October's Paper of the Month

skin sensor.jpg

Electronic skins (E-skins) that can mimic the functions of a human skin have been intensively studied in the past few years. They are expected to have a great impact on the upcoming generation of portable and wearable electronics related to the Internet of Things. Even after many breakthroughs in the material sciences, there are still difficulties in achieving high performance and highly functional sensors durable enough for continuous monitoring of human activity and health.

Advanced micro- and nanoscale materials within polymer-based protective layers have been successfully applied for the E-skins. However, it is not only important to study and understand the behavior and properties of these electronic materials, but also to create smart structures to achieve the desired performance for these devices.

A research group from University of Oulu bypassed common performance and functional issues by combining advanced materials and an ingenious structure to achieve mechanosensitivity to different stimuli, high sensing performance, and functionality within the device. The structure not only provided simultaneous ability to be adhered to a human skin or be attached to clothes or textiles, but also possibilities for precise tuning of the response to achieve optimal performances for different locations in the human body. The devices were able to record human activity and health in a reliable manner, providing adequate long-term durability with machine washability.

The group used a TST350 to test the tensile properties of their sample. When asked about the purpose of the stage, author Jarrko Tolvanen explained: “In our research the Linkam TST350 stage has provided a reliable way to record the stress-strain curve of various materials. Also, this stage has enabled easy and quick way of testing the mechanical properties under controllable temperature and humidity conditions, that could be proven advantageous when further improving and optimizing the performance of a strain sensor.”

However, there is still a long way to go until the commercialisation of wearable products. One of the major difficulties is to achieve a multidirectional sensor that can distinguish the types of stimuli with high selectivity but is also feasible for wireless sensing.

Although more testing is required for long-term durability and environmental stability, the group’s work is a promising start in achieving high performance wearable sensors.

Tolvanen. J, Hannu. J & Jantunen. H, Stretchable and Washable Strain Sensor Based on Cracking Structure for Human Motion Monitoring. (2018) Scientific Reports volume 8, Article number: 13241

August's Paper of the Month

drop-of-water.jpg

The movement of liquid molecules along a solid surface is called hydrodynamic slip. This event is central to understanding how fluids are transported at the smaller scales.

Between a solid and liquid interface, friction is present. When this friction is extremely high, the velocity of the fluid at this interface can be considered zero - this is called the “no-slip” boundary. This can be used to assume fluid flow at macroscopic scales, however there has been much focus in the last few decades to understand this at the more microscopic scale.

A cross-disciplinary research collaboration* aimed to develop a fundamental understanding of the physics of fluid flow. They investigated a long-standing question in fluid dynamics by trying to understand the factors that control friction at a solid/liquid interface. The group did so by conducting experiments using novel techniques that allowed them to precisely measure nanoscale fluid flow.

When discussing their experimental setup, Dr Mark Ilton said: “We use several Linkam stages in the labs, all in the THMS family. The Linkam stages provide a standardised way to thermally anneal our samples across the various labs involved in the collaboration. The simplicity, quick ramp-rates, and remarkable long-term stability are all key features. Since the viscosity of the polymeric fluids, a crucial parameter in our measurements, is highly sensitive to temperature, the precision of the Linkam stages is integral to the experiments. The size of the sample stage provided enough room to have a control sample side-by-side with a sample of experimental interest. This was a crucial part of our experimental protocol and enabled the data quality that supported our conclusions.”

Their experiments demonstrated that solid substrates that are considered “ideal” (coated silicon wafers, where the solid/liquid interactions are weak compared to uncoated substrates) can still have consequential friction due to transient adsorption of liquid molecules. This has important repercussions for products that use such coatings as they may not be as ideal as first thought.

By Tabassum Mujtaba

Bäumchen et al., Adsorption-induced slip inhibition for polymer melts on ideal substrates. (2018) Nature Communications, volume 9, Article number: 1172

*McMaster University, University of Massachusetts Amherst, University of Bordeaux, Global Institution for Collaborative Research and Education, Hokkaido University, Laboratoire de Physico-Chimie Théorique, PSL Research University, Max Planck Institute for Dynamics and Self-Organization & Ecole Polytechnique.

