New analysis finds way to safely conduct heat from graphene to biological tissues
In the future, our health may be monitored and maintained by tiny sensors and drug dispensers, deployed within the body and made from graphene — one of the strongest, lightest materials in the world. Graphene is composed of a single sheet of carbon atoms, linked together like razor-thin chicken wire, and its properties may be tuned in countless ways, making it a versatile material for tiny, next-generation implants.
But graphene is incredibly stiff, whereas biological tissue is soft. Because of this, any power applied to operate a graphene implant could precipitously heat up and fry surrounding cells.
Now, engineers from MIT and Tsinghua University in Beijing have precisely simulated how electrical power may generate heat between a single layer of graphene and a simple cell membrane. While direct contact between the two layers inevitably overheats and kills the cell, the researchers found they could prevent this effect with a very thin, in-between layer of water.
By tuning the thickness of this intermediate water layer, the researchers could carefully control the amount of heat transferred between graphene and biological tissue. They also identified the critical power to apply to the graphene layer, without frying the cell membrane. The results are published today in the journal Nature Communications.
Co-author Zhao Qin, a research scientist in MIT’s Department of Civil and Environmental Engineering (CEE), says the team’s simulations may help guide the development of graphene implants and their optimal power requirements.
“We’ve provided a lot of insight, like what’s the critical power we can accept that will not fry the cell,” Qin says. “But sometimes we might want to intentionally increase the temperature, because for some biomedical applications, we want to kill cells like cancer cells. This work can also be used as guidance [for those efforts.]”
Qin’s co-authors include Markus Buehler, head of CEE and the McAfee Professor of Engineering, along with Yanlei Wang and Zhiping Xu of Tsinghua University.
Typically, heat travels between two materials via vibrations in each material’s atoms. These atoms are always vibrating, at frequencies that depend on the properties of their materials. As a surface heats up, its atoms vibrate even more, causing collisions with other atoms and transferring heat in the process.
The researchers sought to accurately characterize the way heat travels, at the level of individual atoms, between graphene and biological tissue. To do this, they considered the simplest interface, comprising a small, 500-nanometer-square sheet of graphene and a simple cell membrane, separated by a thin layer of water.
“In the body, water is everywhere, and the outer surface of membranes will always like to interact with water, so you cannot totally remove it,” Qin says. “So we came up with a sandwich model for graphene, water, and membrane, that is a crystal clear system for seeing the thermal conductance between these two materials.”
Qin’s colleagues at Tsinghua University had previously developed a model to precisely simulate the interactions between atoms in graphene and water, using density functional theory — a computational modeling technique that considers the structure of an atom’s electrons in determining how that atom will interact with other atoms.
However, to apply this modeling technique to the group’s sandwich model, which comprised about half a million atoms, would have required an incredible amount of computational power. Instead, Qin and his colleagues used classical molecular dynamics — a mathematical technique based on a “force field” potential function, or a simplified version of the interactions between atoms — that enabled them to efficiently calculate interactions within larger atomic systems.
The researchers then built an atom-level sandwich model of graphene, water, and a cell membrane, based on the group’s simplified force field. They carried out molecular dynamics simulations in which they changed the amount of power applied to the graphene, as well as the thickness of the intermediate water layer, and observed the amount of heat that carried over from the graphene to the cell membrane.
Because the stiffness of graphene and biological tissue is so different, Qin and his colleagues expected that heat would conduct rather poorly between the two materials, building up steeply in the graphene before flooding and overheating the cell membrane. However, the intermediate water layer helped dissipate this heat, easing its conduction and preventing a temperature spike in the cell membrane.
Looking more closely at the interactions within this interface, the researchers made a surprising discovery: Within the sandwich model, the water, pressed against graphene’s chicken-wire pattern, morphed into a similar crystal-like structure.
“Graphene’s lattice acts like a template to guide the water to form network structures,” Qin explains. “The water acts more like a solid material and makes the stiffness transition from graphene and membrane less abrupt. We think this helps heat to conduct from graphene to the membrane side.”
The group varied the thickness of the intermediate water layer in simulations, and found that a 1-nanometer-wide layer of water helped to dissipate heat very effectively. In terms of the power applied to the system, they calculated that about a megawatt of power per meter squared, applied in tiny, microsecond bursts, was the most power that could be applied to the interface without overheating the cell membrane.
