Bone-Like Mineral and Organically Modified Bone-Like Mineral Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 26

facilitates efficient endocytosis (Shen, Tan, and Saltzman 2004). Surface morphology and
DNA retention at the mineral surface play an important role in improving transfection
efficiency. SBF concentrations and coprecipitation time can be altered to control the dis-
solution rate of the mineral layer and subsequent release of DNA to improve transfection
efficiency (Luong, McFalls, and Kohn 2009).
DrawbacksofUsingBLM
Although there is merit to using BLM coatings, there are also difficulties that may be
encountered that need to be acknowledged. Challenges that arise when using SBF to coat
implants, especially porous and porous-coated implants and scaffolds, include controlling
coating thickness, preserving substrate stability during functionalization, and translation
to industrial-scale processes. As with all apatite coating techniques, the thickness of the
BLM layer on 3-D scaffolds needs to be controlled to maintain sufficient porosity for mass
transport and angiogenesis. An implant coating that is too thick can occlude pores, which
would interfere with tissue perfusion and vascular infiltration in vivo. Excessively thick
mineral layers can result in delamination of the coating from the underlying substrate.
Substrate surface functionalization is carried out before incubation in SBF to obtain a nega-
tively charged surface for calcium nucleation. However, prolonged treatment for function-
alization can damage the underlying substrate. For instance, PLGA/PLLA materials etched
with sodium hydroxide can undergo considerable hydrolysis resulting in loss of structure,
thereby compromising mechanical stability. The necessity for functionalization creates some
limitations over the types of substrates that can be used for biomimetic precipitation (Table
1.2) as well as the types of methods that could be used to functionalize the surface.
Achieving industrial-scale batch processing could be a problem when working with SBF.
The SBF solution needs to be replenished periodically to maintain pH and ion concentra-
tions near saturation, which would be complex in an industrial setting. Batch processing
implants in large volumes of liquid under sterile conditions to prevent contamination can
also prove to be difficult.
One main disadvantage of using coprecipitation to create organic/inorganic hybrid
materials is the low efficiency of biomolecule incorporation. Although biomolecule reten-
tion on BLM is higher with coprecipitation than with adsorption, only about 10% loading
can be achieved with coprecipitation. Therefore coprecipitation requires large concentra-
tions of biomolecules to elicit a desired response, which becomes expensive for growth
factor administration.
Conclusions
Bone is a complex and dynamic composite tissue that consists of both inorganic and organic
phases, supporting cellular adhesion, proliferation, and differentiation. The technique of
biomimetic calcium phosphate precipitation attempts to simulate aspects of this complexity
by forming a BLM coating on the surface of natural and synthetic substrates. This mineral
layer makes a biomaterial more osteoconductive, as well as enhances mechanical strength