June's Paper of the Month

 Polarised optical photomicrographs of liquid crystals show the change in texture caused by slow cooling.

Polarised optical photomicrographs of liquid crystals show the change in texture caused by slow cooling.

Polymorphism is the existence of more than one form. In the case of liquid crystals, this is when a material can exist in two or more crystal structures. As the structures vary, this in turn affects their function and properties. Finding liquid crystal polymorphs would be advantageous for many different fields including engineering, pharmaceuticals and sensors. 

Real polymorphisms are difficult to find in rod shaped liquid crystals. Previous studies have shown that bent-core liquid crystals, although their phases can vary depending on cooling rate, havesmectic structures and x-ray diffraction patterns that are almost identical. 

A collaborative research effort from the Kent State University and Lawrence Berkeley National Laboratory found a polymorphic bent core liquid crystal that has structurally and morphologically independent liquid crystal phases that are cooling rate dependent. As their structures differ, so does the structural colour, paving way for a range of potential applications. 

The group conducted several different experiments to identify the liquid crystal polymorphs. They used Polarised Light Optical Microscopy to visualise the cooling rate dependant formation. To do so, the team used the Linkam LTS420E to conduct their temperature-controlled experiments, both heating and cooling the samples. 

They found that upon slow cooling oblique columnar phase forms and on rapid cooling, helical microfilament phase forms were produced. This change in structure was also accompanied by a unique colour change. 

This novel finding highlights the ability to control liquid crystal structure through temperature control. The change in colour facilitated by the structural transformation, could be used in future applications of thermal sensors and security tags.  

By Tabassum Mujtaba

Hegmann et al., An unusual type of polymorphism in a liquid crystal. (2018) Nature Communications volume 9, Article number: 714
 

May's Paper of the Month

 Vanadium oxide could have a promising future in applications of smart devices. 

Vanadium oxide could have a promising future in applications of smart devices. 

Vanadium is a transition metal that has 11 oxide phases. Vanadium oxide thin films undergo phase transitions that are stimuli-dependant. This transition can be triggered by temperature or electrical input. An increase in temperature induces a crystal reorientation which causes an insulator-metal transition (IMT). This transition also changes the optical properties of the material, which opens the door for applications in optoelectronic devices. 

One particular oxide, VO2, is theoretically well suited to application in optoelectronics because the phase change occurs at temperatures at which electronics can function, 67°C. Furthermore, the optical transition features a transparent to nearly opaque change at near infra-red wavelengths. These properties can be exploited for various applications including memory devices and smart windows. 

However, VO2 thin film deposition has long suffered from substrate dependency and lack of scalable synthesis. Incorporation into electronic devices relies on special substrates to maintain material functionality. Sensitivity to oxygen levels also proves problematic for large scale synthesis. 

A collaborative effort from RMIT and the university of Adelaide worked towards resolving some of the drawbacks in VO2 fabrication. They found a way to harness its properties in ways that had not been accomplished in the past. 

The group used a magnetron sputtering process to synthesise the material and tested it on glass, quartz and float-zone silica substrates. They used an LTS420 to conduct the optical measurements in situ while heating the films on various substrates. In situ heating with controlled ramps allowed them to take a closer look at optical properties of VO2 thin films at different temperatures.

 Unlike current methods, theirs was shown to be substrate independent, repeatable and less sensitive to oxygen concentration, thereby rendering it a promising method to fabricate VO2 thin films. 

With substrate-independence insulator-to-metal (IMT) behaviour, they can expand on VO2 applications in an electrical context in the form of switching devices and optically in the infrared, microwave and terahertz wavelengths. One near-term application is the so-called “Smart Window”, which is essentially a window made of vanadium dioxide coated glass that can be used to naturally regulate the temperatures inside an office, block, house, room or a building. 