Qin says going forward, implant designers can use the group’s model and simulations to determine the critical power requirements for graphene devices of different dimensions. As for how they might practically control the thickness of the intermediate water layer, he says graphene’s surface may be modified to attract a particular number of water molecules.
“I think graphene provides a very promising candidate for implantable devices,” Qin says. “Our calculations can provide knowledge for designing these devices in the future, for specific applications, like sensors, monitors, and other biomedical applications.”
Researchers have levitated a tiny nanodiamond particle with a laser in a vacuum chamber, using the technique for the first time to detect and measure its “torsional vibration,” an advance that could bring new types of sensors and studies in quantum mechanics.
The experiment represents a nanoscale version of the torsion balance used in the classic Cavendish experiment, performed in 1798 by British scientist Henry Cavendish, which determined Newton’s gravitational constant. A bar balancing two lead spheres at either end was suspended on a thin metal wire. Gravity acting on the two weights caused the wire and bar to twist, and this twisting – or torsion – was measured to calculate the gravitational force.
In the new experiment, an oblong-shaped nanodiamond levitated by a laser beam in a vacuum chamber served the same role as the bar, and the laser beam served the same role as the wire in Cavendish’s experiment.
“A change of the orientation of the nanodiamond caused the polarization of the laser beam to twist,” said Tongcang Li, an assistant professor of physics and astronomy and electrical and computer engineering at Purdue University. “Torsion balances have played historic roles in the development of modern physics. Now, an optically levitated ellipsoidal nanodiamond in a vacuum provides a new nanoscale torsion balance that will be many times more sensitive.”
Findings are detailed in a paper that appeared on Thursday (Sept. 15) in the journal Physical Review Letters.
“This is the first experimental observation of torsional motion of a nanoparticle levitated in a vacuum and represents a very sensitive torque detector,” Li said. “In principle, we could detect the torque on a single electron or a single proton.”
The paper was authored by Purdue postdoctoral research associate Thai M. Hoang; student Yue Ma from Tsinghua University in China; Purdue graduate students Jonghoon Ahn and Jaehoon Bang;Francis Robicheaux, a Purdue professor of physics and astronomy; Zhang-Qi Yin, an assistant research fellow at Tsinghua University; and Li.
The paper details the detection of torsional vibration, a proposal to use the technique for torque sensing and also to achieve torsional “ground state cooling,” which could aid efforts to study quantum theory and realize potential applications in quantum information processing and high-precision measurement for sensors.
This cooling reduces “noise” caused by vibrating molecules and atoms, making it possible to precisely measure torque and probe the relationships between motion and electron “spin.” Electrons can be thought of as having two distinct spin states, “up” or “down,” and this phenomenon might be used in future quantum simulations.
The paper includes experimental and theoretical portions.
“Experimentally, we observed torsional motion, and the theoretical part is a proposal of how to cool down the motion to achieve quantum ground state,” Li said.
The nanodiamonds are about 100 nanometers in diameter, or roughly the size of a virus. Future research will include efforts to achieve ground state cooling.
When one atom first meets another, the precise nature of that interaction can determine much about what kinds of physical properties and behaviours will emerge.
In a paper published today in Nature Physics, a team led by U of T physicist Joseph Thywissen reported their discovery of a new set of rules related to one particular type of atomic-pair interaction. The researchers study interactions between atoms that have been cooled close to absolute zero.
“Ultracold atoms are the stem cells of materials science,” says Thywissen, a Professor of Physics at the University of Toronto and also a Fellow of the Quantum Materials program at the Canadian Institute for Advanced Research. “Just as a stem cell can become a fingernail or a heart cell depending on its context, ultracold atoms can become metals, insulators, superfluids or other types of materials.”
In collaboration with theorists Shizhong Zhang of Hong Kong University and Zhenhua Yu of Tsinghua University, the Toronto experimentalists have been studying “p-wave interactions.” The term “p-wave” refers to the degree to which two atoms twirl around one another – a phenomenon physicists refer to as “angular momentum.”
