Adhesives Terms Links

A term more general than polymer is that of macromolecule. Macromolecules are chemical compounds formed from at least one thousand atoms linked by covalent bonds. They are common as natural substances like cellulose, proteins, lignin, etc., and also as synthetic compounds including plastics, fibers, elastomers, coatings, and adhesives. Many synthetic and some natural macromolecules have repetitive structures and are known as polymers. For example, cellulose is made from p-D-glucose residues interconnected by p-glucoside (1->4) links, polystyrene is made from 1-phenylethylidene units, etc. 

The hematopoietic bone marrow is located primarily in the central portion of the pelvis, ribs, vertebrae, skuU, and femoral and humeral epiphyses. The anatomic structure of the bone marrow is characterized by the central venous marrow sinus, which is linked by coarse vascular sinusoids that intertwine a reticulin mesh where the cells are suspended. Thus hematopoiesis occurs in the extravascu-lar marrow spaces, which also contain endothelial cells, fibroblasts, macrophages, and adipocytes, collectively termed bone marrow stroma. Stromal cells are thought to be important hematopoietic components, providing growth factors, collagen, and cell adhesion proteins.When these cells are combined with accessory cells (lym-phocytes/monocytes) and cytokines, the mixture is referred to as the hematopoietic microenvironment. 

Polymer networks such as epoxies play an increasing role as adhesives in industry. Two properties are of special importance for their application (a) a strong adhesive bond is required between the solidified adhesive and the bonded object, which is often a metal (b) the mechanical stiffness of the adhesive has to be adapted to the desired level. As a consequence, the adhesive has to be selected according to its adhesion properties as well as its mechanical properties. Several studies have shown that both properties are linked as soon as the epoxy polymer layer is sufficiently thin the contact of the polymer with the substrate may induce in the polymer a broad interphase where the morphology is different from the bulk. Roche et al. indirectly deduced such interphases, for example from the dependence of the glass transition temperature on the thickness of the polymer bonded to a metal substrate [1]. Moreover, secondary-ion mass spectroscopy or Auger spectroscopy provided depth profiles of interphases in terms of chemical composition, which showed chemical variations at up to 1 pm distance from the substrate. 

A. Surface Free Energies. Surface free energies must dominate any explanation of the adhesion between different phases which are not mechanically linked. Current levels of understanding of adhesiveness are such that actual adhesive strengths are always much less (1-0.1%) than those predicted by thermodynamic analysis, and often there is apparently little correlation between the two. Further refinement of the theory of adhesiveness will require understanding of the importance of flaws in an adhesive joint and of the relative contributions of polar and dispersive Van der Waal s interactions. The following is an analysis of adhesion in terms of surface free energies. 

Mesenchymal cells are spherical when in suspension and form stable protrusions only upon attachment to a surface [15]. The cells can adhere tightly to the substratum in tissue culture utilizing structures termed focal adhesions [26]. Upon extension of a lamellipod, close contacts to the substratum form, which then convert into localized adhesive clusters that can be detected using interference reflection microscopy [107, 166], These focal contacts have clusters of transmembrane proteins termed integrins that bind to extracellular matrix molecules and link intracellularly to the actin cytoskeleton to form stress fibers [251]. Adhesion to the substratum is necessary for stabilization of the lamellipod if a lamellipod extends over a nonadhesive surface, it then retracts [14],... 

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