It is only fitting that an article relating nanotechnology and national security begin with a quote from a figure central to both – Richard Feynman. After all, it was Feynman who helped launch both the atomic age and the modern nanotechnology revolution. It is also fitting that the 2016 Nobel Prizes for chemistry and physics recognized breakthroughs in molecular machines and exotic materials, both of which reflect strongly upon progress made in nanoscale science and engineering over the half century since Feynman’s famed “There’s Plenty of Room at the Bottom” address to the American Physical Society.
Advances in nanoscale science and engineering promise to reshape how we think about national security. Nano-enabled devices and materials offer both enhanced and, in some cases, completely new defense systems. In the words of one author, “Perhaps no other emerging technology will prove as disruptive in the future as nanotechnology.”  Conversely, these same advances force us
to consider how we protect national interests against sophisticated, nano-enabled threats that are evolving at an accelerating and unpredictable pace.
Convergence of nano-enabled technologies with other domains like biotechnology, autonomy, artificial intelligence andinformatics adds additional layers of complexity and unpredictability.  Beyond the extraordinary applications and defense-related implications of nano-enabled technologies, there are also unintended human and environmental health and safety issues associated with their development, production, use and decontamination/decommissioning.
The Nano Factor: More than Just Size
Much emphasis has been placed on defining a specific size range to encompass what is and is not nanoscale science and engineering. However, what excites us most about nano-enabled technologies really has more to do with the point where unique material properties begin to emerge.
While the emergence of these properties is primarily related to size, it can also be driven by shape and many other factors. So, when we talk about the “nano” factor, we are actually referring to no less than three features whose complex interactions can cause a particular material property to diverge from its expected behavior at the bulk-scale.
Those three features are: 1) the particular material property of interest (e.g., mechanical or thermal behavior that is being observed); 2) the type of matter possessing that property (i.e., the elemental composition); and 3) the physical structure of that matter (i.e., the arrangement of atoms into a structure of varying size and shape, for example).
A classic example of the nano factor is observed with gold nanoparticles. At the bulk scale, gold looks and behaves just as expected – lustrous gold-like color, chemical inertness and malleability. At the nanoscale, somewhere around 200 nm and below, its properties change dramatically – to the point where it causes us to question everything we know about what has defined gold as gold. The classical gold color gives way to a deep ruby red; it becomes catalytic; and it displays unique and tunable optical properties that have inspired entirely new fields like plasmonics. 
Nanoscale silver particles take on a bright yellow/gold color that looks more like what we would expect to see from gold nanoparticles (See Figure 1). And those are just two elements – we have an entire periodic table of elements that, when arranged into structures of varying shape and size, can display behaviors dramatically different from their bulk forms. Even highly stable carbon-based materials display unique electronic, thermal and mechanical behaviors at the nanoscale. 
Figure 1. From right to left: Silver nanoparticles along with gold nanoparticles of decreasing particle size. (Released)
Along with the changes in behavior that occur at the nanoscale come very important physical consequences. Nanoscale particles present new challenges to protective barriers such as filters, seals and joints; protective clothing; and even biological barriers like skin and membranes.
For example, research initiated by the National Institute for Occupational Safety and Health found that while respirators were generally very efficient at removing nanoscale particles from air, the most penetrating particle size for common filter media was between 30 and 100 nm. NIOSH also reported that the size of leaks between respirators and the faces of test manikins had the greatest impact on particle leakage into the respirate facepiece, with nanoparticles being more likely than larger particles to penetrate small leaks. 
Small size also means vastly increased surface area as well as surfaces that are potentially more reactive due, in part, to the increased fraction of atoms on the corners and edges of particles (See Figure 2). Gold, for example, which is usually considered an inert metal, can catalyze multiple chemical reactions when present as nanoparticles of less than 3-5 nm in diameter.  Some of these surfaces can provide suitable scaffolds for conjugation of chemical and biological entities, and thus enable certain types of nanostructures to act as highly-mobile nano carriers or “Trojan Horses.”
