveness. This ability to create powerful adhesives at large scales could open up many new opportunities for useful human products, including new approaches for wall mounting objects, and applications in medicine, apparel, and industrial assembly and design.
Their research was published in the February issue of Advanced Materials . Their innovation was named one of the top five science breakthroughs of 2012 by CNN. The innovation has received widespread media attention as an inspired example of biomimetic design.
For more than a decade, nanoscientists have studied butterfly scales. The nanosized scales of butterflies are well known for their ability to convey color by size and orientation rather than pigment. Many butterflies that appear to have brightly colored wings achieve their colors by a phenomenon that occurs when their scales refract light at specific wavelengths. Butterfly scales are also known for their waterproofing capabilities, which allow butterflies to survive rainstorms.
In 2012, Dr. Shu Yang at the University of Pennsylvania announced the development of a nanomaterial that mimics the waterproof properties of the scales on a butterfly's wing (Figure 11.4). Dr. Yang is professor of Materials Science and Engineering and Chemical and Biomolecular Engineering at UPENN. Her coauthors included Jie Li, Guanquan Liang, and Xuelian Zhu. Their research was published in the April 2012 issue of Advanced Functional Materials .
In addition to waterproofing capabilities – described in science jargon as the development of superhydrophobic surfaces – Dr. Yang's research has achieved what she calls “structural color.” She observes that color can come from periodic patterns and size of structures, while superhydrophobicity can be determined by the surface roughness and texture of a material. When water lands on a hydrophobic surface, she explains, its roughness reduces the effective contact area between water and a solid area where it can adhere, thus increasing the water repellency on the surface.
“In Nature, bioorganisms often possess a hierarchical architecture with multiple functions. For example, a butterfly wing has both structural color and superhydrophobicity, while a gecko's foot hairs are both sticky and superhydrophobic. A lot of research over the last 10 years has gone into trying to create structural colors like those found in nature, in things like butterfly wings, beetle scales and opals,” Dr. Yang explains. “People have also been interested in creating superhydrophobic surfaces which are found in things like lotus leaves and in butterfly wings, too, since they couldn't stay in the air with raindrops clinging to them” .
Mimicking Nature requires ingenuity and technical prowess. Dr. Yang's approach involves an unconventional photolithography technique called “holographic lithography,” which uses a laser to create a cross-linked 3D network from a photoresist material. The regions on the photoresist material that are not exposed to the laser are later removed by a solvent leaving submicrometer-sized “holes” periodically arranged in a 3D lattice, which provides structural color. Applying another solvent causes nanospheres to form within the lattice, which prevents the wetting of water on the surface. She said this process is both a science and an art, since each step requires fine-tuning.
Dr. Yang's colead author on the project, Dr. Guanquan Liang, is an optical physicist. While working to improve the quality of 3D photonic crystals, he experimented with many different solvents, and early in his research he often produced spongy nanospheres that gave the material a rough surface instead of a smooth surface. At first this looked like a failure and was frustrating, but when Dr. Liang showed the results to Dr. Yang, she got excited when she realized that this could be used to create a superhydrophobic surface on photonic film. Furthermore, the structural color would retain its color and would not fade from exposure to light, which happens with many pigments and dyes.
Structural color and waterproofing are features that are needed in a wide variety of applications, including semiconductors, energy-saving materials, and sensors. This technology could be used in traffic signs and consumer electronics. It could even be used as camouflage. These functions can be combined with other functions as well. For example, Dr. Yang's group is currently developing energy-efficient building skins that will integrate optical sensors. Their special focus involves interactions at surfaces and interfaces – what Dr. Yang calls the “Structure–Property Relationship.” Researchers in her group are studying and fabricating a variety of properties that occur in Nature, including wetting, adhesion, biocompatibility, and optical and mechanical properties. Many of these involve multiscaled structures and unique properties. For example, the group has drawn lessons from properties exhibited by human cells and muscles to design adaptive skins for buildings, which responds to environmental changes, thus saving energy.
This fascinating convergence of biomimicry and nanotechnology is creating new types of materials as well as more efficient ways to incorporate color, waterproofing, and other capabilities. Nanoinnovations will continue to replicate biological structures observed in Nature. There is much more yet to come.
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