How small can a radio transmitter be?

Question

Lately, I've heard about a fascinating new technology called DNA Origami, where incredibly tiny machines are folded out of DNA. Using CAD software they are able to build all sorts of interesting things, not the least of which are floating robots small enough to deliver drugs to specific cells, upon receiving a radio signal.

There is a talk about these robots up on youtube.

That got me to wonder, what is the size limitation on building radio transmitters? What factors constrain their size? And could one be built that was the size of a cell? Would it be able to communicate outside the body?

I'd really like to learn more about this!

Answer 1

Smallest transmitter:

(http://www.extremetech.com/extreme/171309-the-worlds-most-smallest-fm-radio-transmitter-made-from-graphene)

I quote "Columbia University have just revealed to us the world’s smallest FM transmitter, which they managed to build around a nanoscale graphene oscillator. Details of their device have just appeared in the current online issue of Nature Nanotechnology."

(also see http://www.nature.com/nnano/journal/v8/n12/full/nnano.2013.232.html)

Smallest receiver.

According to the Guinness World record (http://www.guinnessworldrecords.com/records-10000/smallest-radio/) and I quote "the smallest radio has been built using a single carbon nanotube by scientists at the University of California, Berkeley, USA, the world’s smallest radio receiver measures only 10 micrometres diameter by 100 micrometres long. The carbon tube is electrically charged and can vibrate in harmony with incoming radio waves in the 40 to 400 Megahertz range.

The precise tuning is achieved by changing the voltage applied to the tube to isolate individual radio frequencies. Connecting this receiver to a small loudspeaker, the inventor’s laboratory rang to the sound of Derek and The Dominos' "Layla" when first switched on."

Answer 2

Having a nano sized receiver that has very simple detection circuit for the case of

• "is there" an activation signal OR
• "isn't there" an activation signal

... is relatively much simpler than monitoring a body sensor and transmitting digital data that represents what that sensor is producing.

The nano receiver may only become active when the RF signal strength from the "regular" transmitter is strong enough for the receiver to harvest sufficient energy from the field to power-up the device. This might mean that a nano-transmitter might have to contain its own power source OR have an energy harvesting circuit - this means twice the complexity - it has to receive and transmit on a different frequency - some regular passive low frequency RFID tags can back-modulate the energy from the activation transmitter but, with something so tiny it's doubtful that this would work effectively.

Transmitting data rather than receiving a simple command requires more complexity too. I guess, without appearing to sound too negative, these are sufficient reasons to put a big hurdle in the reality of a nano transmitter. The smallest I've seen is what they tag cats and dogs with, a piece of rice in length and don't ask me if it's basmati, fragrant thai or long-grain: -