Fifteen Eighty Four

The ability to sense and manipulate the body at the level of individual cells has long been a vision for the future of medicine, as well as a staple of science fiction. When it is finally realized, this vision will have a revolutionary impact on human health. For example, consider the treatment of cancer: instead of flooding the body with intravenous chemotherapy, a doctor able to perform cellular-level manipulation could target the tumor cells directly and deliver the medication exactly where it is required, an application known as targeted drug delivery. This method would allow the doctor to treat the tumor with high doses of chemotherapy drugs, at levels that would be toxic to the patient if applied throughout the body. Other intervention at the cellular scale would lead to similarly dramatic advances, such as cellular-level reconstruction of tissues instead of invasive surgery, or real-time, detailed sensing and monitoring of the health of the entire body at extremely fine resolution.

Futuristic applications such as these, operating at the cellular level, are referred to as nanomedicine. Often, these applications envision a swarm of tiny machines, which are themselves the size of individual cells. Remarkably, designing and producing such machines is well within the capability of modern manufacturing, using techniques such as micro electro-mechanical systems (MEMS); while using techniques from systems biology, it is also possible to produce custom biological “machines”, that is, cells with a custom genome that can perform a particular function for which they are designed. We appear to find ourselves already living in a “future” where we can build tiny robots to perform nanomedicine – so what is holding back the “futuristic” applications?

Aside from building the machines themselves devices, the key issue is that a single cell-sized robot has extremely limited capabilities: in the drug-delivery application, the drug payload that such a robot could carry is minuscule compared with the size of a tumor. Most applications involve more power than a single robot can deliver, requiring a swarm of tiny machines – which requires communication. If the drug-delivery robot could coordinate its actions with thousands or millions of its fellows, they could simultaneously release their payloads in a specific location, raising the local concentration of the drug to a much higher level, and deliver the desired effect. Solving the communication problem among nanomachines would enable these swarms to perform complex and powerful tasks.

As it turns out, the kinds of communication systems that have enabled the modern interconnected world are a poor solution at the nanoscale. Almost all of the communication systems that we commonly use are, in some way, electromagnetic, from mobile phones, to wifi, to wired computer networks, and even fiber-optic internet backbones. However, electromagnetic communication requires energy resources (limited at the nanoscale), relatively large antennas (much larger than individual cells), and an appropriate environment for wave propagation (precluded by the highly ionic environment within the human body). So we look to nature for a solution: natural cells have solved the intercellular communication problem in an entirely different way, namely by using molecular communication to share information at very small scales. Using this method, patterns of chemicals are used to form messages, and chemical receptors are used to detect them. This solution has many desirable properties for nanoscale applications: molecular communication is simple and biocompatible, has low energy cost, and works well at very small scales – ideal for coordinating action at the scale of the individual cells in the human body.

Molecular communication relies on vastly different physical principles than electromagnetic communication. Signals molecules propagate from transmitter to receiver via diffusion (or microscopically, via Brownian motion), and as a result, signal propagation is far slower and noisier than in conventional communication systems. Furthermore, the physical principles of a molecular receiver are also unique, relying on ligand receptors that generate an ion current when bound. When designing molecular communication systems, novel techniques must be developed, starting at the physical layer, to account for the different communication environment. Moreover, the constraints imposed by limited energy resources or biological substrates must be accounted for when considering nanomedical applications. All potential means of encoding and transmitting chemical messengers can be considered, from adjusting concentration, to generating waves of chemicals propagating through time, to inscribing information on molecules as in DNA. In many cases, novel physical principles must be developed and modeled, such as when considering the swarming and self-assembly behaviour of nanomachines.

The state of the art in molecular communication has advanced dramatically in the past several years. Researchers have developed novel experimental testbeds and techniques, approaching the complexity needed to tackle nanomedical applications. Moreover, novel applications beyond nanomedicine are actively being explored: for example, molecular communication may have applications in any environment where electromagnetic waves cannot propagate, such as underground or under the ocean, thus enabling important applications in security and search-and-rescue. Interest in these applications has resulted in a new IEEE networking standard, IEEE 1906.1, giving molecular and nanoscale communication a standing similar to familiar IEEE-standardized communication methods such as Bluetooth and WiFi.

The newly-revised second edition of our book, Molecular Communication, explores all areas of this important new form of communication, giving essential scientific foundations for readers of all backgrounds, while introducing modern developments in this exciting and dynamic field. We hope the book inspires interest from a new cohort of researchers, spanning the fields of telecommunications, chemistry, physics, and biology, to help unlock its futuristic potential.

Title: Molecular Communication

Authors: Tadashi Nakano, Andrew Eckford and Tokuko Haraguchi

ISBN: 9781108832762