Smart materials or Active materials or Functional materials are designed materials that have diverse, dynamic features that enable them to adapt to the environment. They have one or more properties that can be significantly changed in a controlled fashion by external stimuli, the stimulus and response may be mechanical, electrical, magnetic, optical, thermal, or chemical.
Smart materials are used to construct smart structures. A smart structure (a.k.a. intelligent structure, adaptive structure, and functional structure) is defined as a structure that is able to sense external stimuli such as pressure, velocity, density, or temperature change. It can process the information and respond in a controlled manner in real time. A smart structure is a system containing multifunctional parts that can perform sensing, control, and actuation; it is a primitive analogue of a biological body. Apart from the use of better functional materials as sensors and actuators, an important part of a “smarter” structure is to develop an optimized control algorithm that could guide the actuators to perform required functions after sensing changes. Many types of actuators and sensors such as piezoelectric materials, shape memory alloys (SMA) (alloys that can remember their original shapes), electrostrictive and magnetostrictive materials, and fiber optics are being considered for various applications.
Numerous examples already exist. In recent MADMEC competition, a team of PhD students, developed a hydrogel that can be added to the surface of windows, making them switch from transparent to opaque in response to temperature. The hydrogel relies on a custom mixture of polymers that turns opaque as it absorbs heat — up until about 34 degrees Celsius — and then turns transparent and releases heat in response to cooling temperatures. “On a cold day, it’s going to be clear, and on warmer day, if it gets really hot out, it’s going to become opaque,” said team member Seth Cazzell, a PhD student in DMSE. “What we have is this passive, self-shading device that responds to ambient temperature.” Another US university project developed liquid crystal technology where liquid crystal display intensity. changes instantly according to the external light intensity.
In the world of structural design there are three primary ingredients to develop a structure: constitute materials the structure will be comprised of, manufacturing and assembly of the constitute materials to produce parts and connections, and design of the structure to meet functional requirements. For smart structure and system, the material should be able to withstand higher load and the weight of the materials should be minimum. In the regard of less weight requirement aluminum and magnesium alloy and composites plays major role.
However, developing Smart materials and products have many challenges like novel fabrication techniques (e.g. dispersion, alignment); in-process metrology; material characterization and in-situ monitoring; yield optimization; accelerated life tests to determine material durability and techniques to advance disassembly and recovery at end of life.
Development of smart or multifunctional materials not only require determining materials behavior, but also manufacturing strategies to design and use the materials to produce multifunctional structures. From understanding material behavior and development of predictive models, analysis tools/instruments are required to establish process parameters for manufacturing of multifunctional materials and structures.
Some of the technologies are composite manufacturing technologies, specifically within the fields of additive manufacturing, fusion bonding of thermoplastic composites, automated fiber placement of thermoplastic composites, and vacuum assisted resin transfer molding (VARTM). Therefore researchers are applying new technologies such as 3D printing, self-assembly and CRISPER for manufacturing these smart materials.
Thus, printed smart materials is an area of increasing interest due to low-cost fabrication, simple integration into devices and possibility of obtaining multifunctional materials over large and flexible areas. The impact of printable smart and multifunctional materials span from the areas of sensors and actuators, to energy generation and storage and tissue engineering applications, among others.
In 2020, Researchers from Texas A&M and the U.S. Army created synthetic materials with morphing abilities that can be 3D printed and self-heal within seconds.
By tweaking the chemistry of a single polymer, researchers at Texas A&M University and the U.S. Army Combat Capabilities Development Command Army Research Laboratory have created a family of synthetic materials that range in texture from ultra-soft to extremely rigid. The researchers said their materials are 3D printable, self-healing, recyclable and naturally adhere to each other in air or underwater. Their findings are detailed in the May 2020 issue of the journal Advanced Functional Materials.
“We have made an exciting group of materials whose properties can be fine-tuned to get either the softness of rubber or the strength of load-bearing plastics,” said Svetlana Sukhishvili, professor in the Department of Materials Science and Engineering and a corresponding author on the study. “Their other desirable characteristics, like 3D printability and the ability to self-heal within seconds, make them suited for not just more realistic prosthetics and soft robotics, but also ideal for broad military applications such as agile platforms for air vehicles and futuristic self-healing aircraft wings.”
