Soft Tissue Replacements - Biomaterials - page 218

rubber control specimens in animal dental studies. Diamond-like carbon surface coatings inhibit infec-
tion, so a combination of hydroxyapatite and diamond-like coatings may be useful and are currently
being investigated (Smith et al., 2006). Indeed, the surface topography has been consistently found to
play a large role in tissue integration, where grooved surfaces seem to promote less infection and better
tissue ingrowth than either smooth or pitted surfaces (Chehroudi and Brunette, 2002). Researchers have
investigated coatings such as laminin-5, which keratinocytes normally produce to enhance migration,
adhesion, and ingrowth (El-Ghannam et al., 1998; Uchida et al., 2004). More work will be necessary to
pinpoint a simple and effective solution to the long-term maintenance of percutaneous implants.
9.2.2.2 Artificial Skins
A major cause of death for severely burned patients is infection as well as fluid and electrolyte loss. A
wide variety of artificial skins have been developed to close burn wounds following excision of necrotic
tissue (Jaksic and Burke, 1987). Criteria for artificial skins include good adherence to the wound
site, elasticity, durability, bacterial resistance, hemostaticity, antisepticity, ease of application, ease of
removal, nontoxicity, biocompatibility, nonantigenicity, durability, and low cost. Artificial skins that are
permanent replacements have the added criteria of being esthetically pleasing.
After many early attempts to make artificial skin using tanned collagens, collagen gels, and collagen-
impregnated PET mesh, a wound-covering dermis with controlled physicochemical properties was
designed using a crosslinked collagen-polysaccharide (chondroitin 6-sulfate) composite membrane
(Chvapil, 1982). This particular configuration was specifically constructed to have tunable porosity,
flexibility, and moisture flux rate, but required an epidermal autograft. Subsequently, temporary arti-
ficial bilayered skin substitutes were developed, where temporary tissue-engineered replacements are
particularly useful alternatives for burns requiring larger area coverage. These devices can be similar to
a synthetic dressing, incorporating a nylon mesh and a silicone rubber component, but also integrat-
ing allogeneic fibroblasts. Of note, in these clinically available devices, the fibroblasts are grown on
the construct; subsequently, the cellular construct is frozen for later transport. Hence, the purpose to
the temporary covering is to stimulate or to allow fibrovascular growth into the wound bed by provid-
ing appropriate cellular products, for example, matrix proteins and growth factors. A related dermal
technology was developed as a “permanent” covering, incorporating allogeneic fibroblasts on a degrad-
able mesh. This product is also cryopreserved for shipping. Implantation of allogenic fibroblast/polymer
constructs has proved to be useful for providing long-term skin replacement. Human fibroblasts are
seeded on the nylon mesh and allowed to proliferate in culture, the concept being that the cells will
release necessary growth factors and thus stimulate healing response in the wound. The cellular meshes
are frozen, thus killing the cells but retaining the factors in the mesh for release following grafting. This
device prevents water evaporation but has not been found to induce fibrovascular growth that may be
necessary for preparing the wound bed for a permanent graft or skin substitute.
Epithelial cells derived from a burn patient may be cultured in vitro to cover a wound area, potentially
a feasible option for superficial or partial thickness wounds. Deep dermal or full thickness skin wounds
can be treated with a sheet of keratinocytes that are grown on mouse cell feeder layers. Although the
mouse cells are not integrated in the substitute, the device is classified clinically as a xenotransplant. A
related type of bilayered, tissue-engineered skin substitute has been developed that includes allogeneic
combinations of human fibroblasts in bovine collagen and human epidermal cells (Hu et al., 2006).
Research is focused on producing similar substitutes without bovine collagen.
9.2.3 Maxillofacial Implants
Maxillofacial implants may be used to correct defects such as those following head or neck cancer surgery
or to correct congenital defects such as cleft palate. Maxillofacial implants are either extraoral, that is, those
used to reconstruct defective regions in the maxilla, mandible, and face, or intraoral, those used to repair
maxilla, mandibular, and facial bone defects. Polymeric implants used in extraoral repair should (1) be