ADAPTIVE SOFT AND BIOLOGICAL MATERIALS
Joerg Lahann, Professor, Chemical Engineering, Materials Science and Engineering, Bio Medical Engineering, University of Michigan.
Dr. Lahann introduced rationale for designing adaptive structural materials modeled on forms that exist in nature. For example, a number of insects and fish are able to dynamically change their exterior color to provide camouflage matching their environment. Such adaptive changes can arise from the activity of multiple distributed autonomous elements: fish melanophores (pigment cells) individually modify their morphology in response to environmental light cues such that their collective action lightens or darkens the animal’s surface.
Similarly, synthetic materials exhibiting smart responses can be developed by engineering appropriate building blocks. For example, Dr. Lahann described work done in his laboratory to construct anisotropic microparticles that combine polymers with several different properties, such as color, magnetic susceptibility and deformation in response to temperature, solvents, electric fields, magnetic fields and pH. Such particles can individually take on a variety of shapes under different environmental conditions. When used together on a surface, such adaptive changes can alter the physical and chemical properties of that surface – for example its color or hydrophobicity. A key concept is the ability to design properties at the smallest element level that lead to useful responses in the bulk. Some bulk/surface properties would not be accessible without the use of this design approach.
Dr. Lahann noted that manufacturing anisotropic micromaterials is challenging, but advances made in microfluidics and chemistry by groups around the world have made it possible to do so consistently. The equipment necessary for such synthesis is not expensive. Forms of this concept have experienced broad adoption, for example in reading devices based on e-ink. Additional practical applications in development or under consideration include security inks, reconfigurable antennas, tunable filters and lenses, camouflage systems and medical diagnostics. While most technical demonstrations of adaptive materials based on anisotropic responsive microparticles have been in the liquid phase, he stated that it is also possible to structure solid materials and membranes with sufficient freedom of movement to take advantage of this approach. “However, I would not use these to paint a car,” Dr. Lahann said.
Research groups whose work was cited by Dr. Lahann are based at academic institutions in the United States, Germany, Israel and Japan. Topics not covered in the presentation included self-healing materials and metamaterials.
LEARNING FROM NATURE: BIOINSPIRED MATERIALS AND STRUCTURES
Vladimir Tsukruk, Professor, School of Materials Science and Engineering, Georgia Institute of Technology.
Dr. Tsukruk defined adaptive materials and structures as comprising the ability to sense external stimuli (ideally with low detection limit and large dynamic range) and actuate a calibrated reaction (e.g., change in organization) governed by a feedback loop to achieve a desired state (e.g. shape, compliance, appearance). He noted that many examples of adaptive and other uniquely beneficial materials and systems exist in nature, including:
• Adaptive colors in butterflies, octopi – photonic, sensing, camouflaging
• Dynamic adhesion in gecko feet – climbing, holding
• Self-healing biological parts – self-healing materials
• Reptilian locomotion – movement on complex terrains
• Dog canine – remote trace chemical sensing
• Silk materials – tough lightweight nanocomposites
• Night vision in some species – thermal sensors
• Wave tracking in seals and fish – underwater monitoring
• Spider hair air flow receptors – mechanical sensors
Devices with unique functionality and performance can be developed by characterizing the mechanisms underlying biological materials and recapitulating them synthetically. Using an example from his own laboratory, Dr. Tsukruk stated that the unique ability of snakes to detect small temperature changes as part of their infrared detection capabilities was traced to a special organ in which thermal expansion of air pockets leads to deformation of a membrane, which is detected in downstream cell signaling. Synthetic nanostructures with such a membrane arrangement were created and exhibited temperature sensitivity of 10 mK, with a spatial resolution of 30 µm when used in an array. Dr. Tsukruk said that at the time they were developed, these sensors showed unique performance compared to state-of-the-art bolometers. Importantly, these sensors are a passive material: temperature changes deform membranes so as to produce changes in reflectivity, which can be assayed as needed. He pointed out that a similar approach was used to mimic the ability of spiders to detect sound waves using unique hair structures, and to mimic the ability of fish to sense fluid velocity. In the latter case, a biomimetic structure had an unprecedented fluid motion detection limit of 2 µm/sec.