Synthetic polymers are made up of long strings of repeating molecular motifs, like beads on a chain. In elastomeric polymers, or elastomers, these long chains are lightly crosslinked, giving the materials a rubbery quality. However, these crosslinks can also be used to make the elastomers more rigid by increasing the number of crosslinks. Although previous studies have manipulated the density of crosslinks to make elastomers stiffer, the resulting change in mechanical strength was generally permanent.
“Crosslinks are like stitches in a piece of cloth, the more stitches you have, the stiffer the material gets and vice versa,” Sukhishvili said. “But instead of having these ‘stitches’ be permanent, we wanted to achieve dynamic and reversible crosslinking so that we can create materials that are recyclable.”
The researchers focused their attention on the molecules involved in the crosslinking. First, they chose a parent polymer, called prepolymer, and then chemically studded these prepolymer chains with two types of small crosslinking molecules — furan and maleimide. By increasing the number of these molecules in the prepolymer, they found that they could create materials stiffer. In this way, the hardest material they created was 1000 times stronger than the softest.
However, these crosslinks are also reversible. Furan and maleimide participate in a type of reversible chemical bonding. Put simply, in this reaction, furan and maleimide pairs can “click” and “unclick” depending on temperature. When the temperature is high enough, these molecules come apart from the polymer chains and the materials soften. At room temperature, the materials harden since the molecules quickly click back together, once again forming crosslinks. Thus, if there is any tear in these materials at ambient temperatures, the researchers showed that furan and maleimide automatically re-click, healing the gap within a few seconds.
The researchers noted that the temperatures at which the crosslinkers dissociate or unclick from the prepolymer chains are relatively the same for different stiffness levels. This property is useful for 3D printing with these materials. Regardless of whether they are soft or hard, the materials can be melted at the same temperature and then used as printing ink. “By modifying the hardware and processing parameters in a standard 3D printer, we were able to use our materials to print complex 3D objects layer by layer,” said Frank Gardea, research engineer in the United States Army Research Laboratory and a corresponding author on the study. “The unique advantage of our materials is that the layers that make up the 3D part can be of vastly different stiffness.”
As the 3D part cools to room temperature, he said, the different layers join seamlessly, precluding the need for curing or any other chemical processing. Consequently, the 3D-printed parts can easily be melted using high heat and then recycled as printing ink. The researchers also noted that their materials are reprogrammable. In other words, after being set into one shape, they can be made to change into a different shape using just heat. In the future, the researchers plan to increase the functionality of their new materials by amplifying its multifaceted properties outlined in the current study.
Optomec Breakthrough in 3D Printing Enables Micron-scale Smart Structures
Optomec, a leading global supplier of production grade additive manufacturing systems for 3D printed electronics and 3D printed metals, today announced its Aerosol Jet Technology can enable 3D polymer and composite structures to be printed at the micron scale with embedded electronics. This breakthrough has significant potential to reduce the cost and size of next-generation products used in the electronics and bio-medical industries.
This new capability is enabled by combining Optomec’s proven Aerosol Jet solution for fine feature printing with a proprietary in-situ curing capability for rapid on-the-fly solidification. Unlike other high resolution 3D printing approaches that deposit material globally, ie: in a powder bed, and then cure locally to define a pattern, the Optomec method relies on both local deposition and local curing. This makes the process more economical, in terms of material consumption, but is also key to enabling the highest resolution features available.
“This breakthrough in 3D printing technology extends additive manufacturing to the creation of micron scale, free-form polymer structures and smart devices,” said Mike O’Reilly, Optomec Director Aerosol Jet Product Management. Early adopter customers have already developed innovative applications for smart devices and micro-fluidic applications. “We continue to place emphasis on innovation such as Aerosol Jet 3D micro-structure printing to address our customer’s next generation product development challenges.”
Using this process high aspect ratio, free-form 3D structures can be printed at the micron-scale, without the need for support structures, from materials including photopolymers and certain composites.
