在这个系列中，一个3D打印机多材料形状记忆的迷你钳子中有形状记忆铰链，自适应触摸技巧和一个螺钉。来源：照片由气（凯文）提供；知识共享署名为非商业性无衍生产品许可证。
从麻省理工学院和新加坡科技设计大学（SUTD）来的工程师们是利用光打印三维结构，来“记住”原来的形状。即使在被拉伸，扭曲，和在极端角度弯曲，从小型线圈和多花的结构，到埃菲尔铁塔一英寸高的复制品---在被加热到一定温度“甜蜜点”时就会恢复到他们原来的形式。
对于一些结构，研究人员能够打印微米尺度，它就如同人类头发直径的尺寸一样小，在尺寸上至少有十分之一可以和别人已经能够实现打印的形状记忆材料一样大。该小组的结果发表在本月早些时候在网上期刊科学报告。
麻省理工学院机械工程系副教授Nicholas X. Fang，说形状记忆聚合物，可以预见变形对温度的响应，这对从使太阳能电池板朝向太阳的软驱致动器，到当感染早期征象时将微小的药物胶囊打开这些大量的应用程序是很有用的。
“我们最终想用体温作为启动装置，”Fang 说。“如果我们能够正确的设计这些聚合物，就可能能够形成一种药物输送装置，只会在发烧时来释放药物。”
Fang的合作者包括前MIT-SUTD研究员齐“凯文”阁，现在是SUTD的助理教授；前麻省理工学院的研究员Howon Lee，目前是罗格斯大学的助理教授，还有乔治亚理工学院其他SUTD。
通用电气说三维印刷形状记忆材料的过程也可以被认为司四维印刷，结构设计随着第四维度——时间的变化。
“我们的方法不仅使4D打印在微米量级，而且建议的配方是打印形状记忆聚合物可以拉伸10倍以上，由商业3D打印机打印，”葛说。“这将推动四维印刷成为更为广泛的实际应用，包括生物医学设备，部署的航空航天结构，并改变形状的太阳能电池。”
对速度的需求
Fang和其他人一直在探索软的使用，来作为可靠的活性材料，柔软的工具。这些新的和新兴的材料，其中包括形状记忆聚合物，在对环境的刺激，如热，光，电，弹力变形明显，——研究人员一直在研究使用在生物医学设备，软机器人，可穿戴式传感器和人肌肉的属性。
形状记忆聚合物特别有趣：这些材料可以在两个状态之间转换——一种是更坚固的，低温，非结晶的状态，一种是软，高温，橡胶的状态。弯曲和拉伸的形状可以在室温下被“冻结”，当被加热时材料将“记住”，并重新回到原来的坚固的形式。
为了制造形状记忆结构，一些研究人员已经研究了3D打印，因为这一技术使他们能够定制设计结构相对较细的细节。然而，使用传统的3D打印机，研究人员已经能够设计结构的细节不小于几毫。Fang说，这种大小的限制也限制了该材料可以恢复其原始形状的速度。
“事实是，如果你能够使它做成更小的尺寸，这些材料实际上可以在几秒钟内回应的非常迅速，”方说。例如，一朵花可以在毫秒内释放花粉。它只能这样做，因为它的驱动机制是在微米尺度内。”
光印刷
为了使打印形状记忆结构有更细的细节，方和他的同事们使用一个他们开创的3D打印过程，称为微粒体光刻，他们使用投影仪上的光照在树脂上的连续层上印刷图案。
研究人员首先使用计算机辅助设计（计算机辅助设计）软件创建了一个结构模型，然后将模型分割成数百个切片，每一个都通过投影仪作为一个位图来发送一个图像文件格式，它表示每一层都是一个非常精细的像素的排列。投影机然后在位图的模式中闪耀光，在液体树脂或聚合物溶液，蚀刻图案的树脂，然后固化。
“我们正在用光一层一层印刷，”Fang说。“这几乎就像牙医如何制成牙齿和填充腔的复制品，除了我们正在用来自半导体行业的高分辨率镜头，这给我们复杂的零件，尺寸可与人类的头发的直径相媲美。
研究人员然后通过科学文献，以确定一个理想的混合聚合物，以创建一个形状记忆材料，以打印他们的光图案的形状记忆材料。他们选择两种聚合物，一个长链，或意大利面状的长链聚合物，和其他更多的类似一种刚性支架。当混合在一起，固化后，材料可以被拉伸和扭曲，而不会断裂。
更厉害的是，该材料在一个特定的温度范围内可以反弹回到它原来的印刷形式-这种情况下是在40至180摄氏度（104至356华氏度）之间。
团队印制各种结构，包括线圈、鲜花和微型的埃菲尔铁塔，其全尺寸对应的是以其复杂的钢梁模式。Fang发现，该结构可以被拉伸到原来的长度的三倍，而不断裂。当他们暴露在40 C到180 C的范围内的热量，他们在几秒钟内就恢复到原来的形状。
“因为我们使用的是我们自己的打印机，它提供更小的像素，我们在几秒钟内看到更快的反应，”Fang说。“如果我们可以推到更小的尺寸，我们也可以推动他们的反应时间，可以毫秒为单位。
软握
为了显示一个形状记忆结构的简单应用，Fang和他的同事们打印了一个有弹性的像钳子一样的爪形器。他们在夹持器底座上连接一个薄的手柄，然后拉长了抓爪的爪子。当周围的空气到至少40℃的高温他们开始用曲柄转动，无论工程师是否放在它下面都会夹闭。
“这个钳子是一个如何用软质材料操作的很好例子，”Fang说。“我们表明，它是可能能拿起一个小螺栓，甚至还鱼蛋和软豆腐。这种类型的软握很可能是非常独特的和有益的。”
在未来，他希望找到聚合物的组合，使形状记忆材料对更低的温度做出反应，接近人体温度的范围内，来设计柔软，活性，可控的药物输送胶囊。他说这种材料也可以被印刷成柔软的，响应性的铰链，以帮助太阳能电池板跟踪太阳。
“经常，过多的热量将建立在太阳电池的背面，所以你可以使用形状记忆材料作为驱动机制来调整太阳电池的倾斜角度，”Fang说。“因此，我们认为可以证明可能会有更多的应用程序。”
文章来源：
转载的材料由麻省理工学院提供。原来的项目是由Jennifer Chu写的。注：内容可编辑风格和长度。
