3D printing reveals the power of shark skin
By Jonathan WebbScience reporter, BBC News
The printed replica included tough \"denticles\" embedded in a smooth, flexible membrane
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Scientists have used a 3D-printed model of shark skin to show how tooth-like scales help the predators to cruise efficiently.
Viewed up close, a shark's skin bristles with tiny teeth or \"denticles\" which aid swimming.
Engineers have tried to mimic the roughness of shark skin when designing swim suits and even racing cars.
But the denticles have never been so well reproduced before, says a report in the Journal of Experimental Biology.
Perhaps counter-intuitively, creating turbulence near the edge of a moving object can reduce drag. In this way, the denticles act like the dimples on a golf ball. Now, researchers have also seen them alter specific currents that help propel the shark through water.
George Lauder and his colleagues took a detailed scan of a tiny square of skin from a mako shark, and built a 3D model of a single denticle just 0.15mm long.
The challenge was then to manufacture a synthetic skin, with thousands of these denticles embedded in a smooth, flexible membrane.
\"It took us about a year,\" said Prof Lauder, of Harvard University.
First the team made a detailed 3D model of a single denticle
3D printing builds up new objects layer-by-layer, following a
computer-generated design. To print the shark skin, the scientists had to use two different materials for the hard, tooth-like structures and for the flexible base - much like the different coloured inks used to print a picture.
The particular shape of the denticles also posed difficulties: \"Because they're overhung, the 3D printers have to print a supporting material, which you then have to remove,\" Prof Lauder told the BBC. \"It took a while to work out all the tricks.\"
The artificial skin has impressed Oliver Crimmen, a fish expert at the Natural History Museum who has previously advised Speedo on swim suit design.
\"I used to think, how on earth would you mimic that complex structure accurately?\" he said. \"3D printing is it - what a marvellous application for it.\"
Because the resolution of even the latest 3D printers is limited, the artificial denticles are about 10 times larger than the real ones seen on the skin of a mako shark.
Nonetheless, when the team stuck the new artificial skin onto a small, flexible paddle and studied it in a water tank, they were able to see the benefit sharks glean from their unusual scales.
A paddle with the new, toothy skin delivered a boost of up to 6.6% in swimming speed, compared to one coated with the smooth membrane alone. The artificial denticles also allowed the paddle to travel the same simulated distance while using 5.9% less energy.
\"That's a huge effect, when factored over the entire lifetime of an animal that is constantly swimming,\" said Prof Lauder.
Sharks' specialised skin appears to help most when they are cruising, rather
than accelerating to catch prey
Mr Crimmen agrees. \"If you think about it, sharks, which don't have a swim bladder... are on the go most of their lives. Swimming's hard work, especially if you're of any size.\"
Interestingly, the advantages were most obvious at relatively slow speeds, when the shark is cruising rather than pouncing. \"It's during the steady, long-distance migrations that you'd really begin to see the benefits,\" Prof Lauder explained.
Using a specialised technique to photograph the flow of water, the team also found that the \"leading edge vortex\by the paddle's movement, was stronger with the denticles than without.
Prof Lauder believes this change in water flow could be crucial. \"It can help suck the fish forward,\" he said. \"One of the things that our flow visualisation has suggested is that the structure of the skin may actually increase the thrust - the engine of propulsion - rather than just reducing the drag.\"
When attached to a paddle and studied in a water tank, the artificial skin strengthened the \"leading edge vortex\" created by a swimming motion. Video courtesy of L. Wen, J. Weaver, G. Lauder
Researchers have studied the fluid dynamics of moulds and real samples of shark skin before. Prof Lauder is especially pleased with this new, 3D-printed model because it moves and bends, just like sharks. \"You have a rigid scale structure embedded into a flexible membrane, that can then swim.\"
Don't expect to be pulling on a denticle-laden swim suit any time soon, however. Transferring this type of design to a textile might take decades, Prof Lauder said. \"But if you could do it, you would see a dramatic effect on swimming performance!\"
The idea of copying design elements from biological systems is known as biomimetics.
3D printing technology has made such mimicry a lot easier and, importantly, it allows the designs to be tweaked. For example, Prof Lauder and his team have already begun to play with the spacing, arrangement and shape of the denticles. \"I want to know what causes this effect,\" he said.
