Applications of Molecular Communication Systems
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A molecular communication system is defined as a system of bio-nanomachines that transmit and receive information using chemical signals or molecules. A bio-nanomachine that constitutes a molecular communication system is made of biomaterials with or without non-biological materials, approximately 1–100 μm in size, and capable of processing molecules. Examples of molecular communication systems are naturally occurring biological systems such as bacterial populations, epithelial sheets, and immune systems where biological cells represent bio-nanomachines. Examples of molecular communication systems also include artificial or synthetic biological systems designed for specific applications such as biomolecular sensing and targeted drug delivery.
Molecular communication research can be divided into its first phase (2005–2009) and second phase (2010–present). The first phase of research (2005–2009) focuses on experimentally recreating functionalities and components of naturally occurring biological systems. This includes the first prototype implementation of molecule transport systems using motor proteins and DNA molecules (Hiyama et al., 2008a). This also includes experimental demonstration of signal amplification mechanisms through calcium signaling (Nakano et al., 2009), self-organized microtubule networks (Enomoto et al., 2006), and encapsulation of information molecules and transport of information molecules from liposomes to biological cells (Moritani et al., 2010).
The second phase of molecular communication research (2010–present) has seen a wider variety of research topics concerning molecular communication. This phase of research includes the use of communication theory and mathematical tools to understand physical properties of molecular communication (Mahfuz et al., 2010; Farsad et al., 2012; Pierobon and Akyildiz, 2013). It also includes design, analysis, and computer simulations of higher-layer mechanisms of molecular communication (Lio and Balasubramaniam, 2012; Felicetti et al., 2014). It further includes standardization efforts to establish molecular communication as a standard framework and to develop simulation tools (Bush et al., 2015).
Biomolecular sensing: Specific molecules in the human body serve as biomarkers for certain diseases or medical conditions. More detailed information such as the spatial distribution of molecules is potentially useful for in-depth diagnosis. For biomolecular sensing, bio-nanomachines may sense their environment, communicate with others, and collectively determine whether specific molecules exist in their environment. Alternatively, bio-nanomachines may transmit sensed information to external devices or control units for diagnosis of their environment (Rogers and shung Koh, 2016; Abdi et al., 2017).
Targeted drug delivery: The delivery of drug molecules to target sites in the human body is expected in nanomedicine; it maximizes the efficacy of drug molecules and at the same time reduces potential side effects at nontarget sites. For drug delivery, bio-nanomachines may be embedded with drug molecules and either injected directly into target sites or intravenously to propagate to target sites. Bio-nanomachines may use molecular communication to collaborate to search for target sites, aggregate at the target sites, and release embedded drug molecules (Okaie et al., 2016; Wei et al., 2013).
Tissue regeneration: In tissue development, biological cells communicate through synthesizing growth factor molecules and transmitting them into the environment. Growth factor molecules propagate in the environment and bind to cell surface receptors of the target cells. The concentrations and types of growth factor molecules modulate migration, proliferation, and differentiation of target cells, leading to the formation of a tissue structure. For the repair or construction of a tissue structure, bio-nanomachines made of living cells may be deployed in the human body. Bio-nanomachines use molecules to communicate with tissue-forming cells, while they divide and grow to help the tissue structure formation.
Internet of Bio-NanoThings: Molecular communication systems may also be interfaced to and integrated with existing communication systems. Future mobile phones or wearable devices may be integrated with bio-nanomachines capable of molecular communication for on-chip analysis of biochemical signals (e.g., molecules in blood or from sweat) (Hiyama et al., 2008b). Further, such devices and molecular communication systems may be integrated into the Internet to form the Internet of NanoThings (Akyildiz and Jornet, 2010) or the Internet of Bio-NanoThings (Akyildiz et al., 2015).
Molecular Communication System Model
- Bio-nanomachines are sensors and actuators to form molecular communication systems. A bio-nanomachine is characterized with the three criteria: material, size, and functionality (Nakano et al., 2012). A bio-nanomachine is composed of biomaterials with or without non-biological materials. The size of a bio-nanomachine ranges from the size of a macromolecule to that of a biological cell (i.e., up to tens of μm). A bio-nanomachine implements a set of biochemical functionalities such as sensing a certain type of molecule to achieve application-dependent goals (e.g., biomolecular sensing). Examples of bio-nanomachines are genetically engineered biological cells.
