Center for Engineered Materials and Manufacturing (CEMM)

Ultralight, Ultrastiff Materials

CEMM researchers are printing aerogel-like materials that can withstand up to 160,000 times their own weight while maintaining a nearly constant stiffness per unit mass density. These materials could have a profound impact on the aerospace and automotive industries, as well as other applications where lightweight, high-stiffness and high-strength materials are needed.

Most lightweight cellular materials have mechanical properties that degrade substantially with reduced density because their structural elements are more likely to bend under applied load. The team's metamaterials, however, exhibit ultrastiff properties across more than three orders of magnitude in density.

To fabricate these materials, the team uses projection microsterolithogtaphy, an additive micro-manufacturing process involves using a micro-mirror display chip to create high-fidelity 3D parts one layer at a time from photosensitive feedstock materials. It allows the team to rapidly generate materials with complex 3D micro-scale geometries that are otherwise challenging or in some cases, impossible to fabricate. The team is able to build microlattices out of polymers, metals, and ceramics.

3D-Printed Optical Glass

CEMM can print transparent glass with a controlled composition and tunable optical, thermal, and mechanical properties. The materials, which are printed at room temperature to prevent residual stress and cracking, exhibit structure and compositional gradients that were previously thought impossible and open the door for a new generation of optical materials.

Optical materials are used in everything from telescope lenses to lasers to consumer electronics, but conventional glassmaking can result in structural inconsistencies that can corrupt a material and its properties and make it hard to optimize. CEMM’s innovative direct ink writing (DIW) method is the first extrusion-based glass fabrication technique to offer the precision of 3D printing alongside the freedom of spatially varying optical properties. It allows researchers to fabricate glassy materials with near-unlimited flexibility in form and structure while preserving the fine features of traditional glassmaking methods.

The team has already produced an array of complex objects once unthinkable using traditional approaches. The process’s free-form flexibility enables fine lattices, solid monoliths, liquid-tight containers, and microfluidic channels, all capable of supporting smooth gradient transitions in color, density, and refractive index.

These structures could become lightweight components in airborne laser-mirror systems or refractive index-graded lenses that could reduce finishing costs up to 90 percent. Maximizing operating parameters while minimizing cost is appealing to aerospace manufacturers and collaborators. However, the utility of tailored glass goes beyond defense-related purposes—the technology promises pristine crystal glass with high refractive index and optical dispersion to increase the luster of luxury goods.

Field-Response Mechanical Metamaterials

CEMM researchers have introduced a new class of 3D-printed metamaterials that can nearly instantly respond and stiffen when exposed to a magnetic field.

While researchers can fabricate materials with a wide range of mechanical properties that don’t exist in nature, those properties are static, so the team considers a metamaterial that can quickly adapt its properties in response to an external stimulus as the next evolution.

The materials employ a viscous, magnetically responsive fluid that is manually injected into the hollow struts and beams of 3D-printed lattices, which are built on LLNL’s Large Area Projection Microstereolithography (LAPµSL) platform. Once the fluid is inside the lattice structures, applying an external magnetic field causes the fluid to stiffen and the overall 3D-printed structures subsequently stiffen almost instantaneously.

Unlike other shape morphing materials, the overall structure does not change. The fluid’s ferromagnetic particles located in the core of the beams form chains in response to the magnetic field, which stiffens the fluid and the lattice structure as a result. This response happens in less than a second. The change is also easily reversible and highly tunable by varying the strength of the applied magnetic field.

The technology could be useful for impact absorption — for example, automotive seats could have fluid-responsive metamaterials integrated inside along with sensors to detect a crash, and seats would stiffen on impact, potentially reducing passenger motion that can cause whiplash. It also could be applied to next-generation helmets or neck braces, housing for optical components and soft robotics, among many other applications.

Energy Absorption Materials and 3D-Printed Foams

CEMM’s cushioning materials can absorb energy at a tunable level and overcome the performance limitations of cellular materials, gels, and traditional forms while being more uniform, consistent, and durable. As an improved alternative to traditional foam structures, CEMM’s 3D-printed materials have controlled, uniform structures and have proven to better retain their mechanical and structural characteristics.

Foams, also known as cellular solids, have applications ranging from thermal insulation and shock-absorbing support cushions to lightweight structural and floatation components. Traditionally, foams are created by processes that lead to a highly non-uniform structure with significant dispersion in size, shape, thickness, connectedness and topology of its constituent cells. However, CEMM can print these materials with predictable structure and performance under both compression and shear.

CEMM reseachers constructed cushions using two different configurations using a direct ink writing process: one in an inline stacked configuration and the other in a staggered configuration. The stacked architecture is stiffer in compression and, with increased compression, undergoes a buckling instability. The staggered architecture is softer in compression and undergoes more of a bending type of deformation. The stacked structure has solid columns of material beneath it to offer more support, while the staggered structure has voids under each filament that offer much less resistance to compression.

