D
Long-Term Feature and Feature Platform Descriptions

This appendix has in-depth feature descriptions of the longer-term features and feature platforms—that is, features that can be implemented in a time frame of more than 7 years. These features are also discussed in Chapter 5. Each feature description includes subheadings dealing with various aspects of the feature:

  • Description—An explanation of the physical principle(s) on which the feature is based. Also, the feature application as visible, machine-readable, applicable to the visually impaired, forensic applicability, and so on is described. Furthermore, the benefits and limitations of the feature are presented; graphics may be included to depict the feature and its operation.

  • Feature Motivation—A summary of the reasons why the feature is highly rated by the committee and reference to its uniqueness.

  • Potential Implementations—A description of scenarios that provide examples of how the feature could be employed to deter counterfeiting.

  • Materials and Manufacturing Technology Options—A summary of the materials and manufacturing process that could be used to produce the feature as well as initial thoughts on how the feature could be integrated into a Federal Reserve note.

  • Simulation Strategies—A discussion of potential ways in which a counterfeiter could simulate or duplicate the feature, and the expected degree of difficulty in attempting to do so.

  • Key Development Risks and Issues: Phase I—A discussion of the durability challenges, feature aesthetics, anticipated social acceptability, and descrip-



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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real D Long-Term Feature and Feature Platform Descriptions This appendix has in-depth feature descriptions of the longer-term features and feature platforms—that is, features that can be implemented in a time frame of more than 7 years. These features are also discussed in Chapter 5. Each feature description includes subheadings dealing with various aspects of the feature: Description—An explanation of the physical principle(s) on which the feature is based. Also, the feature application as visible, machine-readable, applicable to the visually impaired, forensic applicability, and so on is described. Furthermore, the benefits and limitations of the feature are presented; graphics may be included to depict the feature and its operation. Feature Motivation—A summary of the reasons why the feature is highly rated by the committee and reference to its uniqueness. Potential Implementations—A description of scenarios that provide examples of how the feature could be employed to deter counterfeiting. Materials and Manufacturing Technology Options—A summary of the materials and manufacturing process that could be used to produce the feature as well as initial thoughts on how the feature could be integrated into a Federal Reserve note. Simulation Strategies—A discussion of potential ways in which a counterfeiter could simulate or duplicate the feature, and the expected degree of difficulty in attempting to do so. Key Development Risks and Issues: Phase I—A discussion of the durability challenges, feature aesthetics, anticipated social acceptability, and descrip-

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real tion of the key technical challenges that must be addressed during the first phase of the development process to demonstrate the feasibility of the feature idea: that is, demonstrate feature capabilities and determine the usefulness in counterfeit deterrence. (The development phases are defined in Chapter 6.) Development Plan: Phase I—A characterization of the current maturation level of the feature technology, key milestones to be achieved during Phase I, known current and planned related developments external to the Bureau of Engraving and Printing (BEP), and a high-level schedule for Phase I. Estimate of Implemented Production Costs—An initial assessment of additional BEP operational steps that would be required at the BEP to produce a banknote with the feature, incremental cost (higher, lower, the same) relative to the cost of the current security thread, and an indication of whether additional BEP capital equipment would be required for production. References and Further Reading—Selected references relating to the feature and its associated components. Such references could include, for example, papers and conference proceedings for background on any work done relating to this feature. These lists are not exhaustive but are intended to provide a snapshot of current work related to the feature concept. The features described in this appendix are as follows: Anomalous Currency Space Chemical Sensors Digitally Encrypted Substrate Engineered Cotton Fibers e-Substrate NiTi Shape Memory and Superelastic Responsive Materials Smart Nanomaterials Tactilely Active Electronic Features

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real ANOMALOUS CURRENCY SPACE Description The “anomalous currency space,” or ACS (pronounced “ace”), is a materials-based approach that can serve as a platform for a wide range of anticounterfeiting strategies by providing a region or regions that differ entirely in materials composition from the banknote substrate. The primary objective is to provide an eye-catching visual feature that would also possess tactile properties. It is expected that the unique structure and composition of advanced materials incorporated into an ACS would assist forensic investigation as well. This empty space can notionally be thought of as a clear plastic window. However, it does not have to be shaped like a typical window. For example, the shape could be a strip that runs the full length or width of the banknote, or a strip that runs along any or all of the edges of the note, or a series of regions dispersed throughout the currency note. Also, the materials composition of this region does not necessarily need to be a clear plastic or other polymer. In this context, “anomalous” is used to emphasize that there is a physical space within the banknote that differs dramatically in terms of materials composition and behavior from the rest of the bill. Because Federal Reserve notes (FRNs) and their analogues already use a multiplicity of features that differ dramatically in materials structure and properties, the term “anomalous” refers to a macroscopic region of radical discontinuity relative to the bulk composition of the bill or note. Feature Motivation This feature platform offers numerous ways to create an eye-catching visual feature that could also possess other properties, such as a distinctive feel. The ACS provides for the incorporation of heterogeneous materials into the FRN in a manner that would not allow the ACS region to be inconspicuously removed or tampered with; also, these materials would have durability for the lifetime of the banknote. Potential Implementations The clear plastic window or variations on this theme are already in use in some foreign currencies, so it appears that this feature platform concept has been successfully introduced. The concept here is to extend the scope of this feature significantly. The polymeric material itself may be further modified in any number of ways, including the following:

