In spite of its intuitive appeal, harnessing the sun's energy using solar cells has proved to be one of the more costly ways of generating renewable energy.
Traditional photovoltaic cells are made from wafers of semiconducting material such as crystalline silicon. The wafers are specially treated to produce an electric field with one surface positively charged and the other negatively charged, forming a so-called p-n junction. The two sides of the wafer can then be wired together into a circuit. When photons hit the cell, electrons are knocked out of the atoms making up the silicon crystal, creating direct current in the circuit.
Different semiconductors require different amounts of energy to knock out an electron, a property known as the band-gap. This means that a given wafer only generates electricity from the portion of sunlight above a certain frequency. The rest is wasted as heat.
Solar cells are an expensive way of generating energy for two reasons. Growing silicon crystals to make the wafers is costly and the efficiency of solar cells is relatively low (they only convert up to about 25% of solar energy into electricity). Efforts to develop more cost-effective solar cells have generally focused either on alternative materials (which tend to be cheaper to produce but less efficient) or improving the efficiency of crystalline cells (typically by stacking wafers made from semiconductors with different band gaps).
However, Harry Atwater, Michael Kelzenberg and their colleagues at Caltech have made progress towards an alternative solution which uses less silicon without sacrificing efficiency. They created silicon nanowires, each containing a p-n junction, measuring 30 – 100 microns in length and just 1 micron in diameter (about 1/100th the width of a human hair). By embedding an array of these wires in polymer, Atwater's team created a solar cell that can absorb up to 85% of sunlight. Although slightly lower than the 87% achieved by a crystalline cell, the wire array uses just 1% of the amount of silicon. In the cells tested, as little as 2% of the array's volume is taken up by silicon.
This came as a surprise to Kelzenberg, who had initially tried growing the wires close together expecting light falling on the spaces between them to be wasted. However, the team managed to achieve high absorption rates with sparsely spaced wires by trapping light in the array. In a recent issue of Nature Materials, they describe three ways of improving absorption by trapping light: using a back-reflector on the base of the array, coating anti-reflective material on the wires and embedding light-scattering particles in the polymer.
Another advantage of the wire arrays is that they can be embedded in thin, flexible sheets of polymer, which could make them cheaper to install and more versatile than traditional crystalline cells. Although the technique has the potential to produce cheaper solar cells by reducing the amount of silicon needed, it has so far been tested only in tiny cells of a few square centimetres. Atwater and his team are now scaling the arrays up to match the size of traditional solar cells. If they can do so successfully, more affordable solar power could be just on the horizon.
Kelzenberg et al. (2010). Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications. Nature Materials, 9, 239-244.
Tuesday 16 March 2010
Thursday 12 November 2009
Spinal circuits and swimming speed
The ability to move depends on networks of neurons in the spinal cord. Recent research in zebrafish larvae has identified topographic patterns of functional organisation within these networks and shed light on the general principles underlying their development.
Previous research by Mclean et al. has revealed topographic patterns of functional organisation in the spinal cord of the larval zebrafish that underlies its ability to swim at different speeds. In particular, slow movements recruit neurons close to the ventral edge of the spinal cord while more dorsal networks become active as speed increases. Now, Mclean and Fetcho have demonstrated a relationship between the temporal order of differentiation of neurons in the map and the development of motor behaviour.
The fact that the most dorsal neurons are the earliest born suggests a developmental process generating these topographical maps. McLean and Fetcho examined this idea in three stages. First, they filmed Zebrafish larvae to examine developmental changes in swimming movements. This showed that the first movements to develop in 2-day old embryos involve large amplitude lateral head and tail movements characteristic of fast swimming. The smaller movements of the tail characteristic of slow swimming only emerge later in the 4-day old larvae. While larvae make the large movements at high frequencies and the small movements at low frequencies, the embryos made the large movements at all frequencies.