By Tabassum Mujtaba

Bhaskaran et al., Insulator–metal transition in substrate-independent VO2 thin film for phase-change devices. (2017) Scientific Reportsvolume 7, Article number: 17899

April’s Paper of the Month

 The cover of the journal, Chemistry of Materials, highlights a unique phonon projection technique implemented on the yellow emitting phosphor, Y3−xCexAl5O12 (the phosphor applied in most commercial phosphor-converted white LEDs), which provides novel insights into local vibrational dynamics of the crystal and its effects on luminescence properties of the material.

The cover of the journal, Chemistry of Materials, highlights a unique phonon projection technique implemented on the yellow emitting phosphor, Y3−xCexAl5O12 (the phosphor applied in most commercial phosphor-converted white LEDs), which provides novel insights into local vibrational dynamics of the crystal and its effects on luminescence properties of the material.

Phosphor-converted white-light-emitting diodes (pc-WLEDs) are efficient light sources used in displays in electronic devices, lamps for indoor and outdoor lighting, and vehicle indicators, to name a few. The most common type of pc-WLEDs comprises an (In,Ga)N-based blue LED and a yellow phosphor, Y3−xCexAl5O12 (YAG:Ce3+), which is electronically excited by the blue LED and followed by yellow light emission. The admixture of the blue and yellow light appears as white light. Hereby, the luminescence properties of the device such as colour temperature, colour rendering index, efficiency, thermal stability, and so on, are strongly dependent on the luminescence performance of YAG:Ce3+.

In YAG:Ce3+, small amounts of the dopant Ce3+ ions serve as luminescent centers, whose electronic structure, which determines the energy transitions of excitation and emission, is predominantly controlled by the local static and dynamical structural environments of the host material, YAG. 

April’s Paper of the Month, from the Chalmers University of Technology, focus particularly on the vibrational dynamics around the Ce3+ ions using vibrational spectroscopy together with DFT-calculations and a unique phonon projection technique. The phonon projection technique is a novel means to interpret lattice vibrations, which allows the qualitative (symmetry) and quantitative (vibrational amplitude) determination of localized vibrations of individual YO8/CeO8, AlO6, and AlO4 moieties in the Y3−xCexAl5O12 crystal, in terms of symmetry coordinates.

They used the Linkam THMS600 in combination with a commercial Raman spectrometer, to measure temperature-dependent Raman spectra. The results reveal that the studied material, YAG/YAG:Ce3+, remains the same phase in the temperature range of 80 K (-193°C) and 870 K (597°C), however that the frequency of phonon modes changes as a function of temperature. The change in frequency of some specific vibrational modes have been shown to play an important role in the emission colour and luminescence efficiency, especially at high temperature.

The understanding of fundamental structural dynamical properties of one of the most important phosphors in this study, provides a promising design principle, through chemically tuning local static/dynamical structure around the luminescent centers, for developing new phosphors emitting at longer wavelengths, e.g. from greenish-yellow to reddish-yellow emission (to obtain warmer white light from pc-WLEDs), meanwhile exhibiting high luminescence efficiency at high temperature.

Y.-C. Lin, P. Erhart, M. Bettinelli, N. C. George, S. F. Parker, and M. Karlsson, Understanding the Interactions between Vibrational Modes and Excited State Relaxation in Y3–xCexAl5O12: Design Principles for Phosphors Based on 5d–4f Transitions. Chemistry of Materials 2018 30 (6), 1865-1877 DOI: 10.1021/acs.chemmater.7b04348
 

March's Paper of the Month

 An optical image of black phosporus mid-IR photodetector. 

An optical image of black phosporus mid-IR photodetector. 

The element phosphorus has several different allotropes, including the thermodynamically stable form, black phosphorus (BP). BP has interesting properties which make it useful for the optoelectrical field, such as its layered structure, bandgap in the mid-infrared range and high carrier mobility. 

HgxCd(1-x)Te (MCT) is generally regarded as the most popular mid-infrared material, whose composition can be tuned by in material growth process. However dynamical, in-situ tuning of its optical properties has never been achieved, limiting its ability. 