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real Direct integration of electro-optical or other types of materials within the polymer. The material could be anything up to and including complete integrated circuitry. Surface etching or other physicochemical modifications to one or both surfaces (back and front) of the ACS feature to create novel electro-optical or other effects. Complete perforation of the polymer to enable a number of effects, from patterned microholes for physical identification to a type of diffraction grating. Incorporation of “smart” materials (including nanomaterials) whose unusual properties are based on new compositions and/or structures and that are capable of dynamic interaction with the environment: for example, memory polymers that shape-shift on the basis of a change in a physicochemical parameter (for example, temperature). Use of dumb materials (including nanomaterials) whose unusual properties are based on new types of composition and/or structure but do not respond to environmental stimuli: for example, an ultratough polymeric strip that traverses the entire border of the FRN and is impossible to tear. Inclusion of other materials in or on the polymer to create composites with various properties (for example, holographic metal strips). Employment of other materials with unique active and/or passive properties, structures, or behaviors. The window could be composed of two (or more) layers. Polymeric layers, for example, could have any or all of the properties described above. Further, the space(s) between the polymeric layers could contain additional materials. In the simplest case, two polymeric layers would be embedded flush with the two surfaces of the FRN, with each layer being less than 50 percent of the total thickness of the note itself. For example, assume a thickness of ~100 micrometers for U.S. currency and a thickness of 40 micrometers for a single polymeric layer. Assuming the polymer layers do not collapse and adhere to each other, the linear space between them is 20 micrometers in the center region (the ends would be embedded into the substrate). If the “window” is 1 centimeter square, the three-dimensional space between the two polymeric layers creates a volume of 2 microliters. This volume could be (fully or partially) filled with a novel material or composite, including current microscale and nanoscale materials and those in development as part of the National Nanotechnology Initiative (NNI). These materials and composites could be smart or dumb, active or passive, and so on. A larger window or thinner polymer layer would increase the available interlayer volume. Given that U.S. currency exhibits desirable characteristics of materials strength, toughness, and so on with a thickness of ~100 micrometers, it is not unreasonable

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real to assume the existence of materials that provide the same level of performance at less than half that thickness which, in turn, creates the interlayer space discussed about. Since future research will be conducted on supertough materials, it is reasonable to assume that layer thicknesses down to 10 micrometers or even thinner might be achievable. Such thinness would create the opportunity for larger interlayer volumes and/or multiple layers within the window. In the latter case, holographic-like effects and/or color-changing effects should be possible. There are many ways in which a banknote designer could apply the ACS feature platform concept. A few ideas include the following: Strip along outer edge. A strip of ultratough materials around the outer edge of the bill could be easily detected by its distinct look and feel—this region could not be torn or perforated. Distribution of small ACS features throughout the banknote. Distribution of smaller ACSs throughout the bill could produce easily recognizable patterns that could be used for visible, tactile, and possibly instrument-based detection. Such patterns could exhibit dynamic as well as passive behavior if, for example, memory metals or polymers were used. Memory polymers. Memory materials fall into a larger category of smart materials that exhibit unique behavior. Memory polymers (often constructed of bulk copolymers) are capable of dynamic movement and associated shape-shifting when the variable of state is applied (often a change in temperature). For example, spatial distribution of a memory polymer within a certain space could allow the surface to change when it fell above or below body temperature. A simple case would involve reversible surface stippling that would manifest as a change in roughness, which could be detected qualitatively by touch and quantified by instrumentation. Embedded circuitry is specifically excluded from consideration here, so the behavior of these types of smart materials would be strictly dependent on the composition of the material itself and would require no dedicated power source. As a further example, memory polymers have been produced for several biomedical applications, including self-tying surgical sutures. A memory polymer can exist in either of two states: elongated (two-dimensional fiber) or contracted (three-dimensional cylindrical). This material, produced by block copolymerization, could be incorporated into a clear plastic window in such a manner that the cylindrical watermark-like image could be created by the variation in light transmission produced by the three-dimensional state. Suppose the transition temperature was adjusted to ~35°C. The simulated watermark would appear in the window at temperatures below 35°C and disappear above 35°C. It is reasonable to suppose that U.S. currency spends more than 90 percent of its time at temperatures

