In Lewis Carroll’s Alice in Wonderland, Alice comes across the famous cat from Cheshire which discusses philosophy with her. After this talk, it grins and disappears. The cat is gone but the grin stays. Alice exclaims: I have seen a cat with no grin, but never a grin with no cat!
We are all familiar with electronic devices such as integrated circuits, chips and tools that do a hundred useful things. They have become inseparable from our lives.
What do we do once we are done with them? Throw them away, causing what has come to be known as e-waste. It is estimated that e-waste alone accounts for over 70 per cent of toxic wastes currently found in landfills, and we are yet to get a good estimate of how much it is in the seas and oceans.
Again, we use electronic devices such as pacemakers and other sensors that are implanted in our bodies. The trouble with them is that once they are past their use, we need to surgically remove them (and perhaps implant a fresh one). How nice it would be if only they dissolve away and get removed from the body; much the way we discard our body liquids and solids every day!
Yes, it appears possible to do so with the development of a prototype electronic chip that is made to work for a defined period of time, which is implanted into a body and dissolves after use and gets excreted away from the body after it has done its intended job. The job is done and the device is gone – a variation of the famous Cheshire Cat!
Dr. John Rogers of the Material Sciences department of University of Illinois at Urbana-Champaign and his group teamed up with Dr. Fiorenzo Omenetto of the Biomedical Engineering department of Tufts University in Boston, and devised an implantable electronic thermal therapy device which lasts inside a rat’s body for a few weeks before dissolving away. They report this landmark work in the 28th September 2012 issue of Science.
In order that the device is water-soluble, every component in there must be made of molecules and materials which are water-soluble, not the conventional aluminium, rare-earth metal compounds or plastic stuff that are built to last forever. They must be built to stay and work for a stipulated time and then be washed away.
Hence, they used magnesium or Mg as the electrical conductor, MgO and Silicon dioxide as dielectrics, specially fabricated nanomembrane silicon semiconductor and so forth, in order to build the electrothermal device. The entire device including all of its inductor, capacitor, resistor, diode and transistor dissolves away when placed in deionized water.
Next, they packed this device in a sheet of silk, which is specially made so as to stay intact for a set period of time, after which it dissolves in the body water, exposing the electronic device which too dissolves away.
Here then is the ‘proof of principle’ – a silicon-based complementary metal oxide semiconductor (CMOS) device that is an implantable medical device, which can be custom-made or programmed to last for a defined period of time, after which it is resorbed in the body, so that no second intervention for retrieval of the device is needed.
The device they made was meant to be a heater – one that can be placed next to a site where a surgical operation has been done. It is meant to keep the area warmer so as to keep it free of infection from germs. They first implanted it sub-dermally (under the skin) of some mice.
After the programmed three weeks, the implant dissolved away. Only a faint residue was left, which too cleared away, with no inflammation or any side effects.
Reassured of its safety, they next implanted this transient thermal therapy device on rats through surgery. Weeks later, they found no traces of infection after the surgery. Here then is the proof of principle.
Such biodegradable electronic devices can have many uses. They can be sensors placed in fields, reporting what they are meant to, and fade away. One can, in principle, even make portable consumer devices (cell phones?) which, after use, can be dissolved away in water and the starting material retrieved after evaporating the water.
Lewis Carroll used the term Cheshire Cat, apparently after the practice of dairy farmers in Cheshire who would pack and mould cheese in the form of a smiling cat. The cheese was cut and eaten from the tail side of the cat, leaving the smile for last. In any event, Carroll was pre-scient; Cheshire Cats are now an electronic reality.