In a second stage, the authors used whole-cell patch recordings to see if there are corresponding differences in patterns of motor neuron firing between embryonic and larval swimming. They induced episodes of fictive swimming in paralysed zebrafish using a brief electrical shock and took recordings from peripheral motor nerves. Comparing embryos and larvae demonstrated that embryonic motor patterns has a short burst duration closely resembling that of high-frequency larval swimming. However, low-frequency movements generated different motor patterns in embryos compared to the larva. These findings mirror the behavioural results and show that the motor neuron circuits generating large-amplitude, fast swimming movements in the larvae are present and functional at the embryonic stage.
Finally, McLean and Fetcho examined the role of excitatory interneurons using transgenic fish expressing the photoconvertible protein Kaede to track neuronal differentiation in vivo. Tracking the emergence of spinal circuits directly in this way showed that one class of excitatory premotor interneurons (circumferential descending cells or CiDs), which are active during escape and fast swimming movements, emerge very early at dorsal locations with younger neurons appearing more ventrally. Another class of excitatory premotor interneuron (multipolar commissural descending cells or MCoDs), however, which are active only at very slow swimming speeds, differentiate very late and are located most ventrally.
These results demonstrate that spinal circuits responsible for fast escape and swimming movements in zebrafish larvae develop first in dorsal locations. Those neurons generating slower movements are layered on below during development, resulting in topographic patterns of recruitment seen in larvae. McLean and Fetcho suggest that this pattern of development may generalise to other tetrapods, including humans where the earliest movements in utero are large-scale startle responses.
Wednesday 11 November 2009
100 years of drosophila mutant research
In 1910, Thomas Hunt Morgan discovered the white-eyed drosophila mutant and located it to the sex chromosome. Drosophila's contribution to understanding the relationship between genes and behaviour began in the early 1970s when Seymour Benzer and his colleagues isolated mutations that affected circadian rhythms. Later experiments on classical conditioning led to the discovery of a mutant that has specific impairments in learning and memory, which were subsequently found to be caused by impairments to the cyclic adenosine monophosphate (cAMP) signalling pathway. Further screens detected and isolated mutations that affect courtship behaviour, movement, visual perception and ageing leading up to present day research on the role of molecular mechanisms in cognition and behaviour.
Tuesday 5 August 2008
Cannabis might ease the pain
Marijuana may provide symptomatic relief of neuropathic pain associated with HIV, according to a new clinical trial. Participants who smoked cannabis cigarettes experienced greater pain reduction than those who smoked placebo cigarettes with cannabinoids (the psychoactive components of marijuana, including THC) removed.
The results should be interpreted with caution since just 28 HIV patients were examined, according to Prof Igor Grant, director of the Center for Medicinal Cannabis Research at UCSD, which funded the research. However, he continued to say that "converging evidence from several studies point in the same direction ... in the short-term smoked cannabis does have a beneficial effect on neuropathic pain".
Neuropathy is chronic pain due to nerve damage which, in HIV, is caused both by the illness and by common treatment. It affects 30% or more of HIV patients and "is a chronic and significant problem in HIV patients as there are few existing treatments that offer adequate pain management to sufferers", said Prof Ronald Ellis, who led the study.
The trial, conducted at the University of California, San Diego School of Medicine, examined HIV patients whose pain had not responded well to other drugs, opiates included. Since the patients continued taking their conventional medicines, Grant said the study can only tell researchers about the additional (or adjunctive) effect of cannabis on neuropathic pain.
The patients were randomly selected to smoke cannabis or placebo cigarettes over five day periods and the outcome was assessed used standardised tests of pain relief experienced by the patients. To guard against bias, the study used a double-blind approach where neither the patients nor the experimenters knew whether any given patient was smoking cannabis or placebo cigarettes. However, most of the patients taking cannabis guessed that they were not taking a placebo.
Published in the journal Neuropsychopharmacology, the trial showed that 46% of the cannabis smokers reported clinically meaningful pain relief (30% reduction) as compared to 18% of the placebo smokers.
