Cell a million

Solar cells were once a bespoke product, reserved for satellites and military use. In 1977 a watt of solar generating capacity cost $77. That has now come down to about 80 cents, and solar power is beginning to compete with the more expensive sort of conventionally generated electricity. If the price came down further, though, solar might really hit the big time—and that is the hope of Henry Snaith, of Oxford University, and his colleagues. As he described recently in Science, Dr Snaith plans to replace silicon, the material used to make most solar cells, with a substance called a perovskite. This, he believes, could cut the cost of a watt of solar generating capacity by three-quarters.

When light falls on a solar cell, it knocks electrons away from the cell’s material and leaves behind empty spaces called holes. Electrons and holes then flow in opposite directions and the result is an electric current. The more electrons and holes there are, and the faster they flow, the bigger the current will be.

Perovskites are substances composed of what are known as cubo-octahedral crystals—in other words, cubes with the corners cut off. They thus have six octagonal faces and eight triangular ones. Perovskite itself is a naturally occurring mineral, calcium titanium oxide, but lots of other elemental combinations adopt the same shape, and tinkering with the mix changes the frequency of the light the crystal absorbs best.

Dr Snaith’s perovskite is a particularly sophisticated one. It has an organic part, made of carbon, hydrogen and nitrogen, and an inorganic part, made of lead, iodine and chlorine. The organic part acts as a dye, absorbing lots of sunlight. The inorganic part helps conduct the electrons thus released.

It is also cheap to make. Purifying silicon requires high (and therefore costly) temperatures. Dr Snaith’s perovskite can be blended at room temperature. Laboratory versions of cells made from it cost about 40 cents per watt (ie, about half the cost of commercial silicon-based solar cells). At an industrial scale, Dr Snaith expects, that will halve again.

There are caveats, of course. The new perovskite is such a recent invention that its durability has not been properly tested. Many otherwise-promising materials fail to survive constant exposure to the sun, a sine qua non of being a solar cell. And the process of converting a laboratory-made cell into a mass-manufactured one is not always straight forward.

If it leaps these hurdles, though, Dr Snaith’s material will be a strong challenger for silicon. As solar power-generation becomes a mainstream technology over the next few years, the once-strange word “perovskite” may enter everyday language.

First published on economist.com.

Precision strike

In the last 100 years antibiotics have saved millions of lives. However, they are indiscriminate weapons: they kill useful bacteria, such as those in the human gut which extract nutrients, as efficiently as they kill the nasty disease-causing sort. Ben Feringa, of the University of Groningen, in the Netherlands, and his colleagues have devised a method to make antibiotics more selective.

As they report in Nature Chemistry, this was achieved by slapping chemical structures called diazo compounds to a class of antibiotic called quinolones, developed to treat urinary tract infections. Different diazo compounds absorb particular wavelengths of light (it is these structures which give dyes their distinctive colour). They then added the mixture to a Petri dish containing the bacterium E. coli, shielded part of the dish and irradiated the uncovered parts with ultraviolet light. Two of the nine compounds they tested worked as hoped: whereas E. coli remained abundant in the covered parts of the Petri dish, the sections exposed to the light were almost entirely cleared of bacteria.

Crucially, in the half hour or so after exposure to the light, the diazo groups lose energy and revert to their original structure. This, in turn, switches the antibiotic off again. That way, when the antibiotic travels to other parts of the body or is excreted, it does so in its inactive form, and thus remains harmless to the friendly bacteria living in the gut. (And beyond: excreted antibiotics that make their way into sewers have been blamed for spurring the development of drug-resistant bacteria in the wild.)

The brief active window does, however, mean that Dr Feringa’s drug would only be useful in fighting localised infections, where a half-hour antibiotic raid is plenty, rather than more general ones which require a sustained onslaught. And though light can, in principle, be delivered deep inside the body using an endoscope (as happens in some cancer treatments), this is finicky and expensive in practice. As a result, light-activated antibiotics would probably be limited to easily accessible infections on the skin (in wounds, for instance) or in the mouth, ears or nose.

At least for now, that is, for Dr Feringa is working on flipping his chemical switch using sources of energy found inside the body, such as heat or certain energy-rich enzymes. If he succeeds, incidents of antibiotic friendly fire may be consigned to the history of antibacterial warfare.

First published on economist.com.

Image credit: Ben Feringa

Slo-mo mojo

FLIES live shorter lives than elephants. Of that there is no doubt. But from a fly’s point of view, does its life actually seem that much shorter? This, in essence, was the question asked by Kevin Healy of Trinity College, Dublin, in a paper just published in Animal Behaviour. His answer is, possibly not.

How animals perceive time: Slo-mo mojoThe Economist, 21 September 2013.

