In a rubber band–powered airplane, a pretwisted band untwists when the hook used to initially balance its torque is released (see the figure). Such an actuation is based on the elastic recovery of the stretched polymer chains. The material has to be mechanically retwisted to operate but the method is simple and efficient, because the rubber band delivers almost as much energy as needed to twist it. Unfortunately, soft rubber cannot easily provide large stress and cannot be used in modern applications such as robotics, artificial muscles, smart textiles, and new medical devices. But as Haines et al. show on page 868 of this issue (1), the concept of twisted fibers can nevertheless be useful in demanding actuator applications.
The high cost of powerful, large-stroke, high-stress artificial muscles has combined with performance limitations such as low cycle life, hysteresis, and low efficiency to restrict applications. We demonstrated that inexpensive high-strength polymer fibers used for fishing line and sewing thread can be easily transformed by twist insertion to provide fast, scalable, nonhysteretic, long-life tensile and torsional muscles. Extreme twisting produces coiled muscles that can contract by 49%, lift loads over 100 times heavier than can human muscle of the same length and weight, and generate 5.3 kilowatts of mechanical work per kilogram of muscle weight, similar to that produced by a jet engine. Woven textiles that change porosity in response to temperature and actuating window shutters that could help conserve energy were also demonstrated. Large-stroke tensile actuation was theoretically and experimentally shown to result from torsional actuation.
About 60% of graduate students said that they felt overwhelmed, exhausted, hopeless, sad, or depressed nearly all the time. One in 10 said they had contemplated suicide in the previous year.
Like nearly all science graduate students, chemistry blogger See Arr Oh was no stranger to stress. But one afternoon during his third year of a Ph.D. program in organic chemistry, as he sat in his car, he decided the stress might actually be killing him. "All of a sudden I would just be blurred out and tense all over. Everything just seemed to rush at me at once," he says. He went to the emergency room and was told he was suffering from panic attacks.
The 2014 budget suggests that the National Institutes of Health (NIH) is losing its place as first among equals when Congress has additional money to spend on research. Agencies that support the physical sciences grew much faster than did NIH, which receives nearly half of all federal research dollars, and some analysts think that pattern could repeat in 2015.
How do animals see color? We may never know how another animal experiences red or blue, but we do know that sensitivity to ultraviolet light allows bees to see patterns on flowers where we see plain yellow or white. In fact, bee and human color vision are much alike. Both have three spectral types of photoreceptor whose signals are compared by neural “opponent” mechanisms, which are sensitive to the relative amounts of light at different wavelengths, allowing the animal to distinguish the spectrum of a light source from its brightness. Thomas Young (1) recognized in 1802 that having multiple receptors each with a different spectral sensitivity at each point in the image is essential for color vision, but inevitably impairs spatial resolution. He proposed three spectral receptors as the likely compromise. In fact, theory predicts that two to four receptor types are optimal for discriminating the spectra of natural materials and maximizing the number of objects that could be distinguished by color (2). The eyes of many animals seem to follow these principles, with a retina containing two to four spectral receptors combined with a neural mechanism to compare the responses of different receptor types (the color opponent process). It is therefore fascinating to find an animal that sees color in a fundamentally different way, as reported by Thoen et al. on page 411 of this issue (3).
One of the most complex eyes in the animal kingdom can be found in species of stomatopod crustaceans (mantis shrimp), some of which have 12 different photoreceptor types, each sampling a narrow set of wavelengths ranging from deep ultraviolet to far red (300 to 720 nanometers) (1–3). Functionally, this chromatic complexity has presented a mystery (3–5). Why use 12 color channels when three or four are sufficient for fine color discrimination? Behavioral wavelength discrimination tests (Δλ functions) in stomatopods revealed a surprisingly poor performance, ruling out color vision that makes use of the conventional color-opponent coding system (6–8). Instead, our experiments suggest that stomatopods use a previously unknown color vision system based on temporal signaling combined with scanning eye movements, enabling a type of color recognition rather than discrimination.