An Intro to Nanotechnology: What is Molecular Communication?

Nanotechnology is defined in a number of ways. One such definition is provided by Collins English Dictionary. Copyright © HarperCollins Publishers which states nanotechnology as “a branch of technology dealing with the manufacture of objects with dimensions of less than 100 nanometers and the manipulation of individual molecules and atoms”.

Nanotechnology deals with the miniaturization and fabrication of devices on a scale ranging from 1 to 100 nanometers. The prefix nano means one-billionth i.e., (1x10-9). Richard Feynman was the physicist who discussed the possibility of dealing with molecules directly in his lecture "There's Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics" at the annual American Physical Society meeting at Caltech on December 29, 1959

Nanotechnology has a number of applications in multidisciplinary fields such as physics, chemistry, biology, sports, health, computer science, and ICT(Information and Communication Technology) to name a few. This article explores the applications of nanotechnology in the fields of computer science and ICT.

Fabrication of nano-sized devices called nanomachines is made possible through nanotechnology tools. Nanomachines are functional devices and capable of performing trivial tasks like sensing, actuation, computing, and data storage. Single nanomachines are only capable of performing trivial tasks.

Therefore, to perform more complex tasks they must be interconnected to form a network. Nanomachines can be manufactured using three approaches top-down, bottom-up, and bio-hybrid.

Approaches to manufacture nanomachines

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Bottom-up: In this method atoms and molecules are self-assembled into nano- structures. Hence bottom-up refers to the construction of large objects from smaller building blocks, i.e. construction of nanostructures using atoms and molecules, rather than bulk materials.

Top-down approach: Top-down approach is the building of nanostructures and materials by mechanical methods such as lithography, and by bulk technology. The top-down method for processing of nanostructured materials involves starting with a bulk solid and then obtaining a nano-structure by its structural breakdown. Mechanical and thermal methods are also used for the preparation of nanostructured materials from the bulk.

Bio-Hybrid Approach: Bio-Hybrid approach is the assembly of nano-sized biological components and manmade nano-structures, to develop new nano-machines. This process includes positioning of the constituent sub-nano building blocks into an assembly of a functional structure. This approach is very expensive and time-consuming as compared to other states of the art of manufacturing technologies.

Molecular Communication: A nano networking technology

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Nano communication network is a novel communication paradigm that enables communication among nanoscale devices at a scale ranging from 1-100 nm.

At this scale, the receivers and senders used by traditional wired and wireless communication are not workable. Therefore nanomachines have been developed to realize communication at this tiny scale. Nanomachines are tiny devices that are able to perform trivial tasks and can do complex tasks after forming a network (nano network).

Nanomachines can be biological entities or can be synthesized materials.
Molecular communication (MC) is the paradigm in nano communication networks that utilize molecules for communication among nanomachines. Molecular communication is biologically inspired i.e., it adapts the communication mechanisms already existent in nature for communication among living organisms.

The human body is composed of a large-scale heterogeneous network where molecular communication takes place for intrabody communication.

There are many intrabody applications where small-scale communication is necessary e-g targeted drug delivery, BMI (Brain Imaging Interface), tissue engineering and cell repair etc. Various communication and networking aspects of MC are currently being explored by the research community.

Molecular communication is as old as the existence of nature, as communication has been taking place between living organisms since then. However, with the advancement in computer networking, the significance of Molecular communication has been brought to light all over. The research on molecular communication from the networking perspective is almost two decades old but still is immature. There are still a number of open issues regarding the design and mathematical modeling of system components that need to be addressed.

Communication ranges of Molecular Communication

Molecular communication is able to take place in three ranges.

(1) Short-range communication using molecular motors is the mechanism where inter-cell and intra cell communication takes place using molecular motors, which are carriers of information encoded molecule.

(2) Short-range communication using calcium ions is another mechanism of molecular communication where communication might take place either between physically adjacent cells or distant cells using calcium ions (Ca2+).

(3) Long-range communication using pheromones is the communication mechanism that takes place between sender and receiver nanomachines that might be millimeters to kilometers apart. Application domains long-range communication is military field and environmental applications.

Molecular Communication reference models

In this section, some prominent architectures and conceptual models for MC are explored. We know that the OSI ( Open Systems Interconnection) is the conceptual communication model used for traditional wired and wireless networking systems. Researchers in MC have proposed a similar layered model for MC as well.

The molecular communication model consists of five layers i.e., Application Layer, Molecular transport layer, Molecular network layer, Molecular link layer, Signaling sublayer, and Bio-nanomachine sublayer. Nakano et al have described the functionality of each layer in detail and highlight the open research challenges according to each layer.

TCP is a connection-oriented protocol used in traditional communication. TCP-like connection-oriented communication protocol for molecular communication between biological nanomachines is also proposed, to avoid congestion between sender and receiver.

Molecular communication inside the human body has been explored extensively. Intra-body molecular communication channels, i.e., nanoscale neuro-spike communication channel, action potential-based cardiomyocyte molecular communication channel, and hormonal molecular communication channel, are introduced. The architecture of nanonetworks inside the human body is explored to get inspiration for the development of new foundations in molecular communication.

