Biomedical Engineering Reference
In-Depth Information
response was partly dependent on the proteins adsorbed by the implant surfaces (Bender
et al. 2000). The first protein layer adsorbed on the implant surface affects the cellular
adhesion, differentiation, and production of extracellular matrix production (Combes and
Rey 2002; Ducheyne and Qiu 1999). It also affects dissolution (Bender et al. 2000), nucle-
ation, and crystal growth of HA as well as the final fixation between the implant and sur-
rounding tissues (Xie, Riley, and Chittur 2001). Albumin is usually selected for this kind
of study due to its high concentration in blood plasma, favorable diffusion coefficient, and
ability to bind other ions and molecules (Jenney and Anderson 2000). It has been reported
that albumin could slow down the nucleation rate and growth rate of new bonelike apatite
in albumin-containing SBF.
Cell Response to HA Coating
Cell culture methods have been used to evaluate the biological compatibility of a material
for more than two decades. Investigations on cell responses to materials can provide more
details of understanding cell-materials interactions and can aid in establishing actual bio-
logical responses to artificial materials (Knabe et al. 2000). Because HA-coated metallic
implants are used for hard tissue replacement/repairing,
in vitro
models using osteoblas-
tic cells are essential and valuable tools for the initial assessment of candidate implants.
Osteoblastic cells, which arise from pluripotential mesenchymal stem cells, have a set of
distinguishing characteristics that include the ability to synthesize osteoid or bone matrix
and to mineralize the osteoid to get the calcified bone (Aubin et al. 1995). Since then, osteo-
blastic cell lines are commonly employed for
in vitro
cellular assessments of hard tissue
implants.
Initial Cell Attachment on HA-Coated Implant
The fixation of implants to bone is based on the process of osteointegration, which leads
to direct apposition of mature living bone onto the implant surface (Menezes et al. 2003;
Dorota et al. 2005). Since that osteointegration process is strictly mediated by osteoblastic
cells, the fate of such implants is thus determined by the response of cells to the material's
surface. Therefore, the implant should create favorable conditions for osteoblast attach-
ment, spreading, growth, differentiation, and functionalization. Virtually, in a physiologi-
cal environment, all implant surfaces become immediately coated with a 1- to 10-nm-thick
adsorbed protein layer before cells can adhere to the material (Kasemo 1998; Tengvall
2003). The rapid adsorption of proteins effectively translates the structure and composition
of the foreign surface into a biological language, which is also a response to the following
host responses (Wilson et al. 2005). As such, when the cells arrive at the implant surface,
they can only “see” a protein-covered surface. Those adsorbed proteins offer necessary
binding sites to those anchorage-dependent cells, leading to the initial cell attachment
onto the surface of biomaterials. For anchorage-dependent cells (e.g., osteoblastic cells), ini-
tial cell attachment and spreading are crucial prerequisites in determination of long-term
viability of cells on the implant surface, involving DNA synthesis and cell growth (cell
proliferation), differentiation, mineralization, and successful osseointegration (Folkman
and Moscona 1978; Baxter et al. 2002). Therefore, a quick attachment of a certain amount of
osteoblastic cells and the following rapid cell spreading are strongly expected.
Figure 1.12 shows the typical morphological change sequences of osteoblastic cell (MG63)
spreading on sol-gel derived HA coatings during the first 4 h of incubation (Wang, Zhang
et al. 2008). The process of cell attachment and spreading can be described by the follow-
ing steps: (1) adsorption of proteins on coating surface (surface roughness plays a positive