The Paper of the Month for March, discovered black phosphorus could be useful for in-situ tunable mid-infrared applications. They leveraged a thin layer of black phosphorus sandwiched between hexagonal boron nitride (HBN) and applied an electric field to tune its optical properties. This expanded the photo-response of the mid-infrared photodetectors from 3.7 to 7.7 µm. Other than photodetectors, high speed mid-infrared modulators can be readily constructed using the same concept. 

They used the heating and cooling probe stage, the HFS600E-PB4, together with an FTIR spectrometer for the temperature-dependent photo-response measurements. 

Their results prove promising. The layered nature of BP, the high intrinsic mobility and strong photo-response in the broad mid-IR wavelength range make it an ideal material for high-speed mid-IR photodetectors, modulators and spectrometers. 
 

By Tabassum Mujtaba

Xia et al., Widely tunable black phosphorus mid-infrared photodetector. (2017) Nature Communicationsvolume 8, Article number: 1672 doi:10.1038/s41467-017-01978-3

Studying Phase Transitions in Pharmaceuticals with the Linkam DSC450

Dr Asma Buanz

UCL School of Pharmacy, University College London, 29-39 Brunswick Square, London, WC1N 1AX

Background

Polymorphism in pharmaceutical solids has great implications on both the processing and the performance of solid pharmaceutical products. It is the ability of a substance to exist in more than one molecular arrangement and the result is more than one polymorphic forms which differ in their physiochemical properties such as solubility, stability, melting point etc.1 Depending on these arrangements the polymorphic forms could vary in their relative stabilities; with the metastable forms eventually converting to the most stable form.1,2. Studying these phase transformations is important in understanding the properties of these polymorphic forms. Various techniques could be employed for this purpose but Differential Scanning Colorimetry is the most common and efficient technique as it allows following these transformations as a function of temperature or time, in addition to its high sensitivity.3 Nonetheless, sometimes it is difficult to build a clear picture of what is happening to the sample as it goes through a phase transition from just the heat flow signal provided by the DSC, and thus visualising these processes would be valuable. In addition, subtle transitions such as solid-solid transitions could be missed in the DSC if they happen over a wide temperature range.

Method

The Linkam DSC450 stage allows visualisation of the sample during a DSC experiment. Therefore, this system was used in studying flufenamic acid, one of the most polymorphic pharmaceuticals with a record of nine known polymorphic forms2. The aim was to study crystallisation from the amorphous phase obtained by melt quenching. Form I was obtained by spray drying and was first heated in the DSC450  up to the melt, then it was allowed to cool down to room temperature before re-heating at a 10 °C/min heating rate.

Results

As shown in Figure 1, form I melted at ca. 132 °C while the re-heated sample melted at a lower temperature (onset of ca. 122 °C). No re-crystallisation was observed in the second heating cycle, which indicated that upon cooling a metastable form recrystallised from the melt. 

Asma graph 1.jpg

The effect of adding a polymer (PVP) is evident in Figure 2 where it appeared that the sample did not crystallise upon cooling but rather formed an amorphous phase. Heating the amorphous phase caused there-crystallisation of FFA followed by a solid-solid transition and then a melt. These events appear as two exothermic transitions followed by a sharp endotherm. The solid-solid transition is subtle in the DSC thermogram but is very clear from the signal obtained from employing an image analysis technique (Thermal Analysis by Surface Characterization, TASC) shown in Figure 2c. The melting peak has an onset temperature of ca. 119 °C, which is lower than that of the form crystallised from the melt without the presence of the polymer. The TASC signal also shows that melting is detected visually before the DSC signal starts to change.

Asma graph 2.jpg

Conclusion

In this work polymorphic transitions in the pharmaceutical active flufenamic acid were studied with Linkam DSC450 stage, which combines optical microscopy with differential scanning calorimetry. The power of the complementary technique was evident with the increased sensitivity for detecting subtle transitions such as solid-solid transition by analysing the optical images.

References

1. Rodrı́g uez-Spong, B., Price, C. P., Jayasankar, A., Matzger, A. J. and Rodrı́guez-Hornedo, N. r. 2004. General principles of pharmaceutical solid polymorphism: A supramolecular perspective. Advanced Drug Delivery Reviews 56(3): 241-274.

