Biomedical Engineering Reference
In-Depth Information
algae derivative can be used to form NP TE scaffolds. Alginate is a linear copolymer
consisting of repeating blocks of (1,4)-linked
D
-mannuronate and
L
-glucuronate.
Upon submersion in an aqueous environment containing divalent calcium ions,
sodium ions from sodium alginate are displaced by calcium allowing for the forma-
tion of a calcium crosslink between adjacent carboxylic acid groups at neutral pH,
creating a solution to gel transition. However, despite exhibiting the ability to
maintain an NP-like phenotype, reports of shortcomings in utilizing sodium alginate
include the inability of IVD cells to assemble and maintain a functional ECM, as well
as a decrease in the mechanical integrity of the scaffolds that is dependent on the time
in culture [
117
]. It has been suggested that such observations are a result of Na
+
/Ca
2+
ion exchange in culture media, increased cell-mediated degradation of the gels,
cellular Ca
2+
metabolism, or a combination thereof [
117
].
In an attempt to overcome the aforementioned pitfalls, Chou and Nicoll
suggested that methacrylate-modified alginate solutions would create scaffolds
that have improved mechanical stability, maintain NP cell viability, and allow in-
situ free-radical crosslinking via a photo-initiator and UV light exposure [
118
].
Encapsulated bovine NP cells in 3% alginate hydrogels modified with 2.5% meth-
acrylate were shown to be viable and exhibit a round NP cell morphology at
14 days, concomitant with staining positive with immunohistochemistry for chon-
droitin-sulfate proteoglycan. The mean equilibrium Young's modulus for these gels
was ~1.2 kPa, which is slightly lower than that of native human NP tested in
unconfined compression [
114
]. Although the gel modulus increased with increasing
methacrylate incorporation (5-7.5% methacrylation), cell viability decreased. In an
extension of this work, Chou et al. implanted 2% alginate hydrogels modified with
3.5% methacrylate seeded with bovine NP cells into a murine subcutaneous pouch
model to assess the maintenance of phenotype and matrix production [
119
]. Results
indicated that the gels elicited a mild foreign body reaction, as evidenced by thin
fibrous capsule formation; however, the photo-crosslinked gels remained intact. NP
cells maintained collagen type II gene expression while showing a significant
increase in aggrecan expression between 4 and 8 weeks. Equilibrium Young's
modulus significantly increased in cell-encapsulated scaffolds (4.31
1.39 kPA)
in comparison to acellular controls (2.34
0.33 kPa), which could be indicative of
NP-like tissue formation.
Leone et al. developed a new amidic derivative of alginate crosslinked with 1,3
diaminopropane as they hypothesized that a hydrogel more closely mimicking the
physical and chemical properties of hyaluronic acid would be ideal for tissue
engineering the NP [
120
]. A 1% alginate solution resulted in a hydrogel that
reached a swelling equilibrium after 1 h and was capable of swelling up to 250%
its volume. Moreover, rheological analysis indicated that the alginate gel behaved
in similar manner to non-degenerated NP and exhibited a predominantly elastic
solid-like behavior under dynamic conditions with increasing stiffness upon
increasing angular frequency. When seeded with normal primary chondrocytes
from human knee articular cartilage, the gels allowed cell proliferation, mainte-
nance of phenotype, and even appeared to produce low levels of MMP-13, which
may be indicative of a cellular attempt to remodel the scaffold.
