Category: News

Pillalamarri Srikrishnarka
Tampere, Finland

Respiration includes cycles of inhalation and exhalation; is vital for life and by studying the exhaled breath, over 3000 compounds were present. These compounds would suggest the plausible physiological status of that individual. However, measuring the concentration of these species require expensive and exotic instruments such as mass spectrometer which needs expertise high operating costs. Wearable sensors that can target specific components of the breath have been in development in recent times and address the cost, long-time and non-invasive monitoring of health. 

Humidity is the major component of the exhaled breath and the rise and fall in the concentration of humidity is synchronous to the respiration rate. The quest for seamless and intelligent respiratory monitoring is having a great relevance. With this aim, a researchers take a leap forward with the integration of lamellar porous film and GaN optopairs. In this exploration, let’s navigate the scientific intricacies that underpin this revolutionary approach. The seminal work accomplished by the scientists from the Southern University of Science and Technology, Schenzhen, China was published in Nano letters.

One of the major limiting factors in sensing humidity from exhaled breath is the slow response and recovery time, as it will fail to record the minute intricacies in breath changes. To short out this problem, researchers developed an optoelectronic device which showed faster response and recovery time. In view of that, a GaN optoelectronic chip  was fabricated which acts as both as light source and detector. The device is shown below. . This chip was further integrated on a flexible polyimide film  and it exhibited a sensitivity of 13.8 nA/%RH.

(Figure caption: Figure shows the photograph of the packaged microchip for sensing humidity. Copyright © 2023, American Chemical Society)
With this device the authors reported a response and recovery time of 12 and 6 s, which is quicker than the sensors available commercially. Performance of the device compared to commercially available sensor is presented in the figure below. “The compact configuration of a submillimeter size enables the device to be readily integrated with wireless data transfer systems.” the authors reported.

(Figure caption: Performance comparison of the as-fabricated device with a commercial device when measuring changes in humidity in a facemask. The blue, green, and orange shaded areas represent normal breathing, fast breathing, and deep breathing, Respectively. Copyright © 2023, American Chemical Society)

Pillalamarri Srikrishnarka

Chennai: For accurate and fast thermal sensing, scientists have successfully fabricated a flexible temperature sensor.

Non-invasive wearable technologies have been gaining a lot of traction in the recent past. Technologies such as smart-watches can be further embedded with flexible sensors that enable users to capture multi-dimensional data.

Besides monitoring the heart rate, ECG, SPO2, and steps count, the temperature is one crucial parameter where a flexible, fast-responsive sensor can be used. Flexible temperature sensors, ultrathin conformal, and a fast-responsive nature have multiple applications in various fields.

Dr. Alwin Daus and their team recently fabricated a flexible thermal sensor based on molybdenum disulfide (MoS₂) capable of responding in a few microseconds.

Firstly, MoS₂ was grown using chemical vapor deposition (CVD) on a Si@SiO₂ substrate. Later, gold electrodes were evaporated and deposited on this grown MoS₂ for electric contacts. A thin layer of polyimide was spin-coated after curing; this enabled the transfer of the grown MoS₂ with gold contacts onto the polyimide layer. This is a conformal polymer, and thickness can be altered based on the requirement. Alumina was finally deposited on the sensor as a capping agent.

“We found that the microscope light, used for probing the contact pads, led to some charge generation and trapping that persists even after the light was turned off.” To offset this, the authors performed all the experiments in the dark.

MoS₂ is known to be highly reactive with air and moisture when heated, this leads to an increase in conductance. This issue was ingeniously solved by capping the sensor with alumina. “Al₂O₃ is an excellent barrier for gas diffusion and has commonly been used for passivation in flexible electronic devices because dense and high-quality films can be obtained at low temperatures that are compatible with flexible plastic substrates.”

The sensor is sensitive in the temperature range from 27 to 120 ^oC with a response time of 35 microseconds.

“All materials used here are biocompatible, and our temperature sensitivity estimations indicate a suitability for future biomedical applications such as skin or breast cancer detection and wound healing, but we note that other use cases may require sensitivity improvements,” concluded the authors.

