Testing Barriers Slow Sustainable Electronics Production (Part 4) – How can we speed up testing?

mechanical engineer soldering in workshop

If you didn’t catch the previous parts of this series on what electronic tests are and how they relate to sustainability, you can find the first part here! (No really, you’ll be very confused if you read this article otherwise 😁).

But as quick refresher: improved technology is the key to sustainability–from energy-efficient LEDs to environmentally-friendly batteries! But many tests are needed to improve technology and these should look for ‘failure modes’ (the events that lead to electronics breaking), using tests that speed up both burn-in and lifetime under different (usually harsh) conditions. For example, you might run a burn-in test for an integrated circuit chip (computer chip) at 130°C instead of 25°C (even though most computers are probably stored at room temperature instead of 130°C).

There is now a variety of tests used by the industry to test the durability and longevity of their products, such as: Highly Accelerated Stress Test (HAST); Highly Accelerated Lifetime Test (HALT); High Temperature Operating Life (HTOL) Test; Accelerated Lifetime Test, etc. All these tests follow the same principle: the worse the conditions, the faster the degradation. The issue is figuring out how to balance the test accuracy, making sure not to create unrealistic conditions that may not apply to reality. For example, solar panels need to absorb sunlight, and for higher efficiency they need to reflect very little sunlight. To achieve that solar panels must be treated with special antireflective coatings. The coating under intense heat should be able to perform, but at the same time it has to deal with the pressures of cracking, atomic defects, rain, snow, and… bird poo!! How do you go about including all those things in your testing standard??? This is why testing standard organisations exist, like the International Standards Organisation (ISO), the International Electrotechnical Commission (IEC), the Deutsches Institut für Normung (DIN). These organisations have hundreds of committees with experts who have decades of industry experience that decide the specific test requirements for any electronic part (from batteries to supercapacitors to hard drives and beyond).

No really… Polly has NOT been very nice to solar panels (Public Domain)

Keep in mind that these standards aren’t regulations. Manufacturers just use them to show their products’ quality. But test are costly, and as a result manufacturers use some sort of statistical/simulation-based tools to predict the test outcome early. These types of tools are called ‘prognostics’. Many companies are now developing prognostics to estimate the health of electric vehicle batteries after they’ve been in use. This is especially used when products reach the end of their first lifecycle, in order to predict the possibility and potential efficiency of entering into a second lifecycle (e.g. reusing batteries, or parts of bulky electrical equipment such as laundry machines or fridges).

Prognostic tests can be based on statistical simulations. These statistical models find relationships between data and can extrapolate specific relationships between variables, analyse special statistical distributions like the Weibull distribution, and even use machine learning techniques to classify and forecast data. Still, not all tests and electronics will be ‘fixable’ with statistical prognostics alone. For example, some types of electronic products don’t have ANY failure modes that show up for a certain period of time (longer than 100 hours) (e.g., electrical grid components, like insulated transformers / power cables that often withhold their performance for YEARS). Though burn-in tests for 100 hours would catch defective units for many types of electronics, this means some types of important sustainable technology would need even longer burn-in tests.

This pretty much eliminates any type of statistical model where you want to collect some data and then extrapolate a curve of fit forward to predict results.

If you’ve ever seen equipment at electricity stations with those tower-y spirals (transformer ‘bushings’) at the top, those are transformers. (Public Domain)

There’s also another issue with using statistical prognostics to develop sustainable electronics faster. Some electronics won’t have quantifiable data that you can use for statistical modelling solutions. Two reasons for this are that quantifiable test standards haven’t been developed or the electronic product is too complicated.

For instance, test standards for optical coatings in fibre-optic networks (ie. high-speed Internet) have only been around for about 45 years. Coatings are tested to make sure they can resist scratches and don’t easily come off things they’re applied to (like mirrors and lenses). These tests take hundreds to thousands of hours and some parts are quantifiable (ex. 1000 hours at 85°C and 85% humidity). But other parts are just mere ‘visual’ checks: rub a cheesecloth against a coating 50 times and see if you have scratches. It’s hard to gather data about the effect of cheesecloth rubs though. So statistical models aren’t easy to build here.

