Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 2 – The role of gels in instructive putties
In the case of PVA–based putties, the gel was still present in all the explants. When
comparing the in vitro and in vivo data, we could observe a possible relation between
the in vitro dissolution and the in vivo clearance of the gels. It is noticeable that, as
already reported in other studies, BCP granules are not expected to significantly
degrade and/or resorbe in vivo over 12 weeks. [284] However it can be hypothesized
that, when putties were implanted in vivo, the gels started to dissolve, probably due to
the attack of water that started their hydrolysis, and were cleared away by body fluids.
During their clearance, the available space between the BCP granules may have
increased over time allowing soft tissues, including blood vessels, to gradually enter
into the implants. This was possibly followed by colonisation of the micro–structured
BCP surface by (stem) cells which finally may have been instructed to osteogenically
differentiate and trigger heterotopic bone formation (Figures 3, 5). The lack of
osteoinductive potential in PVA putty could be the result of preventing cells to reach
the BCP surface. As already mentioned, after 12 weeks in vivo, the PVA gel was still
completely present amongst BCP granules while no bone formed and cells were
hardly seen in the implant. Meanwhile some residuals of CHI gel were observed and
no traces of CMC and PLU gels were present. Bone and cells were abundant in CMC
and PLU putties but they were less present in CHI putty. From these observations, a
correlation between bone formation and gel dissolution (except ALG, as
discussed here later) is likely. Although the dissolution rate and rapid clearance of the
gels could be important factors controlling bone induction, other factors cannot be
ruled out. For example, putties comprising the ALG gel, which had quicker dissolution
rate than CMC and PLU gels, showed less bone formation. Thus, other factors must
have influenced on osteoinduction, such as the chemistry of the gel or its degradation
residuals. Alginate is a copolymer composed of guluronic (G) and mannuronic (M)
acid blocks in different G/M content ratios that have been shown to influence the
biological properties of the gels. [285–287] According to some studies, high content of G
residuals provokes severe inflammation, [285, 286] while others suggested that it is
caused by low G content. [287] We used native sodium alginate powder with low G
content and did not observe clear signs of inflammatory reaction to ALG putties. The
lower osteoinductive potential of ALG putties may be related to the biological inertia of
alginate. It has been shown that native alginate does not promote significant cell
adhesion [288, 289] while CMC and PLU gels were reported to support the cells adhesion
and bone formation in vitro and in vivo. [290, 291] Thus, not only the dissolvability but also
the chemistry of the gel and its degradation products may play a role in decreasing
the osteoinductivity of some putties.
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