BLOGGER BLOG: ΟΙ ΕΙΔΗΣΕΙΣ ΤΩΝ ΘΕΤΙΚΩΝ ΕΠΙΣΤΗΜΩΝ: Αυγούστου 2018

Παρασκευή 31 Αυγούστου 2018

Nano-pillars fight the build-up of bacteria

30 Aug 2018

Fluorescence microscopy of bacterial adhesion and viability of E. coli on nano-pillar titanium samples after 3 h of incubation in BHI (A, C) or RPMI1640 (B, D) medium. Living bacteria are green; dead bacteria are red. The number of dead adherent bacteria was higher on the surfaces with optimal cell growth conditions. (Courtesy: Biomed. Phys. Eng. Express 4 055002).

Natural cell growth — which is essential for bacterial cell division — appears to favour the destruction of E. coli adhered to cicada-wing-like nano-pillars, report researchers in Germany. By understanding the process in more detail, scientists hope to find new solutions for fighting the build-up of bacteria around medical implants, to assist current drug-based treatments (Biomed. Phys. Eng. Express 4 055002).

“Innovative anti-bacterial implant surfaces, which are effective by their topography, might be a possible strategy,” says team member Manfred Köller from BG University Hospital Bergmannsheil in Bochum. “For example, in orthopaedics, such approaches could help to combat bacterial adherence, which is the first step in the formation of unwanted biofilms.”

The group’s study is inspired by observations made elsewhere that nano-pillar surfaces on cicada wings exert bactericidal effects on certain adherent bacteria. The result, reported back in 2012, continues to fascinate the biomedical engineering community, and researchers are keen to determine exactly how such nano-topographies are able to hinder bacterial colonization and growth.

To find out more, the team — which also includes scientists based at Ruhr University Bochum’s Institute for Materials — fabricated test surfaces (5 x 5 mm) covered with titanium nanopillars to mimic the insect wings. Gram-negative E. coli bacteria were allowed to adhere and proliferate on the nanostructured samples for three hours at 37 °C. The researchers incubated one batch under optimal cell growth conditions (brain heart infusion (BHI) medium), and placed the other in limited growth conditions (RPMI1640 medium). This led to an interesting discovery.

As the bacteria grew — a process that involves cell elongation of their rod-like structure — the titanium nano-pillars appeared to represent more of a threat to the micro-organisms. And the stronger the cell growth, the greater the antibacterial effect of the textured sample surface, as measured by the ratio of adherent dead E. coli to adherent living E. coli.

“Our results show that the bacterial growth of gram-negative micro-organisms adherent to nano-pillar-like structures is somehow involved in the phenomenon known as the cicada wing effect,” Köller comments

The team point out the likelihood that cylindrical elongation during growth induces additional mechanical strain on the cell when the bacterium is entangled by nano-pillars. And it follows that a disruption in the cell wall remodelling processes could lead to a loss of pressure inside the micro-organism, putting the E. coli at risk.

Electron microscopy images taken by the scientists show cell bodies collapsed and laid down on, but not punctured by (at least when viewed from above), the spike-covered test specimens used in the study.

The theory that cell growth plays a role in the overall mechanism is bolstered by previous work by the researchers where they observed a delay in the occurrence of bactericidal effects – a result that can be explained by the time gap between attachment and subsequent cell division.

Next steps

Back in the lab, the group is working on ways to increase the potency of the nanostructures to keep surfaces sterile for longer, which would further benefit implant applications. This involves decorating the nano-pillars with silver-iridium caps, which enhance the antibacterial activity of silver via an electrochemical process. Properties of the system include protection against gram-positive bacteria.

“The combination of both the cicada wing effect and such a sacrificial anode system is the next step forward,” Köller predicts.

James Tyrrell is a freelance science and technology writer based in Bristol, UK

31/8/2018 FROM PHYSICSWORLD.COM

Live cells survive in bioprinted bone

24 Aug 2018 Belle Dumé

A bioprinted construct — A ight micrograph of a bioprinted construct

Researchers in Germany have shown that a material based on calcium phosphate could offer a viable support material for bioprinting replacement bone tissue. The team, led by Michael Gelinksy at the Technical University Dresden, say that the technology opens up new possibilities for plastic and reconstructive surgeries, since it could be used to fabricate patient-specific bone tissue constructs, as well as more complex structures consisting of, for example, bone and cartilage or bone and soft tissue.

According to Gelinsky, calcium phosphate is the ideal scaffold material for these applications because it offers the same mineral structure and mechanical properties as natural bone. His team has been experimenting with scaffolds made from calcium phosphate cement (CPC), a pasty material that is easy to process into various shapes using a low-temperature extrusion-based technique called 3D plotting.