In contrast, integrated circuit chips (computer chips) have had testing standards since the 1970s. But each chip has billions of transistors (electronic switches to let current pass through), capacitors (electronic devices to store static charge), and other components. How do you test if one of them is broken? No magical engineering solutions here, unfortunately. Instead, engineers create what are called ‘test patterns’. These are basically different input, output pairs to test on the computer chip. If you input data and get the expected output, the individual computer chip is working. For example, you could write data to the computer chip and then read it back. If the data you read isn’t what you tried to write, you know the computer chip didn’t save the data properly.

BUT data saved isn’t really a ‘physics’ variable that we can quantifiably test like with other electronics. And even if we tried to test variables like temperature, current, etc. — which of the billions of components in a chip do you take readings from?? This is why statistical models are also hard to build here.

In both cases, though, there is an option for carrying out measurements. Instead of collecting data from optical coatings or computer chips, you collect data from the machines that make them. And you use different data from the factory production environment to optimise the production process and MAYBE make some predictions about the functionality of individual batches of electronics. There are companies working on this very complicated approach. But especially with newer sustainable technology (ex: new types of energy storage electronics or energy production electronics), there’s a lot of potential for new growth!

Certainly, there’s a lot of innovation to be had in creating more sustainable technology for the future… but better testing technology is the pre-requisite to unlocking that innovation!

If you have any questions about this article, feel free to email Voltx’s cofounders: Alishba Imran or Shagun Maheshwari!

Thank you to: 🙏

  • Dr. Jeff Jones from the IEC. I wouldn’t have understood the connections between different electronic products without you!
  • Dr. Darayus Patel from the Nanyang Technological University. I’m grateful for all your enthusiastic support in breaking down semiconductor fabrication with me!
  • Dr. Stefaan Vandendriessche from Edmund Optics. I couldn’t have imagined the issues with testing optical coatings without your tip!
  • Dr. Robert Herrick from Intel. I’m amazed by all your selfless support in answering my endless questions about the optoelectronics industry!

Written by Madhav Malhotra, a 17-year-old developer, designer, and entrepreneur-in-training. To find out more about the author, please visit https://www.madhavmalhotra.com/

Testing Barriers Slow Sustainable Electronics Production (Part 3) – Why does EVERY electronic have slow testing?

macro shot of water drops on leaf

If you didn’t catch the previous parts of this series on what electronic tests are and how they relate to sustainability, you can find the first part here! (No really, otherwise you’ll be very confused if you read this article otherwise 😁).

To briefly summarise: improved technology is the key to sustainability–from energy-efficient LEDs to environmentally-friendly batteries! But many tests are needed to improve technology and they look for complicated ‘failure modes’ (the events that lead to electronics breaking).

Given all these unique types of tests, why did I claim up above that ‘EVERY’ electronic has slow testing? Well, ‘EVERY’ electronic from tiny batteries to giant power cables has two important (but slow) types of tests: accelerated lifetime testing and burn-in testing. As I explained in the first article, these tests are often done by manipulating the exact same variables: temperature, humidity, current, and/or voltage higher and performance over time.

To make it less theoretical, I’ll talk about lifetime testing.

Let’s say you’re a brilliant, laser in a LiDAR sensors that many automated electric vehicles rely on to enable cleaner transportation. You are about to be tested to see if you work! You will be subjected to a 75°C temperature (with all your other laser friends) and see how long it takes you to break. Here’s what I’d see:

Figure 1 Lasers’ performance over time under testing conditions. (Adapted from Lawrence A. Johnson, ILX Lightwave, 2006)

This is a curve like many others in electronics testing. It shows how variable X increases and/or decreases as the time variable increases and a device ages. Basically, as you shine longer, you age more and get tired. You need more ‘fuel’ (i.e., current) to keep you going. Eventually, you’ll need so much ‘fuel’ that you just won’t meet the user’s laser shining needs (and these needs can be important to the user, and can be detrimental from an environmental, economic and social perspective)! Even though, from a technical perspective, you may still be functional you may not offer the desired qualities to the end-user.

Different lasers, have different performance. As shown in Figure 1 lasers indicated with a maroon, purple, and pink colours have a 1000+ hours performance, and it is uncertain as to how long it’ll take for their fuel needs to increase considerably. The Arrhenius equation is often used to figure out what the laser lifetime would be at regular temperatures, given what we know about their lifetime at high temperatures. Warning… CHEMISTRY ahead. Do you remember this monstrosity from high school???