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real below 35°C. Therefore, FRN verification could occur by vigorously rubbing the window between the thumb and forefinger. Because the window would be ≥100 micrometers thick, frictional heating would rapidly raise the temperature, causing the image to disappear. As soon as the window was allowed to cool, the image would reappear. A simpler alternative might be to construct the window so that the image disappeared when the window was physically stretched in one (or any) direction relative to the xyz coordinates of the FRN itself. This could be accomplished by controlling the orientation of the coiled memory polymers during fabrication so that the image was formed much like crosshatched pen and ink work or the engraving process itself. Since the image would disappear rather than becoming distorted, this trait could not be simulated by simply using a window made of an elastic material. Materials and Manufacturing Technology Options The ACS, by definition, has no specific material requirement other than being a completely different material from the bulk of the FRN. The requisite manufacturing technologies would depend entirely on the feature design and the materials selected for the feature application. The ACS provides a flexible mechanism whereby these advanced materials can be incorporated into currency with minimum disruption to the production process. Like the silicon wafer facilities required for integrated-circuit fabrication, extremely-high-technology equipment would be required for the production of some of these new materials, but once the manufacturing process was online the cost per unit would drop to the level of a bulk commodity. In other cases, the properties will depend on exact nanofabrication that will not be possible to counterfeit or simulate without complete knowledge of the molecular structures of the components, once again putting counterfeiting out of the reach of all but the most sophisticated criminals. The spectrum of physicochemical properties that could be incorporated into the ACS is as large as the spectrum of 21st-century materials, which means that the unique property could be physical, chemical, optical, electromagnetic, and so on. Simulation Strategies The ability to simulate an ACS feature will depend on the behavior of the material(s) selected. It is assumed that the wide range of materials that are under development will make it possible to select material performance traits that will make simulation extremely difficult, requiring resources above the level of the opportunist counterfeiter. Using the example of the shape-shifting memory polymer: temperature-sensitive, reversible changes in surface roughness would be extremely difficult to

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real simulate. The reason is that the properties of the feature are based on both the unique composition of the material and the method in which it is integrated into the ACS. Likewise, the ultratough, flexible, lightweight materials currently under development in places such as the Massachusetts Institute of Technology’s Institute for Soldier Nanotechnologies cannot be simulated, since they are new materials with novel properties that are dependent on yet-to-be-created processing methods. It is highly probable that many of these methods will require a very high initial investment in sophisticated instrumentation. Therefore, initially there will be no analogues available for simulation and (for properties such as ultrahigh tensile strength) no way to simulate them. Key Development Risks and Issues: Phase I Since a wide range of highly durable materials will be available for the ACS, as well as rigorous testing methods by which this durability may be characterized, durability will need to be evaluated but probably will not be a major constraint. The ability to integrate a window or other ACS into the FRN is really a question of whether sufficiently strong bonding can be formed between the cotton-linen paper and the material(s) used to create the ACS. Given current and future methods for creating materials composites, it is highly probable that the ACS can be integrated into the FRN in a secure and durable manner, but the details would be part of the development program. In terms of aesthetics, many advanced materials could provide a high-technology gloss to the FRN that would make it appealing to many users. Paper currency that possesses a region that changes shape or contains a perimeter that cannot be ripped, cut with a knife, or even perforated with a bullet would probably be received favorably by most of the public. By specifically limiting any new property to an ACS, the Bureau of Engraving and Printing (BEP) can retain most of the traditional look of the “American greenback.” Retention of the traditional craft involved in creating U.S. currency and the reliability that has accrued to the “brand identification” are, in and of themselves, highly desirable aesthetically. Another key implementation consideration involves the selection of the materials to ensure that they would not pose an environmental or health hazard. Great care will need to be exercised in the selection of any advanced material for the ACS. One simple yet profound example is an ACS capable of degrading into its component nanoparticles. Little is known about the toxicology of nanoparticles. Further, any specific nanofabricated material will have its own set of physicochemical properties. An ACS might meet with all reasonable durability standards for use yet, when burned, release nanoparticulates that could be inhaled (say by a child). Until there is a great deal more experience with these types of materials, the potential to set

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real off an irrational panic response based on the incorporation of any new, advanced material (nanotechnology-based or otherwise) must be considered. The specific key technical challenges with respect to the ACS will depend entirely on the materials selected for it. However, one of the advantages of choosing an ACS component for currency is that many key technical challenges for the materials themselves will be driven by other extremely-high-value applications such as medical implants or biodefense. As a result, most of the research and development (R&D) on the materials themselves can be leveraged by the BEP. Certain technical challenges will be intrinsic to the specific use of these advanced materials in FRNs. The most obvious is the physical incorporation of the ACS into the FRN. Can any ACS feature be physically incorporated into the FRN? Using the clear plastic window as a simple example, it is reasonable to assume that if the material used in the ACS can be fabricated to conform with the physical dimensions of the FRN, then it may be incorporated into the FRN through some form of compositing. It is more likely that the key decision will be the cost of integrating any such compositing step into the current FRN production process. The BEP already has significant experience in incorporating anomalous materials into specific regions of the FRN (for example, specific regions of color-shifting ink and the security thread). In many ways, the ACS may be viewed as a logical extension of such features. Development Plan: Phase I Activity during Phase I must address the key technical challenges with respect to the use of the ACS in U.S. banknotes. There are many opportunities for features within this technology area, so Phase I activities can determine which potential features are of greatest interest for counterfeit deterrence. Feasibility experiments can be conducted, through a combination of laboratory-based work and modeling and simulation analysis, to select the most promising future directions for currency applications. A thorough review of current related work would be the first priority. Several important development programs can be leveraged to create ACS-based features rapidly. This materials revolution is expected to produce new materials with properties that could be of tremendous usefulness in anticounterfeiting efforts. The NNI has already been mentioned. Specifically, the Department of the Treasury is already a member of the Nano-scale Science, Engineering and Technology (NSET) Subcommittee of the National Science and Technology Council that provides a mechanism by which the BEP can obtain information about candidate materials for an ACS. In addition to or in coordination with the NNI, advanced materials are under development at all major government agencies funded by an equally wide range of missions. Obvious candidate agencies would be the Department of