Healthcare applications of Molecular Communication

Applications of MC related to health care have can be classified into two groups namely Diagnosis and Treatment. ADD (Antibody Mediated Drug Delivery Systems) is a therapeutic diagnosis mechanism that has been analytically modeled using molecular communication.

This will be a cost-effective and reliable drug discovery and optimization model. The accuracy of this model has been confirmed by finite element simulations. In the diagnostics area, nano-sensors, nanophotonics and nano cameras are playing a vital role in the early detection of biological and environmental contaminants in the body.

Nanomedicine is the biomedical application that is trending in that top five research areas of nanotechnology. Other biomedical applications of MC include immune system support, medical implants, drug delivery systems, health monitoring, and correcting genetic disorders to name a few.

More applications of nanonetworks can be found here.

Principles of Molecular Communication

Molecular communication protocol consists of information molecules that contain information to be transmitted, sender bio-nano machines that send information molecules, and receiver bio-nano machines that receive information molecules.

Other types of molecules might be included in the system such as transport molecules which move information molecules, guide molecules which guide the movement of transport molecules, and interface molecules for selective transport of information molecules.

Image Source: Science Direct

Different phases of molecular communication are described below:

Encoding: In this phase, sender nanomachines encodes the information into the information molecules in various forms.
Sending: In this phase sender bio nanomachine releases information molecules in the environment.
Propagation: It is the phase in which molecules travel from sender nanomachine towards receiver nanomachine. This transport can be either passive or active. Passive transport is the through the diffusion of molecules in the environment without chemical energy, whereas in active transport information molecules bind to molecular motors.
Receiving: Transmitted molecules are received from the aqueous medium in this phase usually with the help of chemical receptors.
Decoding: In this phase, the captured molecules are decoded by receiver nano-machines into the form of chemical energy.

Molecular Communication Interface

A vesicle-based communication interface provides a mechanism to encapsulate messenger molecules in an aqueous medium, which aims to transport different types of information molecules in diverse propagation environments.

A liposome is an artificially created spherical vesicle composed of a phospholipid bilayer membrane surrounding a discrete aqueous compartment (an inner aqueous phase); its diameter can be controlled from tens of nanometers to tens of micrometer.

The liposomal structure compartmentalizes information molecules from the propagation environment and provides a generic architecture to transport diverse types of information molecules, independent of their biochemical and physical properties. The liposomal structure also protects information molecules from deformation.

Molecular Propagation System

In Molecular Communication, there exist three types of propagation mechanisms. Walkway, Flow-based and Diffusion-based mechanisms.

1. In walkway-based propagation, molecules connect the transmitter to the receiver by traveling on predefined pathways using carrier substances such as molecular motors.

2. In a flow-based mechanism, molecules flow towards the receiver in guided and predicted manner. In a flow-based mechanism, carrier entities can also be used for molecular transmission instead of showing random entities. An example of a flow-based mechanism is the bloodstream in the human body (hormones).

3. In the diffusion mechanism, molecules flow in the fluidic medium and can be affected by non-predictable turbulence present in the fluidic medium. Diffusion-based molecular communication is considered to be the most practical means of wireless communication between nanomachines. In diffusion-based communication, the molecules flow from a high concentration region to low concentration regions in fluidic medium. Key components of diffusion-based transmission are transmitter, propagation system and a receiver.

Transmitter: Each transmitter has a limited store of messenger molecules. Whenever there is a message to be transmitted, a transmitter emits a specific amount of molecules into the environment.

Molecules are released or absorbed by means of molecular flux, which is able to modulate the molecular concentration rate at the transmitter location as a function of time. The amplitude of the transmitted signal defined by the number of released messenger molecules. Each nano-robot has a 0.1 µm3 tankage and has an initial source of 108 molecules of which Q=103-104 molecules are emitted at a time.

Propagation system: The prorogation is accomplished by a thermally activated diffusion mechanism, in which flux is achieved from regions of high to low concentrations via random collision with the underlying medium. Messenger molecules propagate in two ways; normal diffusion where interactions and collision among emitted particles is neglected and anomalous propagation where movements of emitted particles can be affected by the collision among interaction among other particles.

Receiver: The particle reception process is accomplished by sensing the messenger molecules in the receiver sensing area. The receiver sensing area is a small surface around the receiver in which the messenger molecules bind to the receiver chemical receptors with high probability. The received nano-machine biochemically reacts with the received information molecules and an electrical inner cell signal is produced as the output.

Lessons to be learned

There is a plethora of applications of Molecular Communication (MC) that need researchers' attention. Research in MC is still in its infancy and there a lot of open research challenges that need to be addressed.

MC displays potential for enabling complex applications of nanotechnology that require the collaboration of very small entities (nanomachines). MC in nano-scale communication has three advantages.

MC has a number of advantages over traditional electromagnetic communication. It requires very low energy, MC is able to decrease the size of the antenna, and MC has biocompatible characteristics for in vivo systems, as long as harmless molecules are used.

(Featured image source : Juraj Lenhard from Pixabay)