These results have been published recently in ACS Nanoletters.

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Electrochromic device, Vanadium Oxide, Flexible screen

Chennai, India: Whenever we hear the word photochromic device, what comes to our mind is photochromic glasses. The glasses change color to black/brown in the presence of sunlight and return to normal colorlessness in its absence. This active switching protects the users/wearers from the harsh sunlight and soothes their sight. A stimulus of light is given which in response a color change is observed, this response can be activated by other stimuli one such is potential, called as electrochromic device. Electrochromic devices are far more appreciable than conventional photochromic devices due to their vast applications. Especially for smart displays, flexible screens, military camouflage and etc. Conventionally, tungsten oxide particles have been used for electrochromic displays since their discovery back in 1969. “They possess high transmittance contrast, cyclic stability against high temperature and strong light.”

Color tunability of these particles is possible by doping, however, the entire process is time-consuming, could be expensive due to the need of additional material and also affect the overall transmittance of the system.

In this regard, Wang et al., scientists from the University of Science and Technology, China, proposed a simple yet, and scalable method for the fabrication of highly tunable nanowire assembly-based electrochromic devices. Initially, they have synthesized tungsten oxide (W₁₈O₄₉) nanowires and vanadium oxide (V₂O₅) nanowires. These nanowires were then mixed and allowed to self-assemble on a fluorine-doped indium tin oxide coated glass. This process of self-assembly was accomplished by the Langmuir-Blodgett method. Upon drying, the coated glass was tested under various external potentials varying from – 0.5 to 2 V.

Upon application of potential, drastic changes in the chromic response were observed based on the ratio of W₁₈O₄₉ and V₂O₅ mixture. Switchability of the colors were visualized as orange, green and gray, which could be possible by varying the potential. Furthermore, transmittance of the film was tuned by changing the concentration of active material. Having such precise control over switchability and transmittance, these assembled nanowires potentially could be used in the display screen and by varying the substrates, they could be potentially be used as flexible screens also.

References:
[1] J. L. Wang, J. W. Liu, S. Z. Sheng, Z. He, J. Gao, and S. H. Yu, Nano Lett. 2021, 21, 21, 9203–9209

https://pubs.acs.org/doi/full/10.1021/acs.nanolett.1c03061

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Biodegradable plastic, Cellulose nanofibers, Mechanical and Thermal stability

Chennai, India: Plastics have become somewhat inseparable and part and parcel of our day-to-day life. The US flag hoisted on the moon by astronaut Neil Armstrong was made of nylon. The global plastic production has totaled 356 million tons at the end of 2018. From disposable spoons, plates all the way to IV tubes, plastics have become truly insuperable.

However, due to their non-recyclable and nonbiodegradable nature, they end up in landfills, rivers, and oceans. These then pose a grave threat to the survivability of the local ecology, which breakdowns the sustainability of the environment. To address this, there have been few laws imposed that did impact the usage of single usage plastics. However, to meet the growing demands alternatives to plastic needs to be ventured.

Before diving into the substitutes of plastics, let’s think why we are fascinated to use plastics? Plastics are of less weight and offer protection from water and rain. These are also flexible, affordable, possess good mechanical and thermal stability. Moreover cost of plastic is low as compared to many. So, the substitute that we have to find must offer these properties of plastics. Therefore it can be easily replaceable of plastics. However, mass awareness which is very important and highly appreciable to implement this.

We have plastics of various origins, natural, petrochemical and aliphatic polyesters. Petrochemical-derived plastics are albeit most durable, offer very high tensile strength and offer high thermal resistance, are the leading agents for damaging the environment due to their least biodegradability. In this regard, aliphatic polyesters offer some biodegradability but suffer from poor mechanical and thermal properties.

