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Updating Turing’s model of pattern formation

In 1952, Alan Turing published a study which described mathematically how systems composed of many living organisms can form rich and diverse arrays of orderly patterns. He proposed that this ‘self-organisation’ arises from instabilities in un-patterned systems, which can form as different species jostle for space and resources. So far, however, researchers have struggled to reproduce Turing patterns in laboratory conditions, raising serious doubts about its applicability. In a new study published in EPJ B, researchers led by Malbor Asllani at the University of Limerick, Ireland, have revisited Turing’s theory to prove mathematically how instabilities can occur through simple reactions, and in widely varied environmental conditions.

The team’s results could help biologists to better understand the origins of many ordered structures in nature, from spots and stripes on animal coats, to clusters of vegetation in arid environments. In Turing’s original model, he introduced two diffusing chemical species to different points on a closed ring of cells. As they diffused across adjacent cells, these species ‘competed’ with each other as they interacted; eventually organising to form patterns. This pattern formation depended on the fact that the symmetry during this process could be broken to different degrees, depending on the ratio between the diffusion speeds of each species; a mechanism now named the ‘Turing instability.’ However, a significant drawback of Turing’s mechanism was that it relied on the unrealistic assumption that many chemicals diffuse at different paces.

Through their calculations, Asllani’s team showed that in sufficiently large rings of cells, where diffusion asymmetry causes both species to travel in the same direction, the instabilities which generate ordered patterns will always arise — even when competing chemicals diffuse at the same rate. Once formed, the patterns will either remain stationary, or propagate steadily around the ring as waves. The team’s result addresses one of Turing’s key concerns about his own theory, and is an important step forward in our understanding of the innate drive for living systems to organise themselves.

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  1. Malbor Asllani, Timoteo Carletti, Duccio Fanelli, Philip K. Maini. A universal route to pattern formation in multicellular systems. The European Physical Journal B, 2020; 93 (7) DOI: 10.1140/epjb/e2020-10206-3

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Physicists find misaligned carbon sheets yield unparalleled properties

A material composed of two one-atom-thick layers of carbon has grabbed the attention of physicists worldwide for its intriguing — and potentially exploitable — conductive properties.

Dr. Fan Zhang, assistant professor of physics in the School of Natural Sciences and Mathematics at The University of Texas at Dallas, and physics doctoral student Qiyue Wang published an article in June with Dr. Fengnian Xia’s group at Yale University in Nature Photonics that describes how the ability of twisted bilayer graphene to conduct electrical current changes in response to mid-infrared light.

From One to Two Layers

Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, where each hexagon is formed by six carbon atoms at its vertices. Since graphene’s first isolation in 2004, its unique properties have been intensely studied by scientists for potential use in advanced computers, materials and devices.

If two sheets of graphene are stacked on top of one another, and one layer is rotated so that the layers are slightly out of alignment, the resulting physical configuration, called twisted bilayer graphene, yields electronic properties that differ significantly from those exhibited by a single layer alone or by two aligned layers.

“Graphene has been of interest for about 15 years,” Zhang said. “A single layer is interesting to study, but if we have two layers, their interaction should render much richer and more interesting physics. This is why we want to study bilayer graphene systems.”

A New Field Emerges

When the graphene layers are misaligned, a new periodic design in the mesh emerges, called a moiré pattern. The moiré pattern is also a hexagon, but it can be made up of more than 10,000 carbon atoms.

“The angle at which the two layers of graphene are misaligned — the twist angle — is critically important to the material’s electronic properties,” Wang said. “The smaller the twist angle, the larger the moiré periodicity.”

The unusual effects of specific twist angles on electron behavior were first proposed in a 2011 article by Dr. Allan MacDonald, professor of physics at UT Austin, and Dr. Rafi Bistritzer. Zhang witnessed the birth of this field as a doctoral student in MacDonald’s group.

“At that time, others really paid no attention to the theory, but now it has become arguably the hottest topic in physics,” Zhang said.

In that 2011 research MacDonald and Bistritzer predicted that electrons’ kinetic energy can vanish in a graphene bilayer misaligned by the so-called “magic angle” of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology proved this theory, finding that offsetting two graphene layers by 1.1 degrees produced a two-dimensional superconductor, a material that conducts electrical current with no resistance and no energy loss.