文献来源：
Qi Ge, Amir Hosein Sakhaei, Howon Lee, Conner K. Dunn, Nicholas X. Fang, Martin L. Dunn. 4D打印材料的裁剪形状记忆聚合物
。科学报告, 2016; 6: 31110 DOI:
10.1038/srep31110Engineers from MIT and Singapore University of Technology and Design (SUTD) are using light to print three-dimensional structures that "remember" their original shapes. Even after being stretched, twisted, and bent at extreme angles, the structures -- from small coils and multimaterial flowers, to an inch-tall replica of the Eiffel tower -- sprang back to their original forms within seconds of being heated to a certain temperature "sweet spot."
For some structures, the researchers were able to print micron-scale features as small as the diameter of a human hair -- dimensions that are at least one-tenth as big as what others have been able to achieve with printable shape-memory materials. The team's results were published earlier this month in the online journal Scientific Reports.
Nicholas X. Fang, associate professor of mechanical engineering at MIT, says shape-memory polymers that can predictably morph in response to temperature can be useful for a number of applications, from soft actuators that turn solar panels toward the sun, to tiny drug capsules that open upon early signs of infection.
"We ultimately want to use body temperature as a trigger," Fang says. "If we can design these polymers properly, we may be able to form a drug delivery device that will only release medicine at the sign of a fever."
Fang's coauthors include former MIT-SUTD research fellow Qi "Kevin" Ge, now an assistant professor at SUTD; former MIT research associate Howon Lee, now an assistant professor at Rutgers University; and others from SUTD and Georgia Institute of Technology.
Ge says the process of 3-D printing shape-memory materials can also be thought of as 4-D printing, as the structures are designed to change over the fourth dimension -- time.
"Our method not only enables 4-D printing at the micron-scale, but also suggests recipes to print shape-memory polymers that can be stretched 10 times larger than those printed by commercial 3-D printers," Ge says. "This will advance 4-D printing into a wide variety of practical applications, including biomedical devices, deployable aerospace structures, and shape-changing photovoltaic solar cells."
