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11th September 2018
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Nano Futures

Victoria Hattersley, 11 September 2018

018 Nano futures web 2

From monitoring our heart rate and generating renewable energy to keeping astronauts safe in space, a number of novel applications for carbon nanotubes have emerged in recent months, Victoria Hattersley reports.

Image: NASA/ Walter Myers/ Science Photo Library

Academic and industrial interest around carbon nanotubes (CNTs) continues to increase, owing to their exceptional strength, stiffness and electronic properties. Over the years, this interest has mainly focused on creating products that are both stronger and lighter, for example, in the sporting goods sector, but recently many ‘quirkier’ applications are beginning to appear.

At Embry-Riddle Aeronautical University in Prescott, Arizona, for example, researchers are currently working with NASA on new types of nano sensors to keep astronauts safer in space.1 The Embry-Riddle team – along with colleagues at LUNA Innovations, a fibre-optics sensing company based in Virginia, US – have focused on developing and refining smart material sensors that can be used to detect stress or damage in critical structures using a particular class of carbon nanotube (CNT) called ‘buckypaper’.

With buckypaper, layers of nanotubes can be loosely bonded to form a paper-like thin sheet, effectively creating a layer of thousands of tiny sensors. These sensor sheets could improve the safety of future space travel via NASA’s ‘inflatable space habitats’ – pressurised structures capable of supporting life in outer space – by detecting potentially damaging micrometeroroids and orbital debris (MMOD). CNTs coated on a large flexible membrane on an inflatable habitat, for instance, could accurately monitor strain and pinpoint impact from nearby MMODs.

In static tests, the team has already successfully demonstrated dynamic impact detection with the sensors. The next phase will be to embed the smart sensors into a flexible and compliant material that can expand as the modules are inflated in space.

Carbon nanotube containing ‘buckypaper’ could one day improve the safety of future space travel by detecting potentially damaging micrometeroroids and orbital debris

A new kind of yarn called ‘twistron’, made from CNTs, has been shown to generate electricity if stretched or twisted. Sewn into a textile, researchers have shown how it can be used to monitor breathing

Researchers are creating 3D printed scaffolds for nerve growth and repair that incorporate electrically conducting multiwalled carbon nanotubes

More down to Earth, in January 2018 researchers reported on another surprising application for CNTs– to produce disposable sensors from toilet paper.2 To carry out the study, the University of Washington researchers used tissue similar to toilet paper and soaked it in water containing CNTs to make it conductive. When the paper was stretched, its cellulose fibres stiffened and fractured. Fibres in line with the stretching force broke, while those perpendicular to it were reorganised to form crossbar junctions. The perpendicular fibres exhibited piezo resistive and capacitive sensitivity to the stretching force, which could be measured.

The team found that such sensors are capable of detecting heartbeats, finger and eyeball movements. One possible application is for ‘wearable tech’ – in this case, in the form of a disposable, plaster-shaped sensor that could, for example, be placed on the soles of a person’s shoe to measure their gait, or on a pair of glasses to monitor eye movement.

Predicting patterns
Researchers at The Massachusetts Institute of Technology (MIT) have revealed a systematic method to predict the 2D patterns formed by carbon nanotubes through a process of chemical vapour deposition, whereby a solid chemical material is deposited from a gaseous state.5 The team found that the stiffness and strength of CNTs increases by depositing them from drops of acetone or ethanol. If the stiffness and strength can be maintained, while at the same time the CNT arrays can bend without breaking, then the possibilities for industry use will multiply.

‘In the current work, we investigated how a facile solvent evaporation approach, which we call densification, could be used to pattern these macroscopic CNT architectures,’ says graduate student Ashley Kaiser. ‘What makes this work unique is that we use more representative assumptions of the CNT elastic properties when they are manufactured on the macroscopic scale… Early work assumed that the CNTs are straight and ideal, whereas real CNTs contain many defects, disorder, and curvature.’

This research shows how solvent evaporation can be used to predictably transform a nanomaterial to fine-tune its properties for specific applications.

Ultimately, this work could enable CNTs to be patterned with ease and in large scales at reasonable cost. Once pattern formation can be more precisely controlled, the MIT team envisages using the approach to investigate how to improve and tailor the thermal, electrical and mechanical properties of CNT films for mobile electronic devices.

Powering tech
In fact, wearable tech is becoming increasingly popular, whether for navigating your way around, tracking your exercise or monitoring vital bodily functions. Electronic textiles are in development, but how do we go about powering them?

Carter Haines at the University of Texas, Dallas, US, has attempted to answer that question by developing a new kind of yarn called ‘twistron’, made from CNTs.3 The team has shown that ‘twistron’ generates electricity if stretched or twisted. Sewn into a textile, they demonstrated how it could be used to monitor breathing.

‘We combined many individual nanotubes to make a yarn that is both strong and has a lot of surface area,’ Haines says. ‘In an electrolyte, we can store charge on that surface area to make a supercapacitor. The key to our harvesters is that when we stretch a coiled yarn, the twist increases, compressing it and squeezing nanotubes closer together, so it changes capacitance to produce electricity.’

Haines likens this idea to wringing water out of a sponge, when increasing twist decreases internal volume to expel the water. Unlike conventional dielectric capacitor energy harvesters, he explains that these CNT yarn harvesters do not require an external high-voltage source to operate. Instead, the electrochemical nature of the system causes the yarns to become charged simply by placing them in the electrolyte. And there are other intriguing implications for sustainable energy generation. ‘In the future, we envisage large-scale harvesters that convert previously unused mechanical energy into electricity,’ Haines says. ‘Ocean wave harvesting is a particularly exciting possibility since these harvesters can work directly in ocean saltwater electrolyte to produce energy when stretched.’