In a 2019 article in Science Advances, Zhang and Wang, together with Dr. Jeanie Lau’s group at The Ohio State University, showed that when offset by 0.93 degrees, twisted bilayer graphene exhibits both superconducting and insulating states, thereby widening the magic angle significantly.

“In our previous work, we saw superconductivity as well as insulation. That’s what’s making the study of twisted bilayer graphene such a hot field — superconductivity. The fact that you can manipulate pure carbon to superconduct is amazing and unprecedented,” Wang said.

New UT Dallas Findings

In his most recent research in Nature Photonics, Zhang and his collaborators at Yale investigated whether and how twisted bilayer graphene interacts with mid-infrared light, which humans can’t see but can detect as heat. “Interactions between light and matter are useful in many devices — for example, converting sunlight into electrical power,” Wang said. “Almost every object emits infrared light, including people, and this light can be detected with devices.”

Zhang is a theoretical physicist, so he and Wang set out to determine how mid-infrared light might affect the conductance of electrons in twisted bilayer graphene. Their work involved calculating the light absorption based on the moiré pattern’s band structure, a concept that determines how electrons move in a material quantum mechanically.

“There are standard ways to calculate the band structure and light absorption in a regular crystal, but this is an artificial crystal, so we had to come up with a new method,” Wang said. Using resources of the Texas Advanced Computing Center, a supercomputer facility on the UT Austin campus, Wang calculated the band structure and showed how the material absorbs light.

The Yale group fabricated devices and ran experiments showing that the mid-infrared photoresponse — the increase in conductance due to the light shining — was unusually strong and largest at the twist angle of 1.8 degrees. The strong photoresponse vanished for a twist angle less than 0.5 degrees.

“Our theoretical results not only matched well with the experimental findings, but also pointed to a mechanism that is fundamentally connected to the period of moiré pattern, which itself is connected to the twist angle between the two graphene layers,” Zhang said.

Next Step

“The twist angle is clearly very important in determining the properties of twisted bilayer graphene,” Zhang added. “The question arises: Can we apply this to tune other two-dimensional materials to get unprecedented features? Also, can we combine the photoresponse and the superconductivity in twisted bilayer graphene? For example, can shining a light induce or somehow modulate superconductivity? That will be very interesting to study.”

“This new breakthrough will potentially enable a new class of infrared detectors based on graphene with high sensitivity,” said Dr. Joe Qiu, program manager for solid-state electronics and electromagnetics at the U.S. Army Research Office (ARO), an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “These new detectors will potentially impact applications such as night vision, which is of critical importance for the U.S. Army.”

In addition to the Yale researchers, other authors included scientists from the National Institute for Materials Science in Japan. The ARO, the National Science Foundation and the Office of Naval Research supported the study.

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Weighing in on the origin of heavy elements

A long-held mystery in the field of nuclear physics is why the universe is composed of the specific materials we see around us. In other words, why is it made of “this” stuff and not other stuff?

Specifically of interest are the physical processes responsible for producing heavy elements — like gold, platinum and uranium — that are thought to happen during neutron star mergers and explosive stellar events.

Scientists from the U.S. Department of Energy’s (DOE) Argonne National Laboratory led an international nuclear physics experiment conducted at CERN, the European Organization for Nuclear Research, that utilizes novel techniques developed at Argonne to study the nature and origin of heavy elements in the universe. The study may provide critical insights into the processes that work together to create the exotic nuclei, and it will inform models of stellar events and the early universe.

The nuclear physicists in the collaboration are the first to observe the neutron-shell structure of a nucleus with fewer protons than lead and more than 126 neutrons — “magic numbers” in the field of nuclear physics.

At these magic numbers, of which 8, 20, 28, 50 and 126 are canonical values, nuclei have enhanced stability, much as the noble gases do with closed electron shells. Nuclei with neutrons above the magic number of 126 are largely unexplored because they are difficult to produce. Knowledge of their behavior is crucial for understanding the rapid neutron-capture process, or r-process, that produces many of the heavy elements in the universe.

The r-process is thought to occur in extreme stellar conditions such as neutron-star mergers or supernovae. These neutron rich environments are where nuclei can rapidly grow, capturing neutrons to produce new and heavier elements before they have chance to decay.

