As we know, biologic tissues consist of the cells, the extracellular matrix (made up of a complex of cell secretions immobilized in spaces continuous with cells), and the signaling systems, which are brought into play through differential activation of genes or cascades of genes whose secreted or transcriptional products are responsible for cueing tissue building and differentiation. The principal components of scaffolds (into which the extracellular matrix is organized in actual tissues) are collagen biopolymers, mainly in the form of fibers and fibrils. Other forms of polymer organization have also been used (gels, foams, and membranes) for engineering tissue substitutes. The various forms can be combined in the laboratory to create imitations of biopolymer organization in specific tissues. Scaffolds can be enriched with signaling molecules, which may be bound to them or infused into them.