Researchers study these interactions in a highly controlled environment, coaxing a few hundred thousand gas atoms into a “trap,” and cooling them to about -273 Celsius.
Tsinghua University (sometimes abbreviated as THU) is a research university located in Beijing, China, and one of the nine members in the C9 League.
The institution was originally established in 1911 under the name “Tsinghua College” and had been renamed several times since then: from “Tsinghua School” which was used one year after its establishment, to “National Tsinghua University” which was adopted three years after the foundation of its university section in 1925. With its motto of Self-Discipline and Social Commitment, Tsinghua University describes itself as being dedicated to academic excellence, the well-being of Chinese society and to global development. It has been consistently regarded by most domestic and international university rankings as one of the top higher learning institutions in mainland China.
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Why send a message back in time, but lock it so that no one can ever read the contents? Because it may be the key to solving currently intractable problems. That’s the claim of an international collaboration who have just published a paper in npj Quantum Information.
It turns out that an unopened message can be exceedingly useful. This is true if the experimenter entangles the message with some other system in the laboratory before sending it. Entanglement, a strange effect only possible in the realm of quantum physics, creates correlations between the time-travelling message and the laboratory system. These correlations can fuel a quantum computation.
Around 10 years ago, researcher Dave Bacon, now at Google, showed that a time-travelling quantum computer could quickly solve a group of problems, known as NP-complete, which mathematicians have lumped together as being hard.
The problem was, Bacon’s quantum computer was travelling around ‘closed timelike curves.’ These are paths through the fabric of spacetime that loop back on themselves. General relativity allows such paths to exist through contortions in spacetime known as wormholes.
Physicists argue something must stop such opportunities arising, because it would threaten ‘causality’ — in the classic example, someone could travel back in time and kill their grandfather, negating their own existence.
And it’s not only family ties that are threatened. Breaking the causal flow of time has consequences for quantum physics, too. Over the past two decades, researchers have shown that foundational principles of quantum physics break in the presence of closed timelike curves: you can beat the uncertainty principle, an inherent fuzziness of quantum properties, and the no-cloning theorem, which says quantum states can’t be copied.
However, the new work shows that a quantum computer can solve insoluble problems, even if it is travelling along ‘open timelike curves,’ which don’t create causality problems. That’s because they don’t allow direct interaction with anything in the object’s own past: the time travelling particles (or data they contain) never interact with themselves. Nevertheless, the strange quantum properties that permit ‘impossible’ computations are left intact.
“We avoid ‘classical’ paradoxes, like the grandfathers paradox, but you still get all these weird results,” says Mile Gu, who led the work.
Gu is at the Centre for Quantum Technologies (CQT) at the National University of Singapore and Tsinghua University in Beijing. His eight other coauthors come from these institutions, the University of Oxford, UK, Australian National University in Canberra, the University of Queensland in St Lucia, Australia, and QKD Corp in Toronto, Canada.
“Whenever we present the idea, people say no way can this have an effect,” says Jayne Thompson, a co-author at CQT. But it does: quantum particles sent on a timeloop could gain super computational power, even though the particles never interact with anything in the past. “The reason there is an effect is because some information is stored in the entangling correlations: this is what we’re harnessing,” Thompson says.
There is a caveat — not all physicists think that these open timeline curves are any more likely to be realizable in the physical universe than the closed ones. One argument against closed timelike curves is that no-one from the future has ever visited us. That argument, at least, doesn’t apply to the open kind, because any messages from the future would be locked.
Read more: Computing with Time Travel
One team has already made a cat ‘disappear’ with a device that has huge military potential
Mainland scientists are increasingly confident of developing the world’s first invisibility cloak, using technology to hide objects from view and make them “disappear”.
The central government has funded at least 40 research teams over the past three years to develop the idea, which until now has largely been the stuff of science fiction and fantasy novels like the Harry Potter series.
The technology would have obvious military uses such as developing stealth aircraft, but Beijing believes the research could lead to wider technological breakthroughs with broader uses, scientists involved in the research said. The teams involved include researchers at Tsinghua University and the Chinese Academy of Sciences.
The main approaches are developing materials that guide light away from an object, creating electromagnetic fields to bend light away from what one is trying to hide and copying nature to make hi-tech camouflage materials.