This experiment focused on the mercury isotope 207Hg. The study of 207Hg could shed light on the properties of its close neighbors, nuclei directly involved in key aspects of the r-process.

“One of the biggest questions of this century has been how the elements formed at the beginning of the universe,” said Argonne physicist Ben Kay, the lead scientist on the study. “It’s difficult to research because we can’t just go dig up a supernova out of the earth, so we have to create these extreme environments and study the reactions that occur in them.”

To study the structure of 207Hg, the researchers first used the HIE-ISOLDE facility at CERN in Geneva, Switzerland. A high-energy beam of protons was fired at a molten lead target, with the resulting collisions producing hundreds of exotic and radioactive isotopes.

They then separated 206Hg nuclei from the other fragments and used CERN’s HIE-ISOLDE accelerator to create a beam of the nuclei with the highest energy ever achieved at that accelerator facility. They then focused the beam at a deuterium target inside the new ISOLDE Solenoidal Spectrometer (ISS).

“No other facility can make mercury beams of this mass and accelerate them to these energies,” said Kay. “This, coupled with the outstanding resolving power of the ISS, allowed us to observe the spectrum of excited states in 207Hg for the first time.”

The ISS is a newly-developed magnetic spectrometer that the nuclear physicists used to detect instances of 206Hg nuclei capturing a neutron and becoming 207Hg. The spectrometer’s solenoidal magnet is a recycled 4-Tesla superconducting MRI magnet from a hospital in Australia. It was moved to CERN and installed at ISOLDE, thanks to a UK-led collaboration between University of Liverpool, University of Manchester, Daresbury Laboratory and collaborators from KU Leuven in Belgium.

Deuterium, a rare heavy isotope of hydrogen, consists of a proton and neutron. When 206Hg captures a neutron from the deuterium target, the proton recoils. The protons emitted during these reactions travel to the detector in the ISS, and their energy and position yield key information on the structure of the nucleus and how it is bound together. These properties have a significant impact on the r-process, and the results can inform important calculations in models of nuclear astrophysics.

The ISS uses a pioneering concept suggested by Argonne distinguished fellow John Schiffer that was built as the lab’s helical orbital spectrometer, HELIOS — the instrument that inspired the development of the ISS spectrometer. HELIOS has allowed exploration of nuclear properties that were once impossible to study, but thanks to HELIOS, have been carried out at Argonne since 2008. CERN’s ISOLDE facility can produce beams of nuclei that complement those that can be made at Argonne.

For the past century, nuclear physicists have been able to gather information about nuclei from the study of collisions where light ion beams hit heavy targets. However, when heavy beams hit light targets, the physics of the collision becomes distorted and more difficult to parse. Argonne’s HELIOS concept was the solution to removing this distortion.

“When you’ve got a cannonball of a beam hitting a fragile target, the kinematics change, and the resulting spectra are compressed,” said Kay. “But John Schiffer realized that when the collision occurs inside a magnet, the emitted protons travel in a spiral pattern towards the detector, and by a mathematical ‘trick’, this unfolds the kinematic compression, resulting in an uncompressed spectrum that reveals the underlying nuclear structure.”

The first analyses of the data from the CERN experiment confirm the theoretical predictions of current nuclear models, and the team plans to study other nuclei in the region of 207Hg using these new capabilities, giving deeper insights into the unknown regions of nuclear physics and the r-process.

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Clusters of gold atoms form peculiar pyramidal shape

Clusters composed of a few atoms tend to be spherical. They are usually organized in shells of atoms around a central atom. This is the case for many elements, but not for gold! Experiments and advanced computations have shown that freestanding clusters of twenty gold atoms take on a pyramidal shape. They have a triangular ground plane made up of ten neatly arranged atoms, with additional triangles of six and three atoms, topped by a single atom.

The remarkable tetrahedral structure has now been imaged for the first time with a scanning tunnelling microscope. This high-tech microscope can visualise single atoms. It operates at extremely low temperatures (269 degrees below zero) and uses quantum tunnelling of an electrical current from a sharp scanning metallic tip through the cluster and into the support. Quantum tunnelling is a process where electrical current flows between two conductors without any physical contact between them.

