Bone-Like Mineral and Organically Modified Bone-Like Mineral Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 5

Engineering Biomaterial Surfaces
A range of materials used for both prosthetic and regenerative therapies attempts to emu-
late some of the compositional, structural, and/or functional characteristics of the native
bone microenvironment. Whether biomaterials are designed to function in vivo in a tran-
sient or permanent manner, they should integrate with host tissue and not lead to fibrous
encapsulation. Clinical success rates of prostheses correlate with implant integration with
surrounding tissue. Successful implant integration with host bone is characterized by a
bone-like interface that integrates the implant surface with surrounding bone. This inter-
face contains mineral, collagen, and cellular components and functions as a site for bone
formation and resorption. This nanometer-thick interface is observed on implant surfaces
that are conducive to osteogenic cell attachment, proliferation, and differentiation.
Implant and scaffold materials are designed to promote osteoconduction and/or
osteoinduction, thereby improving osseointegration (Albrektsson and Johansson 2001).
Osteoconduction refers to the propensity of a surface to allow bone growth. An osteo-
conductive material implanted at the defect site allows osteogenic precursor migration,
adhesion, proliferation, and differentiation (Alsberg, Hill, and Mooney 2001). Conductive
materials support adhesion of cells migrating from surrounding host tissue or may be
used as a carrier to transplant osteogenic precursors. Therefore, the conductive properties
of a substrate surface do not guarantee osseointegration but simply allow it to take place.
Integration into host tissue is governed by additional factors that direct cells and organic
components to mineralize and form new bone. Therefore, osteoconduction is necessary,
but not sufficient for osseointegration.
Osteoinduction refers to the process by which osteogenesis is induced. More specifi-
cally, this is the process by which osteogenic precursor cells are actively guided to develop
into differentiated osteogenic cells (Albrektsson and Johansson 2001). These differentiated
cells partake in the restructuring of the extracellular matrix and the subsequent forma-
tion of new bone. Osteoinduction is typically achieved via the incorporation of growth
factors, peptides, and/or DNA that interact with cell surface receptors and trigger sig-
nal transduction pathways to recruit and direct cell infiltration into the defect site from
the surrounding tissue or transplanted donor cells. Osteoinductive materials, functional-
ized with biomolecules, actively engage in cell recruitment and direction to enhance the
quality, amount, and rate of bone formation compared to osteoconductive materials alone
(Hirano and Mooney 2004). An osteoinductive material is implicitly osteoconductive since
a biofunctionalized nonconductive material surface would negate the inductive effects
that would have lead to bone formation. Both osteoinductive and osteoconductive proper-
ties of the material play an integral role in osseointegration.
Although bulk properties of a material provide structural stability for both prosthetic
and regenerative therapies, surface characteristics play an equally important role in reg-
ulating conduction and integration (Mitragotri and Lahann 2009; Murphy et al. 2000b;
Liu, de Groot, and Hunziker 2005; Liu, de Groot, and Hunziker 2004). Surface chemistry,
surface roughness, and elasticity can affect biological responses to implanted materials
(Temenoff and Mikos 2008).
Surface chemistry. Atoms at the surface of a material are not bound on all sides like they
are in the bulk. Unbound surface atoms have unfilled valence electrons resulting in surface-
free energy also referred to as surface tension. When implanted into the host, proteins
migrate toward the implant surface to reduce this surface free energy. Two other factors
that regulate protein adsorption or foreign body response are surface charge and surface
hydrophilicity (Temenoff and Mikos 2008). Hydrophilic surfaces demonstrate enhanced