This is the Arrhenius Equation, and if you haven’t seen it since your high school chemistry class, fear not! Here’s the oversimplified version:

  • k is how fast atoms move.
  • T is the temperature.
  • k is equal to a complicated mess that we don’t care about. We just need to know k (how fast atoms move) increases as T (temperature) increases.

This allows us to model how fast atoms move at different temperatures (among other things). And remember what I said in the second article in this series: The faster atoms move the faster electronics degrade. So, the Arrhenius equation can be used to model how long it will take for electronics to degrade at different temperatures: Life is inversely proportional to how fast atoms move (how naughty, and chaotic their life can become), which is complicated to calculate.

In fact, this is exactly why we go to such great lengths to add heat sinks or cooling systems to devices. Cool right? Based on chemistry, and a gallop of physics and math we can model how sustainable technologies like, electric vehicles, solar panels, wind mills, would last while being subjected to environmental conditions (e.g. radiation, heat, humidity and wind) for many years.

Now, what does laser Figure 1 tell us about lasers’ performance before the 100 hours pass? That’s where the burn-in testing happens (e.g. every laser is run for 100 hours to find any defects). The issue is that many failure modes for electronics happen ONLY in that first little while of testing (it’s like you only get chickenpox once— usually when you’re young). But for lasers, it’s more like the atoms in the crystal structure ‘dislocate/diffuse’ to the wrong places when they’re young. The problem with that is that it’s pretty hard to predict all those specific one-time failure modes. Burn-in testing is hard to extrapolate which makes it even harder to prevent potential lower quality electronics to be placed on the market.

EVERY product needs lifetime testing and EVERY unit of every product needs burn-in testing for sustainable electronics and electrical equipment to be placed on the market. Even though, there are considerable ‘costs’ (e.g. energy, carbon emissions, operational and maintenance costs) involved in testing millions of electronic and electrical equipment made at factory, these could be significantly lower than the ‘costs’ associated with electronics and electrical equipment that becomes easily obsolete, especially considering the fact that these become waste very quickly and are alarmingly mismanaged. An in-depth system-based sustainability assessment is needed to demonstrate the impacts involved!

By this point in the series, I hope you realise how important technology innovation and testing is for achieving sustainability in the long-term. With this in mind, I’ll finally wrap up the next (and last) part of this series by looking at solutions to all the major problems I’ve highlighted so far! 🛠️💪

Written by Madhav Malhotra, a 17-year-old developer, designer, and entrepreneur-in-training. To find out more about the author, please visit https://www.madhavmalhotra.com/

Testing Barriers Slow Sustainable Electronics Production (Part 2) – What (specifically) do electronics tests measure?

old gear wheel covered with rust

If you didn’t catch the first part of this series on what electronic tests are and how they relate to sustainability, you can find it here! To summarise: there are different types of electronics tests at different parts of the electronics’ life (e.g. initial design vs. end use). They help us improve technology faster. And improved technology is the key to sustainability–from energy-efficient LEDs to environmentally-friendly batteries!

If you’ve read the first article in this series, you might have noticed how a LOT of tests (for everything from hard drives to solar panels to lasers) involves heating them up. It’s one of the most common variables controlled in electronics testing. Why is that? Basically, modern electronics are made with very SPECIFIC chemistry. All the atoms have to be in just the right place (and the places have weird names, like ‘p-n junction’). At higher temperatures, atoms get ‘excited’ and move about more and more. Eventually, they get so ‘excited’ they move away from the places we want them to be. This ‘naughty’, uncontrolled behaviour causes electronics to break. Which is why temperature is an important variable in all tests, and why electronics have cooling fans, heat sinks, etc. in practical applications. 

🔑 FACT: for every 10° C increase in temperature, an electronic can break twice as fast.

Other common variables that can affect the lifetime of many types of electronics are the current and voltage they receive. Current is the flow of electrons, resistance slows down the flow, and voltage speeds up the flow.

A common analogy of voltage, resistance, and current on the internet. (Source: unknown)

Alongside these basic variables measured, there are some that are more specific to the electronic being measured. For instance, batteries measure a variable called self-discharge when they age, which is when a battery loses energy without being plugged in. Over time, this decreases the amount of energy the battery has stored. And this measure of aging is becoming more important with electric cars. You wouldn’t want to buy an electric car if you didn’t know how many years it would take before you’d have to buy a new battery. Right?

