Designing the Biocompatibility of Biohybrids - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 287

2 Compatibility of the Structural Compound
The structural component of a biohybrid is normally a biomaterial which needs to
prove its biocompatibility in vivo. According to the definition of the Society for
Biomaterials (also known as the ''Williams' definition'' [ 1 ]), biocompatibility is
the ability of a material to perform with an appropriate host response in a specific
application. According to Anderson et al. [ 2 ], placing a biomaterial in vivo always
requires an injection, insertion or surgical intervention which is associated with
damaging tissues and therefore is associated physiologically with inflammation,
wound healing and a foreign body reaction (FBR).
The damage of an intact tissue leads to three episodes of wound healing:
inflammation, proliferation and remodelling. The inflammation caused by a
material inserted in a living tissue can be divided into an acute and a chronic form.
The acute inflammation is identified by the presence of the fast invading cells as
neutrophils, monocytes and lymphocytes. Due to vessel leakages caused by the
implantation method platelets degranulate and will attract leucocytes. Depending
on the age of injury, the dominating cell type differs. The first cells arriving at the
location after implantation are polymorphnuclear cells (PMN), especially
neutrophils. Their recruitment is initiated by cytokines, e.g. chemoattractants from
injured cells or histamine by mast cells. The neutrophils will disappear within the
first two days after the implantation of the biomaterial [ 2 ]. The chronic form of an
inflammation is characterised by the presence of macrophages, the formation of
foreign body giant cells and the encapsulation of the biomaterial with fibrous
tissue. Monocytes invaded from the blood will arrive at the implantation site and
will differentiate into macrophages. They produce several chemokines like TNF-a,
IL-6, G-CSF and GM-CSF to attract additional monocytes and macrophages to
the area of interest. Macrophages adherent to the material surface produce TNF-a,
IL-1 and IL-6 to activate regional T-lymphocytes.
During the last decades, the biomaterial research concentrated on designing
materials that are well tolerated by the living organism, e.g. a newly formed
fibrous capsule that shields the implant from the tissue without initiating a severe
FBR. But today it has been realised, that specific cell-implant interactions could be
advantageous for the acceptance of the implanted foreign body to the host tissue
[ 3 ]. Modulating the implant surface can modulate the immune reaction towards
accepting or tolerating the biomaterial.
The human immune system is divided into two different types: the innate and
the adaptive immune system. The innate immune system is non-specific and
consists of natural barriers, e.g. skin and mucous membranes, phagocytes and the
complement system. The adaptive immune system is mainly comprised of lym-
phocytes, and it can be distinguished between the humoral and the cell-mediated
immunity. The innate and the adaptive immune system work together, hand in
hand, and cannot be separated completely from each other.
Immediately after the insertion of a biomaterial into a living tissue, proteins
from blood and interstitial fluids will adsorb to the biomaterial surface [ 3 ].