Cannabionids relieve pain by acting on pain receptors in the human nervous system where they "mimic the effect of endocannabinoids which are released in a protective manner" to combat pain, said Roger Pertwee, professor of Neuropharmacology at the University of Aberdeen. According to Grant, the UCSD researchers are interested in smoking cannabis since THC taken orally is irregularly absorbed into the bloodstream.
The relatively small scale of the trial "would be enough for a scientific study but drug licensing bodies would require many more", said Pertwee who is also Director of Pharmacology at GW Pharmaceuticals which produces Sativex, a cannabinoid drug for pain relief in multiple sclerosis and certain cancer patients. Sativex is approved in Canada but is not licensed in the UK.
The results should be interpreted with caution since just 28 HIV patients were examined, according to Prof Igor Grant, director of the Center for Medicinal Cannabis Research at UCSD, which funded the research. However, he continued to say that "converging evidence from several studies point in the same direction ... in the short-term smoked cannabis does have a beneficial effect on neuropathic pain".
Neuropathy is chronic pain due to nerve damage which, in HIV, is caused both by the illness and by common treatment. It affects 30% or more of HIV patients and "is a chronic and significant problem in HIV patients as there are few existing treatments that offer adequate pain management to sufferers", said Prof Ronald Ellis, who led the study.
The trial, conducted at the University of California, San Diego School of Medicine, examined HIV patients whose pain had not responded well to other drugs, opiates included. Since the patients continued taking their conventional medicines, Grant said the study can only tell researchers about the additional (or adjunctive) effect of cannabis on neuropathic pain.
The patients were randomly selected to smoke cannabis or placebo cigarettes over five day periods and the outcome was assessed used standardised tests of pain relief experienced by the patients. To guard against bias, the study used a double-blind approach where neither the patients nor the experimenters knew whether any given patient was smoking cannabis or placebo cigarettes. However, most of the patients taking cannabis guessed that they were not taking a placebo.
Published in the journal Neuropsychopharmacology, the trial showed that 46% of the cannabis smokers reported clinically meaningful pain relief (30% reduction) as compared to 18% of the placebo smokers.
Cannabionids relieve pain by acting on pain receptors in the human nervous system where they "mimic the effect of endocannabinoids which are released in a protective manner" to combat pain, said Roger Pertwee, professor of Neuropharmacology at the University of Aberdeen. According to Grant, the UCSD researchers are interested in smoking cannabis since THC taken orally is irregularly absorbed into the bloodstream.
The relatively small scale of the trial "would be enough for a scientific study but drug licensing bodies would require many more", said Pertwee who is also Director of Pharmacology at GW Pharmaceuticals which produces Sativex, a cannabinoid drug for pain relief in multiple sclerosis and certain cancer patients. Sativex is approved in Canada but is not licensed in the UK.
Wednesday 30 July 2008
Bees behave like criminals and could help catch them
New research using bumblebees to refine and test a technique used to catch serial killers has implications for wildlife conservation, fighting malaria and combatting bioterrorism.
Writing in the Journal of the Royal Society Interface, Biologists Dr Nigel Raine and Dr Steven Le Comber at Queen Mary, University of London used a technique called geographic profiling (GP) originally developed by Professor Kim Rossmo from Texas State University, a former detective and co-author, to help police narrow down suspects by looking at the locations of serial crimes.
Although it has been successful in practice, it is difficult for criminologists to test and improve GP scientifically because they can't ethically conduct controlled studies of criminals. But the better the technique, the more precisely the police can locate the criminal.
So Raine and his colleagues took GP into the laboratory and found that it helped to predict the entrance to a beehive from the locations of the artificial flowers visited by the bees. Furthermore, the technique allowed the researchers to distinguish different foraging strategies that the bees might be using. According to the study, for example, one likely strategy simply involves choosing the nearest new flower at each step from the nest.