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real Defense (for example, the Institute for Soldier Nanotechnologies), NASA, and the basic sciences division of the Department of Energy. Since there are so many possibilities of new features, multiple feature concepts can be pursued. Therefore, there could be a base level of activity that is continually at work in Phase I to further develop new ACS ideas. Then, specific ideas that have been determined to have attractive counterfeit-deterrence benefits could be spun out of the base program into distinct, defined projects that would proceed through all the requisite development steps. Estimate of Implemented Production Cost Based on the examples of other currency notes shown to the Committee on Technologies to Deter Currency Counterfeiting, there appear to be no major technical hurdles to the introduction of clear plastic windows and other ACS-based features. However, the introduction of a window or other ACS will likely affect the manufacturing operations at the BEP. Since the ACS is a concept of a feature platform rather than any particular technology, it is not possible to provide estimates of cost. However, many of the technologies that could be leveraged for an ACS-type feature are under development for large-scale industrial, medical, and military applications. So it is reasonable to assume that cost-effective manufacturing (including cost minimization) is a major goal of many of these advanced materials-based projects. Therefore, it is possible that at least some ACS-based features will have the same cost as that for the current security thread. Further Reading While no specific references are given here, numerous examples of smart materials under development may be found at the National Nanotechnology Institute’s Web site at <www.nano.gov>. Accessed February 2007.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real CHEMICAL SENSORS Description Sensors embedded in banknotes could detect a human-produced or gadget-produced chemical and generate a human-detectable signal. Passive sensors would change their appearance directly, while active sensors would require a power source that could be either self-generated on the banknote or obtained from a battery, for instance at the point-of-sale. Active sensors can trigger visual, audible, or tactile responses (see the section below entitled “e-Substrate”). This feature class can enable features for unassisted use or assisted use with simple devices, as well features for the blind. The sensed chemical could be an exhalation gas, activated by breathing on the sensor. Expired air has typically 3.6 percent carbon dioxide as compared with 0.03 percent in ambient air, making it a good target. Expired air also has 6.2 percent water given an atmospheric air water content of 0.5 percent.1 The sensed chemical could also be one of the constituents of perspiration, activated by touching the sensor. Perspiration is 98 to 99 percent water, but also contains (per 100 ml perspiration): lactic acid (45 mg to 452 mg), chloride (30 mg to 300 mg), sodium (29 mg to 294 mg), and potassium (21 mg to 126 mg) as well as numerous other organic and inorganic compounds in smaller quantities.2 The sensed quantity could also be acidity or alkalinity. As an example, a lactic acid sensor could be triggered when someone touched the note, then either the sensor’s appearance would change or the sensor would supply an electric current to activate a light or sound. Care would be required when designing the sensor to ensure that it is reversible, that is, that the sensor returns to its original state after the trigger chemical is removed. Alternatively, a gadget similar to the commonly used “starch pen” could be designed to contain a chemical that produces a temporary change in the banknote’s appearance or to produce an audible or tactile signal. An example could be a pen filled with vinegar or some other inexpensive, nonhazardous substance that could trigger a reversible response when drawn across the banknote. 1 For additional information on the human respiratory system, see <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pulmonary.html>. Accessed February 2007. 2 For additional information on latent fingerprint composition, see the information from the Victoria Forensic Science Centre, Victoria Police, Australia, available at <http://www.nifs.com.au/F_S_A/Latent%20fingerprint%20composition.pdf>. Accessed February 2007.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real Feature Motivation This feature platform received a high rating from the committee because of its potential to deter opportunist, petty criminal, and professional criminal counterfeiters, and because of its potential usefulness for the unassisted general public, cashiers, tellers, and the blind, either unassisted or with the use of an inexpensive device. The primary benefit of this feature platform is the difficulty of reproducing or simulating it. Chemical sensors are difficult to reproduce by opportunist counterfeiters and petty criminals because neither the sensors nor the materials required to make them are readily available in the marketplace. Professional criminal counterfeiters would also be deterred by the difficulty of reproducing these sensors well. The primary limitation of this feature platform is its potential complexity, which might make it expensive to manufacture and limit its robustness over the expected life of the banknote. The sensors must be activated by all people (for example, young, old, healthy, sick) in the full variety of habitable environmental conditions (for example, wide ranges of humidity and temperature). The effectiveness of chemical sensors for banknote authentication depends entirely on how sensitive and robust the sensors are and on the specific implementation of the human-detectable response. In general, active features (those that change in response to a stimulus) should be easily noticed by the general public and should even generate interest in observing the note. Potential Implementations Passive sensors could be formed from chemically activated optical materials sandwiched between porous plastic films; for instance, the material’s refractive index could normally render it transparent, but the material would change to opaque upon exposure to lactic acid or carbon dioxide. Or, the material could change color, polarization, or thickness. Electrical sensors could be formed from electronic elements whose electrical properties would change reversibly when the concentration of a specific chemical was changed. These elements could be transistors, diodes, or passive components. This class of sensor does not produce a detectable signal itself, but rather it provides a trigger to activate a separate, human-sensible device such as a buzzer, light, or raised bump. Specific examples of these sensors are given below.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real materials themselves. These materials exhibit dynamic behavior, but only in a limited range, triggered by changing a simple physicochemical variable such as thermal energy or the presence (or absence) of a specific chemical or biological compound. The result is posited to be a human-perceptible phenomenon easily and rapidly recognized by the targeted class of users. The usefulness of such a feature is that it recaptures the ease of counterfeit detection inherent in the macroscopic optical security features of FRNs prior to the commercialization of cheap yet sophisticated reprographic products. In addition, because it is the materials themselves that create the feature, the potential exists for bioassays other than those involving vision. This possibility could be useful to the vision-impaired. Potential Implementations The Institute for Soldier Nanotechnologies (ISN) at the Massachusetts Institute of Technology (MIT) provides a convenient example of how nanofabrication methods may be used to harness MSA and MM to create technology platforms. There is significant similarity between the performance specifications required for many advanced materials under development for soldier technologies and those required for use as anticounterfeiting features in banknotes. Under field conditions, many of these smart materials will need to be highly durable. In addition, because of the immediacy of the battlefield, many applications (for example: Is there a toxin present? Has a ballistic impact occurred?) will require an almost-instantaneous, unequivocal yes or no, and human-perceptible manifestation. These same traits would make these materials of potential use for anticounterfeit applications. Example 1: Mechanical Actuators Capable of Switching Between States The ISN at MIT is “developing nanomaterials that are capable of mechanical actuation and dynamic stiffness.”14 Either or both of these properties could be incorporated into a windowed currency feature that is both visible and tactile. As part of the soldier’s battlesuit, these adaptive multifunctional materials will improve soldier survivability. According to the ISN: Mechanical actuators embedded as part of a soldier’s uniform will allow a transformation from a flexible and compliant material to a non-compliant material that becomes armor, thus protecting the soldier by distributing impact. Soft switchable clothing can also be transformed into a reconfigurable cast that stabilizes an injury such as a broken leg. Contracting materials can be made to apply direct pressure to a wound, function as a tourniquet, or even perform CPR when needed. Mechanical 14 For further information on this research, see <http://web.mit.edu/ISN/research/team02/index.html>. Accessed February 2007.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real actuators can also be used as exo-muscles for augmentation of a soldier’s physical strength or agility and as wound compresses.15 Smart, self-assembling materials capable of switching between states may be adaptable as a security feature in future generations of FRNs. Example 2: Active, Reactive Fiber Coatings ISN Team 3 is developing smart nanomaterials that will provide protective measures to enable the future soldier to detect and respond to chemical and biological threats.16 The strategy is to develop protective fiber and fabric coatings for integration in the battlesuit. These surfaces will neutralize or significantly decrease bacterial contaminants as well as chemical attack agents such as nerve gas and chemical toxins. For example, some investigations include responsive nanopores that “close” upon detection of a biological agent. In addition, novel organic-inorganic hybrid nanocomposites, consisting of nanoparticles and formed using simple dip processing methods that combine sensing and reactive components.17 While the goal of this work is to develop smart nanomaterials that can act as reactive or responsive protective coatings for fibers and fabrics for soldier technologies, any material that can exhibit “dynamic, reversible behavior” that is human-perceptible (that is, may be bioassayed) is obviously a candidate for a security feature in currency. Materials and Manufacturing Technology Options These features would use materials and manufacturing methodology that is expected to emerge from recent, large-scale investments in nanoscience and nanotechnology. Importantly, these investments are being driven by the need to nanoscale the manufacturing of components for major industries such as the manufacture of defense-related products, of semiconductor devices for computers and communications, more generally, as well as of bioengineered devices for health care applications. Also, the ISN is developing the technology to enable the synthesis of nanotechnologies developed by ISN to provide soldier protection in the field … [using a] broad-based approach to developing processing and fabrication technologies for novel nanomaterials. These technologies must be capable of effectively processing a wide range of components: nanoscale fibers and 15 See <http://web.mit.edu/ISN/research/team02/index.html>. Accessed February 2007. 16 For further information on this research, see <http://web.mit.edu/ISN/research/team03/index.html>. Accessed February 2007. 17 See <http://web.mit.edu/ISN/research/team02/index.html>. Accessed February 2007.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real films; multilayered materials; membranes and microdevices; microfluidic devices; functional hollow fibers; and field-responsive materials and devices. Team 5 has as its goal the fabrication and integration of hierarchically structured materials to achieve multiple and synergistic property combinations.18 Simulation Strategies Smart materials, such as those under development at the ISN, will be virtually impossible to counterfeit or simulate without the ability to nanofabricate—that is, to build at the molecular level with atomic precision. The power of a molecular self-assembly-based material or device lies in its simplicity of operation. This simplicity is based on the fact that certain forms of MSA occur spontaneously if and only if one can nanofabricate the materials components of the device ab initio. It is unlikely that this type of technology will become available to any but the most sophisticated government-sponsored counterfeiters for at least a generation. Nanofabrication facilities necessary to create microprocessor chips or molecular scaffolds for tissue engineering are unlikely to evolve into a format that will make them common household items anytime in the near future. Rather, it is probable that such instrumentation will be controlled by large corporations and major government and/or university research facilities. Key Development Risks and Issues: Phase I Smart nanomaterials comprise an emerging area, and a number of possible features can be imagined. The adaptation of the nanomaterials work to counterfeitdeterrent features leverages ongoing programs. Thus, a key risk that would hinder the future development of these novel features would be the termination of the NNI. A second key risk is related to the need to understand any risks associated with environmental, health, and safety effects of nanotechnology. This is an ongoing program at the national level, and it would be beyond the scope of a currency security feature R&D program. The third risk would be an unfocused program that expended resources without achieving concrete results. Development Plan: Phase I This feature platform would require a long-term commitment to R&D. The most important activity during Phase I would need to be a comprehensive survey of ongoing work and an analysis of the most-promising targets of opportunity for future counterfeit-deterrent features. Once these targets of opportunity were 18 See <http://web.mit.edu/ISN/research/team05/index.html>. Accessed February 2007.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real selected, a specific plan could be developed for each one. The most desirable situation would be for the BEP to be able to highly leverage ongoing work so that BEP funding would be needed only to adopt the technology, not to invent it. Estimate of Implemented Production Cost This technology is too immature to make an estimate regarding production cost. A number of different directions could be pursued. It is possible that this technology could be implemented at some future time for less cost than that of the current security thread. However, usual experience is that new technology costs somewhat more, and a decision must be made about whether the benefit would be worth the added cost. Further Reading See the Web site for the Institute for Soldier Nanotechnologies (ISN) at <http://web.mit.edu/ISN/research/> and for the National Nanotechnology Initiative at <www.nano.gov>. Accessed February 2007. National Research Council. 2006. A Matter of Size—Triennial Review of the National Nanotechnology Initiative. Washington, D.C.: The National Academies Press.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real TACTILELY ACTIVE ELECTRONIC FEATURES Description A number of material classes offer the potential to develop currency features with tactilely active responses—that is, they will produce local changes in shape or tactile nature as a result of a user input. Two promising classes of material for such features are piezoelectric crystals and electroactive polymers. A piezoelectric crystal develops a voltage when strained. Conversely, if a voltage is applied to a piezoelectric crystal, it can respond with a strain resulting in a shape change or deflection. This effect can be used in currency applications to produce a user-generated and detectable change in the tactile feel at a specific location on a note. In the simplest sense, one can envision piezoelectric bumps that would raise or lower on the note when a voltage is applied. The voltage might be supplied by an external source, such as a small battery, but it might also be supplied by an internal source (see the discussion above in the section “e-Substrate”). In practice, while it may be difficult to produce large enough piezoelectric deflections within the constraints of a note, patterns of fine bumps could be produced to generate changes in the tactile nature of the patterned area. The patterning sequence could be varied on notes of different denominations so that each denomination had a unique, readily identifiable pattern. Since most piezoelectric materials require relatively high voltages for activation, the materials would have to be carefully selected for application in currency. At the present time, it is not clear if tactilely active piezoelectric features can be practically integrated into notes. Such features will require significant development to integrate them effectively into notes with electronic substrates. Electroactive polymers (EAPs) are currently being developed for application as artificial muscles and in other actuator applications. The phenomena that make these polymers attractive as active materials, that is, they deform under the influence of an applied external voltage, may also have application for active-response features in currency. Upon the application of the external voltage, an EAP will deform over the period of a few seconds. As an example of the potential of these materials,19 strips 5 cm × 0.6 cm × 0.02 cm (~0.05 inches thick) can display deflections in the range of 60 to 90 degrees and can displace loads in the range of 50 times the weight of the strips at the ends of the strips. The EAP will regain its original shape upon the reversing of the polarity of the voltage. This process can be repeated over a large number of cycles with 19 For more information, see <http://www.polysep.ucla.edu/Research%20Advances/EAP/electroactive_polymers_as_artifi.htm>. Accessed February 2007.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real no apparent degradation in response. The voltages necessary to drive the response are quite low, on the range of a few volts, in the range offered by dry cell batteries (D, AA, AAA, C). In a currency application, one might envision a number of types of active-response features developed from EAPs. These could be in the form of an active bending security strip or strips or, with more development, features that changed shape or topography (including changes in tactile nature). The note response would be designed to be unique to the denomination of the note. Clearly, the response would require a voltage input that could either be self-contained on the note or provided by the user in the form of a readily available battery. Both of these power options would require integration with some level of an electronic substrate (see the section above entitled “e-Substrates”). Feature Motivation The active nature of features based on piezoelectric crystals and electroactive polymers would be a strong deterrent to currency counterfeiting at all levels of counterfeiting. Primitive, opportunist, and petty criminal counterfeiters would not be able to simulate the active response of the features accurately. Furthermore, feature configurations and designs specific to each denomination would render of dubious value attempts to “raise” notes through bleaching and reprinting higher denominations. Professional criminal and state-sponsored counterfeiters would be challenged to duplicate the proper materials and processing to obtain the required response. Furthermore, integration with the necessary electronic substrate would be extremely difficult to duplicate. Active-response features would have a number of advantages for use by the general public. The novel nature of the features would attract attention from the public, leading to people’s making use of the features. Currency users could readily detect shape changes and changes in the tactile nature of the features. Denomination-specific placement and patterning of the features would allow simple identification of the note values, making such features very useful for visually impaired users. Potential Implementations As described above, tactilely active features would allow point-of-sale user verification of a note’s authenticity and denomination by facilitating a user-input/ feature-output response. The user would primarily use the change in shape and tactile nature to evaluate the note. However, other human-perceptible indicators such as change in visual reflectivity might also be detectable. In the simplest form, EAP strips could be integrated into a banknote. Such