Behind all these variables is a complicated physics. It has to do with two key terms: failure modes (events that break an electronic); and failure mechanisms (the causes behind those events). Physics variables can measure when a failure mode occurs. For example, a power cable that transmits electricity underwater would undergo corrosion (failure mechanism). Eventually, there would be so much corrosion that the cable would snap (failure mode). And we would detect the cable snapping when current is no longer flowing through the wire (variable).

This underwater power cable has corroded metal. You can imagine how inconvenient this is to monitor. (Public Domain)

Though we can use current to monitor failure modes of power cables just like we can use it for laser diodes, there are VERY different failure modes for the two electronics. This is the in-depth explanation of something I mentioned in the first article; it’s REALLY hard to test electronics once they’re deployed in the field. For every electronic you want to test, you have to consider ALL the environmental conditions that could trigger ANY of the unique failure modes for that electronic. And you have to simulate the physics of the situation to understand when the failure mode might be triggered!

I hope you now see why electronics testing is a lot more complicated than it first seems. THIS is the complexity that delays electronics testing and slows down rapid innovation for future sustainable technology. In the next part of the series, I’ll describe the SPECIFIC parts of electronics testing which are slow (in case any of you sustainability innovators have ideas for how to speed up the development of green, new technology)!

Written by Madhav Malhotra, a 17-year-old developer, designer, and entrepreneur-in-training. To find out more about the author, please visit https://www.madhavmalhotra.com/

Testing Barriers Slow Sustainable Electronics Production …and they affect EVERY SINGLE device you can imagine.

It currently feels like the words ‘sustainable’ and ‘electronics’ just don’t go together. Every day, there’s a new news report about toxic materials in batteries, or the massive amount of e-waste generated and pollution associated with its mismanagement. Nonetheless, innovation in electronics technology is the KEY to many sustainable innovations: from energy-efficient LEDs to renewable energy from solar panels. And these could lead to future sustainability breakthroughs!

So, what can we do to ensure electronics are developed efficiently? Let’s take batteries. Batteries are needed for all sorts of sustainable innovations: from renewable energy storage to electric vehicles. Yet, their production is onerous process; it can take years from their design to their final acceptance and distribution to the market. Specifically, the manufacture and testing of the performance and safety of a new battery design can make up the longest part of R&D process as shown in Figure 1. So, it’s not that we don’t have better battery technology, it takes years to be released while it’s going through testing!

Most of the time taken to build a new battery is from R&D. Source: Alishba Imran, Voltx

As I’ll soon explain, electronics have testing issues at every step of their development (from the initial design to maintenance while in use).  As a result Incredibly important innovations are being slowed down by testing: solar panels for clean energy, LiDAR for autonomous vehicles, lasers for fibre-optic networks.

This article is the first of a series of four articles on these key questions:

  1. What are ‘electronic tests’ anyways?
  2. What (specifically) do tests measure?
  3. Why is ‘EVERY’ electronic test slow?
  4. How can we speed up testing?

Question #1: What are ‘electronic tests’ anyways?

Certain types of tests are performed in all electronic industries. One of the longest types of testing is reliability testing. I’ll summarise a few reliability tests throughout the device’s life: from designing an electronic to maintaining it when it’s in use. This is shown below:

Overview of the tests I’ll talk about in a product’s life (Source: Madhav Malhotra)

Reliability enhancement tests: These tests happen when electronics are still being designed. The goal is to find the maximum limits of stress (e.g. vibrations, heat, current) that will break a product design. Then, engineers can fix the most common reasons for failure. For example, hard drives (electronics that store data on older computers) have a VERY tiny ‘head’ that reads and stores data on a magnetic disc (details here). It can be as small as a flake of pepper and is suspended over a disc rotating at 130 km/h! Any vibrations can damage this ‘head.’ So engineers should concentrate on trying to design better products by making the head safer.

Engineers have to make this tiny part (as small as a pepper flake) survive for years (CC-BY-2.0)

Accelerated lifetime test: These tests carried out in simulated stressful environments (e.g., 85° C and/or 100% humidity) assess the performance of electronic products before they are placed on the market. Their purpose is to find the specifications to market the product (e.g. warranty and lifetime).

Burn-in / screening tests: These tests happen during the production process. Their purpose is to find units of a product that have manufacturing defects (e.g. cracks, wires not soldered tightly, exposure to contaminants). They do this by setting a ‘challenge’ for all units by forcing them to survive tough conditions for a short amount of time. The theory is that units with defects will degrade in this short amount of time, so they can be separated from functional devices.