Recent work has shown that sensitive bio-components, like growth factors, can be integrated into printed CPC scaffolds without their biological activity being affected. The problem is that live cells cannot be suspended in the same scaffold because they can’t survive in such a solid and stiff support material.

Gelinsky and colleagues have now overcome this barrier by combining 3D plotting of CPCs with cell printing using a specially developed bioink. “Using a mechanically stable, self-setting CPC as a printable support material that nicely mimics the mineral component of bone, as in our work, is a big step forward to when it comes to bioprinting bone tissue constructs,” he says.

Towards stronger scaffolds

Until now, explains Gelinsky, the only scaffold materials that have been used successfully in bioprinting applications have been thermoplastic polymers (such as PCL/polycaprolactone) or highly concentrated biopolymer hydrogels. “The soft hydrogels typically used for cell printing are mechanically too weak for printing constructs for tissues like bone,” he says. “And since bone is a mineralized tissue (more than half its weight by volume comprises the calcium phosphate mineral phase hydroxyapatite), a polymer like PCL is not really a good substitute here either.”

Gelinsky’s team has already optimized a process for fabricating CPC scaffolds using 3D plotting. They have studied the way that the CPC paste solidifies after extrusion, and have found that pre-setting in a humid environment for three days prevents the formation of micro-cracks that compromise the strength of printed scaffolds. “In our previous work, we already showed that we could co-print CPC with cell-free alginate-based hydrogels,” Gelinsky continues. “So it was relatively easy for us to go a step further and co-print CPC with an alginate-based bioink that is laden with live human cells.”

The challenge for Gelinsky and his team was to find a fabrication regime that would enable the live cells to survive the setting process. Their first task was to co-print the CPC with a bioink laden with human mesenchymal stroma cells, which they did with three-channel extrusion printer that alternates printing between the CPC and the bioink. This creates a biphasic scaffold with an open pore structure, which is vital to ensure that oxygen and nutrients can reach the cells and allow them to grow.

However, setting the CPC in a humid environment for three days would kill the cells, so the researchers tested the impact of reducing the setting time on both micro-crack formation and cell viability. They found that a setting period of 20 minutes in a high-humidity environment was sufficiently long to create mechanically strong scaffolds, while also allowing almost all the live cells to survive (Biofabrication 10 045002).

One remaining issue, say the researchers, is that the fresh CPC paste is slightly cytotoxic for cells that are in direct contact within the bioink strands – which is probably caused by a slight pH shift during the cement setting reaction. “We have already come some way in overcoming this problem by using a novel type of bioink in which we haven’t seen dead cells at the crossing points of CPC and bioink strands,” says Gelinsky.

The team also plans to print bi- or tri-layered constructs with different types of human cells. “Until now, we have simply used fluorescent microbeads to demonstrate proof of principle for such complex implants,” notes Gelinsky.

Read our special collection “Frontiers in biofabrication” to learn more about the latest advances in tissue engineering. This article is one of a series of reports highlighting high-impact research published in Biofabrication.

31/08/2018 from physicsworld.com

Where do particle names come from?

Where do particle names come from?

31/8/2018

Designer metal-organic frameworks grow on graphene

31 Aug 2018 Belle Dumé FROM PHYSICSWORLD.COM

3D Metal-organic frameworks (MOFs) are an important class of materials that could be used in applications as diverse as sensing, gas storage, catalysis and optoelectronics. Their 2D versions might even be used as flexible material platforms to realize exotic quantum phases, such as topological and anomalous quantum Hall insulators. The problem is that such 2D sheets are usually synthesized on metal substrates and the strong interactions between the two unfortunately masks the intrinsic electronic properties of the MOF itself. Researchers at Aalto University School of Science in Finland say they have now overcome this problem by fabricating MOFs on graphene (a 2D sheet of carbon), which only weakly interacts with them. The resulting 2D honeycombed frameworks could be used to make MOF-based designer materials with complex, tuneable electronic structures.

“We have shown that we can synthesize 2D MOFs on epitaxial graphene and so probe the intrinsic electronic properties of the MOFs,” explains Peter Liljeroth, who led this study. “This opens the way to making 2D MOFs with exotic electronic properties.”

The researchers began by depositing organic ligand “linkers”, such as dicyanobiphenyl (DCBP) or dicyanoanthracene (DCA) with cobalt (Co) metal atoms on their graphene substrate to produce individual metal-ligand complexes. They then annealed the sample at temperatures below 100°C to form the extended MOFs.

2D band structure in MOF decoupled from the substrate

We characterized the structures using low- temperature scanning tunnelling microscopy (STM) and atomic force microscopy (AFM),” says Liljeroth. “We are able to access the intrinsic electronic properties of the materials and show that the Co-DCA MOF behaves as a 2D system with delocalized states.” This result, which the team backed up with density-functional theory (DFT) calculations, proves that the 2D band structure in the MOF is decoupled from the substrate.