Dr. Tsukruk noted that the development of bio-inspired materials requires close multidisciplinary collaboration involving biologists and engineers. There are no comprehensive catalogs of useful mimicable biological materials/structures, and most are identified through word of mouth. Productive collaborations between engineers and biologists are rare and are usually initiated in the United States (sometimes with goal-driven introductions made by, e.g., DARPA). He stated that foreign biologists, who may not have previously thought about potential bioinspired applications, are drawn into collaborations with US engineers. In several cases they have subsequently established similar collaborations closer to home (examples were provided for Europe). A rough estimate is that fewer than 30% of engineers developing bioinspired materials have biomedical end-goals. Most are pursuing consumer product, defense or energy objectives.
While most of the work described by Dr. Tsukruk was carried out in the United States and Europe, he believed China would also make advances in this area. He provided the example of Peking University, which established a materials science program only ~5 years ago, but has managed to recruit a group of top scientists, most of whom previously held academic positions in the United States. Its graduate program was copied from Georgia Tech. In addition, Russia’s SkyTech and Saudi
Arabia’s KAUST (King Abdullah University of Science and Technology) may be supporting work in this area, according to Dr. Tsukruk.
Heinrich Jaeger, William J. Friedman and Alicia Townsend Professor of Physics, University of Chicago.
Dr. Jaeger opened his presentation by remarking that granular materials exhibit far-from-equilibrium behavior. This behavior can be adaptive, and can yield a smart aggregate response from “dumb” ingredients. His work at the University of Chicago deals with granular1 materials that can be used to form adaptive matter. These include pouring granular materials into a form, designing high strength, low-density structures, and making granular matter jam and unjam based on externally applied stress. This latter project was part of the completed JamBots program at DARPA. Research on granular matter can be traced back many years, but only recently has granular matter become of interest as an adaptive material.
Dr. Jaeger discussed both mesoscopic and macroscopic granular matter and then went on to discuss jamming. He mentioned that jamming is the emergent, cooperative process of a collection of objects getting stuck and becoming rigid. Jamming underlies some of the major scientific challenges in far-from-equilibrium physics, including understanding the glass transition. He noted that some researchers are working on optimizing the packing density of particles. Dr. Jaeger described how there has been rapid recent progress in simulating large collections of non-spherical particles, which has happened due to better computing hardware and algorithms.
Dr. Jaeger described soft robotics where jamming is the enabling technology. He showed different robots that can adapt to the surface of their structures, and also other structures that can adapt themselves to different surfaces (such as gripping a glass of water). An example is a robotic arm that can throw a dart onto a dartboard. These are passive, underactuated type universal grippers, being researched here in the United States.
In summary, Dr. Jaeger stated that granular materials are pervasive across all industries and technology with multitudes of applications. Granular matter is an amorphous collection of many (hard) particles, with interactions dominated by interfaces, and can be viewed as prototypical of far from-equilibrium behavior. Jamming is an important mechanism to produce reversible transformation between soft/rigid states in granular systems. He concluded by noting that with the proper design, granular materials can be adaptable, robust, and possibly “self-healing.”
Q – What is around the corner? A – Jaeger responded with three topics: exploit particle shape, make ingredients “smart,” and adaptive granular matter by design.
Q – Can you create materials that operate in a certain frequency range, and respond to sound waves? A – Yes, he said, research is being conducted to create materials that respond to certain frequency ranges. Sound waves are not yet being thoroughly researched (explorations are just preliminary).
Daniel Inman, Department Chair, Aerospace Engineering and Clarence “Kelly” Johnson Professor, University of Michigan.
Dr. Inman presented to the workshop via video conferencing. He opened his presentation by outlining the areas that he would discuss. They included Morphing Aircraft, Solar Powered Aircraft, Structural Monitoring, Energy Harvesting, Origami Structures and Multifunctional Hybrids. Dr.