Figure 2. Illustration demonstrating the effect of the increased surface area provided by nanostructured materials.  (Nano.gov/ Released)
For example, see work by Park et al.  In some instances, nanoscale materials can be made highly dispersible, enabling them to be broadly disseminated in various types of fluid and semi-porous media. As they move through these media, they can acquire and transport toxicants. For example, the Hochella group at Virginia Tech has reported that naturally occurring nanoparticles of 5-10 nm in diameter played important roles in transporting heavy metals like lead and arsenic in riverine systems. 
Many overviews probe the spectrum and origins of nanoscale phenomena much more thoroughly than can be accomplished here. For additional information on this topic, refer to the recent review by Yan et al on the importance of nano in catalysis.  However, the central point of this article is that nano changes what we know about the physical and chemical world around us. From the way we interact with materials and what we can do with them to the way we perceive the physical world – all of that changes at the nanoscale. Ultimately, these changes have significant impacts on global and national security.
National Security and Defense Applications of Nanotechnology
Some of the most exciting aspects of nanotechnology over the last decade have been the seemingly limitless defense and security related applications they offer. The number and diversity of these applications have even inspired their own conference – NT4D or Nanotechnology for Defense.  According to the federal budget, U.S. Department of Defense spending on nanoscale science and engineering will top $130 million in 2017. 
Applications of nanotechnology in security and defense systems have included advanced chemical and biological detection systems; advanced imaging systems; next generation energetic materials: next-generation camouflage; high-strength/lightweight armor; medicine and human performance enhancement (wound healing, drug delivery, etc.); and advanced weapons systems.
The sections that follow briefly describe a few important and diverse ways that nano is impacting national security and defense.
Nano-bio Interface and Human Performance Enhancement
Nanotechnology has enabled science and engineering to progress at the scale of natural biological machinery, and this capability has led to important breakthroughs in areas like targeted delivery of nanomedicines, disease diagnostics/therapeutics, 3D-printed biomaterials and a growing list of wearable and implantable devices that offer biological feedback and intervention capabilities. Professor John Rogers, for example, who leads the Center for Bio-Integrated Electronics at Northwestern’s Simpson Querrey Institute for BioNanotechnology, pioneered the development of soft, flexible, skin-mounted bio-electronic devices that exploit novel nanoscale material properties to perform a variety of functions from monitoring ultraviolet exposure to mapping electrophysiology
in the heart and brain.
Some have observed that advances in nano-bio devices could, for example, help enhance human performance by detecting changes in neurological behavior or fine motor coordination signaling fatigue, stress, inattention and other behavioral changes that could impact the safety and effectiveness of military personnel.  Recognizing the critical advances underway at the nano-bio interface, the Nano-Bio Manufacturing Consortium was established in 2013 to help mature nano-bio manufacturing technologies with an initial focus on physiological readiness and human performance monitoring priorities set forth by the U.S. Air Force Research Laboratory, the DoD and other partners. 
The Rise of Molecular Machines and Exotic Materials
The awarding of the 2016 Nobel Prizes for physics and chemistry highlighted important examples of advances in nanoscale science and engineering that have important national security implications. While nano has been discussed, hyped and under-/overhyped for more than a decade now, many nanoscientists recognize that in some ways we have really only just begun to explore
the nano realm through our studies of basic nano characterization and fabrication tools and the most fundamental nano-enabled building blocks. Much like children tinkering with Legos for the first time, we are just now exploring how to hold and manipulate them, how to assemble them in meaningful ways, how to integrate mechanical functions that allow the systems to do work, and how to translate those functions to accomplish missions and enhance performance.
As this article goes to print, the news is filled with reports of the awarding of the Nobel Prize in chemistry to Jean-Pierre Sauvage, J. Fraser Stoddart and Ben L. Feringa, “for the design and synthesis of molecular machines.” Upon addition of energy in the form of, for example, changing pH gradients, light or heat, these single-molecule machines can perform simple tasks like rotate around an axis or move up and down in a controllable manner.  Similarly, the 2016 Nobel Prize for Physics was awarded to J. Michael Kosterlitz, Duncan Haldane and David J. Thouless, “for theoretical discoveries of topological phase transitions and topological phases of matter” that could lead to practical applications in new materials, electronics, and quantum computing.