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real strips would change shape when the note was folded across the contacts of an AA or AAA battery (both provide 1.5 volts and are pervasively available). Different denominations would have both the thread and contacts in different locations on the notes, leading to very specific and publicly identifiable denominating. More complex integration into a note would include patterns of either EAPs or piezoelectric crystals that would change local topography on the note with the voltage applied. This patterning could be designed to produce recognizable effects such as buildings, eagles, or numbers specific to the denomination, as illustrated in Figure D-8. The changes in topography would be recognizable either visually or FIGURE D-8 Illustration of the change that would occur in a banknote as the surface relief rose with the application of voltage. In practice the response could be in any direction and still lead to changes in relief.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real by changes in their tactile nature. Although the schematic in Figure D-8 illustrates the surface relief rising with the application of voltage, in practice the response could be in any direction and still lead to changes in relief. Thus, the electroactive material would not have to be orientation-controlled during processing. Ideally, the user input could be generated by an on-note power supply. The changes in tactile nature on the note would be a function of a number of BEP- or supplier-controllable variables, including electroactive material, dot size, and coating. Primitive, opportunist, and petty criminal counterfeiters would not be able to simulate this active change in tactile response accurately. State-sponsored and some professional criminal counterfeiters would be able to reverse-engineer the feature and in time simulate the response. This feature is primarily targeted to assist currency verification by the general public and point-of-sale merchants. The outstanding benefit of this feature is that it is user-induced (preferably by an integrated power source) and detectable by humans. If the power source can be integrated into the note, the authenticity of the note can be subtly checked without visual inspection, allowing a wide range of users to verify the notes, including the visually impaired. Materials and Manufacturing Technology Options Piezoelectric materials are used in a wide range of industrial, consumer, and laboratory applications, and extensive research is currently producing and studying nanoscale piezoelectric materials. For example, piezoelectric print heads are often used as print heads in ink-jet printers; these heads are in fact manufactured through an ink-jet process. Consequently, processing the piezoelectric materials is well established. The key in the current application would be to make the best selection of piezoelectric material to obtain the intended response and to properly integrate the patterned feature with the electronic substrate. A wide range of electroactive polymers is being developed, including ferroelectric polymers, dielectric EAPs, electrostrictive graft elastomers, ionic polymer gels, and ionometic polymer-metal composites, all with advantages and disadvantages. Many of these require the polymer to be saturated with water or immersed in water. Clearly, the need for water to achieve the mechanical response has implications for currency applications. Further advances in EAP technology or flexible encapsulation will be necessary for some EAPs to be integrated into currency. Thus, this is a relatively immature technology that will require considerable development. Simulation Strategies While piezoelectric disks are available for less than $1 at hobby stores, engineered patterns of piezoelectric features would be difficult for many counterfeiters