“2D MOFs have been theoretically predicted to be a flexible platform for realizing various quantum materials, and our work is a first experimental step in that direction,” Liljeroth tells Physics World. “The MOFs we have made are – if you like – just simple semiconductors, but we have shown that it is now feasible to proceed to something more exotic.”

The work also opens the way to making MOF-based designer electronic materials with complex, engineered electronic structures, he adds. And being able to directly grow 2D MOFs on graphene means that these heterostructures might be readily used in applications such as electronics, sensors and catalysts.

The researchers, reporting their work in Nano Letters 10.1021/acs.nanolett.8b02062, say that they will now be looking to expand the types of MOFs that they can synthesize on weakly interacting substrates. “The complexation reactions on these different substrates may work differently compared to those occurring on a metal substrate, and we have much less published literature to go on here,” says Liljeroth. “We will then attempt to incorporate heavy metals into the MOFs since these have high spin-orbit coupling, which should result in the formation of a 2D topological insulator, according to theoretical predictions.”

31/8/2018

Molecules cooled to record-breaking low temperatures by lighting-up dark states

31 Aug 2018 FROM PHYSICSWORLD.COM

Cold molecules — Lighting up: new laser-cooling technique can chill dark states. (Courtesy: iStock/7io)

A dense cloud of optically-trapped molecules has been cooled to record-low temperatures by Lawrence Cheuk and colleagues at Harvard University and the Massachusetts Institute of Technology in the US. Using advanced laser techniques, the researchers were able to cool molecules occupying previously-inaccessible quantum states. Their methods also allowed them to construct images of the ultracold gas.

For decades, physicists have studied the behaviours of clouds of atoms at temperatures just above absolute zero. To cool atoms to these temperatures, atoms are made to absorb and then re-emit photons from a laser beam, losing some energy in the process. After this process occurs, the atom returns to its original electronic state and the cycle repeats; steadily cooling the particle.

For molecules, however, the process is more difficult. Molecules have additional degrees of freedom including vibrational and rotational states, meaning they will not necessarily return to their original states after re-emitting a photon. Instead, they can slip into more complex states that are quantum superpositions of two ground states. In previous experiments carried out by Cheuk’s team, molecules in these “dark” states were not able to absorb incoming laser photons. This prevented the atoms from being cooled any further.

Counterpropagating lasers

Drawing from their earlier research, the team developed a more sophisticated laser-cooling technique to induce cooling in dark-state molecules of calcium monofluoride (CaF). This involved subjecting the molecules to two counterpropagating laser beams separately tuned to the frequencies of the two superimposed ground states comprising the molecules’ dark state. For the first time, the researchers could cause dark-state molecules to scatter incoming photons, allowing them to be cooled to ultralow temperatures.

The technique allowed Cheuk and colleagues to cool a gas of 1300 CaF molecules to just 20 µK – significantly lower than the 60 µK temperatures reached in their previous work. The physicists also maintained a density of 80 million molecules per cubic centimetre in their gas – 10 times higher than in their last study. In addition, each molecule emitted around 2700 photons in total as they cooled – making them 200 times more fluorescent than achieved before. By collecting these photons, the researchers could precisely locate each particle in its optical trap, allowing them to accurately image the CaF gas.

The potential applications for creating and imaging an optically-trapped, ultracold gas of molecules are diverse. By adapting their techniques further, Cheuk’s team hopes their findings will be used in further studies to gain insights into fields ranging from quantum simulations and information processing to precision tests of fundamental physics.

The research is described in Physical Review Letters.

The universe could be caught in a loop and a natural nuclear reactor

23 Aug 2018 Hamish Johnston FROM PHYSICSWORLD.COM

Claims of new evidence that the universe is cyclic and undergoes multiple big bangs is discussed in this episode of the Physics World Weekly podcast. Physics World editors Hamish Johnston and Michael Banks also talk about how a natural underground nuclear reactor could help us deal with radioactive waste.

Exotic quark-matter plus 100 Second Science reaches a century

30 Aug 2018 James Dacey /from PHYSICSWORLD.COM

In this episode of Physics World Weekly, Hamish Johnston is in conversation with the particle physicists Greig Cowan and Tim Gershon, authors of Tetraquarks and Pentaquarks. Published as part of the Physics World Discovery series, the free-to-read ebook explores the latest research on the exotic particles comprising four or five (and sometimes even more) quarks.

Later in the podcast, James Dacey celebrates the landmark of reaching 100 published videos in our 100 Second Science series. Launched in 2012, the series of explainer videos has covered the breadth of physics, from astronomy to quantum entanglement to antimatter, via Noah’s Ark, philosophy and physics education. Watch this site, for more videos in the series soon.

31/8/2018