Acceptance Test: These tests happen right before the electronics are installed for use. Their purpose is to make sure that products meet the standards they’re supposed to. This is more important for larger electronics (like a factory machine) than consumer electronics.

Maintenance/Field Testing: These tests happen after electronics are deployed on the field. Their purpose is to check electronics’ quality and remaining useful life. Remaining useful life is NOT the same as lifetime. Lifetime means: “My phone battery will last 3 years.” Remaining useful life means: “I’ve used my battery for a year and now it has 2 years left to last.”

Lifetime and remaining useful life are hard to predict. Why? Because the real world is messy! You don’t just have a nice simple lab with exactly 50° C of heat, no weather changes, and no humans dropping, throwing or crashing devices ’accidentally’. It’s hard to account for the probability of ANY of those issues happening sometime in the next year. That’s why startups exist just to predict the remaining useful life of important electronics (like electric car batteries).

Given all these steps, no wonder it’s complicated to figure out where these tests can be sped up! And amidst this complicated confusion, we still continue to see important innovations (like batteries) have slower development… whether that be for storing clean energy or just stopping your phone from dying.

In the next part of this series, I’ll dive deeper into understanding what electronics tests are SPECIFICALLY measuring. That’s the first step before finding possible approaches to speed up testing and start innovating faster!

Written by Madhav Malhotra, a 17-year-old developer, designer, and entrepreneur-in-training. To find out more about the author, please visit https://www.madhavmalhotra.com/

Escalating E-Waste Could Turn An Opportunity into A Threat

Waste electrical and electronic equipment, known as e-waste, is the fastest growing solid waste stream globally. This growth is driven by the increasing economic development, urbanization, industrialization and income on the one hand (Debnath et al., 2018), and the planned obsolescence and modernization that make existing technologies redundant and/or out of fashion on the other (Awasthi et al., 2019). The recent UN’s Global E-waste Monitor 2020 report reveals that in 2019 around 53.6 million metric tonnes (Mt) of e-waste was generated globally[1], of which only 17.4% (9.3 Mt) was formally collected and recycled. The report makes no explicit reference to the fate of the remaining 82.6% of e-waste. It suggests that this may be legally (for refurbishment and reuse, often under false pretense) and illegally exported to developing countries (Forti et al., 2020), with much of the e-waste being non-functional and irreparable ‘e-scrap’ (Hinchliffe et al., 2020).

The irreversible environmental, economic and social negative consequences of e-scrap management in developing countries are well documented in the global literature, as are the opportunities for the informal recycling sector (Awasthi et al., 2019, Hinchliffe et al., 2020). For example, informal workers that live in vulnerable, marginal conditions are highly dependent on the income they earn from the sale of valuable resources e.g. copper and gold and components, they extract from e-waste. This income contributes towards the improvement of their livelihoods, and poverty eradication (Hinchliffe et al., 2020). In addition, the repair and reuse of good quality refurbished equipment can provide an affordable source of ICT equipment to a high number of people giving them access to mobile phones and computer facilities at home, school and businesses. This in turn supports the breakdown of the global ‘digital divide’, creating opportunities for social and economic development.

But, the situation is not so straightforward. E-scrap contains hazardous substances such as lead, mercury or brominated flame-retardants that pose high environmental and health risks if not properly managed. Informal recycling practices are suboptimal and are often carried out under inappropriate working conditions, with devastating environmental, economic and human health impacts. Workers do not have the skills and/or access to environmentally sound technologies and personal protective equipment rendering the management of e-waste in developing countries extremely dangerous and unstainable. The health and environmental implications associated with such practices are mounting in urgency due to the expected increase in the production and shipment of e-waste (Hinchliffe et al., 2020).