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real to simulate. The proper patterning and integration with the appropriate leads and power supplies would require processing beyond the realm of all but professional criminal or state-sponsored counterfeiters. Primitive and opportunist counterfeiters would not be able to duplicate and would have a very difficult time simulating the active response of EAPs. Simulations would most likely be based on using elastic polymers to simulate some aspects of the response. However, it is unlikely that such simulations would be reversible, and they would not hold up under even moderate scrutiny. These efforts would be further complicated by more complex patterns that would result in topography changes in the notes. Petty and professional criminals would be challenged in attempting to counterfeit EAPs. The polymer chemistry and processing would require significant technology and equipment to duplicate. It is difficult to envision other approaches to achieving the active response of EAPs that would be any less challenging to the criminal. State-sponsored counterfeiters could, with time, reverse-engineer the technology necessary to obtain the EAPs. However, this would not be a trivial matter. Thus, if proper technology control were in place, state-sponsored counterfeiters would be extremely challenged by active-response features such as EAPs. Key Development Risks and Issues: Phase I The integration of electroactive material features would have some impact on the aesthetics of U.S. currency. It is envisioned that such features would be contained in a moderate-sized strip or patch, approximately the size of the current security strip or of a fingertip. The feature would be noticeably different from the rest of the note, in the same manner that holographic films, clear windows, or metallic ink are noticeably different from the surrounding areas on current currencies. Furthermore, it is envisioned that both internal or external power sources would require recognizable features that would impact the aesthetics of the note. Although aesthetics is a subjective issue, it is likely that such features would be judged to degrade the aesthetics of the currency. Electroactive features would be socially neutral, although any incorrect suggestion that the feature might actually record or track the fingerprints or DNA, or add what you would like, of anyone who touched the note could affect its social acceptability. Many products have been doomed by false rumors about them. Most electroactive polymers are in the early stages of development and are a considerable way from being used as artificial muscles. However, other EAPs are at or near implementation in other actuator technologies. In order for EAPs to be integrated into currency, a number of issues would need to be addressed. Viable EAPs that would be useable in a range of environments, from dry to damp, would need to be developed—possibly requiring new types of EAPs or the encapsulation

