Implantable silk-based microchips
Scientists have developed electronic devices that are almost completely decomposed in the body, which are flexible electronic circuits fixed on a silk substrate. Unlike conventional electronics, which must be carefully isolated from the destructive action of intercellular fluid, new devices do not need protection, and silk ensures their compatibility with living tissues. Gradually, the silk dissolves in the body, and the remaining silicon chips do not cause irritation, since their thickness is measured in nanometers.
On the surface of this thin silk film, about one square centimeter in size, six transistors are fixed. Such flexible devices can be implanted into the mouse's body, as shown in the figure, without harming the animal, and the silk base subsequently completely decomposes. The orange liquid on the coat is a disinfectant used during the operation.
Today, the implantation of electronic medical devices is very limited due to the need to isolate them from the environment of the body and their traditional placement on a solid silicon base. The researchers claim that the new type of microchips they are developing will interact with tissues in a completely different way. They are working on the creation of silk-silicon LEDs that can be used as a kind of "tattoos" that highlight, for example, blood sugar levels, as well as on systems of controlled electrodes designed to ensure the interaction of electronics and the nervous system.
Last year, John Rogers, a professor of materials science and mechanical engineering at the Beckman Institute, part of the University of Illinois (Champaign-Urbana), developed flexible stretchable silicon chips whose performance is not inferior to their rigid counterparts. In order to make their devices biocompatible, Rogers and his colleagues began working together with Fiorenzo Omenetto and David Kaplan, professors of bioengineering from Taft University (Medford, Massachusetts), who last year unveiled optical nanodevices they created from silkworm cocoon proteins.
For the manufacture of new devices, silicon transistors, whose length is about a millimeter and thickness is 250 nanometers, are placed on a special "stamp" with which they are transferred to the surface of the thinnest silk film. Silk holds all devices in place even after implantation into the body and wetting with physiological fluids, and ensures a tight fit of the film to the surface of the tissue. In the article "Silicon electronics on silk as a path to bioresorbable, implantable devices", published in the journal Applied Physics Letters, the authors claim that the implantation of such devices into the body of animals does not cause negative side effects. At the same time, being inside the body does not affect the performance of transistors in any way.
Silk plays a passive but important role in the functioning of silk-silicon electronics. It provides sufficient mechanical strength of the entire device, and when wet, its tight fit to the fabric surface. Silk has already received official approval from the US Food and Drug Administration (FDA) for implantation into the human body, where it eventually completely decomposes into compounds that are safe for health. Silk films are very flexible, they can be stored rolled up and unfolded during surgery, which facilitates the work of surgeons. Changing the conditions for the production of films allows you to change the rate of their degradation in the body from a few minutes to several years after implantation.
The biocompatibility of silicon has not been proven as convincingly as the biocompatibility of silk, but the results of all the studies conducted demonstrate the safety of this material. For implants, the size and shape of silicon parts are of great importance, so developers try to make them as small as possible. The functioning of the devices also requires electrical contacts made of gold and titanium, which are biocompatible, but do not decompose in the body. Rogers is developing biodegradable electrical contacts, so that eventually only silicon parts remain in the body.
Currently, specialists are working on silk-silicon electrodes that will act as an interface between the nervous system and external devices, for example, prostheses. Such electrodes should interact with living tissues much better than existing analogues, which either penetrate the tissue or are located on its surface. Silk-based electrodes can, for example, be wrapped around individual peripheral nerves, which will allow better control of the movements of the prostheses. Complexes of silk-silicon electrodes designed for such purposes as deep brain stimulation, used to suppress the symptoms of Parkinson's disease, are able to repeat the contours of the convolutions of the cerebral cortex, which will stimulate previously inaccessible areas.
Evgeniya Ryabtseva
Portal "Eternal youth" http://vechnayamolodost.ru based on the materials of Technology Review: Implantable Silicon-Silk Electronics.
09.11.2009