{module Science Square (Issue 128)}

First miniature human heart printed

Noor N et al. 3D Printing of Personalized Thick and Perfusable Cardiac Patches and Hearts.  Advanced Science, April 2019.

In a major breakthrough, researchers have “printed” the world’s first 3D vascularized engineered heart using a patient’s own cells and biological materials. This could have huge repercussions for human health: the World Health Organization said that last year, ischemic heart disease and stroke were the world’s leading cause of death for both men and women. Heart transplantation is currently the only treatment available to patients with end-stage heart failure. Given the serious shortage of heart donors, scientists have been trying to develop new “3D organ printing” approaches to regenerate the diseased heart. Past studies were only able to print simple tissues without blood vessels. The new study showed for the first time that an entire heart with cells, blood vessels, ventricles, and chambers could be successfully engineered and printed. The researchers first took a biopsy of fatty tissue from patients and separated the cellular and a-cellular materials of the tissue. While the cells were reprogrammed to become pluripotent stem cells, the extracellular matrix – a 3D network of extracellular macromolecules such as collagen and glycoproteins – were processed into a personalized hydrogel that served as the printing “ink.” After being mixed with the hydrogel, the cells were then robustly differentiated to cardiac or endothelial cells to create patient-specific, immune-compatible cardiac patches with blood vessels and, subsequently, an entire heart. The heart was 3D-printed in about three hours and was too small for humans. It was the size of a rabbit’s heart (~ 2.5 centimeters). But it is completely biocompatible and, most importantly, matches the patient, which reduces the chances of organ rejection inside the body. A human-sized heart might take a whole day to print and would require billions of cells, compared to the millions used to print these mini-hearts. While it’s not clear if a printer can produce hearts that are equal or superior to human ones, perhaps by printing patches there will be a possibility to improve or take out diseased areas in the heart and replace them with something that works. Researchers hope that maybe in 10 years, there will be organ printers in the finest hospitals around the world, and these procedures will be conducted routinely.

Bacterial factories for spider silk

Zhang F et al. Synthetic Biology for Microbial Production of Protein-based Materials, the American Chemical Society (ACS) National Meeting & Exposition, Spring 2019.

Spider silk has always fascinated researchers due to its lightweight and superior strength and numerous applications in areas such as drug delivery, smart textiles, and artificial muscles. It is one of the strongest natural materials in the world. It is thinner than a human hair, but its strength is more than that of steel, pound for pound. Since farming spiders is incredibly inefficient, scientists have been trying for decades to find a way to mass produce the material from genetically modified bacteria, yeast, and even goat milk, but these efforts have always fallen short. The biggest challenge was that the genetic information for dragline silk is a long string of repeating DNA, and, in previously tested organisms, cellular machinery arbitrarily alters or chops up such DNA sequences. To circumvent this problem, researchers precisely separated the repeating DNA into bits and inserted each repeating piece separately into bacterial genome. These smaller DNA pieces produced small peptides that ended up combining in bacteria and formed a strand of silk. The researchers also added to the end of each strand a chemical tag that glued the individual fibers together. This method was able to produce 2 grams of spider silk for each liter of bacteria and the resulting material behaved exactly like dragline silk. Its tensile strength was measured at 1.03 gigapascals, about the same as for naturally produced dragline silk. The engineered silk’s toughness measured 114 megajoules per cubic meter, compared with around 100 megajoules for silk made by spiders. And the engineered silk strands could stretch 18 percent before breaking, the same as natural dragline silk. The new silk was developed in part with NASA funding for applications such as giving astronauts a means of producing tough materials while on Mars. But the substance could be used in designing stronger materials for robotic, medical, or textile applications.

Electricity from falling snow

Ahmet A et al. All printable snow-based triboelectric nanogenerator. Nano Energy, April 2019.