2. López-Mejías, V., Kampf, J. W. and Matzger, A. J. 2012. Nonamorphism in flufenamic acid and a new record for a polymorphic compound with solved structures. Journal of the American Chemical Society 134(24): 9872-9875.

3. Gaisford, S. and Saunders, M. 2012. Physical form i – crystalline materials. Essentials of pharmaceutical preformulation, John Wiley & Sons, Ltd: 127-155.

February's Paper of the Month

Nanomaterials have been found to have interesting electronic, magnetic and optical properties. They can manipulate electromagnetic fields through localised surface plasmon resonance to modulate light interactions. Such plasmonic phenomena are popular in application for the biomedical field. 

February’s Paper of the Month comes from the University of California, Merced and Stanford University. They developed a micro-scale delivery module for various organic and inorganic compounds using nanomaterials. 

Their aim was to create something that would be versatile and capable of encapsulating a range of different materials (drugs, dyes, cells, bacteria, etc.) for many different applications. These could include drug delivery for cancer treatment, releasing dyes in vivo for fluorescence imaging, or tissue engineering. The problem with most current platforms is that they are either leaky, unable to hold the contents without loss for any prolonged period, or they are incapable of releasing contents in a spatially and temporally controlled manner. For example, other cargo delivery systems that use light to activate the release of the cargo need several milliwatts of power over several minutes to achieve the required effect, therefore creating significant localised heating. The group managed to reduce the power required to less than 2 mW and the release time to under 5 seconds. As a result, the total temperature increase at the vicinity of the capsules is only to ~ 40°C, which is well within tolerable limits for many biological systems.

They used an LTS350* for their experiments. When asked on the importance of the hotstage, Dr Ghosh said: “One of the most critical parameters that determine whether a cargo delivery system is viable in vivo is the thermal gradient that is produced because of the photothermal effect when optical excitation used to rupture the shells is in resonance with the plasmonic response of the nanoparticles that make up the shell walls. To estimate this, the first step was to use heat to rupture the shells instead of light. That is where we used the heating stage.”
 

 Fluorescence microscopy images of a Nano-Assembled Microshell loaded with a fuorescent dye on the LTS350 stage. 

Fluorescence microscopy images of a Nano-Assembled Microshell loaded with a fuorescent dye on the LTS350 stage. 

 Their method has proved to be exciting and advantageous. No leakage was seen for over five months after encapsulation, promising a long shelf life. Furthermore, a lower optical intensity was required for shell disintegration compared to other methods. 

Although more work is required to improve future in-vivo applications (such as actuation by near infra-red and reducing overall size of capsule), their work is a promising result for future cargo delivery systems. 

By Tabassum Mujtaba

Ghosh et al., Plasmon-actuated nano-assembled microshells. Sci. Rep. 7. doi:10.1038/s41598-017-17691-6

*The LTS350 has been superseded by the LTS420 offering a large temperature range and better temperature control to 0.01°C.
 

January's Paper of the Month

The efficiency of lead halide perovskites has increased significantly since their introduction in 2009. The high performance as well as cost-effective manufacture make them ideal for photovoltaic applications. However, lead halide perovskites are known for their instability when subjected to changes in light, oxygen, temperature as well as issues with homogeneity. As such it becomes important to test their stability in a practical situation, such as in a device stack, to better understand the degradation mechanics. 

Raman spectroscopy is a powerful probing tool to study the degradation of individual perovskite layers as well as the degradation kinetics. Raman mapping techniques can also be used to investigate the homogeneity of the perovskite film. 

January’s Paper of the Month comes from the University of Swansea. They used a Linkam RH95 and THMS600-H to conduct in-situ Raman Spectroscopy and further understand the degradation kinetics of perovskite materials when temperature and humidity were altered. 

The device stack was first tested for thermal degradation by heating to 150°C. While photo-degradation was found to be dependent on top and bottom layers, thermal degradation was shown to be non-reversible and affects the entire MAPI film. It is determined by the homogeneity of the film rather than structural layers. Raman signals from in-situ humidity experiments show the dihydration of the perovskite to be almost completely reversible once drying occurs. However further analysis of the raman peaks showed dihyration remained in the Au region, suggesting some moisture remained trapped in this region. Device performance may be fully recovered if trapped moisture can be removed.
 