The researchers say their findings have implications for bee conservation. Although there are 25 species of bumblebee in the UK, most of these are in decline as changes in agriculture have eroded their natural habitats. "Since records began, 3 bumblebee species have already become nationally extinct" says Raine. The great yellow bumblebee (Bombus distinguendus), for example, one of 5 species officially listed as priorities for conservation action by the government, is now found in only a handful of places such as the outer Hebrides. GP could help conservationists locate bumblebees' nests, a difficult task according to Raine, as they are usually underground and the bees may travel up to 2km from the nest.
Although this is the first controlled study of GP, Raine explained that "the approach works well for very different animals: from bees and bats to great white sharks". Following on from this work, the scientists plan to use GP "to study the distribution patterns of illegal snares in Zimbabwe" and to examine "how geographic profiling might assist in efforts to fight malaria by locating breeding sites".
GP uses two opposing insights to create a geoprofile that shows where a serial criminal is likely to live. Most crimes are committed near an anchor point, usually the criminal's home. 70% of arsons happen within 2 miles of the arsonists home, for example. However, there is also a buffer zone centred on the anchor point where there are few opportunities to commit a crime. For the bees, there is a energy cost attached to foraging further from the nest while the buffer zone might reflect the cost of attracting parasites and predators to the nest, according to Raine.
The researchers point out that while criminologists have no control over where crimes are committed, biologists can test GP under different conditions by experimentally controlling factors such as the distribution and density of the flowers. According to Rossmo, the research is relevant to analysing "track data from GPS monitored offenders" and "provides a foundation for possible future applications of geographic profiling to bioterrorism".
Writing in the Journal of the Royal Society Interface, Biologists Dr Nigel Raine and Dr Steven Le Comber at Queen Mary, University of London used a technique called geographic profiling (GP) originally developed by Professor Kim Rossmo from Texas State University, a former detective and co-author, to help police narrow down suspects by looking at the locations of serial crimes.
Although it has been successful in practice, it is difficult for criminologists to test and improve GP scientifically because they can't ethically conduct controlled studies of criminals. But the better the technique, the more precisely the police can locate the criminal.
So Raine and his colleagues took GP into the laboratory and found that it helped to predict the entrance to a beehive from the locations of the artificial flowers visited by the bees. Furthermore, the technique allowed the researchers to distinguish different foraging strategies that the bees might be using. According to the study, for example, one likely strategy simply involves choosing the nearest new flower at each step from the nest.
The researchers say their findings have implications for bee conservation. Although there are 25 species of bumblebee in the UK, most of these are in decline as changes in agriculture have eroded their natural habitats. "Since records began, 3 bumblebee species have already become nationally extinct" says Raine. The great yellow bumblebee (Bombus distinguendus), for example, one of 5 species officially listed as priorities for conservation action by the government, is now found in only a handful of places such as the outer Hebrides. GP could help conservationists locate bumblebees' nests, a difficult task according to Raine, as they are usually underground and the bees may travel up to 2km from the nest.
Although this is the first controlled study of GP, Raine explained that "the approach works well for very different animals: from bees and bats to great white sharks". Following on from this work, the scientists plan to use GP "to study the distribution patterns of illegal snares in Zimbabwe" and to examine "how geographic profiling might assist in efforts to fight malaria by locating breeding sites".
GP uses two opposing insights to create a geoprofile that shows where a serial criminal is likely to live. Most crimes are committed near an anchor point, usually the criminal's home. 70% of arsons happen within 2 miles of the arsonists home, for example. However, there is also a buffer zone centred on the anchor point where there are few opportunities to commit a crime. For the bees, there is a energy cost attached to foraging further from the nest while the buffer zone might reflect the cost of attracting parasites and predators to the nest, according to Raine.
The researchers point out that while criminologists have no control over where crimes are committed, biologists can test GP under different conditions by experimentally controlling factors such as the distribution and density of the flowers. According to Rossmo, the research is relevant to analysing "track data from GPS monitored offenders" and "provides a foundation for possible future applications of geographic profiling to bioterrorism".
