Cell adhesion to materials

 

Cell adhesion is when a cell binds to an adjacent cell or a surface via the molecules on the cell surface. When a biomedical product is implanted into a body, there are 3 ways of cell adhesion to the biomaterials in the product. For example, anchoring junction, close contact and extracellular matrix contacts. For anchoring junction, Cells adhere to the extracellular matrix (ECM) film on the biomaterial with a gap of 10 to 15nm which gives a strong adhesion. To connect the cells itself and the ECM film on the biomaterial, a transmembrane protein – Integrin is needed. Integrin connects the fibronectin in ECM to different cytoskeleton filament depending on the type of junction. A type of anchoring junction – focal adhesion has the actin filament connected. There are subsequent behaviours. For focal adhesion, cells involved respond to the stiffness of adhered surface because mechanical forces are transduced to the actin filament in the cytoskeleton of the cell. The response could be increase in focal adhesions, cytoskeletal organisations and cell migration, etc. The image below helps to visualise the concept of cell adhesion with labels on parts such as, focal adhesion, fibronectin and integrin.

Cell adhesion to biomaterial not only is influenced by the types of cell junction but also the ECM surrounding the cell and a range of parameters. The parameters include the roughness of surface and hydrophobicity. The proteoglycans in ECM hydrates the space around cells which optimises the cell adhesion because cell adhesion prefers a hydrophilic surface. The roughness of surface is important to cell adhesion because surface roughness is directly proportional to the level of adhesion. Although smoother surfaces result in higher blood compatibility, the reduction in cell adhesion can cause a blockage of biomedical implant in blood vessels. This issue is very important to the heart devices as blockage of blood vessels in heart would mean potential life loss. Although titanium alloys are excellent biomaterials for its mechanical strength and corrosion resistance in artificial joints, the surface layer of TiO₂ film is not very compatible for cell adhesion with bone tissue

Experiment of cell adhesion has shown that a graphene coated titanium alloy has more vinculin than a titanium alloy without any coating. [2] This shows graphene coated titanium alloy adhere with the muscle tissue through focal adhesions because vinculin is a cytoplasmic actin-binding protein in focal adhesions which signals the coupling between actin cytoskeleton and the ECM-graphene. Therefore, more vinculin indicates a stronger cell adhesion. Subsequently, vinculin transduces signals from the integrins into the nucleus though cytoplasmic actin. The gene expression in the nucleus will be regulated and hence cell behaviours, such as cell migration, will be varied.