 Figure 1: The in situ raman spectra highlights the reversible dihydration when relative humidity is decreased but also indicates the presence of a dehydrated species in the Au region. 

Figure 1: The in situ raman spectra highlights the reversible dihydration when relative humidity is decreased but also indicates the presence of a dehydrated species in the Au region. 

Understanding how humidity affects perovskite layers differentially within a stack has led to the conclusion that targeted drying would help improve and regain device performance. Optimising such performances will have great benefits for the future development of microelectronics and telecommunication.

By Tabassum Mujtaba

Tsoi et al., Probing the degradation and homogeneity of embedded perovskite semiconducting layers in photovoltaic devices by Raman spectroscopy. Phys.Chem.Chem.Phys. 19, 5246 (2017). 

December's Paper of the Month

 Fluorescent labelled E. coli can be seen in the swollen pharynx of some dying C. elegans (P, left) but not others with an atrophied pharynx (p, right). 

Fluorescent labelled E. coli can be seen in the swollen pharynx of some dying C. elegans (P, left) but not others with an atrophied pharynx (p, right). 

With improving healthcare humans are living longer than ever before, but with longer life comes ever more senescence-related pathologies. Understanding the role genes and environment play in the development of such pathologies in the complex system of our bodies is difficult.  The nematode Caenorhabditis elegans is a great model organism and has been used extensively to study the biology of ageing because just like more complex animals, C. elegans also develop senescent pathologies.

Researchers from the University College London and Washington University, MO studied age-related pathologies in C. elegans and their role in limiting lifespan. Understanding these processes in a simple organism may in turn help to further understand the origins of human age-related pathologies.

To investigate the causes of death, the group analysed the corpses of recently expired wildtype C. elegans and found two particular forms with different pharyngeal pathologies. One type, named P ("big P") death, occurred earlier than the other and had increases in the posterior bulb size of 20-120%. The other, named p ("small p") death, showed a shrinking of the posterior bulb by up to 70%.

 The figures above show the age distribution and percentage survival of P and p deaths. 

The figures above show the age distribution and percentage survival of P and p deaths. 

Dissections and RFP labelling of the E. coli food source established that the enlarged posterior bulb in P death individuals was due to E. coli infection in elderly C. elegans. The high pharyngeal pumping rates typical of young nematodes is thought to mechanically damage the cuticle, creating vulnerability to invasion. Their findings suggest there is a narrow time frame in which young nematodes are thus susceptible. Consistent with this, the group found that mutants with reduced pumping rate had fewer P deaths, and lived longer.  However, it was also found that worms dying with P and p death previously had similar pumping rates, so why did some worms but not others get an infected pharynx? 

The difference appeared to be due to the ability of some nematodes (p death type) to heal the cuticle thereby preventing invasion. 

When asked about their work and the role of the PE120 stage, Professor David Gems stated, “to help identify old age pathologies that limit life, we watched nematodes as they aged, measuring a range of pathologies, and then measured their lifespan. By measuring how well each pathology correlated with lifespan we could identify pathologies likely to cause death. But to do this required repeatedly putting immobilized nematodes under the microscope, and we needed to do this in a way that wasn’t so stressful that it shortened their lifespan. By using the PE120 Linkam stage to gently cool the worms, we were able to avoid using stressful anaesthetics. We were able to confirm that repeated viewing of nematodes using the PE120 Linkam stage in this way did not shorten their lifespan.”

The group used a novel approach to understand ageing by analysing and combining pathology and mortality profiles. Further work can now be conducted with a view to understanding how genes that affect lifespan differentially affect worms dying from different causes.

By Tabassum Mujtaba

Zhao, Y. et al. Two forms of death in ageing Caenorhabditis elegans. Nat. Commun. 8, 15458 doi: 10.1038/ncomms15458 (2017).