Cellulose is one of the most abundant natural polymers on earth which is biodegradable. Studies have shown that more than 10¹¹ tons of cellulose can be produced every year through photosynthesis. From this cellulose, cellulose nanofibers can be extracted, that can be used as a structural material due to its unique properties of having high tensile strength comparable to that of steel and high modulus. Owing to exploit these wonderful properties of cellulose nanofibers, Guan et al., of the University of Science and Technology of China from China realized a substitute for the petrochemical-based structural material. [1]

Cellulose nanofibers have a typical diameter in the range of 5-10 nm and these fibers were crosslinked by simply spraying aqueous calcium chloride solution. The resulting hydrogel was pressed under a pressure of ~ 1 MPa for 12 h and finally dried under ~ 50 MPa at 80 ^oC for 1 h. The resulting product was tested for its mechanical and biodegradability studies. They observed that due to crosslinking, the modulus rose to ~ 16 GPa and flexural strength to ~ 300 MPa. The system was so versatile to process, that they could drill holes through the product without damaging the overall structure and even process it into desired complex shapes. The crosslinked product degraded within 90 days when it is placed under soil. These observations are truly inspiring, one day in the near future substitute for plastics is truly possible by modifying the naturally available polymers.

References:
[1] Q. F. Guan, H. B. Yang, Z. M. Han, Z. C. Ling, K. P. Yang, C. H. Yin, and S. H. Yu, Nano Lett. 2021, 21, 21, 8999–9004.
https://pubs.acs.org/doi/10.1021/acs.nanolett.1c02315

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Wearable electronics, Non-invasive, E-textile, hydrophobic, Health monitoring

Chennai, India: Wearable, non-invasive health monitoring sensors have an edge over conventional medical diagnostic tools. Lack of mobility, cost-ineffectiveness, a requirement of physical space and the requirement for trained personnel are some of the preliminary drawbacks of the medical diagnostic tools. With the ad of the internet of things, it has become much conducive for monitoring the necessary biomarkers on a daily basis. However, due to prolonged usage of such wearable sensors, especially on the skin, could lead to discomfort and wet-thermal management is crucial or it could lead to further health complications. Sweat consists of a vast variety of components apart from water, by non-invasively monitoring these components, could help in the overall health assessment of the health of the wearer. Fabrics are by far the most favorable for absorbing sweat, keeping the body cool and protecting one from extreme environment.

Traditional fabrics are ideal for adsorbing the sweat on the skin which helps in keeping the skin dry. However, they can transfer the moisture from the cloth to the skin once, the fabric is completely wet. Fabrics, however, can be customized based on the requirement. When I meant customization, it’s not merely changing the shape and adding colors. Fabrics’ chemical properties can be altered completely transforming the very nature of cloth. For example, we all know that natural fabrics are hydrophilic in nature, i.e., they absorb water. A single droplet of water immediately spreads on it. However, by chemical functionalization this property can be completely altered, transforming the cloth to a hydrophobic one similar to that of a lotus leaf.

He et al., of Shenzen University, China, chose natural fabric silk, then chemically treated it and transformed it into a hydrophobic cloth. [1] Subsequent exposure to oxygen plasma reverted the hydrophobic nature to the hydrophilic one. The hydrophilic part of the cloth comes in contact with the skin and the hydrophobic cloth faces the environment. By doing this, they observed that most of the sweat was absorbed by the cloth and the region where the cloth was in contact was far more uniform when compared to the untreated silk. Now, that one issue of wet-thermal management was addressed, let us think about what else we can use this hydrophobic cloth. Two silk yarns were then chosen, one was coated with conducting carbon paint and the other with Ag/AgCl paint. These two were then stitched on the hydrophilic side of the silk cloth transforming the fabric into E-fabric. Electrochemical measurement was performed using this modified cloth and presence of K⁺ ions, pH, uric acid, as well as glucose in sweat was measured. This E-fabric was connected to a microcontroller worn on a t-shirt and sweat analysis was performed and the concentration of these species was monitored through a mobile application.

Such E-fabrics with higher selectivity and sensitivity towards the various components of sweat could further modernize the field of wearable electronics for health monitoring, owing to its simple fabrication method.

