## 2 Bioinspired Networking: A Brief Overview
### Information Exchange in Cellular Environments  
Living organisms consist of billions of cells interacting with each other in a remarkably harmonic way. While current sensor network protocols suffer from scalability and efficiency issues, intercellular biological networks exhibit a robust and distributed behavior, high efficiency, and self-healing capabilities such as wound healing controlled by local cell clusters inside the organic tissues.  
Signaling in biological systems occurs at multiple levels [27]. The cell communication in the human body can take place between adjacent cells (paracrine signaling) between as well as cells distributed all over the body (endocrine system). The communication between neighbor cells takes the form as an exchange of molecules (signals) via signaling pathways. One cell releases (secretes) cytokines into the intercellular space. The neighboring cell expresses a specific surface molecule, _i.e._, the ligand (the receptor), that is activated by a change in its sterical or chemical conformation when it binds to proper cytokines. The activated receptor molecule is able to further activate intracellular molecules which enters the cell nucleus to alter gene expression. Only cells with a very specific receptor are able to receive the information, _i.e._, the protein binds at the receptor.  
For the communication over longer distances, the cells secrete hormones into blood vessels and the blood carries the molecules to all parts of the body. Signaling pathways are exposed to \"noise\" from interaction with ambient, residual molecules and from interference with other signaling pathways. Inhibitory pathways are interfering with the activatory pathway and the final effect is dependent on the strongest signal.  
The notch signaling pathway, which is present in most multicellular organisms, can be used to model a well-studied cell communication mechanism [28]. The notch protein sits like a trigger spanning the cell membrane, with a part of it inside and a part outside (Figure 2). Because most ligands are also trans-membrane proteins, the receptor is normally triggered only from direct cell-to-cell contact. In this way, groups of cells can organize themselves, such that, if one cell expresses a given trait, this may be switched off in neighbor cells by the intercellular notch signal. In this way, the groups of cells influence one another to distribution \"tasks\" in a large structure.  
A simple model has been presented by [PERSON] _et al._[28], where \(N_{i}\in[0,1]\) denotes the level of notch activation, and \(D_{i}\in[0,1]\) denotes the level of delta activity for the \(i\)th cell. The following set of ordinary differential equations govern the behavior of the \(i\)th cell can be formulated:  
\[\frac{dN_{i}}{dt} = f(\bar{D}_{i})-N_{i} \tag{6}\] \[\frac{dD_{i}}{dt} = \mu(g(N_{i})-D_{i})\,, \tag{7}\]  
where \(\mu\) is a positive constant, \(\bar{D_{i}}\) represents the average delta activity across the neighbors of the \(i\)th cell, and \(f(\bar{D_{i}})\) and \(g(N_{i})\) are functions expressing the production rate of notch and delta activity at the cell, respectively. The rate of production of notch activity increases in response to the increasing level of delta activity in neighboring cells, whereas the rate of production of delta activity decreases with increasing level of notch activity [28].  
Figure 2: The delta-notch mechanism constitutes an effective feedback look between delta and notch ligands in neighboring cells (Adapted from [28] Figure 1).