Tag: babbage

Cell a million

Akshat RathiLeave a comment

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

Akshat RathiLeave a comment

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

Sexual strategies: The numbers game

Akshat RathiLeave a comment

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.