There are also functional or structural classifications, such as whether they are hydrogels (1), injectable (2), surface modified (3,4), capable of drug delivery (5), by specific application, and so on. The breadth of materials used in tissue engineering arises from the multiplicity of anatomical locations, cell types, and special applications that apply. For example, relatively strong mechanical properties may be required in situations where the device may be subjected to weight-loading or strain, or where maintenance of a specific cyto-architecture is needed. In others, looser networks may be needed or even preferable. The type of materials used is also dependent on the anticipated mode of application (open implantation vs. injection or minimally invasive procedure), the nature of any bioactive molecules that might be released, the need for surface functionalization, the needs of the cell types of interest in terms of porosity, and other issues. Despite this broad spectrum of potential materials, there are certain generic properties that are desirable.