Reference:
[1] X. He et al. Nano Lett. 2021, 21, 20, 8880–8887.
https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03426)

Pillalamarri Srikrishnarka

Dielectica traverses through the literature on this topic – and summarizes as they appear.

Key words: Diabetes mellitus, Glucose sensor

Chennai, India: Diabetes mellitus (DM), also known as diabetes is a metabolic disorder caused by the presence of high sugar in blood for prolonged duration. Globally ~ 8.8% i.e., 463 million people suffer from this disorder, classified into two major types, type – I and type – II. Type-I diabetes, also called as “juvenile diabetes” is a rare condition, where the pancreas cannot sufficiently secrete insulin and Type-II is the increased resistance to insulin generally caused due to poor lifestyle. Can lead to ketoacidosis, chronic kidney failure, cognitive impairment, nerve damage and stroke. Therefore regular monitoring of blood glucose level of a diabetic patient is advisable. However, pathological examination of blood sugar level by any patient is hectic. In recent days, glucose is detected by using glucometer, where a small drop of blood is placed on a strip and upon analysis would give the concentration of glucose (mg/dL). Even though, this is a widely used commercialized technology, it has its limitations as the patient needs to constantly monitor his/her blood glucose levels and frequent pricking of blood is uncomfortable. Thus there is a need for noninvasive technique for detection of blood glucose level.

Apart from blood, glucose is also present in tears, sweat and saliva. Monitoring these fluids is noninvasive and could provide some relief to the patients. In this regard, there has been substantial research [1] and few commercial products [2] have also been released which noninvasively monitors glucose. Need of constant sensor replacement and excessive cost are some of the major causes for these commercial products to reach the masses. Even though the amount of sweat or saliva one generates is sufficiently high, due to other components present, it could hamper the detection of glucose. Optical measurements of tear drops can detect the concentration of glucose, usually by fluorescence resonance energy transfer and also by surface plasmon resonance. [1] However, these two techniques require continuous tear generation and also laser, making them difficult for daily monitoring.

To address some of the major limitations and successfully monitor glucose concentrations noninvasively, Jeon et al., of Gwangju Institute of Science and Technology, Republic of Korea fabricated a smart contact lens. [3] This lens is comprised of nanoparticles of cerium oxide nanoparticles and glucose oxidase and polyethylene glycol complex embedded on the lens. As soon as glucose upon contact with the glucose oxidase, it oxidizes and produces hydrogen peroxide. This peroxide converts the Ce³⁺ to Ce⁴⁺ changing the colour of the lens in that process. These lenses are highly biocompatible and were tested for cytotoxicity using the human umbilical vein endothelial and human corneal epithelial cells. The testing rig is extremely simple, all it contains is a CCD camera, with a zoom lens of 0.45 X magnification. A photograph is initially captured of the lens, then by using simple image processing methodologies, the center of lens is tracked, the intensity of the image in red, blue and green channels were calculated and finally an average of all the three intensities estimates the glucose concentration. A schematic explaining the process is shown in the figure below.

Figure: Schematic illustration of colorimetric NECL and optical monitoring system. (a) NECL color was changed to yellow (colorless Ce³⁺ to yellow Ce⁴⁺) (b) A color CCD camera with a zoom lens for the demagnification (0.45×) record NECL surface color change (RGB). Reprinted with permission Copyright © 2021 American Chemical Society.

The system was tested both for rat and human tear, continuously monitored the glucose levels for diabetic rats when they were awake, sleeping due to inhalation and injected anesthesia. Every rats’ blood glucose levels was monitored prior to the experiment using the smart lens. They have observed almost 82 % and 71 % linear correlation between the blood and tear glucose concentration in case of humans and rats, respectively. Thus, this methodology is simple, could alleviate the discomfort and by developing a mobile phone application, it could potentially reach the masses.

References
[1] M. Chung, G. Fortunato, N. Radacsi, J. R. Soc. Interface, 2019, 16, 20190217.
[2] https://www.freestylelibre.co.uk/libre/
[3] H. J. Jeon et al., Nano Lett. 2021, 21, 8933–8940.