Monday 28 July 2008
Is generosity just showing off?
Ever wondered why men usually pay the bill on a first date? An evolutionary psychologist might point out that men on dates also tend to leave unusually large tips and that gives us an important clue.
More specifically, two related evolutionary theories could give us the answer. The first, sexual selection, predicts that because females have a greater investment in parenting, they tend to prefer males who seem likely to provide resources and commitment. Males, meanwhile, have co-evolved to behave so as to give every appearance of being able to provide and care for their partner.
The problem is that, where behaviour is concerned, appearances can be deceptive. Is he really so wealthy and caring? Costly signalling theory, however, predicts that costliness is a reliable indicator of honest behaviours. The more it costs a man to appear capable and committed, for example, the more likely that he really is.
These theories might explain why male chimpanzees use food-sharing to show off to sexually receptive females. But do they really tell us anything about the way we behave? Wendy Iredale, at the University of Kent, and her colleagues decided to investigate by looking at charity donations in the presence of attractive members of the opposite sex.
They asked men and women (heterosexual undergraduates specifically) to play a series of games in which they could earn up to £24. They were then given the option of making a costly act of generosity - donating a percentage of their earnings to charity - either in private or observed by an attractive man or woman.
The results, published last week in the journal Evolutionary Psychology, showed that men are substantially more charitable when observed by the attractive female. As predicted by evolutionary theory, women's donations did not vary. However, scientists still don't know what exactly this generosity signals (caring or wealth, for example) or whether women actually find altruistic men more attractive.
Sexual selection also predicts that males evolve a preference for mates who appear to be fertile and, correspondingly, that females will co-evolve attributes that reliably signal their fertility. An example is breast size and symmetry which, so the argument goes, are honest indicators of fertility. And in case anyone needs convincing that men tend to respond to this particular adaptation, Nicolas Guegen from the Universite de Bretagne Sud studied drivers passing an attractive female hitch-hiker. The results, published in Perceptual and Motor Skills last December, showed that men are more likely to stop if her bra cup size appears to be C rather than A. Women stopped equally frequently in either case.
So what does all this tell us? Obviously, we've also evolved the intelligence and capacity for self-reflection to rise above these kinds of thing to a large extent. They don't fundamentally limit the decisions we make and nor do they excuse poor behaviour. But it's interesting to see residual influences of our evolutionary past still cropping up and being used to test general theories of behavioural evolution. Does anyone have any other examples?
More specifically, two related evolutionary theories could give us the answer. The first, sexual selection, predicts that because females have a greater investment in parenting, they tend to prefer males who seem likely to provide resources and commitment. Males, meanwhile, have co-evolved to behave so as to give every appearance of being able to provide and care for their partner.
The problem is that, where behaviour is concerned, appearances can be deceptive. Is he really so wealthy and caring? Costly signalling theory, however, predicts that costliness is a reliable indicator of honest behaviours. The more it costs a man to appear capable and committed, for example, the more likely that he really is.
These theories might explain why male chimpanzees use food-sharing to show off to sexually receptive females. But do they really tell us anything about the way we behave? Wendy Iredale, at the University of Kent, and her colleagues decided to investigate by looking at charity donations in the presence of attractive members of the opposite sex.
They asked men and women (heterosexual undergraduates specifically) to play a series of games in which they could earn up to £24. They were then given the option of making a costly act of generosity - donating a percentage of their earnings to charity - either in private or observed by an attractive man or woman.
The results, published last week in the journal Evolutionary Psychology, showed that men are substantially more charitable when observed by the attractive female. As predicted by evolutionary theory, women's donations did not vary. However, scientists still don't know what exactly this generosity signals (caring or wealth, for example) or whether women actually find altruistic men more attractive.