Researchers have designed a new device with which we can now obtain electricity from falling snow. This new energy conversion method could become a new source of electricity in the future, especially in remote areas, as it does not need batteries. Researchers called it a Snow-based TriboElectric NanoGenerator, or Snow TENG. It is inexpensive, small, thin, and flexible like a sheet of plastic. After starting a charge from static electricity, energy is generated from the exchange of electrons. Snow is already positively charged by giving up its electrons, while silicone, a rubber-like material which consists of silicon atoms and oxygen atoms, is combined with carbon, hydrogen, and other elements to be negatively charged. When the positive-charged snow falls onto the surface of the silicone, the charges interact, and the Snow TENG captures the charge, which allows it to turn snowfall into electricity. 30% of Earth’s surface is covered by snow each winter, which is also the time when solar panels, one of the most reliable renewable sources of energy, aren’t very effective. Snow accumulation reduces the amount of sunlight that reaches the solar array, which makes them unable to operate. Snow TENG could be integrated into solar panels and provide a continuous power supply, even at a time when it’s snowing. Researchers used 3D printing to design the small device. It consists of a layer of silicone and an electrode which can capture the electric charge. Given that silicone is widely used in the industry, this method could dramatically reduce the global costs of producing electricity.

Many people probably would agree that the transistor was the greatest technological invention of the twentieth century. The first transistor, invented by Jack Kilby and Robert Noyce in 1958, was made from silicon. Even though integrated circuit technology has advanced to the level of putting millions of transistors on a fingernail-sized chip, transistors are still made from silicon. Now, however, both the scientific community and the high-tech industry are very excited about something new: plastic electronics. For many of us, this might sound like an oxymoron, for we know plastic only as an insulator. So how can a protective substrate or a carriage box to the actual electronic device be converted into an electronic device?

The story of plastic electronics started late 1970s with Alan Heeger, Alan Macdiarmid, and Hideki Shirakawa. These scientists demonstrated that the molecular structure of certain polymers (plastics) can be manipulated and then used as conductors. The Swiss Academy of Sciences was somewhat slow in recognizing their work, for they were awarded the Nobel prize in chemistry only in 2000. Nevertheless, the scientific community did not wait for the Nobel committee’s recognition of plastics. Since 1977, plastic has become probably the most common material in our daily lives.

Silicon versus plastic

All computer chips are made out of silicon (semiconductor) and aluminum (metal). Silicon is a great material for integrated circuits, because it is available, can be acquired in an extremely pure state (single crystal or amorphous), and has a very high mobility (the speed that electrons can travel through material). This mobility, in turn, determines the device’s switching speed. However, silicon has one important drawback: It is not easy to process.

Integrated circuit technology can deposit millions of silicon transistors on a single chip. However, the procedure for making these devices usually requires facilities worth billions of dollars, for silicon has to go through complicated photolithography procedures under clean room conditions before it can be incorporated into a device. But is it really worthwhile to spend billions of dollars on such facilities? The answer probably looks obvious, since Intel remains one of the world’s largest companies. However, we do not really need such a high quality in many of the applications for which silicon is used. So if there is something cheaper that can do the job perfectly, why not use it? Plastic is far cheaper, but not so sophisticated an alternative.

Plastic is cheap because billion-dollar facilities are not required to convert it into a device. In fact, the technology needed to process plastic into an electronic device is only slightly more advanced than an ink-jet printer. Electronic circuits are printed on an insulating polymer, and then certain parts of the polymer are exposed to UV light in order to convert the insulating polymer into a conducting polymer. The result is a device in which only the parts that we want to conduct are conducting. Moreover, these conducting parts sit on a protective insulating sheet of plastic. That is pretty much all we need for many applications.

The case for plastic

Plastic has two advantages over silicon: price and flexibility. Like other inorganic elements, silicon has strong covalent bonds between its atoms. As these bonds are rigid, they cannot bend or stretch. Plastic has very loose molecular bonds that can tolerate a significant amount of bending and stretching. Flexibility combined with electronics implies applications like flexible displays that can be rolled up and taken somewhere else, reloadable electronic newspapers that can be bent like paper, disposable mobile phones, and many others.