Image credit: The Economist

The backtrackers

“Viewed as a geometric figure, the ant’s path is irregular, complex, and hard to describe,” wrote Herbert Simon, an American psychologist. But, he added, this is really down to a “complexity in the surface of the beach, not the complexity in the ant”. Or is it?

Ants have the animal kingdom’s biggest brains, relative to their bodies. Brains account for up to 15% of an ant’s total mass in some species (humans weigh in at a meagre 2%). This goes some way to explaining their uncanny knack for finding their way back home from foraging forays. But entomologists have, like Simon, long believed that this apparently complex behaviour is the result of sticking to a handful of simple rules. Ants keep track of distance (for example with an internal pedometer) and direction (based on the position of the sun or scent, of pheromones, for example). If they lose it, they switch to a second startegy and move in a spiral around a centre they think is the nest. If the nest is not found in the first one, then the ant increases the radius and tries another.

Now Antoine Wystrach, of the University of Sussex, proposes that there is more to ants than mindless adherence to simple instincts. As he and his colleagues report in the Proceedings of the Royal Society, at least one species of ant appears to display some hallmarks of intelligence: the ability to integrate different strategies based on experience.

Dr Wystrach captured Melophorus bagoti ants just before they reached their nests after a foraging trip and shunted them into straight tubes which led to random spots about 50 metres away. On exiting the tube, the ants invariably turned around and headed straight back in the direction of the nest. They could not be following the pheromone trail, as that was enclosed in the tube. Nor were they using visual cues: they appeared to backtrack just as well with with their eyes closed. (Cruel as it may sound, the researchers used an opaque paint to cover ants’ eyes.) In other words, the ants appeared to have some sort of internal compass.

Oddly, ants do not always use this device. When Dr Wystrach put ants into the tube when they were farther than two metres away from their nest, however, they used the two basic methods to find home. But when he repeated the procedure, but dropped the ants close to the nest for a few seconds before setting them down the tube, they backtracked just as they had in the first experiment. Backtracking, in other words, appeared to be triggered only when ants possessed a recent memory of their nest. Complexity in the ant, it seems, is a tad greater than Simon would have allowed.

First published on economist.com.

Image credit: Reverend Barry

Midday meals for schoolchildren in India: More good than harm

On July 16th at least 23 children in the Indian state of Bihar died after eating a midday meal that was provided for free by their school. Nearly as many are in critical condition in a local hospital. Tests have revealed that adulterated cooking oil, perhaps containing pesticides, is likely to blame. A government inquiry has determined that the principal of the school, who is in hiding, must be held responsible for the bad ingredients or unsafe methods used in preparing these meals.

This event is horrific, without a doubt. Yet its damage could be even worse, if it raises too many doubts about the value of a largely successful programme. The midday-meal scheme, which began on a small scale decades earlier, received the support of India’s Supreme Court in 2001. Since then most Indian states have adopted it, offering free meals to children in state-run or state-assisted schools. More than 120m children, including many who would otherwise go hungry, receive these meals every school day.

According to a recent analysis by Farzana Afridi of Syracuse University and the Delhi School of Economics, at a cost of three cents per child per school day, the scheme “reduced the daily protein deficiency of a primary-school student by 100%, the calorie deficiency by almost 30% and the daily iron deficiency by nearly 10%.” Ms Afridi also found that, after controlling for all other factors, the meals scheme has boosted the school attendance of girls by 12%. Abhijeet Singh of Oxford University found that, in some parts of India where children were born during a drought, the health of those who had been brought into the meals scheme before the age of six was compensated for earlier nutritional deficits.

What the disaster in Bihar has done, at the very least, is to highlight some of the pitfalls of the scheme. As with any programme of this size in a country rife with corruption, the meals scheme is riddled with problems. The corruptible state is not alone in funding the programme; it is joined by private companies and NGOs. Corruption exists not just among state entities but among the supporting agencies too, as was demonstrated in 2006 when a Delhi NGO was caught dipping into rice that was meant for midday meals. In the states of Bihar and Uttar Pradesh, where the levels of malnutrition are among the highest in the country, it was found that only three-fourths of the food meant for children reached them. Food is often stolen by the administrators’ faking their students’ attendance. Beyond that, reports of adulteration—not only with shoddy or unsafe foodstuffs, but including finding worms, lizards and snakes—are common.

Next month, the Indian government will be voting on a food security bill which aims to provide food to 60% of the entire population, by means of a public distribution system. This one school’s tragedy comes at an especially crucial moment, when officials ought to be forced to inspect the leaky pipeline of distribution. At the same time it will be important to bear in mind: This scheme has done a lot more good than harm.

First published on economist.com.