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real of the EAPs. The durability of EAPS should be quite high, given the thin sections and large deflections currently reported. However, this might be compromised to a degree by encapsulation. The durability of tactilely active piezoelectric features is unknown and would have to be assessed as part of the development of such features. Because such features would be electronic in nature, durable electronic platforms would be necessary, in addition to durable active features. It is envisioned that considerable redundancy of leads and fine-scale piezoelectric bumps would be necessary. Many of the key technological challenges to developing electroactive currency features go hand in hand with those of developing electronic substrates. For example, currently piezoelectric patterns can be produced in manufacturing settings. The difficulty would be to produce patterns integrated with the electronic substrate. Furthermore, the need to power the devices effectively, either through external sources, or preferably, through integrated power sources, would take significant development in order to integrate either EAPs or piezoelectric crystals into currency. Development Plan: Phase I All of the functions critical to implementing electroactive features into currency—piezoelectric devices, EAPs, the electronic substrate, and power supplies—have been demonstrated individually. However, integration of the technologies has not been developed in a currency platform. There are significant industrial and university programs developing e-substrate technologies that would be well suited to developing electroactive features for currency. Also, at the present time some work is going on to develop a number of different variable-friction surface technologies. However, these do not appear to be targeted to currency applications. A key milestone in the development of this technology would be the patterning and encapsulation or coating of the piezoelectric or EAP features in a durable form suitable for integration in currency. At that point the effectiveness of the active changes in shape and tactile nature could be assessed. Estimate of Implemented Production Cost This technology is very immature for currency applications, and thus an analysis of the implementation cost should be included as part of the development program during Phase I. However, it would be expected that features based on this technology will be significantly more costly compared with the current security thread. Since the devices would be embedded in the substrate, the substrate manufacturer would have to make significant investments in process technology.

