Silicon gets silky

Since the 1960s, we have become accustomed to medical science implanting devices into our bodies. But if a new approach fulfils its promise, it will transform the potential for putting electronics inside the body. At the heart of the technique is one of the greatest materials nature has produced – silk. By combining silk and electronics, fields like cardiology and neurology may be transformed over the next decade, by enabling ultra high resolution electrical and chemical interaction with three dimensional biological surfaces. It could also mean that virtually all problems associated with the immune system reacting against the implant are eliminated – and that is because much of the implanted systems dissolve almost completely over time. Arrays of transistors have already been demonstrated working on thin films of silk and, instead of the electronics systems being enclosed to protect them from the body, there is no need for protection; the silk enables the electronics to conform to biological tissue. The silk dissolves over time and because the circuits are so thin, just nanometres thick, they cause no irritation. Pioneering this work is Brian Litt, Associate Professor of both Neurology and Bioengineering at the University of Pennsylvania, who is working with John Rogers, Professor of Materials Science and Engineering at the University of Illinois' Beckman Institute. Here, they have developed flexible, stretchable silicon circuits. Another pair of researchers collaborating are Fiorenzo Omenetto and David Kaplan, Professors of Bioengineering at Tufts University in Medford, who have developed nanopatterned optical devices from silkworm cocoon proteins. To create the silk electronic implants, silicon transistors about 1mm long and 250nm thick are transferred to the surface of a thin film of silk. The silk holds each device in place, even after the array is implanted into a living body and wetted with saline. The silk is very thin and flexible, enabling it to conform to the tissue surface. In a paper published in the journal Applied Physics Letters (5, 133701, 2009), the researchers say devices can be implanted in animals with no adverse effects and the performance of the transistors on silk inside the body doesn't suffer. "The combination of silicon electronics, based on nanomembranes of silicon, with biodegradable thin film substrates of silk protein, yield a flexible system and device that is largely resorbable in the body," the researchers say. "The use of silicon provides high performance, good reliability, and robust operation. Silk is attractive, compared to other biodegradable polymers … because of its robust mechanical properties, the ability to tailor the dissolution, and/or biodegradation rates from hours to years, the formation of noninflammatory amino acid degradation products, and the option to prepare the materials at ambient conditions to preserve sensitive electronic functions." At the core of this collaboration, Jonathan Viventi, a PhD student at UPenn and Dae-Hyeong Kim, at UIUC, work together to design, fabricate and perform in vivo animal testing of devices to translate this novel materials work into practical, patient care applications. They have built flexible sheets of rubber and plastic, some backed with silicon, which are making it possible to put active electronics on the devices. "The important distinction is that, with medical implants today, the active electrical components that communicate with the body are located in a sealed box and connected with a single wire per sensor. This severely limits the number of sensors that can be implanted in the body." Prof Litt explains. "Integrating active electronics on sheets of silk or plastic makes it possible to multiplex the outputs of different sensors, meaning you can put hundreds, or even thousands, of contacts on a sheet." In the brain, many procedures today rely on electrodes that have not changed much – the tissue/electrode interface has hardly altered in 40 years. Now, the new implants hold out the prospect of mapping at very high resolution, down to groups of cells, and then all the way up to much bigger regions, making it possible to localise things like the networks that cause epilepsy. Another possibility is that the implants could be wrapped around depth electrodes and inserted into the brain to stimulate regions responsible for diseases like Parkinson's. Arrays of silk electrodes could conform to the brain's structure and thereby reach otherwise inaccessible areas. "It would be nice to see the sophistication of clinical devices start to catch up with the sophistication of our basic science, and this technology could really close that gap," Prof Litt says. The implants are now being tested in animals and proof of principle has already been demonstrated. Also, MC10, a start up based in Boston, has been formed to commercialise the technology. In July last year, MC10 formed a licensing agreement with the University of Illinois at Urbana-Champaign relating to stretchable silicon technology and the University of Pennsylvania relating to medical applications of this technology. As well as the medical uses described above, MC10 says there are other potential applications, such as stretchable sensor tapes for industrial and healthcare applications, including robotics and ultrathin, lightweight wearable health monitors, and bio inspired 'electronic eye' cameras, providing the basis for ultra compact, high performance imaging systems such as extremely thin mobile phones and lightweight satellites. UIUC's Prof Rogers cites other possibilities. "A lot of things that now have to be done