Additionally, the resulting structures can be metallized with conformal 3D conductive traces and printed functional components, such as antennas and sensors, to create fully functional 3D components all in one manufacturing machine. This direct digital approach optimizes the fabrication process, reducing manufacturing steps and material usage making Aerosol Jet 3D micro-structure printing a cost-effective, green technology.
Aerosol Jet 3D micro-structure printing is capable of ultra-high resolutions with lateral features sizes down to 10 microns, and lateral and vertical build resolutions from 1 micron to 100 nanometers respectively. Aspect ratios of more than 100:1 have been achieved. Additionally, such 3D micro-structures can be printed onto existing components and products, such as semiconductor chips, medical devices or industrial parts.
Making 3-D nanosuperconductors with DNA
Three-dimensional (3-D) nanostructured materials—those with complex shapes at a size scale of billionths of a meter—that can conduct electricity without resistance could be used in a range of quantum devices. For example, such 3-D superconducting nanostructures could find application in signal amplifiers to enhance the speed and accuracy of quantum computers and ultrasensitive magnetic field sensors for medical imaging and subsurface geology mapping. However, traditional fabrication tools such as lithography have been limited to 1-D and 2-D nanostructures like superconducting wires and thin films.
In Nov 2020, S cientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Columbia University, and Bar-Ilan University in Israel reported have developed a platform for making 3-D superconducting nano-architectures with a prescribed organization. As reported in the Nov. 10 issue of Nature Communications, this platform is based on the self-assembly of DNA into desired 3-D shapes at the nanoscale. In DNA self-assembly, a single long strand of DNA is folded by shorter complementary “staple” strands at specific locations—similar to origami, the Japanese art of paper folding.
“Because of its structural programmability, DNA can provide an assembly platform for building designed nanostructures,” said co-corresponding author Oleg Gang, leader of the Soft and Bio Nanomaterials Group at Brookhaven Lab’s Center for Functional Nanomaterials (CFN) and a professor of chemical engineering and of applied physics and materials science at Columbia Engineering. “However, the fragility of DNA makes it seem unsuitable for functional device fabrication and nanomanufacturing that requires inorganic materials. In this study, we showed how DNA can serve as a scaffold for building 3-D nanoscale architectures that can be fully “converted” into inorganic materials like superconductors.”
To make the scaffold, the Brookhaven and Columbia Engineering scientists first designed octahedral-shaped DNA origami “frames.” Aaron Michelson, Gang’s graduate student, applied a DNA-programmable strategy so that these frames would assemble into desired lattices. Then, he used a chemistry technique to coat the DNA lattices with silicon dioxide (silica), solidifying the originally soft constructions, which required a liquid environment to preserve their structure. The team tailored the fabrication process so the structures were true to their design, as confirmed by imaging at the CFN Electron Microscopy Facility and small-angle X-ray scattering at the Complex Materials Scattering beamline of Brookhaven’s National Synchrotron Light Source II (NSLS-II). These experiments demonstrated that the structural integrity was preserved after they coated the DNA lattices.
“In its original form, DNA is completely unusable for processing with conventional nanotechnology methods,” said Gang. “But once we coat the DNA with silica, we have a mechanically robust 3-D architecture that we can deposit inorganic materials on using these methods. This is analogous to traditional nanomanufacturing, in which valuable materials are deposited onto flat substrates, typically silicon, to add functionality.” The team shipped the silica-coated DNA lattices from the CFN to Bar-Ilan’s Institute of Superconductivity, which is headed by Yosi Yeshurun. Gang and Yeshurun became acquainted a couple years ago, when Gang delivered a seminar on his DNA assembly research. Yeshurun—who over the past decade has been studying the properties of superconductivity at the nanoscale—thought that Gang’s DNA-based approach could provide a solution to a problem he was trying to solve: How can we fabricate superconducting nanoscale structures in three dimensions?