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1Dr. Jaeger described granular matter as an amorphous collection of many (hard) particles in which emergent behavior arises from interaction at interfaces, and the internal particle properties are less important.
Inman went on to explain their military significance, including quieter airplanes; improved performance; quiet, all-electric, long-range UAVs; self-powered sensor systems; and structural health monitoring systems. He discussed how birds have a unique ability to maneuver. This behavior has inspired developments in shape changing UAVs and MAVs that offer both stealth and agile maneuver advantages. He also described several different types of morphing structures from wings to engines that promise access to highly unusual flight envelopes. Next he explained how solar-powered use of structural health monitoring (SHM) methods to detect damage while structures are in service improves reliability and offers the possibility for other detections. He further explained that remote sensing, and place-and-forget sensing, require self-power, which implies the area of energy harvesting2 of low power from solar, vibration and thermal gradients. This includes self-powered place-and-forget GPS monitoring systems. Dr. Inman also discussed harvesting mechanical vibration using the piezoelectric effect. The goals were to increase battery life, but it was found that only small amounts of useable electrical energy were produced. He then discussed concepts some ten to fifteen years out, including origami structures actuated by smart materials, autonomous behavior in structural and sensor systems, threat reduction systems that react to incoming threats by changing shape, and multifunctional, and functionally graded, hybrid composites. Dr. Inman concluded his presentation by summarizing that the active aero structures can provide stealth, remote sensing and functionality, improved vehicle performance, unique abilities to expand future capabilities and perhaps enable game changing solutions to security problems.
Q – Which other countries are doing similar research? A – He answered: China, Korea, Switzerland, Israel, and the United Kingdom.
35 YEARS OF ADAPTIVE STRUCTURES
Jay Kudva, Chief Executive Officer, NextGen Aeronautics.
Dr. Kudva opened his presentation by discussing flight from birds, to Icaraus, to the Wright brothers and on to jets. He went on to discuss how examples in biology inspire new projects to be developed. Dr. Kudva discussed a cuttlefish example which led to research areas in underwater vehicles and the use of different polymers. Some of the work was produced by looking at research from overseas and then building a very quiet underwater robot. He argued that an electroactive polymer capable of generating significant force is the holy grail for future research. An enabling vision for the future is a tailorable multifunctional material with stiffness on demand. Dr. Kudva showed another example of a morphing wing. One key point, he noted, was that the designers made several real models and tested them in wind tunnels to get key data. The researchers found that this was better than computational modeling. He pointed out that his company does not have large resources, but is focused, and can quickly catch up with competitors.3 He mentioned that this is true for foreign countries as well. Some of the issues with flexible wings, he noted, were that the flexible skins break fairly quickly. Dr. Kudva mentioned that the ability to have adaptive controls as well as the ability to keep flying despite being damaged is very important. He believes that the United States is ahead in system level design and that smart structures are only 5 years out. He finished by discussing how the United States tends to have multiservice multifunction aircraft and how this can hurt the design of projects.
Q – Where do NextGen employees come from? A – With one degree of separation, all come from universities that Jay Kudva is familiar with (like UCLA). Occasionally, applicants are hired from oversees. However, the vast majority of its employees are from American universities (hired as they finish their engineering degrees).
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2Energy Harvesting as used here refers to capturing low levels of ambient waste energy to convert to useable electrical energy.
3For more information see http://www.nextgenaero.com/.
Q – Competition oversees? A – He argued that American programs are certainly ahead. However, Kudva said, the Chinese are trying to break into NextGen’s servers on a frequent basis, and there has been an explosion of patent applications from oversees.
ADAPTIVE STRUCTURES TECHNOLOGY FOR ADVANCED AIRCRAFT
Ed White, Adaptive Structures Technology Focus Team Leader, Associate Technical Fellow, Boeing Research and Technology.