The researchers used intense plasmas in a complex vacuum chamber setup to sputter gold atoms from a macroscopic piece of gold. “Part of the sputtered atoms grow together to small particles of a few up to a few tens of atoms, due to a process comparable with condensation of water molecules to droplets,” says Zhe Li, the main author of the paper, currently at the Harbin Institute of Technology, Shenzhen. “We selected a beam of clusters consisting of exactly twenty gold atoms. We landed these species with one of the triangular facets onto a substrate covered with a very thin layer of kitchen salt (NaCl), precisely three atom layers thick.”

The study also revealed the peculiar electronic structure of the small gold pyramid. Similar to noble gas atoms or aromatic molecules, the cluster only has completely filled electron orbitals, which makes them much less reactive than clusters with one or a few atoms more or less.

Gold clusters ranging from a few to several dozens of atoms in size are known to possess remarkable properties.

The new discovery helps scientists evaluate the catalytic and optical performances of these clusters, which is relevant for designing cluster-based catalyst and optical devices. Recent applications of clusters include utilisation in fuel cells and carbon capture.

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Brain-like functions emerging in a metallic nanowire network

An international joint research team led by NIMS succeeded in fabricating a neuromorphic network composed of numerous metallic nanowires. Using this network, the team was able to generate electrical characteristics similar to those associated with higher order brain functions unique to humans, such as memorization, learning, forgetting, becoming alert and returning to calm. The team then clarified the mechanisms that induced these electrical characteristics.

The development of artificial intelligence (AI) techniques has been rapidly advancing in recent years and has begun impacting our lives in various ways. Although AI processes information in a manner similar to the human brain, the mechanisms by which human brains operate are still largely unknown. Fundamental brain components, such as neurons and the junctions between them (synapses), have been studied in detail. However, many questions concerning the brain as a collective whole need to be answered. For example, we still do not fully understand how the brain performs such functions as memorization, learning and forgetting, and how the brain becomes alert and returns to calm. In addition, live brains are difficult to manipulate in experimental research. For these reasons, the brain remains a “mysterious organ.” A different approach to brain research?in which materials and systems capable of performing brain-like functions are created and their mechanisms are investigated?may be effective in identifying new applications of brain-like information processing and advancing brain science.

The joint research team recently built a complex brain-like network by integrating numerous silver (Ag) nanowires coated with a polymer (PVP) insulating layer approximately 1 nanometer in thickness. A junction between two nanowires forms a variable resistive element (i.e., a synaptic element) that behaves like a neuronal synapse. This nanowire network, which contains a large number of intricately interacting synaptic elements, forms a “neuromorphic network.” When a voltage was applied to the neuromorphic network, it appeared to “struggle” to find optimal current pathways (i.e., the most electrically efficient pathways). The research team measured the processes of current pathway formation, retention and deactivation while electric current was flowing through the network and found that these processes always fluctuate as they progress, similar to the human brain’s memorization, learning, and forgetting processes. The observed temporal fluctuations also resemble the processes by which the brain becomes alert or returns to calm. Brain-like functions simulated by the neuromorphic network were found to occur as the huge number of synaptic elements in the network collectively work to optimize current transport, in the other words, as a result of self-organized and emerging dynamic processes..

The research team is currently developing a brain-like memory device using the neuromorphic network material. The team intends to design the memory device to operate using fundamentally different principles than those used in current computers. For example, while computers are currently designed to spend as much time and electricity as necessary in pursuit of absolutely optimum solutions, the new memory device is intended to make a quick decision within particular limits even though the solution generated may not be absolutely optimum. The team also hopes that this research will facilitate understanding of the brain’s information processing mechanisms.

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High-performance anode for all-solid-state Li batteries is made of Si nanoparticles

A new study led by NIMS researchers reveals that, in solid electrolytes, a Si anode composed only of commercial Si nanoparticles prepared by spray deposition — the method is a cost-effective, atmospheric technique — exhibits excellent electrode performance, which has previously been observed only for film electrodes prepared by evaporation processes. This new result therefore suggests that a low-cost and large-scale production of high-capacity anodes for use in all-solid-state Li batteries is possible.