“Previously, making 3-D nanosuperconductors involved a very elaborate and difficult process using conventional fabrication techniques,” said Yeshurun, co-corresponding author. “Here, we found a relatively simple way using Oleg’s DNA structures.” At the Institute of Superconductivity, Yeshurun’s graduate student Lior Shani evaporated a low-temperature superconductor (niobium) onto a silicon chip containing a small sample of the lattices. The evaporation rate and silicon substrate temperature had to be carefully controlled so that niobium coated the sample but did not penetrate all the way through. If that happened, a short could occur between the electrodes used for the electronic transport measurements. “We cut a special channel in the substrate to ensure that the current would only go through the sample itself,” explained Yeshurun.
The measurements revealed a 3-D array of Josephson junctions, or thin nonsuperconducting barriers through which superconducting current tunnels. Arrays of Josephson junctions are key to leveraging quantum phenomena in practical technologies, such as superconducting quantum interference devices for magnetic field sensing. In 3-D, more junctions can be packed into a small volume, increasing device power.
“DNA origami has been producing beautiful and ornate 3-D nanoscale structures for almost 15 years, but DNA itself is not necessarily a useful functional material,” said Evan Runnerstrom, program manager for materials design at the U.S. Army Combat Capabilities Development Command Army Research Laboratory of the U.S. Army Research Office, which funded the work in part. “What Prof. Gang has shown here is that you can leverage DNA origami as a template to create useful 3-D nanostructures of functional materials, like superconducting niobium. This ability to arbitrarily design and fabricate complex 3-D-structured functional materials from the bottom-up will accelerate the Army’s modernization efforts in areas like sensing, optics, and quantum computing.”
“We demonstrated a pathway for how complex DNA organizations can be used to create highly nanostructured 3-D superconducting materials,” said Gang. “This material conversion pathway gives us an ability to make a variety of systems with interesting properties—not only superconductivity but also other electronic, mechanical, optical, and catalytic properties. We can envision it as a “molecular lithography,” where the power of DNA programmability is transferred to 3-D inorganic nanofabrication.”
CRISPR gene-editing tool used to trigger smart materials that can deliver drugs and sense biological signals.
Scientists have wielded the gene-editing tool CRISPER to make scores of genetically modified organisms, as well as to track animal development, detect diseases and control pests. Now, they have found yet another application for it: using CRISPR to create smart materials that change their form on command.
The shape-shifting materials could be used to deliver drugs, and to create sentinels for almost any biological signal, researchers report in Science in August 2019. The study was led by James Collins, a bioengineer at the Massachusetts Institute of Technology in Cambridge.
Collins’ team worked with water-filled polymers that are held together by strands of DNA, known as DNA hydrogels. Materials called hydrogels are made of water-filled polymers. Collins and his team turned to a form of CRISPR that uses a DNA-snipping enzyme called Cas12a. (The gene-editor CRISPR–Cas9 uses the Cas9 enzyme to snip a DNA sequence at the desired point.) The Cas12a enzyme can be programmed to recognize a specific DNA sequence. The enzyme cuts its target DNA strand, then severs single strands of DNA nearby.
This property allowed the researchers to build a series of CRISPR-controlled hydrogels containing a target DNA sequence and single strands of DNA, which break up after Cas12a recognizes the target sequence in a stimulus. The break-up of the single DNA strands triggers the hydrogels to change shape or, in some cases, completely dissolve, releasing a payload (see ‘CRISPR-controlled gels’).
The team created hydrogels programmed to release enzymes, small molecules and even human cells — which could be part of a therapy — in response to stimuli. Collins hopes that the gels could be used to make smart therapeutics that release, for example, cancer drugs in the presence of a tumour, or antibiotics around an infection
Furst, professor of chemical and biomolecular engineering and principal investigator of NASA’s InSPACE (Investigating the Structures of Paramagnetic Aggregates from Colloidal Emulsions) project are creating smart materials based on self-assembly, having small building blocks that come together and arrange on their own to rapidly manufacture themselves.
The InSPACE project is using magnetorheological (MR) fluids in microgravity environment of space to study self-assembly. Under Earth’s gravity, the magnetic particles are usually sitting against the bottom of their container, and the friction may prevent the chains from warping the way they do in space.
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