The environment is a three-dimensional space where bio-nanomachines are deployed. It is typically an aqueous environment (e.g., an internal environment of the human body), containing molecules and energy sources for bio-nanomachines to operate. The environment may also contain flow by which the operation of bio-nanomachines can be disturbed or alternatively enhanced (Kadloor et al., 2012; Tavakkoli et al., 2017). The environment may generate events or information of interest, or contain targets that generate such information. For example, in biomolecular sensing applications, a change of the human body into an abnormal state represents such an event, and in targeted drug delivery applications, cancer cells represent a target.
Sources are entities in the environment that provide useful information. A source may be a bio-nanomachine or a group of bio-nanomachines that detects an event or target in the environment. A bio-nanomachine functioning as a source may detect an event or target biochemically and transmit the detected information by propagating information molecules to other bio-nanomachines or sinks.
Sinks are entities that receive and collect information from bio-nanomachines or sources. A sink may be a bio-nanomachine or a group of bio-nanomachines that processes information and performs application-dependent functionalities such as releasing drug molecules. A sink may also be a conventional device capable of traditional communication (e.g., wireless communication) (Nakano et al., 2014). A sink may be made from materials that are not compatible with the environment and may be orders of magnitude larger than bio-nanomachines. A sink may function as a gateway that interconnects molecular communication systems with external systems. Examples of sink devices include implantable medical devices (Kiourti et al., 2014).
Noise exists in molecular communication systems in various forms. The first type of noise is thermal noise. Due to thermal noise, molecular communication among bio-nanomachines and the operation of bio-nanomachines become stochastic. The second type of noise is physical noise. The high viscosity of the environment and fluid in the environment generate physical force, making it difficult for molecules to propagate and for bio-nanomachines to move. The third type of noise is caused by molecules existing in the environment or noise molecules. Due to noise molecules, molecular communication among bio-nanomachines, and the operation bio-nanomachines can be disturbed.
Target Detection and Tracking
Okaie et al. (2016) describes key functionalities of molecular communication systems: target detection and tracking. Target detection is a functionality of molecular communication systems to detect a target in a given environment, while target tracking is aimed at detecting and tracking targets as they move. In nanomedical applications, targets can be disease sites, pathogens, infectious microorganisms, or biochemical weapons that represent a potential threat to the environment; the timely detection of targets and tracking of targets are important to provide immediate treatments or further analysis of the environment.
The mechanisms for target detection and tracking proposed in Okaie et al. (2016) use two types of molecule: repellents and attractants. In search of a target, bio-nanomachines release repellents to quickly spread in the environment; the released repellents form the concentration gradient in the environment, bio-nanomachines move toward lower concentrations of repellents, and therefore they move away from each other to help their search process. Upon detecting a target, they release attractants to recruit other bio-nanomachines in the environment toward the target location; the released attractants also form the concentration gradient in the environment, and bio-nanomachines move toward higher concentrations of attractants, namely, toward the target location.
The current molecular communication research focuses more on theoretical work than experimental one; wet laboratory experiments form an importance piece of future work. Wet laboratory experiments help identify practical issues and gain insight into biologically implementable designs of molecular communication systems. An objective would be to show the collective behavior of bio-nanomachines for a specific application such as target detection and tracking.
Wet laboratory experiments are however challenging for communication engineers, since experimental facilities are often not available to them. To solve this problem, tabletop molecular communication platforms are developed in (Farsad et al., 2013). Tabletop molecular communication platforms are built using macroscale devices such as electronic sprays and sensors, yet communication is performed by propagating molecules between the macroscale devices. Tabletop MC platforms help communication engineers gain experience in molecular communication and develop realistic models to analyze the unique features and characteristics of molecular communication systems.
Nonetheless, theoretical work remains important in future work. Biologically realistic modelling and computer simulations will help reduce the cost and time required to carry out wet laboratory experiments. This will also help us understand the detailed dynamics of molecular communication, such as spatiotemporal concentrations of molecules, which is difficult to observe in wet laboratory experiments. Future work should therefore use an integrated approach of experimental and theoretical investigations in order to design and develop practical applications of molecular communication systems.
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