Si has a theoretical capacity of ~4,200 mAh/g, which is approximately 11 times higher than that of the graphite commonly used as the anode-active material in commercial Li-ion batteries. Replacing the traditional graphite by Si can extend significantly the driving range per charge of electric vehicles. However, its huge volume change (~300%) during lithiation and delithiation — charge and discharge — hinders its practical application in the batteries. In conventional liquid electrolytes, the use of polymeric binders is necessary to hold the active material particles in the electrode together and maintain their adhesion to the surface of metal current collectors. The repeated huge volume change of Si causes the particle isolation and thus leads to losing the active material, which results in a continuous capacity loss. In solid-state cells, the active material is placed between two solid components — solid electrolyte separator layer and metal current collector — , which enables avoidance of tackling the problem — electrical isolation of the active material — . In fact, as reported previously by the team of NIMS researchers, the sputter-deposited pure Si films delivering practical areal capacities exceeding 2.2 mAh/cm2 exhibit excellent cycling stability and high-rate discharge capabilities in solid electrolytes. Nonetheless, cost-effective and industrially scalable synthesis of the anode for all-solid-state Li batteries remains a great challenge.

The team of NIMS researchers has taken another synthesis approach toward develop the high-performance anode for all-solid-state Li batteries with commercial Si nanoparticles, and found a unique phenomenon to the nanoparticles in the solid-state cell: upon lithiation, they undergo volume expansion, structural compaction, and appreciable coalescence in the confined space between the solid electrolyte separator layer and metal current collector to form a continuous film similar to that prepared by the evaporation process. The anode composed of nanoparticles prepared by spray deposition therefore exhibits excellent electrode performance, which has previously been observed only for sputter-deposited film electrodes. The spray deposition method is a cost-effective, atmospheric technique that can be used for large-scale production. Hence, the findings will pave the way for low-cost and large-scale production of high-capacity anodes for use in all-solid-state Li batteries.

Continuing efforts by the team of NIMS researchers to improve the cyclability in the anode having the increased areal mass loading of nanoparticles are in progress to meet the requirements of electric vehicles.

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New biofabrication method creates one scaffold to guide regeneration of multiple tissues

Organs, muscles and bones are composed of multiple types of cells and tissues that are carefully organized to carry out a specific function. For example, kidneys are able to filter waste from the blood because of how their specialized cells and tissues are arranged. Disrupting this organization dramatically affects how cells and tissues do their job effectively.

Another example is articular cartilage, which exists where bones meet at the joints. This type of cartilage provides a cushioning material to protect the ends of bones and is tightly integrated with bone through a gradient region known as the osteochondral interface?osteo means related to bone, chondral related to cartilage. When articular cartilage is absent or damaged, debilitating pain results.

Unlike some tissues, cartilage cannot regenerate. It lacks blood vessels to support such repair. After injury or damage, cartilage degeneration progresses, leading to osteoarthritis, which affects approximately 27 million Americans.

“Medical intervention is the only way to regenerate osteochondral tissue,” says Lesley Chow, assistant professor of Materials Science & Engineering and Bioengineering at Lehigh University. “To successfully regenerate this cartilage and make it functional, we must consider the fact that function is related to both the cartilage and the bone. If the cartilage doesn’t have a good anchor, it’s pointless. You could regenerate beautiful cartilage, but it won’t last if it isn’t anchored to that bone immediately beneath it.”

This presents a huge engineering challenge, says Chow, as it’s difficult to create one organ made up of two very different tissues. What is needed is a tissue engineering method that respects the multi-component and organizational nature of how tissues form in nature, she says, adding: “Then we’d have the ability to create something that’s durable.”

Chow has taken a major step in the field’s efforts to address such a challenge. She and her team at The Chow Lab at Lehigh have demonstrated a new method to fabricate scaffolds presenting spatially organized cues to control cell behavior locally within one material. Their proof-of-concept paper, published in Biomaterials Science, is called: “3D printing with peptide-polymer conjugates for single-step fabrication of spatially functionalized scaffolds.” This work was led by Lehigh graduate students Paula Camacho (Bioengineering) and Hafiz Busari (Materials Science & Engineering) with co-authors Kelly Seims (Materials Science & Engineering), Peter Schwarzenberg (Mechanical Engineering & Mechanics), and Hannah L. Dailey, assistant professor of Mechanical Engineering and Mechanics at Lehigh. Their publication shows how their platform can be used to create continuous, highly organized scaffolds to regenerate two different tissues, such as those found in the osteochondral interface.