So if plastic is that good, why is it not the electronics industry’s standard material? For one simple reason: Plastic’s loose molecular bonds, which make the material so flexible, make it more difficult for the electrons to travel through it. Thus, plastic devices are slower than silicon devices. Until several years ago, the mobility of a typical conducting plastic used to be around 0.1 cm2/volts, whereas crystalline (best) silicon could reach 1000 cm2/volts at room temperature. Recently, a new class of polymers (pentacene) has been found in which molecules tend to self-organize. As a result, the mobility has been pushed up to 3 cm2/volts. Scientist working on pentacene estimate a number close to 50 cm2/volts as the limit of achievable mobility for this special polymer.

These expectations are not just wishful thinking of some optimistic scientists. In fact, even now there are some significant outcomes of plastic technology.

Current developments

John Rogers and coworkers from Bell Labs (Lucent Technologies) have patterned 256 polymer transistors on the back-plane of a flexible optical display. Richard Friend and coworkers from University of Cambridge have produced thin film transistor circuits using a high-resolution inkjet printing. Dago de Leeuw and colleagues at Philips Research Laboratories in Eindhoven, The Netherlands, have developed a new technique called photochemical patterning. In photochemical patterning, a light sensitive-polymer is exposed to ultraviolet light through a mask shaped in the form of the desired circuit. The ultraviolet light changes the polymer from a conducting state to a non-conducting state. In this process, the polymer’s resistance can increase as much as 11 orders of magnitude (100000000000). The advantage of this technique over the patterning techniques used for silicon is that it does not need any vacuum and can be used on flexible substrates. The problems with photochemical patterning are that it is not significantly cheaper than photolithography and etching used for silicon, and it can be used only for light-sensitive polymers. Different research groups have developed various techniques that have pros and cons compared to photochemical patterning. However, many of the techniques cannot print features that are small enough for electronic circuits. The critical length is the distance between the transistor’s source and drain, typically 0.01 mm. This is the distance that the field-induced charges have to travel. As the drive current and switching speed of the device depend on this distance, having too large of a distance reduces the capabilities of the device. Another technique that pursues quite a different approach is microcontact printing. Developed by the Bell Labs group, microcontact printing with rubber-like stamps can make small enough features. Scientists have used this technique to make a flexible display in which a transistor controls each pixel. The key point in developing this technique was using gold pads, instead of a polymeric material, to deposit the transistor’s source and drain. Even though this device is not completely plastic, it is a step toward that goal. When a special kind of ink was applied to a thin film of gold, it formed some sort of self-assembled layer on the gold, which then produced well-defined patterns and sharp edges. This provided the required resolution to make small enough features necessary for an electronic device having a reasonable speed. These two techniques show two important aspects of the problem. In photochemical printing, we have a device that is completely plastic and so has the important advantage of flexibility. However, it is not as cheap as it could be and does not have the required resolution. In the second technique, the outcome is not a 100 percent plastic device, so it is not as flexible as a purely plastic circuit. However, it can be manufactured very cheaply and has a better resolution (and thus a higher switching speed).

Conclusion

Many other approaches are being employed to develop this new and exciting technology. If plastic electronics does become standard for at least some applications, it probably will be a hybrid of these different techniques. If the optimistic group of scientists working on plastic electronics prove to be right, one day we might see TV screens curling around the walls of our rooms and even reloadable electronic newspapers that can be folded and carried like regular newspapers. Who knows what new inventions will come with this new technology?

References

• Garnier, F., (et al). Science 265 (1994): 1684-86.
• Gelinck, G., T. Geuns, and D. de Leuw. Applied Physics Letters 77 (2000): 406-8.
• Levi, Barbara G. ‘New Printing Technologies Raise Hopes for Cheap Plastic Electronics.’ Physics Today (February 2001). Online at: www.physicstoday.org/pt/vol-54/iss-2/p20.html.
• Nobel Focus: Electricity through Plastic.’ Physical Review Focus (24 October 2000). Online at: http://focus.aps.org/v6/st18.html.
• Scott, Campbell. ‘Electronics Put It on Plastics.’ Physics in Action. (October 1998). Online at: www.physicsweb.org/article/world/11/30/3/1.
• Voss, David. ‘Cheap and Cheerful Circuits.’ Nature 407 (28 September 2000).