Aside from that curiosity, Prof Lauder enjoys learning from nature. \"It pays us to understand how the natural world works,\" he told BBC News. \"Millions of years of evolution give us solutions to problems that we may not have thought of.\"
3D打印解密鲨鱼皮的作用
BBC新闻
乔纳森·韦伯报导
被打印出的仿制品上有一层坚韧的“齿”，它们嵌入光滑的，柔韧的细胞膜表面。
Figure 1被打印出的仿制品上有一层坚韧的“齿”，它们嵌入光滑的，柔韧的表面。
科学家应用3D打印出的鲨鱼皮模型来研究齿形鳞片是如何帮助捕食者更有效的巡游。
近处观察发现，鲨鱼皮上有很多直立的牙或小齿，这些有助于它在水中游动。
于是，工程们在设计游泳衣甚至赛车时试图模拟鲨鱼皮的粗糙度。
但是小齿在过去从未被仿制的这么好，一位试验生物学日报的记者如是说
或许这是反直觉的，在移动物体边缘制造涡流会减少阻力。在此种情况下，小齿的作用相当于高尔夫球上的坑凹。现在，科研人员认为他们改变了流体的流动从而帮助推动鲨鱼在水中移动。
乔治兰黛和他的同事对灰鲭鲨皮进行了一次小范围详细地扫描，并且建立了只有0.15mm长的单齿3D模型。
接下来的挑战是制造人造皮，就是把成千上万个小齿嵌入到一个柔韧的、光滑的膜上。“这个过程耗时一年，”哈佛大学兰黛教授如是说。
Figure 2制作出的详细的单齿3D模型
3D打印技术是根据计算机设计要求，一层一层堆积的来制造新产品。为了打印出鲨鱼皮，科学家们必须使用两种不同的材料，一种用来制造硬的、齿形的结构，一种用来制造柔韧的基体，就好像是在打印一幅图像时应用不同颜色的墨水。
小齿的特定形状也为研究制造了困难。“因为它们是悬挂式的，3D打印机必须打印出一种支撑结构，而起到支撑作用的这部分材料在稍后的进一步加工时需要被移除，”兰黛博士对BBC这样说。“寻求这些问题的解决办法花了我们一些时间。”
奥利弗克里曼，国家历史博物馆的一名渔业专家对这种人造皮很感兴趣，他曾经建议速比涛在泳装上采用此种设计。
“我过去总是在想，在地球上何种技术可以精确地仿制这么一个复杂的结构？”他说“3D打印技术可以，它的应用是多么惊人啊。”
因为即便是最新的3D打印技术，其分辨率也是有限的，人造小齿大约是实际灰鲭鲨
表皮上小齿的十倍大小。
尽管如此，当团队把人造皮贴到一个小的、柔韧的船桨表面上并在水箱中进行实验时，他们便发现了鲨鱼从这种不寻常的鳞片中得到的益处。
拥有齿形表皮的船桨与普通船桨相比会使船在水中的移动速度得到6.6%的提升。人造小齿也会使船在移动相同的模拟距离时使用节省5.9%的能量。
“对于一个一生都在水中不停游动的海洋动物来说，这影响非常巨大，”兰黛博士说。
Figure 3研究显示，鲨鱼特殊的皮主要是帮助它去巡游，而不是帮它在捕食中加速
克里曼先生同意这种说法。“如果你仔细想一想，鲨鱼没有气囊却必须在它们的一生在水中四处游动。游泳是一件困难的工作，尤其是当你具有各种体型。”
有意思的是，这些益处在相对的低速时比较明显，它们在鲨鱼巡游时而不是在突袭时效果明显。“在稳定的长距离的迁徙中，你才能真正看到这些益处”兰黛博士解释说。
在使用专门技术对水流进行拍照之后，这个团队发现了有齿时“前缘涡”（一种船桨移动过程中产生的小压力的漩涡）比无齿船桨产生的更剧烈。
兰黛博士相信这种在水中的改变是至关重要的。“这会把鱼向前吸”他说。“我们的流动可视化显示的信息之一就是这种皮的结构可能实际上推高了推力—驱动的引擎—而不仅仅是减小阻力。”
Figure 4当把有齿材料贴到船桨上并进行水箱实验时，人造皮会在划动过程中产生更强的前缘涡。G·兰黛，L·温，J·韦弗的图像资料
研究者认真地研究了流体动力学模型和真实鲨鱼皮试样。兰黛博士对他的新研制的3D打印模型很满意，因为它可以像真的鲨鱼那样弯曲、移动。“你把坚硬的鳞嵌入到柔韧的膜上，它们便能在水中游动。”
然而，不要奢望在近期内能穿上满载小齿的泳衣。把这种设计转移到纺织品上可能需要数十年，兰黛博士说。“但是如果你能成功应用这种设计，你就会发现这将对游泳产生引人瞩目的影响。”
从生物系统中提取设计要素被称作仿生学。
3D打印技术使得仿制变得容易得多，它使得我们的设计允许进步优化。例如，兰黛博士和他的团队已经开始考虑小齿的间隔、排布、形状。“我想知道是什么引起了这种结果，”他说。
除了因为好奇，兰黛博士也很享受从自然界中学东西。“这使我们逐渐懂得自然界是如何工作的，”他告诉BBC。“数以百万年的进化给了我们许多未知问题的答案。”
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