Chow’s lab creates biomaterial scaffolds made of biodegradable polymers, which are long chains of molecules that can degrade over time in the body. Scaffolds are widely-used in tissue engineering to provide cells with structural support, as well chemical cues that “tell” the cells what type of cell to become or tissue to form. Used in the early stages of tissue regeneration, scaffolds are designed to be implanted in the body and then degrade as new tissue forms.

Chow’s team uses 3D printing technology to control the deposition of “inks” with different material compositions. These inks are prepared by mixing a biodegradable polymer with peptide-modified polymers. The peptides, composed of amino acids, provide the bioactive cues to the cells.

“We know from literature and nature what amino acid sequences we want,” says Chow. “We can take a segment that we know plays a specific and important role in telling cells to grow new tissue and, in a sense, steal from Nature. We take a peptide and attach it onto a polymer and add that in while we are constructing our scaffolds. We use 3D printing as a way to control the organization of these peptide-functionalized polymers as well as the scaffold’s architecture.”

Once the team fabricates the scaffold, they “seed” them with cells, such as human mesenchymal stem cells that can be “coaxed” in response to the peptides into becoming different cell types.

As Chow explains, changing the scaffold’s properties is simply a matter of changing the inks loaded in the printer. The team can modify peptide concentration as well as location, and they can do this with more than one ink composition.

“What we are doing is creating an environment that fosters the regeneration of two different tissues simultaneously in one scaffold,” says Chow. “We make a scaffold that has the correct cues?one that promotes cartilage, one that promotes bone?all in one material. You then have a single scaffold where you don’t have to worry about mechanical failure at the interface because you have a single material rather than “gluing” two separate scaffolds together and just hoping for the best.”

In the paper, the authors demonstrate the effectiveness of their method using two very familiar peptides. They describe how peptide-modified polymer conjugates were synthesized with the cell adhesion motif RGDS or its negative control RGES. To demonstrate spatial control of peptide functionalization, multiple printer heads were used to print both conjugates into the same construct in alternating patterns. As designed, cells preferentially attached and spread on RGDS(biotin)-polymer conjugate fibers compared to RGES(azide)-polymer conjugate fibers. This illustrated how spatial peptide functionalization influenced local cell behavior within a single biomaterial. This preferential attachment demonstrates that the technique has real potential for creating scaffolds that enable scientists to direct “where cells are going to stick.”

According to Chow, most scaffold fabrication techniques involve modification after it is created, which can lead to unwanted outcomes, such as the distribution of chemistries in a uniform concentration. Yet, native tissues are not organized this way.

“Our platform is designed to really control how cells arrange themselves,” says Chow. “It’s like building a house and then seeing which house the cells like best. And, we found that the cells really notice. They notice the two different cues. They notice whether the cues are organized or not organized.”

“It is so important for us to have fine-tuned control to make the cells do what we want them to do,” adds Camacho.

One of Camacho’s current projects is applying the team’s scaffold biofabrication platform to engineer osteochondral tissue formation. Camacho and her colleagues culture the cell-seeded scaffolds in an incubator held at body temperature (37?C?or 98.6?F) with 5% carbon dioxide in order to mimic the conditions inside the human body. They evaluate what type of tissue forms and how the cells behave at different points in time. This offers them a glimpse into which scaffolds are most likely to be successful.

“Right now I’m testing two different peptides,” says Camacho. “One is to coax the human mesenchymal stem cells to differentiate into chondrocytes, or cartilage cells. And the other peptide is trying to get them to differentiate into bone. I build these scaffolds with one peptide or both peptides that are organized in different ways. And I want to see how the cells react to it?if they like one more than the other. I characterize what they are doing up to 42 days in culture.”

While the team is working on a few specific projects, including the osteochondral work, their goal is for other researchers to be able to use the platform and, ultimately, to help move the field forward.

“We believe this presents a versatile platform to generate multifunctional biomaterials that can mimic the biochemical organization found in native tissues to support functional regeneration,” says Chow.

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