Need for speed
Fang and others have been exploring the use of soft, active materials as reliable, pliable tools. These new and emerging materials, which include shape-memory polymers, can stretch and deform dramatically in response to environmental stimuli such as heat, light, and electricity -- properties that researchers have been investigating for use in biomedical devices, soft robotics, wearable sensors, and artificial muscles.
Shape-memory polymers are particularly intriguing: These materials can switch between two states -- a harder, low-temperature, amorphous state, and a soft, high-temperature, rubbery state. The bent and stretched shapes can be "frozen" at room temperature, and when heated the materials will "remember" and snap back to their original sturdy form.
To fabricate shape-memory structures, some researchers have looked to 3-D printing, as the technology allows them to custom-design structures with relatively fine detail. However, using conventional 3-D printers, researchers have only been able to design structures with details no smaller than a few millimeters. Fang says this size restriction also limits how fast the material can recover its original shape.
"The reality is that, if you're able to make it to much smaller dimensions, these materials can actually respond very quickly, within seconds," Fang says. "For example, a flower can release pollen in milliseconds. It can only do that because its actuation mechanisms are at the micron scale."
Printing with light
To print shape-memory structures with even finer details, Fang and his colleagues used a 3-D printing process they have pioneered, called microstereolithography, in which they use light from a projector to print patterns on successive layers of resin.
The researchers first create a model of a structure using computer-aided design (CAD) software, then divide the model into hundreds of slices, each of which they send through the projector as a bitmap -- an image file format that represents each layer as an arrangement of very fine pixels. The projector then shines light in the pattern of the bitmap, onto a liquid resin, or polymer solution, etching the pattern into the resin, which then solidifies.
"We're printing with light, layer by layer," Fang says. "It's almost like how dentists form replicas of teeth and fill cavities, except that we're doing it with high-resolution lenses that come from the semiconductor industry, which give us intricate parts, with dimensions comparable to the diameter of a human hair."
The researchers then looked through the scientific literature to identify an ideal mix of polymers to create a shape-memory material on which to print their light patterns. They picked two polymers, one composed of long-chain polymers, or spaghetti-like strands, and the other resembling more of a stiff scaffold. When mixed together and cured, the material can be stretched and twisted dramatically without breaking.
What's more, the material can bounce back to its original printed form, within a specific temperature range -- in this case, between 40 and 180 degrees Celsius (104 to 356 degrees Fahrenheit).
The team printed a variety of structures, including coils, flowers, and the miniature Eiffel tower, whose full-size counterpart is known for its intricate steel and beam patterns. Fang found that the structures could be stretched to three times their original length without breaking. When they were exposed to heat within the range of 40 C to 180 C, they snapped back to their original shapes within seconds.
"Because we're using our own printers that offer much smaller pixel size, we're seeing much faster response, on the order of seconds," Fang says. "If we can push to even smaller dimensions, we may also be able to push their response time, to milliseconds."
Soft grip
To demonstrate a simple application for the shape-memory structures, Fang and his colleagues printed a small, rubbery, claw-like gripper. They attached a thin handle to the base of the gripper, then stretched the gripper's claws open. When they cranked the temperature of the surrounding air to at least 40 C, the gripper closed around whatever the engineers placed beneath it.
"The grippers are a nice example of how manipulation can be done with soft materials," Fang says. "We showed that it is possible to pick up a small bolt, and also even fish eggs and soft tofu. That type of soft grip is probably very unique and beneficial."
Going forward, he hopes to find combinations of polymers to make shape-memory materials that react to slightly lower temperatures, approaching the range of human body temperatures, to design soft, active, controllable drug delivery capsules. He says the material may also be printed as soft, responsive hinges to help solar panels track the sun.
"Very often, excessive heat will build up on the back side of the solar cell, so you could use [shape-memory materials] as an actuation mechanism to tune the inclination angle of the solar cell," Fang says. "So we think there will probably be more applications that we can demonstrate."
Story Source:
Journal Reference:
- Qi Ge, Amir Hosein Sakhaei, Howon Lee, Conner K. Dunn, Nicholas X. Fang, Martin L. Dunn. Multimaterial 4D Printing with Tailorable Shape Memory Polymers. Scientific Reports, 2016; 6: 31110 DOI:10.1038/srep31110
发表于 2016-8-30 11:04:46