Figure 3. “A single chain of water molecules lines the cavity inside a carbon nanotube porin, which is embedded in a lipid bilayer.”  (Image by Y. Zhang and Alex Noy, Lawrence Livermore National Laboratory/Released)
Finding Nano: Nano Tracking and Forensics and Their Importance to National Security
One of the greatest security challenges of a nano-enabled future is the difficulty involved in locating and characterizing nano-scale systems once released into human and environmental systems. How would a faulty nano-biomedical implant device be found and retrieved from the body? How would a nano-scale weapon, or even a swarm of nano-scale weapons for that matter, be distinguished from naturally occurring aerosols and dust particles and eliminated on a future battlefield?
For answers to these types of questions we can learn from interdisciplinary fields like environmental nanotechnology that offer tools and techniques to discriminate signals attributable to nanoscale contaminants from vast amounts of biological and environmental “noise.” Trying to detect engineered nanoparticles in complex environments is a daunting task. The minuscule scale at which nanoparticles exist relative to the scale of human and environmental systems makes searching for a needle in a haystack seem remarkably simple.
For perspective, the task of searching for a single 50 nm nanoparticle contaminant in your morning cup of coffee would be equivalent to searching for a needle in a haystack large enough to fill the Grand Canyon more than 30 times (See Figure 4)!
To complicate matters further, the chemical composition of many nanoparticles, carbon nanotubes and fullerenes for example, is virtually indistinguishable from materials like soil and plant matter that co-occur in natural environments.
Figure 4. Searching for a 50 nm nanoparticle in your morning cup of coffee? Good luck, that task would be equivalent (by volume) to searching for a needle in a haystack large enough to fill the Grand Canyon—more than 30 times! (Released)
Practical Nano Security Scenarios
As best we can tell, current to near-term nano security scenarios are much more limited and manageable than those that can be imagined based on the trajectories of nano- as well as other emerging and converging technologies. But it is a waiting game, and the gap between science fiction and reality has shrunk rapidly over the last decade. The tangible progress in molecular
machines noted earlier is proof enough of that. For the most part though, current embodiments of nanoscale materials appear more like building blocks for increasingly sophisticated material and devices of the future, and less like the “grey goo” they were once feared to be.  Nevertheless, present day nano security concerns do exist, and we consider three of these below:
• Nano-enhanced delivery of chemical and biological agents: Chemical and biological agent attacks remain a very real threat to global and national security. The potential for nanoscale agents to be deployed to enhance the efficacy of such attacks is one practical and near-term concern. As noted earlier, researchers have already demonstrated that nanoscale particles can act as ubiquitous carriers of toxic chemicals. A NATO report on the security implications of nanotechnology noted that:
“The potential for [nanotechnology] innovations in chemical and biological weapons is particularly disquieting, as NT can considerably enhance the delivery mechanisms of agents or toxic substances. The ability of nanoparticles to penetrate the human body and its cells could make biological and chemical warfare much more feasible, easier to manage and to direct against specific
groups or individuals. Dr. Sean Howard, in his work on NT security implications, has even called the threat of chemical and biological warfare a ‘real nano goo.’” 
• Limited nano detection/forensic capabilities: A major security concern and unmet need lies in our limited ability to determine forensically, whether and to what extent a particular nano threat may have been deployed. Additionally, there exists a clear lack of field deployable and scalable tools capable of detecting and monitoring nanoscale threats beyond laboratories and clean-rooms. Scientific and engineering-based approaches can be taken to address these gaps. For now, capabilities suitable for enhanced detection/ mitigation of nanoscale tracking devices or nano-enabled “Trojan Horse” delivery threats, for example, remain limited.
