Biomedical Engineering Reference
In-Depth Information
the high risks of leukaemias raise concerns about gene delivery mediated
by viral vectors. Conversely, non-viral vectors do not ensure the required
transfection efficiency. Furthermore, gene carriers can exacerbate the host
response already elicited by biomaterials. therefore, the future of gene
delivery in bone tissue engineering is very uncertain.
14.5 Scaffolds for bone repair
the favourable cell and mineralisation substrate offered by ceramics, polymers
and composites as well as the stimulation by specific growth factor needs to
be accompanied by architectural features similar to those of bone. natural
bone consists of cortical and trabecular morphologies, the latter having
variable pore sizes. Most of the studies performed so far have focussed on
mimicking this structure through engineering 3D macroporous scaffolds with
the aim of providing the growing bone with an architectural structure where
cells can proliferate and deposit a new extracellular matrix. in addition, to
allow the integration of the tissue engineering construct, scaffolds have to
favour the in-growth of the surrounding tissues (i.e. bone and vessels). this
can be achieved through suitable porosity (range 100-300 mm diameter
pores), degradation rate and remodelling potential. Various techniques are
currently available to produce 3D scaffolds bearing these properties. almost
all the materials discussed in Section 14.2 can be processed into 3D porous
structures.
14.5.1 Three-dimensional scaffold fabrication
techniques
the most used methods for processing biodegradable biomaterials into 3D
porous scaffolds are (Chung and Park, 2007):
1.
Fibre bonding.
In this technique fibres of polymers can be assembled by
generating crosspoints with a second polymer in the solution in which
the first polymer is embedded. This technique suffers from relatively
poor control of the porosity.
2.
Emulsion freeze drying.
this method consists in freeze drying an
emulsion solution where the organic polymeric phase is dispersed in a
aqueous phase. Sublimation of the water phase leads to the formation
of a polymeric structure with interconnected pores up to 200 mm.
3.
Solvent casting/particulate leaching
. this can be considered one of the
most suitable methods for producing scaffolds. it consists of adding a
given amount of salt granules to a polymer solution in organic solvent.
after solvent evaporation, the salt granules are removed from the matrix
by immersion in an aqueous solution, obtaining a porosity up to 90%.