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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real The impact on manufacturing operations within the BEP might be minimal, but it is premature to state this unequivocally. Further Reading Bar-Cohen, Y. 2001. ElectroActive Polymers—EAPs. Available at <http://www.azom.com/details.asp?ArticleID=885#_Ferroelectric_Polymers>. Accessed February 2007. Bar-Cohen, Y. 2006. WorldWide Electroactive Polymer Actuators Webhub. Available at <http://eap.jpl.nasa.gov>. Accessed February 2007. Cohen, J.Y. Electroactive Polymers as Artificial Muscles—A Primer. Available at <http://www.polysep.ucla.edu/Research%20Advances/EAP/electroactive_polymers_as_artifi.htm>. Accessed February 2007. Free electricity from nanogenerators. MIT Technology Insider. Available at <http://www.techreview.com/read_article.aspx?id=16746&ch=nanotech>. Accessed February 2007. Galassi, C., M. Dinescu, K. Uchino, and M. Sayer (eds.). 2000. Piezoelectric Materials: Advances in Science, Technology and Applications. NATO Science Partnership Sub-Series: 3: High Technology, Vol. 79. Norwell, Mass.: Kluwer Academic Publishers. Price, B., and C. Blankenship. 2003. NASA Langley Research Center is offering to license its intellectual property in electroactive polymers to prospective industrial clients for commercialization opportunities. Available at <http://www.teccenter.org/electroactive_polymers/index.html>. Accessed February 2007. Wang, Z.L., and J. Song. 2006. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 312(April 14): 242-246.