Is the breakdown of digital divide and poverty reduction a justification for the increasing production and shipment of e-waste to developing countries in spite of the environmental degradation and health implications? Where is the silver lining to such practices, and how should action be prioritized to reduce the environmentally destructive practices associated with the e-waste management practices? With the global volume of e-waste expected to increase over the next years, a holistic approach must be urgently sought after to identify the right solutions, and avoid the risk of undermining efforts to promote sustainable development alongside the sustainable recovery of resources from e-waste. This requires a holistic understanding of the system, looking at the design, production, use, disposal and management of e-waste, and the balancing of multi-dimensional values that span the political, environmental, economic, social and technical domains (Iacovidou et al., 2017). Currently, much of the attention and discussions are focused on the political and economic spheres that seem to bear little (if any) positive impact in curbing the e-waste management problems. Developing countries are still the backyards of developed ones, serving corporations at the back of impoverished people that seek to improve their well-being. Unless action is taken, the deleterious effects of inappropriate production, use, disposal and management of e-waste will soon become a global threat to our natural, social and economic systems.

A systems based approach could play a key role in understanding the drivers and barriers of e-waste production-use-management system and identifying ways of recovering maximum value for e-waste, whilst inflicting the lowest possible environmental, economic, social and technical impacts (Iacovidou et al., 2017). As described in (Iacovidou et al., 2017), the geographical scale and context and the consideration and selection of values from different stakeholders (incl. consumers and their behavioral traits) and policy-makers perspectives is essential to creating a clear picture of the e-waste issues and enabling the development of an integrated e-waste management strategy centered around the 3R’s principle of reduce, reuse, recycle for recovering maximum value from the e-waste stream, whilst promoting circularity and sustainability (Figure 1).

Figure 1: Overview of the key elements involved in the development of an integrated e-waste management plan

Developing and implementing an integrated e-waste management plan requires data, a good understanding of the relevant ecological, economic, social/ behavioural, political, and organizational drivers, and the development of a supportive regulatory and political landscape to encourage change. To that end, a global multi-national collaboration between regulators and governments, and other stakeholders is needed to revise, reform and promote social security and development, environmental protection and conservation, and regulatory and economic reconstruction of the e-waste production-consumption-management system (Iacovidou et al., 2017). It also requires the development of skills and capacity building to improve product design upstream, and facilities for e-waste management downstream of the e-waste system. This could also involve the employment of new environmentally sound technologies, given that there is space for establishing and maintaining a well-functioning market for sustainable and second-hand electrical and electronic equipment, and recycled materials. This is imperative for averting future irreversible consequences, and ensuring the scientific knowledge sharing, behavioral change based on awareness raising campaigns and good communication techniques; essential ingredients in promoting a sustainable management of e-waste resources alongside efforts to achieve circularity and sustainable development (Awasthi et al., 2019).

[1] Continental contribution: Asia (24.9 Mt), Americas (13.1 Mt), Europe (12 Mt), Africa (2.9 Mt), and and Oceania (0.7 Mt)


AWASTHI, A. K., LI, J., KOH, L. & OGUNSEITAN, O. A. 2019. Circular economy and electronic waste. Nature Electronics, 2, 86-89.
DEBNATH, B., CHOWDHURY, R. & GHOSH, S. K. 2018. Sustainability of metal recovery from E-waste. Frontiers of Environmental Science & Engineering, 12, 2.
FORTI, V., BALDE, C. P., KUEHR, R. & BEL, G. 2020. The Global E-waste Monitor 2020: Quantities, flows and the circular economy potential. Bonn, Geneva and Rotterdam: United Nations University/United Nations Institute for Training and Research, International Telecommunication Union, and International Solid Waste Association.
HINCHLIFFE, D., GUNSILIUS, E., WAGNER, M., HEMKHAUS, M., BATTEIGER, A., RABBOW, E., RADULOVIC, V., CHENG, C., DE FAUTEREAU, B., OTT, D., AWASTHI, A. K. & SMITH, E. 2020. Case studies and approaches to building Partnerships between the informal and the formal sector for sustainable e-waste management. Solving the E-waste Problem (StEP) Initiative.
IACOVIDOU, E., MILLWARD-HOPKINS, J., BUSCH, J., PURNELL, P., VELIS, C. A., HAHLADAKIS, J. N., ZWIRNER, O. & BROWN, A. 2017. A pathway to circular economy: Developing a conceptual framework for complex value assessment of resources recovered from waste. Journal of Cleaner Production, 168, 1279-1288.

Dr Eleni Iacovidou
Division of Environmental Sciences
College of Health, Medicine and Life Sciences
Brunel University London,
Kingston Ln, London,
Uxbridge UB8 3PH, UK

Dr Abishek Kumar Awasthi
School of Environment,
Nanjing University,
Nanjing 210023, China