• Complacency amidst a silent arms race: The number of state-sponsored nanotechnology initiatives globally signifies a clear arms race to assume a dominant position in nano-enabled science and technology. While not as visible as the nuclear threat, this race is every bit as important to national and global security. A major threat to U.S. national security on this front is the potential to become complacent and to prematurely reduce federal investments into nano and convergent technologies. The United States has established itself as a global leader in nanoscale science and engineering research, scholarship and commercialization. Nevertheless, failure to maintain strategic, long-term investments in these areas, particularly rapidly evolving infrastructure and human capital, could severely impact U.S. innovation in nano-enabled industries and many other emerging technology fields that are simultaneously enhanced by progress in nanotechnology. Attrition of U.S. intellectual and infrastructural capabilities across nanotechnology-related programs would weaken U.S. defense and security interests in the future, when strategic nanoscale science and engineering investments are expected to yield their greatest payoffs.
Off Buttons and Erasers: Integrating Security Features into Nano-enabled Technologies
A critical security feature of any technology is the ability to turn it off, undo it, deactivate it or otherwise separate the harm it might cause from those it might harm. Even the humble pencil has evolved to include an eraser for undoing its mistakes. But, mankind has endured a host of challenges that arise when new technologies yield unintended consequences – the persistence of consumer plastic goods has left debris scattered across the Earth’s oceans; the use of nuclear weapons and runaway reactor cores have rendered cities uninhabitable for thousands of years; and the use of CFCs in coolant systems migrated unabated to the stratosphere where they’ve depleted the earth’s ozone layer.
The recent Galaxy Note 7 battery fire controversy coupled with growing use of lithium ion batteries in mobile devices underscores the importance of technology that can be turned off. At present, it is unclear how persistent nanostructures and the unique behaviors that may accompany them will be in biological and environmental systems, and that should be alarming.
An unprecedented dialogue around responsible nanotechnology has yielded progress, but feasible safeguards have been limited at best. Researchers have called for more green chemistry/nanotechnology approaches to help address some of these issues,  but those are likely to be effective only in situations where they clearly do not compromise performance of nano-enabled materials and devices.
Nano and National Security: Key Considerations for the Future
Looking ahead, nanoscale science and engineering will continue to impact security both nationally and globally in significant and far-reaching ways. The following list summarizes some key opportunities for the nano defense and security community:
• Translate nano properties to human scale devices and systems. Much of the hype surrounding nanotechnology has been muted by a lack of real-world examples demonstrating how unique
nanoscale material properties can be translated into materials and devices with performance capabilities that are vastly enhanced relative to their bulk counterparts.
• Perfect nanoscale power systems. Realization of some of the most exciting security and defense applications of nanotechnology requires innovative strategies to power and mobilize nano devices against ambient molecular forces that are far greater at the nanoscale than they are at the human scale. To nanomachines, molecules of air, water and biological fluids appear as impenetrable walls of infinite thickness.
• Enhance nano forensic capabilities. Analytical capabilities, particularly in the field, have not kept pace with nanoscale innovations. As researchers continue to perfect and advance nano-enabled systems, those systems will possess capabilities to perform increasingly sophisticated functions – delivery, communications, listening, imaging and others. As these functions are acquired, the need for national security and law enforcement personnel to have readily available nano forensic tools and procedures will become more vital to ensure justice, public safety, and security.
• Convergence complicates future threat assessment and mitigation. As multiple technology domains converge into more sophisticated systems, they link not only the benefits of those disparate
technologies but also their risks. Consequently, threat assessment must stay ahead of convergence through interdisciplinary programs linking nano to other emerging technology domains like bio, big data, artificial intelligence, and autonomy (See Figure 6).
• Prepare for every eventuality. Robust emergency preparedness and response planning and procedures are needed. While much progress has been made to better understand the risks of emerging technologies, translation of this knowledge into coordinated emergency response procedures has been lacking. The absence of dialogue between researchers investigating the hazards posed by new technologies and emergency managers is alarming, and poses risks to numerous stakeholders (e.g., students, research professionals, first responders, the general public) and national security.
Figure 6. Convergent technologies may integrate threats that we have traditionally managed individually. (Released)
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