Evaluation of Biocompatibility Using In Vitro Methods: Interpretation and Limitations - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 135

which time small proteins and proteins with low affinity are rearranged and
replaced by those with higher affinity [ 2 , 43 , 68 , 76 , 128 ]. Total protein adsorption
and the final adsorbed protein composition is affected by surface charge and
surface energy (surface wetting capability) [ 4 , 93 , 107 , 120 , 157 ].
Protein denaturation may induce cascades of unwanted events, resulting in a
foreign body reaction. Cells adhere to the implant surface through accessible
integrin binding motifs of adsorbed proteins. Integrins are cell membrane proteins
that mediate the contact of the cell with these proteins. Cells express several types
of integrins. The relative expression of these integrins is defined by the type of
surface [ 126 ] and is probably defined by the type of accessible integrin binding
motifs of the adsorbed proteins as specific ligands. Different proteins adsorbed to
the surface result in different cell adherence profiles [ 143 ]. Furthermore, the
integrin expression pattern is related to the functional differentiation of the cells
[ 82 , 101 ]. The cell be able to form a stable connection between cells and implant
surface only if integrins can cluster to focal adhesions. The ability to form focal
adhesions is defined by the intermolecular spacing size of the surface-bound
integrin adhesion ligands [ 48 ]. A spacing of above 90 nm has been reported to
inhibit focal adhesion formation [ 121 ]. This might also be one reason why cells
have difficulty in adhering to certain (sub-) micrometer structured surfaces [ 21 ].
Upon maturation of these focal adhesions, actin filaments network are strength-
ened and the cytoskeleton reorganizes accordingly. Finally, with sufficient cellular
forces (cytoskeletal contractility and/or globular actin motion), adhesions mature
into long-lasting entities. This last step is essential for cell contractility and
decisive underpinning of mechanosensing and cellular physical integrity [ 84 ].
Whereas the effects of nanostructuring are probably mainly protein-based, the
effects of microstructure are assumed to be based on surface-induced modifi-
cation of cell shape [ 23 , 38 , 75 ]. The cell shape is thought to directly affect cell
functionality [ 23 , 90 , 124 ]. Although much research has been carried out, so far
the crucial cues that steer and define cell functionality cannot be exactly iden-
tified. Furthermore, cell stiffness adapts depending on the elastic modulus of the
biomaterial surface. It is known that cells can adapt to a variation in the stiffness
of their environment within 0.1 s [ 94 ] and a modification of the substrate's
elastic modulus is known to affect cell morphology, cytoskeleton structure
and adhesion [ 159 ]. The elasticity of the material surface defines the strain in
the cytoskeleton of the cells, which affects cell physiology in vitro and in vivo
[ 35 , 138 ].
Since tissue formation adjacent to a material is directly affected and defined by
the cell-surface interactions, it may be assumed that material surfaces which best
mimic the targeted tissue environment perform best if all influences such as
chemistry, mechanical moduli and topography are adapted and considered

[ 130 , 154 ].
In vitro, bioactivity can be measured on a molecular level by assessing specific
gene expressions as a function of time and/or synthesis of certain proteins. On the
cellular level, it is evaluated by measuring cell adherence, proliferation and/or
differentiation.