The researchers envision using their novel energy absorbing materials in many applications, including shoe and helmet inserts, protective materials for sensitive instrumentation and in aerospace applications to combat the effects of temperature fluctuations and vibration.

Catalytic Materials

At high current densities, alkaline water electrolysis generates gas bubbles that can get stuck in the pores of Nickel electrodes and block active sites for catalysts. To solve this problem, CEMM researchers developed and additively manufactured Ni electrodes with periodic structures that facilitates bubble transport and release, which prevents them from getting trapped and makes the process more efficient.

In another project, CEMM researchers printed fluoropolymer gas diffusion layers with tunable micropore characteristics, macroscale structures and surface morphology that make the CO2 reduction reaction much more effective and efficient. The materials could accelerate CO2 capture as an alternative to fossil fuels while providing a platform for designing new catalytic materials for the process.

Cellular Fluidics

Inspired by the way plants absorb and distribute water and nutrients, CEMM researchers developed a groundbreaking method for transporting liquids and gases using 3D-printed lattice design and capillary action phenomena. The method, known as cellular fluidics, involves 3D-printing micro-architected structures capable of containing and flowing fluids to create extensive and controlled contacts between liquids and gases.

The ordered, porous and open-cell structures facilitate surface tension-driven capillary action (the movement of liquid though small pores due to adhesion and cohesion forces) in the unit cells — akin to a tree pulling water from soil or a paper towel soaking up a spill — and enable liquid and gas transport throughout the structures.

Leveraging years of Lab research into 3D-printed, hierarchical lattice design and LLNL-developed Large Area Projection Micro-stereo Lithography (LAPuSL) technology — a light-based printer that can produce extremely tiny features over a large scale — researchers built various fluid-filled structures to study different kinds of multiphase transport and reaction phenomena.

Researchers said the breakthrough technique could have transformative and broad-ranging impacts on numerous fields involving multiphase (gas/liquid/solid) processes, including electrochemical or biological reactors used to convert carbon dioxide or methane to energy, advanced microfluidics, solar desalination, air filtration, heat transfer, transpiration cooling and the delivery of fluids in low- or zero-gravity environments.

In addition, cellular fluidics concepts could improve current microfluidics technology by allowing for controlled fluid transport in complex geometries in 3D. It also showed promise for applications in outer space, where it would allow for fluid transport in the absence of gravity, and in aerosol sample collection and gas filtration, due to the ability to precisely control contact between liquid and gas phases. The concepts could also improve heat transfer by incorporating lattice designs that allow structures to remain cooled over extended periods of time.

Next-Generation Lightweight Supercapacitors

CEMM researchers have successfully 3D-printed supercapacitors using an ultra-lightweight graphene aerogel, opening the door to novel, unconstrained designs of highly efficient energy storage systems for smartphones, wearables, implantable devices, electric cars and wireless sensors.

CEMM’s printed graphene-based aerogels are much smaller, thinner and more lightweight than other supercapacitors and show record performance in terms of storage, capacity and charging speed. The materials could revolutionize energy storage, solve the traditional trade-off of charging speed and storage in supercapacitor design and enable a new generation of energy-efficient storage.

Using a direct-ink writing and a graphene-oxide composite ink designed at the Lab, the team was able to print micro-architected electrodes and build supercapacitors able to retain energy on par with those made with electrodes 10 to 100 times thinner.

The supercapacitors also can charge incredibly fast, in theory requiring just a few minutes or seconds to reach full capacity. They’ve since doubled their performance by sandwiching a lithium ion and a percholorate ion between layers of graphene in aerogel electrodes. The devices’ capacity has substantially improved while maintaining an excellent rate capability.

The next step is creating more complex architectures using aerogels, enabling more powerful supercapacitors that could someday be used in custom-built electronics.The researchers believe the 3D-printed supercapacitors will be used to create unique electronics that are currently difficult or even impossible to make using other synthetic methods, including fully customized smartphones and paper-based or foldable devices, while achieving unprecedented levels of performance.

Biomaterials

For the first time, CEMM was able to print bacterial biofilms using projection microstereolithography. The printed biofilms allow researchers to make complex structured cultures of microbes in a controlled and reproducible way, opening the door to new studies on microbial behavior and environmental response, as well as advances in biotechnology and environmental remediation.

With these sophisticated structures, researchers will be able to conduct controlled experiments to extend basic scientific understanding of microbial interaction from the flat, unnatural setting of Petri dishes to dynamic, 3D environments that reflect natural patterns.

Previous efforts to characterize the biodiversity and community stability of micro-environments have been constrained by lack of control over growth geometry. Microbes systematically grown on the same agar resort to interspecies competition over nutrients rather than expand in new directions to exhibit colony spacing and nuanced biological relationships. Exerting precise control over microbial cultures in increasingly complex settings will reveal insights into behaviors that have so far eluded researchers.