The researchers report generating a voltage after attaching the yarn between a float and a weight on the ocean floor, but acknowledge that the high cost of producing twistron remains a limitation.

3D printed CNTs
In the area of pharmaceuticals, meanwhile, CNTs have long been of interest, from drug delivery systems to therapeutic monitoring. However, new 3D printing technologies have dramatically expanded the number of potential applications.

At George Washington University (GWU) in Washington DC, US, for example, researchers are using 3D printing to create a porous nerve scaffold incorporating electrically conducting multiwalled carbon nanotubes (MWCNTs).4

3D nerve scaffolds are a promising alternative to stem cell transplants. Seeded with neural stem cells, such scaffolds can mimic the extracellular architecture of neural tissue to encourage cell proliferation and – therefore – nerve repair.

‘I previously used a table-top stereolithography 3D printer to fabricate a biocompatible hydrogel scaffold to enhance neural cell proliferation and differentiation,’ says Se-Jun Lee, a PhD student at GWU: ‘Afterwards, I found out that CNTs have great potential in neural engineering application due their unique electrical and mechanical properties.’

Researchers in Canada have recently reported a way to control both the orientation and size of single-walled carbon nanotubes deposited on a surface – a big step forward in making possible a CNT-based transistor

Lee and his team used a bioink containing a 60% polyethylene glycol diacrylate (PEGDA) solution and varying concentrations of CNTs to fabricate scaffolds. Upon exposure to a 355nm UV laser during printing, a photoinitiator in the bioink acts as a curing agent, polymerising the PEGDA and transforming it into an insoluble and biologically inert hydrogel.

In Lee’s more recent work, double-stranded DNA was used to homogenously disperse CNTs in a hydrogel mixture. The DNA-assisted CNT-hydrogel bioink has shown enhanced electrical properties as well as excellent cellular biocompatibility. Aside from nerve rebuilding, the combination of 3D printing and CNT-based material could be used in various electrical engineering applications such as to make 3D-printed electrodes or micro-supercapacitors.

‘So far, I have only demonstrated biocompatibility of CNTs in a short culture period – less than 14 days. I plan to demonstrate long-term biocompatibility of CNT and perform an in vivo study to fully show its therapeutic potential in treating nerve damage.’

However, one major limitation Se-Jun Lee mentions for CNTs in biomedicine is their toxicity, owing to their structural similarity to asbestos. Until this safety issue can be adequately addressed, many in the field believe that nanotubes may never realise their full potential.

CNT-based computing
Researchers have long realised that we are fast approaching the limits of what is possible with silicon-based semiconductors. The small size of CNTs allows more data to be condensed into tighter spaces, consuming considerably less power. They could also pave the way for new flexible displays or even artificial ‘smartskin’ that could mimic human skin’s sense of touch.

Now it appears that researchers are finally making headway on this idea, with a team of researchers at the University of Waterloo, Ontario, Canada,6 recently reporting a way to control both the orientation and size of single-walled carbon nanotubes deposited on a surface – a big step forward in making possible a CNT-based transistor.

The team’s ‘Alignment Ray Technique’ had to overcome two major barriers: how to purify and sort CNTs based on size and shape; and how to align them between electrodes.

‘We developed the Alignment Ray Technique (ART) to tackle both of these problems in one step,’ says Derek Shipper, assistant professor at the University of Waterloo. ‘The biggest innovation on the chemistry side is the ability of the ART to align organic molecules on a substrate surface. Normally this is done through self-assembly, which relies on small molecules packing tightly together.

‘But not all molecules are capable of self-assembly, and even minute changes at the molecular level can impact the final structure. The ART relies on the synthesis of a specially designed organic molecule that acts as a “nanotweezer”, capable of strong interactions with CNTs and of selectively binding specific sizes of CNTs.

We created a way of aligning these nanotweezers on a surface… allowing us to deposit nanotubes on the “nanotweezers”. Because the nanotweezers were aligned, they pass the alignment to the CNTs, hence the alignment relay.’

While the team is not the first to look at potential solutions to work with CNTs as opposed to silicon for electronic devices, this is the only study to tackle the above-mentioned problem of alignment. The ART relies on liquid crystals to pass alignment information to a substrate’s – such a metal oxide – surface. Small aromatic molecules known as ‘iptycenes’ can then bond to the surface, which changes the orientation pattern and switches the transistor’s capability. The team has found that any molecules that containing an ‘iptycene’ motif can be aligned using this method.

‘The extent of the alignment can further be tuned through altering various parameters in the alignment process,’ Shipper adds. ‘By enabling such alignment we have enabled many potential new processes and technologies, one of which we have demonstrated with the alignment and sorting of CNTs.’

However, he stresses that much more research is needed if CNT transistors are ever to rival silicon-based electronics.

References
1 J. Ohanian, N. Garg and M. A. Castellucci; doi:10.1117/12.2260106
2 J. Zhang, et al; doi.org./10.1002/admt.201700266
3 C. S. Haines, et al; Science, 2017, 357, 773.
4 S. J. Lee, et al; doi.org/10.1089/ten.tea.2016.0353
5 A. L. Kaiser, et al; Phys. Chemis. Chem. Phys., 2018, 20, 3876.
6 D. J. Schipper and S. Selmani; doi.org/10.1002/anie.201712779

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