Biomedical Engineering Reference
In-Depth Information
that resembles an extracellular matrix surrounding cells in vivo . Results revealed
that HRNs coated on and embedded in hydrogels signifi cantly improved osteo-
blast adhesion even at a low concentration of 0.001mg/ml [106]. These HRN
incorporate hydrogels can also solidify at body temperatures, thus, allowing
for aqueous injection and in situ bone healing. Thus, it has been anticipated
that such nanostructured hydrogels are promising for bone tissue engineering
applications.
7.3.4 Nanostructured Composites
Bone is a natural nanostructured matrix composite made from calcium-phosphate
apatite mineral which is reinforced with collagen fi bers. Cells in the body are
accustomed to interacting with composite materials that have nanostructured
features, thus, one way to closely mimic the properties of natural bone tissues is by
using nanocomposite biomaterials. Such composite biomaterials are engineered
materials consisting of two or more distinct materials with signifi cantly different
physical or chemical properties that provide desirable overall mechanical, chemi-
cal, biological, and physical properties. Such composites may contain reinforcing
phases embedded within a matrix phase. The reinforcing material can be either
fi bers or particles, for example, carbon nanofi bers, carbon nanotubes, helical rosette
nanotubes (HRN), or ceramic nanoparticles (such as hydroxyapatite or calcium
phosphates). The matrix can be metal, ceramic, or polymer, but most biomedical
composites have polymeric matrices. The majority of composites used in biomate-
rial applications are intended for the purposes of bone repair.
Polymer composite materials offer a desired high strength and bone-like
elastic properties, potentially leading to a more favorable bone remodeling
response. For example, polyethylene (PE) is a polymeric material which, in differ-
ent grades — namely, HDPE (high - density polyethylene), UHMWPE (ultra - high -
molecular - weight polyethylene), and LDPE (low - density polyethylene) — can be
used as an implant [107]. Unfortunately, osseointegration of this polymer is low.
Thus, normally its attachment to bone should be mechanical in nature. However,
in many studies bioactivated PE has been obtained by adding bioactive materials
to PE, such as, hydroxyapatite-reinforced high-density polyethylene (HAPEX).
In addition, some studies investigated nanostructured titanium as a bioactive
coating on UHMWPE and polytetrafl uoroethylene (PTFE) [134]. For instance,
Reising et al. demonstrated that a nanostructured Ti coating created by a new
ionic plasma deposition (IPD) method can signifi cantly improve osteoblast func-
tions compared to conventional Ti or the uncoated polymer (Figure 7.10) [134].
Minerals of calcium phosphates are frequently used as an additive to poly-
meric matrices. Calcium phosphates (CaP) are a major constituent of natural
bone. Therefore, they show great potential as a supplement in orthopedic applica-
tions, including bone replacements, bone cements, and scaffolds for tissue engi-
neering [108-109]. Thus, they are very popular as bioactive ceramic materials. For
example, the chemical composition of hydroxyapatite (HA) makes it more stable
against resorption. On the other hand, tricalcium phosphate (TCP) is partially
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