Harnessing microbes’ natural metabolic processes will hasten the development of new technologies across diverse fields. By exploiting special metabolic processes—oftentimes via fermentation—bioplastics and energy-rich biofuels such as ethanol and biodiesel could be more efficiently derived from biomass, relieving dependence on finite petroleum resources.

Encasing cells in printed bioresins could effectively capture and neutralize pollutants as part of bioremediation efforts in contaminated areas. Similarly, printed microbial structures could be used to absorb and purify rare-earth elements during mining operations, a capability that promotes resource independence.

An Electronic Injury Response System

CEMM designed and printed a piece of artificial skin mechanically mimic the body’s injury response and healing process. Using a combination of uses memristors, LEDs and a triboelectric generator, the device can sense pain, show signs of injury and eventually heal the way real skin does.

Deformable Materials

These printed materials deform under external stimuli like temperature, light, magnetism and radiation and return to their original state over time or under different conditions. Researchers can program the original and the deformed shape and use them to induce functional changes in a device. The materials are easy to process, lightweight and inexpensive compared to their metal counterparts and exhibit unique mechanical properties.

The materials were developed by CEMM’s Jennifer Rodriguez, Cheng Zhu, Eric Duoss, Thomas Wilson, Christopher Spadaccini and James Lewicki.

Multiscale Metallic Metamaterials

CEMM engineers have achieved unprecedented scalability in 3D-printed architectures of arbitrary geometry, opening the door to super-strong, ultra-lightweight and flexible metallic materials for the aerospace and automotive industries.

The materials have multiple layers of fractal-like lattices with features ranging from the nanometer to centimeter scale, resulting in a nickel-plated metamaterial with a high elasticity not found in any previously built metal foams or lattices.

The lattices were initially printed out of polymer, using a one-of-a-kind Large Area Projection Micro-Stereolithography (LAPuSL) printer invented at LLNL. The lattice structure was then coated with a nickel-phosphorus alloy and put through post-processing to remove the polymer core, leaving extremely lightweight, hollow tube structures.

Taking inspiration from the hierarchical structure of materials found in nature, such as wood and bone, the finished material incorporates multiple levels of feature size, from nanoscale films to the macroscale unit cells and lattices, within a single piece roughly five centimeters long. While testing the nickel-plated material, researchers found that structures fashioned with walls 700 nanometers thick broke under a 5 percent strain, while those with 60-nanometer walls stretched about 20 percent before failing.

Besides flexibility, researchers said the concept could overcome the current limitations of 3D-printed micro and nano-architectures by addressing the usual tradeoff between high resolution and build size and extend to a variety of large-scale applications, including aircraft parts, and batteries.

Negative Thermal Expansion Materials

CEMM researchers helped develop a new class of metamaterials that are designed to contract instead of expand when exposed to heat. In addition, the materials can be "tuned" to shrink over a large range of temperatures by varying the thermal expansion coefficient difference between constituent beams and geometrical arrangements.

The structure, printed from polymer and a polymer/copper composite material, can flex inward, causing the structure to contract when exposed to heat over a range of tens to hundreds of degrees—possibly the first experimental demonstration showing large tunability of negative thermal expansion (NTE) in three Cartesian directions of microlattice structures. The thermal expansion also can be zero or positive depending on how the geometry and topology of the structure are engineered.

Possible applications could come in securing parts that tend to move out of alignment under varying heat loads, including microchips and high precision optical mounts. The microstructured metamaterial could be used in dental fillings, which tend to move or crack when a person eats something hot, to fill in small gaps in bridges or buildings that are normally left open to account for thermal expansion, or in precision devices such as atomic-force microscopes.

Stiff Isotropic Lattices Beyond the Maxwell Criterion

CEMM researchers have looked beyond the conventional wisdom and designed stable microstructures using anisotropic lattices—mechanical trusses that are not equilibrium. By superimposing complimentary anisotropic lattices, the center can create microstructures that rival their isotropic counterparts and open the door to new designs and materials with unprecedented properties.

Hierarchically Nanoporous Materials

CEMM researchers were the first to additively manufacture nanoporous structures with controllable properties across scales. These materials have shown greatly improved mass transport and reaction rate for liquids and gases, and demonstrated the ability to control the integration of nanopores in the dealloying process.

Spatial Control of Structure and Composition in Printed Multimaterials

CEMM researchers use a two-step curing process to spatially control the structure and composition of composite materials during printing. Multiple materials can be printed within the same ink, but one is cured before the other to maximize stiffening and create new, previously unattainable structures with applications in soft robotics, functional materials and biomedical devices.

The innovative composite printing method combines electrophoretic deposition and light patterning to direct particles toward illuminated parts of the electrode. Since the electrode pattern can change throughout the print, it allows researchers to rapidly and precisely pattern multiple materials over a large area and produce a wide range of composites.