The heart of the problem21 May 2019
The vast majority of semiconductors in medical devices are made from silicon, which, though the cheapest material available, is not the most efficient. Now, a team of engineers at the Massachusetts Institute of Technology (MIT) has developed a new way to fabricate ultra-thin semiconducting films made from a host of exotic materials. Michael Shaw talks to Jeehwan Kim, the university’s class of 1947 career development assistant professor in the departments of mechanical engineering and materials science and engineering, about the potential applications of this research.
The baby was delivered stillborn and Mark C Lidwill was running out of ideas. Artificial respiration and injections of adrenalin had failed; now, the only option left open to the doctor and his assistant was a highly experimental technique of cardiac therapy. Together they plunged a needle straight into the baby’s chest and into the ventricle of his tiny heart. The spindle was connected to a generator, which – Lidwill hoped – would deliver the 16V necessary to restart the patient’s pacemaker.
Remarkably, the procedure worked. “The heart responded to each impulse,” Lidwell wrote in an article describing the procedure. “At the end of 10 minutes the current was stopped and it was found the heart would beat of its own accord.” The child would eventually leave the Royal Prince Alfred Hospital, in Sydney, Australia, fully recovered, and living proof of the first successful artificial pacemaker.
Lidwell’s actions in 1928 were groundbreaking but quickly forgotten. The contraption he’d jerryrigged with his assistant was incredibly complicated to operate, and any patient who needed to have the electrical impulses in their pacemaker regulated instead of jump-started would be tied to it, needle and cable, for life. What was needed was a wearable device, or better yet an implanted artificial pacemaker that could regulate the patient’s heartbeat autonomously. It would not be until the 1950s, with the advent of the silicon transistor, to make this a reality.
And yet, even as this development led to a succession of medical devices with ever-greater capabilities, another wall has inevitably been hit. In a medical market that yearns for more flexible electronics, affordable manufacturing methods that produce the thin semiconducting films necessary for such devices remain elusive. The electronics industry’s continued marriage of convenience with silicon – which, as the seventh-most abundant element on the planet, remains the cheapest semiconductor out there – is also a sad indictment of the sector’s lack of progress in fulfilling its true potential.
Why? The mobility of electrons in silicon is simply not as good as that encountered in other compounds, like gallium arsenide or gallium nitride. In fact, ‘not as good’ is a radical understatement; the way electrons zoom through gallium arsenide makes their passage through silicon look like a leisurely stroll, and it’s the reason why the compound can be found in almost every mobile phone, the better for it to parse the radio waves beamed down to it via satellite into interpretable speech.
And yet, for all its advantages, compounds like gallium arsenide remain prohibitively expensive to manufacture compared with silicon, which comprises just over a quarter of the Earth’s crust in weight. By comparison, making an 8in wafer of gallium arsenide – a compound of gallium and arsenic, this time occupying a more productive role than its traditional application as a poison – costs up to $5,000, a thousand times more than its silicon equivalent. What is needed, it seems, is a revolutionary new method of manufacturing semiconductor films, one that Professor Jeehwan Kim and his colleagues at MIT may have just discovered.
The key to the professor’s new manufacturing method is, as it turns out, including another muchvaunted compound in the process – graphene. First synthesised by two scientists at the University of Manchester, UK, after they accidentally removed slices of carbon from graphite using scotch tape, the substance has since been hailed as a miracle material thanks to its own remarkable conductive potential.
Even so, graphene is also prohibitively expensive to synthesise. What Kim and his team discovered in their research, however, was that the thinness of graphene – the molecule is only one atom thick – lends itself to the wholesale fabrication of other compounds. Using a manufacturing method called ‘remote epitaxy’, they could produce new and exotic semiconducting films inexpensively and at scale.
The team began by taking a layer of graphene and placing it over an existing semiconductor compound, such as a gallium nitride wafer. Constituent atoms of this compound were then placed on top of the graphene. These particles began to form a pattern that mirrored the crystalline structure of the underlying wafer at the bottom of the structure, until an entirely new copy of the bottom section could be peeled from the top of the stack. This process, says Kim, is called remote epitaxy, and effectively allows the user to “copy and paste the crystal information of the underlying substrate”, he explains. “In principle, you can do this on and on, because you go on the same material on the thin graphene and peel off… the wafer just becomes a template.”
Kim and his team quickly discovered limits to this new manufacturing method. While they experienced success in their creation of gallium arsenide and gallium nitride films, when they decided to conduct new experiments on the same lines with germanium and silicon, they were not able to peel off any new layers of either material from the middle graphene section.
The reason why remote epitaxy failed to copy anything similar was, according to the professor, down to the ionic bonding properties of both materials. “Silicon does not have polarity,” explains Kim, referring to the interactions between the atoms in the substrate and those lain on top of the graphene. The professor and his team deduced that they weren’t able to copy and paste germanium and silicon because the ionic charge in the substrate and the top layer was identical, as they were from the same atomic group on the periodic table. Gallium and nitrogen, meanwhile, retain opposite charges. Not that the professor was overly concerned.
“Silicon’s a chip anyway,” says Kim, referring to the ubiquity of semiconductors made out of the material in multitudes of devices around the world. In the grand cost-benefit analysis, using remote epitaxy to create copied versions of an original silicon substrate might not make as much of a difference as an equivalent made out of gallium arsenide.
What difference, then, would remote epitaxy make to the field of medical devices? According to Kim, by laying each of these films produced through remote epitaxy on to one another, it could be possible to design new and more flexible types of devices.
“Typically, the electrical and optical properties of single crystalline films are much better than any other electronic functional materials,” he says. The use of these films in medical device design could, Kim argues, lead to the first viable implants embeddable inside skin. After all, the largest organ in the body yields a variety of symptoms and signs to clinicians that might be more easily interpreted with such devices.
“In order to sense that kind of information from the skin pore, you need to have high-quality silicon conductors,” explains Kim. “But there’s no way to put those semiconductors on the skin, because the semiconductor is rigid. By using our technology, you can make this and put it on the skin. By putting [a device in] the skin patch, you can then acquire instant information from the body through the skin pore.”
Kim also envisions health-monitoring devices implanted in or near major organs, the better to communicate with individual cells. He concedes, though, that this is all speculation. The full practical potential of remote epitaxy is yet to be realised.
“I think it will be five to 10 years,” says Kim, on how long before medical OEMs will be able to use this new manufacturing method. Even so, the work of Kim and his team at MIT points towards new and exciting developments in the form and potential of medical devices. Just as it took the invention of the silicon chip to popularise the pacemaker and revolutionise the medical device industry, so new manufacturing methods like remote epitaxy might lead to the flexible clinical implants and wearables to transform healthcare.