Mr. White’s talk on adaptive structures technology for advanced aircraft began by his discussing what is stopping adaptive airframes. He believes that solutions based on kinematics of rigid structures are usually too heavy and introduce concerns with added complexity, reliability, and maintainability. He then discussed the barriers that adaptive airframes have at the platform performance level. These include limited ability to do multidisciplinary analysis and optimization (MDAO) with adaptive airframe elements. Mr. White noted that researchers can create point designs for adaptive airframe solutions, but not within an MDAO environment, but he felt that this will need to change in the future.
Mr. White presented three key technology developments that are needed. These developments included compact, highly weight efficient variable geometry primary load paths; skins to provide fairings and gap closeouts to support the variable geometry; and highly integrated, multi-degree-of-semi freedom actuators. Next White discussed key implementation indicators. Some of these indicators included large-scale testing of any of the technology needs area previously mentioned, and testing beyond a technology readiness level of 6.
To summarize, White suggested that adaptive structures (applied to large structures) require technology development in the three key technology development areas. He also believes that design of adaptive airframes must be performed as part of a system level trade off and that the development of these tools lag significantly behind the current research on adaptive structural materials.
Manfred Wuttig, Professor, and Director of Graduate Program, Department of Materials Science and Engineering, University of Maryland.
Professor Wuttig noted that shape memory alloys are a general class of materials that remember their original shape. They are potentially useful in actuators that may be designed to change shape, stiffness, position, natural frequency of vibration, and other mechanical characteristics in response to temperature or electromagnetic fields. He stated that magnetic shape memory alloys are ferromagnetic materials that exhibit tensile strain (with a measurable relative physical extension that can be as large as 10%) when a magnetic field is imposed on the material. The physical extension depends on the magnetic anisotropy of the material, where the magnetic domains line up much more readily in one direction than in the other. The main advantage of magnetic shape memory alloys over conventional shape memory alloys, he argued, is that the former can respond faster to changes than the latter. Magnetic field effects operate on a faster time scale than thermal effects.
Dr. Wuttig noted that the leading foreign research work in magnetic shape memory alloys is being carried out primarily in Europe (University of Helsinki, University of Dresden, and University of Barcelona), Russia (Kiev University), and Asia (Tohoku University and Tsing-Hua University). In the United States, he said, this kind of work is being carried out at Caltech, MIT, and the University of Maryland.
Dr. Wuttig stated that most of the work in this area is still in its infancy. The synthesis of ferromagnetic materials for dynamic adaptive applications is still a challenge. The unit volumetric change for applied input energy remains too high. Use of magnetic shape memory alloys in large
adaptive structures in the aerospace industry, for example, is nonexistent. However, he noted, hobby applications in model toy aircraft where the flight surfaces are controlled by magnetic shape memory alloys are probably feasible.
MULTIFUNCTION FOR PERFORMANCE TAILORED STRUCTURES
Leslie Momoda, Director, Sensors and Materials Laboratory, HRL Laboratories.
Dr. Momoda began with a discussion of multifunctional structurally adaptive materials, including those that have controllable electrical, thermal, and acoustic transmission or that exhibit color or surface texture adaption. A potential approach is to embed the responsive materials in a structural matrix (one that supports mechanical stresses well) such as a composite, foam or microlattice material. The system benefits from such an advance, she noted, would be significant, including lighter, longer-range vehicles, more payload capacity, mission adaptive behavior (including camouflage), and self-diagnosis, healing and targeted maintenance. Her presentation reviewed developments in macro-scalable integrated thermal management, self-healing structures based on micro-vascular concepts, meta materials for acoustic and EMI shielding, soft materials including shape memory polymers and variable stiffness composites. Dr. Momoda discussed multifunctional battery work including batteries that can be painted on a surface (Rice University), carbon fiber batteries (KTH, Sweden), fiber reinforced lithium ion batteries (HRL) and a BAE fiber-based Ni-MH chemistry system. She reviewed the many challenges that will be encountered in realizing these concepts and discussed potential applications in small unmanned systems and robotics. She stated that tracking developments in these areas is likely to provide early warning of potentially game-changing advances by adversaries. Her presentation also included a comprehensive bibliography of relevant research papers and reports.