Image credit: GlobalPartnership for Education

Social entrepreneurs in India: Water for all

Nearly three-fourths of all diseases caused in India are due to water contaminants. Despite that, one in eight Indians still lacks access to clean drinking water. The poor now realise that paying for clean water can save much more in health-care costs later. It was this market that Sarvajal, a social enterprise in India, wanted to cater to.

Founded in 2008, Sarvajal—which in Sanskrit means “water for all”—now sells clean drinking water to more than 70,000 people in rural India. In bigger villages, it employs local people to man filtration plants and sell water. In small villages it installs solar-powered water dispensing machines (pictured) that use prepaid (or pay-as-you-go) smart cards that can be topped up just like a mobile phone. The machines send data to a central server via SMS, which helps Sarvajal ensure regular supply of clean water.

Sarvajal started with some help from the Piramal Foundation, a charity. And it is not alone: Water Health International was launched with an investment from the Acumen Fund and the Naandi Foundation’s not-for-profit company was backed by a charity with the same name. What sets Sarvajal apart is that it has stayed away from government subsidies while still keeping the price of water low. It sells 10 litres of water for four pence (or six cents), just as much or lower than its competitors.

“Subsidies are not a long-term solution,” says Anand Shah, Savajal’s founder, who grew up in America and moved to India to become a social entrepreneur. It took a healthy bit of tinkering to lower the price of installation and maintenance for its water supply infrastructure. It costs on average $2,500 to install a filtration plant, which is about half the expense of similar projects. Sarvajal claims to recover those costs within three years.

Setting up its project was not easy. Savajal needed to deal with things that few businesses in rich countries have to worry about: lack of proper roads in villages, irregularity of power supply, unreliability of water sources and devising a system of money transfer. Having reached a respectable size, Mr Shah is hopeful that scaling up his business further will be less challenging.

Apart from villages, Sarvajal’s other obvious market is the urban poor. Nearly 100m people live in very densely populated slums in India’s cities. They are more willing to pay a higher price for water than villagers who have a much smaller disposable income. But Mr Shah says that “water barons”, sellers of bottled-water, have been trying to block Sarvajal’s entry into cities. After many months of efforts, this time not without help from the government, Sarvajal will soon be launching its first filtration plant in Delhi.

First published on economist.com.

Image credit: Sarvajal

Sexual strategies: The numbers game

In 1948 Angus John Bateman, an English geneticist, proposed that females invest more in producing and caring for their offspring than males because sperm are cheaper than eggs. Since then, however, many species, in particular egg-laying ones, have been found to violate what became known as Bateman’s principle. Such role reversal has left evolutionary biologists baffled.

Some suggeseted that species in which females lay eggs that are big compared to their bodies may need more time to recover after laying eggs and males perform nest chores to compensate. Others fingered high levels of nest predation, which prompts females to seek more males to mate with, in order to produce more offspring, and leave nests untended; again, males pick up the slack. Neither hypothesis had robust data to back it up.

In 2000 Tamas Szekely, an ornithologist at the University of Bath, put forward an alternative explanation. What determines the role adopted by each sex, Dr Szekely contends, is the ratio of males to females. Typically, females outnumber males. This means a male mates with a female once and goes off in search of another willing partner, leaving the mother to tend the nest. Where the ratio favours males, however, the fathers might care for the young rather than face stiff competition to woo another female. Since the supply of males is low, females compete for them instead.

This idea remained untested, however, mainly because finding reliable data on animal sex ratios is tricky. But Andras Liker, Dr Szekely’s colleague at the University of Sheffield, believes he has found some. For over 20 years researchers around the world have been painstakingly collecting data on waders. Studies by Dr Liker and Dr Szekely showed that the data were good enough to test the sex ratio hypothesis.

As they report in Nature Communications, wading birds’ sex roles are indeed correlated with the sex ratio in 16 of the 18 species they tested. In the five species in which females outnumber males (ruffs and northern lapwings, for instance) mothers care for their brood. In the 11 male-dominant species, including Jesus birds and greater painted snipes, by contrast, it is the fathers who look after the nestlings.

Sex ratios are, of course, in part determined by precisely the sort of behavioural traits Drs Szekely and Liker strive to explain. The reason this does not lead to a chicken-and-egg problem, as it were, is that sex ratios are also a function of other factors, like different mortality rates among adult males and females, themselves the result of things like body size.

Dr Szekely’s idea may help explain why sex-role reversal seldom happens in mammals, where sex ratios tend to favour females (though mammalian males also lack females’ ability to produce milk). It might even, Dr Liker speculates, shed light on other social behaviour in animals, such as homosexual pairing, possibly triggered byof a shortage of available partners of the opposite sex.

First published on economist.com.

Image from Greg Schneider