inside the box could be done outside on the implanted sheet." Also, since these devices can be made with micron thickness and are foldable and rollable, they can be introduced into body with minimal invasiveness, a major benefit. "Silk allows you to have a little bit of a stiff backing to get them into where you want, and then it dissolves away," Prof Rogers says. "Think of it as sinking into the wrinkles of the brain, or conforming to the walls of the bladder, or wrapping around nerves. Of course, these are active devices that also offer potentially far greater resolution then has been possible previously." It will be possible to put a device inside someone and take a reading from it just by holding an inductor coil over the skin. Also, there are ways to implant a device that chemicals would bind to, so this could be used to monitor the region for any molecules that might signal the return of a cancer, for example. Work on silk electronics by Omenetto and Kaplan at Tufts University evolved as a result of Kaplan asking Omenetto – whose background is principally in photonics and optoelectronics – if a laser could be used to make tiny, precise cuts in a silk based material he was using to make a replacement cornea. Since then, the two have made several advances in combining silk and optoelectronics. To make the silk into an optical material, they take conventional silk thread, boil it down to purify the protein in it and pour it in a mould. After it dries and crystallises, it can be peeled off. Then, by using tuned lasers, they have placed nanometre sized patterns on the silk material. Because the wavelength of visible light ranges between 400 and 700nm, it is an ideal medium for manipulating light. One potential application is detecting harmful bacteria in food. A silk optic material would have a pattern of nanoscale peaks and troughs, with each trough containing a substance that reacted to the bacteria. If the bacteria were present, the troughs would fill, and like a butterfly wing when its structure is altered, change colour, revealing the presence of bacteria. Medical monitoring is another possibility. A specific case would be monitoring glucose concentration – using a silk based photonic monitoring system implanted under the skin that stays there for maybe a month, and would change colour depending on what was happening. "The optical properties change depending on the biological activity of what is inside the optical material," Omenetto notes. Components like enzymes or proteins could be mixed in with the liquid silk solution and used as biological markers for oxygen or pH levels. When the components are added to the silk as it is drying, the silk locks the component into its structure and, within the hardened element, the enzyme or protein retains its function, says Omenetto. "We're trying to reinvent silk as a high technology platform," Omenetto says. "Silk has already been used a lot for tissue engineering applications – it's an FDA approved material and there are several companies purifying silk fibres to make them physiologically acceptable. These are being woven into substitutes for ligaments. "Silk is a material that interfaces extremely well with the body, causing no immune problems, which is almost unique. You can interface with planar electronic technology, and this gives you lots of control. It's also very green, basically requiring water based processing at room temperature, and it is of course already a commodity, because of the textile industry. It has a spectacular confluence of properties. Other biopolymers are very good at doing specific things but it's like everything comes together with silk." No one knows where silk and electronics might ultimately go. One intriguing possibility is electronic tattoos – silk based leds that can be implanted under the skin and activated by, for example, touch. Science fiction? Certainly – read Ray Bradbury's book The Illustrated Man. But maybe soon to become fact: electronics giant Philips, no less, has already created a video showing what might be possible. And how about these: implanted GPS, with a map readout on the back of the wrist? Or chips that cover your eyeballs and darken down when the sun is shining too bright? Or finally, the ultimate combination of capitalism and self obsession: a full body display used for advertising? US based designer Jim Mielke, in his entry to the 2008 Greener Gadgets Competition, suggested that implantable electronics could be used to create subcutaneous mobile phones or implanted health monitors. He believes Bluetooth based devices could be implanted permanently beneath the skin. Made from flexible silicon and silicone, the devices would be inserted through a small incision and unfurled beneath the skin. Two small tubes might be attached to a blood supply, feeding a coin sized fuel cell which converts glucose and oxygen in the bloodstream into electricity needed to power the device. The surface of the implant, a touchscreen control that faces the underside of the skin, is covered with a matrix of field producing pixels that active a matching matrix of pixels tattooed on the skin above the implant. Rather than use ink, tiny clusters of microscopic spheres would be injected into the skin; each sphere filled with a field sensitive material that changes from clear to black when a field in the matrix is turned on. Implanted medical devices could communicate wirelessly with the outside world, as well as with other devices implanted in the same body. Because it is always present and always on, the device could monitors for blood disorders continually, alerting the person of a health problem.