Sexual selection also predicts that males evolve a preference for mates who appear to be fertile and, correspondingly, that females will co-evolve attributes that reliably signal their fertility. An example is breast size and symmetry which, so the argument goes, are honest indicators of fertility. And in case anyone needs convincing that men tend to respond to this particular adaptation, Nicolas Guegen from the Universite de Bretagne Sud studied drivers passing an attractive female hitch-hiker. The results, published in Perceptual and Motor Skills last December, showed that men are more likely to stop if her bra cup size appears to be C rather than A. Women stopped equally frequently in either case.
So what does all this tell us? Obviously, we've also evolved the intelligence and capacity for self-reflection to rise above these kinds of thing to a large extent. They don't fundamentally limit the decisions we make and nor do they excuse poor behaviour. But it's interesting to see residual influences of our evolutionary past still cropping up and being used to test general theories of behavioural evolution. Does anyone have any other examples?
Friday 22 February 2008
The art of gliding
How gliding mammals avoid crash landings
The development of the aeroplane owes much to its humbler relative, the glider. The Wright brothers explicitly acknowledged the efforts of nineteenth century enthusiasts such as Otto Lilienthal and Sir George Cayley. However, other mammals first evolved the ability to glide many millions of years ago. For a tree-dwelling creature, gliding offers considerable advantages in terms of conserving energy, avoiding injury and evading predators.
An example is the colugo. One of the closest evolutionary relatives to primates, colugos are also known as flying lemurs although they are neither lemurs, which are primates, nor do they fly. These squirrel-sized beasts are one of the more successful mammalian gliders, using membranes of skin between their outstretched limbs to cover distances of 150 metres between trees (sometimes even carrying young).
However, in order to enjoy the benefits of gliding, mammals such as the colugo must actively manipulate forces during take-off and landing to reach their target and avoid injury due to high-impact landings. To date, most research has been conducted under artificial conditions in the laboratory and, consequently, has failed to shed light on how they achieve these feats. First, there are discrepancies in the relative magnitude of take-off and landing forces observed depending on the rigidity and orientation of the artificial platforms. No-one knows what colugos use in their natural environment. Second, take-off and landing forces increase with distance travelled. However, this may be due to the small range of short distances examined since aerodynamic theory predicts that gliders reach an equilibrium velocity (where weight is balanced by drag) beyond which such forces no longer increase.
The fact that gliding mammals are nocturnal has hampered previous efforts to study them in the wild using videography. However, in a paper published in the Proceedings of the Royal Society B, Greg Byrnes from the University of California, Berkeley, and his colleagues made novel use of technology to discover how colugos glide in their natural habitat. The researchers captured colugos in Singapore and, before returning them to the wild, glued tiny rucksacks to their backs. These contained an electronic accelerometer, similar to those used in the Wii computer game controllers, which records 3-d acceleration. Also included was a radio tag, allowing Byrnes to track the whereabouts of each animal as well as recover the device when it eventually fell off (between one and four weeks later).
In contrast to some laboratory studies, the data retrieved from these gadgets indicated that landing forces (up to 17 times the animal’s body weight) exceed take-off forces (up to 13 times thereof) suggesting that colugos mainly encounter rigid landing surfaces. Furthermore, over the wider range of natural distances studied, take-off forces did not vary with glide duration. Take-off speed varied widely, probably reflecting natural variation in the stiffness and orientation of the branches colugos glide from.
However, the most surprising outcome was that landing forces show an inverse relationship with glide duration: longer glides result in lower landing forces, in contrast both with previous laboratory research and aerodynamic theory. It turns out that this counterintuitive relationship is due to aerial braking: colugos change their body posture dynamically in order to slow down throughout a glide. This is particularly significant just before landing and allows the colugo to reduce its speed by 60% on average. Short glides simply do not provide enough time to change body posture and result in high impact landings.
Byrnes and his colleagues suggest that this ability to brake forms a key step in the evolution from leaping to gliding. By using the accelerometer to study and compare other species in their natural habitats, they hope to discover more about the evolution of mammalian gliding.
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