# The Role of Extracellular Matrix (ECM) Adhesion Motifs in Functionalised Hydrogels
## 2 ECM Components That Contribute to Tissue Engineering Strategies  
The ECM is an interconnected, 3D network of non-cellular components that form a porous, physical scaffold within which cells can adhere and embed [52]. The architecture of the ECM provides structural stability for cells, whilst allowing nutrients, oxygen, growth factors and waste products to diffuse freely [53]. Although the primary components of the ECM are water, proteins and polysaccharides, each tissue possesses a highly specific ECM composition and topography, due to biochemical and biophysical crosstalk between tissue-specific cells and the surrounding dynamic environment [54]. This crosstalk results in constant remodelling of the ECM by both enzymatic and non-enzymatic actions [55]. Despite the complex nature of any unique ECM, its architecture remains highly ordered and organised, reflecting the significance of biochemical and biophysical properties for directing the behaviour of tissue-specific cells [54].  
In addition to providing a physical scaffold for cells, the ECM is responsible for initiating essential biochemical and biomechanical signals needed for tissue development, homeostasis and fundamental cell processes [56]. This is made possible as the ECM adopts a bidirectional relationship with cells, where information is passed in a reciprocal fashion from ECM to cells through specialised receptors [55], which will be discussed in Section 4.1. Within the established cellular microenvironment, signalling cascades are coordinated by intrinsic and extrinsic factors that direct cellular processes such as differentiation [57]. These factors play a crucial role in directing the cell phenotype, with changes in this microenvironment interaction causing variations in gene expression and cell behaviour [10]. In order to control the myriad of biochemical and biophysical cues presented within any microenvironment, the ECM can act as a reservoir for these bioactive molecules. Through storing and isolating particular growth factors and cytokines, concentration gradients canbe established and maintained, leading to spatial and temporal regulation of molecule bioavailability [56].  
The ECM is a hydrated composite containing a \"core matrisome\" of around 300 proteins [56], belonging to three main families of molecular effectors: (1) insoluble hydrated macromolecules, examples of these include (a) fibrillar proteins such as collagen, (b) non-collagen glycoproteins such as laminin or fibronectin and (c) hydrophilic proteoglycans; (2) soluble macromolecules such as growth factors and cytokines; (3) proteins resident on the surface of cells [56]. Interactions between the ECM and the molecular effectors outlined above will modulate cell behaviour [11], and these merit individual discussion to more fully describe their potential roles as adhesion motifs later in this review.  
The molecules that make up the ECM of specific tissues and how they are arranged ultimately determine the final ECM structure and organisation. This resulting ECM framework constitutes distinct features that aim to mirror and promote the functional capacity of the tissue in question [54].